Microbiology and

Immunology
Dr. Chirag Sheth Dra. Verónica Veses

Immunology for Medical Students

Contact Details

Email: chirag.sheth@uch.ceu.es
Room 305, Faculty of Health Sciences Building
https://www.uchceu.es/directorio/chirag-sheth
Consultancy hours:
Monday 1200-1300
Thursday 1200-1300
Friday 1200-1300

2
Main Elements of this Course

Topic 1: Overview of the immune response. Cells and

lymphoid tissues of the Immune system. Cytokines.
Inflammation
Topic 2: Cell Markers. Cells. Mucous membranes and skin
Topic 3: Antibodies
Topic 4: TCR and MHC molecules
Topic 5: Regulation of cellular and humoral immune
response
Topic 6: Immune response to infections and cancer
Topic 7: Allergies and hypersensitivity
Topic 8: Disorders of the immune system and
immunogenetics
Topic 9: Alloimmunity and pregnancy
Topic 10: Biotechnology. Vaccines.

3
Course Textbook

Kuby Immunology - Seventh Edition ©2012

Judy Owen (Haverford College) , Jenni Punt (Haverford College) ,
Sharon Stranford (Mount Holyoke College)
ISBN-10: 1-4292-1919-X
ISBN-13: 978-1-4292-1919-8
Paperback, 574 pages

4
Seminars

S1: 4th October: Immunology

S2: 31s October: Immunology
S3: 15th November: Immunology
S4: 28th November: Microbiology

5
Practical Sessions

Consult the online schedule

6
Course Evaluation

The course is evaluated by combining a final exam (theoretical, 50%) with a

continuous evaluation system (practical-theoretical, 50%), divided as follows:
A. Theoretical Examination (50%):
50% of the final subject grade. Students must score a minimum of 5
overall in order for the grades from the continuous evaluation
system to be taken into account. The test will include approximately
15% integrated medicine type exam questions.
B. Continuous Assessment (50%):
5% - Active participation and attendance to classes
20% - Attendance to (2) Los Miercoles Hablamos de Salud Seminars
and (1) Seminar in Fundacion Valenciana de Estudios Avanzados;
Seminar Assignment Scores
25% - Mid-Term Exam (Micro. and Immuno.)
To pass the course, students should achieve a minimum of 5 by adding the
scores of each of the assessment types (A and B).

7
Los Miércoles Hablamos de Salud

You must attend (mandatory) the following two seminars, held

on Wednesdays from 1200-1400 in the Aula Magna of the Faculty
of Health Sciences Building:
Artritis reumatoide
22 noviembre 2017
Pacientes trasplantados hepáticos
29 noviembre 2017
Attendance will be taken

8
Overview of the Immune System

LECTURE 1

9
Overview of the Immune System

What is immunity?
Immunity is the state of protection against foreign pathogens
or substances (antigens)
Latin term immunis meaning e empt is the so rce of
the English word immunity
Observations of immunity go back over 2000 years
Thucydides, an ancient historian, wrote in 430 BC of a
plague in Athens where those who had recovered could
safely nurse the currently ill

10
Co-evolution of Microorganisms and Mammals

Plants m a Bacteria m a Fish m a Mammals m a H mans m a

11
12
 A historical perspective of immunology

Can we generate immunity without inducing

disease? YES h o gh accina ion
Vaccination prepares the immune system
to eradicate an infectious agent before it
causes disease
Widespread vaccine use has saved many
lives
Classic examples: rabies vaccine and
eradication of smallpox

13
Edward Jenner and the Smallpox Vaccine

14
Importance of Vaccination

15
 Important concepts for understanding the mammalian
immune response

Pathogens fall into four major categories

Immune responses are quickly tailored to the type of organism involved

16
17
Important concepts for understanding the mammalian
immune response

Immune responses rely on recognition molecules

Germ-line encoded (pattern recognition receptors, PRRs)
These bind to pathogen-associated molecular patterns
(PAMPs)―generic molecules found on many different
types of pathogens (e.g., peptidoglycan)
Randomly generated (B and T cell receptors)
These bind to very specific antigens, rather than generic
molecules found on many pathogens

18
A historical perspective of immunology

A portion of immunity involves both humoral and cellular

components
Humoral immunity combats pathogens via antibodies
Antibodies are produced by B cells
Antibodies can be transferred between individuals to
provide passive immunity
Cell-mediated immunity involves primarily T lymphocytes
These can eradicate pathogens, clear infected self-cells, or
aid other cells in inducing immunity
A historical perspective of immunology

Clonal selection
Individual B and T cells each have an individual specificity for a
single antigen
This is due to each cell having many copies of a receptor on
their surface that only bind to one type of antigen
These are randomly generated by DNA rearrangements
Many of these are nonviable and are deleted during
development
When a B or T cell interacts with its specific antigen, it is selected
and becomes activated
Activation results in a proliferation, producing a large number of
clones
Each clone is reactive against the antigen that initially
stimulated the original lymphocyte
21
Important concepts for understanding the mammalian
immune response

Tolerance ensures that the immune system avoids destroying host

tissue
Many of the random rearrangements used to create B and T
cell receptors could be anti-self
Tolerance helps to keep these anti-self recognition
molecules/cells from circulating in the bloodstream

22
 Important concepts for understanding the mammalian
immune response

In response to pathogens, vertebrate immune systems use two interconnected

systems
Innate immunity
Adaptive immunity

23
Important concepts for understanding the mammalian
immune response

Innate immune responses

First line of defense
Fast, but nonspecific
Uses germ-line encoded recognition molecules
Also uses phagocytic cells

24
Important concepts for understanding the mammalian
immune response

Adaptive immune responses

Humoral and cell-mediated responses
Using B and T lymphocytes, respectively
Slower to develop
5 6 days (or more)
Use randomly generated antigen receptors
Highly specific to individual antigen molecules

25
Important concepts for understanding the mammalian
immune response

Innate and adaptive immunity work cooperatively

Activation of innate immune responses produces signal
molecules (often cytokines)
These signal molecules stimulate and direct adaptive immune
responses

26
Important concepts for understanding the mammalian
immune response

Memory is the hallmark of adaptive immunity

Primary response is initiated upon first exposure to an antigen
Memory lymphocytes are left behind after antigen is
cleared
A second exposure to the same antigen re-stimulates memory
lymphocytes . Reactivation yields faster, more significant,
better response
Memory is NOT present in innate immunity

27
Immune Memory

28
 The good, bad, and ugly of the immune system

Dysfunctions of immunity―two broad categories

Overly active or misdirected immune responses
Allergies/asthma
Autoimmune disease (e.g., multiple sclerosis,
Crohn s disease
Immunodeficiency
Primary (genetic) loss of immune function
Secondary (acquired) loss of immune function
Opportunistic infections (e.g., oral thrush) can occur
in people with impaired immune responses

29
The good, bad, and ugly of the immune system

Transplanted tissues
A rare case where we want to AVOID an immune response
(rejection)
The bod s nat ral response to foreign tiss e is to attack it
and destroy it
Cancer
A situation where the dangerous cells we want to target are
our own self cells
Generally tolerated and hard to generate immunity
against

30
 Summary

Immunity is a complex subject, broken down into many different

layers and areas
This is just a quick summary of the basics of the field― he e
MUCH more to come!
Understanding how immunity works allows us to:
Exploit it to prevent infections (vaccination)
Exploit it to treat illness (shutting down autoimmune disease or
ramping up anti-cancer responses)
Provide safer organ and tissue transplants

31
Review Questions

Wh as Jenner s accine s perior to pre io s methods for

conferring resistance to smallpox?
What are the four major categories of pathogen, and amongst
these, which are likely to be most homogenous and diverse in
form?
What is meant by the term tolerance?
What is an antigen? What is an antibody? How are they related to
one another?

32
Cells and Lymphoid Tissue of the Immune System

LECTURE 1

33
Cells of the immune system

Immune responses result from coordinated activities of many

cells, organs, and microenvironments in the body
Hematopoietic stem cells (HSCs) have the ability to differentiate
into many types of blood cells
HSCs are rare 1:50,000 bone marrow cells, huge proliferative
potential
All red and white blood cells develop from a pluripotent HSC
during hematopoiesis
Hematopoiesis is a highly regulated process

34
35
Cells of the immune system

In adult vertebrates, hematopoiesis occurs in the bone marrow

Within the bone marrow, HSCs are constantly renewed and
directed to differentiate into two major types of progenitors
Common myeloid progenitor cells
Common lymphoid progenitor cells

36
Cells of the immune system

Four main types of cells develop from common myeloid

progenitors
Red blood cells (erythrocytes)
Monocytes
Granulocytes
Megakaryocytes

37
Cells of the immune system

Four main types of cells develop from common myeloid

progenitors
Red blood cells (erythrocytes)
Contain high concentrations of hemoglobin
Anuclear in mammals (nuclei lost during time in bone marrow)
Oxygen delivery to cells and tissues
Monocytes
Granulocytes
Megakaryocytes

38
Cells of the immune system

Four main types of cells develop from common myeloid progenitors

Red blood cells (erythrocytes)
Monocytes
Migrate into tissues and differentiate into macrophages
function to repair/remodel, destroy pathogens, present antigens;
2 new classes:
» inflammatory (tissue distribution) - osteoclasts (bone);
microglial cells (CNS), alveolar macrophages (lung)
» patrolling (blood vessel crawling)
Can also differentiate into dendritic cells HIGH degree of
function as ingesters of antigens, followed by presentation to
naïve T lymphocytes for initial activation
Granulocytes
Megakaryocytes

39
40
Cells of the immune system

Macrophages and neutrophils are specialized for phagocytosis

Macrophages can also present antigens to T cells via MHC
molecules
Immature dendritic cells capture antigen, then mature and
migrate out of that location to another to present antigen to T
cells
Dendritic cells are the most potent antigen-presenting cells
for activating naïve T cells

41
Cells of the immune system

Four main types of cells develop from common myeloid

progenitors
Red blood cells (erythrocytes)
Monocytes
Granulocytes
Ne trophils direct harm to pathogens
Basophils mast cells inflammation allergies
Eosinophils anti iral acti it antiparasite acti it
Megakaryocytes

42
43
44
Cells of the immune system

Four main types of cells develop from common myeloid

progenitors
Red blood cells (erythrocytes)
Monocytes
Granulocytes
Megakaryocytes :bone marro resident form platelets cell
fragments placed into circulation for clotting

45
EPO: eosinophil peroxidase ; MBP: Major basic protein; ECP: Eosinophil cationic protein; EDN: eosinophil derived neurotoxin; RANTES: Regulated
on Activation, Normal T Expressed and Secreted; MIP: Macrophage inflammatory protein

46
Cells of the immune system

Three main types of cells develop from the common lymphoid

progenitor
B lymphocytes
T lymphocytes
There is a subset that appears more similar to Natural
Killer cells that are known as NKT cells
NK cells
Lymphocytes constitute 99% of cells in the lymph
Lymphocytes appear visually very similar, but different sets carry
different cluster of differentiation (CD) molecules on their surface

47
48
Common

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49
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50
Cells of the immune system

Three main types of cells develop from the common lymphoid

progenitor
B lymphocytes
T lymphocytes
There is a subset that appears more similar to Natural Killer
cells that are known as NKT cells
NK cells
Closely related to B and T cells
No antigen specific receptors
Distinguished by cell surface marker NK1.1 and presence of granules
Efficient at detecting and killing altered or infected self cells.
Detects absence of MHC class I
Lymphocytes constitute 99% of cells in the lymph
Lymphocytes appear visually very similar, but different sets carry
different cluster of differentiation (CD) molecules on their surface

51
 P ima l mphoid o gan Whe e imm ne cell
develop

B lymphocytes
develop in the niche
of the bone marrow
Endosteal niche:
quiescent HSCs
Vascular niche:
mobilized HSCs
(active)

52
Primary lymphoid organs―Where immune cells develop

T cells develop initially in the bone marrow, but then

migrate to the thymus to achieve full maturity

53
Seconda l mphoid o gan Whe e he imm ne
response is initiated

Areas where lymphocytes encounter antigen, become activated,

undergo clonal expansion, and differentiate into effector cells
Secondary lymphoid organ areas include:
Lymph nodes
Spleen
Mucosa-associated lymphoid tissue (MALT)
Other diffuse and loosely organized areas
These are connected to each other via the blood and lymphatic
circulatory systems

54
55
Seconda l mphoid o gan Whe e he imm ne
response is initiated

Lymph nodes and spleen are the most highly organized secondary
lymphoid organs
T-cell and B-cell activity are separated into distinct
microenvironments
The cells will actively migrate toward each other during
activation events for their required interactions

56
Seconda l mphoid o gan Whe e he imm ne
response is initiated

Differentiation into effector cells takes place in follicles of

secondary lymphoid organs
T lymphocytes
CD4+ T cells differentiate into helper T cells that assist in
B-cell differentiation
CD8+ T cells differentiate into killer (or cytotoxic) T cells
that attack and destroy virally infected cells

57
Seconda l mphoid o gan Whe e he imm ne
response is initiated

Differentiation into effector cells takes place in follicles of

secondary lymphoid organs
B cells further mature in germinal centers in such tissues
Antigen affinity is increased
Class switching can take place
Both B and T lymphocytes will develop into long-lived memory
cells in these areas, as well

58
59
 Afferent lymphatic vessels
carry DCs and antigen to the
lymph nodes

Paracortex is
supported by
fibroblastic reticular
cells which guide the
migration of APCs and
T cells.

The B cell follicle

contains a network of
FDCs which perform
the same role as FRCs

60
Secondary lymphoid organs―Where the immune
response is initiated

The spleen is the first line of defense against blood-borne

pathogens
Red blood cells are compartmentalized in red pulp
White blood cells are segregated in white pulp
A specialized region of macrophages and B cells known as the
marginal zone borders the white pulp

61
62
Secondary lymphoid organs―Where the immune
response is initiated

Mucosa-associated lymphoid tissue (MALT)

Important layer of defense against infection at mucosal and
epithelial layers
Organizes responses to antigens that enter mucosal tissues
Includes a network of follicles and lymphoid
microenvironments associated with the intestines (gut-
associated lymphoid tissue, or GALT)

63
64
Seconda l mphoid o gan Whe e he imm ne
response is initiated

MALT
M cells in the lining of the gut are unique
They function to deliver antigen from the intestinal spaces
to lymphoid cells in the gut wall

65
 Secondary lymphoid organs―Where the immune
response is initiated

Various loosely organized and

diffuse lymphoid tissue are
also found under the skin,
mucosae, and tertiary tissues
at sites of infection

66
Summary

While blood cell development is a necessary part of immune

responses it s onl a first step
Multiple other organs and tissues of the body must receive those
blood cells and interface them with each other to achieve proper
immune responses
These interfaces and the tissues involved are complex and
m ltifaceted as sho n in this chapter s materials

67
Topic 2: Antibodies.
Regulation of cellular and
humoral immune response.
Receptors and Signaling: B- and T-
Cell Receptors
Antibodies

• Antibodies, also known as immunoglobulins, are proteins

in the blood that are created by B cells in response to
proteins called antigens, which the body recognizes as
‘non-self’.
• Imagine you’re in your clinic and a patient comes to see
you with a fever, muscle aches, a sore throat and a general
sense of lethargy. bacteria
– These symptoms might¡suggest that your patient’s
been infected with Mycoplasma pneumoniae, which
should be treated with antibiotics as quickly as possible
– Or they might indicate influenza, which should never
be treated with antibiotics because it’s caused by a
virus.
Antibodies

• One way to tell the difference between the two, or

any other pair of illnesses, is to look at your patient’s
blood to see what type of antibodies they are
producing. This should reveal which antigens are in
their system.
• Antibodies also play an important part in the
development of several diseases. They can render
infectious organisms harmless by attaching to their
antigens.
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• The term Antikörper (‘antibodies’) was introduced in Paul Ehrlich’s
‘Experimental Studies on Immunity’ in 1891.
– At first, he fed the mice cocaine; later, he started testing their
resistance to different amounts of ricin, one of the world’s
deadliest toxins.
– The amount of ricin that it takes to kill a laboratory mouse is
extremely small, but Ehrlich gradually increased the doses for
individual mice until they could resist doses that would have been
lethal if they had been administered straight away.
Antibodies

• The underlying concept was similar to smallpox

vaccination, introduced by Edward Jenner in 1798.
What separated Ehrlich’s work from Jenner’s was his
insights into the mechanisms of immunity,
particularly the side-chain theory that he proposed in
1900.
• The theory suggested that side chains within the cells
react with antigens and bind them – fitting like a lock
and key – to create antibodies, which then travel
around the body in the blood.
 The structure of antibodies
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 The structure of antibodies

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• Some heavy-chain domains are separated by
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• Signals are usually generated by the binding of a ligand to
a complementary cell-bound receptor
• A cell can become more or less susceptible to actions of a
ligand by increasing or decreasing expression of the
receptor for that ligand
Essential Concepts – 2 stand
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• The ligand may be a soluble molecule or a peptide,

carbohydrate, or lipid presented on the cell surface
• The ligand may travel long distances from its entry
point in either the bloodstream or lymphatics before
it reaches a cell bearing the relevant receptor
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transcriptional program of the target cell
-

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receptors are required to effect particular outcomes
• Integration of all signals received by a cell occurs at
the molecular level inside the recipient cell
 Receptor-ligand interactions

• Receptor-ligand binding occurs via

multiple noncovalent bonds
– Each individual bond may be
weak
– Many such bonds occur between
receptors and ligands, providing
great cumulative bond strength
Receptor-ligand interactions

• Antigen-immune system receptor interactions are

enhanced by co-receptor binding
– These are separate receptor-ligand interactions
that may take place near the original interaction
– Often times, a single type of interaction may be
insufficient to lead to an activation event
• A co-receptor interaction may provide a
second signaling interaction to further signal
the cell to proceed with activation
Receptor-ligand interactions

• Receptor-antigen interactions are usually multivalent

– Multivalency increases avidity of the interactions
• Individual interactions have an affinity―a
strength of that individual pairing
• Avidity is the combined strength of multiple
interactions
• As such, an interaction may have weak affinity,
but high overall avidity
 Receptor-ligand interactions

Ratio of time spent in “ON” vs “OFF” state defines the affinity of the receptor
 Receptor-ligand interactions

• Receptor and ligand expression can vary during the course of an

immune response
– An example: white blood cells (blue) treated with an activating
mitogen show upregulation of the receptor for cytokine IL-2
(yellow-orange)
 Receptor-ligand interactions

• Local concentrations of cytokines and other ligands may be

extremely high
– A cell may direct its secretion machinery toward a recipient for maximum
effect
• An example: blue dendritic cells secreting cytokine IL-12 (pink) to T cells
(green); note the localization of the cytokine-filled, vesicle-rich area in the
dendritic cell
 Common stages used in many signaling
pathways

• Binding of antigen to receptor induces:

– An internal signaling cascade, which:
• Leads to cellular alterations in:
– Motility
– Adhesive properties
– Transcriptional programming
• These cascades are behind the various cellular changes that take
place during an immune response against an antigen
• Often, the same players/proteins are used in different cell
types―triggering receptors may be different
 Common stages used in many signaling
pathways

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– Src-family kinases (phosphorylate tyrosines) protein otner EN a

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start
transductor
• These kinases become activated themselves
cotno
when phosphorylated ( scaffoid )
sirve
andamio
Summary

• Signal transduction is a complex subject

• The trick is to look for similarities and differences
– Both B and T lymphocytes use similar strategies
once the processes are underway
– The differences lie at the level of the antigen
receptors and the early membrane-linked events
• By understanding antigen receptor and early
membrane event differences, we can better
understand how signaling (and activation) works in
each lymphocyte cell type
 nnltinerizatior
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 table
Component B-cells memoria this
T-cells exceptqqstqros.me
Receptor B-cell receptor – similar to T-cell receptor – heterodimer,
antibody structure. CH structure either ab (more common, binds
modified to bind to cell MHC) or gd (mucosal localiztion,
membrane binds MHC and other ligands).
Short cytoplasmic tail
Accessory proteins Iga and Igb with ITAM (Immuno- CD3 complex (3 dimers – de, ge
-

and [either zz or zh]. Intra-

-

(signal transduction function) receptor tyrosine activation

RAMS
motifs) cytoplasmic tails of CD· complex
contain ITAMs
Ligand Antigens (sometimes bound to
- -
MHC class I and II

-0--0
complement components eg C3d)
Co-receptors O
CD19, CD21 (C3d binding) and CD4 (MHC class II) andO
-
CD8
CD81
O (MHC class I) - bind non-
-

polymorphic regions of the MHC

moleculegtabilire
$ hola

①
CD28: naïve T-cells need CD28
co-stimulation – CD4 and CD8
-

alone are not enough.

Key tyrosine kinases Syk and Lyn (membrane Lck (membrane associated) and
associated) ZAP-70
Key adaptor proteins
O .O
Phospholipase C; BLNK; LAT, SLP76 and GADS, PLC
- -
-0
Key effector molecules NFAT, NF-kB, AP1 NFAT, NF-kB, AP1
Topic 2: Regulation of the
Cellular and Humoral
Response
Receptors and Signaling: Cytokines
and Chemokines
 General properties of cytokines and
chemokines

• Cytokines are proteins that mediate the

effector functions of the immune system
– They can act in several different ways
• Endocrine action―released into
the bloodstream to effect distant
cells
• Paracrine action―released to
effect nearby cells
• Autocrine action―released, but
then bind to receptors on the cell
that produced them
 General properties of cytokines and
chemokines

⑦
• Most cytokines are soluble proteins

O
– Released by one cell a

– Bind to receptors on another cell

= – Small amount can induce biological
-

effects
- – High affinity 1 cytokinett
receptor

• Some cytokines are membrane-bound

forms (TNF family)
 General properties of cytokines and
chemokines
special fact
Type of cytokines with
a
are a
chenokines
El
'

of chenoaHrach con . . .

• Cytokines can have numerous biological functions

– Changes in differentiation state, proliferation, or inducing
survival (or death!) of target cells
– Interplay is especially important in immunity
Cytokines are produced
• Innate immune system cytokines ←
by innate El adaptive
inmune Cell
– IL-1, TNF-α, CXCL8 types
-

J
– Secreted by cells of innate immunity (e.g., neutrophils,
macrophages)
• Adaptive immunity cytokines
– IL-2, IL-4, IL-17
-

– Secreted by T and B lymphocytes

Xneed to memoriae
General properties of cytokines and
chemokines

• Cytokines can mediate the activation, proliferation, and

differentiation of target cells
agtokines
– Effects can include Whan you combina
• Pleiotropy —one cytokine produces multiple effects
• Redundancy—more than one cytokine induces the
same effect
• Synergy —two (or more) cytokines work together
to induce an effect
• Antagonism —one cytokine can inactivate the
effect of another
General properties of cytokines and chemokines
The effect
of a

at
cytokire
docsnt stop

the cell this

÷
the ⑦ of ebe
somethíng
,

ce " neill go on ⑦
ttt
Fort her produce
→ casca de
cytokires
efecto

÷: O
÷: -

O o

O
General properties of cytokines and chemokines

1
Cytokines vs Chemokines

• Cytokines
– Soluble (although some are membrane bound)
– Effects include: changes in expression of surface
adhesion molecules, enzyme activity modulation,
transcriptional control, apoptosis
– Classification: Interleukins (classification and failure –
TNF)
• Chemokines
– Are cytokines (chemoattractants)
– Effects limited to mobilization of immune cells –
cytoskeletal effects
– Classification: Based on biochemical structure
 Six families of cytokines and associated
receptor molecules essential for
corre
at functioning

the
inmune
of

System
lo
• Each family is distinguished by: escrito
A prenderse y
– Structure of ligand and receptor Subrayado dice
que
.

lo
important
es
que
.

– Nature of signaling pathways induced el que

es

• 00
000 -
e- - a-

- +
• cendea
otros
B-
00 *
mires
•

• O lrepnlsion
Six families of cytokines and associated
receptor molecules
IL -

1 IL -

18 , IL
-

23-3
,

• Cytokines of the IL-1 family promote inflammation -

inftanatqi
→
pro

– IL-1 stimulated by viral, parasitic, or bacterial antigens

• Secreted very early in immune responses by
cells)
macrophages and dendritic cells ( usnalhgimate inmune
Ist
acliuated

• Acts locally on capillary permeability and to pull

leukocytes to infected tissues tq
• Acts systemically to signal the liver to produce
acute phase proteins
• Can help to activate adaptive immune responses
p with inmune
inteact adaptive system
 Six families of cytokines and associated
receptor molecules

y.EE • Cytokines of the IL-1 family promote

inflammation
– IL-1α and IL-1β
-

III.Estáis →
EastEnders • 31 kDA precursors trimmed to a 17
kDa active form in secreting cells
• Binds to IL-1RI/IL-1RAcP (receptor
accessory protein) heterodimer
receptor
'
v
– Has inhibiting ligand/receptor pair to
÷:* shut down function
• IL-1 receptor antagonist (IL-1Ra)
① irons
binds to IL-1RI and prevents it from
binding with its functional partner
* '
sin
€ but for
this
IL-1RAcP
Same soabo
'

JI
 Six families of cytokines and associated
receptor molecules

• Cytokines of the IL-1 family promote

inflammation O :
s

– IL-18—an example of redundancy

• Also processed in secreting cells
to become active
• Uses same receptor family and
signaling as IL-1
• Expressed by macrophages and
dendritic cells early in responses
– Inhibition through binding of
cytokine before it binds to signaling
O
receptor
 Six families of cytokines and associated
receptor molecules

" is
• Cytokines of the IL-1 family promote
$
µ It inflammation allergies
receptoresRÉEÍÉÍÉEÍÍ
implicatiors
in
TO
Strong

①
,
– IL-33 asuma
broncoespasmo
①
• Constitutively expressed in smooth

{
muscle and bronchial epithelia
• Induces TH2 cytokines that promote
T- cell interactions with B cells, mast
cells, and eosinophils ÷.si?IiisErowa
→

• Implicated in asthma, allergies, and

bowel diseases
• Receptor is also a heterodimer with
IL-1RAcP
– Inhibitory receptors also exist to shut
down signaling
 Six families of cytokines and associated
receptor molecules
②

.iq/- -
• Signaling from IL-1 receptors
Í
– Adapter protein MyD88 at engaged
si
receptor recruits kinases
– Involves use of IL-1 receptor activated
kinases (IRAKs) and TGFβ-associated
kinase 1 (TAK1)
÷
– Results in activation of AP-1 and NF-κB
transcription factors, leading to
cytokine expression
 Six families of cytokines and associated
receptor molecules

• Each family is distinguished by:

– Structure of ligand and receptor
– Nature of signaling pathways induced
Six families of cytokines and associated
receptor molecules
• Hematopoietin (Class I) family cytokines
– Originate from and affect many cells
• Signal
– Differentiation (e.g. TH cell – IL-4 vs IL-12)
– Proliferation (e.g. of B- and T- cells – IL-2)
– Ab secretion (Plasma B-cell – IL-6)
– Share common structural motifs
Imp • Four-helix bundle motif organized in four antiparallel
helices
• Can be further grouped based on helix length
– IL-2, IL-4, IL-3 have short helices of 8–10 amino
acids
– IL-6, IL-12 have long helices of 10–20 amino
acids
Hematopoietin Family Cytokine Examples
Esaam
Cytokine Functions
IL-2 T- and B- cell proliferation
IL-6 B-cell differentiation into plasma
cells and antibody secretion
IL-4 vs IL-12 Determination of differentiation of
Th cells into one pathway or another
GM-CSF, G-CSF Differentiation of leukocytes
 Six families of cytokines and associated
depend
receptor molecules subfamilias
divided in
the raptor on

← ( know)
Hematopoietin (Class I) Receptors → cnaractois
¡¡
"
•
neic
receptor
C)
g-subunit family nave

-
b-subunit family
– gp130 subunit family

€
-

• Receptors for these cytokines include two types of protein domains

÷÷÷÷÷÷÷ :
– An Ig-like domain of β sheets
– Domains with homology to FNIII domain of fibronectin a.
.
• A common feature is .

multiple subunits
– Common subunits paired
with distinct cytokine-
binding subunit to form
ubfamilies of receptors
-00
→
Six families of cytokines and associated
receptor molecules

=
- -

● g-subunit bearing subfamily

– IL-2R is the stereotypical member of this
family
: g-subunit transduces signal

8 oo
O
o

:÷÷÷÷
E

diferentes
si
• Exists in low, intermediate, and high-
affinity forms
y
• Different forms marked by presence of
accessory receptor chains
qq.at • Lymphocytes shift to expression of high-
affinity form during activation events
• XSCID patients lack g-subunit – result is
loss of IL-2 receptor from T- and NK cells.
 Six families of cytokines and associated
receptor molecules
O
•
•
β-subunit bearing subfamily
Includes receptors for IL-3, IL-5, and GM-CSF
000
ú
– Each cytokine binds to a unique α subunit weak

– The β subunit is a shared signal-transducing

protein
strong
• These cytokines exhibit redundancy
– Act on HSCs - proliferation O O 05
– Induce megakaryoctye differentiation
– Induce differentiation and degranulation of
eosinophils / basophils
– Stimulate blood-cell differentiation
– Activate monocytes
 Six families of cytokines and associated
receptor molecules
• gp130 subunit subfamily
-

• Includes IL-6 (initiation of the immune response) and IL-

12 (Th1 cell differentiation pathway) receptors
EE

⑦ • Critically important; targeted disruption is lethal in

embryonic mice
• Ligand-specific dimers or trimers are expressed with the
l l l gp130 subunit
is
• Further subdivided into receptors for:
of
f p – Monomeric cytokines (e.g., IL-6)

Yard
Ha
– Dimeric cytokines (e.g., IL-12)
• Signaling pathways induced are similar to those induced
by interferons (discussed next)
 Six families of cytokines and associated
receptor molecules

• Each family is distinguished by:

– Structure of ligand and receptor
– Nature of signaling pathways induced

27
Me
Six families of cytokines and associated
with
receptor molecules infectad
{
¡f
cell
TU HC express
liras
ios
of
the
Surface
P protein
on
infectad destroyer
ce" te that is
d El
others detecte
• Interferon (Class II) cytokine family Show beof

Type I interferons – Increase MHC expression on target cells indicator

it
Cellular
can
so the
of
• terralStage ce
"
- in
– 18–20 kDa dimers with antiviral effects exposingniralyinfeoted aelb
– Secreted by activated macrophages, dendritic cells and virally infected cells
• Interferons α, a family of about 20 related proteins
• Interferon β
• Type II interferon (aka interferon γ)
– Dimer produced by activated T/NK cells
• Potent modulator of adaptive immunity, produced by activated T and NK
cells
• Drives T cell differentiation towards TH1 type
• Induces macrophage activation
• Anti-intracellular pathogen activity
• Used therapeutically in leprosy and toxoplasmosis treatments
• Type III interferon family
– Newer (2003); three current members (IL-29, IL-28A, IL-28B)
secreted by Special Dcs
-

( plasma agtoid Dcs) .

-

Antiviral ftx)
Six families of cytokines and associated
receptor molecules

• Interferon (Class II) cytokine family

¢
híghplastiúhs
the
– Receptors for interferons are heterodimers btw .

receptores
be
$ can

• Share conserved cysteine residues in similar re

combined
to Form
locations as Class II receptors dif receptor

• Family has at least 12 chains capable of binding

to 27 or more different cytokines, depending
on the receptor chain pairing utilized
 Six families of cytokines and associated
receptor molecules

• Each family is distinguished by:

– Structure of ligand and receptor
– Nature of signaling pathways induced

femeninos§
cellswuial
de
→
apoptosis
Six families of cytokines and associated
receptor molecules

• The tumor necrosis family (TNF) of cytokines

regulates many activities
– Development
– Effector function $ )
survival (apoptosis
Cell death
– Homeostasis of cells of the Cell

• Skeletal
• Neuronal
• Immune systems
 Six families of cytokines and associated
receptor molecules

→
• TNF cytokines may be soluble or membrane-

*µµ]g¡
bound
• Generally Type 2 transmembrane proteins (short
intracytoplasmic tail) that form as trimers
– TNF-α is proinflammatory and produced by
activated macrophages/other cell types
– TNF-β (or lymphotoxin-α) is produced by
activated lymphocytes, delivering signal to
leukocytes and endothelial cells
• Physiologically significant, membrane-bound
members are FasL and TNF-R1
Six families of cytokines and associated
receptor molecules

• TNF receptors
– Most are Type I membrane proteins (N termini
outside cell)
• Some are cleaved to become soluble variants birds
so
it
Enen
• Others are anchored to the membrane by
glycolipids siiiii
←
• Soluble variants can intercept cytokine before
it reaches an actual signaling receptor on a cell
Six families of cytokines and associated
receptor molecules

• Signaling through TNF receptors

– Not fully defined yet
– Some signals lead to cell death; others lead to cell
proliferation
• Signaling through the Fas receptor leads to
death
• Signaling through TNF-R1 may lead to death or
survival
 Six families of cytokines and associated
receptor molecules

• Each family is distinguished by:

– Structure of ligand and receptor
– Nature of signaling pathways induced
Six families of cytokines and associated
receptor molecules

• IL-17 family cytokines are proinflammatory

– Expressed by a variety of cells
– Receptors found on neutrophils, keratinocytes,
and other nonlymphoid cells
– Tend to work at the interface of innate and
adaptive immunity
– Generally exist as homodimers
• Monomers range from 17.3–22.8 kDa
• Primarily β sheet in structure, stabilized by
intrachain disulfide bonds
Six families of cytokines and associated
receptor molecules

Adaptive
 Six families of cytokines and associated
receptor molecules
x

• IL-17 family receptors

– Composed of five protein chains arranged in different ways
• Share fibronectin domains with Class I/II families of receptors
• Each subunit is a single-pass transmembrane protein
• All contain a cytoplasmic SEF/IL-17R domain for signal transduction
 Six families of cytokines and associated
receptor molecules

• Each family is distinguished by:

~

– Structure of ligand and receptor

– Nature of signaling pathways induced

El chenorepulsión
 Six families of cytokines and associated
receptor molecules
• Chemokines direct leukocyte migration
– Chemokine structure
• Small (7.5–12.5 kDa) proteins
• Possess highly conserved disulfide bonds that dictate both
structure and category (six categories)
 Six families of cytokines and associated
receptor molecules

• Chemokines direct leukocyte migration

– Chemokine receptors are an example of G-
protein-coupled receptors (GPCRs)
• Thread through the plasma membrane seven
produced by inrate inmune
times
cells , pdynorphonudeotides ,

DCS • Transduce signals via interactions with a

polymeric GTP/GDP-binding G protein
• These are a rare case where one type of
receptor may bind to several different
chemokine ligands
 evolved by
Cytokine antagonists pathogers strategies
and
Fond gubstarces
producimosevade cytokíne Mis
$ tio
oc

son

{
• Molecules that inhibit cytokine activity may be listos
– Natural (e.g., IL-1Ra inhibiting IL-1R activation)
– Pathogen-derived (e.g., viral proteins)

C )
Renembet son e

escanples
Cytokine-related diseases tod
so
diagnostico
• Inflammatory cytokine levels may be increased in disease
states
– Septic—common and potentially lethal
– Bacterial toxic shock induced by superantigens
– Potential involvement in rheumatoid arthritis and Type
2 diabetes
– Implication in lymphoid and myeloid cancers
– 1918 H1N1 influenza pandemic and cytokine storms
– Evidence of cytokine involvement in Severe acute
respiratory syndrome (SARS)
Cytokine-based therapies

• If cytokine levels are related to disease, therapies to

reduce cytokine levels can treat disease
– Rheumatoid arthritis
• Soluble TNF-α receptor (Enbrel) → reduce
synptons
• Monoclonal antibodies against TNF-α µ
(Remicade and Humira)
– Prevention of transplantation rejection
• Monoclonal antibodies against the α chain of
IL-2R (Simulect and Zenapax)
Summary

• Understanding what cytokines and their receptors are

is important to understanding cell/cell
communication channels
• There are many different types of cytokines and their
receptors, making this a complex field
• By understanding these molecules, we can better
understand disease states and how to intervene to
treat them, reducing their severity
Summary

• Understanding what cytokines and their receptors are

is important to understanding cell/cell
communication channels
• There are many different types of cytokines and their
receptors, making this a complex field
• By understanding these molecules, we can better
understand disease states and how to intervene to
treat them, reducing their severity
Topic 3: Innate Immunity.

Innate Immunity
 Innate Immune System

• Anatomical barriers

Sepecific

Barrios
(X cekresporse)
-

Cellular responsa

2
 Anatomical barriers to infection

• Several barriers, both physical and chemical, exist to

prevent pathogens from gaining access to deep tissues
– Should those barriers be breached, innate immune
system receptors recognize the threat
÷
• Conserved pathogen-associated molecular patterns
Éi \ (PAMPs) found on microbes ( protein Dave
a-

– Aging, dead, or damaged self structures can also be

o
recognized mate ¡ immneqtsyoten
_
• Damage-associated molecular patterns (DAMPs) ②
+

pos _

– Pattern recognition receptors (PRRs) recognize these

structures and target them for clearance

3
 Anatomical barriers to infection
that produce
) [glards flora of
cellular Normal
the skin
(Nor
-

Mechanical
barrios

Cellular barrios
1 A / /
destroy
EXC
EPFÍÓNIS
pathens

§
- -
Esca
INC

←
sutaae presentation

'
÷a÷:&
←

← O
{ noicalior
d
btw systems
cells

4
 Anatomical barriers to infection

• Barriers are just one difference between innate and

adaptive immune responses

Faster takes ti me to deuelop

memory
v
×
-

÷
T range
of Cell types
broad
5
 Anatomical barriers to infection

• Epithelial barriers prevent pathogen entry into the body’s

interior exterior
– Skin strong
ases
→
• Epidermis (keratin) and dermis (leukocytes)
• Secretions (fatty acids, pH)
– Mucosal membranes (GI, respiratory, urogenital)
• Densely packed cells X allow aacesbtw .
cells

ontains
• Tight junctions <

anlinioobilal maleantes
t
P T q no

pathoges
os

• Secretions (mucus, urine, saliva, tears, milk) w

6
 into
this table down →
go
eran
Copy

swfactant

7
l

8
 Anatomical barriers to infection

• Epithelial layers produce protective

substances
– Acidic pH
– Enzymes and binding proteins
– Antimicrobial peptides of >100
amino acids (e.g. psoriasin)
sbto to
antinivdáal
-

like
E
coli
Kill
.

9
renenber the Ones underlineal

for nívobes
need
"
-

that is why
Chelato r1 were
is so
little
in the
iron
Free it
to ( in
body of
boat .

growth
$ furgi

@ lungs

10
 Phagocytosis
• Defined as engulfment and internalization of materials such as
microbes for their clearance and destruction

11
Proceses of
phagocytosis

12
13
 would it
Phagocytosis phagocytosis
we
without inmune
able to ⑦ innata
be

s5ienppRs@7.ts
• Microbes are recognized by receptors on phagocytes
– Macrophages, neutrophils and DCs in tissues
– Monocytes in blood
– May recognize PAMPs directly
– May recognize soluble opsonin protein bound to
microbes t
Small protein elements
by
°
' produced inmune System

nos
Live ✓ of the
sale
→
coatlcouer
in
order
blood
with pathogersmore inmune
ri inuading then the
e
to
nake by
reconizcable
System

14
 DC
-

NOX

II. \ € -
dieotodió
⑦
→ =

opsonírj
• coatthepathogeh
tindired adios

ltlspeed -

15
 "
°"
°
"

)
Phagocytosis "

Lípopdysacharides
aomporertób
cells
Fergal
@
→
(
Mannose
¿
PRR
g.
g) • Microbes are recognized

}
by receptors on
phagocytes
– Recognize PAMPs

€ a
O °
directly
recorre– Recognize soluble
← *
-
bound
µ
vinos
opsonin protein
ergios
cóoPivote bound to microbes
÷÷:
⑨
16
-
Ísidues -

which indicat -

¥:*:* -
-

-
-

- -

) that
recogí protein (opsonins
this stick
are made in our
body but topathogeus

17
 Phagocytosis

:*
:O
. O
÷
• Ingested materials are taken into phagosomes
of
– Phagosomes are fused with lysosomes or granules at this step this
phagoagtosis aceres

• Destruction occurs through enzyme degradation, antimicrobial d

Impo¥stepFrkiUbacte
proteins, and toxic effects of reactive oxygen and reactive nitrogen
species (ROS and RNS)
leoelsto chronic
it are × able to produce them this
you
in flammation . 18
 Induced cellular innate responses
Structure of the ex .
seenbefore

• Families of PRRs recognize a wide variety of PAMP ligands

– TLRs – Toll like receptors
• Bind a large range of PAMPs (bacteria, fungi and viruses)
– CLRs – C-type lectin receptors to detectfurgi
→

• Bind carbohydrates on extracellular surface of pathogens

– RLRs – Retinoic acid-inducible Gene-I-like receptors todetectnirus
→

• Bind intracelluar viral dsRNA (recently, cytosolic viral DNA)

the
– NLRs – Nod-like receptors out
of
the

• Recognise a variety of PAMPs and DAMPs →

if
seer
it
neanstbat( danageel
Cell broker to

recognition
has
Cell
• Cell wall breakdown products creed
the problem
SO

then • Signaling pathways are activated, contributing to Fix

innate/inflammatory responses lat of

→
thingshappen
nexturtil
luto
NFAT ¡ AP -11 NF
-

KB
19
 Induced cellular innate responses

• Toll-like receptors (TLRs) recognize many types of pathogen

molecules discouered ir

– Homologous to fruit fly Toll receptor

– Dimers with extracellular leucine-rich (LRR) domains that bind

[ g.
PAMPs and DAMPs

Flexibilizan
giues

– Expressed on all immune cell subtypes

ost
fourd
and some non immune cell types (skin, ahuays
os
dimos

÷÷÷÷÷
mucosal, glandular epithelial cells, .

fibroblasts, vas. endo. cells,

¿ 20
Induced cellular innate responses

• Toll-like receptors (TLRs) recognize many types of

pathogen molecules
– Of 13 TLRs in mice and humans, some are in
lysosomes and some are surface bound
– Location helps determine what each binds

.IE#i*zrats:sentEt
Same are
round inside the cell

21
 CM

Ueside

vesi de

22
 Induced cellular innate responses

• Toll-like receptors (TLRs) recognize many types of pathogen

molecules
– Location helps determine what each TLR will bind

Some
← oíside
-

inside
gane
usnally →
homodímero
① ③ Cell

/
IIE

23
Induced cellular innate responses

• TLR binding of PAMPs activates signaling pathways

– Different TLRs recruit different adapter proteins to the
TIR domain
• Different adapter proteins lead to different events
• Pathways include:

inmune
– NF-κB transcription factor activation (Genes
me
altract
to
the
site →
regulated: Inflammatory cytokines, chemokines,
-

<
en

-
immune effector molecules, cell survival factors)
– Interferon regulating factor (IRF) pathways (Ifinfecto
viral
)

– MAP kinase pathway downstream transcription

factors such as AP-1 (Oncogene (2001) 20,
2390-2400)

24
 Induced cellular innate responses
to
memoriae
Xneed

Oo
25
Induced cellular innate responses

• C-type lectin receptors (CLRs)

– Expressed on monocytes, macrophages, DCs, inmune
, on
neutrophils, B- and T- cells Only expre Ssee
ceis
– Heterogeneous population of surface PRRs
-1
– Recognize cell wall components
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INC specific
• Sugars/ polysaccharides of bacteria/ fungi/ allergens
– Trigger a variety of pathways
• Some similar to those activated by TLRs
– Function as phagocytic receptors, induce intracellular
signaling and transcription activation

26
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Induced cellular innate responses

• RIG-I-like receptors (RLRs)

O-u.IT
– RNA helicases
– Function as cytosolic PRRs
– Recognize viral double-stranded RNAs (very
recently, cytosolic viral DNA)
– Trigger signaling pathways that activate:
• Interferon Regulatory Factors (IRFs) to trigger
antiviral interferon responses
• NF-κB transcription factor

28
-

29
 r

Induced cellular innate responses

• NOD-like receptors (NLRs)

– Large family of cytosolic PRRs
– Activated by intracellular PAMPs IDAMPS
– 23 NLR genes in human genome
– Can sense changes in intracellular environment –
breakdown products of viruses or bacteria stimulate
this system detect self infectad
-

cells

• Activates caspase-1 protease

• Caspase-1 cleaves IL-1/IL-18 into active forms for
release
30
 Induced cellular innate responses

• PRR signaling pathways activate expression of a large

variety of genes
– Antimicrobial peptides
– Type I interferons (potent antiviral activity)
– Cytokines (inflammatory IL-1, TNF-α, and IL-6)
– Chemokines
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Induced cellular innate responses

32
33
Inflammatory responses

• Proinflammatory cytokines and chemokines triggered

by innate responses to infection, damage, or harmful
substances
• Early components of inflammation include:
– Increased vascular permeability
– Recruitment of neutrophils and other leukocytes
from the blood to the site of damage/infection

34
Inflammatory responses

35
÷ ?
or DC
or DC

36
 Inflammatory responses

• Later stages of inflammation are the acute phase responses

(APRs)
– Induced by proinflammatory cytokines (IL-1, TNF-α, and IL-
6) -

the live
– APR involves: Cuork more on

• Increased synthesis/secretion of antimicrobial proteins

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are di
– CRP C- reactive protein

÷
– Complement components
• Liver acute phase proteins activate other processes
that help eliminate pathogens

37
 Natural killer (NK) cells

• NK cells are lymphocytes with innate immune functions

– Express a set of receptors for self proteins induced by:
• Infections
• Malignant transformations
°

Scar $ • Other stresses

,
→
bots – Activated NK cells perform one of two functions:
µµ
d

destruir
por
ir

F • Kill the altered self cell

→
• Produce cytokines that induce adaptive responses
against the altered self cell

38
 Regulation and evasion of innate and
inflammatory responses

• Regulation and control of these responses are important

– Defects in PRRs and signaling pathways increase
susceptibility to infections
– Defects that allow the systems to remain abnormally
“turned on” contribute to inflammatory disorders
• These can be cases where more and more of a
good thing ends up being unhealthy and damaging
– Regulation includes both
• Positive feedback mechanisms
• Negative feedback mechanisms

39
Regulation and evasion of innate and
inflammatory responses

÷
40
Interactions between the innate and adaptive
immune systems

• A constant interplay between the two systems exists

– Several innate systems have been co-opted by
adaptive immunity to contribute to antibody-
mediated pathogen elimination
• Opsonization
• Complement activation
– Some lymphocytes express TLRs, but use them as
costimulatory receptors

41
Interactions between the innate and adaptive
immune systems

• A constant interplay between the two systems exists

→ can presenta
– Dendritic cells are a key bridge → APC to B
¢ Tcelb

• They bring antigens from the site of infection

and present them to T cells in lymph nodes
• This activates the T cells, allowing them to
differentiate into particular pathogen-specific
subsets for the best antigen clearance
– TH cell subsets
– TC cells

42
Interactions between the innate and adaptive
immune systems Dcs are able

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43
Ubiquity of innate immunity

• Innate immunity is evolutionarily “older” than

adaptive
– TLRs are unique to animals, BUT
– PRRs with leucine-rich repeats (LRRs) are found in
virtually ALL plants and animals

44
45
Summary

• Basic barrier mechanisms and simple biochemical defenses

are the frontline of immunity
• Innate immune responses depend on recognition of
“general” pathogen molecules
– Responses are varied, but include:
• Phagocytosis
• Triggering of inflammatory responses
• Direct destruction by natural killer cells
• Initiation of adaptive immune responses
• Learning the first defense strategies of immunity helps to
better understand the later adaptive methods

46
Topic 3: Innate Immunity.

The Complement System

The complement system

• A group of serum proteins circulating in inactive form

– Once activated, multiple possible outcomes
• Target cell membrane lysis
• Chemotaxis
• Opsonization to enhance phagocytosis

48
 Ciraíate in blood in

② form

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49
 Sunnary
The complement system

50
The complement system

51
The complement system

52
53
The complement system

• A group of serum proteins circulating in inactive form

– Three activation pathways exist
• Classical
• Lectin
• Alternative

54
 proteasa "
si : ó

¡÷
→

- -

MAC

- -

55
 The major pathways of complement activation

• The classical pathway is initiated by antibody binding

– IgM or IgG binds to a multivalent antigen
– This allows the binding of C1q, beginning the process of
complement deposition

56
 The major pathways of complement activation

• The classical pathway is initiated by antibody binding

– C1 binding is followed by cleavage of C4, then C2
– C4b2a bound to the cell surface is C3 convertase

57
 The major pathways of complement activation

• The classical pathway is initiated by antibody binding

– A thioester bond is exposed when C4 is cleaved
– This bond is reactive, binding to amino groups on the surface of
a target
– If no target is present, the bond is quickly hydrolyzed and
rendered inert

58
 The major pathways of complement activation

• The classical pathway is initiated by antibody binding

– C3 convertase cleaves many C3 proteins
– Some combine with C3 convertase to form C5 convertase
– C5 convertase cleaves C5 protein

59
60
The major pathways of complement activation

• The lectin pathway is initiated when soluble proteins

recognize microbial antigens
– Lectins (e.g., mannose-binding lectin, or MBL) bind
to microbial surfaces
– These lectins can serve as docking sites for MBL-
associated serine proteases (MASPs)
• MASPs cleave C4 and C2 to form the C3
convertase
• Subsequent steps are the same as in the
classical pathway

61
62
¡n blood

63
The major pathways of complement activation

• The alternative pathway is initiated in three ways

1. The alternative tickover pathway
– Small amounts of C3 are always being cleaved
• Usually quickly inactivated if nothing is around
for it to bind to
– Activated C3b binds to membrane of target cell
• Factor B binds and is cleaved by Factor D
• C3bBb at the membrane is the C3 convertase
• Properdin stabilizes the C3 convertase

64
The major pathways of complement activation

1. The alternative tickover pathway

– After properdin stabilizes the C3
convertase, it cleaves many more
C3 proteins
– Newly active C3b binds to C3bBb to
form C3bBbC3b, (a.k.a. C5
convertase complex)
– C5 convertase cleaves many C5
proteins
65
 The major pathways of complement activation

2. The alternative properdin-activated pathway

– Properdin can directly bind to a surface
– This can then recruit C3b and Factor B
• Factor D is recruited and cleaves Factor B into Bb
• The resultant C3bBb is an active C3 convertase
• Subsequent steps are identical to the remainder of the alternative
tickover pathway

66
The major pathways of complement activation

3. The alternative protease-activated pathway

• Initiation of clotting cascades has also been shown
to stimulate cleavage of complement proteins
– Thrombin cleaves C3 and C5 in vitro
– Platelet activation releases ATP, calcium ions,
and serine/threonine kinases that could
stabilize C3b in fluid phase
• This indicates that strong inflammatory reactions
could potentially activate complement systems

67
 The major pathways of complement activation

• The three complement pathways converge at the formation of the C5

convertase complex
• C5 initiates the generation of the Membrane Attack Complex (MAC)
– Membrane attack complex is the result of deposition of C5b, C6, C7,
C8, and C9 in target cell membranes
– This pore structure disrupts osmotic integrity, resulting in cell death

68
The diverse functions of complement

69
The diverse functions of complement

• Complement receptors connect complement-tagged

pathogens to effector cells
– Receptors on host cells allow for discrete and
differentiated responses

70
71
The diverse functions of complement

• Complement receptors connect complement-tagged

pathogens to effector cells
– CR1 on leukocytes and erythrocytes
• On erythrocytes, helps to bring immune complexes
to the liver for clearance by phagocytes
• On phagocytes, helps bind to complement-coated
bacteria to enhance ingestion and destruction
• On B cells, helps bind to complement-coated
antigens, enhancing ingestion for processing and
presentation to helper T cells

72
The diverse functions of complement

• Complement receptors connect complement-tagged

pathogens to effector cells
– CR2 on B cells
• Binds to C3d on opsonized bacteria/antigens
• Helps provide secondary signals to B cells
through BCR complex for more efficient
activation

73
74
The diverse functions of complement

• Complement receptors connect complement-tagged

pathogens to effector cells
– C3aR/C5aR on granulocytes
• Stimulates release of proinflammatory
cytokines and granule components from
basophils, eosinophils, neutrophils

75
76
The diverse functions of complement

• Complement enhances host defense against infection

– MAC-induced cell death
– Promotion of inflammation
– Promotion of opsonization
• This is particularly important
• Opsonized microbes easier to ingest/destroy
• Opsonized immune complexes easier to clear

77
 Destruction of opsonized pathogens

Clearance of immune complexes

78
The diverse functions of complement

• Complement mediates the interface between innate

and adaptive immunities
– Increases uptake of antigens by antigen-
presenting cells
– Can help provide secondary signals to B cells for
activation and increase their uptake of antigen to
present to helper T cells
– Can help to stimulate T cells to release cytokines,
further potentiating innate inflammatory and
adaptive immune responses

79
The diverse functions of complement

• Complement aids in the contraction phase of the

immune response
– As lymphocytes are no longer required,
complement aids in disposal of apoptotic cells and
bodies
– Complement also aids in removal/disposal of
immune complexes formed during responses
• These responses avoid damaging inflammation
induction in the absence of antigens following
clearance of an infection

80
The regulation of complement activity

• Complement activity is passively regulated by protein

stability and cell-surface composition
– Short half-life of C3 convertase unless stabilized by
properdin
– Self cells possess different carbohydrate
structures that are more effectively bound by
fluid-phase proteases
• These more readily inactivate C3b through
hydrolysis, protecting self cells

81
 Numerous regulatory proteins help to prevent the
complement system from harming self cells
The regulation of complement activity

82
 The regulation of complement activity

• The C1 inhibitor, C1INH, promotes dissociation of C1

components
– Binds in the active site of serine proteases
– Causes C1r2s2 to dissociate from C1q
– No further cleavage of C4 or C2 is possible

83
 Numerous regulatory proteins help to prevent the
complement system from harming self cells
The regulation of complement activity

84
 The regulation of complement activity

• Decay accelerating factors promote decay of C3 convertases

– Several different proteins with similar activities
• DAF (CD55), CR1, C4BP (C4 binding protein)
– Work to accelerate the decay of C4b2a (C3 convertase) on the surface
of host cells

85
 Numerous regulatory proteins help to prevent the
complement system from harming self cells
The regulation of complement activity

86
 The regulation of complement activity

• Factor I degrades C3b and C4b

– Soluble, constitutively active serine protease
– Cleaves membrane-associated C3b and C4b into inactive fragments
• Requires MCP and CR1 (found on membranes of host cells) to
function

87
 Numerous regulatory proteins help to prevent the
complement system from harming self cells
The regulation of complement activity

88
 The regulation of complement activity

• Protectin inhibits the MAC attack

– Binds C5b678 complexes deposited on host cells
• Prevents their insertion into the plasma membrane
• Also blocks C9 recruitment, preventing MAC formation
– Similarly, soluble complement S protein (vitronectin) binds fluid phase C5b67
to prevent insertion into host cell plasma membranes

89
 Numerous regulatory proteins help to prevent the
complement system from harming self cells
The regulation of complement activity

90
 The regulation of complement activity

• Carboxypeptidases can inactivate the anaphylatoxins C3a and C5a

– Enzymes remove arginine residues from the C termini of C3a
and C5a
• Creates des-Arg (without arginine) inactive forms
• Helps to shut down unnecessary or dangerous chemotactic
and inflammation induction

91
Complement deficiencies

• Genetic deficiencies have been described for each of the

complement components (see PDF: Complement
Deficiencies)
– Outcomes vary
• Patients often present with immune complex
disorders due to inadequate clearance
• Some may exhibit greater frequency of infections
by encapsulated bacteria due to inefficient
opsonization and phagocytosis
• Animal models exist for most complement deficiencies,
allowing for further study

92
Microbial complement evasion strategies

• Different mechanisms exist, and are highly varied

– Some interfere with the first step of Ig-mediated
complement activation
– Microbial proteins may bind and inactivate
complement proteins
– Microbial proteases destroy complement proteins
– Some microbes mimic or bind complement
regulatory proteins

93
Microbial complement evasion strategies

94
 The evolutionary origins of the complement
system

• Genes for complement components belong to five families

– Alternative pathway
genes appear first in
evolution
– Terminal complement
components appear last
• Complement served to
assist phagocytosis
prior to evolution of
adaptive immunity

95
 The evolutionary origins of the complement
system

• Five families of genes for complement components

– Alternative pathway genes appear first in evolution
– Terminal complement components appear last
• Complement served to assist phagocytosis prior to evolution of adaptive
immunity

96
Summary

• The complement system serves many different

purposes, helping to link innate and adaptive immune
responses
• It is tightly controlled, but enhances many other
responses once activated
• It also provides a window into the evolution of
immunity
• Understanding the system and its methods of
activation and regulation helps us to better
understand innate immunity and evolution

97
 T cells

Topic 4: |
cells
B.
-
Immunoglobulins, TCR
and MHC molecules APCI →

SCIFCY
The Organization and Expression of
Bcely
Lymphocyte Receptor Genes
• Isotypes have different heavy
chains. They are represent
classes of antibody.

• Allotypes have the same

constant regions with minor,
but immunologic differences.
Different individuals have
different allotypes.

• Idiotypes are antibodies that

recognize different specific
epitopes. Each idiotype is
composed of several idiotopes
or combining sites.
2
Antibody Isotypes (Classes)

• Five major classes of antibody

– IgM
– IgD
– IgG
• Subclasses: IgG1, IgG2, IgG3, IgG4
– IgE
– IgA
• Subclasses: IgA1, IgA2

3
 Characteristics of the Antibody Isotypes
Light Chains Heavy Chains Other Chains

IgM Kappa Chain Mu Chain J Chain

Lambda Chain

IgD Kappa Chain Delta Chain

Lambda Chain

IgG Kappa Chain Gamma1 Chain

Lambda Chain Gamma2 Chain
Gamma3 Chain
Gamma4 Chain
IgE Kappa Chain Epsilon Chain
Lambda Chain

IgA Kappa Chain Alpha1 Chain J Chain

Lambda Chain Alpha2 Chain Secretory
Component

4
Structures of Different Isotypes (Classes) of Antibodies

The different antibody

isotypes (classes) differ
in
1. Number of
subunits
2. Number of
domains,
3. Number and
location of
carbohydrate
chains,
4. Presence or
absence of hinge
region,
5. Number and
location of
disulfide bridges.

5
Antibody Isotypes in the Serum

IgM IgD IgG IgE IgA

No. of monomers 5 1 1 1 1-3

Molecular Weight 900 kD 150 kD 150 kD 190 kD 150 kD

450 kD
600 kD
Serum Conc (mg/ml) 1.5 0.03 13.5 0.0003 3.5

Serum Half-life (days) 5 3 8-23 2.5 6

Placental Transfer No No Yes No No

6
 Biological Activity of Antibody Isotypes

IgM IgD IgG IgE IgA

Complement Fixation ++++ - + - -

Mast Cell/Basophil - - - ++++ -

Degranulation
Bacterial Lysis +++ - + - +

Antiviral Activity + - + - +++

Toxin Neutralization - - +++ - +++

7
IgM

• Monomeric IgM is expressed on the surface of B cells

where it serves the role of antigen recognition.
• Pentameric IgM is expressed in the serum (5-10% of
total serum Ab).
• Pentameric IgM is secreted into mucosal secretions.

8
IgM Structure

• The pentameric structure of

IgM is stabilized by a J chain
(joining chain) and by
disulfide bonds.
• Secretory IgM has a
secretory component that
assists in transport across
the epithelial membrane.

9
IgM Structure

• Note that for IgM, the pentameric structure is shown

as a flat molecule (like a hand flat on a surface).
• In reality, the disulfide bonds would all be the same
length. In its binding form, the molecule would look
more like a hand poised on a surface with only its
fingertips touching the surface.
• This allows for multiple interactions with repeated
epitopes on a given antigen.

10
Biological Functions of IgM

• Good at virus neutralization

• Excellent at bactericidal activity
• Excellent at causing agglutination of antigens
• Excellent at causing precipitation of antigens
• Excellent at complement fixation
• Does not bind to Macrophage Fc receptors
• Poor at toxin neutralization

11
Medical Considerations of IgM

• Elevated levels indicate a recent infection or other exposure to

antigen
• Can be present in bodily secretions
• Does not cross placenta
• Not useful for protecting immunocompromised individuals
(short half-life and lower specificity than IgG)
• Not useful as blocking antibody to block TNF production
• Not useful as blocking antibody to block allergens
• Not present in interstitial fluids (too big)

12
IgD

• Monomeric IgD is expressed on the surface of B cells where it

serves the role of antigen recognition.
• IgD is a very minor component of serum Ig (0.2%)
• Biological Functions: Not well known, still under debate.
Immuno-stimulatory / surveillance role proposed
• Medical Consideration: some multiple myeloma patients over-
express IgD.

13
IgG

• IgG is the predominant antibody synthesized during a secondary

immune response (anamnestic response)
• IgG is the most abundant class in serum, constituting 80% of
serum Abs.
• Although it cannot be transported across the epithelium, it is
produced in the distal pulmonary tract and is important in
pulmonary secretions.
• There are four subclasses of IgG.

14
 Four Subclasses of IgG

Note the much longer

Gamma chain on IgG3

15
 IgG Subclasses

IgG1 IgG2 IgG3 IgG4

Serum Conc (mg/ml) 900 300 100 50

Serum Half-life (days) 23 23 8 23

Complement Fixation + +/- ++ -

Crosses Placenta + +/- + +

Binds to M Fc ++ +/- ++ +
Receptors
16
Biological Functions of IgG

• Good at virus neutralization

• Excellent at toxin neutralization
• Good at bactericidal activity
• Good at causing agglutination of antigens
• Good at causing precipitation of antigens
• Good at complement fixation
• Binds to M Fc receptors

17
Medical Considerations of IgG

• Crosses placenta providing protection to the fetus.

• Can mediate hemolytic disease of the newborn (blue baby
syndrome, Rh mismatch).
• Can be used to protect immunocompromised (gamma globulin).
• Can be used as blocking antibody to block TNF production
(rheumatoid arthritis).
• Can be used as blocking antibody to block allergens
(desensitization to hypersensitivity)
• Present in interstitial fluids
• Not present in most bodily secretions

18
IgA

• IgA is the predominant Ig in bodily secretions.

• IgA in blood can be monomeric, dimeric, or trimeric. When
multimeric, the J chain stabilizes the interactions of the two or
three monomers.
• IgA in bodily secretions is dimeric and combined with the J chain
and with secretory component.

19
Biological Functions of IgA

• Excellent at virus neutralization

• Excellent at toxin neutralization
• Good at bactericidal activity
• Good at causing agglutination of antigens
• Good at causing precipitation of antigens
• Does not bind to M Fc receptors

20
Medical Considerations of IgA

• Daily production of IgA is greater than any other Ig.

• B cells that will produce IgA migrate to subepithelial tissue of
most mucosal epithelia and of glandular epithelia.
• Present in bodily secretions.
• Present at very high levels in colostrum and present in breast
milk. Provides an excellent level of protection of newborns
against respiratory and intestinal infections.

21
IgE

• There is very little IgE in the blood (0.02% of Ig).

• Most of the IgE is bound to Fc receptors on mast cells in the
epithelial tissue and on basophils in the blood.

22
Biological Functions of IgE

• Cross-linking of IgE molecules on the surface of a mast cell or

basophil causes the release of histamine; the synthesis of
prostaglandins, leukotrienes, and other chemokines; the
production of various cytokines.
• IgE plays a major role in combating parasitic infections.
• IgE plays a role in combating pulmonary fungal infections.

23
IgE-Mediated Release of Histamine

• The epitope of an antigen binds

to an IgE paratope.
• If another epitope of the antigen
binds to the paratope of another
IgE molecules, this causes cross-
linking.
• The cross-linking changes the
character of the binding of the Fc
receptors to the mast cell,
initiating a trans-membrane
signal.
• The mast cell degranulates.

24
Medical Considerations of IgE

• Individuals who express allergies to certain antigens over-

produce IgE to those antigens.
• This causes a high level of expression of IgE with the same
paratopes (recognize same epitope on antigen) on given mast
cells. This makes it easier to cross-link two IgE antibodies.
• When antigen is present, many mast cells are degranulated,
resulting in an over-stimulation of the immune system that is
manifested as an allergic reaction (Type 1 hypersensitivity).
• IgE plays an important role in asthma

25
The puzzle of immunoglobulin gene structure

• Antibodies can be produced for an enormous variety of different

specific antigens (107–1011 different specificities!)
– But how can a limited amount of DNA genetic information
produce such an extensive variety of possible antibodies?
• The mechanisms used by B cells to produce this vast repertoire
were elucidated by very elegant experiments in the 1970s

26
Features of the Antibody Genes

• Antibodies are composed of heavy and light chains.

• As is seen for most eukaryotic genes, the heavy and light chains
of the immunoglobulin genes are each composed of segments
(exons) that must be joined together to form the
immunoglobulin genes.
• For immunoglobulin genes, the joining of a number of the exons
occurs via a rearrangement of the gene segments at the level of
the DNA, rather than at the level of the mRNA.
• There are multiple copies of each of the various segments of the
heavy and light chains of the immunoglobulin genes, with one of
each of these segments becoming sequentially rearranged to
form the heavy and light chain genes.

27
The Keys to Antibody Diversity

• Antibody diversity is generated during genetic rearrangement by

mixing and matching one of each of the various gene segments
for the heavy and light chains in a combinatorial manner.
• Antibody diversity is generated by errors incorporated at the
joining sites for the various segments of the heavy and light
chains.
• Antibody diversity is generated by hypermutation in one of the
gene segments (variable regions) of the heavy and light chains
during proliferation of B cells.
• Antibody diversity is generated by mixing and matching heavy
and light chains in a combinatorial manner.

28
The Immunoglobulin Heavy Chains

• There are two identical immunoglobulin heavy chains in each

antibody.
• Each of the immunoglobulin heavy chain genes is assembled
from Variable (V), Diversity (D), Joining (J), and Constant (C)
gene segments.
• There are multiple C gene segments (constant regions) that give
rise to different isotypes.
– IgD
– IgM
– IgG: 4 heavy chains, IgG1, IgG2, IgG3, IgG4
– IgE
– IgA: 2 heavy chains IgA1, IgA2

29
Heavy Chain Rearrangement

30
Heavy Chain Diversity

• 39 V gene segments
• 23 D gene segments
• 6 J gene segments

• 5,382 combinations of heavy chain VDJ segments

31
The Immunoglobulin Light Chains

• There are two types of immunoglobulin light chains.

– light chain
– light chain
• Each of the immunoglobulin light chain genes is assembled by
the rearrangement of V, J, and C gene segments.

32
Light Chain Rearrangement

33
Light Chain Diversity

• light chain diversity

– 40 V gene segments
– 5 J gene segments
– 1 C gene segment
• light chain diversity
– 30 V gene segments
– 4 J gene segments
– 4 C gene segments

• 200 combinations of light chain VJC segments

• 480 combinations of light chain VJC segments
• 680 combinations of light chain VJC segments
34
Antibody Diversity from Rearrangements

• From heavy chains: 5,382 combinations

• From and light chains: 680 combinations
• 3.6 x 106 Combinations of light and heavy
chains
• This is far short of the 1010-1011 different
antibodies that are postulated to occur.

35
How Does Rearrangement Occur?

• Rearrangement occurs between specific sites on the DNA called

recombination signal sequences (RSSs).
• Rearrangement is catalyzed by two recombination-activating
genes: RAG-1 and RAG-2.

36
Recombination Signal Sequences

• The specific recognition sequences, called recombination signal

sequences (RSSs), indicate the sites of recombination.
• These signal sequences composed of a 7 bp sequence and a 9 bp
sequence are separated by one turn of the DNA (12 bp spacer)
or by two turns of the DNA (23 bp spacer).
• The RSSs are inverted repeats that allow the DNA to form a
stem-loop with the RSSs aligning on the stem.

37
Rearrangement Process

38
Consequences of Rearrangement and P- and N-
Nucleotide Addition

• Positive consequence of imprecise joining of Ig gene segments

– A productive rearrangement occurs if the number of
nucleotides added across the joining region allows the
genetic code to be read in phase.
– This results in the generation of additional diversity.

39
Consequences of Rearrangement and P- and N-
Nucleotide Addition

• Negative consequence of imprecise joining of Ig gene segments

– A nonproductive rearrangement occurs if the number of
nucleotides added across the joining region causes the
genetic code to be read out of phase (the majority of times).
– This results in an incomplete antibody (run into stop codons)
– The B cell may be able to productively rearrange the
immunoglobulin gene on the other chromosome.
– Otherwise, it will result in the death of the B cell.

40
Alleleic Exclusion

• Alleleic exclusion occurs when only one of two alleles is

expressed.
• This is the case with immunoglobulin molecules.
• This ensures that a given B cell will make antibody molecules
with only a single specificity.
• Antibodies may be made from maternal and paternal
chromosomes:
– Heavy-M:Light-M; HP:LM; HM:LP; HP:LP.
– Note: If the first allele makes a non-functional antibody, the
second allele will undergo rearrangement.

41
Immunoglobulin Molecules Expressed on Mature B
Cells

• Mature (but not activated) B cells initially express IgD and IgM on
their external cell membranes.
– The choice of IgD versus IgM occurs at the level of processing of
mRNA, so a given B cell can both express IgD and IgM.
• As mature B cells are activated to divide and differentiate by their
cognate antigen, they switch from membrane-bound IgD and IgM to
secretory IgM.
– This switch occurs at the level of processing of mRNA
transcripts.
• As they continue to divide and differentiate, they may undergo
additional class switching: IgM => IgG => IgE => IgA.
– These switches occur at the level of rearrangements of the
DNA.

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Topic 4:
Immunoglobulins, TCR
and MHC molecules
The Major Histocompatibility Complex
and Antigen Presentation
The structure and function of MHC molecules

• Class I molecules
– Member of the Ig superfamily
– Larger 45 kDa glycoprotein α chain
• Three external domains, each approximately 90 amino
acids in length
• A transmembrane domain of approximately 25 amino
acids
• Cytoplasmic anchor section of 30 amino acids
• The α1 and α2 domains form a cleft region that binds a
8–10 amino acid–long peptide fragment from an antigen
• Smaller 12 kDa β2-microglobulin protein

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The structure and function of MHC molecules

• Class II molecules
– Member of the Ig superfamily
– Heterodimeric
• A 33 kDa α chain
• A 28 kDa β chain
– Both chains pass through the plasma membrane
– A peptide-binding cleft is formed by the pairing of the α1 and
β1 domains
• Accommodates peptides of 13–18 amino acids in length

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The structure and function of MHC molecules

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The structure and function of MHC molecules

• Class I and II molecules exhibit polymorphism in the peptide-

binding region
– Several hundred different allelic variants in humans
• Up to 6 class I and 12 class II molecules expressed per
person
• How does this limited group of molecules present the
vast diverse array of possible antigen peptide
fragments?
– A given MHC molecule can bind numerous different
peptides, and some peptides can bind to several
different MHC molecules

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The structure and function of MHC molecules

• Class I MHC-peptide interactions

– Present peptides to CD8+ T cells
– Peptides derived from endogenous intracellular proteins
– Some amino acids anchor the peptide into the groove
– Other amino acids are available to interact with a TCR
– Provides a way for “checking” that cells are self and are
generally healthy
– Can also be used to show which cells have been infected
with viruses or are abnormal

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The structure and function of MHC molecules

• Class II MHC-peptide interactions

– Usually derived from exogenous extracellular processed
antigens
– Present antigen peptides to CD4+ T cells
• Also exhibit anchor amino acid residues that lock them
into the groove
• Extends other amino acid residues from groove to
interact with TCRs
– Expression is more restricted; generally found on cells
involved in immune responses
– Helps to direct responses against threats―things that
shouldn’t be in our systems

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The role of the MHC and expression patterns

• MHC class I expression is found throughout the body

– Essentially on all nucleated cells
• MHC class II is primarily restricted to antigen-presenting cells

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General organization and inheritance of the MHC

• Allelic forms of MHC genes are inherited in linked groups called

haplotypes
– Each individual inherits one haplotype from each parent
– Inbreeding of mice has allowed for more standardized MHC
genotypes to control variability
• Syngeneic
• Congenic

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The Human HLA Complex

• MHC locus encodes three major classes of molecules

– Class I MHC genes
– Class II MHC genes
– Class III MHC genes
• Complement and inflammation proteins

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General organization and inheritance of the MHC

• MHC alleles are codominantly expressed

– Both maternal and paternal MHC genes are expressed in
offspring cells
– This gives the best chance for an organism to have SOME
capability of presenting all the possible antigen peptides it
encounters
– This can also make transplantation somewhat difficult, as
humans are heterozygous at each locus
• Nonmatching MHC patterns will result in rejection of
transplanted tissues
• Some of these characteristics were originally defined in
inbred mouse strains

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The role of the MHC and expression patterns

• MHC expression can change with changing conditions

– Genetic regulatory components
• Promoters that drive up transcription during times of
infection
– Viral interference
• Viruses like to shut down MHC Class I expression because it
targets the cells they’re in for destruction
– Cytokine-mediated signaling
• Some cytokines expressed during infection/disease can
drive up/down MHC expression
– Class II MHC alleles and antigen presentation
• Different capability to present antigens may dictate overall
strength of response from individual to individual

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The role of the MHC and expression patterns

• T cells are restricted to recognizing peptides presented in the

context of self MHC alleles
– CD8+/CD4+ T cells can only recognize peptides presented by
MHC class I/II, respectively
• The MHC haplotype of the presenting cell must match
the haplotype of the T cell
– Whole antigens can’t be recognized by T cells
– Only processed antigen peptide fragments can be
recognized

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The role of the MHC and expression patterns

• Evidence suggests different antigen processing and presentation

pathways
– Class I presentation requires cytosolic or endogenous
processing
– Class II presentation requires exogenous processing

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The role of the MHC and expression patterns

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The endogenous pathway of antigen processing and
presentation

• Peptides are generated by

protease complexes called
proteasomes
– Ubiquitin proteins are
used to “tag” intracellular
proteins for degradation
– Tagged proteins are fed
into proteasomes that
cleave proteins into
fragments that pair better
with MHC molecules

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The endogenous pathway of antigen processing and
presentation

• Peptides are transported

from the cytosol to the
rough endoplasmic
reticulum (RER)
– Transporter associated
with antigen processing
(TAP) molecules in the
RER membrane move the
fragments
– MHC class I molecules lie
in wait in the RER after
their translation by
ribosomes on its surface

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The endogenous pathway of antigen processing and
presentation

• Chaperones aid peptide/MHC class I assembly

– Calnexin, calreticulin, and tapasin help fold MHC class I and
put it in close proximity to TAP
– ER aminopeptidase ERAP1 trims long peptides to a suitable
size for MHC class I grooves

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The exogenous pathway of antigen processing and
presentation

• Peptides are generated

from internalized antigens
in endocytic vesicles
– Particles are taken in
within endosomes
• Endosomes are fused
with lysosome
• Contents are degraded
– Simultaneously, MHC
class II molecules are
produced and exported
from the ER in vesicles

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The exogenous pathway of antigen processing and
presentation

• Invariant chain guides transport of class II MHC molecules to

endocytic vesicles
– Invariant chain (Ii, CD74) prevents peptides from binding to
the groove too early in the ER
– Ii also uses sorting signals in its cytoplasmic tail to direct
MHC class II molecule–containing vesicles to peptide-
containing endocytic compartments

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The exogenous pathway of antigen processing and
presentation

• Peptides assemble with class II MHC molecules

– Ii is initially degraded by proteolytic activity within endocytic
compartments to class II–associated invariant chain (CLIP)
– HLA-DM exchanges CLIP out of the groove for a peptide
fragment

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The exogenous and endogenous pathways compared

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Cross-presentation of exogenous antigens

• Dendritic cells appear to be the primary cross-presenting cell

type
– Exogenous antigens are redirected to the exogenous
presentation pathway
– This allows for their presentation on MHC class I molecules,
priming CD8+ T-cell responses
– Dendritic cells are the only APCs (so far) to exhibit this
activity in vivo

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Cross-presentation of exogenous antigens

• Mechanisms and functions of cross-presentation

– Actual redirection method unclear
– To prevent redirection of self antigens into APC pathways,
dendritic cells may need “license”
• If DC can present foreign antigen to CD4+ helper T cell, it
gets “license” to redirect exogenous Ag into the
endogenous pathway
• “License” might be back/forth cytokine signal between
the APC/helper T cell
– indicating situation is right for cross-presentation

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Cross-presentation of exogenous antigens

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Presentation of non-peptide antigens

• Some nonprotein Ag can be recognized by T cells

– Presentation not via classic MHC molecules
– The CD1 family of nonclassical class I
molecules can present lipids
• Five human CD1 genes are known
• Very little polymorphism is displayed
• Most function similarly to MHC class II
molecules

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Summary

• T cells must have antigen peptides presented to them for

activation
• Understanding the molecules that perform this presentation is
crucial to understanding T cell activation
• Also, understanding how intracellular and extracellular antigens
are processed for presentation gives us insight into why
different T cells are stimulated/activated during immune
responses

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