INNATE IMMUNITY

Innate Immunity - characteristics

•Most primitive type of immune system found in
Virtually all multicellular animals/in born

•First line of defense against infection

•no need for prolonged induction

•act quickly
immediate direct response 0-4
hrs rapid induced 4-96 hrs

•antigen-independent
 Innate Immunity – characteristics (contd.)

•always present and active, constitutively expressed

(some components can be up-regulated)

•Nonspecific; not specifically directed against

any particular infectious agent or tumor

•no clonal expansion of Ag specificity

•Same every time; no ‘memory’ as found

in the adaptive immune system
 Components of Innate Immunity

First line Second line

1 Physical barriers A- cells/cellular barriers
2 Chemical & biochemical barriers 1- Natural killer
3 Biological barriers (Normal flora) 2- Phagocytes
3- inflammatory cells

B- Soluble factors
C- Inflammatory barriers
 Anatomical /Physical/Mechanical
Barriers
System or Organ Cell type Mechanism

Skin Squamous epithelium Physical barrier

(intact skin)
Desquamation
Mucous Membranes Non-ciliated epithelium (e.g. Peristalsis
GI tract)
Ciliated epithelium, Mucociliary elevator,
hairs (e.g. respiratory Coughing, sneezing
tract)
Epithelium (e.g. Flushing action of
nasopharynx) tears, saliva, mucus,
urine; blinking of
eye lids
 Biological Factors

System or Organ Component Mechanism

Skin and mucous Normal microflora Antimicrobial

membranes substances
Competition
for nutrients
and
colonization
 Chemical Factors
(at surfaces and in body cavities)

System or Organ Component Mechanism

Skin Sweat Anti-microbial fatty

acids, high salt conc.
Mucous Membranes HCl (parietal cells) Low pH
Tears and saliva Lysozyme and
phospholipase A
Defensins (respiratory & Antimicrobi
GI tract)
Sufactants (lung) Opsonin
 Chemical Factors
(Humoral Components)
Component Mechanism
Complement Lysis of bacteria and some viruses,
Opsonin Increase in vascular permeability
Recruitment and activation of phagocytic cells
Coagulation system Increase vascular permeability, Recruitment of phagocytic
cells, -lysin from platelets – a cationic detergent,
antibacterial
Acute phase protein Antibacterial

Lysozyme Breaks down bacterial cell walls

Lactoferrin and transferrin Compete with bacteria for iron

Cytokines Various effects

Interferon Anti-viral protein

Pathways of complement activation

CLASSICAL LECTIN ALTERNATIVE

PATHWAY PATHWAY PATHWAY
antibody antibody
dependent independent

Activation of C3 and
generation of C5
convertase

activation
of C5

LYTIC ATTACK
PATHWAY
 THE ACUTE PHASE PROTEINS
‘Acute phase proteins’ are a large group of plasma proteins whose
concentration increases (or decreases) (by 25% or more) during
inflammation (acute or chronic), such as injury and in disease
states.
Proinflammatory cytokines stimulate hepatocytes in the liver to
synthesize
and secrete acute phase proteins.
Examples of acute phase proteins are CRP, MBP, haptoglobin,
SAA,
fibrinogen, α1-antitrypsin, and complement components C3 and
C4.
C-reactive protein (CRP) binds to membrane phospholipids in
microbial
membranes.
Mannose binding protein (MBP) binds to mannose sugars found
in many
bacteria and fungi.
These functions as opsonins, soluble pattern-recognition
receptors, activate the complement pathway or be involved in
sequestration of essential nutrients.
 Antimicrobial Peptides (AMPs)
• Antimicrobial peptides forms pores in the cytoplasmic
membrane of a variety of bacteria causing leakage of
cellular needs, e.g. lysozymes, lactoferrins, defensins,
protegrins, granulozymes, etc

• Lysozyme, in serum, mucus, plasma, tissue fluids and

tears,
breaks down the bacterial cell wall (peptidoglycan)

• Beta-defensins are short peptides found in blood plasma

and
mucous.

• Transferrin & lactoferrin competitively binds iron in

blood, tissues and milk thereby preventing its
availability to microorganisms
 Interferons (IFN)
• Interferons (IFNs) comprise a family of secreted α-helical cytokines
induced in response to specific extracellular biomolecules of viruses
or other pathogens through stimulation of Toll-like receptors (TLRs).
• Act in paracrine or autocrine modes for regulating innate and
acquired immunity, resistance to viral infections, and normal and
tumor cell survival and death.
•There are five types of human interferon: alpha, beta, gamma,
delta and
omega (α -IFN, β -IFN, γ-IFN, ξ-IFN, and ω-IFN, respectively) .
• Virus infected cells produce IFN-α and IFN-β.
•Interferons are host-cell-specific, but not virus-specific
• Gamma-IFN, also called as immune interferon, activates neutrophils,
NK cells
and macrophages.
• Interferons also result in
resistance to viral replication; induce enzymes to degrade
viral mRNA increased MHC I expression
activate NK cells, T-cells and macrophages
 Interferons

Virus-infected cells (double-stranded RNA)

Interferon
Uninfected cells
Induces enzymes (inactive) capable of degrading
mRNA Uninfected cells become infected with a virus
Activation of enzymes
Degradation of viral and cellular mRNA
Blocks viral and cellular protein
synthesis

Death of the infected cell

 Cellular Components

Cell Functions

Neutrophils Phagocytosis and intracellular killing

Inflammation and tissue damage
Macrophages Phagocytosis and intracellular killing
Extracellular killing of infected or altered self targets
Tissue repair

NK cells Killing of virus-infected and altered self targets

Eosinophils Killing of certain parasites

Inflammation
 Inflammation

• Inflammation is an attempt by the body to

eliminate the noxious agent and restore
and maintain homeostasis after injury to a
tissue.

• The injury is often caused by invading

organisms.

• It is the second line of defense.

Inflammation (contd.)
Chapter 21, Immune System
 The Good Side of Inflammation
The inflammatory response to tissue damage is
of great value by:

•isolating the damaged area

•mobilizing effector cells and molecules
to the site, and
•in the late stages — promoting healing

Inflammation protects the body

(innate immunity)
 The Bad Side of Inflammation

Often the inflammatory response is out of

proportion to the threat it is dealing with.
The result can be more damaging to the
body than the agent itself would have
produced.
Allergies and Autoimmune Diseases are
examples of inflammation in response to
what should have been a harmless, or at
least noninfectious, agent
Phagocytosis
 Phagocytosis
Phagocytosis is the ingestion of microorganisms or
particulate matter by a cell.
Phagocytosis is performed by phagocytes—
certain types of white blood cells or derivatives
of them.

All phagocytes
eat, digest
and extrude
 Phagocytic Cells
Myeloid Macrophage-
Basophils Monocyte
Neutrophils Eosinophils

•Rapid phagocytosis, but cannot •Slow but can

phagocytose
phagocytose repeatedly ANTIGEN

•Has granules which contain

bactericidal enzymes

•Short lived

•NO ABILITY TO PRESENT

 repeatedly.

• Contain bactericidal
enzyme.

• Long lived

• Selected cells HAVE

ability to present Ag.
Mechanism of Phagocytosis

25
Chemotaxis & attachment
• Attraction by chemotactic substances (microbes, damaged
tissues,
complement components, vasoactive amines, etc )

• Attachment by receptors on surfaces of phagocytes.

Ingestion and phagosome formation

• Phagocytes’ produce pseudopodia surrounding organism

forming
phagosome

• Opsonins and co-factors enhance phagocytosis

Phagolysosome formation
• Fusion of phagosome with lysosomal granules of
phagocyte take place by help of cytoskeleton followed by
the release digestive and degradative enzymes
 The Process of Phagocytosis - ingestion

Following attachment, polymerization and depolymerization of actin

molecules send pseudopods out to engulf the bacterium
 The Process of Phagocytosis - phagosome formation

Following engulfment, the bacterium is placed in a vesicle called a phagosome.

Lysosomes move along the cytoskeleton and fuse with phagosomes to form
phagolysosomes.
 The Process of Phagocytosis - Destruction

The lysosome, its digestive enzymes and microbicidal chemicals fuses with
the phagosome containing the ingested bacteria to form a phagolysosome
and the bacterium is killed.
Intra-cellular killing (two microbicidal routes)

• Oxygen-dependent system (powerful microbicidal

agents) Oxygen converted to superoxide anion,

hydrogen peroxide,
activated oxygen and hydroxyl radicals.

• Oxygen-independent system (anaerobic conditions)

Digestion and killing by lysozyme, lactoferrin, low pH,

cationic
proteins and hydrolytic and proteolytic enzymes
 Pathways of Intracellular Killing

Intracellular
Killing
oxygen- oxygen-
depenedent independent
myloperoxidase- myeloperoxidase-
independent dependent
 Respiratory Burst
Respiratory
Burst
Oxygen-dependent Myeloperoxidase-independent Reactions

Glucose +NADP+ Pentose-P

G-6-P-dehydrogenase + NADPH

NADPH + O2 -
NADP++ O2
Cytochrome b558
-
2O2 + 2H+
H2O2 + 1O 2
Superoxide dismutase
- -
2O2 + H2O2 .OH + OH + 1O2

Toxic compounds – Superoxide anion (O2 -), Hydrogen peroxide (H2O2),

Singlet oxygen (1O2) and Hydroxyl radical (OH*)
 Respiratory Burst

Oxygen-dependent Myeloperoxidase-dependent Reactions

H2 O2 + Cl-
OCl- + H 2O
myeloperoxidase
- -
- 1
2OCL + H 2O O2 + Cl +
H2O
Toxic compounds
– Hypochlorous acid (OCl-)
- Singlet oxygen (1O2)
 Respiratory Burst

Detoxification Reactions

-
Superoxide dismutase
2O2 + 2H+ H2O2 + O2

Catalase
2 H2O2 H2O + O2
Mediators of Oxygen-independent Killing in
the Phagolysosome

Effector Molecule Function

Cationic proteins (cathepsin) Damage to microbial
membranes
Lysozyme Hydrolyses mucopeptides
in the cell wall
Lactoferrin Deprives pathogens of
iron Hydrolytic enzymes (proteases) Digests killed organisms
Nitric Oxide Dependent Killing

Some
cytokines can
also
induce
phagocytic
Nitric Oxide cells,
particularly
macrophages,
to produce
nitric oxide (NO),
which is toxic to
microorganisms
and malignant
cells
 Nitric Oxide Dependent Killing

Nitric oxide also possess antiviral properties:

• inhibition of viral RNA synthesis
• inhibition of viral protein accumulation
• inhibition of virus release from infected cell
Extracellular Destruction of Bacteria by a Phagocyte

• If the phagocyte is
overwhelmed with
microorganisms,
the phagocyte will
empty the contents of its
lysosomes by a
process
called
degranulation in order to
kill the microorganisms
or cell extracellularly.
These released
lysosomal contents,
however, also
kill surrounding
host cells and tissue.
Most tissue destruction
associated with infections
is a result of this process
 FACTORS AFFECTING PHAGOCYTOSIS

OPSONINS “natural ketchup”

Proteins which coat the antigen to facilitate phagocytosis
e.g. Antibodies
Complement
Components Certain Liver
proteins

Generally both bacteria and cells that are suspended in body fluids
have negative charges (Zeta Potential). Therefore they tend to
repel each other. Opsonins, provide positive charges and
coating with opsonins promotes phagocytosis.
ANTIGEN OPSONIN EFFICIENCY OF
PHAGOCYTOSIS

None +

Antibody
+++

Complement
+++

++++
Antibody Plus Complement
Fever
 Fever
Activated macrophages and other leukocytes
release
proinflammatory cytokines such as TNF-alpha, IL-6, and
IL-1
These cytokines stimulate the anterior hypothalamus
of the brain to produce prostaglandins that lead to
an increase in body temperature – fever.
Fever increases the physiological temperature above
the optimum growth temperature for many
microorganisms.
Fever leads to the production of heat shock proteins
resulting in the production of inflammation-
promoting cytokines.
Fever elevates the temperature of the body
increasing the rate of enzyme reactions, and
speeding up metabolism within the body
 Fever and Immune Activation

LYMPHOCYTE ACTIVATION BACTERIA

(T, B, NK)
Alternate pathway of C’ SLEEP APPETITE
Mac
MOTILITY OF MUSCLE
NEUTROPHILS IL-1, IL-6, TNF-
PROTEOLYSIS
GROWTH OF BACTERIA
INCREASED
FEVER AMINO ACIDS
IN BLOOD
INCREASES
NEUTROPHILS

LOWERS Stimulates
Stimulates IRON, ZINC Hepatocytes
Fibroblast IN BLOOD
Proliferation
ACUTE PHASE
PROTEINS
How innate Immunity Recognizes?
 Pattern-Recognition Receptors
Innate immunity recognize a few highly conserved
structures present in many different microorganisms -
the pathogen- associated molecular patterns (PAMPs) as
well as danger signals released from damaged or dying
(necrotic) cells (DAMPs)
These PAMPs/DAMPs are recognized by pattern recognition
receptors (PRRs), expressed on/in the innate immunity cells.
PRRs can also recognize host molecules containing
damage- associated molecular patterns (DAMPs), molecules
that are often released from necrotic cells damaged by
invading pathogens
These PRRs include Toll-like receptors (TLRs), nucleotide-
binding domain (NOD) and leucine-rich repeat containing
receptors (NLRs), and retinoic acid-inducible gene-I
(RIG-)-like receptors (RLRs). lectins, and scavenger
receptors.
 PAMPs
Lipopolysaccharide
Peptidoglycan
Lipoteichoic acids
Mannose-rich glycans
Flagellin
Pilin
Bacterial nucleic acid
N-formylmethionine,
Double-stranded
RNA
Lipoteichoic acids, glycolipids, and
zymosan Phosphorylcholine and other
lipids
 PAMPs binding to PRRs on defense cells

The PRRs recognize approximately 103 molecular patterns

 Innate immune responses encountered by microbes.

Microbes are detected by pattern recognition receptors (PRRs) expressed in

innate immune cells, such as macrophages. The detection of microbes by the
PRRs rapidly activates signalling cascades and generates inflammatory
responses. Microbial encounter also leads to maturation of macrophages and
dendritic cells into antigen presenting cells. PAMP, pathogen- associated
molecular pattern; TCR, T-cell receptor.
 Biological
PAMP PRR Consequence of
Interaction
Microbial cell Complement Opsonization;
wall components Complement activation
Mannose-containing Mannose-binding protein Opsonization;
carbohydrates Complement activation
Lipoproteins of Gram TLR-2 (Toll-like receptor 2) Macrophage activation;
positive bacteria, Secretion of
yeast cell wall inflammatory cytokines
components
Double stranded RNA TLR-3 Production of
interferon (antiviral)
LPS (lipopolysaccharide TLR-4 Macrophage activation;
of Gram –ve bacteria Secretion of
inflammatory cytokines
Flagellin (bacterial TLR-5 Macrophage activation;
flagella) Secretion of
inflammatory cytokines
 DAMPs
• Damage-associated molecular patterns (DAMPs) are
endogenous danger molecules that are released released
upon cellular stress or tissue injury from damaged or dying
cells and activate the innate immune system by interacting
with pattern recognition receptors (PRRs).
• DAMPs activate the innate immune system by inducing
potent
inflammatory responses during non-infectious inflammation
• These DAMPs are recognized by macrophages, and
inflammatory responses are triggered by different
pathways, including TLRs and inflammasomes
• DAMPs can originate from different sources and include:
– extracellular proteins, e.g. biglycan and tenascin C,
– intracellular proteins, e.g. high-mobility group box 1
(HMGB1),
histones, S100 proteins, heat-shock proteins (HSPs), and
– plasma proteins, e.g. fibrinogen, Gc-globulin, and serum
amyloid
A (SAA)
Induction of immune response by DAMPs
 Toll-like receptors (TLRs) - 1

Play a major role in innate immunity and the

induction of adaptive immunity.
Different combinations of TLRs appear in different
cell
types and seem to appear in pairs; 13 recognized so
far
Different TLRs directly or indirectly bind
different microbial molecules.
TLRs are found both on the surface and within the
phagolysosomes of phagocytes.
Surface TLRs recognize molecules on the surface
of
microbes such as cell wall components
Internal TLRs recognize microbial molecules
released upon phagocytosis of the microbe.
.