Immunoprophylaxis

Vaccines 17
The purpose of immunization in one indi- vaccines, there remains the need to improve
vidual is to prevent diseases. The objective the available vaccines as regard to safety and
of immunization program has been respon- efficacy and cost-effectiveness, especially
sible for spectacular advances in combating in developing country. The World Health
the infectious diseases such as smallpox, Organization (WHO) estimates that 20% to
poliomyelitis, tetanus, diphtheria, etc. Since 35% of infant deaths, in the world, are due
1977 smallpox has been eradicated, equal- to diseases that could be prevented by exist-
ly encouraging is the predicted eradication ing vaccines. Some potential vaccines have
of polio. unacceptable side effects and others' worsen
the disease, especially for the immunodefi-
HISTORICAL OVERVIEW cient subjects. Therefore, stringent testing is
The discipline of immunology had its roots in an absolute necessity, as the vaccines will be
the early vaccination trials of Edward Jenner given to healthy subjects.
and Louis Pasteur. Edward Jenner's introduc-
tion of vaccinations with cowpox virus (1796) ACTIVE AND
to protect against smallpox was the first docu- PASSIVE IMMUNIZATION
mented use of a live, attenuated viral vaccine Immunity against infections can be achieved
and the beginning of modern immunization. by active and passive immunization. In ac-
Robert Koch was the pioneer to demon- tive immunization, the immunity is achieved
strate the specific bacterial cause of anthrax naturally by suffering, clinically or subclini-
(1876). Subsequently, the causes of several cally from the disease or by artificial means,
common illnesses were rapidly identified. such as vaccines (Table 17.1). Similarly, in
The control of number of the diseases passive immunization, the immunity is ob-
that cause significant mortality has made out- tained naturally from mother to fetus by
standing progress, but there remains an ur- transfer of antibody (from mother to infant
gent need for vaccines against others. Every via milk or artificial injection of preformed
year, millions are dying from malaria, tuber- antibody) (Table 17.2).
culosis (TB) and acquired immune deficiency
syndrome (AIDS) for which there are no ef- Passive Immunization
fective vaccines. Jenner and Pasteur are the stalwarts who pio-
In addition to the challenges presented neered vaccines. So, also Emil von Behring
by diseases for which there are no effective and Hidesaburo Kitasato are well recognized
 Immunoprophylaxis Vaccines 233

by their contributions to passive immunity. the immune system plays active role by pro-
In fact these investigations, for the first time, ducing antigen-reactive T or B cells. Some
showed that the immunity could be trans- of them remain as memory cells. The recom-
ferred passing from one individual to other mended program of childhood immuniza-
by serum. Common agents used for passive tion in India is outlined in Table 17.1.
immunization are mentioned in Table 17.2.
Routine Immunization
Active Immunization While introducing immunization schedule,
The goal of the active immunization is to certain factors are taken into consideration.
elicit protective immunity and immunologi- The schedule is based on the prevalence of
cal memory. When active immunization is the infectious diseases, their public health
successful, a subsequent exposure produce importance, availability of suitable vaccines,
heightened response, leading to the elimina- the cost-benefit factors and the logistics. In
tion of pathogen or prevention of the disease India, the Expanded Program on Immuniza-
mediated by its products. Active immuniza- tion (EPI) and the Universal Immunization
tion can be achieved by natural infection Program (UIP) has been able to afford pro-
with the microorganism or it can be acquired tection against vaccine-preventable diseases
artificially by administration of vaccines. In (VPDs). The National Immunization Sched-
active immunization, as the name implies, ule, in force, in India shown in Table 17.1.

Table 17.1 National Immunization Schedule (India)

Age Vaccine
At birth Bacille Calmette-Guérin (BCG), oral polio vaccine-0 (OPV-0)
6 week BCG-2, diphtheria, pertussis, tetanus, (DPT-1), OPV-1
10 week DPT-2, OPV-2
14 week DPT-3, OPV-3
9 month Measles
16–24 month DPT, OPV
5–6 year Diphtheria-3 (DT-3)
10 year Tetanus toxoid-4 (TT-4)
16 year TT-4
For pregnant women TT-1 or buster
1 month after TT-1 TT-2
Note:
1. For institutional birth only. OPV-0 is additional and not to be counted for the primary course of third doses starting
at 6 week.
2. Only for infant not given BCG at birth.
3. A second dose of DT to be given to children with no documentary evidence or history of primary DPT im-
munization.
4. A second dose of TT to be given after month to those with no record or history of prior DPT, DT or TT immunization.
5. For prevention of tetanus in the neonate primarily, but also in the mother.
234 Textbook of Immunology

DESIGNING VACCINES OF Table 17.2 Classification of common

ACTIVE IMMUNIZATION vaccines for humans
The designer must keep in mind certain fac- Disease or pathogen whole organisms
tors for developing successful vaccine. The Bacterial cells Types of vaccine
first requirement is the development of ap-
Anthrax Inactivated
propriate immune response in order to deal
with the organism, what is often critical that, Cholera Inactivated
which branch of the immune system antibody Pertussis* Inactivated
-mediated immunity (AMI) or cell-mediated Plague Inactivated
immunity (CMI) is to be activated, depend- Tuberculosis Live attenuated BCG†
ing on the structural contents and the nature
of the microorganisms. A second factor is the Typhoid Live attenuated
development of immunological memory. If a Viral particles
vaccine induces only protective primary re- Hepatitis A Inactivated
sponse and fails to induce memory cells, the
Influenza Inactivated
host is not protected on subsequent infec-
tion by the same microorganisms. The role of Measles Live attenuated
memory cells also, is dependent on the incu- Polio (Sabin) Live attenuated
bation period of the pathogen. For example, Polio (Salk) Inactivated
in the case of influenza virus, which has a
Rabies Inactivated
short incubation period (1 or 2 day). The dis-
ease symptoms are already on the way be- Rotavirus Live attenuated
fore the memory cells are activated. Effective Rubella Live attenuated
protection against influenza can be achieved Varicella zoster Live attenuated
by repeated immunization to maintain a high (chickenpox)
level of neutralizing antibody. For pathogen,
Yellow fever Live attenuated
with long incubation period (poliovirus), re-
quires more than 3 days to begin to infect Whole organisms purified macromolecules
central nervous system (CNS). An incuba- Toxoids
tion period of this length gives the memory
Diphtheria Inactivated exotoxin
B cells more time to respond by producing
high level of serum antibody. Tetanus Inactivated exotoxin
In addition to the factors already men- Capsular poly-
tioned, the effective protection against the saccharides
intended pathogen must occur without dan- Haemophilus Polysaccharide protein
ger of causing disease or producing severe influenzae type b carrier
side effects. Besides, the vaccine must be Neisseria Polysaccharide
economically feasible for production and be meningitidis
suitably stable for storage, transport and use. Streptococcus 23 distinct capsular
pneumoniae polysaccharides
Whole Organism Vaccines Surface antigen Recombinant surface
Many of the common vaccines, which are in antigen
use fall into inactivated (killed) or live, but *There is also acellular pertussis vaccine.
attenuated (avirulent) bacterial cells or viral †
Bacille Calmette-Guérin (BCG) is an avirulent strain of
particles (refer Table 17.2). Mycobacterium bovis.
 Immunoprophylaxis Vaccines 235

Attenuated Bacterial or Viral Vaccines is the presence of other virus as contaminants

In some cases, microorganisms can be at- [simian virus-40 (SV40)], oncogenic virus in
tenuated, so that the pathogenicity is de- monkey kidney cultures, used for Sabin vac-
stroyed, but antigenic property is retained. cine production). Attenuated vaccines may
Attenuation can be achieved by growing a rarely, be associated with complications sim-
pathogenic bacteria or virus for a prolonged ilar to those seen in natural diseases. There
periods under abnormal culture conditions. may be also vaccine-mediated immune sup-
This procedure selects mutants, which are pression, as that happens in measles vaccine
capable of growing in abnormal culture con- (Edmonston-Zagreb strain).
ditions and therefore, less capable of growth Presently, genetic engineering techniques
in natural host. The common example is the play important role to attenuate viruses ir-
BCG vaccine, which is the attenuated strain reversibly by selectively removing the genes
of Mycobacterium bovis. This vaccine was that are necessary for virulence (herpes virus).
prepared by growing M. tuberculosis on a More recently, a vaccine for rotavirus has
medium containing increasing concentra- been developed using genetic engineering
tion of bile. Several examples, such as Sabin techniques to modify an animal rotavirus to
polio, measles and other vaccines have been contain antigens present in human rotavirus.
successfully attenuated.
Advantages: The attenuated vaccines have ad- Killed Bacterial or Viral Vaccines
vantages and disadvantages. Because of their Another approach to prepare vaccine is to
capacity for transient growth, such vaccines inactivate the bacteria by heat or chemicals.
provide prolonged immune system exposure Heat inactivation may be unsatisfactory due
to the epitopes of the attenuated vaccine, so to extensive denaturation of protein. Chemi-
that there is increased immunogenicity and cal inactivation with formaldehyde or various
production of memory cells. Therefore, boost- alkylating agents has been successful. Salk
er doses may not be required. This property polio vaccine and the pertussis (whooping
of the vaccine is more relevant in develop- cough) vaccines are the examples of chemi-
ing country like India, where majority do not cally (formaldehyde)-inactivated vaccines.
turn up for the second and subsequent doses The inactivated vaccines, unlike live at-
of the vaccines. The other advantage of the tenuated vaccines, are given in repeated
live attenuated vaccine is the production of booster doses, because, the organisms are
CMI local immunity. The attenuated Sabin killed and do not multiply in the tissue. They
vaccine colonizes the intestine and enables are usually given intramuscularly and they
to induce production of secretory immuno- produce predominantly humoral immunity.
globulin A (IgA), which serves an important If inactivation is improper this may lead to
defense against naturally acquired poliovirus. serious untowards complications. A serious
The vaccine induces both IgM and IgG class. complication (paralytic polio) with first Salk
Disadvantages: One of the important disad- vaccines arose, when formaldehyde failed to
vantage of the attenuated strain is the possibil- kill all the viruses. Pertussis vaccine may also
ity of their reversion to virulent form. The rate cause encephalitis, if not inactivated prop-
of reversion of the Sabin polio vaccine (OPV) erly. Therefore, presently acellular pertussis
is one case in 4 million doses of vaccine. vaccine (Table 17.2) has replaced the whole
Another danger with the attenuated vaccine organism vaccines.
236 Textbook of Immunology

Purified Macromolecules ful exotoxins, which cause lesions. Vaccine

as Vaccines against diphtheria and tetanus can be made
Three types of vaccines are currently avail- by purifying the bacterial exotoxin and inac-
able. They are capsular polysaccharides, in- tivating the toxins with formaldehyde to form
activated exotoxins and recombinant surface toxoids. The toxoid when given in vaccine,
antigens (refer Table 17.2). induces antitoxoid antibodies, which are
also capable of neutralizing the toxin. One of
Polysaccharide Vaccine the problems was difficulty in obtaining the
exotoxins in large quantity. This impediment
The virulence of the capsulated bacteria de-
has been overcome by cloning the exotoxin
pends on the presence of capsules, which
genes and impressing them in easily grown
prevent phagocytosis. But if the capsulated
bacteria are coated with antibodies and/or host cells. In this way, large quantities of exo-
complement (opsonin), they can be easily toxins can be produced, purified and subse-
phagocytosed by macrophages and neutro- quently inactivated.
phils. This is the principle of purified poly-
Recombinant Antigen Vaccines
saccharide vaccines.
The genes encoding any immunogenic pro-
Presently, polysaccharide vaccines are
tein can be cloned and be expressed in bac-
available for pneumococcus, Neisseria
teria, yeasts and other mammalian cells by
meningitidis and %=AIKLDEHQO EJŃQAJV=A
recombinant deoxyribonucleic acid (DNA)
Vaccine for pneumococcus, which causes
technology.
pneumococcal pneumonia consists of 23
antigenically capsular polysaccharides. The A number of genes from surface antigen
vaccine induces opsonizing antibodies. One of viruses, protozoa and other pathogens
limitation of polysaccharide capsular anti- have been successfully cloned into bacterial
gen is that they cannot activate T helper cells cell, insects, yeasts and the expressed antigen
(Thcell). They activate B cells in thymus-in- may be used in development of vaccines.
dependent type-2 manner, resulting in IgM The first such recombinant antigen vac-
production. There is no class switching and cine, approved and used, is the hepatitis-B
development of the memory cells. One way surface antigen, this was developed in yeast
to involve the Thcells directly in the response cells. The recombinant yeast cells are grown
to a polysaccharide antigen, is to conjugate in large fermenters and hepatitis-B surface
the antigen to some sort of protein carrier. For antigen (HBsAg) accumulates intracellularly.
example, the vaccine for %EJŃQAJV=Atype These yeast cells are then harvested, disrupt-
b (Hib), which is a major cause of bacterial ed in high pressure, releasing the recombi-
meningitis, in children, under 5 years of age, nant antigens. Subsequently, they are puri-
consists of type ‘b' capsular polysaccharide fied by biochemical methods.
covalently linked with a protein carrier teta-
nus toxoid (TT). Recombinant Vector Vaccines
Genes that encode major antigens of virulent
Toxoid Vaccines pathogens can be introduced to attenuated
In some bacterial diseases, the pathogens are viruses and bacteria. The attenuated organ-
toxigenic rather than invasive. The common isms can serve as a vector, replicating within
examples are Corynebacterium diphtheriae the host and expressing the gene products.
and Clostridium tetani. They produce power- The organisms, which serve as vectors and
 Immunoprophylaxis Vaccines 237

replicate in the host cells include vaccinia including those for, malaria, AIDS, influ-
virus, canarypox virus, attenuated polio- enza and herpes virus. Cytokine-producing
virus, adenovirus and attenuated strains of genes, such as interleukin-12 (IL-12) gene,
Salmonella. Vaccinia virus is the most suit- can be combined with DNA vaccine to pro-
able vector widely used. This large complex duce optimal immune response. The expres-
virus can carry several dozens of foreign sion of IL-12 at the site of immunization will
genes without impairing its capacity to in- stimulate Th1 type of immunity, induced by
fect host cells and replicate. Genetically the vaccine.
engineered vaccinia vector vaccines can be
administered, simply by scratching the skin, Certain drawbacks of DNA vaccine do
causing localized infection in host cells. The exist. First, only protein antigen can be en-
procedure for producing a vaccinia vector coded and secondly, inability to use the DNA
that carries a foreign gene from pathogen is vaccine to provide mucosal immunity as that
outlined in Figure 17.1. happens in oral and nasal spray vaccine.

Deoxyribonucleic Acid Vaccine Synthetic Peptide Vaccines

Recently, plasmid DNA encoding antigenic Although, initially, the use of synthetic pep-
proteins are directly injected into the mus- tide as vaccines appeared very promising, it
cle of the recipient. DNA is taken up by the had not progressed much, as expected. The
muscle cells and the expressed protein pro- reason being, they are not immunogenic as
duce desired humoral immunity or CMI. The proteins and they are not effective in produc-
DNA, inside the muscle cells, remains inte- ing both humoral and CMI. Their immunoge-
grated with the host cell DNA or in an epi- nicity can be enhanced by conjugating adju-
somal form. In addition to muscle cells, the vants, but there are some other impediments,
dendritic cells around the area, also take up which have prevented their use. To produce
the plasmid DNA and express viral antigen. a desired humoral or CMI, an understanding
Deoxyribonucleic acid vaccine, present- of the nature of T cell and B cell epitopes are
ly, proves better because of certain distinct required. In most pathogens, though the ami-
advantages over other vaccines. The encoded no acid sequence of many important antigens
protein expression occurs, in its natural form, is known, their three-dimensional structure
without denaturation or modification. DNA is unknown. An effective memory response
vaccines induce both humoral and CMI. The for both humoral and CMI, the generation of
DNA vaccine caused prolonged expression population of memory Th cells is imperative.
of antigen, therefore, generating immunologi- A successful vaccine must therefore include
cal memory cells. Refrigeration is not manda- immunodominant T cell epitopes. Synthetic
tory, which helps the cost-effectiveness. peptides that represent immunodominant T
An improved method, for administering or B cell epitopes are being evaluated as vac-
these vaccines, is to coat microscopic gold cines for several diseases.
beads with the plasmid DNA and then de-
liver the coated particles through the under- Multivalent Subunit Vaccines
lying muscle with an air gun (gene gun). This One of the limitations of synthetic peptides
will allow rapid delivery of vaccines to large and recombinant vaccine is that they are
population without the requirement of large weakly immunogenic. In addition, they pro-
number of needles and syringes. duce only humoral immunity, but not CMI.
At present, there are animal and human The most ideal synthetic peptide vaccine
trials under way with several DNA vaccines, should contain both immunodominant B and
238 Textbook of Immunology

Fig. 17.1: Production of vaccinia vector vaccine. The gene that encodes the desired antigen (green) is
inserted into a plasmid vector adjacent to a vaccinia promoter (yellow) and flanked on either side by the
vaccinia thymidine kinase (TK) gene (reddish orange). When tissue culture cells are incubated simultane-
ously with vaccinia virus and the recombinant plasmid, the antigen gene and promoter by homologous re-
combination at the site of the non-essential TK gene, resulting in a TK recombinant vaccinia virus, selected
by addition of 5-bromodeoxyuridine (BUdR), which kills TK cells.

T cell epitopes. Secondly, for better cytolytic T tides are processed and presented with class-
lymphocyte (CTL) response, the vaccine must I major histocompatibility complex (MHC)
be delivered intracellularly, so that the pep- molecules. A number of new techniques
 Immunoprophylaxis Vaccines 239

are being applied to develop multivalent different monoclonal antibodies to the solid
vaccines that can present multiple copies of matrix, it has become possible to bind mix-
a given peptide or mixture of peptides to the ture of peptides or proteins, composing im-
immune system (Fig. 17.2). munodominant B and T cell epitopes. These
One approach is to prepare solid matrix multivalent complexes produce marked hu-
antigen-antibody (SMAA) complexes by at- moral and CMI. Their particulate nature pro-
taching monoclonal antibodies (mAbs) to vides better immunogenicity.
particulate solid matrices and saturating the Another approach to produce multiva-
antibody with desired antigen. The formed lent vaccines is to use detergent to incorpo-
complexes are used as vaccines. By attaching rate protein antigens or synthetic antigenic

Figs 17.2A to C: Multivalent subunit vaccines. A. Solid matrix antibody-antigen complexes can be de-
signed to contain synthetic peptides representing both T cell epitopes and B cell epitopes; B. Protein mi-
celles, liposomes and immunostimulating complexes (ISCOMs) can all be prepared with extracted antigens
or antigenic peptides. In micelles and liposomes, the hydrophilic residues of the antigen molecules are
oriented outward. In ISCOMs, the long fatty-acid tails of the external detergent layer are adjacent to the
hydrophobic residues of the centrally located antigen molecules; C. ISCOMs and liposomes can deliver
antigens inside cell, so they mimic endogenous antigens. Subsequent processing by the cytosolic pathway
and presentation with class-I MHC molecules induces a cell-mediated response. (TAP, transporter associ-
ated with antigen processing; ER, endoplasmic reticulum).
240 Textbook of Immunology

peptides into protein micelles, into lipid ves- SUGGESTED READING

icles (liposomes) or into immunostimulating 1. Black JG. Microbiology: Principles and
complexes (ISCOMs). applications, 3rd edition. USA: Printice Hall
Membrane proteins from various patho- College div; 1996.
gens such as influenza, measles and hepa- 2. Coligan JE, Kruisbeek AM, Margulies DH, et
titis B viruses have been incorporated into al. Current Protocols in Immunology. New
micelles, liposomes and ISCOM and are cur- York: Wiley; 1997.
3. Daniel P Stites. Basic and Clinical Immuno-
rently addressed as potential vaccines.
logy, 8th edition. USA: Lange (Medical
STUDY QUESTIONS Book); 2007.
4. Goldsby RA, Kindt Thomas J, Osborn Barbara
Essay Questions A. Kuby Immunology, 6th edition. New York:
WH Freeman and Company; 2007.
1. Classify the common vaccines, which
5. Jawetz, Melnick and Adelberg's Medical
are in use for human beings. How do
Microbiology, 25th edition. Lange Basic
vaccines and toxoids differ?
Science, USA: McGraw Hill; 2010.
2. List the major differences among vac- 6. Male David, Brostoff Jonathan, Roth David,
cines. What are the major benefits and et al. Immunology, 7th edition. Mosby
hazards of active immunization? Elsevier; 2006.
7. Thao Doan, Roger Melvold, Susan Viselli,
Short Notes
et al. Lippincott's illustrated reviews: Imm-
1. Live vaccines. unology. Ist Indian print. Baltimore, USA:
2. Polysaccharide vaccines. Lippincott Williams and Wilkins; 2008.
3. Recombinant antigen vaccines. 8. Tortora Gerard J, Funke Berdel R, Case chri-
4. Subunit vaccines. stine L. Microbiology: an introduction, 10th
5. DNA vaccines. edition. USA; 1998.