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Rationale for
Vaccine Development
Vaccine Types
Experimental Approaches to Vaccine Development
Rationale for Vaccine Development
Vaccination involves deliberate exposure to antigen under conditions where disease should not result. Humans first attempted to manipulate the immune system by exposing themselves to infectious organisms in order to develop immunity. (Mothers still did this in the 1950's by exposing their children to neighbors with measles or chickenpox!). Improved hygiene and discovery of antibiotics have resulted in greatly reduced deaths from infectious organisms in developed nations. However, vaccination still provides one of the best means for preventing, rather than treating, infectious disease. Vaccination has eliminated the smallpox virus worldwide; no cases have been observed for over 20 years. The World Health Organization is now conducting a vaccination campaign designed to eliminate polio early in the 21st century. Current vaccine recommendations can be found at the CDC National Immunization Program site: http://www.cdc.gov/nip/.
Vaccines are developed after rigorous testing for effectiveness and safety. A vaccine must be of the proper dose and chemical characteristics to stimulate protective immunity, which may involve secretion of neutralizing antibody or production of memory CTL or Th1 cells. Immunity should last several years. A good vaccine should not induce autoimmunity or hypersensitivity. Ideally it will be inexpensive to produce, store and administer. The final vaccination schedule is arrived at after several years of use dictate number of doses and optimal intervals between doses.
Vaccines must also be perceived to be safe. Bordetella pertussis causes whooping cough, which in small infants results in significant hospitalization (32% of cases), pneumonia (10% of cases)and death (0.2% of cases). The whole cell vaccine against Bordetella pertussis was developed in the 1930's and childhood vaccination in the US reduced the annual rate of infection from 200/100,000 in the 1940's to less than 2/100,000. Whole cell vaccine, given with tetanus and diphtheria toxoids, was associated with inflammation at the injection site. In a few children, high temperature and persistent crying occurred; very rarely, seizures or a transient unresponsive state were seen. Anecdotal reports that irreversible brain damage might be a rare consequence of pertussis vaccination, coupled with two deaths in Japan, lead to a decline in vaccination rates in the late 1970's and a rise in whopping cough and death due to pertussis infection, especially in Japan and in Great Britain. Careful studies did not confirm that pertussis vaccination was a primary cause of brain injury, but in response to public concerns an acellular vaccine was developed containing purified antigens that induce protective immunity. This vaccine is as effective as the whole cell vaccine and does not induce the common side-effects of the original vaccine. Recent anecdotal reports of association between childhood vaccination (particularly with MMR) and autism have raised concerns in parents; worldwide studies have found no association between the incidence of vaccination and autism.
Infants commonly experience a dip in serum antibodies between 3-12 months of age. Maternal IgG acquired in utero is eliminated from the baby's circulation, and infants only become able to make their own IgG at about 6 months of age. Infants during this period are more susceptible to infection, especially premature babies who are born with lower levels of maternal IgG and are slower at producing their own IgG.
The original Salk polio vaccine is an example of an inactivated (killed) vaccine. It is made by growing virulent polio virus in tissue culture, then treating the virus with formaldehyde so that it cannot reproduce in the person who receives the vaccine. Polio virus does not undergo rapid mutation, so a mixture of three serotypes of polio virus is sufficient to generate good protective immunity against all common polio viruses. Neutralizing antibody produced to polio virus is very efficient at blocking the ability of the virus to infect host cells and offers good protection from infection. The advantages of this vaccine are that the risk of infection is very low if inactivation is efficient; it can be used safely on people whose immune systems are compromised by chemotherapy for cancer, transplantation anti-rejection drugs, or other immune deficiencies; and use of the whole virus stimulates immunity to antigens in their natural conformation on the virus surface (essential for neutralizing antibodies). The disadvantages are that since the virus cannot multiply, a large number of virions are required to stimulate immunity; periodic boosters must be given to maintain immunity; only humoral immunity can be induced; and since the vaccine must be injected, it is costly to administer. Influenza virus vaccine is another inactivated virus vaccine. It has the additional disadvantage that flu viruses mutate rapidly, so that new antigenic specificities must be included in each year's vaccine. Predictions are made in the spring about which antigens will predominate the following year; if those predictions are wrong, the vaccine will not be protective.
The Sabin oral polio vaccine and the measles, mumps, and rubella (MMR) vaccine are examples of attenuated (weakened) vaccines. To make an attenuated vaccine, the pathogen is grown in animals or tissue culture under conditions that make it less virulent. Advantages of whole virus attenuated vaccines are that infectious virus can stimulate generation of memory cellular as well as humoral immune responses; the ability of the virus to multiply in the host means that less virus must be injected to induce protection; and use of the whole virus stimulates response to antigens in their natural conformation. Additional advantages of the Sabin vaccine are that it can be administered orally, which is less expensive than giving injections; and oral administration induces mucosal immunity and IgA synthesis, which gives more protection at the normal site of virus entry. Disadvantages of attenuated vaccines are that the virus may very rarely revert to its virulent form and cause disease; and live virus vaccines cannot be given safely to immunosuppressed people. Because the incidence of vaccine-acquired polio is much higher than that of naturally acquired polio in the US, vaccination recommendations changed recently so that infants will receive killed polio vaccine prior to receiving the oral vaccine. The oral vaccine is being used in the WHO polio eradication campaign.
Subunit vaccines contain purified antigens rather than whole organisms; an example is the Bordetella pertussis antigens included in the acellular DPT vaccine. Subunit vaccines are not infectious, so they can safely be given to immunosuppressed people; and they are less likely to induce unfavorable immune reactions that may cause side effects. The disadvantages of subunit vaccines are that the antigens may not retain their native conformation, so that antibodies produced against the subunit may not recognize the same protein on the pathogen surface; and isolated protein does not stimulate the immune system as well as a whole organism vaccine. Other protein vaccines that induce good protective immunity are the diphtheria and tetanus toxoid components of DPT. These are toxins that have been treated to eliminate their toxicity; they are still able to induce antibodies that can neutralize the native toxins.
The effectiveness of subunit vaccines in increased by giving them in adjuvants. Adjuvants slow antigen release for a more sustained immune stimulation, bind toll-like receptors on macrophages and dendritic cells to stimulate production of inflammatory cytokines, and activate APC to express B7. Alum (aluminum salts) is a common adjuvant used in human vaccines; it aggregates proteins to make them easier for phagocytes to engulf. Pertussis toxin, one of the components of the acellular DPT, acts as an adjutant in that vaccine. Some bacterial components used as adjuvants in animals but which cause too much inflammation to be safe in humans are whole Mycobacterium tuberculosis, muramyl dipeptide from Mycobacterial cell walls, and bacterial DNA. Animal studies have demonstrated that injecting mice with Leishmania major plus IL-12 stimulates T cells and NK cells to make IFNg and induces a protective Th1 response.
Conjugate vaccines have been developed to pathogens whose polysaccharide capsules protect them from phagocytosis: Haemophilus influenzae B (HiB), Streptococcus pneumoniae, and Neisseria meningitidis. Immunity to these organisms requires opsonizing antibodies, but infants make poor T-independent responses to polysaccharide antigens. By covalently linking the polysaccharides to protein carriers, they are converted into T-dependent antigens and protective immunity is induced. For example, HiB polysaccharide is complexed with diphtheria toxoid to increase its immunogenicity in infants.
A new approach to developing vaccines to parasites is to isolate parasite peptides from host cell MHC and use those peptides (synthesized in bulk in the lab) to induce immunity. Peptide vaccines target particular peptides to which a protective response can be developed. Peptides have no native structure and do not bind the pattern recognition molecules on phagocytes that promote pathogen uptake. Peptide immunogenicity can be increased by giving them in ISCOMS, lipid micelles which transport the peptides directly into the cytoplasm of dendritic cells for presentation on Class I MHC. One limitation of the peptide approach is that it is tightly linked to particular HLA alleles, so some peptides may not be universally effective at inducing protective immunity.
Recombinant vaccines are those in which genes for desired antigens are inserted into a vector, usually a virus, that has a very low virulence. The vector expressing the antigen may be used as the vaccine, or the antigen may be purified and injected as a subunit vaccine. Advantages of recombinant vaccines are that the vector can be chosen to be not only safe but also easy to grow and store, reducing production cost. Antigens which do not elicit protective immunity or which elicit damaging responses can be eliminated from the vaccine, and proteins expressed on a virus, even if it is not the usual pathogen, are more likely to have their native conformation. Disadvantages of recombinant vaccines are their cost to develop, since the genes for the desired antigens must be located, cloned, and expressed efficiently in the new vector. The only recombinant vaccine currently in use in humans is the Hepatitis B Virus (HBV) vaccine, which is a recombinant subunit vaccine. Hepatitis B surface antigen is produced from a gene transfected into yeast cells and purified for injection as a subunit vaccine. This is much safer than using attenuated HBV, which could cause lethal hepatitis or liver cancer if it reverted to its virulent phenotype.
DNA vaccines are the newest vaccines and are still experimental, although to date they seem to be very effective and their safety record is good. Like recombinant vaccines, genes for the desired antigens are located and cloned. In the case of DNA vaccines, however, the DNA is injected into the muscle of the animal being vaccinated, usually with a "gene gun" that uses compressed gas to blow the DNA into the muscle cells. Some muscle cells express the pathogen DNA to stimulate the immune system. Both humoral and cellular immunity have been induced by DNA vaccines.
Passive immunization is used when exposure to pathogen has already occurred and there is not enough time to induce active protective immunity, or prophylactically in children with inherited immune deficiencies or undergoing cancer chemotherapy (which suppresses the immune system). Passive immunization involves administration of specific antibody which has been produced in a human or animal in response to vaccination or environmental exposure to the pathogen. Examples include antivenin for snake bite; Rhogam (human anti-Rh) to block formation of IgG anti-Rh antibodies in Rh negative mothers to Rh positive erythrocytes from their babies; and human gamma globulin given to children who have humoral immune deficiencies. Passive immunization can confer adequate protection in the short term, but antibodies have a half-life of only a few weeks in serum and they cannot be replaced except by active immunization or another dose of passive antibody. When the antibodies come from another species, serum sickness (Type III hypersensitivity) may also result from complexes of the foreign Ig with antibodies made against it.
Experimental Approaches to Vaccine Development
With our expanded understanding of how the immune system works, experimental approaches to vaccine development are aimed at developing vaccines that target protective immune responses. One area of interest is the development of better vaccines to stimulate mucosal immunity, since most pathogens enter the body through mucosal membranes. The oral polio vaccine is an example of a vaccine that enters by the pathogen's normal route and stimulates protective neutralizing antibody. Difficulties with oral vaccine administration include antigen destruction in the stomach or intestines and risk of inducing tolerance.
Another area of vigorous research is targeting antigens to APC. Antigens have been covered with mannose to bind macrophage mannose receptor and made into immune complexes to stimulate uptake by FcR+ cells. Pathogen DNA has been complexed with CTLA-4 to promote its uptake and expression by B7+ APC. ISCOMs target antigen to Class I MHC, while antigen coupled to a particular signal peptide can be used to move antigen into endosomes for processing and presentation on Class II MHC. The outer membrane protein of Salmonella typhimurium binds M cells and may be useful for targeting antigen to the mucosal immune system.
Finally, the ability of vaccination or cytokine administration to control ongoing infection is being studied. Chronic infections occur with Herpes simplex viruses, hepatitis B and C viruses, Mycobacterium tuberculosis and M. leprae, and the parasites Leishmania, Plasmodium, and Schistosoma. Persisting infections lead to prolonged infectivity, tissue damage from immune hypersensitivity, and tumor development. Established immune responses are very difficult to modify or eliminate; but there is hope that with a properly-targeted vaccine boost the immune system may be able to completely eliminate pathogen.
References
CDC National Immunization Program http://www.cdc.gov/nip/
World health Organization Polio Eradication Initiative http://www.polioeradication.org/
Practice Quiz
Pick the one BEST answer for each question by clicking on the letter of the correct choice.
1. Advantages of the oral polio vaccine compared with the killed polio vaccine are all of the following EXCEPT that it
a. elicits IgA as well as IgG synthesis.
b. induces cellular as well as humoral immunity.
c. induces secretion of protective mucosal neutralizing antibodies.
d. is less expensive to administer.
e. is safer to give to immunosuppressed children.
2. To make a recombinant vaccine to a virus, the virus DNA is expressed
a. covalently linked with a polysaccharide.
b. in a genetic variant of the same virus.
c. in a less virulent vector, which may be a virus or a bacterium.
d. in muscle cells of the immunized person.
e. on an ISCOM.
3. An advantage of giving passive immunization is that it provides ___________ immunity.
a. cellular
b. faster
c. longer lasting
d. more specific
e. safer
4. Bacterial polysaccharide vaccines are conjugated to proteins so that
a. the polysaccharide can act as an adjuvant to induce better immunity to the protein.
b. the polysaccharide looks less like blood typing antigens to the immune system.
c. the protein can act as an adjuvant to upregulate B7 expression on APC.
d. the protein can stimulate T cell help for polysaccharide-specific B cells.
e. none of the above.
5. Hepatitis B surface antigen vaccine, a recombinant vaccine produced in yeast cells,
a. cannot be given to immunosuppressed children.
b. employs the yeast cells as an adjuvant to increase its immunogenicity.
c. is less safe to administer than killed Hepatitis B virus vaccine.
d. is more immunogenic than the whole Hepatitis virus would be.
e. requires several boosters because it is also a subunit vaccine.
6. Passive immunization to a bacterial toxin would be recommended if
a. exposure to the toxin had already occurred in an unimmunized person.
b. the person who needed the immunization had been recently immunized with the toxoid.
c. the toxin could not reproduce in the body.
d. the toxin was not immunogenic.
e. the toxoid was available only as part of a conjugate vaccine.
7. ISCOMs
a. are DNA vaccines.
b. are safe replicating vectors for recombinant vaccines.
c. stimulate APC to upregulate their B7 levels.
d. transport peptide vaccines into APC cytoplasm so they can be presented on Class I MHC.
d. transport killed viruses into T cell cytoplasm so they can induce cellular immunity.
Problems
1. A close friend of yours has recently become a parent, and states that the baby will not be immunized because immunizations are dangerous and anyway no one in this country gets "those diseases" anymore. Based on what you have learned in this course, what is your response to your friend and upon what evidence do you base your advice?
