Avid Bioservices Inc (CDMO)

[jazzbeerman]

great read

I highlighted some of it. Anyone who understands how bavi works will be thrilled.



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Correlates of Immune Protection and the Development of a Human Immunodeficiency Virus Vaccine


Norman L. Letvin1,

1 Harvard Medical School, Boston, MA 02115, USA



The development of antiviral vaccines is historically rooted in efforts that were not based on an understanding of the immunopathogenesis of disease. The first successful antiviral vaccine, the vaccine to prevent smallpox, was administered by Edward Jenner in 1796 without knowledge of the etiologic agent of the disease, an understanding of how smallpox is transmitted, or what physiologic mechanisms might prevent the acquisition of infection (Jenner, 1801). Learning that milkmaids were protected from developing smallpox and hypothesizing that exposure to cowpox might confer protection against smallpox, Jenner exposed young James Phipps to fluid extracted from a cowpox lesion and showed that the boy was subsequently protected from acquiring smallpox. Thus, a sophisticated understanding of how specific immune effector mechanisms might mediate clearance of the smallpox virus was not employed at the time to develop an effective smallpox vaccine.

Although recent decades have witnessed an explosion in our understanding of the immune mechanisms that can mediate the containment of viral replication, vaccines in the modern era have been developed based on studies in which only a single immune effector response, an antiviral IgG titer, is monitored. Effective vaccines against diverse viruses such as poliovirus and hepatitis B virus were developed by benchmarking a critical antiviral IgG titer that was associated with protection and by devising vaccination strategies that reproducibly and safely generated those antibody responses (Nathanson et al., 1962). This approach to vaccine development did not require an understanding of the differences in disease pathogenesis associated with infections by different viruses.

Viral infections can be prevented by a variety of vaccine strategies (Table 1) (Plotkin and Orenstein, 1999). Building on Jenner's experience in the 18th century, a number of effective live-virus vaccines have been developed. A pathogenic virus is passaged in vitro, which results in an accumulation of mutations. The resulting mutated virus retains the capacity to replicate in vivo but has an attenuated pathogenic potential. Vaccination with such a nonpathogenic virus initiates an infection that induces a robust immune response comprised of not only antibody and CD4+ T lymphocyte responses, but also CD8+ T lymphocyte responses. These CD8+ T lymphocyte responses are generated because of the MHC class I processing of viral proteins that occurs in the setting of active replication of the vaccine virus in vivo. The measles and chickenpox vaccines currently in use were created by this approach. A major limitation of this vaccine strategy is the possibility that a vaccine virus that has the capacity to mutate may revert to a pathogenic state in inoculated individuals and could potentially be transmitted from a vaccinee to immunologically naive individuals who come into contact with the vaccinee.

Table 1. Vaccine Strategies that Confer Protection against Virus Infections in Humans

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A number of antiviral vaccine strategies have also been developed that make use of nonreplicating immunogens, including inactivated viruses, recombinant proteins, and virus-like particles (VLPs). Chemical treatment or irradiation of a virus can eliminate the ability of that virus to replicate while leaving it in a conformation that can present protective epitopes to cells of the immune system. The currently used hepatitis A vaccine is such an immunogen. Viral proteins expressed in cell lines can also be administered as immunogens. The hepatitis B vaccine is based on this approach. Finally, the recently developed papillomavirus vaccine is a VLP, an immunogen created through the expression of structural viral proteins that come together in vitro to form nonreplicating, noninfectious particles. The inactivated virus-, recombinant protein-, and VLP-based vaccines all induce antibody and CD4+ T lymphocyte responses because these immunogens undergo MHC class II processing. However, because they do not replicate in vivo, these vaccine antigens do not undergo MHC class I processing and therefore do not elicit CD8+ T lymphocyte responses.

Although the application of these established approaches for vaccination has provided many successful immunogens, these strategies have not been able to elicit immune responses that confer universal control against other viruses that cause considerable morbidity and mortality in human populations. Although the available vaccines against influenza virus can provide protection against some circulating viral isolates, they cannot confer protection against all circulating viruses. No successful vaccine has been developed for use against hepatitis C virus (HCV). Interestingly, influenza virus and HCV share the property that they cannot be contained in a human population by a vaccine that induces an antibody response to a single conserved region of a viral protein. Antibodies specific for the influenza principal neutralizing determinant (PND), the hemagglutinin protein, can neutralize the virus, blocking its ability to infect a target cell. However, the hemagglutinin undergoes rapid and continuous mutational evolution, which allows it to escape from a vaccine-elicited neutralizing antibody response. Like influenza, HCV generates mutant progeny virions at a rapid rate, permitting it to evade a vaccine-induced neutralizing antibody response. Moreover, there is no evidence that HCV even has a PND. Therefore, viruses of this type cannot be consistently controlled by vaccine-induced antibody responses.

The human immunodeficiency virus (HIV) is the best-studied virus for which an established technology is unlikely to yield an effective vaccine. Like influenza virus and HCV, HIV mutates at an extraordinarily rapid rate, allowing it to evade a neutralizing antibody response (Wei et al., 2003). Therefore, currently available nonreplicating vaccine prototypes will not generate an HIV antibody response that will neutralize a diversity of HIV isolates. In fact, accruing evidence suggests that CD8+ T lymphocytes are of central importance in HIV containment (Schmitz et al., 1999). Although live-virus vaccines can induce effective anti-HIV CD8+ T lymphocyte immunity, studies in the simian immunodeficiency virus (SIV) macaque model for the acquired immunodeficiency syndrome (AIDS) have shown that prototype live attenuated vaccines can rapidly mutate from a nonpathogenic to a pathogenic form, making this approach unsafe for HIV vaccination (Whatmore et al., 1995). Additional technologies for inducing CD8+ T lymphocyte immunity to HIV are therefore currently being evaluated, including the use of plasmid DNA and live recombinant vectors. However, nonhuman primate studies suggest that such CD8+ T lymphocyte-based vaccination approaches may generate immune responses that partially contain HIV replication after the initiation of infection but may not be able to block infection by the virus (Letvin et al., 2006). Thus, it is possible that neither established nor recently developed vaccine technologies will be able to confer absolute protection against infection by HIV.

Frustrated by the difficulties being encountered in attempts to create an effective HIV vaccine, the AIDS vaccine research community has turned its attention to defining immunologic and virologic correlates of HIV containment. Investigators anticipate that an effective HIV vaccine strategy might emerge from a better understanding of both the biology of HIV transmission and the immunologic events that are triggered by early virus replication. These investigators assume that a better understanding of HIV immunobiology will uncover specific vulnerabilities of the virus or previously unexplored immune effector mechanisms that might be harnessed through vaccination.

Studies being pursued to define these correlates of protection include explorations of the virology, genetics, and immunology of early HIV replication. Investigators are searching for examples of the precise virus that is transmitted from infected to uninfected individuals, hoping to demonstrate idiosyncratic sequences or qualitative properties that might distinguish the transmitted virus from viruses isolated from chronically infected individuals (Derdeyn et al., 2004). These studies are being done with the hope that there may be properties of the transmitted virus that can be specifically targeted through vaccination. There is a well-documented heterogeneity in the success that various individuals have in controlling early HIV replication, with some manifesting poor and others manifesting effective early control of virus. To elucidate the mechanisms accounting for this heterogeneity, genetic determinants of early viral control are being explored through genomic analyses of defined populations of acutely infected individuals (Fellay et al., 2007). These genomic analyses may uncover novel immune effector mechanisms that might be harnessed through vaccination to control early HIV replication. Most transmission of HIV occurs across a mucosal surface during sexual contact, so considerable effort is being devoted to characterizing the early events in mucosal transmission of HIV (Morrow et al., 2007). These studies are focused on clarifying how HIV traverses a mucosal membrane. Is the virus crossing a mucosal membrane by transcytosis through mucosal cells, by movement between cells, or by trafficking across a tear in the mucosal membrane? The answers to questions such as this will determine how important it might be to generate mucosal immunity to HIV through vaccination. Finally, because of concern that adaptive immune effector cells from a vaccine-primed memory population of lymphocytes emerge too slowly to contain the early spread of HIV after the transmission event, attention is being focused on determining whether innate immune mechanisms might contribute to HIV control in the acutely infected individual. Investigators hope that such innate antiviral mechanisms might be effectively harnessed through novel vaccination strategies.

A striking example of protection against HIV infection that has received considerable attention in studies of AIDS immunopathogenesis is the phenomenon of exposed, uninfected individuals. Cohorts of commercial sex workers in Africa have been described that are repeatedly exposed to HIV through sexual contact but do not become infected by the virus (Fowke et al., 1996). This phenomenon, if real, represents sterile protection or abortive infections rather than simply improved control of viral replication after the initiation of infection. The biologic events underlying this protection would provide an important target for vaccine development, so considerable effort has been focused on determining whether the protection is a result of a measurable immunologic effector function. To date, both local and systemic HIV-specific immune responses have been described in these individuals. Moreover, this immunity includes both cellular and humoral responses. However, the immunity that has been described in these individuals is sporadic and the cellular immune responses are of low frequency.

Correlates of immune protection are also being sought in studies of macaques that are vaccinated and then challenged with a pathogenic SIV isolate. Although experiments in the SIV macaque model of HIV infection have not demonstrated a safe vaccine strategy that confers sterile protection from infection, impressive viral control has been documented in monkeys that have received live, attenuated vaccines and in monkeys that have been infected with one strain of SIV and are then superinfected with another strain of the virus (Wyand et al., 1999). Efforts are therefore being focused on characterizing the immune mechanisms that contribute to this protection.

Some investigators are evaluating HIV controllers, individuals who are infected with HIV but are able to contain viral replication for periods of many years without antiretroviral drug therapy (Deeks et al., 2007, in this issue of Immunity). These studies are focusing on host genetics, immune responses, and the properties of the infecting virus. The hope is that a vaccine might be developed that mobilizes the same mechanisms that mediate protection against HIV replication in these rare individuals and, in so doing, facilitate control of HIV replication in people who become infected.

The most ambitious studies currently being pursued to define correlates of immune protection against HIV are a series of human clinical HIV vaccine trials that are being sponsored by the National Institutes of Health. These include trials of a recombinant adenovirus vaccine developed by Merck Research Laboratories and a plasmid DNA prime with recombinant adenovirus boost developed by the Vaccine Research Center (VRC), National Institutes of Health (Dubey et al., 2007, Catanzaro et al., 2006, Graham et al., 2006). The Merck vaccine is being evaluated in two separate trials of 3000 volunteers, and the VRC vaccine is being evaluated in a trial of 8000 volunteers. Because these vaccine approaches have been specifically developed to elicit cellular immunity, considerable attention has been focused on devising durable and reproducible assays to evaluate vaccine-elicited cellular immune responses. The assays developed for these clinical trials include highly quantitative elispot and intracellular cytokine staining (ICS) assays. Peripheral blood lymphocytes of vaccinees are exposed to pools of peptides spanning the HIV proteins employed in the vaccines, and cytokine production by the lymphocytes is measured by these technologies.

These vaccine trials employ not only unique assays but also unique clinical endpoints. Impressive SIV control has been observed in vaccinated monkeys as a consequence of the early emergence of a potent virus-specific cytotoxic T lymphocyte response after viral infection (Letvin et al., 2006). Associated with that virologic control is a striking protection against the loss of activated memory CD4+ T lymphocytes. Based on these findings in nonhuman primate studies, the human vaccine trials include as endpoints not only acquisition of virus but also the control of virus and memory CD4+ T lymphocyte loss after infection. It is hoped that associations will become apparent between the quantity and quality of vaccine-elicited cellular immune responses and the degree of clinical protection conferred by those immune responses in the human vaccinees.

Correlates of immune protection are also being defined to guide the development of an effective vaccine against HCV. Like for HIV infections, experimental work suggests that CD8+ lymphocytes may be critical for containing the replication of HCV. This has been demonstrated in nonhuman primate studies both by showing poor control of virus in animals depleted of CD8+ lymphocytes and by showing protection against virus challenge in animals after vaccination with a live recombinant vector expressing only nonstructural proteins of HCV. Experimental systems are being developed to clarify the role of neutralizing antibody responses in the containment of HCV. The selection of vaccine strategies to evaluate for protection against HCV will be made based on the findings in these studies to define correlates of immune protection against this virus.

It is important to recognize that vaccines can sometimes provide protection against infection by mechanisms that differ from those that are operative in the setting of natural infection. For example, it has recently been shown in rhesus monkeys that in vivo B cell depletion has no effect on measles virus clearance, whereas CD8+ lymphocyte depletion substantially impedes clearance of the virus (Permar et al., 2004). These observations suggest that cellular immunity rather than humoral immunity is of central importance in the containment of measles spread. Nevertheless, human vaccine trials have shown that an inactivated virus vaccine, an immunogen that can elicit antibodies but not CD8+ CTL populations, can confer protection against acquisition of measles (Guinee et al., 1966). Thus, viral clearance in the setting of natural infection is mediated by cellular immunity, whereas vaccine-induced antibodies can confer protective immunity. These observations demonstrate that there may not be only a single correlate of protection against a particular viral infection.

Further, vaccine-induced immune responses can confer protection by mechanisms that are not associated with the transmission of the virus. The systemic antibody response elicited by the inactivated-virus polio vaccine does not block polio virus transmission. Rather, it diminishes poliovirus replication systemically after mucosal transmission has occurred, blocking viral seeding of the central nervous system (Bodian, 1955). This highly effective vaccine, therefore, aborts the natural course of a mucosally acquired poliovirus infection without acting at the site of transmission.

Underlying any consideration of strategies for developing a vaccine for HIV is the question of what should we expect an HIV vaccine to do? Specifically, can a vaccine be developed that actually confers sterile protection against the virus? Many highly effective antiviral vaccines that are in use worldwide protect against the development of disease but do not protect against infection by the virus. In studies done in monkey AIDS models, SIV infection cannot be blocked by live attenuated virus vaccines or prior infection with a modestly divergent SIV strain (Wyand et al., 1999). Although sterile protection against a simian human immunodeficiency virus (SHIV) challenge has been described in monkeys infused with neutralizing monoclonal antibodies, these antibodies must be present at extraordinarily high titers in the experimental animals. These studies all raise the possibility that sterile protection against HIV may not be achievable (Mascola et al., 1999). The efforts to define correlates of protection against HIV replication will provide an important test of the concept that an intense study of the immunopathogenesis of a viral infection will inform the development of an effective vaccine against that virus.


http://www.immunity.com/content/article/fulltext?uid=PIIS1074761307004190




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