HIV/AIDS

AIDS Vaccine Research Subcommittee Meeting - September 16-17, 2008

The AIDS Vaccine Research Subcommittee (AVRS) met in public session on September 16-17, 2008, in the Natcher Conference Center on the campus of the National Institutes of Health (NIH) in Bethesda, MD.

AVRS members present: Eric Hunter (chair), James Bradac (executive secretary), Jay Berzofsky (ex officio), Larry Corey (ex officio), Susan Buchbinder, Deborah Birx, Kevin Fisher, Barton Haynes (ex officio), Paul Johnson, Jeffrey Lifson, Margaret Liu, Bonnie Mathieson (ex officio), Nelson Michael (ex officio), Gary Nabel (ex officio), Louis Picker, Nina Russell, Jerald Sadoff, Jack Whitescarver (ex officio).

Other NIH personnel participating:

• Carl Dieffenbach, Director, Division of AIDS (DAIDS), National Institute of Allergy and Infectious Diseases (NIAID);
• Peggy Johnston, Director, Vaccine Research Program (VRP), DAIDS, NIAID
• Jorge Flores, Vaccine Discovery Branch (VDB), VRP, DAIDS, NIAID;
• Nancy Miller, Preclinical Research and Development Branch (PRDB), VRP, DAIDS, NIAID;
• Michael Pensiero, PRDB, VRP, DAIDS, NIAID.

Speakers:

• Ronald Desrosiers, Harvard Medical School;
• Justin Greene, University of Wisconsin;
• John Harding, National Center for Research Resources (NCRR), NIH;
• Norman Letvin, Harvard Medical School;
• Matthew Reynolds, University of Wisconsin;
• Guido Silvestri, University of Pennsylvania School of Medicine;
• Ronald Veazey, Tulane University School of Medicine.

Call to Order

Dr. Hunter called the meeting to order and asked the committee members and observers to introduce themselves.

Post-vaccine summit activities and plans

Peggy Johnston reported that the findings of the STEP trial are still being analyzed, with final results to be delivered at the AIDS Vaccine 2008 meeting in Cape Town on October 13-15. However, the preliminary results point to a failure of the product and raise questions about the concept. Based on recommendations from this group and others, Dr. Anthony Fauci made the decision not to proceed with the PAVE 100 trial of the Vaccine Research Center (VRC) candidate (as outlined in Science (25 Jul 2008). A vaccine summit was arranged by NIAID leadership in March, 2008and the broad consensus reached by members of the research community was the need to return “back to basics” with a new emphasis on discovery research, with priorities in six areas:

1. Early events;
2. Adaptive and innate immune responses;
3. 3D structure of HIV envelope trimer;
4. Neutralizing antibodies;
5. Animal models; and
6. Correlates of vaccine-induced immune protection.

There will also be a greater emphasis on the need for new young investigators.

NIAID will use targeted initiatives to bolster HIV vaccine discovery research and has reorganized the Vaccine Research Program accordingly, adding a Vaccine Discovery Branch with Jorge Flores as acting chief. NIAID will launch an initiative for Basic HIV Vaccine Discovery, with R01 grants totaling $10 million in the first year, with an RFA published in August 2008 and application deadline November 10. In addition, NIAID will fund an initiative for Highly Innovative Technologies to Interrupt Transmission of HIV (HIT-IT), with R01 grants totaling $4 million in year 1 and a deadline of January 5. Both initiatives will make their first awards in July 2009. Johnston also announced a new B-Cell Biology Network for HIV-1 Vaccine Development and a Phased Innovation Award Program for AIDS Vaccine Research, which have been approved for FY 2010.

Johnston summarized recent progress in the six priority areas, which suggests that these may be fruitful avenues of research, and announced that the AIDS Research Advisory Committee (ARAC) has given permission to extend the HIV Vaccine Research and Design (HIVRAD) Program, which supports discovery-related preclinical research that will be relevant to these priorities. DAIDS is working with its partners to support and improve work in animal models, with a workshop on nonhuman primates (NHPs) on November 12-13 and another on systems biology on December 9, in conjunction with the 26th Annual Symposium for NHP Models for AIDS. Changes in the scientific agenda of the HIV Vaccine Trials Network (HVTN) will include greater emphasis on studies to explore correlates of immune protection, improvements in NHP studies, better assays of T-cell and antibody functions, and better understanding of STEP and Phambili data, with the goal of increasing mucosal responses and the breadth and magnitude of T-cell responses. These new discovery activities will be funded primarily by reducing or redirecting funding for development activities.

In response to questions, Johnston stated that the level of funding for the new discovery initiatives could be as much as $100 million over the next five-years; as solicited proposals, they will be reviewed by NIAID rather than the Office of Scientific Review. There will be no new vaccine manufacture solicitations in 2009. However, NIAID will maintain the ability to move promising discoveries quickly into animal models, manufacture and clinical trials, when justified. The development of new animal models (other than nonhuman primates) will be followed closely by the new Vaccine Discovery Branch and might be the subject of a future meeting of AVRS.

The PAVE 100 protocol is currently being revised and will not be reviewed again until after January 2009. Several AVRS members indicated that they had no desire to rehash the VRC proposal for a third time – they gave it a green light if there is no new information except for trials design. However, they would like a report on the status of PAVE 100 plans at their January meeting.

Description and plans for the newly formed Vaccine Discovery Branch (VDB)

Jorge Flores characterized VDB as a new combination of existing pieces and activities, arranged in such a way as to encourage the broadest possible collaboration and to bring new discoveries and technologies to bear on AIDS vaccines. VDB will develop its grant programs in close collaboration with other DAIDS programs and NIAID divisions. The Basic Vaccine Discovery (BVD) initiative is designed in part to attract investigators from other fields, including new investigators, and to encourage cross-disciplinary research; the HIT-IT initiative is designed to elicit innovative, unconventional concepts and will have no requirement for preliminary data. Similarly, the B-Cell Network will stimulate networking and the sharing of knowledge, reagents and protocols that will advance the field. In addition to systems approaches, VDB will also support discovery informatics to identify and apply new knowledge from other fields. Since the traditional paths to vaccine discovery do not appear to be applicable to HIV vaccine discovery, VDB will foster the development of a new path based on a deeper understanding of pathogenic and immunologic mechanisms.

In response to questions, Flores added that VDB, like the rest of NIAID and NIH, faces the problem of increased duties without increases in staff, posing challenges to communications and collaboration. VDB’s structure and activities are meant to be a new mechanism for capturing and synthesizing new information relevant to HIV vaccine discovery and development. NIAID has reprogrammed some funds for these activities, but Flores hopes that NGOs will take on part of the challenge and even fund some of the ideas independently. He agreed that a flexible pool of money will be needed to pursue promising discoveries; this might be done through competitive supplements, either alone or in cooperation with Gates and IAVI. The two new initiatives will fund a total of 25 or 30 projects over the next few years, and it will be very interesting to see what comes of this effort.

Workshop on NHPs for AIDS vaccine research

Nancy Miller reviewed the priorities for NHP research that were proposed at the Vaccine Summit:

• Earliest events in SIV infection;
• Vaccine-related enhancement of infection;
• Mechanisms of control of SIV, especially elite controllers;
• Apathogenesis of SIV in some NHP species;
• Viral and host factors involved in disease progression;
• Correlates of vaccine-induced immune protection; and
• Role of innate/mucosal immunity in SIV infection and vaccine-induced protection.

A post-summit workshop was to be held on November 12-13 to refine these priorities and answer three central questions:

1. How best to invest resources in NHPs for HIV research;
2. Priority areas where NHP models can answer significant research questions; and
3. Resources and reagents needed to pursue these priorities.

In today’s workshop, she hoped to engage AVRS on these and three additional questions:

4. What are the critical questions to be addressed in NHP models?
5. What vaccine questions cannot be addressed in NHPs?
6. What questions need to be pursued in parallel humans and NHPs?

Jeffrey Lifson reminded participants that there is more than one NHP model of AIDS, but rather a number of NHP models, and that the goal should be a better NHP model. In humans, HIV is a relatively recent (and highly pathogenic) xenobiotic infection. The three “natural” hosts to SIV are African green monkeys, the sooty mangabey, and the chimpanzee. Each species has its own version of SIV, but in every case SIV is a nonfatal infection is a species that has coevolved with the virus over millions of years. In macaques, on the other hand, infection with either SIV or HIV produces disease that looks and acts very much like AIDS. As a result, the NHP “model” of HIV consists of a specific NHP, infected by a specific virus, that manifests as a specific disease, and has specific (but never universal) applicability to the human disease. Different models have their strengths and weaknesses, and it is vital to select the best model for the scientific question of interest (e.g., pathogenesis, therapy, prophylaxis).

In addition, HIV does not productively infect NHPs in any experimentally useful way, so the current model involves infecting macaques (one of three species) with SIV or the hybrid SHIV. Investigators have defined the history and characteristics of the SIV found in U.S. colonies, which through serial passage have evolved several pathogenic phenotypes. SHIV, on the other hand, proliferates and then dies out, although it provides a good enough model for some experimental designs. Selecting one or the other involves a tradeoff between viral load and time to progression. The resulting infection mimics HIV faithfully in both its direct effects (depletion of CD4+ T-cells in the gut) but also in its indirect effects (collagen deposits and disruption of lymphatic tissues). The phenomenon of “elite controllers” is also mirrored in this model. In addition, investigators currently have both the antibodies and assays needed to study the neutralization of SIV, although at present these tools aren’t broad enough to reflect the diversity of HIV isolates.

In using NHPs to test HIV vaccines, the difficulty is to select an NHP model that is stringent enough without being too stringent. There are multiple NHP models (more every day), and investigators have to pick the right model for the question at hand, matching it to the specific study. This involves picking not only the right NHP and the right virus, but also the right challenge (both site and inoculum), the right number of challenges, and the right clinical endpoint(s). In addition, a vaccine might succeed in a monkey but fail in a man, just as it might fail in the monkey but succeed in a man – the predictive value of an animal model can only be demonstrated when the vaccine has also proven its clinical efficacy in humans.

Louis Picker reported that the NHP model is not merely a preclinical stand-in for humans. SIV in macaques (SIVmac) is a “compelling recreation” of HIV in humans, more faithful to the human disease than almost any other animal model. For this reason, “this model (or rather, models) has the capacity to reveal the fundamental mechanisms of human immunity and pathogenesis – mechanisms that will critically inform therapeutic development.” The viral dynamics are the same, as is the disregulation of CD4+ replacement and regeneration, mutational escape from immune responses, and opportunistic infections (OIs) with the same or homologous organisms. The crucial events in both infections reveal a progressive failure of homeostasis: SIV and HIV infection initiates a persistent hyperproliferation among CD4+ and CD8+ T cells, which however deteriorates over time until the host is susceptible to OIs.

As a result, the NHP model can provide a reliable guide to HIV research. For example, experiments with “naïve-deficient” (athymic) macaques have shown that the CD4+ naïve compartment does not play a crucial role in maintaining the central memory compartment in SIV infection. This in turn focuses new attention on the CD4+ central memory population and the factors that regulate its homeostasis, such as IL-7. Similarly, macaques in which CD8+ populations were depleted with monoclonal antibodies (mAbs) showed higher viral loads and more rapid disease progression than controls. In addition, CD8+ depletion also induced a sudden increase in the proliferation of CD4+ effector memory T cells in the blood, markedly increasing the number of “optimal” SIV targets just as viral replication is ramping up. In this and other instances, the NHP model allows investigators to experimentally dissect these complex mechanisms in ways that could never be done in humans.

The NHP model has only begun to demonstrate its value in delineating immune mechanisms, and further development and exploitation of this model will be expensive. However, core funding for primate research has been flat for many years, and the overall availability of animals is falling behind demand. A significant long-term investment must be made in breeding facilities and infrastructure, as well as training and model development. In the short term, we must preserve and focus existing NHP expertise on the “big questions” of AIDS immunology.

In response to questions, Picker added that NHP funding increased by only 9 percent during the doubling of the overall NIH budget and has remained flat since. Further experiments with naïve-deficient macaques would cost about $350,000. Demand is forcing up the price of macaques (currently about $10,000 apiece) and it will take years to ramp up breeding. As a result, an R01 grant to study NHP models (for example, how to manipulate mucosal immunity) represents an expensive gamble, but switching to the mouse or another animal model might give misleading results. NCRR, for its part, appears to be sympathetic but is unable to assist unless the new emphasis on NHP research toward an AIDS vaccine results in increased funding. Several participants noted that NHPs represent a new and better way to study how a vaccine manipulates the immune system, since the results are more easily validated in a model that allows full-body access.

Guido Silvestri reported on efforts to understand how “natural hosts” to SIV control infection and progression, in hopes of applying those mechanisms to HIV. There are at present some 42 known “natural hosts” and counting; his work is with sooty mangabeys and their version of the virus, SIVsm. In sooty mangabeys, SIVsm infection is not followed by CD4+ T cell depletion, despite a high viral load. Investigators also found that the CD8+ T cell count following SIVsm infection than is the case in HIV, which suggests that attenuated immune activation reduces the number of target cells and thus protects against progression. Since the resistance of a natural host is independent of adaptive immune response, however, it would be difficult for a vaccine to elicit a protective response in a human. Investigators also learned that the CD4+ cells of NHP hosts have low levels of CCR5, especially in the gut, which suggests there may be a pool of “protected” CD4+ cells that don’t home in on in SIV-related inflammation. Finally, sooty mangabeys have proven “exquisitely resistant” to vertical transmission (mother to infant), which may be the evolutionary rationale for some of these adaptations.

Based on these and other observations, Silvestri concluded that progression to AIDS is not a consequence of viral load or the direct killing of cells by the virus, but rather the result of a global dysfunction in the human immune system. He plans to pursue this research in NHP models using classical vaccine approaches, but he would consider alternative approaches if NHP models are out of reach. Resources that would facilitate such research include increased funding for primate centers, validation and standardization of assays and reagents, full genomic sequences for both host animals and viruses, and changes in conservation status for “endangered” NHPs that can be successfully bred in captivity for research purposes.

In response to questions, Silvestri added that one research goal would be to uncouple CD4+ activation from CCR5 expression – low CCR5 expression blocks vertical transmission but not horizontal, and blocking CCR5 will block HIV transmission but not SIV. This may involve coreceptors, but the CCR5 regulatory pathways are extremely complex. New and standardized reagents will be needed to explore these molecular mechanisms.

Ronald Veazey gave examples of the kinds of studies that are possible using NHP models, including infectious diseases, therapeutics and drug studies, and immunology. The major disadvantage of NHPs is expense: direct costs of between $4,200 and $7,000 per animal and total costs of as much as $11,500 apiece. Depending on species, however, between 93 and 98 percent of NHP genes are similar to human, and about 75 percent of human antibodies, chemokines and cytokines are cross-reactive in macaques. In addition, most of the assays and reagents used to study HIV can also be used to study SIV in macaques. Given the many similarities between SIV and HIV (transmission, pathogenesis, immune collapse, OIs, etc.), this makes it possible to do vital research in NHPs that could never be done in humans, including:

• Inoculation with well-characterized or molecularly engineered viruses designed to address specific aspects of transmission and pathogenesis;
• Control variables such as genetics, diet, behavior, and antiviral therapy;
• Examine tissues difficult to access in humans;
• Examine very early events in infection (minutes to days after infection or treatment);
• Repeatedly biopsy to follow tissue responses at regular intervals;
• Test new or radical vaccines and therapies or administer agents designed to manipulate pathogenesis or immune response (e.g., CD8+ depletion); and
• Euthanize animals at strategic timepoints to collect, compare and archive tissue and cells.

NHP studies have already led to major breakthroughs, including the discovery that the gut is the major site of viral replication and persistence, creating a “vast reservoir” of virus in the GALT rather than the PBMCs. Both SIV and HIV preferentially replicate in mucosal tissues, even in “elite controllers” and nonprogressors, suggesting that stimulating mucosal immune responses will be important to vaccines. Similarly, high doses of topical microbicides that block CD4 and/or CCR5 binding completely blocks vaginal transmission of SIV and HIV, in the absence of trauma or inflammation, suggesting that the combination of a vaccine that induces central memory and a vaginal microbicide might provide reinforced protection against HIV. Additional basic studies in SIV-infected NHPs will be needed to clarify remaining questions and determine the correlates of immunity. At present, the only known correlate of protection is the preservation (or restoration) of intestinal memory CD4+ T cells, which “nature” has achieved by changing the immune system of nonprogressing hosts to tolerate persistent infection.

Norman Letvin addressed the predictive value of NHP immunogenicity studies, particularly for vaccines that had been used or proposed for human trials. In the case of the Merck rAd5 vaccine used in the STEP trial, the preclinical NHP results were “spot on,” faithfully predicting the strength and breadth of human responses. In the case of the Wyeth plasmid DNA vaccine used in HVTN-060, however, the human response to HIV gag was far weaker than would have been expected from the NHP response to SIV gag, probably because SIV antigens are far more immunogenic. The Sanofi-Pasteur DNA-recombinant pox vaccine showed no immunogenicity in NHP or human. And the VRC pDNA-rAd5 vaccines, NHP immunogenicity was comparable to humans in magnitude, durability and polyfunctional profiles. Overall, then, NHP responses were very predictive of human responses to the same immunogen, making it important that the two studies are examining the same vaccine construct. Other participants objected that NHPs often overestimate the human response, for example in flu vaccines, but in the case of Ad35 the NHP results seem to have underestimated the human response.

Letvin also reported that the NHP Core Cellular Immunology Laboratory at Beth Israel Deaconess Medical Center is making good progress toward Good Laboratory Practice (GLP) compliance, with GLP certification expected in the spring of 2009. This will ensure the quality and duplicability of all assays and data, and is required if the data is to be used for regulatory submissions. It also provides a vital resource that will allow the NHP field to make further progress.

Letvin also reported on the CHAVI-Gates collaboration to investigate the mechanisms of protection conferred by live attenuated SIV vaccination, and the role of adaptive immune responses. Results to date (from Jörn Schmitz) show that CD8+ T cells are responsible for most of the protection conferred by vaccination, particularly viral control, and that animals that do not express the MHC-1 allele Mamu-A*01 had much higher levels of neutralizing antibodies. This led to a further study using Mamu-A*01-negative animals, in which vaccination protected against SIV challenge even when B cells were depleted. These results demonstrate that CD8+ T cells provide partial protection against SIV challenge, that live attenuated vaccination results in protection against challenge, and that B cell responses do not play a major role in live attenuated vaccine protection. In response to questions, Letvin cautioned that these live attenuated vaccine studies are still very much in progress. Many questions remain, most of which can be answered by additional assays and refinements in experimental design.

Discussion

AVRS members agreed that NHPs should be used to make fundamental discoveries about the mechanism of HIV, not just as preclinical test for human vaccines and therapeutics. They noted that several groups have already begun working in this way, and recognized that further breakthroughs will depend on a big increase in breeding, to make animals cheaper and more available. However, they also suggested that there should also be a clearer sense of how the various players will interact, and what mechanisms might be needed to coordinate and synergize their efforts, if this agenda brings additional funding. For example, NIH puts money into basic, long-term research, while Gates and IAVI look for more immediate results; NHP breeders need support from both groups, and it’s important that they not work at cross purposes. In addition, there needs to be a clear and compelling statement of the overall research agenda in NHP immunology, including:

• Things we don’t understand;
• Answers we know we need (e.g., correlates of protective);
• What epitopes are recognized at different stages, especially when we see protection;
• Where does the response need to be (in the gut, in mucosal membranes, where).

More work is also needed in the correlation of NHP and human results, both to refine the NHP model and to seek clinical applications of NHP discoveries. This part of the discussion should probably include more investigators working in humans, if only to clarify the complementarities and identify which scientific questions to ask. The field also needs more assays and reagents that bridge the gap between NHP and human.

Addressing the needs of the research community

Jack Harding reported that NCRR supports eight National Primate Research Centers (NPRCs) with a total budget of $79 million in FY2008. This covers 30 to 50 percent of their costs, the balance coming from program income, mostly in the form of NIH grants. Of this total, some $17 million supports colonies of macaques, primarily of Indian origin. Each NPRC’s base grant (P51) funds the basic infrastructure of the center – animal facilities and related services, core laboratories (available to outside researchers), animal welfare, administration, pilot projects – but does not directly fund R01-type research. At present the NPRCs house 28,000 animals, of which 17,000 are rhesus macaques, including 4,800 specific pathogen free (SPF) animals set aside for AIDS research. About 20 percent are part of breeding colonies; a number that can be increased only if new facilities are available.

A working group has determined that there is essentially no reserve of Indian-origin rhesus macaques, either SPF or non-SPF. To increase their number, we need to consider expanding the breeding colonies, finding new sources, reserving Indian-origin macaques for AIDS research, building a central reserve of animals, and determining whether SPF animals are needed. All of these alternatives would be problematic in an era of flat budgets, but a great deal can be accomplished through better colony management, genetic screening and genome banking, and consortium-based activities that decrease redundancies. However, it will be impossible to enlarge the colonies of SPF rhesus and other NHPs with increasing funding for facilities construction. The NIH Office of AIDS Research (OAR) has provided supplements for construction of breeding facilities in FY2007 and FY2008.

In response to questions, Harding added that it takes three or four years to get larger numbers of animals into the pipeline. The NPRC currently have about 4 percent excess breeding space but no excess containment space. They can breed specific MHC types (e.g., Mamu-A*01 positive or negative) if there is a need, and the centers are already sequestering elite controllers.

Correlates of protection studies in the live attenuated SIV model

Ronald Desrosiers provided an historical overview of live attenuated vaccine NHP studies, which are conducted with virus strains (largely based on SIV-239) in which the deletion of a part of the genome produces a virus with reduced virulence. Several factors may affect the results of vaccination, including the MHC of the recipient, the size of the inoculum, and the recipient’s health at day zero. However, while other vaccine approaches have achieved at best a 1.5-log reduction in viral load, live attenuated vaccines a 4.0- or 5.0-log reduction against homologous challenge. However, this protection does not extend to other SIV strains, and it tends to weaken over time, seldom persisting up to four years. Other experiments have produce contradictory results – antibody titer may or may not affect protection, persistence may or may not be important, CD8+ cells may or may not play a role.

In response to questions, Desrosiers admitted that investigators might not be getting the answers they seek because they aren’t asking the right questions. It could be useful to do another experiment in which they try to achieve sterilizing immunity, or provide a second does of attenuated virus in hopes of a booster effect. In addition, protection appears to weaken with increasing attenuation of the vaccine strain. Both cellular and humoral responses are likely to be involved.

Paul Johnson pointed to these questions as examples of just how complex the live attenuated virus experiments can be. Most such studies have been relatively crude and underpowered, and they probably need a systems biology approach in order to understand all of the variables at work:

• Combination of mechanisms;
• Host genotype affects protection;
• Outcome of challenge (and correlates of protection) depends on strains used to immunize and challenge;
• Mechanism of protection against homologous challenge is unknown;
• High efficiency of protection makes it difficult to identify correlates;
• SIV reactivated after CD8+ depletion.

Several studies suggest that both CD8+ cell responses and neutralizing antibodies contribute to protective immunity following live attenuated vaccination. More recent studies of mucosal protection against vaginal challenge suggest that protection is affected by multiple variables, including host genotype, challenge stock and route of challenge. In addition, multiple immune effector mechanisms appear to act in concert to mediate protection. As a result, the correlates of delayed kinetics of protection remain to be defined, although ongoing stimulation by the challenge stock appears to play a role in mediating protective immunity.

Louis Picker described the activities of the Live Attenuated Consortium, an ongoing collaboration between Gates- and IAVI-funded researchers to determine the mechanisms contributing to live attenuated SIV vaccine-mediated protection. They have taken three different approaches:

1. Correlation of “immune” parameters in protected and unprotected animals;
2. Deletion of specific immune functions by in vivo manipulation; and
3. Mimicking specific attributes of live attenuated SIV vaccines in non-lentivirus vectors.

Picker’s lab is responsible for parts 1 and 3. Part 1 will involve a total of 120 animals that will receive comprehensive evaluation of genetic, virologic, innate and adaptive immune variables. It is currently 40 days into the first cohort of 15 animals, and preliminary analysis has identified 139 genes whose expression is significantly different after vaccination. Many of these genes are associated with transcription factors, indicating that vaccination results in a profound and specific change in host gene regulation, but investigators don’t yet know the meaning of the pattern. Part 3 focuses on the use of cytomegalovirus (CMV) as a vector that mimics live attenuated SIV in its preferential induction of “effector memory”-dominant T cell responses. In early tests, rhesus CMV vectors with SIV inserts were able to permanently infect both naïve and immune macaques, establishing a robust and long-lasting T cell response to the SIV inserts. This SIV-specific T cell response was decidedly “effector memory” in phenotype and function, and it successfully delayed or inhibited infection by repeated, low-dose rectal challenge. When infection did occur, however, this response was less effective in suppressing viral load but did not make the infection worse. Clearly more development and experimentation are needed, but these results strongly suggest that TEM-biased responses can decrease the incidence of HIV infection after sexual contact.

In response to questions, Picker said that it is difficult to measure effector cells in rectal tissue, but he would welcome ideas or proposals for ways to measure them. He suggested that sterilizing immunity comes in part from changes in the local microenvironment that prevent the virus from going systemic. He is not sure where CMV is expressing antibodies – possibly in the spleen? Picker said that Gates is supporting 78 percent of this work, IAVI 22 percent, and that each investigator is well aware of what the others are doing. They are careful to avoid overlap, fill gaps, and seek complementarity.

Matthew Reynolds presented result from a study in which MHC-characterized macaques were vaccinated with live attenuated SIVmac239Δnef and then received a heterologous challenge with SIVmacE660, which differs from 239 in about 15 percent of its genome. In this case, the T cells induced by vaccination were effective in controlling early infection but did not control viral replication in the long term. When investigators looked at the MHC profile vs. viral control, they found that both B08 and B17 controlled early but faded. Future studies will study the effect of host MHC profile on live attenuated virus induced immunity against homologous and then heterologous challenge.

Justin Greene described studies of adoptive transfer in Mauritian-origin cynomologous macaques (MCMs). In the mouse model, adoptive transfer can transmit immunity from vaccinated to naïve animals. There has been no equivalent in primates, however, because the transferred cells are cleared within hours after transfer. In vitro, however, a MHC match is much the same as an autologous transfer. MCMs are a genetically isolated population with only seven haplotypes, making it easy to identify MHC-matched, unrelated individuals. This NHP resource makes it possible to test the feasibility of MHC-matched adoptive transfer of vaccinated animals to naïve animals, a model that might also be possible in humans. Development of this model is still in early days, and investigators are still trying to determine the optimum cell type (including minor antigens) and cell numbers (inoculums), but early results show that PBMCs transferred between unrelated MHC-matched macaques persist for at least 14 days and are trafficked from the blood to peripheral lymphoid tissues within 12 hours of transfer. IAVI is developing a pilot study to see if this technique can be used to transfer T cells, and possibly immunity as well.

AVRS members asked if transferred PBMCs would home to mucosal tissues, were protection is most needed; perhaps the experiments could be repeated using mucosal cells for transfer. MCM differ from rhesus macaques in their response to SIV, including both infection rate and set point. Mixed bulk transfers represent the best way to validate the model and observe protection; then investigators can break down the components of the transfer. Certainly it will be worth following the progress of this research, if only to find out what can and can’t be done.

Discussion

Dr. Hunter asked members to focus discussion on resource needs in NHP research and gaps in live attenuated virus research. One member suggested that there is a need to ensure the quality of SIV stocks, especially challenge virus, and to ensure that what happens in the NHP model truly mimics what happens in HIV infection. This might mean reducing the size of the challenge inoculums; repeated low-dose challenge is a step in right direction. Another suggested that the focus of research should be on aborting a productive infection, not merely controlling it. Sterilizing control certainly seems to be the goal of most NHP research. Ironically, AIDS therapy seems to moving in the opposite direction, controlling viral load rather than killing the virus.

In terms of the number of NHPs needed, that depends entirely on how many it will take to answer the fundamental questions. When polio was the target, it was easy to do 2,000 animals in Puerto Rico, but Louis Picker’s 120-animal study is enormous in today’s environment. Financial and political constraints make it more important than ever that other AIDS researchers help the NHP community make the case for the value of the NHP model and the need for additional resources. The high priority of an HIV vaccine, and the recent emphasis on NHP models, are helpful in this regard.

More research is needed on persistent vectors, and the NHP model is an ideal testbed for this work. It would be highly desirable to find a system that would “preposition” effector cells without stimulating CD4+ proliferation; here again the NHP model is an invaluable opportunity to dissect the phenomenon in exquisite detail. Increased dialogue between the NHP and human communities can only reveal additional opportunities for cross-fertilization.

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