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  National Institute of Allergy and Infectious Diseases
National Institutes of Health
Friday, June 9, 2006
Results of two new studies sponsored by the National Institute of Allergy and Infectious Diseases (NIAID), a component of the National Institutes of Health (NIH), suggest that even if an HIV vaccine offers imperfect protection against the virus, it might provide vaccinated individuals with an important benefit: a significant survival advantage after infection.
Such a survival advantage was observed in monkey studies conducted by two teams of researchers, one led by Norman L. Letvin, M.D., of Beth Israel Deaconess Medical Center, Harvard Medical School and the NIAID Vaccine Research Center (VRC), and the other by Mario Roederer, Ph.D., of the VRC. The researchers found that monkeys vaccinated against simian immunodeficiency virus (SIV)--a close relative of HIV that causes an AIDS-like disease in monkeys--and then exposed to the virus survived significantly longer than unvaccinated animals exposed to SIV.
"The worldwide need for an HIV vaccine is profound," says Elias A. Zerhouni, M.D., director of the NIH. "In 2005, more than 11,000 people became infected with HIV every day. If that rate continues unchecked, the virus is going to infect another 40 million people during the coming decade."
"Although our ultimate goal is to have a vaccine that completely blocks HIV infection, this research suggests a potential benefit of even a partially effective vaccine," says NIAID Director Anthony S. Fauci, M.D.
Published in this week's issue of Science and this month's issue of the Journal of Experimental Medicine, the studies also identified a measurable marker of SIV vaccine effectiveness in monkeys--something known as an immune correlate of vaccine efficacy. Further study is needed to determine if the immune correlate could predict the effectiveness of a vaccine against HIV in humans.
"Having an immune correlate of vaccine efficacy could markedly reduce the time it takes to evaluate whether a candidate HIV vaccine works in people," says VRC Director Gary J. Nabel, M.D., Ph.D. "The significance of this discovery is clearly worth evaluating in humans and may considerably accelerate future efficacy trials."
The SIV vaccine regimen used in the two studies was a simplified version of a preventive human HIV vaccine strategy developed by VRC scientists and currently undergoing Phase II human clinical trials in the United States, the Caribbean and sub-Saharan Africa. Current plans call for testing the efficacy of the vaccine in large-scale human clinical trials some time next year.
To examine the theory that some imperfect HIV vaccines may still allow infected people to live longer and healthier lives, Drs. Letvin and Roederer and their colleagues sought to determine if SIV vaccines confer such a survival advantage to monkeys.
They found that the best way to predict survival after a vaccinated monkey is infected with SIV is by measuring, early in infection, levels of a specific subset of immune cells known as the memory CD4+ T cells. Memory CD4+ T cells are T cells that have been activated by bacteria and viruses upon first exposure and are primed to act more quickly upon reinfection. Of the approximately one trillion T cells in the average adult, more than half are memory cells.
Normally, a rapid and significant loss of these memory CD4+ T cells occurs early on in SIV infection: about ten days into the infection, when the levels of virus in the bloodstream are at their peak, up to 80 percent of the memory CD4+ T cells in some tissues became infected, and ultimately, nearly all of those memory CD4+ T cells are lost.
But vaccinating the monkeys can lessen this damage to the immune system, Dr. Roederer and his colleagues found. In their study of six vaccinated monkeys and six unvaccinated monkeys exposed to SIV, the vaccinated group had about 3 to 5 times fewer memory CD4+ T cells infected and destroyed. "If the virus wipes out only a fraction of the memory CD4+ T cells that it might otherwise destroy, that should allow [the animals] to live longer," Dr. Roederer says. Likewise, he adds, if HIV vaccines can prevent the destruction of these memory cells in humans, it may be possible to provide people with longer, healthier lives.
In Dr. Letvin's study, he and his colleagues looked at the effect of preserving the memory CD4+ T cells over the long term. A total of 30 monkeys--24 vaccinated and six unvaccinated controls--were infected with SIV and followed for nearly three years. The vaccine helped control the infection for the first 112 days, but thereafter, the virus levels and total CD4+ counts in the vaccinated and unvaccinated animals did not differ significantly.
But the vaccine protected the memory CD4+ T cells from the virus early on, and the levels of memory CD4+ T cells remained at significantly higher levels in the vaccinated animals for the 850 days they were studied.
"This [early protection] had huge consequences for the development of disease," says Dr. Letvin. "When infection did occur, the monkeys preserved their memory CD4+ T cells better and lived longer."
Moreover, the researchers found that measuring a subset of the memory CD4+ T cells, so-called central memory CD4+ T cells, could help predict how the monkey would fare in the long run. Since these new studies indicate that the central memory CD4+ T cell counts appear to be a crucial predictor of long-term health, blood samples from human clinical trial participants might now be examined for this marker. That way, says Dr. Letvin, scientists can gauge how well a vaccine will perform simply by measuring the central memory cell levels in the first few months after infection.
NIAID is a component of the National Institutes of Health. NIAID supports basic and applied research to prevent, diagnose and treat infectious diseases such as HIV/AIDS and other sexually transmitted infections, influenza, tuberculosis, malaria and illness from potential agents of bioterrorism. NIAID also supports research on basic immunology, transplantation and immune-related disorders, including autoimmune diseases, asthma and allergies.
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References: N Letvin et al. Preserved CD4+ central memory T cells and survival in vaccinated SIV-challenged monkeys. Science DOI: 10.1126/science.1124226 (2006).
J Mattapalli et al. Vaccination preserves CD4 memory T cells during acute SIV challenge. Journal of Experimental Medicine DOI: 10.1084/jem.20060657 (2006).
Preserved CD4+ Central Memory T Cells and Survival in Vaccinated SIV-Challenged Monkeys
Science 9 June 2006:
Vol. 312. no. 5779, pp. 1530 - 1533
Norman L. Letvin,1,2* John R. Mascola,1 Yue Sun,2 Darci A. Gorgone,2 Adam P. Buzby,2 Ling Xu,1 Zhi-yong Yang,1 Bimal Chakrabarti,1 Srinivas S. Rao,1 Jorn E. Schmitz,2 David C. Montefiori,3 Brianne R. Barker,2 Fred L. Bookstein,4,5 Gary J. Nabel1
Vaccine-induced cellular immunity controls virus replication in simian immunodeficiency virus (SIV)-infected monkeys only transiently, leading to the question of whether such vaccines for AIDS will be effective. We immunized monkeys with plasmid DNA and replication-defective adenoviral vectors encoding SIV proteins and then challenged them with pathogenic SIV. Although these monkeys demonstrated a reduction in viremia restricted to the early phase of SIV infection, they showed a prolonged survival. This survival was associated with preserved central memory CD4+ T lymphocytes and could be predicted by the magnitude of the vaccine-induced cellular immune response. These immune correlates of vaccine efficacy should guide the evaluation of AIDS vaccines in humans.
1 Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
2 Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
3 Duke University Medical Center, Durham, NC 27710, USA.
4 Department of Statistics and Department of Psychiatry and Behavioral Sciences, University of Washington, Seattle, WA 98195, USA.
5 Department of Anthropology, University of Vienna, Austria.
The protection afforded by vaccines can be mediated by humoral or cellular immune responses. Because a strategy to elicit broadly neutralizing antibodies to HIV-1 has not yet been developed, HIV-1 vaccine candidates now entering advanced phase clinical testing are designed to induce potent cellular immunity (1). It is hoped that such vaccines will generate populations of virus-specific T lymphocytes that rapidly expand following infection. By this means, viral spread might be contained, leading to reduced chronic viral replication and prolonged disease-free survival. The expectation that such vaccines will confer clinical benefit comes from the association of lower set-point plasma viremia with prolonged survival in HIV-1-infected humans (2). In some nonhuman primate models, particularly those using the chimeric simianhuman immunodeficiency viruses (SHIVs), vaccines that elicit potent cellular immune responses have resulted in reduced viremia and preserved CD4+ T lymphocytes after viral challenge (3-5). However, the biology of these SHIVs differs from the usual transmitted primary HIV-1 isolates (6). It is possible that the pathogenic simian immunodeficiency (SIV) model may provide a more predictive and rigorous test of vaccine efficacy. In some SIV challenge models, vaccine-induced reductions in viremia and progression to AIDS have been shown (7-9), although in the case of the commonly used SIVmac239/251 strain, only transient reductions in viremia were observed. Because chronic set-point viremia was presumed to predict long-term survival (10), these SIV challenge studies were terminated before the deaths of the experimental animals, and it was suggested that T cell-based vaccines may not confer clinical benefit to humans (11, 12).
We evaluated the clinical benefit afforded by an SIV-specific cellular immune response elicited in rhesus monkeys through a plasmid DNA prime/recombinant E1-deleted, E3-inactivated adenovirus serotype 5 (rAd) boost vaccination regimen (13-16). The monkeys selected for study did not express the MHC class I allele Mamu-A*01 because of its association with particularly efficient control of SIV/SHIV replication (17, 18). Six monkeys in each of five experimental groups received different vaccination regimens (Fig. 1) (16). The monkeys were monitored prospectively using pooled peptide Elispot and intracellular cytokine staining assays as measures of cell-mediated immunity, and for neutralizing antibody responses (13-15). Monkeys were then challenged by intravenous route with SIVmac251 and monitored for viral replication and changes in CD4+ T lymphocyte subsets (16, 19).
The vaccination regimens elicited high-frequency SIV-specific cellular immune responses, with greater responses detected in the monkeys that received the DNA prime/rAd boost than those that received only rAd immunizations (Fig. 1). After virus challenge, there were no statistically significant differences in any parameters of clinical outcome between the various groups of experimentally vaccinated monkeys. Therefore, all 24 experimentally vaccinated monkeys were subsequently treated as a single group and compared to the 6 sham-vaccinated control monkeys.
Plasma SIV RNA levels were significantly lower in the experimentally vaccinated monkeys compared with the controls at the peak of viral replication, with this difference persisting through day 112 after virus challenge (Fig. 2, A and B). Thereafter, no significant difference was observed in plasma viral RNA levels between the experimentally vaccinated and control monkeys. Although the prediction would be that comparable set-point viral load measurements in the vaccinated and control monkeys would be associated with comparable survival, we elected to follow the monkeys for another 750 days. In fact, as late as day 850 after virus challenge, the vaccinated monkeys maintained a statistically significant survival advantage over the control monkeys (Fig. 3A) (16). Therefore, the set-point plasma viral RNA levels did not predict the relative duration of survival in these vaccinated monkeys.
We next evaluated whether other measurements of viral replication or immune function might serve as predictors of survival. A quantitative measure of viral replication during the first 126 days after infection was determined for each animal by integrating the plasma SIV viral RNA levels between days 3 and 126. The vaccinated monkeys had significantly lower values than the controls (Fig. 3B), which suggests that differences in total viral replication during this early phase of infection are associated with differences in long-term survival.
There is increasing evidence that the loss of memory CD4+ T lymphocytes during acute infection is important in AIDS pathogenesis (20, 21). To examine this phenomenon in the present study, peripheral blood memory CD4+ T lymphocytes were assessed at a single time point, and their association with clinical outcome in the cohort of SIV-infected monkeys was evaluated. We first assessed these values on day 126 after challenge (Fig. 4), a time point at which plasma virus RNA levels were comparable in the vaccinated and control monkeys (Fig. 4A). Although the proportions of naive and effector memory CD4+ T lymphocyte populations were comparable in the peripheral blood of the vaccinated and control monkeys, central memory CD4+ T lymphocyte populations were larger in the vaccinated group (Fig. 4, B to D, and fig. S1). Similar immunologic data were obtained on day 112 after challenge (fig. S2). Moreover, when animals were divided into terciles based on magnitudes of absolute peripheral blood CD4+ T lymphocyte counts on day 126 after challenge, no differences in survival were seen (Fig. 4E). Yet, when a similar analysis was done dividing monkeys into terciles based on central memory CD4+ T lymphocyte counts, a clear survival advantage was apparent for the monkeys with the highest counts (Fig. 4F). Similar results were seen in an evaluation of data generated on day 112 after challenge (fig. S3). These data sets also revealed an association between the survival of infected animals and the preservation of central memory CD4+ T lymphocytes after infection in a Cox proportional-hazards model (P = 0.05, likelihood ratio test). No association was observed in these monkeys between central memory CD8+ T lymphocyte counts and survival (fig. S4). Thus, although set-point plasma viral RNA levels were indistinguishable between the vaccinated and control monkeys, a single determination of the peripheral blood central memory CD4+ T lymphocyte count was associated with increased survival in these animals.
We then assessed the ramifications of this preservation of total central memory CD4+ T lymphocytes in the vaccinated, challenged monkeys on the SIV-specific immune response. On day 203 after challenge, the total central memory CD4+ T lymphocytes remained better preserved in the vaccinated than in the control monkeys, and the SIV Gag-specific CD4+ and CD8+ T lymphocyte IFN-, TNF-, and IL-2 responses were greater in the vaccinated than in the control monkeys (figs. S5 and S6). Interestingly, however, no difference was observed in the magnitude of the SIV neutralizing antibody responses in the vaccinated and control monkeys as late as 126 days after challenge (fig. S7). Therefore, the preservation of total central memory CD4+ T lymphocytes in the vaccinated monkeys was associated with preserved virus-specific T lymphocyte responses.
Finally, we evaluated the contribution of vaccine-elicited immunity to survival in this cohort of monkeys. Dividing the 16 monkeys that received Gag immunogens into halves based on the magnitude of peak vaccine-elicited SIV Gag-specific total T lymphocyte responses measured by IFN-y Elispot assay (Fig. 4G) and IFN-y CD4+ T lymphocyte responses measured by ICS assay (Fig. 4H), a survival advantage was apparent for the monkeys with the highest frequency vaccine-elicited cellular immune responses. The Elispot data also revealed an association between vaccine-elicited cellular immune responses and survival in a Cox proportional-hazards model (P = 0.015, likelihood ratio test). Therefore, the magnitude of the immune responses generated by the vaccine was associated with survival after virus infection.
These findings suggest that vaccine protection against high levels of viral replication during only the first weeks following an AIDS virus infection may provide sufficient protection against central memory CD4+ T cell loss to confer a survival advantage to infected individuals. Moreover, current models of large human HIV-1 vaccine efficacy trials propose the use of set-point viral load and total CD4+ T lymphocyte count as surrogate markers for a beneficial vaccine effect (22, 23). It has been presumed that a lower set-point viral load or a higher set-point CD4+ T lymphocyte count after infection will portend a better AIDS-free survival. The results of the present study indicate that set-point viral load and total CD4+ T lymphocyte count may not have predictive value in this setting. Rather, the quantitation of central memory CD4+ T cells in a vaccine trial several months after infection may be an important immune correlate of long-term protection and predict the efficacy of an HIV-1 vaccine.
Most important, this cohort of vaccinated monkeys followed for 850 days after challenge with the highly pathogenic SIVmac251 provides a distinctive data set for exploring the mechanisms underlying the vaccine-associated survival.
The demonstration of an association between the magnitude of the vaccine-elicited immune responses and the duration of survival after challenge provides a framework for understanding the immune protection conferred by cellular immune-based vaccines. Moreover, the prolonged survival conferred by a vaccine that stimulates T cell immunity provides support for pursuing clinical efficacy trials of such HIV-1 vaccines, even if they do not induce sterilizing immunity.
Vaccination preserves CD4 memory T cells during acute simian immunodeficiency virus challenge
Journal of Experimental Medicine June 5, 2006
Joseph J. Mattapallil1, Daniel C. Douek1, Alicia Buckler-White2, David Montefiori3, Norman L. Letvin4, Gary J. Nabel1, and Mario Roederer1
1 Vaccine Research Center and 2 Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Disease (NIAID), National Institutes of Health (NIH), Bethesda, MD 20892
3 Duke University Medical Center, Durham, NC 27710
4 Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215
Acute simian immunodeficiency virus (SIV)/human immunodeficiency virus infection is accompanied by a massive destruction of CD4 memory T cells across all the tissue compartments. These early events set the course toward disease progression and immunodeficiency. Here, we demonstrate that prior vaccination reduces this destruction during acute SIV Mac251 infection, leading to better survival and long-term outcome. Systemic vaccination with a DNA-prime recombinant adenovirus boost regimen preserved memory CD4 T cells throughout the body. The vaccine regimen induced broad CD4 and CD8 T cell responses in all tissues examined and, importantly, induced antibodies that neutralized the primary isolate of SIV used for challenge. Finally, we demonstrate that the extent of preservation of the CD4 memory compartment during the acute phase provides a strong predictor for subsequent progression to death. Our data provide a mechanism to explain clinical observations that acute-phase viral loads predict long-term disease progression and underscore the need for interventions that protect against early destruction of CD4 memory T cells during acute infection.
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