Conference Highlights - HIV Pathogenesis and Vaccine Development Volume 13 Issue 1 March/April 2005

HIV Pathogenesis and Vaccine Development

R. Paul Johnson, MD

New information on the crystal structures of the HIV and the simian immunodefi- After screening different combinations ciency virus (SIV) envelopes represented one of the scientific highlights of the 12th of gp120 core proteins with an intact V3 Annual Conference on Retroviruses and Opportunistic Infections. Numerous presen- loop complexed with CD4 and different tations also underscored the increasing recognition of the central role of gut-associ- antibodies able to bind to the envelope ated lymphoid tissue in AIDS pathogenesis and helped reveal a better understand- only after CD4 binding (CD4-induced ing of the multiple mechanisms underlying CD4+ T lymphocyte depletion in AIDS. antibodies), the Kwong laboratory iden- Progress on vaccine development was largely incremental but was strongly influ- tified a crystal that could be used for enced by the impact of an expanding array of flow cytometric assays that have structural analysis. Several features of revealed significant functional and phenotypic differences in virus-specific CD8+ the resulting predicted structure of the cells. The interplay between host cellular and humoral immune responses and virus V3 loop in the context of an envelope evolution was another prominent theme, and it underscored the challenge facing trimer help explain its relative immuno- host immune responses and vaccine developers in attempting to thwart an ever- mutating virus. genicity, including the fact that this loop is relatively exposed in the envelope trimer and the lack of intermolecular New Insights into Envelope In the unliganded form, the tip of the hydrogen bonds, a feature that confers Structure and HIV-specific CD4 binding site is exposed at the top more flexibility on the loop to interact Neutralizing Antibodies of envelope. Following initial contact with antibodies. with CD4, the resulting conformational Only a handful of monoclonal anti- Designing strategies to foil the ability of change brings together the other bodies are able to provide relatively HIV to evade neutralizing antibodies rep- residues of the CD4 binding site as well broad neutralization of primary HIV iso- resents one of the primary (and still elu- as the chemokine receptor binding site, lates. Based on the premise that elucida- sive) goals of HIV vaccine research. The locking in the remodeled envelope tion of the structure of these rare anti- crystal structure of HIV-1 gp120 pub- around CD4, and exposing the newly bodies could provide clues to the design lished in 1998 analyzed the conforma- formed chemokine receptor binding of modified envelope immunogens that tion of a modified envelope stripped of site. These changes shift a portion of the would be more effective in inducing most of its sugar residues and bound to V1-V2 loop that interacts with gp41, broadly neutralizing antibodies, the CD4 and to an antibody that mimicked thereby helping to release gp41 for sub- structures of several of these antibodies the chemokine coreceptor binding site sequent fusion with the host cell. have been determined over the past sev- (ie, a liganded envelope). Although this The investigators also identified a eral years. However, to date, the hope structure has been quite valuable for deep hydrophobic pocket in the inner that this information would lead to bet- advancing our understanding of HIV domain of the unliganded envelope ter vaccines has yet to be realized. Most envelope function, it does not provide where resistance mutations have been of these antibodies (notably b12 and information on several key points, mapped for a newly identified antiviral 2G12) have relatively unusual structures including the structure of the envelope compound (BMS-78806) that has been that are likely to be difficult to elicit by in its native state prior to binding to shown to inhibit HIV entry. This class of immunization. However, Kwong high- CD4, the point at which it is most sus- inhibitor is therefore likely to inhibit entry lighted the fact that the recently deter- ceptible to neutralizing antibodies. by binding to this pocket and stabilizing mined structure of the 2F5 monoclonal Using crystals of a fully glycosolated the unliganded form of gp120. Although antibody might provide a logical path to SIV gp120 (which was more stable than significant conformational changes in the a better immunogen. The antigen-bind- the HIV version) stripped of the V1-V2 envelope were clearly anticipated prior to ing region of 2F5 contains an extended and V3 variable loops, the Harrison lab the determination of this structure, the hydrophobic region that allows the anti- determined the crystal structure of the availability of a 3-dimensional model to body to bind to a hydrophobic region of unliganded envelope (Abstract 7). The visualize these changes should help guide gp41 that is relatively close to the virion most notable finding was the dramatic efforts to develop new means to interfere surface. In fact, binding of 2F5 to the conformational shift observed in the with viral entry by pharmacologic or HIV envelope is strongly enhanced by inner domain of the unliganded enve- immunologic means. the presence of membrane, suggesting lope compared with the previously Another gap in our understanding of that presentation of the 2F5 epitope in determined liganded envelope structure. envelope structure has been the lack of the context of membrane may be a key solid information on the V3 loop, which factor in eliciting similar antibodies. was deleted in the molecule used for the This observation suggests that use of Dr Johnson is Associate Professor of previously reported crystal structure. a modified envelope immunogen, which Medicine at Harvard Medical School and Kwong (Abstract 110) described recent is locked into place (by introduced disul- Chair of Immunology at the New England success in generating crystals of HIV-1 fide bonds) and presented in the context Primate Research Center. envelopes containing an intact V3 loop. of a membrane (eg, by a virus-like parti-

9 International AIDS Society–USA Topics in HIV Medicine

cle or proteoliposome), might be a more gp41. As noted above, these antibodies controversial. In a plenary lecture, Douek effective means to induce antibodies to are atypical, both with respect to their (Abstract 127) provided an insightful and this epitope. structure and ability to mediate relative- entertaining review of different mecha- Alternative approaches to under- ly broad neutralization of multiple HIV nisms contributing to CD4+ cell deple- stand how to elicit broadly neutralizing strains, and are not representative of tion at different stages of HIV infection. antibodies have focused on analysis of neutralizing antibodies found in most Several studies in SIV-infected monkeys antibodies in HIV-infected people that HIV-infected subjects. The subjects, who had previously documented depletion of bind to conserved functional targets, were preselected for HIV isolates sensi- CD4+ cells in the gut within 2 weeks such as the chemokine coreceptor bind- tive to these antibodies, received all 3 after infection. However, there were ing site. Decker (Abstract 87) utilized a neutralizing antibodies 1 day prior to some who interpreted these studies as novel approach to screen for antibodies discontinuation of antiretroviral therapy representing peculiarities of the SIV/ in patient sera able to bind to the and received weekly infusions for a total macaque model rather than insights into chemokine receptor binding site by of 12 weeks. The 8 chronically infected HIV pathogenesis. Recent studies in HIV- preincubating either HIV-1 or HIV-2 patients had undergone previous treat- infected patients, including one from the reporter viruses with soluble CD4 to ment interruptions, thereby allowing a Douek laboratory, have provided com- expose the coreceptor binding site. comparison of the increase in viremia pelling evidence for rapid and profound These CD4-induced neutralizing anti- following monoclonal antibody treat- depletion of CCR5+CD4+ cells in the bodies were able to mediate neutraliza- ment with that observed following previ- gut of patients in the first several weeks tion across numerous HIV-1 clades and ous interruptions. of HIV infection. This depletion of acti- even neutralization of HIV-2 at titers of A significant delay in the rebound of vated CCR5+CD4+ cells occurs most up to 1:100,000 or more. Competition viremia was observed in 2 of the 8 noticeably in the gut, the major reservoir experiments with an antibody (19c) chronically infected patients. The 6 for activated CD4+ cells in the body, but known to bind to the chemokine recep- acutely infected patients had such based on studies in macaques, also tor binding site confirmed that this anti- rebound significantly later than did a ret- occurs at other mucosal sites, including body activity largely or exclusively rospective control group of patients with the pulmonary tract and female repro- reflected antibodies to the chemokine acute HIV infection who underwent ductive tract. Whether these cells are receptor binding site. These results prob- treatment interruption and were not killed by direct infection or indirect ably have greater significance for under- treated with neutralizing antibodies. mechanisms has been much debated. standing of envelope function and evo- Interestingly, the development of resis- Douek presented data from acutely lution than vaccine design, because tance in the resurgent virus to the 2G12 infected macaques documenting SIV CD4-inducible neutralizing activity is monoclonal antibody was observed in 11 infection of 30% to 60% of memory only detected after the virus binds to of 13 patients. Overall, 7 of 14 patients CD4+ cells. Although this high level of CD4. Thus, the access of antibodies to had either a delayed or a decreased infection was most striking in the gut, it this binding site is stearically inhibited rebound in viremia after interruption of was also observed in peripheral blood following the interaction between HIV therapy. These data provide a proof-of- lymphocytes as well. Interestingly, mem- and CD4+ target cells. However, these principle demonstration of the ability of ory CD4+ cells that appeared to lack data suggest that viruses are largely con- neutralizing antibodies to help contain surface expression of CCR5 were also strained to CD4 dependence in order to viral replication in a subset of HIV-infect- found to be infected with SIV at relative- avoid neutralization by these relatively ed patients. However, the fact that rela- ly high rates. These extraordinary rates potent antibodies. tively high amounts of 3 potent neutral- of infection are over 100-fold higher Virtually all studies that have attempt- izing antibodies in a highly idealized than those observed in chronic infec- ed to understand protective immunity setting had no effect on viral rebound in tion. These data document the remark- against HIV in humans have relied on at least half of the study subjects serves able finding that approximately 50% of correlative studies, and there have been as a compelling reminder of the chal- all the body’s memory CD4+ cells are relatively few opportunities to examine lenge of neutralizing HIV in vivo. Also, killed within 2 weeks after infection, a the ability of specific immune responses the observation that evolution of resis- loss which appears to be largely, if not to block HIV replication in vivo. Trkola tance to 2G12 occurred in some patients exclusively, due to the direct effects of (Abstract 94LB) investigated whether 3 without evidence of significant suppres- viral infection. potent neutralizing antibodies (2G12, sion of viremia suggests the possibility However, distinct mechanisms appear 2F5, and 4E10) could suppress viral that neutralizing antibodies might exert to contribute to the depletion of CD4+ rebound in HIV-infected subjects with selective pressure on envelope sequence cells during chronic HIV infection, in either acute (n=6) or chronic (n=8) diversity while mediating only modest which levels of infected memory CD4+ infection who underwent interruption of suppression of viral replication. cells are much lower. Several mecha- their antiretroviral therapy. These anti- nisms contribute to the progressive bodies represent 3 of the 5 well-charac- The Guts of HIV Pathogenesis CD4+ cell depletion in chronic HIV terized antibodies with broadly neu- infection, including the induction of an tralizing activity against HIV-1: 2G12 Although CD4+ cell depletion has long inflammatory state in lymph nodes recognizes a carbohydrate epitope on been recognized as the defining feature associated with the deposition of colla- gp120; 2F5 and 4E10 both recognize of AIDS, the mechanisms that mediate gen, suppression of thymic output, and epitopes on highly conserved regions of this relentless depletion have remained chronic immune activation with its asso-

10 Conference Highlights - HIV Pathogenesis and Vaccine Development Volume 13 Issue 1 March/April 2005

ciated increased rates of CD4+ and bility class I allele Mamu-B*17, which vation of the consensus sequence. They CD8+ cell apoptosis. Douek also pro- has been shown to present a number of went on to demonstrate that the Nef 83 posed that breakdown of local immune relatively conserved SIV CD8+ cell epi- residue lies in a previously undefined responses at mucosal sites could con- topes. These animals do possess relative- B*57/B*5801-restricted epitope and tribute to chronic immune activation. ly broadly directed SIV-specific CD4+ appears to represent an escape mutation Ta ken together, these observations paint and CD8+ cell responses, but the fre- that has increased in frequency to a complex picture of CD4+ cell deple- quency of these responses does not nec- become the consensus sequence. A sim- tion in AIDS, in which direct infection of essarily distinguish these animals from ilar situation was demonstrated for an CD4+ cells leads to a substantial deple- other animals who have not achieved HLA-B*51-restricted epitope. In several tion of memory CD4+ cells within the long-term control of SIV replication. instances, they documented transmis- first couple weeks of infection and in Watkins postulated that there may be sion of these escape sequences to sub- which multiple direct and indirect mech- distinct differences in the ability of SIV- jects who lacked the restricting allele anisms contribute to the depletion of specific CD8+ lymphocytes specific for and did not observe reversion of the CD4+ cells during the chronic phase. different epitopes to suppress SIV repli- sequence, an observation that strongly cation both in vitro and in vivo, and that suggests that the presence of this muta- Protective Immunity Against these differences may be observed in tion does not exact a cost to the virus in HIV: Lessons from Macaques settings where no cytotoxic T lympho- terms of decreased fitness. cyte (CTL) escape has occurred. The dynamic relationship between Lack of solid information on protective CTL escape and replication fitness was immunity against HIV remains one of Escape from HIV- and reinforced by macaque studies reported the major impediments to the develop- SIV-specific CTL Responses by Barouch (Abstract 131). Using a virus ment of an AIDS vaccine. Watkins stock derived from monkeys that (Abstract 95) provided an overview of The ability of HIV and SIV to rapidly expressed the Mamu-A*01 major histo- insights into protective immunity from mutate to escape CTL responses has compatibility complex (MHC) class I studies in macaques. He rejected the been well documented in numerous set- allele and contained an escape mutation notion that it was realistic to expect that tings. There is increasing evidence that in the dominant A*01-restricted Gag an AIDS vaccine could provide sterile immune selection pressure is shaping epitope p11c, Barouch observed consis- protection and instead highlighted the the sequence of HIV on a population tent reversion to wild-type p11c epitope importance of trying to suppress viral level as well. sequences in monkeys that did not replication in the postacute phase of Altfeld (Abstract 91) analyzed CTL express A*01. These data confirm prior infection to less than 1000 copies/mL, a recognition of an HLA-A2–restricted epi- reports that escape mutations in this epi- benchmark that would reduce transmis- tope in Vpr. Most HLA-A2–positive indi- tope exact a significant fitness cost to sion and significantly delay disease pro- viduals do not generate a response to the virus. Interestingly, in A*01-positive gression. the consensus sequence of this epitope. animals, transient reversions to wild- He also highlighted the fact that However, a subset of individuals infected type p11c sequences were observed, macaques infected with attenuated with a less-frequent variant (I60L) did which then triggered an increase in the viruses such as SIV∆nef, which has pro- mount a response against this epitope frequency of CTL specific for this epi- vided some of the strongest protection in that was detectable relatively early in tope, followed by an increase in the fre- the SIV/macaque model to date, are dis- the course of infection. This I60L variant quency of CTL escape variants. tinguished not so much by high-frequen- epitope had a higher affinity for HLA-A2 Ta ken together, these data reinforce cy CD8+ cell responses but by their rel- and was better recognized by CTL than the notion that HIV and SIV have evolved atively high-frequency and broadly the consensus sequence, suggesting that both in individuals and on a population directed SIV-specific CD4+ cell respons- this immunodominant epitope may level to develop mutations in immun- es. He further postulated that the induc- have already been lost due to virus odominant CTL epitopes. The implica- tion of strong virus-specific CD4+ cell escape driven by the relatively common tions for vaccine design would be that responses would likely prove to be a cru- HLA-A2 allele. vaccines should attempt to elicit re- cial characteristic of an effective AIDS A similar scenario was presented by sponses to epitopes for which escape vaccine, especially with regard to the Leslie (Abstract 92), who demonstrated mutations exact a significant fitness cost ability of CD4+ cell responses to provide a negative association between the pres- to the virus and are therefore less likely to help for antibody and CD8+ cell respons- ence of HLA-B*57/*5801 and conserva- become fixed in the general population. es. He also highlighted insights into the tion of a consensus glycine at residue 83 role of CD8+ cell responses in control- of Nef. Most previous studies on Natural Hosts of Primate ling SIV replication gleaned from the immune escape from CTL have focused Lentiviruses study of 161 macaques at the University on the documentation of a positive asso- of Wisconsin. Of these animals, 11 were ciation between the presence of a spe- Natural hosts of primate lentiviruses, termed elite controllers: animals able to cific HLA allele and a change from the such as sooty mangabeys and African achieve sustained control of plasma consensus HIV-1 sequence, whereas green monkeys, rarely develop immun- viremia to less than 1000 copies/mL. these authors reported the opposite odeficiency, despite viral loads that are Seven of these 11 animals express the effect—that is, the presence of a specific often as high as those in HIV-infected relatively infrequent major histocompati- HLA allele was associated with a preser- people or SIV-infected macaques with

11 International AIDS Society–USA Topics in HIV Medicine

AIDS. Notwithstanding the intense inter- ferent conclusions, their data on the contrast, activation of type 1 IFN est surrounding this topic, the mecha- magnitude of SIV-specific CD8+ cell responses was observed in lymphocytes nisms that underlie the lack of disease responses were in fact overlapping from HIV-infected subjects. progression in natural hosts of SIV rather than diametrically opposed and Previous work from Veazey and col- remain obscure. Previous studies from clearly documented that most infected leagues has demonstrated that the several groups have shown that species animals generate a significant virus-spe- intestinal tract is a major target for SIV such as sooty mangabeys lack the gener- cific CD8+ cell response. This debate infection and CD4+ cell depletion in alized immune activation and increased highlighted the limitations of trying to SIV-infected macaques. Could the rela- T-cell turnover found in HIV-infected draw conclusions on the role of virus- tive absence of these activated T cells in humans or SIVmac-infected macaques. specific immune response by correlative gut-associated lymphoid tissue of natu- One interpretation of the absence of and phenotypic studies and emphasized ral hosts in part underlie their resistance generalized immune activation in natural the need for additional interventional to immunodeficiency? hosts of SIV is that this reflects a marked studies such as CD8+ lymphocyte deple- To address this issue, Veazey (Abstract attenuation of the host response to SIV tion in these animals to better assess the 153), compared CD4 and CCR5 expres- and would be associated with weak or role of CD8+ cells. sion in lymphocytes from the intestinal absent SIV-specific CD8+ cell responses. An alternative hypothesis to explain tract, lymph nodes, and blood of uninfect- Two oral presentations presented differ- the lack of immune activation and ed sooty mangabeys, African green mon- ing views of this basic question. CD4+ cell depletion in naturally infected keys, and rhesus macaques. As previously Silvestri (Abstract 155) described the hosts suggests that the early interaction reported, approximately 50% of T cells in results of intracellular cytokine staining of SIV with the host innate immune the intestines of macaques express CD4 using lymphocytes from 74 infected responses and dendritic cells may differ and 50% of these CD4+ cells express sooty mangabeys that were stimulated fundamentally from those observed in CCR5. However, less than 10% of T cells with peptide pools corresponding to the susceptible hosts. Staprans (Abstract in the intestines of African green monkeys SIVmac239 Gag, Pol, Env, and Nef pro- 152) analyzed differences in myeloid express CD4 and of these, only approxi- teins. Although SIV-specific CD8+ cell and plasmacytoid dendritic cell subsets mately 12% also express CCR5. The fre- responses were detected in 65% of all during acute SIV infection of sooty quency of CD4+ cells in the gut of sooty animals tested, responses were low mangabeys and rhesus macaques. Acute- mangabeys was higher than in African (<0.2%) or absent in 78% of all ani- ly infected macaques had significantly green monkeys, but essentially none of mals tested. No correlation between the higher levels of expression of the these cells express CCR5. magnitude of the CD8+ cell response chemokine receptor CCR7 on myeloid These observations suggest the and plasma viremia was found. and plasmacytoid dendritic cells, but lit- hypothesis that natural hosts of SIV In contrast, Wang (Abstract 154) tle CCR7 was observed on either den- infection may have evolved to reduce described a more robust SIV-specific dritic subtype in acutely infected the number of CD4+CCR5+ cells in CD8+ cell response in naturally infected mangabeys. Consistent with the known gut-associated lymphoid tissue that sooty mangabeys using a combination of role of CCR7 in mediating homing of serve as the primary fuel for SIV replica- enzyme-linked immunospot (ELISPOT) lymphoid cells to secondary lymphoid tion. Although this observation would and intracellular cytokine staining tissue, there was a preferential accumu- initially appear to be at odds with the assays. Positive ELISPOT responses to lation of dendritic cells in lymph nodes fact that many of the natural hosts have SIV peptide pools were detected in 25 of of acutely infected rhesus macaques but viral loads that are as high or higher than 25 SIV-infected mangabeys studied, with not in mangabeys. those observed in susceptible hosts, it is the highest responses observed to SIV A significant difference among possible that viral replication in gut-asso- structural proteins. The overall magni- species was also observed in regard to ciated lymphoid tissue may be responsi- tude of the total SIV-specific ELISPOT the response of their dendritic cells to ble for initiating the indirect effects of response ranged from 240 to 5200 spot- SIV stimulation. Both rhesus macaque CD4+ lymphocyte depletion. As sug- forming cells per 106 peripheral blood and human dendritic cells produced rel- gested by Douek’s plenary talk, this pro- mononuclear cells, a value comparable atively large amounts of interferon (IFN)- cess may be initiated by immune activa- to that obtained in SIV-infected rhesus α following stimulation with SIV, but tion induced by increased mucosal macaques. sooty mangabey dendritic cells had rela- inflammation associated with the deple- Reasons for the apparently dis- tively little response. These data suggest tion of mucosal CD4+ lymphocytes. crepant results were not immediately that the lack of dendritic cell activation clear but may be related to the greater and type 1 IFN production in SIV-infect- Update on the Quest for a Safe sensitivity of ELISPOT assays in the ed sooty mangabeys may lead to an and Effective AIDS Vaccine detection of responses in sooty attenuated inflammatory response and mangabeys. Although this group also a significant reduction of the indirect Recent advances in HIV vaccine research noted no correlation between the overall bystander effects of SIV infection. The were highlighted in a symposium encom- magnitude of the response and the con- relative lack of induction of a type 1 IFN passing basic science, nonhuman pri- trol of viremia, these results are in fact response to SIV infection was also veri- mate trials, and human clinical trials. The similar to those observed in SIV-infected fied by microarray analysis of peripheral past decade has witnessed a significant macaques or HIV-infected humans. blood lymphocytes from uninfected and expansion in the number of viral vectors Although these 2 groups arrived at dif- infected mangabeys (Abstract 323). In able to induce cell-mediated immune

12 Conference Highlights - HIV Pathogenesis and Vaccine Development Volume 13 Issue 1 March/April 2005

responses. Johnson (Abstract 111) summa- with a comparable SIV construct yielded eration and cytolytic ability are often rized the strengths and shortcomings of significant levels of SIV-specific CD8+ impaired. viral vectors currently under study as can- cell responses in nonhuman primates, Although nonhuman primates have didate AIDS vaccines. the overall level of immunogenicity of served as a primary platform for preclin- Notable advantages of these vectors the HIV-1 clade-A constructs was dis- ical evaluation of AIDS vaccines, the abil- include their track record in inducing appointing: regardless of the DNA dose ity of monkey studies to predict results in relatively robust cellular immune re- or the MVA boosting schedule, only 14% humans has been a source of recurring sponses in mice and nonhuman pri- to 20% of volunteers had a positive debate. Letvin (Abstract 113) reviewed evi- mates, as well as their ability to deliver ELISPOT response, and the majority of dence as to whether results from nonhu- multiple viral antigens. However, he these responses were not sustained. man primate studies are likely to be pre- highlighted the challenge of overcoming More encouraging results have been dictive of either the immunogenicity or immune responses to these vectors obtained with the human trials of aden- efficacy of AIDS vaccines in humans. He induced by prior administration or natu- ovirus vectors. McElrath particularly highlighted 2 studies—the Oxford/IAVI ral infection. This limitation has been highlighted recent results from the Ad5 DNA/MVA study described in McElrath’s well documented in a trial of an aden- Gag/Pol/Nef vaccine, which were also talk and a DNA/recombinant Ad5 study ovirus serotype 5 (Ad5) HIV-1 Gag vac- reported in detail in a subsequent oral sponsored by the National Institutes of cine, which demonstrated a significant presentation (Abstract 135). This vac- Health Vaccine Research Center—in decrease in the frequency of ELISPOT cine was immunogenic in approximate- which disappointing immunogenicity responses in subjects with high levels of ly 70% of subjects, and the majority of results had been obtained in humans. In titers of neutralizing antibodies to the responders developed a response to each case, although initial testing of sim- Ad5 vector backbone, especially when more than 1 vaccine antigen. ilar SIV vaccines had yielded promising administered at lower doses. McElrath also addressed the issue of results in macaques, subsequent testing Several different options for over- heterogeneity of virus-specific CD8+ of the HIV vaccine constructs that had coming preexisting or induced antivec- cell function. Although much of recent yielded poor results in humans also tor immunity are being pursued. In AIDS vaccine development has been demonstrated low levels of cell-mediat- some cases, the effect of preexisting focused on increasing the magnitude of ed immune responses in macaques. immunity can be overcome by increas- HIV-specific CD8+ cells induced by vac- These results highlight the fact that HIV ing the vector dose, as has been demon- cination, she highlighted a number of immunogens should be evaluated for strated for the adenovirus vectors. How- qualitative aspects of CD8+ cell function immunogenicity in macaques even with- ever, the necessity of increasing the vac- that might affect efficacy, including the out the ability to carry out an effective cine dose by 100-fold to 1000-fold has a breadth of responses, epitope avidity, challenge. significant impact on vaccine cost and and their ability to home to mucosal Answering the question of whether production, as well as reactogenicity. sites, as well as to proliferate following vaccine efficacy trials in macaques will Alternative strategies that are being antigenic stimulation. predict efficacy results in humans will pursued include the use of less-frequent As one approach to determine what await results from additional phase 2B serotypes of viral vectors (eg, adenovirus parameters of HIV-specific CD8+ cell or phase 3 efficacy studies of candidate serotypes 11 and 35 or adenoviruses function might correlate with control of AIDS vaccines. There has been consider- from other species, such as chimpanzees) viral set point after acute infection, 21 able controversy as to whether the use and the use of viral vectors derived from patients with primary HIV infection of CXCR4-tropic simian/human immun- viruses that infrequently infect humans were studied in detail with respect to the odeficiency virus (SHIV) strains such as (eg, vesicular stomatitis virus or Venezue- magnitude, breadth, and avidity of their SHIV 89.6p, which induces rapidly pro- lan equine encephalitis virus). A final HIV-specific CD8+ cell response. In fact, gressive depletion of naive CD4+ cells strategy that is being pursued is that of none of these parameters correlated in 2 to 4 weeks, are likely to serve as a boosting with heterologous viral vectors, with viral set point, and the functional better predictor of vaccine efficacy in an approach that is currently being pur- avidity of individual epitopes did not humans than the use of CCR5-tropic sued in a collaborative trial that is exam- correlate with the magnitude of the viruses such as SIVmac239 or SIV- ining the immunogenicity of a combined response for a given epitope. mac251, which induce a slower progres- canarypox (ALVAC) and adenovirus vac- Although it remains unclear which sion to AIDS over a period of 1 to 2 cine regimen. parameters of CD8+ cell function best years. McElrath (Abstract 112) summarized correlate with control of HIV replication, Letvin presented data from a DNA CD8+ cell responses elicited in several McElrath underscored the finding that prime/adenovirus boost vaccine trial in recent candidate AIDS vaccine trials. A HIV-specific CD8+ cells from long-term SIV-infected macaques using a regimen trial jointly sponsored by Oxford Univer- nonprogressors typically maintain a high that had previously generated significant sity and the International AIDS Vaccine perforin content and the ability to prolif- protection from disease induced by Initiative (IAVI) examined the safety and erate following antigen stimulation. Thus, SHIV 89.6p. When challenged with SIV- immunogenicity of an HIV-1 clade-A although HIV-specific CD8+ cells general- mac251, vaccinated animals had an DNA vaccine followed by boosting with ly persist with disease progression and approximate 1-log10 decrease in viral an HIV-1 clade-A modified vaccinia retain the ability to secrete IFN-γ or tumor load and improved survival. However, Ankara (MVA) vaccine in seronegative necrosis factor (TNF)-α, their functional only a modest and marginally statistical- volunteers. Although preclinical trials characteristics with respect to prolif- ly significant effect against CD4+ cell

13 International AIDS Society–USA Topics in HIV Medicine

depletion was observed, in contrast to pipeline) to block vaginal infection of essentially complete protection against dramatic protection against CD4+ cell macaques with the R5 SHIV strain mucosal or even intravenous infection depletion induced by a similar vaccine 162p3. Previous studies of a lower dose with SIV. However, more recent studies following challenge with SHIV 89.6p. of this compound had demonstrated using lower doses of tenofovir (4-10 These results provide some encour- that a low intravaginal dose (0.6 mg) of mg/kg/day) have demonstrated more agement that T-cell–based vaccines may CMPD-167 was able to decrease peak modest protection against oral or rectal provide protection against disease in a viremia in macaques but was only rarely challenge, although they have still setting of challenge viruses such as SIV- able to induce complete protection. Fol- demonstrated a statistically significant mac251, which appear likely to provide lowing a reformulation of the compound reduction in the risk of infection per a better model of HIV-induced disease in to allow a 5-fold increase in dose, the exposure. Although several clinical trials people. However, they still leave open investigators observed complete protec- of preexposure prophylaxis employing the question of which of the different tion from SHIV infection in 7 of 8 ani- tenofovir have been planned or initiated, SIV and SHIV challenge models is most mals studied, whereas all 5 placebo- controversy regarding these studies likely to predict results of vaccine effica- treated control animals were infected. has resulted in some of these studies cy in humans. The relatively high doses of this com- being cancelled, notably in Cambodia, Most vaccine trials in nonhuman pri- pound (and other vaginal microbicides Cameroon, and Nigeria. Grant highlight- mates have focused on the use of a sin- such as PSC-RANTES) that are required ed the fact that the assessment of the gle high-dose challenge. However, there suggest that diffusion of these com- effects of preexposure prophylaxis has been increasing interest in the use of pounds into the vaginal epithelium or should extend beyond simply measuring repeated low-dose mucosal challenges submucosa may be required for inhibi- acquisition rates of HIV infection and that may better model the relative ineffi- tion of viral entry. include assessment of the development cient transmission of HIV through sexu- Promising preliminary findings for of viral resistance to the drug(s), potential al contact observed in humans. Butera an alternative microbicide approach effects of drug resistance mutations on and colleagues (Abstract 134) analyzed were reported by Cristofaro (Abstract viral replication and transmission fitness, the ability of a DNA prime/MVA boost 129), who demonstrated the ability of potential effects on risk-taking behavior, regimen to protect against repeated low- liposomal delivery of small interfering and whether abortive infections might dose rectal challenge. Animals were vac- RNA (siRNA) to downregulate vaginal induce immune responses that could sub- cinated with the Gag, Pol, and Env anti- expression of the nuclear membrane sequently protect against infection (as gens of an HIV-1 CRF02_AG primary iso- protein lamin A/C (which was selected has been observed in macaque studies). late using a DNA/MVA regimen previous- as a proof-of-principle target) or CCR5 in Given the controversy and difficult ly shown to induce relatively strong cell- mice. Sustained downregulation was ethical questions that have surrounded mediated immune responses. Vaccinat- observed for up to 7 days after a single these trials, it is also clear that involve- ed animals and naive controls then had a application. Although these results pro- ment of community members in the repeated low-dose rectal challenge with a vide an encouraging demonstration of planning stages of future studies of pre- recombinant SHIV isolate expressing an the feasibility of using siRNAs as a exposure prophylaxis will be a key factor R5 clade-B envelope (SHIV SF162p3). microbicide, practical questions related in their successful execution. Although most vaccinated animals to toxicity, cost, and optimal dosing were ultimately infected after repeated schedules will have to be addressed, Therapeutic Vaccination intrarectal exposure to SHIV, there was including nonhuman primate studies, a significant increase in the number of before this strategy can move forward to The prospect that immunization of HIV- intrarectal exposures required to infect clinical trials. infected individuals could improve their the vaccine recipients as opposed to Although the concept of providing ability to contain viral replication has naive controls. Naive controls were infect- preexposure prophylaxis to people at generally offered more promise than ed after an average of 3.5 rectal expo- increased risk for acquisition of HIV hard results. Two studies provided sup- sures, but 44% of vaccinated animals infection has been controversial, given port for the ability of therapeutic immu- remained virus free after 10 intrarectal the grim prospects of developing an nization regimens to improve control exposures and 5 of these remained unin- AIDS vaccine, this approach has received over viremia but also highlighted the sig- fected after a total of 18 exposures. increasing attention and provoked ongo- nificant challenges that this approach ing controversy. Grant (Abstract 137) faces. In a late breaker (Abstract 133LB), Vaccine Alternatives: reviewed the rationale behind preexpo- Levy offered a report on the ANRS 093 Microbicides and Preexposure sure prophylaxis. The planned or recent- trial through 100 weeks of follow-up. Prophylaxis ly initiated clinical trials of preexposure In this trial, HIV-infected subjects prophylaxis have employed tenofovir, with CD4+ counts greater than 350 In light of the clear challenges to devel- which has a number of characteristics cells/µl and HIV-1 RNA levels less than 50 oping an effective HIV vaccine in the well suited for this purpose, including copies/mL were randomized to antiretro- next decade, there has been a resur- once-daily dosing and an excellent safe- viral therapy alone or to antiretroviral gence of interest in microbicides. Veazey ty profile. therapy plus immunization with the (Abstract 128) reported on the ability of Several trials of tenofovir in canarypox vector ALVAC vCP1433 (which a CCR5 coreceptor inhibitor (CMPD-167, macaques employing relatively high expresses HIV-1 gag, pol, env, and nef) which is no longer in the clinical doses (20-30 mg/kg) had demonstrated and HIV-1 lipopeptides in association

14 Conference Highlights - HIV Pathogenesis and Vaccine Development Volume 13 Issue 1 March/April 2005

with low-dose subcutaneous interleukin ment interruptions, the magnitude and therapeutic immunization is generally (IL)-2. All subjects then underwent a duration of benefit was relatively modest. short lived, they do offer some hope that treatment interruption at week 40 and Additional evidence for the potential more potent therapeutic immunization were restarted on therapy if HIV-1 RNA of therapeutic immunization was provid- regimens may be able to offer an extend- levels rose to more than 50,000 copies/ ed by a presentation from Pavlakis ed drug-free period to at least a subset of mL at 4 weeks or more than 10,000 (Abstract 132) on SIV-infected macaques patients. Whether this increased period copies/mL at subsequent time points. In that received an SIV DNA vaccine while of time off therapy will justify the effort the second phase of this trial, patients on antiviral therapy. SIVmac-infected of therapeutic immunization is at present who had resumed antiretroviral therapy macaques were treated with a combina- unclear. underwent a second treatment interrup- tion of tenofovir, didanosine, and stavu- tion with the restart criteria as defined dine for 13 to 23 weeks. During treat- Financial Disclosure: Dr Johnson has no above. Vaccinated patients did have a ment, the animals received intramuscu- affiliations with commercial organizations significant increase in their time off lar injections with optimized DNA vec- that may have interests related to the content treatment as compared with controls tors, and in a subset of animals, with an of this article. (177 days vs 89 days) and also had IL-15 DNA vector as well. Compared with slightly lower levels of viremia during unvaccinated controls that underwent a the interruptions. Better control was similar treatment interruption regimen, A list of all cited abstracts appears on associated with a positive lymphoprolif- vaccinated animals had an approximate pages 45 to 50. erative response to one of the HIV pep- 1-log10 decrease in viral load off therapy, tides used for vaccination and with and a subset of 3 animals had prolonged higher HIV-specific ELISPOT responses. control of viremia off therapy. Animals Top HIV Med. 2005;13(1):9-15 Although these results clearly docu- with better control had relatively high Copyright 2005, International AIDS Society–USA ment the potential of this therapeutic vac- levels of Gag and envelope-specific cination regimen to enhance control of ELISPOT responses. Although these viral replication in the setting of treat- results also suggested that the benefit of

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