TAG HIV Basic Science, Vaccines, and Cure Project Blog

First Human Trial of AAV as a Delivery Vehicle for HIV Neutralizing Antibodies Gets Underway

Over the past few years there has been growing interest in the use of adeno-associated virus (AAV) as a vehicle for generating anti-HIV neutralizing antibodies in humans. The approach is different from traditional vaccination, in that AAV is used essentially as a gene therapy: the AAV vector is designed to take up residence in cells and then act as a factory for churning out broadly neutralizing antibodies against HIV (genes that encode these antibodies are inserted into the vector). This novel idea may be able to circumvent the challenging problem of inducing the production of broadly neutralizing antibodies with traditional vaccines, and could potentially offer significant protection against HIV acquisition. 

As covered previously on the blog, there are two main research groups working on AAV-based HIV prevention. The laboratory of David Baltimore has named their approach vectored immunoprophylaxis (VIP) and last Sunday they published a study in Nature Medicine demonstrating protection against vaginal HIV transmission in a humanized mouse model (the study was supported by the National Institute of Allergy and Infectious Diseases, who issued a press release drawing attention to the findings). Meanwhile Philip Johnson and colleagues at the Children’s Hospital of Philadelphia are collaborating with the International AIDS Vaccine Initiative (IAVI) on a phase I clinical trial of an AAV vector encoding the HIV neutralizing antibody PG9, and, according to clinicaltrials.gov, the trial started recruiting participants last month. This is a major milestone for the research, as there were many challenges associated with obtaining regulatory approval for a human trial and Johnson and IAVI have been working toward this goal for many years. The estimated date for completion of the study is January 2016. 

Nature Medicine (2014) doi:10.1038/nm.3471

Vectored immunoprophylaxis protects humanized mice from mucosal HIV transmission

Alejandro B Balazs, Yong Ouyang, Christin M Hong, Joyce Chen, Steven M Nguyen, Dinesh S Rao, Dong Sung An & David Baltimore

Received 03 October 2013 Accepted 07 January 2014 Published online 09 February 2014

The vast majority of new HIV infections result from relatively inefficient transmission of the virus across mucosal surfaces during sexual intercourse. A consequence of this inefficiency is that small numbers of transmitted founder viruses initiate most heterosexual infections. This natural bottleneck to transmission has stimulated efforts to develop interventions that are aimed at blocking this step of the infection process. Despite the promise of this strategy, clinical trials of preexposure prophylaxis have had limited degrees of success in humans, in part because of lack of adherence to the recommended preexposure treatment regimens. In contrast, a number of existing vaccines elicit systemic immunity that protects against mucosal infections, such as the vaccines for influenza and human papilloma virus. We recently demonstrated the ability of vectored immunoprophylaxis (VIP) to prevent intravenous transmission of HIV in humanized mice using broadly neutralizing antibodies. Here we demonstrate that VIP is capable of protecting humanized mice from intravenous as well as vaginal challenge with diverse HIV strains despite repeated exposures. Moreover, animals receiving VIP that expresses a modified VRC07 antibody were completely resistant to repetitive intravaginal challenge by a heterosexually transmitted founder HIV strain, suggesting that VIP may be effective in preventing vaginal transmission of HIV between humans.

Free Database of Broadly Neutralizing HIV Antibodies

Following up on the prior post about broadly neutralizing antibodies (bNAbs) against HIV, there is now a free database available (http://bnaber.org) that offers detailed information on all known bNAbs. The genesis and purpose of the database is described in an open access paper in the journal Nucleic Acids Research. Although not a focus of the paper, it's worth noting that the discovery of growing numbers of bNAbs has been facilitated by major investments in the research, particularly by National Institutes of Health funding for the Center for HIV/AIDS Vaccine Immunology and, more recently, two Centers for HIV/AIDS Vaccine Immunology & Immunogen Discovery (at Duke University and the Scripps Institute) and also by the non-profit International AIDS Vaccine Initiative (IAVI) though its Neutralizing Antibody Consortium. 

Nucleic Acids Res. 2013 Nov 7. [Epub ahead of print]

bNAber: database of broadly neutralizing HIV antibodies. (open access)

Eroshkin AM, Leblanc A, Weekes D, Post K, Li Z, Rajput A, Butera ST, Burton DR, Godzik A.

Abstract

The discovery of broadly neutralizing antibodies (bNAbs) has provided an enormous impetus to the HIV vaccine research and to entire immunology. The bNAber database at http://bNAber.org provides open, user-friendly access to detailed data on the rapidly growing list of HIV bNAbs, including neutralization profiles, sequences and three-dimensional structures (when available). It also provides an extensive list of visualization and analysis tools, such as heatmaps to analyse neutralization data as well as structure and sequence viewers to correlate bNAbs properties with structural and sequence features of individual antibodies. The goal of the bNAber database is to enable researchers in this field to easily compare and analyse available information on bNAbs thereby supporting efforts to design an effective vaccine for HIV/AIDS. The bNAber database not only provides easy access to data that currently is scattered in the Supplementary Materials sections of individual papers, but also contributes to the development of general standards of data that have to be presented with the discovery of new bNAbs and a universal mechanism of how such data can be shared.

Exploring the Therapeutic Potential of Neutralizing Antibodies

HIV is notoriously stubborn in its ability to thwart attacks by antibodies. The sugary outer envelope of the virus is so effective in cloaking its vulnerabilities that, for many years, the number of antibodies known to have broad neutralizing activity against HIV could be counted on one hand. But recently, collaborative research efforts aiming to identify and isolate more broadly neutralizing antibodies (bNAbs) have paid extraordinary dividends, increasing the number well into double digits and discovering several bNAbs with significantly greater breadth and potency than ever seen before (each isolated antibody is assigned a name, e.g. VRC01, PGT121, etc.). Although the primary impetus for this work is the development of an effective preventive HIV vaccine, there is also interest in exploring whether infusions of these new bNAbs may have therapeutic potential. A study published last year in the journal Nature reported encouraging signs of efficacy in a humanized mouse model, and this past Wednesday two research teams reported similarly promising results in macaque monkeys, drawing considerable media attention. 

The take-home message from both sets of studies is that infusions of either single or combined bNAbs were capable of suppressing viral replication in macaques. The viruses used in the experiments were lab-created hybrids that include the envelope of HIV combined with SIV in order to facilitate the study of anti-HIV antibodies in this animal model; both viruses are pathogenic and cause simian AIDS in macaques. In the paper published by Dan Barouch and colleagues, several additional observations are made that have contributed to the excitement surrounding the work: 

• A subset of macaques (3 of 18) showed prolonged control of SHIV replication to undetectable levels after a single bNAb infusion, which persisted long after the antibodies declined to undetectable levels. This control was associated with increased functionality of virus-specific CD8 T cell responses and enhanced levels of natural neutralizing antibodies in the animals, suggesting that the bNAb infusions had both direct antiviral effects and also indirectly improved virus-specific immune responses. Notably, however, these three macaques had relatively low viral loads at baseline.
• The bNAb infusions were associated with significant declines in SHIV DNA in blood and tissues, indicating a reduction in the persistent viral reservoir, and offering hope that bNAbs could have a role in HIV cure research.
• The declines in viral load appeared faster than is typically seen with antiretroviral therapy (ART), hinting that the bNAbs were accelerating the clearance of virus-infected cells via antibody-mediated effector mechanisms. These mechanisms involve antibodies binding to viral proteins on infected cells and thereby flagging them for clearance by the immune system.

There are a number of potentially important caveats, however. The second paper, by Masashi Shingai and colleagues, involved macaques with more advanced, symptomatic disease and resistance to the bNAbs appeared to develop more easily in this setting (although there were still benefits in terms of reduced viral loads and improved CD4 T cell counts). The Barouch study also found that bNAbs were unable to fully suppress viral load in two macaques with high viral loads at baseline. More generally, it is not yet known if there might be differences between HIV and the lab-created SHIVs when it comes to their ability to develop resistance to bNAbs. Additionally, chronically infected HIV-positive people may harbor more diverse viruses and therefore be more likely to have baseline resistance to some bNAbs compared to SHIV-infected macaques. Early efforts to explore bNAb infusions as therapies were stymied by the development of resistance, but those clinical trials involved first-generation antibodies far less potent than the new crop.

In terms of whether bNAbs might be able to help target HIV-infected cells for destruction, a commentary by Steve Deeks and Louis Picker that accompanies the papers notes that this effector mechanism depends on expression of the viral envelope protein, and “the extent to which viral envelope protein is exposed on the surface of long-lived infected cells is unknown.” At the IAS conference earlier this year there were two presentations that may address this question to a limited degree: on the upside, Victor Garcia showed that in a humanized mouse model, a strategy targeting infected cells via envelope protein expression significantly reduced cell-associated HIV RNA levels when added to ART. On the downside, Rick Koup reported that it was difficult to identify infected CD4 T cells isolated from HIV-positive individuals using envelope-specific bNAbs, even though the cells appeared to be actively transcribing HIV RNA (Koup’s presentation is covered in a report from the conference by David Margolis for NATAP).

Even if productively infected cells can be targeted by bNAbs via envelope expression, Deeks and Picker point out that latently infected cells would not be affected unless the thorny problem of activating latent HIV can be solved. So while bNAbs may be able to offer advantages over ART in the context of cure research, possibly even inducing prolonged containment of viral load in some cases (in the very optimistic scenario in which the results in macaques with low baseline viral loads are duplicated in humans), it is highly unlikely that bNAbs could be fully curative.

Definitive answers regarding the therapeutic efficacy of the new generation of bNAbs can only be obtained from human trials, and it is good news that, this past August, the first such trial got underway at the National Institutes of Health Clinical Research Center. The trial will recruit HIV-positive individuals both on and off ART and evaluate the effects of escalating doses of the bNAb VRC01.

Lastly, there is an overarching issue regarding monoclonal antibody therapies that deserves mention: they are extremely costly to manufacture and, as a consequence, very expensive. Delivery via infusion is also not particularly practical, even though they are typically given intermittently. In part due to these problems, scientists are exploring novel delivery methods such as the use of adeno-associated virus (AAV) vectors as gene therapies that would create long-lived bNAb-making factories in muscle tissue. Examples are David Baltimore’s vectored immunoprophylaxis (VIP) approach (covered previously on the blog) and a construct developed by Philip Johnson from the Children’s Hospital of Philadelphia in collaboration with the International AIDS Vaccine Initiative (IAVI). In the latter case, a phase I clinical trial in HIV-negative volunteers is now pending according to the clinicaltrials.gov database. Although the AAV-based strategy is currently being pursued for prevention, it is perhaps possible that it could be adapted for therapeutic use if bNAb infusions were found to be successful.

Nature (2013) doi:10.1038/nature12744

Therapeutic efficacy of potent neutralizing HIV-1-specific monoclonal antibodies in SHIV-infected rhesus monkeys

Dan H. Barouch, James B. Whitney, Brian Moldt, Florian Klein, Thiago Y. Oliveira, Jinyan Liu, Kathryn E. Stephenson, Hui-Wen Chang, Karthik Shekhar, Sanjana Gupta, Joseph P. Nkolola, Michael S. Seaman, Kaitlin M. Smith, Erica N. Borducchi, Crystal Cabral, Jeffrey Y. Smith, Stephen Blackmore, Srisowmya Sanisetty, James R. Perry, Matthew Beck, Mark G. Lewis, William Rinaldi, Arup K. Chakraborty, Pascal Poignard, Michel C. Nussenzweig et al.

Abstract

Human immunodeficiency virus type 1 (HIV-1)-specific monoclonal antibodies with extraordinary potency and breadth have recently been described. In humanized mice, combinations of monoclonal antibodies have been shown to suppress viraemia, but the therapeutic potential of these monoclonal antibodies has not yet been evaluated in primates with an intact immune system. Here we show that administration of a cocktail of HIV-1-specific monoclonal antibodies, as well as the single glycan-dependent monoclonal antibody PGT121, resulted in a rapid and precipitous decline of plasma viraemia to undetectable levels in rhesus monkeys chronically infected with the pathogenic simian–human immunodeficiency virus SHIV-SF162P3. A single monoclonal antibody infusion afforded up to a 3.1 log decline of plasma viral RNA in 7 days and also reduced proviral DNA in peripheral blood, gastrointestinal mucosa and lymph nodes without the development of viral resistance. Moreover, after monoclonal antibody administration, host Gag-specific T-lymphocyte responses showed improved functionality. Virus rebounded in most animals after a median of 56 days when serum monoclonal antibody titres had declined to undetectable levels, although, notably, a subset of animals maintained long-term virological control in the absence of further monoclonal antibody infusions. These data demonstrate a profound therapeutic effect of potent neutralizing HIV-1-specific monoclonal antibodies in SHIV-infected rhesus monkeys as well as an impact on host immune responses. Our findings strongly encourage the investigation of monoclonal antibody therapy for HIV-1 in humans.

Nature (2013) doi:10.1038/nature12746

Antibody-mediated immunotherapy of macaques chronically infected with SHIV suppresses viraemia

Masashi Shingai, Yoshiaki Nishimura, Florian Klein, Hugo Mouquet, Olivia K. Donau, Ronald Plishka, Alicia Buckler-White, Michael Seaman, Michael Piatak, Jeffrey D. Lifson, Dimiter Dimitrov, Michel C. Nussenzweig & Malcolm A. Martin

Neutralizing antibodies can confer immunity to primate lentiviruses by blocking infection in macaque models of AIDS^{1, 2, 3, 4}. However, earlier studies of anti-human immunodeficiency virus type 1 (HIV-1) neutralizing antibodies administered to infected individuals or humanized mice reported poor control of virus replication and the rapid emergence of resistant variants^{5, 6, 7}. A new generation of anti-HIV-1 monoclonal antibodies, possessing extraordinary potency and breadth of neutralizing activity, has recently been isolated from infected individuals⁸. These neutralizing antibodies target different regions of the HIV-1 envelope glycoprotein including the CD4-binding site, glycans located in the V1/V2, V3 and V4 regions, and the membrane proximal external region of gp41 (refs 9, 10, 11, 12, 13, 14). Here we have examined two of the new antibodies, directed to the CD4-binding site and the V3 region (3BNC117 and 10-1074, respectively), for their ability to block infection and suppress viraemia in macaques infected with the R5 tropic simian–human immunodeficiency virus (SHIV)-AD8, which emulates many of the pathogenic and immunogenic properties of HIV-1 during infections of rhesus macaques^{15, 16}. Either antibody alone can potently block virus acquisition. When administered individually to recently infected macaques, the 10-1074 antibody caused a rapid decline in virus load to undetectable levels for 4–7 days, followed by virus rebound during which neutralization-resistant variants became detectable. When administered together, a single treatment rapidly suppressed plasma viraemia for 3–5 weeks in some long-term chronically SHIV-infected animals with low CD4+ T-cell levels. A second cycle of anti-HIV-1 monoclonal antibody therapy, administered to two previously treated animals, successfully controlled virus rebound. These results indicate that immunotherapy or a combination of immunotherapy plus conventional antiretroviral drugs might be useful as a treatment for chronically HIV-1-infected individuals experiencing immune dysfunction.

Circulating Memory T Follicular Helper Cells Correlate with the Development of Broadly Neutralizing Antibody Responses Against HIV

A study published on September 12th by the journal Immunity ties together two emerging areas of HIV vaccine research. In recent years, scientists have discovered that a small proportion of chronically infected individuals develop antibody responses capable of broadly neutralizing a diverse array HIV isolates. These antibody responses typically take years to develop, and are not present at sufficient titers to offer noticeable benefit to the infected individuals they are isolated from, but there is reason to believe that if they could be induced by a vaccine they could protect uninfected people against HIV acquisition. A potential complement to this line of investigation has been the discovery of T follicular helper cells (Tfh), a specialized CD4 T cell subset that plays a critical role in providing help to B cells, thereby facilitating antibody production. Researchers have posited that Tfh may have an important role in the generation of broadly neutralizing antibodies against HIV, but direct evidence has been lacking.

In the Immunity paper, Michela Locci and colleagues report that there is a circulating population of Tfh that can be identified using a combination of surface markers, and that in a large cohort of HIV-positive individuals the frequency of these cells correlated with the development of broadly neutralizing antibodies against HIV. The data suggest that inducing this type of Tfh response should be a goal for vaccines aiming to create neutralizing antibodies against HIV (or potentially any other pathogen).

In a helpful example of kismet, the September 13th issue of the journal Science featured an article by Jon Cohen describing progress in discovering broadly neutralizing antibodies to HIV, along with a review on the same topic and a podcast interview with the senior author of the review, Michel Nussenzweig.

Immunity, 12 September 2013 doi:10.1016/j.immuni.2013.08.031

Human Circulating PD-1+CXCR3−CXCR5+ Memory Tfh Cells Are Highly Functional and Correlate with Broadly Neutralizing HIV Antibody Responses 

Michela Locci1, 2, Colin Havenar-Daughton1, 2, Elise Landais4, Jennifer Wu1, Mark A. Kroenke1, Cecilia L. Arlehamn1, Laura F. Su6, Rafael Cubas7, Mark M. Davis6, Alessandro Sette1, Elias K. Haddad7, International AIDS Vaccine Initiative Protocol C Principal Investigators8, 9, Pascal Poignard4, 5 and Shane Crotty1, 2, 3

1 Division of Vaccine Discovery, La Jolla Institute for Allergy and Immunology, La Jolla, CA 92037, USA

2 Department of Medicine, School of Medicine, University of California, San Diego, La Jolla, CA 92037, USA

3 Center for HIV/AIDS Vaccine Immunology and Immunogen Discovery, La Jolla, CA 92037, USA

4 International AIDS Vaccine Initiative, Neutralizing Antibody Center, La Jolla, CA 92037, USA

5 Department of Immunology and Microbial Science, The Scripps Research Institute, La Jolla, CA 92037, USA

6 Department of Microbiology and Immunology and the Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94304, USA

7 Vaccine and Gene Therapy Institute of Florida, Port St. Lucie, Florida 34987, USA

8 International AIDS Vaccine Initiative, New York, NY 10038, USA

Summary

The vast majority of currently licensed human vaccines work on the basis of long-term protective antibody responses. It is now conceivable that an antibody-dependent HIV vaccine might be possible, given the discovery of HIV broadly neutralizing antibodies (bnAbs) in some HIV-infected individuals. However, these antibodies are difficult to develop and have characteristics indicative of a high degree of affinity maturation in germinal centers (GCs). CD4+ T follicular helper (Tfh) cells are specialized for B cell help and necessary for GCs. Therefore, the development of HIV bnAbs might depend on Tfh cells. Here, we identified in normal individuals a subpopulation of circulating memory PD-1+CXCR5+CD4+ T cells that are resting memory cells most related to bona fide GC Tfh cells by gene expression profile, cytokine profile, and functional properties. Importantly, the frequency of these cells correlated with the development of bnAbs against HIV in a large cohort of HIV+ individuals.

Lack of Efficacy and Safety Concerns Blight Adenovirus Vaccine Vectors

Vaccine vectors based on adenoviruses—which are common in nature and cause severe colds—were once viewed as promising for HIV and other intractable diseases due to their ability to induce antigen-specific CD8 T cell responses in the majority of recipients. The subsequent Icarus-like trajectory of the approach in HIV has been covered in some detail on this blog, starting with the results of the first efficacy trial of Merck’s now-discontinued adenovirus serotype 5 (Ad5) candidate (the Step study), which showed that the vaccine significantly increased the risk of HIV acquisition and did not significantly lower viral load or preserve CD4 T cell counts in participants who became infected.

The HIV vaccine section of TAG’s new 2013 Pipeline Report covers the latest round of grim news, which emerged earlier this year with the announcement that immunizations in the lone ongoing HIV vaccine efficacy trial, HVTN 505, were being stopped at the recommendation of the Data Safety Monitoring Board (DSMB). An interim analyses conducted by the DSMB on April 22, 2013 found that the vaccine regimen—comprised of three priming immunizations with a DNA vaccine and a booster shot with an Ad5 vector (both designed by the NIH’s Vaccine Research Center)—did not show efficacy either in terms of preventing HIV infection or lowering post-infection viral load set points, and there was no possibility of efficacy being demonstrated if the trial ran to completion. The total number of infections was greater in the vaccine arm compared to placebo (41 vs. 30), but many of these infections occurred prior to the administration of the Ad5 boost and the difference was not statistically significant. Post-boost, there were 27 infections in the vaccine arm and 21 in the placebo arm.

On the heels of this announcement came word that extended follow up from the second efficacy trial of Merck’s Ad5-based HIV vaccine, the Phambili trial in South Africa, had documented significantly more infections among vaccine recipients compared to placebo recipients. The data were described by principal investigator Glenda Gray at the HIV Vaccine Trials Network meeting in Washington DC on May 7th, and reported by Jon Cohen in Science on May 10th. Although it went largely unnoticed at the time, on May 14th the National Institute of Allergy and Infectious Diseases (NIAID) issued a bulletin reporting that regulatory authorities had been notified of the Phambili findings. The bulletin notes that while the initial published analysis of the trial results did not reveal significant differences between vaccine and placebo arms, “in the current analysis of the full 3.5 year follow-up period, the researchers found that 100 study participants became HIV-infected: 63 participants who received the investigational vaccine and 37 participants who received a placebo injection. The increased number of HIV infections among the vaccinated recipients was greatest among men and more pronounced roughly 30 months after initial vaccination.” There are some potential caveats: the additional infections occurred after the study was unblinded, and there was a substantial drop-out rate that was slightly greater in the placebo arm. Also, in contrast to the Step trial where the enhanced HIV risk has been reported to have waned over time, the increased rate of infection occurred late (>2 years post vaccination). 

The accumulation of negative data has left a cloud of suspicion hanging over all investigational adenovirus-based vaccines, including those based on alternate human serotypes to Ad5 and chimpanzee-derived adenovirus serotypes. The mechanism of the apparent enhancement of HIV acquisition risk in the Step and Phambili trials remains unclear. Importantly, it is not yet known if the adenovirus vectors were somehow solely responsible or if the effect was interlinked with the immune responses to HIV that were induced by the constructs (as one small but as yet unconfirmed macaque study has suggested). Complicating matters further, evidence from the Step study indicated that the degree of pre-existing immunity to natural Ad5 (as measured by antibody titers) was correlated with the magnitude of the vaccine-associated enhancement of HIV risk. In the absence of clarity regarding these issues, there remains the worrying possibility that experimental adenovirus-based vaccines for other diseases (such as TB, malaria, and hepatitis C) could place recipients at increased risk of becoming HIV infected if they are exposed to the virus.

Currently the various stakeholders in the development of adenovirus-based vaccine candidates, including major sponsors such as NIAID and the International AIDS Vaccine Initiative (IAVI), are conducting meetings to discuss the new findings and their implications for the future. It is not yet known if planned or ongoing trials will be affected. In a related development—as reported by Mike Frick in the TB vaccine section of TAG’s Pipeline Report—plans for a large efficacy trial of an Ad35-based TB vaccine candidate have been significantly scaled back due to the observation that the approach is less immunogenic than preliminary results had suggested.

Countervailing Selective Forces Shape HIV

It’s well documented that T-cell immune responses targeting HIV can cause selection of viral variants that evade recognition (immune escape). But some studies have reported that, surprisingly, HIV variability in regions targeted by T cells is lower compared to other locations in the genome, a phenomenon not seen with other RNA viruses. In a paper published last week in PLoS Biology, Rafael Sanjuán and colleagues offer a potential explanation. Based on mathematical modeling studies, they suggest that selection may favor the conservation of parts of HIV that efficiently activate CD4 T cells, because this provides the virus with target cells in which to replicate. This suggestion is consistent with evidence that HIV itself is often a major driver of immune activation (e.g. activation precipitously declines when HIV replication is suppressed by therapy), and with the finding that HIV preferentially infects HIV-specific CD4 T cells. The theory also fits with recent data showing that SIV can cause immune activation and progression to simian AIDS in the absence of any contribution from microbial translocation. 

The overall message is that while effective CD8 T-cell responses can drive the development of immune escape mutations and thus promote viral variation, HIV’s need to activate CD4 T-cell targets may act as a countervailing force selecting for conservation. The strength of the selective pressure favoring conservation, Sanjuán and colleagues note, will depend on the extent to which other stimuli—such as co-infections and/or microbial translocation—are contributing to immune activation (the greater their contribution, the less HIV needs to activate CD4 T cells itself).

One implication of the study, the researchers argue, is that vaccines that induce HIV-specific CD4 T-cell responses might create conditions favorable to viral replication. They suggest that in order to provide the CD4 T-cell help required to generate functional CD8 T-cell responses, non-HIV antigens that activate CD4 T cells should be included in vaccines. However, the model does not distinguish between the different types of CD4 T cell that may respond to HIV antigens (e.g. naïve or the many possible types of memory cells) and an alternative solution to this conundrum may be to try to design vaccines that create HIV-specific memory CD4 T cells that are relatively resistant to infection. Evidence that CMV-specific specific memory CD4 T cells are highly resistant to HIV suggests that this might be possible.

PLoS Biol 11(4): e1001523. doi:10.1371/journal.pbio.1001523

Immune Activation Promotes Evolutionary Conservation of T-Cell Epitopes in HIV-1

Rafael Sanjuán, Miguel R. Nebot, Joan B. Peris, José Alcamí

Abstract

The immune system should constitute a strong selective pressure promoting viral genetic diversity and evolution. However, HIV shows lower sequence variability at T-cell epitopes than elsewhere in the genome, in contrast with other human RNA viruses. Here, we propose that epitope conservation is a consequence of the particular interactions established between HIV and the immune system. On one hand, epitope recognition triggers an anti-HIV response mediated by cytotoxic T-lymphocytes (CTLs), but on the other hand, activation of CD4+ helper T lymphocytes (TH cells) promotes HIV replication. Mathematical modeling of these opposite selective forces revealed that selection at the intrapatient level can promote either T-cell epitope conservation or escape. We predict greater conservation for epitopes contributing significantly to total immune activation levels (immunodominance), and when THcell infection is concomitant to epitope recognition (trans-infection). We suggest that HIV-driven immune activation in the lymph nodes during the chronic stage of the disease may offer a favorable scenario for epitope conservation. Our results also support the view that some pathogens draw benefits from the immune response and suggest that vaccination strategies based on conserved TH epitopes may be counterproductive.

PLoS Biol 11(4): e1001521. doi:10.1371/journal.pbio.1001521

HIV Plays (and Wins) a Game of T Cell Brinkmanship

Roland G. Roberts

Viruses are locked into an adaptive arms race with the host immune system: the immune system adapts to recognize the virus, the virus adapts to evade the immune system, the cycle repeats. But in the case of HIV, it seems, sometimes it's good to be recognized. 

Can a Route to Broadly Neutralizing Antibodies Be Traced?

In recent years, new technologies have facilitated the discovery of an expanding number of antibodies capable of neutralizing a broad array of primary HIV isolates from different clades. As covered previously on the blog, these broadly neutralizing antibodies (bNAbs) have been fished from the plasma of individuals with chronic HIV infection and, in most cases, do not seem to be present at titers sufficient to control viral load or retard disease progression; however, there are reasons to hope that if similar antibodies could be induced by vaccination, they could rebuff the relatively small amount of HIV that enters the body during a typical exposure.

A common feature of the bNAbs is that the B cells that produce them have gone through many more rounds of somatic hypermutation than is typically seen in other infections. Somatic hypermutation is the process by which the B cell’s antibody-producing genetic code is progressively revised, potentially leading to an increase in the affinity of the antibody for its target. The genetic code that the B cell starts out with is known as the germline sequence (or unmutated common ancestor or UCA), and it is typically altered by around 5–15% to produce antibodies against common infections, whereas the range is 19–46% for the bNAbs against HIV. This requirement for extensive mutation appears to be connected to the unusual shapes the bNAbs must form to access the hard-to-reach conserved areas of the HIV envelope (Env) protein, which are cloaked by highly variable decoy targets.

In a paper published yesterday in the journal Nature, researchers report tracking the development of a bNAb response in an HIV-positive person, in parallel with documenting the evolution of the infecting virus. The study shows that the Env protein of the virus at the time of acute infection was able to activate B cells with a germline sequence that then underwent progressive somatic hypermation, leading to the appearance of antibodies with increasing breadth of activity against a panel of HIV isolates during weeks 41-92 of follow-up. Driving the B-cell somatic hypermutation process was stimulation of the cells by the ever-mutating Env protein of the infecting virus, which evolved and became more diverse over time (as is typical in untreated HIV infection). The researchers were able to demonstrate that the diversification of the Env protein preceded the appearance of bNAb response.

This brief description greatly simplifies a complicated study, but the implication for HIV vaccines is that it may be possible to try and mimic the process observed in this individual using sequential immunization with vaccines containing similar Env proteins of increasing diversity. The hope would be to initially activate the right B cell, and then push it along a somatic hyermutation pathway that would lead to the eventual generation of bNAbs.

Whether this is actually feasible, however, remains to be seen. Because there is a degree of randomness involved, it may be that the relatively rare individuals who develop bNAbs represent instances of B cells essentially hitting the somatic hypermutation jackpot as a result of repeated stimulation. But, given the implications for HIV vaccines if bNAbs could be successfully induced with some reliability, it will be essential to fully pursue the idea. In addition to the Nature paper, several other recently published studies report data relevant to this pursuit (abstracts and links also appended below).

Nature (2013) doi:10.1038/nature12053

Co-evolution of a broadly neutralizing HIV-1 antibody and founder virus

Hua-Xin Liao, Rebecca Lynch, Tongqing Zhou, Feng Gao, S. Munir Alam, Scott D. Boyd, Andrew Z. Fire, Krishna M. Roskin, Chaim A. Schramm, Zhenhai Zhang, Jiang Zhu, Lawrence Shapiro, NISC Comparative Sequencing Program, James C. Mullikin, S. Gnanakaran, Peter Hraber, Kevin Wiehe, Garnett Kelsoe, Guang Yang, Shi-Mao Xia, David C. Montefiori, Robert Parks, Krissey E. Lloyd, Richard M. Scearce, Kelly A. Soderberg, Myron Cohen, Gift Kamanga, Mark K. Louder, Lillian M. Tran, Yue Chen, Fangping Cai, Sheri Chen, Stephanie Moquin, Xiulian Du, M. Gordon Joyce, Sanjay Srivatsan, Baoshan Zhang, Anqi Zheng, George M. Shaw, Beatrice H. Hahn, Thomas B. Kepler, Bette T. M. Korber, Peter D. Kwong, John R. Mascola & Barton F. Haynes

ABSTRACT

Current human immunodeficiency virus-1 (HIV-1) vaccines elicit strain-specific neutralizing antibodies. However, cross-reactive neutralizing antibodies arise in approximately 20% of HIV-1-infected individuals, and details of their generation could provide a blueprint for effective vaccination. Here we report the isolation, evolution and structure of a broadly neutralizing antibody from an African donor followed from the time of infection. The mature antibody, CH103, neutralized approximately 55% of HIV-1 isolates, and its co-crystal structure with the HIV-1 envelope protein gp120 revealed a new loop-based mechanism of CD4-binding-site recognition. Virus and antibody gene sequencing revealed concomitant virus evolution and antibody maturation. Notably, the unmutated common ancestor of the CH103 lineage avidly bound the transmitted/founder HIV-1 envelope glycoprotein, and evolution of antibody neutralization breadth was preceded by extensive viral diversification in and near the CH103 epitope. These data determine the viral and antibody evolution leading to induction of a lineage of HIV-1 broadly neutralizing antibodies, and provide insights into strategies to elicit similar antibodies by vaccination.

Nature (2013) doi:10.1038/nature12091

HIV: Roadmaps to a vaccine

Hugo Mouquet and Michel C. Nussenzweig

More than 30 years since the AIDS pandemic began, there is still no effective vaccine. But analysis of broadly acting, potent human antibodies obtained from single cells suggests a rational approach to vaccine development.

Science. 2013 Mar 29. [Epub ahead of print]

Rational HIV Immunogen Design to Target Specific Germline B Cell Receptors.

Jardine J, Julien JP, Menis S, Ota T, Kalyuzhniy O, McGuire A, Sok D, Huang PS, Macpherson S, Jones M, Nieusma T, Mathison J, Baker D, Ward AB, Burton DR, Stamatatos L, Nemazee D, Wilson IA, Schief WR.

Department of Immunology and Microbial Science, The Scripps Research Institute, La Jolla, CA 92037, USA.

ABSTRACT

Vaccine development to induce broadly neutralizing antibodies (bNAbs) against HIV-1 is a global health priority. Potent VRC01-class bNAbs against the CD4 binding site of HIV gp120 have been isolated from HIV-1-infected individuals; however, such bNAbs have not been induced by vaccination. Wild-type gp120 proteins lack detectable affinity for predicted germline precursors of VRC01-class bNAbs, making them poor immunogens to prime a VRC01-class response. We employed computation-guided, in vitro screening to engineer a germline-targeting gp120 outer domain immunogen that binds to multiple VRC01-class bNAbs and germline precursors and elucidated germline-binding crystallographically. When multimerized on nanoparticles, this immunogen (eOD-GT6) activates germline and mature VRC01-class B cells. Thus, eOD-GT6 nanoparticles have promise as a vaccine prime. In principle, germline-targeting strategies could be applied to other epitopes and pathogens.

J Exp Med. 2013 Mar 27. [Epub ahead of print]

Engineering HIV envelope protein to activate germline B cell receptors of broadly neutralizing anti-CD4 binding site antibodies.

McGuire AT, Hoot S, Dreyer AM, Lippy A, Stuart A, Cohen KW, Jardine J, Menis S, Scheid JF, West AP, Schief WR, Stamatatos L.

Seattle Biomedical Research Institute, Seattle, WA 98109.

ABSTRACT

Broadly neutralizing antibodies (bnAbs) against HIV are believed to be a critical component of the protective responses elicited by an effective HIV vaccine. Neutralizing antibodies against the evolutionarily conserved CD4-binding site (CD4-BS) on the HIV envelope glycoprotein (Env) are capable of inhibiting infection of diverse HIV strains, and have been isolated from HIV-infected individuals. Despite the presence of anti-CD4-BS broadly neutralizing antibody (bnAb) epitopes on recombinant Env, Env immunization has so far failed to elicit such antibodies. Here, we show that Env immunogens fail to engage the germline-reverted forms of known bnAbs that target the CD4-BS. However, we found that the elimination of a conserved glycosylation site located in Loop D and two glycosylation sites located in variable region 5 of Env allows Env-binding to, and activation of, B cells expressing the germline-reverted BCRs of two potent broadly neutralizing antibodies, VRC01 and NIH45-46. Our results offer a possible explanation as to why Env immunogens have been ineffective in stimulating the production of such bNAbs. Importantly, they provide key information as to how such immunogens can be engineered to initiate the process of antibody-affinity maturation against one of the most conserved Env regions.

Proc Natl Acad Sci U S A. 2013 Mar 22. [Epub ahead of print]

Structural basis for HIV-1 gp120 recognition by a germ-line version of a broadly neutralizing antibody. (free full text)

Scharf L, West AP Jr, Gao H, Lee T, Scheid JF, Nussenzweig MC, Bjorkman PJ, Diskin R.

Division of Biology, California Institute of Technology, Pasadena, CA 91125.

ABSTRACT

Efforts to design an effective antibody-based vaccine against HIV-1 would benefit from understanding how germ-line B-cell receptors (BCRs) recognize the HIV-1 gp120/gp41 envelope spike. Potent VRC01-like (PVL) HIV-1 antibodies derived from the VH1-2*02 germ-line allele target the conserved CD4 binding site on gp120. A bottleneck for design of immunogens capable of eliciting PVL antibodies is that VH1-2*02 germ-line BCR interactions with gp120 are uncharacterized. Here, we report the structure of a VH1-2*02 germ-line antibody alone and a germ-line heavy-chain/mature light-chain chimeric antibody complexed with HIV-1 gp120. VH1-2*02 residues make extensive contacts with the gp120 outer domain, including all PVL signature and CD4 mimicry interactions, but not critical CDRH3 contacts with the gp120 inner domain and bridging sheet that are responsible for the improved potency of NIH45-46 over closely related clonal variants, such as VRC01. Our results provide insight into initial recognition of HIV-1 by VH1-2*02 germ-line BCRs and may facilitate the design of immunogens tailored to engage and stimulate broad and potent CD4 binding site antibodies.

Cell. 2013 Mar 28;153(1):126-38. doi: 10.1016/j.cell.2013.03.018.

Somatic Mutations of the Immunoglobulin Framework Are Generally Required for Broad and Potent HIV-1 Neutralization.

Klein F, Diskin R, Scheid JF, Gaebler C, Mouquet H, Georgiev IS, Pancera M, Zhou T, Incesu RB, Fu BZ, Gnanapragasam PN, Oliveira TY, Seaman MS, Kwong PD, Bjorkman PJ, Nussenzweig MC.

Laboratory of Molecular Immunology, The Rockefeller University, New York, NY 10065, USA.

ABSTRACT

Broadly neutralizing antibodies (bNAbs) to HIV-1 can prevent infection and are therefore of great importance for HIV-1 vaccine design. Notably, bNAbs are highly somatically mutated and generated by a fraction of HIV-1-infected individuals several years after infection. Antibodies typically accumulate mutations in the complementarity determining region (CDR) loops, which usually contact the antigen. The CDR loops are scaffolded by canonical framework regions (FWRs) that are both resistant to and less tolerant of mutations. Here, we report that in contrast to most antibodies, including those with limited HIV-1 neutralizing activity, most bNAbs require somatic mutations in their FWRs. Structural and functional analyses reveal that somatic mutations in FWR residues enhance breadth and potency by providing increased flexibility and/or direct antigen contact. Thus, in bNAbs, FWRs play an essential role beyond scaffolding the CDR loops and their unusual contribution to potency and breadth should be considered in HIV-1 vaccine design.

Selected Research Highlights from CROI 2013

Coverage of some subjectively selected presentations from the recent CROI meeting in Atlanta, in addition to the prior post on the case report of a potentially functionally cured infant.

Gene Modification of Virus-Specific CD4 T Cells 

Patrick Younan from the Fred Hutchinson Cancer Research Center delivered an interesting talk about transplantation of gene-modified stem cells in pigtailed macaques (abstract, webcast—third in session). The experiment used lentiviral vectors to deliver the gene for a virus entry inhibitor, C46 (also known as M87o, it has a similar mechanism of action to the approved antiretroviral Fuzeon), into stem cells along with a green fluorescent protein (GFP) marker to make the modified cells identifiable. Four macaques underwent stem cell transplantation, with two given C46-modified cells and two controls given cells with only the GFP marker. All were subsequently challenged with the dual-tropic SIV/HIV hybrid virus SHIV89.6P. The two controls displayed typical rapid CD4 T-cell loss, and one was euthanized at week 32 due to the onset of simian AIDS. In contrast, recipients of the modified cells recovered CD4 T-cell counts after an initial dip and showed significantly lower viral loads. Levels of gene-modified CD4 T cells peaked at around 90% during acute infection, but subsequently declined to pre-challenge levels (around 20% in one animal, 55% in the other) during follow-up. Despite the decline in modified CD4 T cells, levels of unmodified cells improved over time, suggesting a protective effect of the intervention on the overall CD4 T-cell pool.

Further studies revealed that superior SHIV-specific CD4 T-cell and antibody responses were associated with the salutary outcome. SHIV-specific CD4 T cells were not detectable in either of the controls. Younan found that a striking 85% of the SHIV-specific CD4 T cells in treated animals were gene-modified, suggesting that protection of these cells had allowed them to better perform their role of providing help to B cells and CD8 T cells. One potentially encouraging implication of this work is that gene therapy approaches might not have to protect all susceptible cells from HIV infection in order to offer benefit; if sufficient numbers of HIV-specific CD4 T cells can be protected, it is possible that these cells will do a better job of coordinating the immune response against HIV, leading to improved control of viral replication. This possibility is being investigated in ongoing trials of Sangamo BioSciences SB-728-T gene therapy, which aims to protect CD4 T cells by abrogating expression of the CCR5 coreceptor. 

Engineering SIV to Spare the CD4 T-Cell Help

Adrienne Swanstrom from the University of Pennsylvania described a novel approach to defining the role of infection of CD4 T cells in SIV pathogenesis, using an engineered version of the highly pathogenic SIVmac239—named iMac-ΔD385—that does not bind the CD4 molecule (abstract, webcast—seventh in session). In a preliminary experiment involving just two macaques, Swanstrom found that the modified virus replicated to levels comparable to the wild-type SIVmac239 during acute infection and targeted a variety of non-CD4 cell types, but was then robustly controlled by the immune response. Strong neutralizing antibody responses were detected, which is unusual in pathogenic SIV infection, and ongoing work is now looking at CD8 T-cell responses. The findings, at least so far, suggest that sparing CD4 T-cell help led to more effective CD4 T cell–dependent immune responses and superior control of SIV replication. 

CMV Vector Vaccination Leads to Apparent Clearance of SIV Infection

Louis Picker from the Vaccine and Gene Therapy Institute at Oregon Health & Science University gave an update on results obtained in macaques with a CMV-based vaccine against SIV (abstract, webcast—fourth in session). As Picker has shown in published work, the vaccine consistently facilitates strict control of a pathogenic SIVmac239 challenge in around 50% of immunized macaques. The remarkable news shared by Picker at CROI is that, over time, these protected animals appear to clear SIV infection. This claim is based on multiple criteria, including loss of detectable virus, waning of CD8 T-cell responses to viral antigens not included in the vaccine, and the failure to transmit infection to uninfected macaques despite transfer of over 50 million blood and/or tissue white blood cells (in contrast, similar transfers from elite controller animals or those on suppressive ART reliably transmitted infection). Picker noted that the CD8 T-cell responses in the vaccine recipients appear to “violate all the rules” in that they target very large numbers of different SIV epitopes and, in many cases, their ability to recognize the virus involves MHC class II molecules, which normally facilitate antigen recognition by CD4 T cells rather than CD8 T cells. Picker is now working to shed further light on these findings, as well as collaborating with the Vaccine and Gene Therapy Institute of Florida to try and develop a CMV vaccine vector that can safely be studied in humans.

Results from Multidose Vorinostat Trial

Sharon Lewin from Monash University in Melbourne debuted data from the first multidose trial of vorinostat (also known as SAHA) as a potential anti-HIV latency drug (abstract, webcast—ninth in session). Consistent with results published by David Margolis from a single-dose trial, expression of HIV RNA significantly increased among the twenty participants after 14 days of vorinostat. However, there was no evidence for reduction in the size of the latent HIV reservoir, suggesting that additional approaches will be needed to facilitate the elimination of the infected cells. As expected, vorinostat side effects were more prevalent as a result of the multiple dosing, particularly fatigue and lethargy.

Curing HIV Removes the Scars of the Past

One small but encouraging piece of news about the lone adult considered cured of HIV, Timothy Brown, was to be found in a presentation by Joyce Sanchez from the University of Minnesota (abstract, webcast—second in session). Sanchez’s study focused on lymphoid structure abnormalities in people with HIV, particularly fibrotic (scarring) damage to lymph tissue resulting from persistent HIV replication and associated immune activation. The extent of lymph tissue fibrosis can be quantified by measuring collagen deposition using imaging techniques. Sanchez showed that in gut lymph tissue, even HIV controllers (individuals with low viral loads in the absence of ART) have levels of collagen deposition that are higher than those of uninfected individuals (15.9% compared with 7%). However, samples from Timothy Brown showed levels of collagen deposition comparable to the uninfected study participants (6.8%), consistent with studies showing no HIV activity in his body (despite the occasional detection of viral genetic material that was reported last year).  

CD4 T Cells Specific for Different Pathogens Vary in Susceptibility to HIV Infection

When it comes to susceptibility to HIV infection, not all CD4 T cells are created equal. There are a variety of factors that have been shown to influence how readily HIV gains entry and replicates, with the best-described distinction being between activated CD4 T cells, which are highly susceptible, and resting CD4 T cells, which are relatively resistant. The particular types of cytokines and chemokines that a CD4 T cell makes have also been shown to influence the efficiency of HIV infection. Less well understood is the influence of antigen specificity—the pathogen being targeted by the CD4 T cell. Evidence has been published showing that TB-specific CD4 T cells are highly susceptible, whereas those responding to the viral infection CMV are relatively resistant (with this resistance being associated with the release of beta-chemokines capable of binding the CCR5 receptor and blocking HIV entry). A study by Haitao Hu and colleagues from the U.S. Military HIV Research Program has now explored this issue further by looking at the susceptibility of various pathogen-specific CD4 T cells and the relationship with the genes that the different cells express.

The results confirm that CMV-specific CD4 T cells are highly resistant to HIV, but not just due to the release of beta-chemokines: gene expression analyses revealed that the cells also upregulate several innate antiviral factors such as IFIT1 (a protein that recognizes a particular form of viral RNA). Because of these factors, CMV-specific CD4 T cells displayed reduced susceptibility to HIV infection even when the researchers used antibodies to neutralize any effect of beta-chemokines. In contrast, CD4 T cells specific for tetanus toxoid (TT) and Candida exhibited a pro-inflammatory Th17-type profile and were highly permissive to HIV infection. The researchers suggest that these differences in susceptibility may contribute to the well-documented relationship between differing levels of immune deficiency and risk of specific opportunistic infections; i.e., candidiasis is typically an early sign of immune deficiency whereas active CMV disease occurs almost exclusively at extremely low CD4 T-cell levels.

Another important implication of the study is that HIV vaccines should aim to induce HIV-specific CD4 T cells with a profile that resembles CMV-specific responses. The researchers speculate that there may be a connection between their observations and the fact that the best results obtained to date in the stringent SIV/macaque model of vaccination have involved a CMV-based vaccine vector. Several research groups—including those of Louis Picker at the Oregon Health & Science University and Rafick-Pierre Sékaly at the Vaccine & Gene Therapy Institute of Florida—are now working to develop CMV-based HIV vaccine vectors with the aim of conducting human studies in both the therapeutic and preventive contexts.

Blood. Published online before print, December 20, 2012. doi: 10.1182/blood-2012-07-446278

Distinct gene expression profiles associated with the susceptibility of pathogen-specific CD4 T cells to HIV-1 infection

Haitao Hu1, Martin Nau1, Phil Ehrenberg1, Agnes-Laurence Chenine1, Camila Macedo1, Yu Zhou2, Z. John Daye3, Zhi Wei4, Maryanne Vahey1, Nelson L Michael1, Jerome H Kim1, Mary Marovich1, and Silvia Ratto-Kim1

1 US Military HIV Research Program, Walter Reed Army Institute of Research, Silver Spring, MD, United States;

2 Department of Pathology and Laboratory Medicine, Temple University, Philadelphia, PA, United States;

3 Division of Epidemiology and Biostatistics, University of Arizona Mel and Enid Zuckerman College of Public Health, Tucson, AZ, United States;

4 Department of Computer Science, New Jersey Institute of Technology, Newark, NJ, United States

ABSTRACT

In HIV infection, the CD4 responses to opportunistic pathogens like Candida albicans are lost early, but cytomegalovirus (CMV)-specific CD4 response persists. Little is currently known about HIV infection of CD4 T cells of different pathogen/antigen specificity. CFSE-labeled PBMC were stimulated with CMV, Tetanus Toxoid (TT) and Candida antigens and subsequently exposed to HIV. HIV infection was monitored by intracellular p24 in CFSE-low population. We showed that while TT- and Candida-specific CD4 T cells were permissive, CMV-specific CD4 T cells were highly resistant to both R5 and X4 HIV. Quantification of HIV DNA in CFSE-low cells showed a reduction of strong-stop and full-length DNA in CMV-specific cells compared to TT- and Candida-specific cells. β-chemokine neutralization enhanced HIV entry and infection in TT- and Candida-specific cells, whereas HIV infection in CMV-specific cells remained low despite increased HIV entry by β-chemokine neutralization, suggesting post-entry restriction of HIV replication by CMV-specific cells. Microarray analysis (GEO Accession Number: GSE42853) revealed distinct transcriptional profiles that involved selective upregulation of comprehensive innate antiviral genes in CMV-specific cells, whereas TT- and Candida-specific cells mainly upregulated Th17 inflammatory response. Our data suggest a mechanism for persistence of CMV-specific CD4 response and earlier loss of mucosal Th17-associated TT- and Candida-specific CD4 response in AIDS.

David Baltimore and Vectored Immunoprophylaxis

In November 2011, the laboratory of Nobel laureate David Baltimore published encouraging results from humanized mouse studies of an approach to HIV prevention they have named vectored immunoprophylaxis (assigned the slightly uncomfortable acronym VIP). The strategy involves the use of adeno-associated virus (AAV) as a vector to deliver genes that make broadly neutralizing antibodies against HIV after delivery into muscle tissue. The ultimate goal is to make an end-run around the well-documented challenges associated with designing a vaccine capable of persuading the human immune system to generate broadly neutralizing antibodies. Instead, AAV would serve as a factory for the production of these antibodies, a task to which it is thought to be well-suited because it can persist for very long periods in the episome of cells. The idea was first developed by Phillip Johnson at the Children’s Hospital of Philadelphia, who is collaborating with the International AIDS Vaccine Initiative (IAVI) to launch phase I human testing of an AAV encoding the broadly neutralizing antibody PG9 (listed as AAV1-PG9 on the IAVI portfolio page). Johnson’s preclinical research in the SIV/macaque model has been covered previously on the blog.

Baltimore’s VIP work is now receiving renewed attention due to the posting of a video by Cara Santa Maria, who runs the appealingly named “Talk Nerdy to Me” science blog on Huffington Post. Inadvertently, the distillation of the research into a short-video-interview format suggests a narrative in which the VIP idea is unprecedented and unique, created by Baltimore and the scientists in his lab. Baltimore’s opening statement, “no one has ever tried this before,” is true in that VIP has yet to be tried in humans, but it risks glossing over the contributions of Johnson and many others who have long been wrestling with the complex issues involved in attempting to develop the technology for clinical testing. In essence, the approach is a gene therapy intended for healthy HIV-negative people, and it is very likely—and entirely appropriate—that plans for human trials will be closely scrutinized by regulators prior to approval. 

Beyond just safety, there are other issues pertaining the use of AAV as a vector that the video does not explore. A study published in Nature Medicine in 2007 demonstrated that humans—unlike mice or monkeys—typically possess AAV-specific CD8 T-cell responses that target the virus for elimination. These CD8 T-cell responses are thought to have contributed to the poor performance (and in some instances, toxicity) of AAV as a gene transfer vehicle in human trials for other conditions, such as hemophilia B. The presence in humans of AAV-specific CD8 T cells may also explain why IAVI-sponsored trials of AAV as a traditional vaccine vector found that it was poorly immunogenic (only around 20% of those receiving the highest dose showed low-level immune responses to the HIV antigens encoded by the AAV-based vaccine—starkly different from the promising-looking immunogenicity seen in macaques).

In their November 2011 Nature paper, David Baltimore and colleagues note that the AAV serotype they have chosen to work with, AAV8, has some differences compared to the more commonly used AAV2. They write:

"Studies in non-human primates have shown that the elicitation of capsid-specific cytotoxic T-lymphocytes is limited to AAV capsids that exhibit heparin-binding activity [26]. Interestingly, serotypes lacking heparin-binding activity, including AAV8, did not induce CTL responses, suggesting that AAV8-based vectors, like the one we have used, may circumvent previously observed immunological barriers to long-term transduction." 

However, they fail to cite the 2007 paper mentioned above, which is explicit that: “alternative serotypes currently being considered for clinical applications (for example, AAV-1, AAV-8) are processed in a manner appropriate for presentation of the same epitope(s), at least by human cells, resulting in expansion of functional CD8+ T cells that are indistinguishable from those elicited by AAV-2 capsid.”  

So far it appears that the only clue as to how this might play out in humans comes from a study of a hybrid AAV vector comprising the backbone of AAV2 with an AAV8 capsid, used to deliver the gene for factor IX. In a small clinical trial for the treatment of hemophilia B in which the vector was given via peripheral vein (to target the liver), encouraging evidence of therapeutic efficacy was observed, which was not the case in a prior trial involving a vector consisting entirely of AAV2. But most participants did show some evidence of T cell responses against the AAV8 capsid, indicating that the hope that this serotype would be completely invisible to CD8 T cells has not borne out. 

Thankfully though, that is not the end of the story. A study published just a couple of days ago in the journal Blood suggests that additional modifications to AAV can significantly reduce CD8 T cell recognition. And last year in the same journal, researchers reported that the AAV2 vector used to deliver the gene for factor IX in the first hemophilia B clinical trial could still be detected in muscle tissue ten years after administration. Although the level of factor IX production achieved in this trial was too low to be therapeutic, the authors conclude: “these data do establish that AAV2-mediated gene transfer to human skeletal muscle can persist for up to a decade. The use of alternate serotypes and/or delivery techniques that can perfuse muscle more extensively may improve therapeutic efficacy for this approach." This conclusion also offers hope for VIP because, as David Baltimore stresses in the video interview, his lab’s humanized mice experiments suggest that protection against HIV infection might be achievable with relatively low levels of broadly neutralizing antibodies.