HIV / AIDS Update: Prevention, Treatment and Beyond

Journal Articles

Dan H. Barouch

Barouch DH, Kik SV, Weverling GJ, et al. International seroepidemiology of adenovirus serotypes 5, 26, 35, and 48 in pediatric and adult populations. Vaccine 2011;29(32):5203-5209.

Barouch DH, Liu J, Li H, et al. Vaccine protection against acquisition of neutralization-resistant SIV challenges in rhesus monkeys. Nature 2012;482(7383):89-93.

Barouch DH, O'Brien KL, Simmons NL, et al. Mosaic HIV-1 vaccines expand the breadth and depth of cellular immune responses in rhesus monkeys. Nat. Med. 2010;16(3):319-323.

Timothy Cardozo

Gorny MK, Sampson J, Li H, et al. Human anti-V3 HIV-1 monoclonal antibodies encoded by the VH5-51/VL lambda genes define a conserved antigenic structure. PLoS One 2011; 6(12):e27780.

Nicolas Chomont

Chomont N, El-Far M, Ancuta P, et al. HIV reservoir size and persistence are driven by T cell survival and homeostatic proliferation. Nat. Med. 2009;15(8): 893-900.

Donald Forthal

Forthal DN, Gilbert PB, Landucci G, et al. Recombinant gp120 vaccine-induced antibodies inhibit clinical strains of HIV-1 in the presence of Fc receptor-bearing effector cells and correlate inversely with HIV infection rate. J. Immunol. 2007;178(10)6596-6603.

Forthal DN, Landucci G, Cole KS, et al. Rhesus macaque polyclonal and monoclonal antibodies inhibit simian immunodeficiency virus in the presence of human or autologous rhesus effector cells. J. Virol. 2006;80(18):9217-9225.

Hessell AJ, Hangartner L, Hunter M, et al. Fc receptor but not complement binding is important in antibody protection against HIV. Nature 2007;449(7158):101-104.

Liu J, O'Brien KL, Lynch DM, et al. Immune control of an SIV challenge by a T-cell-based vaccine in rhesus monkeys. Nature 2009;457(7225):87-91.

J. Victor Garcia-Martinez

Denton PW and Garcia JV. Mucosal HIV-1 transmission and prevention strategies in BLT humanized mice. Trends Microbiol. 2012; epub ahead of print.

Melkis MW, Estes JD, Padgett-Thomas A, et al. Humanized mice mount specific adaptive and innate immune responses to EBV and TSST-1. Nat. Med. 2006;12(11):1316-1322.

Nelson L. Michael

Rerks-Ngarm S, Pittisutthithum P, Nitayaphan S, et al. Vaccination with ALVAC and AIDSVAX to prevent HIV-1 infection in Thailand. New Engl. J. Med. 361(23):2209-2220.

Christopher J. Miller

Van Rompay KK, Berardi CJ, Dillard-Telm S, et al. Passive immunization of newborn rhesus macaques prevents oral simian immunodeficiency virus infection. J. Infect. Dis. 1998;177(5):1247-1259.

Alan M. Schultz

Winstone N, Wilson AJ, Morrow GE, et al. Enhanced control of pathogenic Simian immunodeficiency virus SIVmac239 replication in macaques immunized with an interleukin-12 plasmid and a DNA prime-viral vector boost vaccine regimen. J. Virol. 2011; 85(18):9578-9587.

Koff WC, Johnson PR, Watkins DI, et al. HIV vaccine design: insights from live attenuated SIV vaccines. Nat. Immunol. 2006;7(1):19-23.

Susan Zolla-Pazner

Almond D, Kimura T, Kong X, et al. Structural conservation predominates over sequence variability in the crown of HIV type 1's V3 loop. AIDS Res. Hum. Retroviruses 2010;26(6):717-723.

Hayes BF, Gilbert PB, McElrath J, et al. Immune-correlates analysis of an HIV-1 vaccine efficacy trial. New Engl. J. Med. 2012; 366(14):1275-1286.

Zolla-Pazner S, Cardozo T. Structure-function relationships of HIV-1 envelope sequence-variable regions refocus vaccine design. Nat. Rev. Immunol. 2010;10(7):527-535.

Zolla-Pazner S, Kong XP, Jiang X, et al. Cross-clade HIV-1 neutralizing antibodies induced with V3-scaffold protein immunogens following priming with gp120 DNA. J. Virol. 2011;85(19):9887-9898.

Additional Resources

Arthos J, Cicala C, Martinello E, et al. HIV-1 envelope protein binds to signals through integrin α4β7, the gut mucosal homing receptor for peripheral T cells. Nat. Immunol. 2008;9(3):301-309.

Chun T-W, Finzi D, Margolick J, et al. In vivo fate of HIV-1 infected T cells: quantitative analysis of the transition to stable latency. Nat. Med. 1995;1(12):1284-1290.

Fischer W, Perkins S, Theiler J, et al. Polyvalent vaccines for optimal coverage of potential T-cell epitopes in global HIV-1 variants. Nat. Med. 2007;13(1):100-106.

Haase AT. Early events in sexual transmission of HIV and SIV and opportunities for interventions. Annual Rev. Med. 2011;62:127-139.

Lohse N, Hansen AB, Pedersen G, et al. Survival of persons with and without HIV infection in Denmark. Ann. Int. Med. 2007;146(2):87-95.

Zak DE, Aderem A. Overcoming limitations in the systems vaccinology approach: a pathway for accelerated HIV vaccine development. Curr. Opin. HIV AIDS 2012;7(1):58-63.

Sponsors

Grant Support

This program is supported by an educational grant from Gilead Sciences, Inc.

Although infection rates are declining, approximately 33 million people worldwide are infected with HIV. Highly Active Antiretroviral Therapy, HAART, has successfully reduced the viral load in patients with HIV infection and has significantly increased both the quality and longevity of life. HAART does not, however, eradicate HIV. Although viral particles in the blood are reduced to below detectable levels, these particles can hide in viral reservoirs so infection rebounds to pre-therapy levels when HAART is discontinued. Overall life expectancy increases with HAART, but it does not reach the level of non-infected individuals; therefore, there remains a strong need to develop curative treatments and to prevent infection. Current research aims to prevent infection by vaccinating to produce an effective neutralizing antibody response, with antibodies binding to invading HIV viral particles and blocking their entry into human cells. If achieved, this would reduce infection rates and early viral load. Obstacles to developing such a vaccine still exist, but the recent success of the RV144 clinical trial provides early promise for employing a vaccine approach.

To date, only three approaches to vaccine development have been tested. A 2003 trial tested the AIDSVAX Env gp120 vaccine in high-risk populations in both the U.S. and Thailand. This vaccine was developed to target a protein subunit of HIV, the envelope protein gp120, to generate type-specific and neutralizing antibodies; however, in clinical trials, only type-specific antibodies were generated—neutralizing antibodies were not—and the vaccine failed to reduce the HIV infection rate. A second trial conducted in 2007, the Step or Merck V520-023 trial, tested an adenoviral vector (Ad5), incorporating HIV gag, pol, or nef proteins, in a population of homosexual men at high risk for HIV infection. The Step vaccine was intended to reduce infection and viral load early on by producing a robust humoral immune (B cell antibody) response to HIV proteins. While this vaccine did produce high antibody titers in some individuals, surprisingly this high level correlated with an increase in the HIV infection rate. Although the reasons underlying this failure are unclear, one hypothesis suggests that the Step vaccine did not include all the necessary HIV protein targets (such as Env): without blocking the proper protein targets, HIV retained its ability to infect human cells. The 2009 RV144 trial combined these two previous approaches, employing a recombinant canarypox vector vaccine (ALVAC-HIV [vCP1521]) with two booster injections of gp120-directed AIDSVAX vaccine administered 6 months later. This combination achieved the first successful HIV vaccine, reducing the infection rate by 31% in at-risk heterosexuals compared to placebo. Further analysis to investigate this vaccine's mode of action is ongoing, but these results are a promising step towards producing an effective vaccine that is capable of reducing the incidence and burden of HIV infection worldwide. Many questions remain, however, regarding the efficacy and compatibility of this and similar vaccines for generalized use.

RV144 HIV vaccine trial results: treating at-risk patients with the ALVAC-HIV [vCP1521]) AIDSVAX dual vaccine combination reduced the rate of HIV infection by 31% over 3.5 years. (Image courtesy of Nelson Michael)

Speakers:
Nelson Michael, Walter Reed Army Institute of Research
Timothy Cardozo, NYU School of Medicine
Donald Forthal, University of California, Davis

Highlights

• The RV144 achieved a 31% decrease in the HIV infection rate employing a combination of two single-agent vaccines.
• Participants with the lowest infection rate exhibited high levels of Env V1V2 loop IgG antibodies, while those with the highest infection rate exhibited high levels of Env IgA antibodies.
• IgG antibodies target the V1/V2 and/or V3 loops of the HIV gp120 env protein.
• Neutralizing antibodies recognize conformational epitopes, but do not recognize sequence epitopes, within the V2 and V3 loops.
• Non-neutralizing antibodies that engage the Fc receptor may play a key role in cellular anti-viral responses.

The RV144 Trial: Updates and steps for the future of HIV vaccine development

Nelson Michael, from the Walter Reed Army Institute of Research, opened the symposium and set the stage to discuss the future of HIV vaccine development. He summarized the recent results of the RV144 clinical trial, also called the Thai trial, conducted in Thailand in 2009 with 16,000 at-risk, HIV-negative participants. This was the first trial to successfully demonstrate that a vaccine can prevent infection, with vaccination resulting in a 31% decrease in infection rate. Limitations to the data do exist: for example, members of this population were infected with only one clade, or subtype, of HIV, raising questions about the general efficacy of this vaccine against other HIV clades. In addition, although the vaccine produced a high initial antibody response, this was not sustained, and antibody titers fell tenfold over a 6-month period. Increasing the longevity of the antibody response may be possible by administering additional boosts, but the feasibility of this approach remains unclear. Another limitation of the current study lies in the types of tissue samples obtained: in the future, additional sample types (e.g., mucosal samples) will be necessary to better determine the mechanisms underlying vaccine action and other components (e.g., genetic) contributing to the success and generalization of a vaccine approach. These findings highlight the need for further investigation into vaccine development.

High IgG antibody titers correlate with low infection rates. (Image courtesy of Nelson Michael)

The discussion then turned to the molecular characteristics of the antibody response. Analysis of blood samples from trial participants revealed that those with high infection rates produced high levels of IgA antibodies, which are primarily found in the nose, eyes, ears, saliva, tears, digestive tract, and vaginal tract, while those with low infection rates produced high levels of IgG antibodies, found in the blood. Although the significance of these high IgA titers is still unclear, it is possible that plasma IgA monomers bind to the HIV env protein and block the neutralizing IgG antibodies from binding. Neutralizing IgG antibodies appear to target the variable region of the HIV gp120 envelope protein—specifically, epitopes within the V2 and V3 loops. Timothy Cardozo and his colleagues from NYU School of Medicine reported on their work with the epitopes and conformational structures in V2 and V3 loops that are crucial for eliciting an effective neutralizing antibody response. Evidently, although these structures contain high sequence variability, the V2 and V3 regions have important structural conservation: both regions form beta folds with highly conserved amino acids, forming a conformational epitope. As a result, chemically-designed antibodies with defined structural motifs may be more effective and efficient than sequence-designed antibodies. Questions about the generalized use of antibodies still exist, but Cardozo suggests that designing antibodies to target the V3 loop may be the most easily applicable approach in broad populations because V3 loops in HIV viral particles, isolated from clades in diverse locations throughout the world, show a high degree of structural overlay (highly conserved structure) and of antibody cross-reactivity.

Emerging data demonstrate that three variable regions within gp120, the V1, V2, and V3 regions, form a trimer association domain (TAD), and that targeting this TAD appears to increase protection from HIV infection, making this configuration a potentially useful vaccine target. It remains challenging, however, to create a neutralizing antibody that is able to access the epitope and maintain specificity while also broadly targeting many clades of HIV. Accessibility and masking are critically important because many of the epitope sites cannot be accessed by antibodies, lowering vaccine efficacy and posing significant challenges to overcome in future vaccine development.

Engaging the FcR may effectively eliminate HIV-infected cells. Binding Fc receptors can induce both phagocytosis and chemokine production, resulting in the elimination of HIV-infected cells. (Image courtesy of Donald Forthal)

In addition to neutralizing IgG antibodies, non-neutralizing antibodies may also play an important role in vaccine responses. Donald Forthal, from the University of California, Davis, discussed a number of studies that indicate that non-neutralizing antibodies, which bind via the constant region, Fc region, to the Fc receptor on effector cells, can reduce viral infectivity. These non-neutralizing antibodies activate the complement system or initiate phagocytosis by mobilizing macrophages, monocytes, natural killer (NK) cells, or dendritic cells (DCs). But in the case of the Vax004 trial with the Env gp120 vaccine, engaging FcγRIIIa receptors of a particular genotype increased the infectivity of HIV; an obviously undesirable outcome.

Forthal closed with a discussion of some unpublished data on the interaction of neonatal Fc receptors (FcRn) with the antibodies produced in response to viral particles. The findings may help explain the poor efficacy of single vaccine approaches.

Speakers:
Christopher Miller, University of California, Davis
Dan Barouch, Beth Israel Deaconess Medical Center, Harvard University
Alan Schultz, National Institute for Allergy and Infectious Disease
J. Victor Garcia-Martinez, University of North Carolina, Chapel Hill

Highlights

• Testing the vaccine used in the Step trial in Rhesus monkey models faithfully reproduces results obtained in humans—including the production of a high IgG titer correlating with a high infectivity rate—helping scientists to understand molecular mechanism underlying human infection.
• Novel adenoviral vectors (Ad35 and Ad26) containing mosaic antigens with common sequences are safe and effective for use in humans.
• SHIVs (simian/human engineered viral particles) are useful tools for understanding human infection, but must be used carefully with clearly defined parameters.
• Bone Marrow Liver Thymic mice (BLT mice), a small, easily manipulated model organism, are useful for testing how human diseases affect the human immune system.

In the second session, attention turned to the development of animal models to accurately mimic human diseases. Presenters discussed simian/human engineered viral particles (SHIVs); Rhesus monkey models used to mimic human infection; and a humanized mouse model created to express a human immune system, which that can be infected with human viruses. Animal models are particularly useful to help answer key questions and to determine viral mechanisms of action before conducting human studies, helping scientists predict human outcomes and uncover unexpected or unwanted side-effects. It is critical, however, to use these models carefully, asking specific questions that will provide highly predictive results and correlate well with human studies.

Rhesus monkeys, a model for studying human disease

Christopher Miller, from the University of California, Davis, began the session discussing a Rhesus monkey model. His data demonstrate that this model faithfully reproduces the biology of HIV infection in humans. When the Ad5 vector used in the Step trial was tested in this model, the results, including the correlation between high Ad5 IgG neutralizing antibody titers and high infection rate, were reproduced. Further molecular analysis of these findings revealed a strong antibody response to the gag protein, but little antibody response to either the pol or nef proteins. These data may help researchers explain why the Step trial Ad5 vector vaccine failed and may provide insight to help identify appropriate antibody targets to limit infection. The model is particularly useful for mimicking human conditions and lifestyles: for example, researchers used it to mimic human sexual practices to test the effect of low-dose, repeated viral exposure.

The discussion turned next to the development of novel vectors and approaches. Dan Barouch, from the Beth Israel Deaconess Medical Center at Harvard University, related how additional vectors produced more desirable clinical responses and reduced HIV infection when he used the Rhesus monkey model to mimic human sexual practices and to test the efficacy of novel adenoviral vectors in the presence of low dose, repeated viral exposure. New adenoviral vectors, Ad35 and Ad26, incorporate mosaic antigens consisting of commonly found gag, pol and env epitope-specific sequences from different HIV-1 clades. These vectors are immunogenic and produce both CD8 and CD4 proteins, triggering cytotoxic T cell (CD8) and T helper cell (CD4) responses. By comparing the physiological responses and gene expression profiles of T cells exposed to different adenovirus vectors, researchers can uncover signature genes that elicit positive responses, leading to novel targets for vaccine development. Adenoviral vectors are considered safe and effective for use in animal models and in first-in-human clinical trials; and these initial clinical trials testing adenoviral vectors in humans are currently underway.

SHIVs: An effective laboratory tool

SHIV vectors—engineered vectors that contain both simian and human viral genes—are also widely used as models to test HIV vaccines. The inclusion of genes from both species allows these vectors to be used in monkey models to help explicate HIV clade diversity and to test vaccine efficacy against human HIV, and makes these models useful for investigating the development and the location of viral reservoirs.

Alan Schultz, from National Institute for Allergy and Infectious Disease, explained that SHIV models are quite useful and effective, provided that appropriate (and carefully considered) experimental questions are asked. It is crucial to consider the clade, neutralization tier, primary viremia intensity, duration of viremia, and persistence of disease, when selecting appropriate SHIV vectors; otherwise the data will be meaningless. SHIV models do not faithfully reproduce the trajectory of human disease; therefore they cannot and should not be used to ask questions about latency or persistent disease. These models can, however, be used to answer questions about the early stages of immune cell infection, to identify resistance, and to develop viral reservoirs. They can also be helpful for testing early steps in HIV infection. More SHIVs are needed to develop better models able to target more HIV clades, including, for example, Clade AE and a larger variety of Clade C.

Humanized mouse model: The latest player

Researchers working to eradicate HIV aim to develop an informative model system to study its infectivity and persistence, and to test novel therapeutics to fight it. According to J. Victor Garcia-Martinez, from the University of North Carolina, Chapel Hill, a humanized mouse model has clear benefits for studying HIV: allowing researchers to study how the human immune system is affected by the human disease, while using a small, easily manipulated, and easily inoculated non-human host.

In 2006, the Garcia laboratory created the BLT (bone marrow–liver–thymus) humanized mouse. Immune-deficient NOD/SCID mice were implanted with a chimeric organoid consisting of human fetal liver and thymus tissue; homologous CD34+ stem cells were isolated from the human fetal liver cells and transplanted intravenously, after irradiation, into the organoid-containing NOD/SCID mice; and stem cell transplantation successfully reestablished a human immune system in the mouse. Polychromatic flow cytometry analysis of peripheral blood from the BLT mouse confirms that more than 50% of cells were human (CD45+), and that human immune cells were also present in the female reproductive tract (vagina, cervix, and uterus).

BLT mice are highly susceptible to contracting HIV infection through the same human routes of entry—oral, rectal, vaginal, and intravenous—allowing researchers to investigate these different routes of transmission and to study the progression of viral infection from the site of entry to the plasma. When infected with HIV, BLT mice exhibit the same characteristics as humans and respond to anti-HIV retroviral therapy, ART, which suppresses HIV viral load to below detectable levels in the peripheral blood. As in humans, withdrawal of ART results in the virus multiplying and causes a rebound of viremia from established HIV reservoirs. The BLT mouse model is a useful tool for testing novel prevention approaches: for example, Garcia-Martinez demonstrated a clear reduction in HIV acquisition rates when testing the model with topical or systemic tenofovir treatment. Introducing the humanized mouse model adds another useful tool to the arsenal for scientists striving to understand the complexities of HIV infection and to develop novel methods for treatment, prevention, and eradication.

Bone Marrow–Liver–Thymus (BLT) mice were created to express a human immune system in an immunocompromised mouse. Shown here is the schema used to generate this model. These mice have human immune cells in the peripheral blood (PB), bone marrow (BM), spleen (Spl), lymph nodes (LN), liver, lung, and fetal liver/thymus organoid). (Image courtesy of J. Victor Garcia-Martinez)

Speaker:
Nicolas Chomont, Vaccine and Gene Therapy Institute of Florida

Moderator:
Dan Barouch, Beth Israel Deaconess Medical Center & Harvard University

Highlights

• HIV preferentially infects memory T cells and integrates as a provirus into the chromosome.
• HAART reduces viral load in the plasma to below detectable levels, but does not eliminate viral reservoirs in tissue.
• Early treatment (within weeks of infection) with HAART or megaHAART reduces the establishment and size of viral reservoirs.
• Future treatment protocols for HIV infection may involve early HAART followed by megaHAART.

Highly Active Antiretroviral Therapy, HAART, successfully reduces the HIV viral load in peripheral blood, but it does not eradicate HIV: instead, HAART alleviates AIDS symptoms and reduces the amount of HIV DNA in plasma to below detectable levels. HAART does not prevent HIV DNA from integrating at low levels into the human chromosomes as a provirus—creating a latent reservoir that is able to reestablish infection if HAART is discontinued—and viral rebound occurs quickly, no matter how long HAART is provided (months or decades). Reservoirs, found in the brain, lymph nodes, gastrointestinal tract, bone marrow, and genital tract, are detectable as early as one week after the onset of infection and grow larger as infection persists.

HAART and megaHAART treatment during the initial acute phase of infection reduces reservoir size. (Image courtesy of Nicholas Chomont)

Nicolas Chomont, from the Vaccine and Gene Therapy Institute of Florida, discussed how HIV reservoirs are created and proposed novel treatment protocols to prevent or eliminate their development. HIV preferentially infects memory CD4+ T cells and can infect these cells more efficiently as they differentiate, so highly-differentiated effective memory T cells (T_EM) exhibit higher levels of HIV DNA reservoirs than less-differentiated central memory cells (T_CM) and transitional memory cells (T_TM). HAART reduces the levels of total and integrated HIV DNA in all differentiated T-cell subsets. Although some HIV DNA does persist in T_CM and T_TM cells, the level of DNA in reservoirs falls if HAART is provided early after infection, suggesting that early HAART treatment may effectively prevent these reservoirs from becoming established. Preliminary results from the RV254 clinical trial suggest that just such early treatment with HAART or mega-HAART, a 5 drug anti-viral cocktail, does reduce the amount of integrated HIV DNA and prevent latent reservoir development. Small patient sample sizes currently limit our interpretation of these data, but future analysis of larger data sets will hopefully confirm initial findings reported here.

The future

The symposium ended with a spirited mock-debate moderated by Dan Barouch. The question posed: will the first FDA-approved HIV vaccine be a neutralizing or a non-neutralizing antibody? Speakers were divided into two camps: those who think that neutralizing antibodies would be most effective in an HIV vaccine versus those who think that non-neutralizing antibodies would be most effective. Although this was a mock-debate—and researchers perhaps exaggerated their views—several important points were highlighted, including the feasibility of generating a robust immune response and how to administer a vaccine. In the end, the vote among audience members remained the same. It seems that neither side was able to ultimately sway their views.

The focus of the HIV vaccine development field has shifted considerably from developing T-cell vaccines—previously deemed to be the best chance for finding a cure—to creating neutralizing antibodies to prevent or to limit infection. Based on emerging data, it now appears that targeting early events (infection, lymph node involvement, and CD4+ transmission) may be the most effective method to prevent infection and to limit disease transmission in early stages. New models, including humanized mice and SHIV vectors, are effective tools with which to develop and test new antibodies and vaccines; however, vectors are of limited use and should be used cautiously to answer very specific questions. There is a clear need for ongoing development of new models to study HIV and to test new treatment options before human clinical trials. The symposium raised key questions facing researchers in the vaccine science field. Chomont, for example, asked: "Is early treatment with HAART, followed by more intensive mega-HAART at a later stage of infection, an effective way to limit the establishment of latent reservoirs and potentially cure HIV?" As new data emerges, important questions like this will be answerable.

A proposed schema for the treatment of HIV. Can new methods be devised to treat during early stages of infection to limit viral load and the establishment of HIV reservoirs? (Image courtesy of Nicolas Chomont)

Will a neutralizing antibody vaccine approach be generalizable in a larger population infected with different substrains or clades of HIV?

The RV144 vaccine produced a high initial response. How can this effect be prolonged? Are additional booster injections feasible and will they be efficacious?

What is the mode of action underlying the RV144 antibody response? Are the antibodies targeting a specific protein or protein structure? Can this be identified and exploited in future vaccines?

Are non-neutralizing antibodies produced? Do these contribute to the response? If so, how do they function? If not, what is the function of a non-neutralizing antibody?

Can reliable animal models be produced to effectively test these questions?

If HIV viral levels can be lowered to below the level of detection, are there ways to reduce the reservoirs that exist? Can several methods be employed for early treatment and elimination of HIV reservoirs?

Is there a cure for HIV on the horizon?

Will the first FDA-approved HIV vaccine be a neutralizing or non-neutralizing antibody?