TAG HIV Basic Science, Vaccines, and Cure Project Blog

Microbicide Trial Halted

News emerged today that a phase III efficacy trial of a microbicide candidate called Ushercell (cellulose sulphate) has been halted. The decision to stop the trial was made after a preliminary analysis revealed a slightly higher rate of HIV infection in women using the microbicide compared to a placebo (inactive substance). A second study of Ushercell was also halted as a precaution, although in that trial there was no evidence suggesting an increased risk of HIV infection in Ushercell recipients. Keith Alcorn provides excellent coverage of the story in his article for AIDSmap. Ushercell belongs to a class of microbicide candidates called adsorption inhibitors; in vitro these compounds block the binding of HIV to target cells. Efficacy trials of two other adsorption inhibitors - Carraguard and PRO2000 - are ongoing, and interim analyses have not revealed any safety concerns necessitating the early termination of these studies.

amfAR Announces New RFP

The Foundation for AIDS Research (amfAR) has just issued a new request for proposals for research on optimizing the treatment of HIV infection, see the following announcement and link for more information:

amfAR Announces New Funding For AIDS Research

The Foundation for AIDS Research (amfAR) announces that funding is available for grants and fellowships for research on optimizing the treatment of HIV infection. Research proposals can explore biomedical, preclinical, clinical, epidemiological, social, behavioral, ethical or policy issues relevant to the topic.

amfAR's grants give faculty-level researchers affiliated with a nonprofit institution one year of support at up to $120,000. The two-year fellowships provide $125,000 in total funding to postdoctoral or equivalent investigators who have limited experience and are sponsored by an experienced investigator.

The deadline for submitting a letter of intent in response to this request for proposals is March 1, 2007. Please visit www.amfar.org/rfp for details on areas of interest, funding levels, and application requirements.

For more information, contact:

amfAR
Grants Management Department
120 Wall St., 13th Fl.
New York, NY 10005-3908 USA
Phone: 212/806-1600
email: [email protected]

Gag Rules?

A study in the January 2007 issue of Nature Medicine reports that the core HIV protein Gag is the most important target for virus-specific CD8 T cell responses in people with HIV. The researchers analyzed HIV-specific CD8 T cell responses targeting a broad array of HIV proteins in a cohort of 578 infected individuals in South Africa, and found that only responses against Gag showed inverse correlations with viral load (i.e. the greater the Gag-specific CD8 T cell response, the lower the viral load). In contrast, CD8 T cell responses targeting Envelope (Env) and Accessory/Regulatory proteins (Rev, Tat, Vif, Vpr, Vpu and Nef) showed positive correlations with viral load.

These findings lead the study authors to suggest that the efficacy of HIV-specific CD8 T cells may vary depending on the viral protein being targeted, and that this may have implications for designers of HIV vaccines. However, while the data in this study (and another recent study published in J Virology) suggest that Gag-specific CD8 T cell responses are critically important in controlling viral load, the investigators acknowledge that it may not be possible to extrapolate these findings directly to preventive vaccines. CD8 T cells depend on help from CD4 T cells to develop (or “mature” in immunological parlance) into fully functional “memory” CD8 T cells. In an unvaccinated, HIV-infected individual, this maturation process inevitably occurs in the presence of a virus known to infect and compromise CD4 T cells. In an HIV-negative vaccine recipient, however, the maturation of CD8 T cells occurs in a setting of uncompromised CD4 T cell help. Because of this difference, it remains unknown whether memory CD8 T cell responses in a vaccinated individual would behave the same way as the responses in this study.

Fortunately, two large human trials of HIV vaccines that can induce HIV-specific memory CD8 T cell responses should help answer this question within the next few years. One ongoing trial is investigating the protective efficacy of Merck’s Ad5 vaccine and this construct includes the Gag, Pol and Nef proteins from HIV, while a second efficacy trial (due to start soon) involves a DNA/Ad5 prime-boost regimen that includes Gag, Pol, Nef and three different Env proteins (from subtypes A, B & C). This latter vaccine was designed by researchers at the NIH’s Vaccine Research Center.

Abstracts:

Nat Med. 2007 Jan;13(1):46-53. Epub 2006 Dec 17.

CD8(+) T-cell responses to different HIV proteins have discordant associations with viral load.

Kiepiela P, Ngumbela K, Thobakgale C, Ramduth D, Honeyborne I, Moodley E, Reddy S, de Pierres C, Mncube Z, Mkhwanazi N, Bishop K, van der Stok M, Nair K, Khan N, Crawford H, Payne R, Leslie A, Prado J, Prendergast A, Frater J, McCarthy N, Brander C, Learn GH, Nickle D, Rousseau C, Coovadia H, Mullins JI, Heckerman D, Walker BD, Goulder P.

HIV Pathogenesis Programme, Doris Duke Medical Research Institute, University of KwaZulu-Natal, Durban 4013, South Africa.

Selection of T-cell vaccine antigens for chronic persistent viral infections has been largely empirical. To define the relationship, at the population level, between the specificity of the cellular immune response and viral control for a relevant human pathogen, we performed a comprehensive analysis of the 160 dominant CD8(+) T-cell responses in 578 untreated HIV-infected individuals from KwaZulu-Natal, South Africa. Of the HIV proteins targeted, only Gag-specific responses were associated with lowering viremia. Env-specific and Accessory/Regulatory protein-specific responses were associated with higher viremia. Increasing breadth of Gag-specific responses was associated with decreasing viremia and increasing Env breadth with increasing viremia. Association of the specific CD8(+) T-cell response with low viremia was independent of HLA type and unrelated to epitope sequence conservation. These population-based data, suggesting the existence of both effective immune responses and responses lacking demonstrable biological impact in chronic HIV infection, are of relevance to HIV vaccine design and evaluation.

J Virol. 2006 Dec 20; [Epub ahead of print]

CD8 T cell recognition of multiple epitopes within specific Gag regions is associated with maintenance of a low steady-state viremia in HIV-1 seropositives

Geldmacher C, Currier JR, Herrmann E, Haule A, Kuta E, McCutchan F, Njovu L, Geis S, Hoffmann O, Maboko L, Williamson C, Birx D, Meyerhans A, Cox J, Hoelscher M.

Mbeya Medical Research Programme, Referral Hospital, Mbeya, Tanzania; Department of Infectious Diseases and Tropical Medicine, University of Munich, 80799 Munich, Germany; The US Military HIV Research Program, Rockville, MD20851, USA; Division of Medical Virology, Institute of Infectious Disease and Molecular Medicine, University of Cape Town, South Africa; Institute of Virology, University of Saarland, 66421, Homburg, Germany; Department of Internal Medicine, University of Saarland, 66421 Homburg, Germany.

The importance of HLA class I restricted CD8 T cell responses in the control of HIV infection is generally accepted. While several studies have shown an association of certain HLA class I alleles with slower disease progression, it is not fully established whether this effect is mediated by HIV-specific CD8 T cell responses restricted by these alleles. In order to study the influence of the HLA class I alleles on the HIV-specific CD8 T cell response and on viral control, we have assessed the HIV-specific epitope recognition, the plasma viral load and the expression of HLA class I alleles in a cohort of HIV seropositive bar workers. Possession of the HLA class I alleles B5801, B8101 and B0702 was associated with a low median viral load and simultaneously with a broader median recognition of Gag epitopes compared to all other HLA alleles (2-fold increase; P = 0.0035). We further found an inverse linear relationship between the number of Gag epitopes recognized and the plasma viral load (R = -0.36, P = 0.0016). Particularly, recognition of multiple epitopes within two regions of Gag (aa001-aa075 and aa248-aa500) was associated with the maintenance of a low steady-state viremia, even years after acute infection.

Immunology TV

Over the past month or so, two amazing imaging studies have emerged from the laboratory of Ron Germain at the National Institute of Allergy and Infectious Diseases (NIAID). The techniques involved are fairly recent innovations which allow the activity of mouse immune system cells to be visualized in vivo or in excised tissue sections using special microscopes and fluorescent labels to tag the cells and tissues of interest. In one study, the technique is used to monitor the movement of T cells in lymph nodes. Germain and colleagues show that, in contrast to initial suggestions that T cells wander randomly in these tissues, they in fact move along string-like pathways of fibroblastic reticular cells (FRC). These pathways form a complex traffic system with crossroads, junctions and dead ends. Dendritic cells, whose job it is to present antigens to potentially responsive T cells, gather at the crossroads like streetcorner salesmen plying their wares.  The dead ends of the FRC pathways act to prevent T cells from being able to travel into the B cell zone of the lymph nodes. Germain and colleagues also show that B cells have their own similar traffic system, the follicular dendritic cell network in the B cell zone of the lymph node. Although access to the full paper requires a subscription to the journal Immunity, the video images can be viewed on the journal’s website free of charge.

In a second paper in the Journal of Experimental Medicine, the techniques are used to investigate the behavior of dendritic cells in the gut mucosa. While it is known that dendritic cells can reach through the epithelial barrier of the gut to sample antigens in the GI tract, the precise nature of this process has not been well elucidated. Germain’s stunning images show how the long tentacles of dendritic cells can balloon out through epithelia to grab passing antigens before seeming to deflate back behind the mucosal barrier. Additional experiments also demonstrate that the prevalence of this activity varies in different locations in the GI tract, and that it is robustly enhanced by microbial stimuli (such as Salmonella infection). Given that dendritic cells can capture HIV and act as Trojan horses that transport the virus to CD4 target cells, these findings may be very relevant to the mucosal transmission of HIV infection. Again, while access to the full text of the paper is restricted, the movies can be viewed online free of charge (all the movies require a free media player such Windows Media Player or Quicktime).

Abstracts:

Immunity. 2006 Dec;25(6):989-1001. Epub 2006 Nov 16.

Stromal cell networks regulate lymphocyte entry, migration, and territoriality in lymph nodes.

Bajenoff M, Egen JG, Koo LY, Laugier JP, Brau F, Glaichenhaus N, Germain RN.

Lymphocyte Biology Section, Laboratory of Immunology, NIAID, National Institutes of Health, Bethesda, Maryland 20892, USA.

After entry into lymph nodes (LNs), B cells migrate to follicles, whereas T cells remain in the paracortex, with each lymphocyte type showing apparently random migration within these distinct areas. Other than chemokines, the factors contributing to this spatial segregation and to the observed patterns of lymphocyte movement are poorly characterized. By combining confocal, electron, and intravital microscopy, we showed that the fibroblastic reticular cell network regulated naive T cell access to the paracortex and also supported and defined the limits of T cell movement within this domain, whereas a distinct follicular dendritic cell network similarly served as the substratum for movement of follicular B cells. These results highlight the central role of stromal microanatomy in orchestrating cell migration within the LN.

J Exp Med. 2006 Dec 25;203(13):2841-52. Epub 2006 Dec 4.

Dynamic imaging of dendritic cell extension into the small bowel lumen in response to epithelial cell TLR engagement.

Chieppa M, Rescigno M, Huang AY, Germain RN.

Lymphocyte Biology Section, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), Bethesda, MD 20892.

Cells lining the gastrointestinal tract serve as both a barrier to and a pathway for infectious agent entry. Dendritic cells (DCs) present in the lamina propria under the columnar villus epithelium of the small bowel extend processes across this epithelium and capture bacteria, but previous studies provided limited information on the nature of the stimuli, receptors, and signaling events involved in promoting this phenomenon. Here, we use immunohistochemical as well as dynamic explant and intravital two-photon imaging to investigate this issue. Analysis of CD11c-enhanced green fluorescent protein (EGFP) or major histocompatibility complex CII-EGFP mice revealed that the number of trans-epithelial DC extensions, many with an unusual "balloon" shape, varies along the length of the small bowel. High numbers of such extensions were found in the proximal jejunum, but only a few were present in the terminal ileum. The extensions in the terminal ileum markedly increased upon the introduction of invasive or noninvasive Salmonella organisms, and chimeric mouse studies revealed the key role of MyD88-dependent Toll-like receptor (TLR) signaling by nonhematopoietic (epithelial) elements in the DC extension response. Collectively, these findings support a model in which epithelial cell TLR signaling upon exposure to microbial stimuli induces active DC sampling of the gut lumen at sites distant from organized lymphoid tissues.

Leaking Bacteria & HIV Pathogenesis

In the December issue of Nature Medicine, Daniel Douek and colleagues publish evidence supporting their theory that HIV’s depletion of gut CD4 T cells causes a phenomenon known as “microbial translocation.” This theory holds that HIV infection damages the integrity of the gut mucosa in a way that allows friendly commensal bacteria to leak from the gut into the circulation; the leaked bacteria then cause the immune activation seen in HIV infection. The researchers support their theory by showing that people with chronic HIV infection have higher levels of lipopolysaccharide (LPS) in their bloodstream than uninfected controls, and that LPS levels correlate with markers of immune activation (CD38 expression on CD8 T cells). LPS is typically a product of gut bacteria although it can also be produced by other sources (such as pathogens). LPS levels are often used as a marker for microbial translocation in other settings (such as inflammatory bowel disease), although recently researchers have also begun using DNA PCR to look directly for bacteria in the bloodstream; Douek's research group plans to evaluate this technology in future studies.

The paper also notes that LPS levels were elevated in 10 out of 11 SIV-infected macaques that the researchers studied at two timepoints (baseline and 100 days post-infection). Two macaques treated with antibiotics showed a transient (2 week) decline in LPS levels and bacteria in their feces, leading the researchers to state that: “These data suggest that the origin of plasma LPS in SIV-infected rhesus macaques and, by inference, in HIV-infected humans is translocation from the gastrointestinal tract.” It is perhaps worth noting that a considerable amount is being inferred from two macaques. The researchers also suggest that LPS levels are not elevated in early HIV infection because of anti-LPS antibodies – when immunodeficiency worsens, they argue, these antibodies wane and levels of LPS therefore increase. They support this argument with data showing an inverse correlation between anti-LPS antibodies and LPS levels. Additionally, the authors found reduced anti-LPS antibody levels in people with acute/early HIV infection compared to uninfected controls and a further reduction in anti-LPS antibody levels in chronic HIV infection versus acute/early infection. These data suggest that HIV-related immunodeficiency impairs the anti-LPS antibody response.

The effects of ART on LPS levels were also investigated. Out of 28 individuals studied, 24 showed a decline in LPS levels after 48 weeks of ART. There was also a significant correlation between the reduction of LPS and the magnitude of CD4 T cell reconstitution, although the researchers note that there was no correlation between LPS levels and CD4 T cells counts prior to the initiation of ART.

The paper concludes by addressing the seemingly confounding data on gut CD4 T cell depletion in SIV-infected Sooty Mangabeys. These animals rarely develop immunodeficiency but, surprisingly, do show evidence of severe CD4 T cell loss from the gut. An investigation of LPS levels in SIV-infected Sooty Mangabeys found no difference compared to uninfected controls, leading the authors to argue that while these animals experience an apparent loss of gut CD4 T cells, they are protected from immune activation because microbial translocation does not occur (although they do not yet have an explanation as to why this might be the case).

Taken together, the results reported in this paper are very interesting and provocative, but additional research will be needed to confirm that microbial translocation occurs and is responsible for immune activation in HIV infection.

Study abstract:

Nat Med. 2006 Dec;12(12):1365-71. Epub 2006 Nov 19.

Microbial translocation is a cause of systemic immune activation in chronic HIV infection.

Brenchley JM,Price DA, Schacker TW, Asher TE, Silvestri G, Rao S, Kazzaz Z, Bornstein E, Lambotte O, Altmann D, Blazar BR, Rodriguez B, Teixeira-Johnson L, Landay A, Martin JN, Hecht FM, Picker LJ, Lederman MM, Deeks SG, Douek DC.

Human Immunology Section, Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.

Chronic activation of the immune system is a hallmark of progressive HIV infection and better predicts disease outcome than plasma viral load, yet its etiology remains obscure. Here we show that circulating microbial products, probably derived from the gastrointestinal tract, are a cause of HIV-related systemic immune activation. Circulating lipopolysaccharide, which we used as an indicator of microbial translocation, was significantly increased in chronically HIV-infected individuals and in simian immunodeficiency virus (SIV)-infected rhesus macaques (P </= 0.002). We show that increased lipopolysaccharide is bioactive in vivo and correlates with measures of innate and adaptive immune activation. Effective antiretroviral therapy seemed to reduce microbial translocation partially. Furthermore, in nonpathogenic SIV infection of sooty mangabeys, microbial translocation did not seem to occur. These data establish a mechanism for chronic immune activation in the context of a compromised gastrointestinal mucosal surface and provide new directions for therapeutic interventions that modify the consequences of acute HIV infection.