TAG HIV Basic Science, Vaccines, and Cure Project Blog

New Research on Gut CD4 T-Cell Depletion and HIV Pathogenesis

In both HIV and SIV infections, it has been shown that the most rapid and extensive loss of CD4 T cells occurs in the gut. As a consequence, a theory has emerged positing that gut CD4 T-cell depletion plays a central causative role in driving HIV pathogenesis; the proposed mechanism is that gut wall integrity becomes compromised, leading to the leakage of normally friendly bacteria from the digestive tract and into systemic circulation, which in turn contributes to persistent immune activation and, ultimately, progression to AIDS. However, some scientists have remained skeptical of this theory, suggesting instead that gut CD4 T-cell depletion is an effect of HIV infection, but not necessarily the primary cause of disease progression. The skeptics have gained some support from studies showing that severe gut CD4 T-cell depletion occurs during acute SIV infection in monkey species that experience no apparent ill effects from the virus (sooty mangabeys and African green monkeys). A paper published recently in the Journal of Virology now shows that the opposite phenomenon is also possible: a modified SIV that does not cause loss of gut CD4 T cells nevertheless causes persistent immune activation and progression to simian AIDS in rhesus macaques.

Published ahead of print, 14 November 2012, doi: 10.1128/JVI.01928-12

Loss of a Tyrosine-Dependent Trafficking Motif in the SIV Envelope Cytoplasmic Tail Spares Mucosal CD4 Cells but Does Not Prevent Disease Progression

Matthew W. Breed1, Andrea P.O. Jordan2, Pyone P. Aye1, Cornelis F. Lichtveld1, Cecily Midkiff1, Faith Schiro1, Beth S. Haggarty2, Chie Sugimoto1, Xavier Alvarez1, Netanya G. Sandler3, Daniel C. Douek3, Marcelo J. Kuroda1, Bapi Pahar1, Michael Piatak Jr4, Jeffrey D. Lifson4, Brandon F. Keele4, James A. Hoxie2 and Andrew A. Lackner1 1 Tulane National Primate Research Center, Covington, LA, USA 2 University of Pennsylvania, Philadelphia, PA, USA 3 Human Immunology Section, Vaccine Research Center, National Institutes of Allergy and Infectious Diseases, NIH, Bethesda, Maryland, USA 4 SAIC-Frederick Inc., Frederick National Laboratory for Cancer Research, Frederick, MD, USA

ABSTRACT

A hallmark of pathogenic SIV and HIV infection is the rapid and near complete depletion of mucosal CD4+ T-lymphocytes from the gastrointestinal tract. Loss of these cells and disruption of epithelial barrier function are associated with microbial translocation, which has been proposed to drive chronic systemic immune activation and disease progression. Here we evaluate in rhesus macaques, a novel attenuated variant of pathogenic SIVmac239, termed ΔGY, which contains a deletion of a Tyr and a proximal Gly from a highly conserved YxxØ trafficking motif in the envelope cytoplasmic tail. Compared to SIVmac239, ΔGY established a comparable acute peak of viremia, but only transiently infected lamina propria and caused little or no acute depletion of mucosal CD4+ T-cells and no detectable microbial translocation. Nonetheless, these animals developed T-cell activation, declining peripheral blood CD4+ T-cells and ultimately progressed with clinical or pathological features of AIDS. ΔGY-infected animals also showed no infection of macrophages or CNS tissues even in late stage disease. Although the ΔGY mutation persisted, novel mutations evolved including the formation of new YxxØ motifs in two of four animals. These findings indicate that disruption of this trafficking motif by the ΔGY mutation leads to a striking alteration in anatomic distribution of virus with sparing of lamina propria and a lack of microbial translocation. Because these animals exhibited wildtype levels of acute viremia and immune activation, our findings indicate that these pathological events are dissociable, and that immune activation unrelated to gut damage can be sufficient for the development of AIDS.

HIV Replication and Persistence in T Follicular Helper Cells

Following up on the recent post on T follicular helper cells (TFH), a study published on December 17 in the Journal of Experimental Medicine reports that TFH represent a major site of HIV replication and persistence. The finding is not necessariy surprising, because lymph node studies conducted in the late 1980s and early 1990s showed that CD4 T cells in areas named germinal centers bore a great burden of HIV infection, and it is now known that this is where TFH locate during an immune response. But it was not previously understood that these CD4 T cells comprise a discrete subset, and TFH are now becoming increasingly well characterized in terms of both identifying features (such as surface markers and cytokine secretion) and their function in providing help to B cells.

Matthieu Perreau and colleagues from the University of Lausanne investigated TFH in three different groups of HIV-positive individuals:

• 23 with untreated chronic infection and viral load >5,000 copies/mL
• 14 on antiretroviral therapy (ART) with viral load <20 copies/mL
• 3 long-term nonprogressors (LTNP) with low viral load (<1,000 copies/mL)

A control group of 13 HIV-negative individuals was also included. The study confirmed the prior finding that TFH are significantly expanded in the lymph nodes of people with HIV compared to controls and that this expansion correlates with alterations in B-cell subsets (a decline in long-lived resting naive and memory B cells and an increase in short-lived activated B cells). The percentage of TFH correlated significantly with peripheral blood HIV viral load (R = 0.6035, P = 0.002). Suppression of HIV replication by ART reduced TFH levels and shifted B-cell subsets back toward the distribution observed in controls. Echoing and extending the previous research by Madelene Lindqvist, a substantial proportion of the TFH were found to be specific for HIV, with responses to Gag and Pol proteins more commonly detected than those against Env (based on a test evaluating cytokine production after stimulation with Gag, Pol, or Env peptides).

An analysis of the different CD4 T-cell populations present in lymph nodes revealed that TFH contained the greatest amount of HIV DNA: assuming one HIV DNA copy per cell, approximately five percent of the TFH population was infected. After 72 weeks of ART there was a 1.5–2 log drop in the number of TFH containing HIV DNA, but a reservoir of infected cells remained detectable. Cell culture experiments showed that TFH were highly susceptible to HIV infection and replication compared to other CD4 T-cell subsets, with the difference being most notable among the LTNP. The expression of the proliferation marker Ki67 by TFH was found to correlate with HIV DNA levels and ability to support HIV replication in vitro. Consistent with the elevation in immune activation that occurs in HIV, the proportion of TFH expressing Ki67 was 50 percent higher in the untreated HIV-positive individuals than in HIV-negative controls.

The discussion section of the paper highlights several implications of the results, including:

• HIV itself—as opposed to other potential factors, like microbial translocation—appears to be the main driving force behind TFH activation and expansion, and the associated dysregulation of B cells.
• The preferential infection of TFH likely contributes to viral persistence, because germinal centers have been shown to offer limited access to CD8 T cells (which might otherwise kill HIV-infected cells). Follicular dendritic cells in lymph nodes have also long been known to be sites of extensive trapping of HIV virions, providing an ongoing source of viruses to infect TFH.
• The involvement of TFH in responses to new pathogens and vaccines offers an explanation for the reported association between intercurrent infections and vaccinations and transient HIV viral-load increases (activation of TFH by these stimuli provides additional targets for HIV replication).
• The expression of certain cell-surface markers (such as PD-1) by TFH may offer a means of targeting this population with interventions designed to reduce HIV persistence. A human trial of an antibody targeting PD-1 is currently in the works at the AIDS Clinical Trials Group (for more background, see this recent short review "Novel Approaches to Curing HIV" by the principal investigator for the trial, Hiroyu Hatano).

Journal of Experimental Medicine

Published December 17, 2012

The Rockefeller University Press, doi: 10.1084/jem.20121932

Article

Follicular helper T cells serve as the major CD4 T cell compartment for HIV-1 infection, replication, and production

Matthieu Perreau1, Anne-Laure Savoye1, Elisa De Crignis1, Jean-Marc Corpataux2, Rafael Cubas5, Elias K. Haddad5, Laurence De Leval3, Cecilia Graziosi1, and Giuseppe Pantaleo1,4

1Divisions of Immunology and Allergy and 2Thoracic Surgery, 3Institute of Pathology, and 4Swiss Vaccine Research Institute, Lausanne University Hospital, University of Lausanne, CH-1011 Lausanne, Switzerland

5Vaccine and Gene Therapy Institute of Florida, Port St. Lucie, Florida

M. Perreau and A.-L. Savoye contributed equally to this paper.

ABSTRACT

In the present study, we have investigated the distribution of HIV-specific and HIV-infected CD4 T cells within different populations of memory CD4 T cells isolated from lymph nodes of viremic HIV-infected subjects. Four memory CD4 T cell populations were identified on the basis of the expression of CXCR5, PD-1, and Bcl-6: CXCR5−PD-1−Bcl-6−, CXCR5+PD-1−Bcl-6−, CXCR5−PD-1+Bcl-6−, and CXCR5+PD-1+Bcl-6+. On the basis of Bcl-6 expression and functional properties (IL-21 production and B cell help), the CXCR5+PD-1+Bcl-6+ cell population was considered to correspond to the T follicular helper (Tfh) cell population. We show that Tfh and CXCR5−PD-1+ cell populations are enriched in HIV-specific CD4 T cells, and these populations are significantly increased in viremic HIV-infected subjects as compared with healthy subjects. The Tfh cell population contained the highest percentage of CD4 T cells harboring HIV DNA and was the most efficient in supporting productive infection in vitro. Replication competent HIV was also readily isolated from Tfh cells in subjects with nonprogressive infection and low viremia (<1,000 HIV RNA copies). However, only the percentage of Tfh cells correlated with the levels of plasma viremia. These results demonstrate that Tfh cells serve as the major CD4 T cell compartment for HIV infection, replication, and production.