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
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.
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.