One of the studies presented at the recent AIDS Vaccine 2011 conference has now been published in the Journal of Clinical Investigation. It addresses a longstanding uncertainty in HIV research regarding the role of virus-specific CD4 T cells in controlling viral replication. On the one hand, basic immunology research conducted over the last two decades has made it increasingly clear that CD4 T cell responses play a vital role in sustaining functional CD8 T cell and B cell responses in most settings. In HIV infection, however, it is known that HIV-specific CD4 T cells are preferentially infected by the virus, raising the concern that these responses may enhance rather than limit viral replication. Although the issue is still debated, the preponderance of evidence now suggests that functional, broadly targeted—and particularly Gag-specific—HIV-specific CD4 T cells are vital for sustaining effective HIV-specific CD8 T cell responses and are therefore typically beneficial rather than harmful.
The new study by Guido Silvestri and colleagues adds to this evidence by demonstrating that depletion of CD4 T cells from rhesus macaques profoundly impairs their ability to reduce SIV viral load after it peaks during acute infection. In discussing the results the authors note that, to inform vaccine development, it will be important to better define the elements that constitute a protective CD4 T cell response:
“The current study suggests that, in addition to humoral and/or CD8+ T cell–mediated immune responses, the antiviral CD4+ T cell response may play an important role in limiting HIV replication, a finding that could be exploited in the design of candidate AIDS vaccines. As CD4+ T cell activation induced by an AIDS vaccine has the intrinsic potential of generating more targets for HIV (and SIV) infection, it will be essential to investigate at the phenotypic and functional level what type of CD4+ T cell responses mediate the protective antiviral effect revealed in this study.”
J Clin Invest. 2011 Oct 17. pii: 46023. doi: 10.1172/JCI46023. [Epub ahead of print]
Ortiz AM, Klatt NR, Li B, Yi Y, Tabb B, Hao XP, Sternberg L, Lawson B, Carnathan PM, Cramer EM, Engram JC, Little DM, Ryzhova E, Gonzalez-Scarano F, Paiardini M, Ansari AA, Ratcliffe S, Else JG, Brenchley JM, Collman RG, Estes JD, Derdeyn CA, Silvestri G.
CD4+ T cells play a central role in the immunopathogenesis of HIV/AIDS, and their depletion during chronic HIV infection is a hallmark of disease progression. However, the relative contribution of CD4+ T cells as mediators of antiviral immune responses and targets for virus replication is still unclear. Here, we have generated data in SIV-infected rhesus macaques (RMs) that suggest that CD4+ T cells are essential in establishing control of virus replication during acute infection. To directly assess the role of CD4+ T cells during primary SIV infection, we in vivo depleted these cells from RMs prior to infecting the primates with a pathogenic strain of SIV. Compared with undepleted animals, CD4+ lymphocyte-depleted RMs showed a similar peak of viremia, but did not manifest any post-peak decline of virus replication despite CD8+ T cell- and B cell-mediated SIV-specific immune responses comparable to those observed in control animals. Interestingly, depleted animals displayed rapid disease progression, which was associated with increased virus replication in non-T cells as well as the emergence of CD4-independent SIV-envelopes. Our results suggest that the antiviral CD4+ T cell response may play an important role in limiting SIV replication, which has implications for the design of HIV vaccines.