Several new studies suggest a role for the PD-1–PD-L pathway in exhaustion of virus-specific CD8 T cells during HIV infection. The study by Petrovas, et al. (on p. 2281
]), in this issue, and studies by Day et al. (24
) and Trautmann et al. (25
), published recently in Nature
and Nature Medicine
, respectively, show that PD-1 expression is elevated on HIV-specific CD8 T cells and that blocking the PD-1–PD-L pathway leads to increased T cell proliferation and effector cytokine production (illustrated in ). Previous work has shown that PD-L1 is up-regulated in HIV infection (21
). Collectively, these observations suggest that the PD-1– PD-L pathway may indeed be operating during chronic HIV infection.
Figure 2. Reinvigorating exhausted T cells. (A) Microbial products and cytokines produced in response to microbes activate APCs and stimulate expression of CD80 and CD86. Engagement of CD28 by CD80/CD86 stimulates the expansion and differentiation of naive T cells (more ...)
A large percentage of HIV-specific CD8 T cells expressed PD-1, and the expression of this receptor was elevated on a per cell basis. A large proportion of HIV-specific CD8 T cells also expressed CD27 and CD45RO, indicating previous activation. These CD8 T cells had lost expression of the costimulatory receptor CD28 and perforin and expressed only low levels of CCR7 and CD127 (IL-7 receptor α), which are important molecules for the maintenance of memory T cells. This phenotype suggests that the T cells are poorly functional, are not transiting into memory cells, and are particularly receptive to inhibitory signals.
CD8 T cells in individuals infected with HIV have previously been shown to be dysfunctional with reduced proliferative capacity and effector function (13
). Day et al. show that disease severity, as judged by viral load and declining CD4 counts, correlated with both the level of PD-1 expression on HIV-specific CD8 T cells and the percentage of cells expressing PD-1, providing a marker on CD8 T cells that correlates with disease severity (24
). The level of PD-1 expression was also associated with decreased CD8 T cell proliferation in response to in vitro stimulation with HIV antigen. Collectively, these findings show that the level of PD-1 correlates with the extent of T cell exhaustion. Day et al. (24
) and Trautmann et al. (25
) further demonstrated that PD-1 expression on HIV-specific CD8 T cells was reduced in patients undergoing effective highly active antiretroviral therapy, consistent with the notion that high antigen levels drive PD-1 expression and functional exhaustion. According to Trautmann et al., the CD8 dysfunction did not result from changes in TCR expression or stimulation with altered peptide ligands (25
There were striking differences in the levels of PD-1 expression on virus-specific T cells in chronic versus resolved infection. T cells specific for vaccinia virus, which causes a self-limiting acute infection, expressed little PD-1 compared with high levels of PD-1 expressed by HIV-specific T cells. In contrast, T cells specific for the chronic viruses, CMV and Epstein Barr virus, expressed moderate to high levels of PD-1, respectively. This suggests that sustained viremia and antigen presentation maintain the high levels of PD-1 expression. It will be important to determine if up-regulation of PD-1 and PD-L is a consequence of the antiviral interferon response, an indirect effect of T cell activation and inflammatory cytokine production, or whether HIV proteins directly up-regulate their expression.
All three papers demonstrate functional significance of PD-1 expression on HIV-specific T cells. Blocking PD-L1 with a monoclonal antibody led to increased T cell proliferation and production of TNF-α, IFN-γ, and granzyme B, indicating an overall increase in effector function. Although short-term blockade of the PD-1–PD-L pathway in a 6-h cytokine assay had little effect on HIV-specific CD8 T cell function, blockade of this pathway during a 6 d proliferation assay enhanced the proliferation of both HIV-specific CD8 and CD4 T cells and resulted in more functional T cells at the end of the culture. Whether proliferation is a prerequisite for the recovery of other T cell functions remains to be determined. Cytolytic activity, for example, was not examined and will be of great interest. It is also unclear whether the recovery of proliferative potential observed in the LCMV and HIV systems reflects greater cell division, less death, or both.
Petrovas et al. used a plate-bound, PD-1-specific polyclonal antibody to show that engagement of PD-1 reduces HIV-antigen specific T cell proliferation (23
). They examined apoptosis in the PD-1+
HIV-specific CD8 T cells and found increases in both spontaneous and Fas-mediated apoptosis, suggesting that cross-talk may occur between PD-1 and Fas receptors. The interpretation of these findings, however, is complicated by the finding that PD-1−
HIV-specific CD8 T cells also have increased susceptibility to apoptosis. It is possible that other factors such as the level of T cell activation are involved. Cells expressing very high levels of PD-1 were much more susceptible to death signals, suggesting that PD-1 expression leads to a survival defect in vivo.
Because of the link between CD4 help and CD8 function, Day et al. also examined the effects of PD-1 ligand blockade on CD4 T cell expansion (24
). Remarkably, in five of six patients with undetectable CD4 proliferative responses to HIV p24 antigen, blocking PD-L1 restored vigorous CD4 T cell expansion, suggesting that HIV-specific CD4 T cells may be present but so functionally impaired that they are undetectable in standard assays.