Contrasting roles of the PD-1 signaling pathway in dendritic cell-mediated induction and regulation of HIV-1-specific effector T cell functions.

Eliciting highly functional CD8+ cytotoxic T lymphocyte (CTL) responses against a broad range of epitopes will likely be required for immunotherapeutic control of HIV-1 infection. However, the combination of CTL exhaustion and the ability of HIV-1 to rapidly establish CTL escape variants represent major hurdles towards this goal. Our previous work highlighted the use of monocyte derived, mature, high IL-12-producing type-1 polarized dendritic cells (MDC1) to selectively induce more potent effector CTLs derived from naïve, rather than memory, CD8+ T cell precursors isolated from HIV-1 positive participants in the Multicenter AIDS Cohort Study. In this study, we report that these highly stimulatory antigen presenting cells also express enhanced levels of the co-inhibitory molecule programmed cell death ligand 1 (PD-L1), the ligand for PD-1, which is further upregulated upon subsequent stimulation with the CD4+ T helper cell-derived factor CD40L. Interestingly, blocking the PD-1 signaling pathway during DC1 induction of HIV-1-specific CTL responses inhibited the priming, activation, and differentiation of naïve CD8+ T cells into T-box transcription factor high (Tbethi ) and eomesodermin (Eomes)+ effector T cells. In contrast, PD-1 blockade enhanced the overall magnitude of memory HIV-specific CTL responses and reversed the exhausted memory phenotype from a T-betlow /Eomes+ to a Tbethi /Eomes+ phenotype. These results indicate that the PD-L1/PD-1 signaling pathway has a previously unappreciated dual role in the induction and regulation of HIV-1-specific CTL immunity, which is greatly determined by the context and differentiation stage of the responsive CD8+ T cells. IMPORTANCE Targeting the PD-1/PD-L1 immune checkpoint axis with signaling inhibitors has proven to be a powerful immunotherapeutic strategy to enhance the functional quality and survival of existing antigen-specific effector T cells. However, our study brings to attention that the context and timing of PD-1 signaling in T cells greatly impacts the outcome of the effector response. In particular, we show that PD-1 activation plays a positive role during the DC-mediated initiation stage of the primary T cell response, while it serves as an inhibitory mechanism during the effector phase of the response. Therefore, caution should be taken in the design of therapies that include targeting of the PD-1/PD-L1 signaling pathway in order to avoid potential negative impacts on the induction of de novo T cell responses.