PD-1 (CD279) Contributes to the Exhaustion of Gamma/Delta-T Cells in Tumor-Bearing Mice

Programmed death-1 (PD-1)/CD279 is an immunoinhibitory receptor that can be physiologically expressed on activated antigen-specific alpha/beta T-cells and is thought to play a role in maintaining a balance between T-cell activation and tolerance. Recently, both in vitro and in vivo, it has been shown that disrupting the interaction between PD-1 and its ligands can improve antitumor effects in preclinical and clinical models, this suggesting an important role played by this pathway in escape from immune surveillance. In comparison to healthy donors, gamma/delta-T cells found in tumor-bearing hosts can be diminished in number, or can be functionally impaired in a variety of important ways. While the mechanisms accounting for these numeric or functional defects have remained unclear, here we examine the extent to which PD-1 expression on gamma/delta-T cells may play a role in this process.

We first noted that peripheral blood gamma/delta-T cells are diminished in numbers in patients newly diagnosed with cancer. In addition, these gamma/delta-T cells expanded poorly when cultured ex vivo. Similar to humans, in tumor-bearing mice, we found that peripheral blood gamma/delta-T cells are diminished in number and likewise, expand poorly when cultured ex vivo. Using FACS analysis of mouse peripheral blood, we first determined that a substantial proportion of gamma/delta-T cells are actively undergoing apoptosis in tumor-bearing mice compared to healthy mice. Further analysis revealed that PD-1 is significantly upregulated on gamma/delta-T cells taken from tumor-bearing mice compared to gamma/delta-T cells taken from healthy mice. In contrast, no difference of PD-1 expression was seen when comparing alpha/beta-T cells taken from tumor-bearing and healthy mice. Using in vitro co-culture studies, we next determined that apoptosis in gamma/delta-T cells can be induced by direct contact with malignant cells, but not by contact with non-malignant cells. We then showed in these cultures that PD-1 is upregulated on gamma/delta-T cells co-cultured with tumor cell lines. Moreover, we were able to determine that the PD-1-positive gamma/delta-T cells in these cultures were undergoing apoptosis to a greater extent than PD-1-negative gamma/delta-T cells in these same cultures. Finally, in vitro using CFSE-based methods, we showed that while gamma/delta-T cells isolated from healthy mice readily proliferate upon mitogen stimulation, in contrast, gamma/delta-T cells from tumor-bearing mice proliferate poorly under the same conditions. However, in spleen cell cultures derived from tumor-bearing mice, upon addition of a monoclonal antibody directed against PD-L1 (B7-H1), a ligand for PD-1, substantial restoration of gamma/delta-T cell proliferation occurs.

Until now, the role played by PD-1 in the exhaustion of tumor-reactive gamma/delta T-cells has not been explored. Using in vitro and in vivo models, we show that the PD-1 pathway is a potentially important mechanism by which gamma/delta T-cells are either functionally impaired or otherwise exhausted in tumor-bearing mice. These findings suggest that by disrupting the PD-1 pathway, it may be possible to “revive” or “rescue” gamma/delta T-cells in tumor-bearing hosts.

No relevant conflicts of interest to declare.