Abstract 4359: Rescue of exhausted CD8 T cells by PD-1 targeted therapies is CD28-dependent

T cell exhaustion was first described in mice during chronic lymphocytic choriomeningitis virus (LCMV) infection and later shown to occur in humans during persistent infections and cancer. Sustained expression of the inhibitory receptor programmed cell death-1 (PD-1) is a hallmark of T cell exhaustion. Therapeutic blockade of PD-1 rescues proliferation, cytokine production and cytotoxicity of exhausted T cells. PD-1 targeted therapies have shown efficacy for multiple cancer types and changed the landscape of cancer treatment, but the requirements for T cell rescue by blockade of the PD-1 pathway remain elusive. T cell activation ensues when positive signals overcome negative signals. CD28 is the most prominent positive co-receptor for T cell activation, and human CD8 T cells become CD28neg upon continuous stimulation. We analyzed tumor specimens from non-small cell lung cancer (NSCLC) patients by flow cytometry and found heterogeneous CD28 expression on tumor-infiltrating CD8 T cells. In most NSCLC patients, CD28neg cells were prevalent among CD8 T cells co-expressing inhibitory receptors (such as PD-1 and Tim-3). To address the role of the CD28/B7 pathway on CD8 T cell rescue by PD-1 targeted therapies, we used the LCMV chronic infection model. When B7 signals were impeded by CTLA-4-Ig or B7 blocking antibodies, blockade of the PD-1 pathway failed to rescue virus-specific CD8 T cells. To investigate a cell intrinsic requirement of CD28 on exhausted CD8 T cells, we used an adoptive transfer strategy of cells genetically deficient in CD28. We found that CD28-deficient LCMV-specific CD8 T cells do not expand after blockade of the PD-1 pathway. Furthermore, to ensure appropriate development and differentiation of CD8 T cells, we used mice in which the CD28 gene can be conditionally deleted. We confirmed that CD28neg exhausted CD8 T cells failed to proliferate after blockade of the PD-1 pathway even when CD28 gene deletion was induced on already exhausted T cells. These data demonstrate that CD28 engagement is required for CD8 T cell rescue, and CD28 status may constitute a predictive biomarker for PD-1 targeted therapies. Citation Format: Alice O. Kamphorst, Andreas Wieland, Shu Yang, Tahseen Nasti, Ruan Zhang, Daniel L. Barber, Bogumila T. Konieczny, Lydia Koenig, Ke Yu, Gabriel Sica, Taofeek K. Owonikoko, Rathi Pillai, Suresh S. Ramalingam, Arlene H. Sharpe, Gordon J. Freeman, Laurence A. Turka, Koichi Araki, Rafi Ahmed. Rescue of exhausted CD8 T cells by PD-1 targeted therapies is CD28-dependent. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4359.