Toll like receptor agonist effects on human CD8+ T cell activation and expression of T cell checkpoint receptors

We have previously found that murine CD8+ T cells activated with cognate antigen increase the expression of PD-1 and other checkpoint receptors, and this can be attenuated in the presence of specific Toll-like receptor (TLR) agonists. In the current report, we sought to investigate whether TLR agonists similarly affect the expression of T cell checkpoint receptors on human CD8+ T cells and their function. Cryopreserved human peripheral blood mononuclear cells (PBMCs) containing CD8+ T cells reactive to human cytomegalovirus (CMV) or Epstein-Barr virus (EBV) were stimulated with virus-specific peptide epitopes in the presence of TLR agonists. Activation of human CD8+ T cells in the presence of TLR agonists did not further expand tetramer-positive CD8+ T cells or significantly alter the expression of checkpoint receptors (PD-1, CTLA-4, LAG-3, TIGIT, TIM-3, or CD160). Activation of CD8+ T cells by viral antigen-loaded dendritic cells (DC) in the presence of a TLR3 agonist increased the number of antigen-specific IFNγ-secreting cells and the magnitude of IFNγ expression. DCs were found to secrete IFNβ and IL-12 in response to stimulation with TLR3 agonist and activation of CD8+ T cells in the presence of IFNß and IL-12 led to a slight decrease in expression of CTLA-4 and TIGIT. These findings demonstrate that, among TLR agonists, TLR3 agonists can enhance the activation of human CD8+ T cells and might be preferred TLR agonists for use as adjuvants in human anti-cancer vaccines or other immunotherapies aimed at activating antigen-specific CD8+ T cells.