BACKGROUND

CD8 cells are key players in the identification and elimination of cancer cells. Cancers can escape an effective T cell response by inducing an exhausted cell state, which limits the cytotoxic capacity of the effector cells. Among other mechanisms, new checkpoint inhibitors reactivate exhausted, dysfunctional T cells. CD8 T cells can eliminate tumor cells after presentation of tumor-specific antigens via antigen-presenting cells (APCs). APC-mediated tumor recognition is mainly stimulated by Toll-like receptors (TLRs).

OBJECTIVE

This study investigates the effect of TLR agonists on APCs as well as stimulatory and inhibitory signaling pathways of the T cell-APC interaction.

MATERIALS AND METHODS

Gene expression of interleukin (IL)12 and programmed death ligand 1 (PD-L1) was analyzed by quantitative polymerase chain reaction (qPCR) after 0, 8, 24, and 48 h of CD14 cell stimulation with CpG. Protein expression of inhibitor of nuclear factor kappa B (IκBα) after CpG stimulation was investigated by western blot. CD8 T cells were stimulated for 72 h with or without programmed cell death protein 1 (PD-1) checkpoint blockade and analyzed for expression of PD‑1, Tim‑3, CTLA4, and Lag3 by flow cytometry.

RESULTS

TLR stimulation (by unmethylated CpG DNA) of APCs upregulates immunostimulatory signals such as IL12 expression but also activates immunoinhibitory signaling pathways such as PD-L1 expression. This signaling is NF-κB dependent. After blockade of the PD-1/PD-L1 signaling pathway, overexpression of other immune checkpoint inhibitory receptors was observed-a potential explanation for lacking therapeutic responses after TLR stimulation with PD‑1 checkpoint blockade.

CONCLUSION

TLR stimulation causes APCs in the tumor microenvironment to upregulate PD-L1 in an NF-κB-mediated fashion, thereby contributing to CD8 T cell exhaustion. The effect of PD‑1 blockade after TLR stimulation might be impaired due to upregulation of other checkpoint inhibitors.