Elevated body temperatures (regardless of the cause) lead to the activation of HSF1 (heat shock factor 1), which is a major mediator of transcriptional responses to proteotoxic stress (including heat stress), frequently overexpressed in cancer. PDCD1 (Programmed cell death protein 1) is a cell surface receptor that inhibits the excessive response of antigen-activated T cells to prevent autoimmune tissue damage. Thus, the PDCD1 has been named an “immune checkpoint”, referring to its role as a gatekeeper of immune responses. In chronic infections or cancer, lasting antigen exposure leads to permanent PDCD1 expression that can limit immune-mediated clearance of pathogens or degenerated cells. Consequently, blocking PDCD1 can enhance T cell function, which is the basis of cancer immune checkpoint therapy.

The effect of proteotoxic stress (heat shock, bortezomib) on PDCD1 expression was studied in various human cell lines, peripheral blood mononuclear cells (PBMC), and mouse tissues by RT-qPCR, western blot, or flow cytometry. HSF1 binding to chromatin was studied by chromatin immunoprecipitation. Functional HSF1 knockout was obtained using the CRISPR/Cas9 editing system.

PDCD1 expression can increase as early as a few hours after temperature elevation. We observed this at the mRNA and protein levels in human leukemic and lymphoblastoid cell lines (such as Jurkat, THP1, HL-60, and GM07062). Transcriptional upregulation of the PDCD1 gene was associated with the binding of heat shock factor 1, HSF1, to the promoter. In contrast, HSF1 knockout in HL-60 cells resulted in the inhibition of PDCD1 activation. Although PDCD1 transcript levels increased in the thymus and spleen of heat-shocked mice, PDCD1 mRNA did not increase in human PBMC and NK-92 (natural killers cell line). Nevertheless, we observed a heat shock-dependent increase in glycosylated (and therefore active) PDCD1 protein levels also in these cells. This is associated with PDCD1 exposure on the cell membrane and may lead to the loss of cytotoxic properties of NK-92 cells. We propose different mechanisms of heat-induced up-regulation of PDCD1 in cancer and normal cells: at the transcriptional and post-transcriptional levels.

Our observation suggests that the immune response could be attenuated in various physiological conditions accompanied by increased temperatures (infection, heat stroke, etc.). For this reason, fever (as well as pharmacological fever reduction) can have unexpected consequences depending on the disease state. This observation may have clinical implications, and therefore, further research is warranted to understand the importance of fever and PDCD1 in various disease states, as well as their interaction with treatment.