In tumor cells, thyroid hormone analogues non-immunologically regulate PD-L1 and PD-1 accumulation that is anti-apoptotic.

The PD-1/PD-L1 immune checkpoint involving tumor cells and host immune defense lymphocytes is a well-studied therapeutic target in oncology. That PD-1 and PD-L1 may have additional functions within tumor cells that are independent of the checkpoint is indicated by actions of a thyroid hormone analogue, L-thyroxine (T), on these checkpoint components. Acting at a cell surface receptor on plasma membrane integrin αvβ3, T stimulates intracellular accumulation of PD-L1 in cancer cells. In these thyroid hormone-treated cells, T-induced PD-L1 is non-immunologically anti-apoptotic, blocking activation of p53. Several laboratories have also described accumulation of PD-1 in a variety of cancer cells, not just immune defense lymphocytes and macrophages. Preliminary observations indicate that T stimulates intracellular accumulation of PD-1 in tumor cells, suggesting that, like PD-L1, PD-1 has non-immunologic roles in the setting of cancer. Where such roles are anti-apoptotic, thyroid hormone-directed cancer cell accumulation of PD-1 and PD-L1 may limit effectiveness of immunologic therapy directed at the immune checkpoint.