PD‐L1 is a key immune checkpoint ligand that suppresses antitumor immunity by engaging PD‐1 on T cells. While therapeutic blockade of PD‐L1/PD‐1 interactions has shown clinical benefit, many patients fail to respond, indicating modulation by other factors. Here, we identified a novel regulatory axis in which the membrane‐organizing protein tetraspanin‐4 (TSPAN4) modulates PD‐L1 in melanoma cells. Using cell surface proximity biotinylation coupled with mass spectrometry, we discovered that TSPAN4 physically associates with PD‐L1, with both proteins colocalizing on migrasomes and retraction fibers. Mechanistically, we show that TSPAN4 negatively regulates PD‐L1 protein levels by enhancing its degradation and restricting its lateral mobility at the plasma membrane. Loss of TSPAN4 stabilized PD‐L1, promoted its interaction with CMTM6, and increased PD‐L1 surface availability for PD‐1 binding. Functionally, TSPAN4 knockdown in melanoma cells led to more efficient immune checkpoint blockade through PD‐1 on T cells. This study identifies TSPAN4 as a negative regulator of PD‐L1 at the cell surface of melanoma cells suggesting that targeting TSPAN4 may offer a new therapeutic strategy to enhance immune checkpoint blockade in melanoma and other cancers.
Franken et al. (Mon,) studied this question.