Identification of CMTM6 and CMTM4 as PD-L1 protein regulators
Riccardo Mezzadra1*, Chong Sun1*, Lucas Jae2*, Raquel Gomez1, Evert de Vries1, Yanling Xiao1, Jannie Borst1, Thijn Brummelkamp#2 and Ton Schumacher#1
1Division of Immunology, 2 Division of Biochemistry, The Netherlands Cancer Institute-Antoni van Leeuwenhoek, Amsterdam 1066 CX, the Netherlands
*shared first author
#shared corresponding author
The clinical benefit in patients with metastatic cancers that is observed upon blockade of the PD-1 - PD-L1 interaction has highlighted the importance of this axis in the suppression of tumor-specific T cell responses. In spite of the central role of PD-L1 expression within the tumor micro-environment, our understanding of the regulation of this protein is still highly limited. Through a haploid genetic screen, we here identify CMTM6, a poorly described multi-transmembrane protein, as a regulator of PD-L1 protein expression. Knock out or knock down of CMTM6 results in impaired PD-L1 protein expression in all tumor cell types tested and also within myeloid cells. A repeat genetic screen in CMTM6 deficient haploid cells and rescue experiments demonstrate that, contrary to CMTM1, 3, 5, and 7, also CMTM4 modulates PD-L1 protein expression. Notably, CMTM6 modulates the PD-L1 protein pool without affecting PD-L1 transcript levels. Furthermore, we demonstrate that CMTM6 is expressed at the cell surface and associates with PD-L1, as shown by co-immunoprecipitation experiments. Finally, T cell inhibitory capacity of PD-L1 expressing tumor cells is significantly reversed by CMTM6 removal.
Collectively, our data reveal that PD-L1 relies on CMTM6/4 to carry out its inhibitory function with the tumor micro-environment, and suggest potential new avenues to block this pathway.