New Advances in the Study of CMTM6, a Focus on Its Novel Non-Canonical Cellular Locations, and Functions beyond Its Role as a PD-L1 Stabilizer

Simple Summary Recently, CMTM6 has been found to stabilize PD-L1 on the plasma membrane of some tumor cells, favoring tumor immune system evasion. Until the present, it was unknown whether this protein could be present in a soluble form in the plasma of patients with cervical cancer (CC) or, additionally, if it could be found in atypical subcellular compartments in cell lines derived from CC. The results of our present research confirm that CMTM6 is increased in the plasma of patients with CC and associated, but not exclusively so, with exosomes, and that exosomal CMTM6 levels are correlated with exosomal PD-L1 levels. In addition, in CC cell lines, CMTM6 was found to be secreted and present both on the membrane and intracellularly. CMTM6 derived from CC in soluble, exosomal membrane and intracellular forms could have an impact on the biology of tumor cells by affecting the expression of molecules associated with tumor development and progression, such as PD-L1.Abstract CMTM6 is a membrane protein that acts as a regulator of PD-L1, maintaining its expression on the cell surface, and can prevent its lysosome-mediated degradation. It is unknown if CMTM6 is present in the plasma of patients with cervical cancer, and if it has non-canonical subcellular localizations in cell lines derived from cervical cancer. Our objective was to determine whether CMTM6 is found in plasma derived from cervical cancer patients and its subcellular localization in cell lines. Patient plasma was separated into exosome-enriched, exosome-free, and total plasma fractions. The levels of CMTM6 in each fraction were determined using ELISA and Western blot. Finally, for the cellular model, HeLa, SiHa, CaSki, and HaCaT were used; the subcellular locations of CMTM6 were determined using immunofluorescence and flow cytometry. Soluble CMTM6 was found to be elevated in plasma from patients with cervical cancer, with a nearly three-fold increase in patients (966.27 pg/mL in patients vs. 363.54 pg/mL in controls). CMTM6 was preferentially, but not exclusively, found in the exosome-enriched plasma fraction, and was positively correlated with exosomal PD-L1; CMTM6 was identified in the membrane, intracellular compartments, and culture supernatant of the cell lines. These results highlight that CMTM6, in its various presentations, may play an important role in the biology of tumor cells and in immune system evasion.