CMTM6 maintains the expression of PD-L1 and regulates anti-tumour immunity

Abstract

Cancer cells exploit the expression of the programmed death-1 (PD-1) ligand 1 (PD-L1) to subvert T-cell mediated immunosurveillance1,2. The success of therapies that disrupt PD-L1 mediated tumour tolerance has highlighted the need to understand the molecular regulation of PD-L1 expression1. Using a genome-wide CRISPR/Cas9 screen we identified the uncharacterized protein CMTM6 to be a critical regulator of PD-L1 in a broad range of cancer cells. CMTM6 is a ubiquitously expressed, protein that binds PD-L1 and maintains its cell surface expression. CMTM6 is not required for PD-L1 maturation but co-localizes with PD-L1 at the plasma membrane and in recycling endosomes where it prevents PD-L1 from being targeted for lysosome-mediated degradation. Using a quantitative approach to profile the entire plasma membrane proteome we find that CMTM6 displays remarkable specificity for PD-L1. Importantly, CMTM6 depletion decreases PD-L1 without compromising cell surface expression of MHC Class I. CMTM6 depletion, via the reduction of PD-L1, significantly alleviates the suppression of tumour specific T-cell activity in vitro and in vivo. These findings provide novel insights into the biology of PD-L1 regulation, identify a previously unrecognised master regulator of this critical immune checkpoint and highlight a new potential therapeutic target to overcome immune evasion by tumour cells.

Novel regulators of PD-L1 expression in cancer: CMTM6 and CMTM4—a new avenue to enhance the therapeutic benefits of immune checkpoint inhibitors

“…CMTM6 was found to be a key regulator of T lymphocytemediated anti-tumor immunity, in diseases that respond to immune checkpoint inhibitors through modulation of the PD-L1 expression ex vivo (in humans) and in vivo (mouse), respectively (19). Further basic and translational studies should confirm these findings and enable novel approaches to enhance the effectiveness and clinical benefits of the immune checkpoint inhibitors in humans.…”

“…CMTM6 is established as a new regulatory target to specifically vary the cell surface expression of this critical immune checkpoint molecule. Although CMTM6 is found within cytosol and plasma membrane, it is not required for the trafficking of PD-L1 from the endoplasmic reticulum to the cell surface, but is required for stable expression of PD-L1 at the cell membrane (19,20). CMTM6-deficient tumor cells were also more susceptible to antigen-specific cytotoxic T-lymphocytes in vitro and were associated with longer survival in a murine transplantable melanoma model.…”

“…CMTM6-deficient tumor cells were also more susceptible to antigen-specific cytotoxic T-lymphocytes in vitro and were associated with longer survival in a murine transplantable melanoma model. Another interesting observation from these studies is that CMTM6 protects PD-L1 from ubiquitination and helps it be recycled and evade lysosomal degradation (19). Also, STUB1, an E3 ubiquitin ligase, has been identified as a negative regulator of PD-L1.…”

“…CMTM6 belongs to a family of proteins, primarily encoded by two distinct gene clusters located on chromosome 16 (CMTM1-4) and chromosome 3 (CMTM6-8). MARVEL domain proteins have been implicated in regulating trafficking of transmembrane and secretory proteins (19). Depletion of CMTM6 led to a marked reduction of PD-L1 protein level, but not its transcriptional levels.…”

“…In this context, two potential pathways (CMTM6 and CMTM4), considered as PD-L1 regulators, have been recently discovered in maintaining antitumor immunity (19,20 (19,20). CMTM6 is a ubiquitously expressed protein with previously unknown function; it appears to play a role in limiting antitumor immunity.…”