Checkpoint Protein Propped Up by Cell-Surface Sidekick

Many cancer cells duck the immune system’s T cells by displaying programmed death-ligand 1 (PD-L1), a cell-surface protein that engages programmed cell death protein 1 (PD-1), another cell-surface protein, this one displayed by T cells, and thereby signals T cells to stand down. Although the PD-L1/PD-1 connection is well studied and is the inspiration behind a new class of anticancer drugs called checkpoint inhibitors, it isn’t as well understood as scientists would like. Outstanding questions include: Why is PD-L1 more abundant in some cancers than in others? And why is checkpoint therapy more helpful in some patients than others?

Hoping to shed light on such questions, two teams of scientists decided to investigate the PD-L1/PD-1 connection. Both teams – one based in The Netherlands, one in Australia – focused on PD-L1 and how it comes to be expressed, and how it manages to persist, on the surface of cancer cells.

Although the teams used different methods, they arrived at a similar result—another cell-surface protein has PD-L1’s back. This PD-L1 partner is called CMTM6. It co-localizes with and binds to PD-L1, protecting it from degradation. Now that PD-L1’s hidden enabler has been revealed, it could become a prognostic marker, indicating which patients may respond best to checkpoint inhibitor drugs, or it could become a drug target itself.

Both teams published their findings August 16 in the journal Nature. One paper, from the Netherlands Cancer Institute, was entitled “Identification of CMTM6 and CMTM4 as PD-L1 Protein Regulators.” The other paper, from the Peter MacCallum Cancer Centre, was entitled “CMTM6 Maintains the Expression of PD-L1 and Regulates Anti-Tumour Immunity.” Each team used a different loss-of-function screening method. The Netherlands-based team used a haploid genetic screen; the Australia-based team, a CRISPR/Cas9 screen.

“Here we identify the uncharacterized protein CMTM6 as a critical regulator of PD-L1 in a broad range of cancer cells, by using a genome-wide CRISPR–Cas9 screen,” wrote the authors of the paper prepared by the Netherlands-based team. “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.”

“We always thought PD-L1 was a loner at the cancer cell surface, but it turns out that it binds to another protein,” said the Netherlands Cancer Institute’s Ton Schumacher, Ph.D. “This other protein, called CMTM6, stabilizes PD-L1 and thereby increases the capacity of cancer cells to inhibit the immune response.”

In addition to being highly relevant for understanding how the PD-L1-checkpoint works, this discovery might also have uncovered a new target for immunotherapy. “You can imagine that blocking CMTM6 could reactivate immune cells just like the currently used PD-L1 blockers can,” added Dr. Schumacher. Blocking both molecules could even be superior. It remains to be seen whether it will eventually deliver a therapy, but this is clearly something we are eager to test.”

The newly identified companion of PD-L1 may also help clinicians predict whether patients will benefit from treatment with checkpoint inhibitors. Dr. Schumacher and his colleagues are currently investigating this in patients who are treated with the current PD-L1-blockers. “The amount of PD-L1 itself can predict treatment success to a limited extent and we have some hope that CMTM6 can improve the precision of this prediction.”

The Netherlands-based team also evaluated CMTM6’s family members, and it found that CMTM4, but no other CMTM proteins, shared CMTM6’s function—reducing the ubiquitination of PD-L1 and increasing its half-life.

“Notably, CMTM6 depletion decreases PD-L1 without compromising cell surface expression of MHC class I,” added the authors of the paper prepared by the Australia-based team. “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 insights into the biology of PD-L1 regulation, identify a previously unrecognized master regulator of this critical immune checkpoint and highlight a potential therapeutic target to overcome immune evasion by tumour cells.”