Cancer cells are adept at escaping death. One of the ways in which cancer cells prevent death from fatal attacks by cytotoxic T lymphocytes (CTLs) is by efficiently repairing damage to their cell membranes.
With vivid live-cell videographic micro-imagery, a new study published in the journal Science “ESCRT-mediated membrane repair protects tumor-derived cells against T cell attack“, uncovers mechanistic insights into how cancer cells escape death, which could increase the effectiveness of current immunotherapies for cancer.
Senior author Ira Mellman, PhD, vice president of oncology research at Genentech, said, “This study shows cancer cells use a generalized membrane repair mechanism to protect themselves against immune attack. Expected and unexpected at the same time. Evolutionarily, this may help cells guard against unwanted or ‘accidental’ killing under healthy conditions, but in cancer, it may explain at least a portion of inherent resistance to immunotherapy. Understanding and solving these mechanisms will contribute to more effective therapeutic approaches.”
CTLs zero in on abnormal cells such as virus-infected cells and tumor cells, killing them by shooting dual bullets: perforin and granzymes. These are protein toxins. Perforin punctures tiny holes in the outer membrane of the targeted aberrant cell. Granzymes, which are proteolytic enzymes, pass through the openings and induce programmed cell death (apoptosis). The mechanism by which cancer cells escape this deadly immune attack was unclear until now.
Mellman said his team was inspired to take on this investigation “to understand precisely what transpires between a CTL and its target at the moment of cell killing. Remarkably, despite decades of interest in the problem, this precise issue has remained unexplored.”
Another realization that intrigued Mellman’s team stemmed from their imaging and functional studies: not all cell contacts were productive. “We wondered whether cells had intrinsic mechanisms that could protect them against CTL attack,” Mellman said. “This brought us to ESCRTs [Endosomal Sorting Complexes Required for Transport], which have for years been implicated in membrane repair.”
Alex Ritter, PhD, a postdoctoral researcher in Mellman’s laboratory in the department of cancer immunology at Genentech, and his colleagues combined high-resolution imaging data with functional analysis in live cells to demonstrate cancer cells overcoming CTL attack through a class of proteins called ESCRTs, which repair openings created by perforin in the target cell’s outer membrane. This efficient repair mechanism delays or prevents the entry of killer granzymes into the cancer cell.
“We found that ESCRT proteins were precisely recruited in target cells to sites of CTL engagement immediately after perforin release,” the authors noted.
To exploit this newfound mechanism for therapeutic advantage, the investigators reasoned, inhibition of the target cell’s ESCRT-mediated pore-repairing strategy should make it easier for CTLs to kill them.
As earlier studies have shown extinguishing the ESCRT pathway for extended periods can itself kill cells, the researchers used two alternative approaches to inhibit the ESCRT pathway in target cells: they used CRISPR to knock out the gene Chmp4b, a component of the ESCRT pathway that sorts engulfed cell-surface receptors into vesicles. They also over-expressed a dominant mutant form of a phosphorylating enzyme, VPS4, in the ESCRT pathway…..
