T Cell Topography Enhances Target Perforation

Researchers in Australia have described a mechanism by which cytotoxic T cells use mechanical force to deliver a killer punch to cells that are infected or cancerous.  

Cytotoxic T cells, also known as killer T cells, are specialized immune cells armed with lytic granules containing two key components for immune attack: perforin, which are proteins that punch holes in the target cells, and granzymes, which gain access via these holes and ultimately kill disease-causing cells. T cells snuggle up to targeted diseased cells and form an intimate junction between the two, called the cytotoxic immunological synapse.

In a recent study, a research team at University of New South Wales (UNSW) Sydney’s EMBL Australia Node in Single Molecule Science in the School of Biomedical Sciences found that mechanical forces generated by T cells influence how effectively perforin can punch through tumor cell membranes. In a paper published recently in the journal Developmental Cell, they describe the cell interactions and the integration of forces at both the front and rear of the cell. They detected physical forces within T cells that propel lytic granules toward the immunological synapse where their payloads are released. These forces also enable T cells to grab onto regions of the cancer cell membrane where the membranes of both immune and target cells are pulled and manipulated.

“It was very exciting to discover that, in addition to its mechanical tension and biochemical configuration, the shape of the target cell membrane plays an important role in T cell mediated cancer cell killing,” says Dr. Daryan Kempe at UNSW Medicine & Health.

Essentially, by stretching and bending the membranes of tumor cells in a certain direction, T cells made it easier for perforin to punch through, but only if the membranes were bent in the right direction. Using human melanoma cell lines, the UNSW group demonstrated that perforin preferentially perforated outwardly curved tumor cell membranes, rather than inwardly curved ones. The authors think that this bias ensures that the killer payload is deliver to its intended recipient and could also be another level of protection for the T cells from their own assault.

“As the granules arrive, their contents will be emptied at this region of the membrane that is very highly curved. That there was a bias between positively curved and negatively curved membranes was completely unexpected,” says EMBL Australia Group Leader, Associate Professor Maté Biro at UNSW Medicine & Health.

Biro said that most of the experiments in the study relied on delicate biophysical assays with cancer cell lines, and T cells isolated from healthy blood donors and mice. They used high precision microfluidic pumps, computer-controlled micromanipulators and micropipettes in which the pressure could be controlled independently. “This technique really allows us to tease apart the whole integrated process because it is such a controlled method,” Biro says. “One micropipette picks up a T cell and another picks up a tumor cell, and we bring them into contact on a microscope. We image the entire cytotoxic process. At the same time, because we control and know the exact pressure inside each of the micropipettes, we can also measure the mechanical properties of the cells as they are interacting and engaging in the process.”

This study adds to the understanding of fundamental mechanisms involved in how T cells destroy disease-causing or compromised cells in our bodies. Knowing that mechanical forces are also at play when pore-formers, like perforin, punch through target cells could also help researchers investigating how these proteins work at the molecular level.