Australian-led research has discovered the mechanism that enables T-cells to punch through the outer membrane of cancerous cells.
The study, published September 16th in Developmental Cell, investigated the integration of forces at either end of the cytotoxic T-cells (CTLs) whereby they targeted and approached tumorous cells, before forming an intimate junction between the two called a ‘cytotoxic immunological synapse’ (CIS).
CTLs are armed with lytic granules containing two key components for immune attack: perforin, proteins that punch holes in a target cell; and granzymes, which gain access via these holes and kill disease-causing cells.
The research team detected the physical forces within the CTLs that propelled these lytic granules toward the CIS – actomyosin dynamics (contractions), and the ATP powered protein motors, dynein and myosin II.
Dr Daryan Kempe, from the University of New South Wales’ (UNSW) Department of Medicine and Health, who co-led the research, explained that these forces also enabled CTLs to manipulate the membranes of both cells to form the CIS and help transport the lytic granules.
“Our findings show that upon cognate target contact, CTLs exhibit a biphasic contraction of the tail and increase in surface tension, with the first phase driven by rho-associated protein kinase (ROCK) mediated actomyosin contractility, and the second phase dependent on dynein activity,” Dr Kempe said.
“Myosin II and dynein cooperatively contribute to the transport of lytic granules, with the former driving the anterograde motion of lytic granules from the rear of the cell toward the nucleus, and the latter responsible for transporting lytic granules around the nucleus and to the synapse.”
Using human melanoma cell lines, the researchers also found that perforin preferentially perforated convex cancerous cell membranes, rather than those with a concave wall.
“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 CTL mediated cancer cell killing,” Dr Kempe said.
“Remarkably, perforin is degranulated at regions of the synapse with high membrane curvature, where it preferentially disrupts the convex membrane on the tumour cell side. By stretching and bending the membranes of tumour cells in a certain direction, CTLs made it easier for perforin to punch through – but only if the membranes were bent in the right direction.
“Lytic granules were ultimately delivered to degranulation pockets of negative membrane curvature, which arise due to the action of dynein motors at the synapse and induce positive membrane curvature bulges on the target cell.”
Senior author and EMBL Australia Group Leader, Associate Professor Maté Biro from UNSW’s department of Medicine and Health, explained that this preference not only ensured that lytic granules were delivered to their intended recipient, but could also be another level of protection for the CTLs against their own assault.
“Such a mechanism could constitute an additional layer of protection against auto-perforation for T cells, beyond the lipid order and charge of their plasma membrane that were recently shown to increase their tolerance to perforin,” Professor Biro said.
“Membrane curvature can influence lipid composition, and it is therefore possible that highly curved synaptic pockets cause local changes in lipid composition, which regulates susceptibility for perforation.”