Lysis of tumor cells by natural killer cells in mice is impeded by platelets.

TLDR

NK cells reduce metastasis, yet some tumor cells metastasize by inducing platelet aggregation, which may counteract NK activity—a role not previously considered. The study demonstrates that platelets directly protect tumor cells from NK lysis in vitro and in vivo. Platelet aggregates form a surface shield around tumor cells, blocking NK cell access. Platelet depletion lowered metastatic seeding in NK‑sensitive tumors, while platelet aggregation inhibited NK cytotoxicity, demonstrating that platelets protect tumor cells from NK lysis across mouse strains.

Abstract

Natural killer (NK) cells provide effective antitumoral activity in the blood stream of mice, leading to reduced metastasis. There are, however, tumor cells that metastasize despite the presence of an intact NK system. The capability of tumor cells to induce platelet aggregation, on the other hand, correlates with their enhanced metastatic potential. A counteractive role of platelets for the NK function in metastasis has never been conceived. Here we demonstrate for the first time that platelets directly protect tumor cells from NK lysis in vitro as well as in vivo. Using three different tumor cell lines in a mouse model of experimental metastasis, tumor seeding in the target organs was reduced when the host was platelet depleted, but only if the tumor cells were NK sensitive. Aggregation of platelets around tumor cells also inhibited in vitro NK tumorilytic activity. This protection of tumor cells by platelets was mouse strain independent and was equally observed with platelets from beta2-microglobulin-deficient mice, excluding a NK inhibitory function of MHC class I on platelets. Thus, even if tumor cells are NK susceptible and cytotoxic NK cells threaten their survival in the blood, platelets are capable of protecting them from cytolysis, thereby promoting metastasis. Surface shielding by platelet aggregates seems to be the main mechanism of this protection.