Platelets Promote Activation of the Complement System in Ovarian Cancer

Platelets promote metastasis and growth of ovarian cancer. We have shown that platelets extravasate into the tumor microenvironment (TME) and increase proliferation and epithelial-mesenchymal transition (EMT) in ovarian cancer cells. We have also shown that activation of the complement system in TME of ovarian cancer enhances tumor growth. Ovarian cancer cells secrete complement proteins that upon activation in the TME increase proliferation of cancer cells and promote EMT via an autocrine pathway. The activators of the complement system in the TME have not been identified. We have demonstrated that upon activation platelets activate the complement system on their surface. In the current study, we examined whether extravasated platelets inside tumors contribute to the complement activation in the TME.

1) We examined the effect of antiplatelet reagents on platelet extravasation into TME, using murine models of ovarian cancer. Tumors induced by injection of ovarian cancer cells into the peritoneum of Nu/Nu mice were resected after 6-8 weeks and the number of extravasated platelets was determined by immunostaining tumor sections and counting the number CD42 (GPIb) positive cells that were outside the blood vessels (CD31 positive). We found that platelet extravasation is an active process and platelet inhibition by aspirin or ticagrelor reduces the number of extravasated platelets. Furthermore, P2Y12 deficient platelets extravasate less than normal platelets. In all of these experiments, the number of extravasated red blood cells were significantly less than extravasated platelets and was not affected by the inhibition of platelets.

2) We examined the effect of platelet inhibition on the activation of the complement system in the TME. We immunostained resected tumors from aspirin- or ticagrelor-treated tumor-bearingmice and from P2Y12-deficient tumor-bearingmice for the endproductof complement activation (C5b-9 or membrane attack complex). Inhibition of platelet function by aspirin or ticagrelor,or the presence of hypoactive platelets in P2Y12 deficient mice reduced the amount of C5b-9 deposited in the tumors induced in murine models of ovarian cancer.

Our result showed that complement activation in the TME is at least partially dependent onextravasated platelets. We propose that platelets in addition to directly increasing proliferation of ovarian cancer cells, also enhance tumor growth by activating the complement system in the vicinity of cancer cells. Our study links platelets, complement activation, and ovarian cancer growth; and raises the possibility of using antiplatelet reagents and complement inhibitors as novel synergisticanti-tumor reagents in ovarian cancer.