An approach to kill high-grade serous ovarian cancer cells by complement

Abstract

Epithelial ovarian cancer is the sixth leading malignancy amongst women. High-grade serous ovarian cancer (HGSOC) is the most common and fatal subtype. Most patients develop ascites, which promotes metastasis through the peritoneal cavity by increased pressure and contains cytokines that stimulate immune escape. However, ascites may also contain components of an active immune system, such as complement factors and tumor-directed antibodies. The tumor cells protect themselves from complement killing through, for instance, overexpression of the complement inhibitors CD59 on membranes and Factor H in the fluid phase. Our goal is to study whether local inhibition of these complement inhibitors could allow antibodies and complement in ascites to kill tumor cells.

We obtained ascites fluid samples (n=101) from ovarian cancer patients treated at the Helsinki University Hospital Women’s Clinic. The ascites samples were screened for the presence of tumor-directed IgG antibodies by immunoblotting. The presence of classical pathway (CP) complement factors in ascites was examined by in-house sandwich ELISA. We observed a wide diversity between patients in both levels and types of antibodies and CP activity in ascites samples. Hemolytic assays showed that monoclonal rat IgG2 anti-human CD59 (YTH53.1) allowed about 25% of ascites fluids to kill red blood cells at a comparable level as normal human serum. YTH53.1 and anti-Factor H increased HGSOC cytotoxicity by ascites fluids only slightly, but some ascites fluid samples, rich in tumor-directed IgG seem to result in higher cytotoxicity in combination with classical pathway complement-containing NHS.

In summary, ascites fluids differ strongly in levels of HGSOC-directed IgG and classical complement activity. Ascites fluids with strongly reactive HGSOC-directed IgG seem to have more HGSOC cytotoxic capacity, especially in combination with NHS, which is slightly increased by anti-CD59 and anti-FH antibodies.