Abstract 4619: CD47 promotes ovarian cancer progression by inhibiting macrophage phagocytosis

CD47 is highly expressed in many malignant diseases including ovarian cancer, and renders malignant cells resistant to phagocytosis by macrophages. Anti-CD47 mAbs have shown anti-cancer effects in several types of cancers. However, it remains controversial whether anti-CD47 mAbs promote phagocytosis by the inhibition of CD47 function or by CD47-SIRPα independent opsonization. We tried to provide more evidence in our study by knocking down CD47. CD47 expression of 93 patients was tested by flow-cytometry. We confirmed a positive correlation between high CD47 levels and poor clinical characteristics, such as poorly differentiated and advanced FIGO stages. A cohort of 29 FIGO stage III-IV patients with poorly differentiated serous adenocarcinoma were followed up for 50 months. Median survival for CD47 high patients was 19 months, while for CD47 low patients was 43months, p=0.0435. To further evaluate the mechanism of CD47 in phagocytosis, CD47 expression on SK-OV-3 cell was knocked down using shRNA. No change in tumor cell viability, proliferation, and migration was observed. Macrophage phagocytosis index increased from 10.2% to 43.9% in vitro. In xenograph experiments, average tumor number decreased from 75.6 to 50.4 and the animals also experienced significantly longer survival (p=0.0017). Similar sets of experiments were done using an anti-CD47 mAb B6H12 clone while adopting both control IgG and anti-HLA ABC as control antibodies. Consistent with the results from the knockdown experiments, anti-CD47 mAb treatment led to an increase in macrophage phagocytosis index in vitro of both SK-OV-3 cells (from 8% and 9.4% to 37.1%) and primary cancer cells (from 5.6% and 7.4% to 35.1%). In vivo xenograft experiments also showed increased macrophage infiltration (6.5% compared to 1.5% in control group). In five different ovarian cancer cell lines, we found that CD113+ tumor initiating cells (TICs) expressed higher levels of CD47 compared to CD133- cancer cells. Tumorigenic capabilities of TICs were confirmed by colony-forming assay and ALDEFLUOR assay. Upon anti-CD47 mAb treatment, macrophage phagocytosis of TICs was enhanced significantly in vitro (phagocytosis index increased from 10% and 12.7% to 47.3%). In conclusion, CD47 inhibits macrophage phagocytosis of both ovarian cancer cells and TICs. Downregulation of CD47 or inhibition by mAb was able to reverse this effect, and it may involve Fc independent mechanism. Thus, CD47 antibody therapy may be a promising strategy for not only eliminating tumor cells but also preventing cancer relapse triggered by TICs, benefiting the patients’ short-term and long-term prognoses. Citation Format: Yuting Huang, Peng Gao, Yifeng He, Yuchi Ma, Shangwen Dong, Zhijun Li. CD47 promotes ovarian cancer progression by inhibiting macrophage phagocytosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4619. doi:10.1158/1538-7445.AM2017-4619