Abstract 5067: Inhibition of CD47-SIRPα axis to enhance immunity against different molecular subtypes of breast cancer
Authors: Ma Xingru , Hartman Zachary , Tsao Li-Chung , Wang Tao , Liu Cong-Xiao , Yang Xiao , Lei Gangjun , Wei Junping 
Published: April 2023

Abstract
Background: The CD47-SIRPα axis is an innate immune checkpoint that primarily regulates myeloid cells. In the context of cancer, tumor cells upregulate CD47 to engage SIRPα inhibitory signaling to block phagocytosis. Thus, inhibition of this axis may enhance myeloid cell phagocytosis and adaptive immune responses against cancer. While our previous studies identified that CD47 mAbs enhance HER2-specific mAb therapy (Tsao et al., 2019), clinical trials have revealed that CD47-targeting mAbs are associated with severe side effects of anemia and thrombocytopenia in patients. We thus hypothesized that targeting SIRPα, which is more restrictively expressed on myeloid cells, may achieve similar but less toxic treatment effects.
Methods: To ascertain the impact of SIRPα inhibition against different molecular subtypes of breast cancer, we utilized and validated a novel SIRPα KO mouse, along with SIRPα mAbs in our previously utilized models of Triple-Negative Breast Cancer (TNBC) and HER2+ Breast Cancer (Crosby et al., 2018; Tsao et al., 2019). Using both in vitro and in vivo tumor models, we tested the therapeutic impact of SIRPα inhibition/blockade combined with immunogenic chemotherapy (doxorubicin) in TNBC or combined with HER2-specific mAbs in HER2+ Breast Cancer (HER2+ BC).
Results: After validating the loss of SIRPα in KO mice, we found that E0771 TNBC cells had reduced rates of engraftment in SIRPα KO mice compared to controls, but comparable rates of growth once tumors were established. We then treated E0771 engrafted SIRPα KO and wildtype mice with doxorubicin. This treatment elicited tumor rejection in the majority of SIRPα KO mice (70% of injected tumors), compared to only 10% rejection in SIRPα WT mice. To study tumor antigen-specific immunity, we expressed different forms of OVA (secreted, cytoplasmic, membrane) in tumor cells. We found that membrane expressed OVA led to universal tumor rejection (10/10 mice) with high levels of OVA-specific Abs, in contrast to non-membrane expressed forms of OVA. This suggested the importance of tumor-specific Abs in eliciting tumor rejection. To specifically test the antitumor effect of tumor-specific antibodies in the context of SIRPα blockade, we evaluated the impact of a HER2 mAb (Trastuzumab) on HER2+ BC cells. We found enhanced phagocytosis and antitumor effects by SIRPα KO macrophages in comparison to wild-type macrophages, suggesting the potential of targeting SIRPα along with the use of standard-of-care mAb.
Conclusion: Our study demonstrates that SIRPα loss can prevent tumor engraftment but may not have a strong effect against established tumors without secondary immune stimulation. However, addition of immunogenic cell death-eliciting chemotherapy or tumor-specific Abs promote tumor phagocytosis and stimulate anti-tumor immunity. These results support further investigation of SIRPα-targeting combination therapies for cancer.
Citation Format: Xingru Ma, Zachary Hartman, Li-Chung Tsao, Tao Wang, Cong-Xiao Liu, Xiao Yang, Gangjun Lei, Junping Wei. Inhibition of CD47-SIRPα axis to enhance immunity against different molecular subtypes of breast cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5067.