Focal tumor radiation therapy (RT) induces an immunogenic cell death associated with endogenous adjuvants that promote uptake of cancer antigens by conventional dendritic cells (cDCs). The expression of CD47 is markedly increased in a high proportion of breast cancers (BC) and correlates with poor-prognosis molecular subtypes. CD47 interaction with SIRPα expressed at the surface of cDCs and macrophages provides a negative signal that inhibits phagocytosis of dying cancer cells, hindering cross-presentation of tumor-derived antigens and activation of anti-tumor T cells. Thus, we hypothesized that the “don't-eat-me” signal mediated by CD47/SIRPα acts as a barrier to RT-induced anti-tumor immunity. Using two mouse BC models refractory to immune checkpoint blockade, we investigated the effects of RT combined with SIRPα; blockade on the development of anti-tumor immunity. BALB/c mice were inoculated subcutaneously in one or both flanks with the syngeneic TSA BC cells. RT was delivered to one tumor in 8 Gy doses on three consecutive days once tumor volume reached 60-80 mm3. SIRPα; blocking (MY1, OSAKA University [Garcia, et al. J Immunol. 2011;187:2268]) or isotype control antibodies (Abs) were given one day prior to RT and every 3 days thereafter. Some mice also received PD-L1 blocking Abs (BioXcell) every 3 days starting one day after RT completion. Mice were followed for tumor growth and splenic immune cells were analyzed by flow cytometry. Similar experiments were performed in the AT-3 mouse model of triple-negative BC (C57BL/6 background). SIRPα; blockade alone did not inhibit TSA tumor growth, but it significantly improved tumor response to RT (p<0.05), leading to complete regression of the irradiated tumor in 50-60% of the mice. In addition, the combination of SIRPα; blockade with RT significantly inhibited the growth (p<0.05) of a contralateral non-irradiated tumor (abscopal response), suggesting that it induced a systemic anti-tumor immune response. Consistent with systemic immune activation induced by SIRPα; blockade, the percentage of PD1+ CD4 T cells was increased in the spleen of mice treated with SIRPα; Abs compared to control Abs (p<0.01), regardless of RT. In the myeloid compartment, SIRPα; blockade led to a reduction in PD-L1 expression on type 2 conventional DC (cDC2) whereas cDC1 and macrophages showed increased PD-L1 expression. However, addition of anti-PD-L1 to RT+anti-SIRPα; did not further improve abscopal responses. The ability of SIRPα; blockade to significantly improve tumor control induced by RT was also seen in the AT-3 model (p<0.01).Overall, obtained results support the hypothesis that SIRPα; blockade potentiates the ability of RT to induce anti-tumor T cell responses and improve local and systemic tumor control. We are currently investigating the mechanisms of RT interaction with SIRPα; blockade to provide the rationale for translating these findings to the clinic.

Citation Format: Claire Lhuillier, Maud Charpentier, Sergio Trombetta, Sandra Demaria. Blocking SIRPα on myeloid cells synergizes with radiation therapy in mouse breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB168.