SIRPα blockade improves the antitumor immunity of radiotherapy in colorectal cancer.

High-dose hypofractionated radiotherapy (HRT) is an important anticancer treatment modality that activates antitumor host immune responses. However, HRT for oligometastases of colorectal cancer (CRC) has shown frustrating results in the clinic. As part of immune evasion, myeloid cells express signal regulatory protein α (SIRPα) to inhibit phagocytosis by phagocytes in the tumor microenvironment (TME). We postulated that SIRPα blockade enhances HRT by alleviating the inhibitory action of SIRPα on phagocytes. We demonstrated that SIRPα on myeloid cells was upregulated in the TME after HRT. When SIRPα blockade was administered with HRT, we observed superior antitumor responses compared with anti-SIRPα or HRT alone. When anti-SIRPα was administered to local HRT, the TME could become a tumoricidal niche that was heavily infiltrated by activated CD8 + T cells, but with limited myeloid-derived suppressor cells and tumor-associated macrophages. While CD8 + T cells were required for the effectiveness of the anti-SIRPα + HRT combination. The triple therapy with anti-SIRPα + HRT + anti-PD-1 had superior antitumor responses compared with the combination of any two therapies and established a strong and long-lasting adaptive immunological memory. Collectively, SIRPα blockade provides a novel way to overcome HRT resistance in oligometastatic CRC patients. Our results herein provide a valuable cancer treatment strategy that has the potential to be translated into clinical practice.