Targeted release of a bispecific fusion protein SIRPα/Siglec-10 by oncolytic adenovirus reinvigorates tumor-associated macrophages to improve therapeutic outcomes in solid tumors.

Background: The pleiotropic roles of tumor-associated macrophages (TAMs) render them attractive targets in antitumor drug development. CD47/SIRPα (signal regulatory protein alpha) and CD24/Siglec-10 (sialic acid-binding immunoglobulin-like lectin 10) signaling pathways have been found to suppress macrophage phagocytosis of malignant cells. Systemic blockade of CD47/SIRPα has shown severe side effects. Intratumoral delivery of a CD47 inhibitor by oncolytic viruses (OVs) may circumvent this hurdle.

Methods: To identify the characteristics of recombinant adenovirus (AdV)-SIRPα/Siglec-10, we conducted CCK8 assay, quantitative PCR, western blot, competitive binding assay, in vitro cytotoxicity assay, ELISA and phagocytosis assay. We investigated the antitumor immune responses of AdV-SIRPα/Siglec-10 using flow cytometry, various tumor-bearing mouse models, humanized tumor-bearing mouse models, immune cell depletion, RNA sequencing, and in vitro T cell activation assay.

Results: Here, we developed a novel AdV encoding a fusion protein composed of the extracellular domains of murine or human SIRPα and Siglec-10 (SIRPα/Siglec-10), termed AdV-mSS or AdV-huSS. The SIRPα/Siglec-10 was effectively secreted by cells infected with AdV-mSS and functioned as a dual blocker of CD47 and CD24, thereby significantly enhancing macrophage phagocytosis. In a series of tumor models, including subcutaneous and ascitic H22 hepatocellular carcinoma (HCC), subcutaneous Hepa1-6 HCC, MC38 colorectal carcinoma, and Lewis lung carcinoma, AdV-mSS treatment markedly enhanced antitumor efficacy. Mechanistically, AdV-mSS reprogrammed TAMs toward an antitumor phenotype and enhanced the expression of major histocompatibility complex (MHC)-I/II, promoting CD8+T cell proliferation and activation. Depletion of either macrophages or CD8+T cells abrogated the antitumor efficacy of AdV-mSS. Similarly, in a humanized LM3 HCC mouse model, AdV-huSS significantly inhibited tumor growth and prolonged survival.

Conclusions: Dual SIRPα/Siglec-10 inhibitor delivered intratumorally by AdV not only reinvigorated the TAM-CD8+T cell axis but also potentially reduced the risk of off-target effects. Further investigation of AdV-huSS in patients with cancer is warranted in the near future.