(225f) Macrophage Checkpoint Blockade: From Cell Therapy and CRISPR Models to Acquired Immunity

The inhibitory receptor SIRPα on macrophages binds to the ‘marker of self’ CD47 expressed by all cells to impede phagocytosis. Blocking this macrophage checkpoint to enhance phagocytosis of cancer cells is an emerging cancer therapy, but clinical trials indicate that anti-CD47 monoclonals (i) are ineffective as monotherapies, (ii) have limited success in solid tumors, and (iii) deplete healthy blood cells. Preclinical development of anti-CD47 biologics against liquid tumors in immunocompromised mice do not account for these challenges. Therefore, we used the B16 murine melanoma model, which is syngeneic and weakly immunogenic in immune competent C57 mice, and first deleted CD47 in B16 cells with CRISPR/Cas9. Knockout of CD47 enhanced phagocytosis of B16 cells by primary mouse macrophages more than twofold, but opsonization with a mouse monoclonal against the melanoma antigen Tyrp1 was required for phagocytosis. Knockout tumors were repressed by systemic injections of anti-Tyrp1 at doses that had no effect on control tumors. Approximately 40% of mice survived knockout tumors with treatment, and more than half of these surviving mice rejected tumors when re-challenged. Treated mice generated IgG that recognized B16 surface antigens and functioned as opsonins. To target tumors expressing wild-type levels of CD47, we treated marrow-derived monocytes ex vivo with anti-SIRPα and co-injected with anti-Tyrp1, which extended survival while avoiding anemia caused by anti-CD47. Thus, solid tumors can be safely targeted with macrophage checkpoint blockade in combination with pro-phagocytic IgG antibodies – first administered as a combination therapy and later generated endogenously through adaptive immunity.