CD47, a "don't eat me" signal for phagocytic cells, is expressed on the surface of all human solid tumor cells. Analysis of patient tumor and matched adjacent normal (nontumor) tissue revealed that CD47 is overexpressed on cancer cells. CD47 mRNA expression levels correlated with a decreased probability of survival for multiple types of cancer. CD47 is a ligand for SIRPα, a protein expressed on macrophages and dendritic cells. In vitro, blockade of CD47 signaling using targeted monoclonal antibodies enabled macrophage phagocytosis of tumor cells that were otherwise protected. Administration of anti-CD47 antibodies inhibited tumor growth in orthotopic immunodeficient mouse xenotransplantation models established with patient tumor cells and increased the survival of the mice over time. Anti-CD47 antibody therapy initiated on larger tumors inhibited tumor growth and prevented or treated metastasis, but initiation of the therapy on smaller tumors was potentially curative. The safety and efficacy of targeting CD47 was further tested and validated in immune competent hosts using an orthotopic mouse breast cancer model. These results suggest all human solid tumor cells require CD47 expression to suppress phagocytic innate immune surveillance and elimination. These data, taken together with similar findings with other human neoplasms, show that CD47 is a commonly expressed molecule on all cancers, its function to block phagocytosis is known, and blockade of its function leads to tumor cell phagocytosis and elimination. CD47 is therefore a validated target for cancer therapies.

During oncogenesis, tumors develop mechanisms to avoid rejection by the immune system. Recent studies have identified CD47 as an anti-phagocytic “don't eat me” signal tat cancer cells employ to inhibit macrophage-mediated destruction. Here, we modified the 14 kDa binding domain of human SIRPα, the receptor for CD47, for use as a CD47 antagonist. Using in vitro evolution via yeast surface display, we engineered high-affinity SIRPα variants with up to a 50,000-fold increase in affinity for human CD47 relative to wild-type SIRPα. As high-affinity SIRPα monomers, the variants potently antagonized CD47 on cancer cells, but to our surprise, they did not induce macrophage phagocytosis on their own. Instead, the high-affinity SIRPα monomers exhibited remarkable synergy with all tumor-specific monoclonal antibodies tested by increasing phagocytosis in vitro and enhancing anti-tumor responses in vivo. This novel “one-two punch” directs immune responses against tumor cells while lowering the threshold for macrophage activation, thereby providing a universal method for augmenting the efficacy of therapeutic anti-cancer antibodies.

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Antibodies against CD47, which block tumor cell CD47 interactions with macrophage signal regulatory protein-α, have been shown to decrease tumor size in hematological and epithelial tumor models by interfering with the protection from phagocytosis by macrophages that intact CD47 bestows upon tumor cells. Leiomyosarcoma (LMS) is a tumor of smooth muscle that can express varying levels of colony-stimulating factor-1 (CSF1), the expression of which correlates with the numbers of tumor-associated macrophages (TAMs) that are found in these tumors. We have previously shown that the presence of TAMs in LMS is associated with poor clinical outcome and the overall effect of TAMs in LMS therefore appears to be protumorigenic. However, the use of inhibitory antibodies against CD47 offers an opportunity to turn TAMs against LMS cells by allowing the phagocytic behavior of resident macrophages to predominate. Here we show that interference with CD47 increases phagocytosis of two human LMS cell lines, LMS04 and LMS05, in vitro. In addition, treatment of mice bearing subcutaneous LMS04 and LMS05 tumors with a novel, humanized anti-CD47 antibody resulted in significant reductions in tumor size. Mice bearing LMS04 tumors develop large numbers of lymph node and lung metastases. In a unique model for neoadjuvant treatment, mice were treated with anti-CD47 antibody starting 1 wk before resection of established primary tumors and subsequently showed a striking decrease in the size and number of metastases. These data suggest that treatment with anti-CD47 antibodies not only reduces primary tumor size but can also be used to inhibit the development of, or to eliminate, metastatic disease.L eiomyosarcoma (LMS) is a neoplasm of smooth muscle cells that can arise in the uterus or in soft tissue throughout the body. Currently, there exist limited therapeutic options for patients diagnosed with LMS, and the lack of actionable prognostic markers and a limited understanding of the biological mechanisms underlying LMS complicate the clinical management of these tumors (1). The rate of metastatic relapse for these tumors following local treatment is ∼40% at 5 y, leading to, in most cases, an incurable condition (2, 3).Macrophages are monocyte-derived phagocytic cells that play crucial roles in adaptive and innate immunity. Tumor-associated macrophages (TAMs) also play important roles in tumor behavior, depending on their polarization. M1, or "classically activated" TAMs, can mediate anticancer effects by eliciting antitumor-adaptive immunity mechanisms that include phagocytosis. In contrast, M2, or "alternatively activated" TAMs, suppress adaptive immunity and promote a tumor microenvironment (TME) that can augment cancer progression. In many types of carcinomas, TAMs function as promoters of cancer progression, presumably via their ability to mediate tumor angiogenesis, increase extracellular matrix breakdown, aid in tumor invasion, and augment the capacity of tumor cells to form distant metastases (4-6). The TME's role as a nonneoplastic co...