Exciting progress in the field of cancer immunotherapy has renewed the urgency of the need for basic studies of immunoregulation in both adaptive cell lineages and innate cell lineages. Here we found a central role for major histocompatibility complex (MHC) class I in controlling the phagocytic function of macrophages. Our results demonstrated that expression of the common MHC class I component β2-microglobulin (β2M) by cancer cells directly protected them from phagocytosis. We further showed that this protection was mediated by the inhibitory receptor LILRB1, whose expression was upregulated on the surface of macrophages, including tumor-associated macrophages. Disruption of either MHC class I or LILRB1 potentiated phagocytosis of tumor cells both in vitro and in vivo, which defines the MHC class I–LILRB1 signaling axis as an important regulator of the effector function of innate immune cells, a potential biomarker for therapeutic response to agents directed against the signal-regulatory protein CD47 and a potential target of anti-cancer immunotherapy.

Signaling through the immune checkpoint programmed cell death protein-1 (PD-1) enables tumor progression by dampening antitumor immune responses. Therapeutic blockade of the signaling axis between PD-1 and its ligand programmed cell death ligand-1 (PD-L1) with monoclonal antibodies has shown remarkable clinical success in the treatment of cancer. However, antibodies have inherent limitations that can curtail their efficacy in this setting, including poor tissue/tumor penetrance and detrimental Fc-effector functions that deplete immune cells. To determine if PD-1:PD-L1-directed immunotherapy could be improved with smaller, nonantibody therapeutics, we used directed evolution by yeast-surface display to engineer the PD-1 ectodomain as a high-affinity (110 pM) competitive antagonist of PD-L1. In contrast to anti-PD-L1 monoclonal antibodies, high-affinity PD-1 demonstrated superior tumor penetration without inducing depletion of peripheral effector T cells. Consistent with these advantages, in syngeneic CT26 tumor models, high-affinity PD-1 was effective in treating both small (50 mm 3 ) and large tumors (150 mm 3 ), whereas the activity of anti-PD-L1 antibodies was completely abrogated against large tumors. Furthermore, we found that high-affinity PD-1 could be radiolabeled and applied as a PET imaging tracer to efficiently distinguish between PD-L1-positive and PD-L1-negative tumors in living mice, providing an alternative to invasive biopsy and histological analysis. These results thus highlight the favorable pharmacology of small, nonantibody therapeutics for enhanced cancer immunotherapy and immune diagnostics.protein engineering | cancer immunotherapy | PET imaging | PD-1 | PD-L1

Cancer immunotherapy has emerged as a promising therapeutic intervention. However, complete and durable responses are only seen in a fraction of patients who have cancer. A key factor that limits therapeutic success is the infiltration of tumors by cells of the myeloid lineage. The inhibitory receptor signal regulatory protein-α (SIRPα) is a myeloid-specific immune checkpoint that engages the "don't eat me" signal CD47 expressed on tumors and normal tissues. We therefore developed the monoclonal antibody KWAR23, which binds human SIRPα with high affinity and disrupts its binding to CD47. Administered by itself, KWAR23 is inert, but given in combination with tumor-opsonizing monoclonal antibodies, KWAR23 greatly augments myeloid cell-dependent killing of a collection of hematopoietic and nonhematopoietic human tumor-derived cell lines. Following KWAR23 antibody treatment in a human knockin mouse model, both neutrophils and macrophages infiltrate a human Burkitt's lymphoma xenograft and inhibit tumor growth, generating complete responses in the majority of treated animals. We further demonstrate that a bispecific anti-CD70/SIRPα antibody outperforms individually delivered antibodies in specific types of cancers. These studies demonstrate that SIRPα blockade induces potent antitumor activity by targeting multiple myeloid cell subsets that frequently infiltrate tumors. Thus, KWAR23 represents a promising candidate for combination therapy.