CD200-CD200R as a myeloid immune checkpoint in solid tumors: mechanisms, context-dependent functions, and therapeutic opportunities.

CD200 and its receptor CD200R form a myeloid-centered immune checkpoint that normally restrains myeloid activation and limits tissue damage, but many solid tumors co-opt this pathway to suppress antitumor immunity. CD200-CD200R signaling is highly context-dependent, varying even among models of the same cancer type. In melanoma and breast carcinoma, it can either suppress protumor inflammation or promote tumor growth and metastasis depending on the model, whereas in neuroblastoma; glioblastoma and other central nervous system tumors; cutaneous and head and neck squamous cell carcinoma; colorectal cancer; and several endocrine, ovarian, and pancreatic cancers, it consistently suppresses macrophage, dendritic cell, natural killer cell, and CD8+ T-cell activation, impairs phagocytosis and antigen presentation, and drives immunosuppressive infiltration. Non-small cell lung cancer and renal cell carcinoma emerge as two additional, less-characterized but clinically important contexts warranting further investigation. CD200R1-CD200 has also recently been identified as a macrophage phagocytosis checkpoint that functions independently of CD47-SIRPα, placing it within the expanding family of innate immune checkpoints. Rather than treating CD200-CD200R as a universally protumorigenic target, we propose a context-dependent framework-incorporating tumor type, microenvironmental architecture, and dominant myeloid and lymphoid programs-to guide tumor-specific therapeutic strategies in solid cancers.