Neuropilin-1: a checkpoint target with unique implications for cancer immunology and immunotherapy

Checkpoint blockade immunotherapy established a new paradigm in cancer treatment: for certain patients curative treatment requires immune reinvigoration. Despite this monumental advance, only 20%-30% of patients achieve an objective response to standard of care immunotherapy, necessitating the consideration of alternative targets. Optimal strategies will not only stimulate CD8⁺ T cells, but concomitantly modulate immunosuppressive cells in the tumor microenvironment (TME), most notably regulatory T cells (T_reg cells). In this context, the immunoregulatory receptor Neuropilin-1 (NRP1) is garnering renewed attention as it reinforces intratumoral T_reg cell function amidst inflammation in the TME. Loss of NRP1 on T_reg cells in mouse models restores antitumor immunity without sacrificing peripheral tolerance. Enrichment of NRP1⁺ T_reg cells is observed in patients across multiple malignancies with cancer, both intratumorally and in peripheral sites. Thus, targeting NRP1 may safely undermine intratumoral T_reg cell fitness, permitting enhanced inflammatory responses with existing immunotherapies. Furthermore, NRP1 has been recently found to modulate tumor-specific CD8⁺ T cell responses. Emerging data suggest that NRP1 restricts CD8⁺ T cell reinvigoration in response to checkpoint inhibitors, and more importantly, acts as a barrier to the long-term durability of CD8⁺ T cell-mediated tumor immunosurveillance. These novel and distinct regulatory mechanisms present an exciting therapeutic opportunity. This review will discuss the growing literature on NRP1-mediated immune modulation which provides a strong rationale for categorizing NRP1 as both a key checkpoint in the TME as well as an immunotherapeutic target with promise either alone or in combination with current standard of care therapeutic regimens.