Regulation of Anti-Tumor Immunity by B7-H3

Abstract Checkpoint blockade, which targets inhibitory receptors that suppress successful T cell responses to tumors, demonstrates the critical role that the immune response plays against cancer. For example, many tumors increase surface expression of a protein PD-L1 (originally called B7-H1) on their surface which binds to an inhibitory receptor on T cells, PD-1. Thus, by increasing expression of PD-L1, tumors develop a mechanism to escape the immune system. One of the most successful immunotherapies targets the PD-1/PD-L1 pathway by using blocking antibodies, which enhance the immune response to tumors. However, not all tumors upregulate PD-L1 and not all cancers respond to immunotherapy or checkpoint blockade targeting PD-1 or CTLA-4. Thus, other mechanisms by which tumors evade the immune response must be identified to be able to design novel immunotherapies that can disrupt these interactions and enhance anti-tumor immune responses. B7-H3, which is related to PD-L1, is overexpressed on many different tumor types and can inhibit the immune response. However, although B7-H3 was cloned twenty years ago, the identity of the immune inhibitory receptor(s) which bind to B7-H3 is not currently known. The inability to identify the inhibitory receptors through which it exerts its immunomodulatory effects during tumor responses have greatly impeded progress toward developing effective therapeutics, as it is unclear whether any of the B7-H3 antibodies examined in clinical trials functionally block the interaction of B7-H3 with inhibitory receptors. Siglecs (Sialic acid binding immunoglobulin like lectins) are a family of transmembrane proteins that are expressed almost exclusively on immune cells. Many but not all Siglecs are inhibitory and contain an intracellular ITIM (immunoreceptor tyrosine-based inhibitory motif) and ITSM (immunoreceptor tyrosine-based switch motif) in their cytoplasmic tails. Intriguingly, these are the same motifs that are found in the cytoplasmic tail of PD-1 which exerts immune inhibition. Here, we now identify that human B7-H3 binds to the inhibitory receptor Siglec-9 which is found on human NK cells and T cells. Novel monoclonal antibodies against B7-H3 which block this interaction with Siglec-9 promotes NK cell killing in vitro and anti-tumor CART-mediated killing in NSG mice. The focus of this proposal is to define the biochemical basis for B7-H3/Siglec-9 interactions and its role in inhibiting anti-tumor immunity. The well characterized pre-clinical tumor models in NSG mice capture key features of human disease and enables mechanistic investigations not feasible in human subjects. The work here is foundational to understanding how B7-H3-expressing tumors limit an effective immune response