MDB-73. Inhibiting the B7-H3 Checkpoint in Medulloblastoma

Abstract INTRODUCTION Tumors express immune checkpoints to evade detection and clearance by the immune system. Immune checkpoint inhibition (ICI) allows for tumor clearance. Medulloblastoma (MB) has not seen success with ICI and relapsed/refractory tumors remain incurable. B7-H3 is an immune checkpoint that is highly and specifically expressed by many cancers including MB. It is implicated in immune evasion in cancer. We hypothesize inhibiting B7-H3 in MB will enhance the anti-tumorigenic properties of immune cells. METHODS Murine MB cells, which mimic aggressive disease, were transduced with a CRISPR/Cas9 lentivirus (courtesy of Zhang lab) to create a B7-H3 knockout (B7-H3–). Unsuccessful iterations of the same CRISPR/Cas9 sgRNA were used as a B7-H3+ control. B7-H3+ and B7-H3– cells were co-cultured for 5 days with splenic, bead isolated, CD4-T cells from C57BL/6J mice. Proliferation of the T cells was measured with Cell Trace Violet via flow cytometry using a Cytek Aurora. RESULTS We saw comparable tumor cell inhibition of T cell proliferation when cultured with B7-H3+ and B7-H3– tumor cells, as measured by CTV elution. CONCLUSIONS We saw similar amounts of T cell proliferation inhibition from both B7-H3 – and + tumor cells. This could indicate a few possibilities. First, B7-H3 may not be contributing to CD4-T cell inhibition, in which case it is important to explore its role in other immune cells. Another option is that B7-H3 is targeting the function of these cells, rather than the proliferation. To examine this, we could look at markers of effector function. Lastly, B7-H3 could be mediating inhibition of CD4-T cells via other cell(s) in the tumor microenvironment (TME) of MB, which we could test in vivo. If B7-H3 promotes ant-tumorigenic immune responses, it would be a promising approach relapsed/refractory medulloblastoma that presently has no cure, as well as other cancers that highly express it.