OSTEOPONTIN AS A NOVEL IMMUNE CHECKPOINT

Abstract

The host adaptive immune system functions to discriminate self from non-self, eliminating threats from viral infection to tumors. Cytotoxic lymphocytes (CTLs) are the primary effector arm of adaptive immunity. To prevent aberrant activation and autoimmunity, immune checkpoints function physiologically to restrain the CTL response. Tumors pathologically express these checkpoints, preventing immune-driven tumor clearance. Accordingly, immune checkpoint inhibitors (ICIs) have shown remarkable clinical success. However, many types of malignancies, as well as many individual patients with responsive tumor types, fail to benefit from current ICI immunotherapies. This conundrum suggests that as-yet undiscovered immune checkpoints exist. We observed that mice deficient in the transcription factor interferon regulatory factor eight (IRF8) tolerated allogenic tumor grafts and demonstrated impaired CTL activation with an accumulation of CD44hi memory-like CTLs. We sought to investigate the mechanism of this immunosuppression. Conditional deletion of IRF8 in T cells, as well as a mixed chimera model, demonstrated that IRF8 did not directly control CTL activation or differentiation into a CD44hi population. Instead, global loss of IRF8 lead to an expansion of an immature myeloid CD11b+Ly6G+Ly6Clo population which highly expressed osteopontin (OPN), a physiological ligand for CD44. Elevated levels of OPN were shown to suppress murine CTL activation and proliferation. A similar IRF8-OPN-CD44 axis was observed in murine and human colorectal cancer, which is refractory to current ICI therapies. Malignant cells and human patients displayed enhanced OPN levels relative to healthy donor controls. This was shown to be mediated by loss of IRF8 expression, which directly bound to the OPN promoter to repress its transcription. Elevated levels of OPN similarly prevented human CTL activation, and higher levels of OPN were correlated with decreased survival in human patients. We have shown that the IRF8-OPN-CD44 axis functions as a novel immune checkpoint in both myeloid and tumor cells. Blockade of OPN may have potent anti-tumor activity, expanding the pool of patients responsive to ICI therapy.