Soluble CD80 Protein Delays Tumor Growth and Promotes Tumor-Infiltrating Lymphocytes.

Tumor cells use various immune-suppressive strategies to overcome antitumor immunity. One such method is tumor expression of programmed death ligand-1 (PD-L1), which triggers apoptotic death or anergy upon binding programmed death-1 (PD-1) on T cells. Our previous cellular studies with human and mouse PD-L1 tumor cells demonstrated that a soluble form of the costimulatory molecule CD80 prevented PD-L1-mediated immune suppression and restored T-cell activation by binding PD-L1 and blocking interaction with PD-1. We now report that treatment of established syngeneic PD-L1 CT26 colon carcinoma and B16F10 melanoma tumors with CD80-Fc delays tumor growth and promotes tumor-infiltrating T cells. Studies with PD-1 and CD28 mice demonstrate that soluble CD80 acts by simultaneously neutralizing PD-1 suppression and activating through CD28. We also report that soluble CD80 mediates its effects by activating transcription factors EGR1-4, NF-κB, and MAPK, downstream signaling components of the CD28 and T-cell receptor pathways. Soluble CD80 binds to CTLA-4 on activated human peripheral blood mononuclear cells. However, increasing quantities of CTLA-4 antagonist antibodies do not increase T-cell activation. These results indicate that soluble CD80 does not suppress T-cell function through CTLA-4 and suggest that CTLA-4 acts as a decoy receptor for CD80, rather than functioning as a suppressive signaling receptor. Collectively, these studies demonstrate that soluble CD80 has therapeutic efficacy in mouse tumor systems and that its effects are due to its ability to inhibit PD-1-mediated suppression while concurrently activating T cells through CD28. .