Summary Combination therapy with anti-CTLA-4 and anti-PD-1 mAbs has emerged as the most potent and durable cancer immunotherapy. However, the autoimmune adverse effect associated with the combination therapy is considerably more severe, with 50-90% of melanoma patients developing grade 3 and 4 immunotherapy-related adverse effect. A major challenge in cancer immunotherapy is how to reduce adverse effects of the combination therapy without affecting therapeutic efficacy. This is in part related to the fact that the mechanism by which combination therapy exacerbate irAE is not well understood. To address this issue, we have developed a novel model in which combination therapy with anti-mouse PD-1 in conjunction with either Ipilimumab or Tremelizumab recapitulates the severe irAE observed in clinic. Importantly, different anti- human CTLA4 antibodies differ dramatically in irAE in this preclinical model. We have carried out extensive preliminary studies to elucidate the molecular basis of irAE-prone vs non-prone antibodies and have shown the antibody-mediated degradation of membrane-associated CTLA-4 as a major feature of irAE-prone antibodies. Meanwhile, it has long been established that soluble CTLA-4 (sCTLA4) molecule is protective against autoimmune diseases in the mice. We have obtained data that showed strong correlation between the levels of sCTLA4 and irAE in the clinic. We further established a striking correlation between high binding to soluble CTLA4 protein and the severity of irAE in human CTLA4KI mice. Based on these unexpected observations and the strong therapeutic effect of CTLA4-Fc (Abatacept, marketed as Orencia) for autoimmune diseases including irAE, we hypothesize that both cell surface and sCTLA-4 confers protection against irAE and that sCTLA4 that evades clearance by irAE-inducing anti-CTLA-4 antibodies is a potential therapeutic for prevention and treatment of irAE. Our proposal challenges the prevailing paradigm that autoimmunity and cancer immunity are based on the same mechanism and thus intrinsically linked. More importantly, our work will have a transforming impact in immune-oncology research, as it may offer prophylactic and treatment for irAEs associated with the most effective immunotherapy in use in the clinic.

Project Narrative Immunotherapy related adverse events (irAE), especially the irAE associated with combination of anti- CTLA4 and anti-PD1/PD-L1, are serious clinical problems that affect the cancer treatment efficacy and compliance, as well as patients' quality of life. Our hypothesis is reduction of cell surface and soluble CTLA4 by anti-CTLA-4 antibodies may contribute to irAE. Our work will not only establish a fundamental mechanism of irAE but also a treatment for adverse effect associated with anti-PD-1 and anti-CTLA-4 combination therapy.