The advent of checkpoint inhibitor (CPI) therapy in human cancer has dramatically changed the landscape of treatment strategies in recent years. Previously, cancers with largely ineffective therapeutics and a poor prognosis, such as melanoma, have experienced significant advances of patients able to extend survival and/or progression free disease. Within the tumor microenvironment, a number of immunosuppressive molecules are overexpressed, including CTLA-4, PD-1 and its ligand PD-L1. Simply put, inhibiting these immunosuppressive pathways causes the reactivation of ‘exhausted’ cytotoxic and helper T cells and subsequent attack of the tumor. However, patients treated with CPIs have experienced a variety of untoward side effects of immune mediated adverse events (irAEs), many leading to significant morbidity and uncertainties in balancing the management of both the autoimmunity and the cancer. These adverse events (of grades 1-5) include colitis, pneumonitis, neuropathies, endocrinopathies, nephritis, dermatitis, and arthritis. It is reported that upwards of 80% of CPI treated patient will experience some form of irAE. As the numbers of patients treated with CPIs increases, coupled with the use of CPIs in combination therapies as well as the development of new generations of CPIs, the treatment of irAEs will complicate medical care in these patients. The simple goal of the present proposal is to develop a Luminex based bioassay of the key protein biomarkers to which CPI patients elicit autoantibodies both prior to, and as the appearance of autoimmune syndromes arise. A multiplexed Luminex system can detect as many as 50 biomarkers and be simply modified as new biomarkers may emerge in the future of CPI therapy. The project is strengthened by a PI who directs a CLIA certified diagnostic laboratory in the Department of Medicine at Yale University with close ties to the Section of Medical Oncology and with an existing serum library collection of both patients treated with CPIs and of patients with spontaneous autoimmune disease. Phase I studies will develop and perform early specificity studies with the panel of biomarkers while phase II studies will be expanded to include a screening of larger cohorts of patients on specific CPI types as well as a robust, sensitive and specific bioassay system.