175 Background: The success of PD-1/PD-L1 inhibition is clearly dependent on optimal patient selection. Developing validated biomarkers that identify patients with an increased probability of response to these antibodies remains a challenge. DTH responses are a standard method to evaluate cellular immune responses in animal and human immunotherapy trials. The trans-vivo delayed-type hypersensitivity (tvDTH) methodology, using the SCID mouse footpad swelling response to human PBMC (peripheral blood mononuclear cells) + antigen injection, and using antibodies to block the regulatory pathways, was developed in our laboratory and has been used to characterize regulation systems in tolerant transplant recipients and more recently in prostate cancer. Methods: In this pilot project we aim to investigate the mechanisms of regulatory immune responses in patients with advanced NSCLC receiving anti-cancer therapies using the tvDTH, and to correlate tvDTH results with responses to checkpoint inhibitor blockade therapy. So far we have enrolled 4 NSCLC patients. PBMCs are obtained by blood draw and lung cancer antigen (LCA) is prepared from malignant pleural effusion cellular pellet or fresh frozen tumor. 1 of 4 patients’ sample was negative for malignancy. Results: Our preliminary data suggests that all NSCLC patients with pleural effusions positive for malignancy had a PD-1-dependent exhausted immune response towards their LCA. Remarkably, 1 of these patients not only showed PD-1-dependent exhaustion, but also PD-1-dependent bystander suppression, whereby a positive control immune response was reduced by co-injection of LCA, suggesting the existence of immunosuppressive mechanisms. Unlike our previous results with prostate cancer, no patient showed CTLA-4-dependent exhaustion or suppression. Importantly, an antigen preparation from a NSCLC that showed no malignant cells by cytology did not induce a swelling response. Conclusions: We plan to expand our study patient population and follow up our patients in order to correlate tvDTH results with clinical outcomes.