Abstract Despite advances in surgery, radiotherapy, and molecular targeted therapies, mortality in non-small cell lung cancer (NSCLC) remains high. Immune checkpoint inhibitors (ICI) targeting the PD-1/PD- L1 axis have been approved for the treatment of advanced NSCLC, yet >70% of the patients experience little clinical benefit due to a variety of immunosuppressive barriers in the tumor microenvironment (TME). Myeloid suppressor cells exert potent immunosuppressive activity in the TME that promotes tumor progression, mediates resistance to immunotherapy, and confers poor outcomes in a variety of cancer types. We have shown that low dose stereotactic body radiation therapy (SBRT) in combination with ICI confers durable anti-tumor immunity resulting in marked tumor regression and improved survival in mouse models of NSCLC. Unexpectedly, we uncovered that SBRT-mediated activation of lung resident Scgb1a1+ club cell secretome is necessary for improving the efficacy of ICI. We identified a set of 8 club cells secretory factors which in combination with PD-1 blockade elicited significant tumor control and improved survival. Mechanistically, club cell factors inhibit myeloid suppressor cells, reduce pro-tumor inflammatory mediators to improve the effectiveness of ICI. Consistent with the preclinical findings, NSCLC patients who responded to neoadjuvant SBRT/ICI therapy showed increased plasma CC10, a member of the club cell secretome factor. These findings have led to the hypothesis that club cell factors selectively inhibit immunosuppressive and proinflammatory function of myeloid suppressor cells to increase the efficacy of immune checkpoint blockade. We further hypothesize that club cell factors may serve as biomarkers of response and recurrence to SBRT/ICI therapy in human NSCLC. Using an integrated preclinical and clinical approach, we will test this hypothesis through two specific aims. Aim 1 will determine the exact identity of the core club cell factor/s and elucidate mechanisms by which they attenuate myeloid suppressor cells to generate durable anti-tumor immunity, and Aim 2 will leverage samples from a clinical trial to determine if club cell-based biomarkers are associated with response therapy and metastatic recurrence following ICI/SBRT therapy in human NSCLC. This work will develop a mechanism-based rationale for the development of club cell factors as selective inhibitors of myeloid suppressor cells and for improving the efficacy of ICI in NSCLC.