Background: Elevated frequency of peripheral myeloid cell populations has consistently been associated with poor response to immune checkpoint inhibitors (ICI) in metastatic non-small cell lung cancer (mNSCLC). The mechanisms underlying this relationship remains poorly understood. Thymic stromal lymphopoietin (TSLP), a cytokine involved in T-cell maturation, has been implicated in a complex feedback loop leading to tumor growth and expansion of myeloid populations. We hypothesized that TSLP levels directly correlate with the presence and expansion of myeloid derived suppressor cell (MDSC) populations and sought to explore their association with response to PD-1 inhibitors in mNSCLC. Methods: mNSCLC patients treated with ICIs underwent baseline and serial blood collection. Peripheral blood mononuclear cells (PBMC) were analyzed by high-dimensional flow cytometry using validated panels to evaluate T/B/NK-cell, Treg and myeloid populations. Plasma cytokines including TSLP were analyzed using ELISA and Luminex assays. Cox and logistic regressions were utilized to correlate biomarkers with progression-free survival (PFS), overall survival (OS) and radiographic response. Results: 30 mNSCLC patients treated with single-agent ICI were included in the analysis. TSLP level was significantly associated with expansion of monocytic(M)-MDSCs in response to ICI treatment (p=0.02). M-MDSC frequency after a median of 20 days of ICI treatment was significantly associated with progressive disease (PD), reduced PFS and OS (all p<0.05) whereas no correlation was seen with baseline M-MDSC frequency. Patients with a doubling of M-MDSCs (n=11) after treatment had a primary PD rate of 64% vs 24% (OR 7.0, p=0.04) and significantly worse median PFS (2.5 vs 7.8 months, HR 2.6 p=0.04). Conclusions: Early expansion of circulating M-MDSCs after treatment with PD-1 inhibitors is associated with elevated baseline TSLP levels and primary disease progression following ICI therapy in mNSCLC. These findings suggest that elevated TSLP and early expansion of myeloid populations may represent an important mechanism of primary resistance to PD-1 inhibitors in mNSCLC.