Background: Bintrafusp alfa is a first-in-class bifunctional fusion protein composed of the extracellular domain of TGF-βRII receptor (TGF-β“trap”) fused to a human IgG1 mAb blocking PD-L1. In preclinical models, bintrafusp alfa treatment promoted CD8+ T cell and NK cell activation, and both immune cell (IC) populations were required for optimal bintrafusp alfa mediated tumor control. However, the effect of bintrafusp alfa on TIME in humans has not been reported. Methods: In this unplanned interim analysis of a biomarker expansion cohort (NCT 02517398), patients (pts) with advanced non-small cell lung cancer (NSCLC) underwent paired biopsies (bx) before and on treatment with bintrafusp alfa (~ 50 days apart). The objective was to evaluate frequency and localization of tumor infiltrated ICs by IHC. Out of 12 pts, 7 had matched (Pre vs Post) tumor-containing specimens sufficient for multiplex immunofluorescence (MxIF) analysis of TIME. Four pts were excluded as Post bx histology for 3/12 [2 PR (partial response), 1 SD (stable disease)] was negative for tumor (necrosis or fibrosis) and 1/12 did not have a Post bx performed. Results: TIME study shows CD8 T cell infiltrates were increased in Post compared to Pre bx (median 161 vs 62/mm²; interquartile range [IQR] 65–396/mm² vs 31–135/mm²; p = 0·04). While M2 macrophages were also increased (median 800 vs 367/mm²; IQR 776–1131/mm² vs 171–831/mm²; p = 0·04), the ratio of M1/M2 was reversed in pts with SD (↑) compared to pts with PD (↓). Other ICs such as CD4, T-regs, NK cells and M1 macrophages were not changed. On average compared to baseline, M2 macrophages were > 2 fold closer to every other IC in pts with PD, but > 2 fold further from any IC in pts with SD. Tregs were relatively closer to other IC in PD pts. Linear Discriminant Analysis was also performed and results indicate that differential IC densities (mainly M1 macrophages and CD4 T cells) do perform as classifiers between long ( > 5 months) and short ( < 5 months) term responses. Conclusions: This study suggests that bintrafusp alfa not only can enhance intratumoral effector IC infiltrates (CD8) but also has a modulating effect on the spatial distribution of both M1/M2 macrophages within the NSCLC TIME. The differential proximity of M2 macrophages to other IC infiltrates and changes in M1/M2 ratios in association with response suggests that an M1/M2 macrophage balance is directly involved in response and/or resistance to bintrafusp alfa. Given the limited number of patients in this cohort, we intend to study effects of bintrafusp alfa in a larger cohort of patients. Clinical trial information: 02517398.