3025 Background: Axi-cel is an autologous anti-CD19 CAR T cell therapy. ZUMA-1 is a multicenter, registrational trial of axi-cel in patients (pts) with refractory/aggressive B-cell non-Hodgkin lymphoma (NHL). In a pre-specified interim analysis, ZUMA-1 met its primary endpoint with 76% objective response rate and 47% complete response (Blood 2016;128:LBA-6). We describe, for the first time, a tumor microenvironment immune gene signature associated with CAR T cell treatment (tx) of NHL pts. Methods: Paired biopsies, pre- and within 3 weeks post-axi-cel tx, were analyzed by digital gene expression followed by a pre-specified bioinformatics algorithm applied to IGES15 and IGES21 genes involved in immune-mediated tumor regression (Immunosign; Galon Immunity 2013). Immunosign profiles expression of a pre-defined set of effector T cell, Th1, chemokine, and cytokine genes. Expression analysis and hierarchical clustering were used to define an axi-cel-related tumor immune gene signature. Wilcoxon signed rank test with multiple test correction by FDR (Benjamini-Yekutieli) was used. Results: Gene expression profile comparisons of pre- and post-axi-cel tx biopsies from 14 pts showed profound changes in gene expression within the tumor environment after infusion. The most upregulated genes post-axi-cel tx were CCL5 (RANTES), CTLA4, and GZMA (log₂ fold change > 2, P< 0.05, FDR < 0.050). Immune checkpoints PD-L1 and LAG3 were also upregulated post-axi-cel (log₂ fold change > 1.6, P< 0.05, FDR < 0.055). Other genes associated with T cell proliferation, homing, and effector function were also upregulated: IL-15, GZMK, CXC3CL1 (Fractalkine), CD8A, and STAT4 (log₂ fold change > 1.6; P< 0.05, FDR < 0.074). Additional baseline tumor characteristics and associative analysis will be presented. Conclusions: We define a mechanistic tumor immune gene signature in NHL pts associated with axi-cel tx. This signature comprises upregulation of T cell activation, effector, chemokine, and immune checkpoint genes. These data will potentially lead to rational optimization of T cell interventions in cancer Clinical trial information: NCT02348216.