148 Background: Andecaliximab (ADX) is a monoclonal antibody that inhibits matrix metalloproteinase 9 (MMP9). Preclinical studies suggest that MMP9 inhibition relieves immune suppression and promotes T cell infiltration to potentiate checkpoint blockade. In the phase 2 study combining ADX with nivolumab (N) versus N monotherapy (NCT02864381), addition of ADX to N did not improve objective response rate, progression-free survival (PFS), or overall survival (OS). Methods: Archival tumor samples were collected from all patients (n = 141). CD8 and PD-L1 (28-8 DAKO) were assessed by immunohistochemistry. CD8+ cell density was measured in the tumor area. PD-L1 was prospectively scored by a pathologist for tumor cell (TC) and associated immune cell (IC) positivity. IFNg, Teffector (Teff), and activated CD8+ T cell (ActT) gene signatures were assessed by RNA sequencing. Due to a small number of responders, treatment arms were combined to evaluate response. Cox proportional hazards models were used for survival analyses. Nominal p-values are reported. Results: Baseline biomarkers of T cell infiltration and activation did not differentiate responders from non-responders (IFNg, Teff, ActT, CD8+; p > .10). None of the evaluated biomarkers were associated with PFS or OS for all treated patients or per treatment arm (IFNg, Teff, ActT; p > .10), with the exception of CD8+ (PFS HR = .43, p = .02). The majority of baseline samples were positive for IC PD-L1 (< 1%, n = 36; 1-10%, n = 50; > 10-25%, n = 32; > 25%, n = 20) and negative for TC PD-L1 (H = 0, n = 88; H < 1, n = 27; H > 1; n = 27). Comparing ADX/N to N, there was a trend toward longer OS for the PD-L1+ (TC + IC ≥ 1%) population (n = 102, HR = .621, p = .098), the TC H < 1 group (HR = .464, p = .08) and the IC > 10-25% (HR = .466, p = .08). Conclusions: Neither CD8+ cell density nor IFNg, Teff or ActT gene signatures were associated with response or survival to checkpoint blockade. While TC was low, IC intermediate and TC + IC ≥ 1% PD-L1+ groups trended toward better survival for the ADX+N arm, consistent with the hypothesis that ADX potentiates N activity; this did not translate into better outcome. Clinical trial information: NCT02864381.