We investigated 90 immuno-oncology (I-O) related serum proteins in 78 patients with PDAC treated with nivolumab with or without ipilimumab combined with radiotherapy in a randomized phase 2 study (CheckPAC, NCT02866383)....Patients with clinical benefit (partial response (n=7) or stable disease (n=15)) had significantly higher levels of fas ligand (FASLG) and galectin 1 (Gal-1), and decreased C-C motif chemokine (CCL) 4 compared to patients without clinical benefit. High Gal-1 and FASLG and low angiopoietin-2 and mucin-16 were associated with longer progression-free survival in univariate analysis. In multivariate analysis, the association remained significant for Gal-1 (Hazard ratio (HR) = 0.25, confidence interval (CI) 0.12-0.56, p<0.001), and borderline significant for FASLG (HR=0.52, CI: 0.26-1.04, p=0.06).