Background: Immune checkpoint inhibitors demonstrated poor efficacy in MSS mCRC. Cetuximab (anti-EGFR inhibitor) could initiate, independently from RAS mutation, an immunogenic tumor cell death and mediate antitumor immune response. In this trial, we aim to explore the clinical efficacy and safety of avelumab (anti-PDL1) combined with cetuximab and irinotecan for treatment refractory MSS mCRC and to understand its mechanisms of action through associated translational research. Methods: AVETUXIRI trial (NCT03608046) enrols MSS, chemorefractory (fluoropyrimidine, oxaliplatin, irinotecan and anti-EGFR treatment if RAS wt tumor) mCRC patients in 2 cohorts (A: RAS-wt, n = 10 – B: RAS-mut, n = 13). Primary endpoints are safety and tumor response rate ((i)RECIST1.1). Secondary endpoints include disease control rate (DCR), progression-free survival (PFS) and overall survival (OS). According to a Simon 2-stage design, 23 patients have been included in the first stage of the trial. Multiplex immunofluorescence and RNA sequencing were realized on metastasis biopsies performed before, during and after the treatment. Densities of CD3+ (T cells) and CD8+ (cytotoxic) were quantified and analyzed with to generate an immunoscore (IS). RNA-seq data was used to perform differential expression analysis (DESeq2), gene set enrichment analysis (GSEA), deconvolution analysis (Consensus^TME) and gene ontology analysis (GO). Results:: No unexpected safety signals were observed. 3/10 tumor responses were observed in cohort A, 0/13 in cohort B. DCR was 60.0% and 61.5% in cohort A and B, respectively. 6-months PFS and 12-months OS rates were respectively 40.0% and 50.0% (cohort A) and 38.5% and 46.2% (cohort B). Independently of RAS mutation, patients with a high IS (metastasis biopsy, baseline) had significantly higher tumor shrinkage (OR = 18.67 p = 0.019), median PFS (6.9 vs 3.4 months; HR = 0.16, p = 0.002) and median OS (13.7 vs 7.9 months, HR = 0.26, p = 0.009). Similarly, tumor shrinkage and survival outcome (PFS > 6 months, OS > 12 months) were associated with upregulation of an adaptive immune response signature (including Th1, chemokine, adhesion molecules, immune checkpoints and T-cell activation genes, p. adj = 0.009) and the GSEA hallmark of epithelial to mesenchymal transition (p. adj = 0.045). Few modifications of IS and gene expression profiles were observed on the different metastasis biopsies performed overtime in the included patients. Conclusions: AVETUXIRI met its preliminary primary efficacy endpoint for RAS-wt mCRC pts, justifying its current continuation. Encouraging survival data observed in RAS-mut cohort allow the opening of a new cohort (PFS as primary endpoint). IS and adaptive immune response signature evaluated on metastases biopsies were associated with treatment efficacy and survival. Clinical trial information: NCT03608046.