Background: Antibiotics (ATB) deviate the gut taxonomic microbiota composition and have a deleterious impact on survival in ICI-treated pts. ATB downregulate the ileal mucosal addressin cell adhesion molecule-1 (MAdCAM-1), leading to the recirculation of immunosuppressive enterotropic T cells into the tumor and ICI resistance. Here, we aimed to assess the prognostic impact of MAdCAM-1 in ICI-treated mRCC pts. Methods: The GETUG-AFU 26 NIVOREN phase II trial (NCT 0301335) is a multicentric study that assessed the activity and safety of Nivolumab in pts with clear cell mRCC after anti-angiogenic therapy. We measured serum soluble MAdCAM-1 (sMAdCAM-1) levels (correlating ileal MAdCAM-1 transcripts) using Human Luminex Discovery Assay in the available plasma. Progression-free survival (PFS), overall survival (OS) and clinical benefit rate (CBR) were assessed using sMAdCAM-1 median as a cut off value (high if >median and low if <median). Multivariate Cox analysis adjusted for established risk factors: ATB, gender, age, international Metastatic RCC Database Consortium (IMDC) score, number of previous lines, hypoalbuminemia, and brain, bone and liver metastasis. Two independent cohorts of metastatic lung and bladder cancer patients validated the data. Results: Overall, 212 pts were included. Median age was 64 years (22-87), and pts were mostly male (82%). ATB users had lower levels of sMadCAM-1. Low sMadCAM-1 pts had diminished OS compared to high sMadCAM-1 pts [HR 2.97 (95%CI 1.99-4.44), p<.0001]: 13.3 (95%CI 9.8-14.9; 72/106) versus 25.3 (95%CI 24.1-NR; 36/106) months. Also, low sMadCAM-1 pts had diminished PFS [HR 1.92 (1.43-2.57), p<.0001]: 2.6 (95%CI 2.4-2.8; 99/106) versus 5.2 (95%CI 4.6-5.7; 86/106) months. Interestingly, sMAdCAM-1 was independently associated with OS [HR 2.40 (1.52-3.80), p=0.0002] and PFS [HR 1.55 (1.13-2.13), p=0.0071] in multivariate analysis. Low sMadCAM-1 pts had lower CBR (37% versus 63%, p=0.0004). Serum sMAdCAM-1 also predicted OS in 381 ICI-treated patients with lung and bladder cancer. Conclusions: Low sMAdCAM-1 is associated with ATB intake, ATB-independent gut dysbiosis and worse outcomes, in ICI-treated pts with metastatic lung, bladder and RCC cancer. ELISA determination of sMAdCAM-1 might guide the selection of patients amenable to microbiota-centered interventions, such as Akkermansia sp., and fecal microbiota transplantation. Clinical trial information: NCT0301335.