Faecalibacterium prausnitzii a member of the Clostridium IV group of the Firmicutes phylum that is abundant in the intestinal microbiota, has anti-inflammatory effects. The relative level of F prausnitzii is decreased in fecal samples from patients with inflammatory bowel diseases (IBD), compared to healthy individuals. Reduced F prausnitzii was correlated with relapse of Crohn’s disease after surgery. We identified, in human colonic mucosa and blood, a population of Tr1-like T-regulatory (Treg) cells that express CD4 and CD8alpha (DP8alpha T cells) and are specific for F prausnitzii.