Faecalibacterium prausnitzii (F. prausnitzii), a dominant bacterium of the human microbiota, is decreased in patients with inflammatory bowel diseases (IBD) and exhibits anti-inflammatory effects. In human, colonic lamina propria contains IL-10-secreting, Foxp3-negative regulatory T cells (Treg) characterized by a double expression of CD4 and CD8alpha (DP8alpha) and a specificity for F. prausnitzii. This Treg subset is decreased in IBD. The in vivo effect of DP8alpha cells has not been evaluated yet. Here, using a humanized model of NOD.Prkcscid IL2rgamma-/- (NSG) immunodeficient mouse strain that expresses the human leucocyte antigen D-related allele HLA-DR*0401 but not murine class II (NSG-Ab degrees DR4) molecules, we demonstrated a protective effect of a HLA-DR*0401-restricted DP8alpha Treg clone combined with F. prausnitzii administration in a colitis model. In a cohort of patients with IBD, we showed an independent association between the frequency of circulating DP8alpha cells and disease activity. Finally, we pointed out a positive correlation between F. prausnitzii-specific DP8alpha Tregs and the amount of F. prausnitzii in fecal microbiota in healthy individuals and patients with ileal Crohn's disease.