The invention relates to a method comprising a step of determining the number, concentration and/or proportion of T lymphocytes with a CD4+ CD8??low phenotype and further expressing CCR6 and/or CXCR6, for (i) diagnosing, (ii) prognosing outcome of, or (iii) predicting the risk of developing a disease related to a decrease of F. prau. The invention also concerns the treatment of said disease by administering a population of these specific T lymphocytes.rnThe Inventors have indeed identified two markers, CCR6 and CXCR6, enabling to select a population of F. prau-specific cells among CD4+ CD8??low T lymphocytes, from a blood sample and without needing to assess their F. prau specificity. T lymphocytes with a CD4+ CD8??low CCR6+ CXCR6+ phenotype are for example significantly decreased in IBD patients.rnThe disease related to a decrease of F. prau is particularly an inflammatory bowel disease (IBD), such as Crohn’s disease.