Abstract: Among regulatory T cells, natural regulatory T cells (nTregs) ensure the control of self-tolerance and are currently tested in clinical trials in autoimmune diseases and allogeneic hematopoietic stem cell transplantation. Here the inventors show that based on CD39/CD26 markers, the human nTreg population is comprised of 5 major cell subsets each representing a distinct state of maturation. Phenotypic and genetic characteristics of the subsets illustrate the structural parental maturation between subsets which further correlates with expression of regulatory factors. Importantly, the inventors also show that blood nTreg CD39/CD26 profile, remaining constant over a 2year period in healthy persons but varying between individuals, represents a novel biomarker for monitoring chronic diseases, as illustrated in their preliminary study on AI (dermatomyositis, rheumatoid arthritis and leukemias).