Abstract: The present invention relates to methods for detecting, assessing severity and treating multiple sclerosis. The inventors showed an impairment of the function of CD8+ Treg cells in MS patients and they demonstrated here that several criteria correlated with the disease severity i.e. the percentage of CD8+CD45RCintCD161lowValpha7? T cells in the blood, the secretion of IFNg and IL10 and the suppressive activity of the CD8+CD45RCintCD161lowValpha7? T cells. In particular, the present invention relates to a method for determining whether a subject has or is at risk of having multiple sclerosis comprising i) determining the percentage of CD8+CD45RCintCD161lowValpha7? T cells in a biological sample obtained from the subject, ii) comparing the percentage determined at step i) with a predetermined reference value and iii) detecting differential in the percentage determined at step i) with the predetermined reference value indicates that the subject has or is at risk of having multiple sclerosis.