This is potentially relevant for MS, regarding the well-documented role of PD-1 and its ligand PD-L1 in the regulation of T-cell immunity and tolerance.31 It has been suggested and supported by evidence that autoimmune disease results from a general dysfunction of immune homeostatic mechanisms and peripheral tolerance breakdown.49 There is an established correlation between EBV viral load in acute infectious mononucleosis and PD-1 expression. Here, PDCD1 is linked to autoimmune disease.