Epstein-Barr virus reactivation is associated with altered immune cell profiles in peripheral blood and cerebrospinal fluid of treatment-naive multiple sclerosis patients

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system, with autoimmune and neurodegenerative components, recently linked with Epstein-Barr virus (EBV) infection. To investigate immunological alterations associated with EBV reactivation (EBV-R) in MS we analyzed immune cell profiles in matched peripheral blood (PB) and cerebrospinal fluid (CSF) samples from treatment-naive MS patients, in relation to plasma EBV serology markers. In our Hellenic MS cohort, 12 of 33 patients (39 %) exhibited serological evidence of EBV-R (VCA IgG+ with VCA IgA+, VCA IgM+, or EA(D) IgG+), and 7 of these 12 also tested positive for plasma BZLF1 mRNA, indicative of lytic infection. EBV-R patients showed reduced CD19+ B cells in PB and equal proportions in CSF compared to patients with latent EBV infection. Conversely, EBV-R patients showed increased cytotoxic CD56^dim NK cells and CD14⁺ monocytes in PB, while cytotoxic CD56^dim NK cells remained absent in the CSF, regardless of EBV status. Proportions of regulatory CD56^bright NK cells were reduced in both PB and CSF during EBV-R. Our results reveal that MS patients with serological evidence of EBV-R show altered immune responses, with reduced B cell proportions in the PB in the presence of expanded cytotoxic NK cells, and unaffected B cell proportions in CSF in the absence of cytotoxic NK cell surveillance.