Levels of Interleukin-15-Expressing Blood Mononuclear Cells Are Elevated in Multiple Sclerosis

Interleukin-15 (IL-15) is a novel IL-2-like cytokine expressed by cells of the monocyte/macrophage and epithelial lineages. Cytokines might be involved in the pathogenesis of multiple sclerosis (MS). Using immunocytochemistry, we analysed spontaneous expression of IL-15 by peripheral blood (PB) and cerebrospinal fluid (CSF) mononuclear cells (MNC) from patients with MS, other neurological diseases (OND) and healthy controls. IL-15- positive peripheral blood mononuclear cells (PBMNC) were elevated in patients with MS compared to healthy controls (P < 0.05). The elevation of IL-15- positive PBMNC was restricted to patients with chronic progressive MS and not observed in patients studied during the relapsing-remitting phase of MS. The numbers of IL-15- expressing PBMNC correlated with the duration and disability of MS (r = 0.45, P < 0.001, and r = 0.39, P < 0.01, respectively). IL-15 was undetectable in CSF MNC, and ELISA showed low CSF levels of IL-15 in occasional patients with MS and OND. IL-15 is a potent growth factor for gammadelta T cells, but there was no correlation between IL-15 expression by PBMNC and percentage of gammadelta T cells in blood from the MS patients. Together, these data demonstrate that IL-15 expression by PBMNC is upregulated in the chronic stage of MS.