Immunoglobulin-like transcript 3, an inhibitor of T cell activation, is reduced on blood monocytes during multiple sclerosis relapses and is induced by interferon β-1b

Immunoglobulin-like transcripts (ILTs) are immunoregulatory proteins that either activate or inhibit immune responses. ILT3 is inhibitory and is expressed preferentially by antigen-presenting cells. When its extracellular domain binds to an unidentified ligand of activated T cells, the T cell is silenced. Our objective was to study the expression of ILT3 on circulating monocytes in RRMS. Freshly isolated peripheral blood mononuclear cells were analyzed by multicolored flow cytometry. The proportion of ILT3+CD14+ monocytes in blood, and ILT3 levels expressed by them, is lower in untreated multiple sclerosis in relapse than in: (1) untreated multiple sclerosis in remission (p < 0.009); (2) stable interferon β-treated relapsing—remitting multiple sclerosis (p < 0.001) and; (3) healthy controls (p < 0.009). Glatiramer acetate-stimulated CD4 + T cells, co-cultured with freshly isolated monocytes, proliferate significantly better (p = 0.0017 for multiple sclerosis; p = 0.0015 for controls) when T cell interaction with monocyte-expressed ILT3 is blocked by anti-ILT3 antibody. Interferon β is beneficial in multiple sclerosis; why so remains unclear. Interferon β-1b markedly increases ILT3 expression in vitro by monocytes from multiple sclerosis patients and controls. These findings identify a putative novel mechanism for the therapeutic benefit bestowed by Interferon β and a new target for therapeutic intervention in relapsing—remitting multiple sclerosis.