Increased Soluble 4‐1BB Ligand (4‐1BBL) Levels in Peripheral Blood of Patients with Multiple Sclerosis

4‐1BB ligand (4‐1BBL; CD137L) is a member of the tumour necrosis factor superfamily expressed primarily on antigen presenting cells such as B cells, macrophages and dendritic cells. Its engagement with the receptor 4‐1BB (CD137) has been shown to promote T‐cell activation and regulate proliferation and survival of T cells. The role of the costimulatory molecule in multiple sclerosis (MS) remains unclear. In this study, the expression of 4‐1BBL and soluble 4‐1BBL (s4‐1BBL) protein levels were analysed in peripheral blood of MS patients. Compared with healthy controls, MS patients had an increase in both plasma s4‐1BBL protein levels and expression of 4‐1BBL in CD14+ monocytes. In contrast, myelin basic protein‐reactive T‐cell proliferation was not found to be inhibited by the use of an anti‐4‐1BBL antibody. The elevated s4‐1BBL protein levels in the MS patients may function as a self‐regulatory mechanism of 4‐1BB/4‐1BBL interaction and costimulation.