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Expression of myelin basic protein (MBP) epitopes in human non‐neural cells revealed by two anti‐MBP IgM monoclonal antibodies

Two monoclonal antibodies (1H6.2 and 45.30) were raised against MBP purified from human brain under experimental conditions that allowed MBP to retain binding to surrounding myelin lipids (human lipid‐bound MBP (hLB‐MBP)). 1H6.2 and 45.30 MoAbs were selected on the basis of their different binding properties to: hLB‐MBP, human lipid‐free‐MBP (hLF‐MBP) and bovine lipid‐free‐MBP (bLF‐MBP). Although the isotype of both MoAbs was IgM, their specificity, as tested in ELISA assays against chemical haptens and unrelated protein antigens, was restricted to MBP. 1H6.2 and 45.30 MoAbs stained MBP from human brain white matter tissue extracts, as well as bLF‐MBP, in Western blot assays. Both MoAbs stained oligodendrocytes and myelin in immunohistochemical analysis of white matter from human brain. Tissue sections from human peripheral nerves were labelled by 1H6.2 only, however, demonstrating that the MoAbs recognize two different epitopes. Epitopes recognized by 1H6.2 and 45.30 MoAbs were also expressed by a wide array of human non‐neural cells of either normal or pathological origin, as evidenced by cytofluorimetric assays. In particular, MBP epitopes (MEs) were expressed by lymphoid cells as well as by cells which play a pivotal role in immune homeostasis and in the immune response, such as thymic epithelial cells and professional antigen‐presenting cells. Both MoAbs were efficiently internalized by cells from a human B cell line, suggesting trafficking of MEs along the endocytic pathways. These findings support hypotheses regarding the role of MEs expressed by non‐neural cells in establishing self‐tolerance and/or in triggering the immune response against MBP antigen.