Although
multiple sclerosis (MS) is thought to be an
autoimmune disease, the mechanisms by which
immunodominant epitopes are generated and lymphocytes are activated are not known. Here,
myelin basic protein-component 1 (MBP-C1) from MS tissue was shown to undergo autocatalytic cleavage at slightly alkaline pH. Importantly, one of the major
peptides released contained the
immunodominant epitope 84-89. Interestingly, MBP isolated from MS patients showed a faster time course of cleavage and a more robust release of
epitope 84-89 than MBP isolated from normal individuals. The cleavage reaction was not inhibited by
protease inhibitors, except for
phenylmethanesulfonyl fluoride (PMSF), a
serine protease inhibitor. Since PMSF inhibition suggested a role for a
serine residue in the cleavage, we labeled
myelin basic protein with diisopropyl
fluorophosphate (
DFP), known to bind active site
serine residues. Mass spectrometry was used to identify the labeled
peptide, which consisted of residues 140-152. Since this
peptide contained a single
serine residue, we concluded it to be the active
serine. The importance of this cleavage mechanism is that it provides for a ready source of the immunodominant
peptide for sensitization of T-cells. It is not necessary to invoke other mechanisms such as molecular mimicry.