Epitope mapping of anti-alpha-fodrin autoantibody in juvenile Sjögren's syndrome: difference in major epitopes between primary and secondary cases.

Objective: Juvenile Sjögren's syndrome (SS) is an early-onset type of SS. Autoantibody against the N-terminal 120 kDa form of a-fodrin is a specific and sensitive disease marker for both juvenile and adult SS. We investigated the initial and major determinants of a-fodrin in SS.

Methods: Sera were obtained from patients with juvenile SS, 10 with primary SS and 10 with secondary SS. Epitope specificities of IgG antibodies were examined by dot-blot analyses using overlapping fusion proteins of the N-terminal part (561 amino acid residues) of a-fodrin as antigens.

Results: All sera from patients with primary SS reacted with amino acid residues 1 to 98 and 36 to 150, but not with 91 to 199. Epitope mapping using fusion proteins with subfragments, each consisting of about 50 amino acid residues, showed reactivity with amino acid residues 27-80 and 79-132, suggesting that at least 2 epitopes are contained in the first 150 amino acid residues. All 3 cases with neurological complications had additional epitope specificities. Sera from patients with secondary SS showed more diversified specificities and strongly reacted with amino acid residues 1-98 and 334-432, whereas the reactivities to 36-150, a major epitope in primary SS, were minimal.

Conclusion: Major and initial B cell epitopes specifically reside in N-terminal amino acids 36-132 and could be used as a diagnostic tool for primary SS. The epitope subsequently expands to other regions of a-fodrin in association with the development of neurological complications or disease progression. Secondary SS has distinct epitope specificities.