Autoantibodies to the Amino-Terminal Fragment of β-Fodrin Expressed in Glandular Epithelial Cells in Patients with Sjögren’s Syndrome

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Abstract
Sjögrens’s syndrome (SS) is an autoimmune disease characterized by destruction of lacrimal and salivary glands, but the mechanisms underlying the disease process are unclear. By immunoscreening a HepG2 cDNA library with serum from an SS patient we isolated a cDNA encoding amino-terminal 616 aa of β-fodrin, a membrane skeleton protein associated with ion channels and pumps. Serum Ab to the amino-terminal fragment of β-fodrin was frequently detected in SS patients compared with rheumatic disease patients without SS or healthy controls (70 vs 12 or 4%; p < 0.00001). All the anti-β-fodrin-positive sera recognized the amino-terminal fragment with no homology to α-fodrin. Anti-β-fodrin Abs in patients’ sera as well as mouse polyclonal sera raised against the amino-terminal β-fodrin fragment did not react with intact β-fodrin, but recognized the 65-kDa amino-terminal fragment generated through cleavage by caspase-3 or granzyme B. When expression of intact and fragmented β-fodrin in lacrimal glands was assessed by immunohistochemistry, the antigenic amino-terminal fragment was distributed diffusely in acinar epithelial cell cytoplasm, whereas the carboxyl-terminal fragment and/or intact β-fodrin were localized in peripheral cytoplasm, especially at the basal membrane, in SS patients. In contrast, intact β-fodrin was detected primarily at the apical membrane of epithelia, and the amino-terminal fragment was scarcely detected in control patients with chronic graft-vs-host disease. These findings suggest that cleavage and altered distribution of β-fodrin in glandular epithelial cells may induce impaired secretory function and perpetuate an autoimmune response to β-fodrin, leading to autoantibody production and glandular destruction in SS.