Molecular Analysis of the Human Autoantibody Response to α-Fodrin in Sjögren's Syndrome Reveals Novel Apoptosis-Induced Specificity

Abstract

Lymphocyte infiltration of salivary and lacrimal glands leading to diminished secretion and gland destruction as a result of apoptosis is thought to be pivotal in the pathogenesis of Sjögren's syndrome (SS). The cytoskeletal protein alpha-fodrin is cleaved during this apoptotic process, and a strong antibody (Ab) response is elicited to a 120-kd fragment of cleaved alpha-fodrin in the majority of SS patients, but generally not in other diseases in which apoptosis also occurs. Little is known about the anti-alpha-fodrin autoantibody response on a molecular level. To address this issue, IgG phage display libraries were generated from the bone marrow of two SS donors and a panel of anti-alpha-fodrin IgGs was isolated by selection on alpha-fodrin immunoblots. All of the human monoclonal Abs (hmAbs) reacted with a 150-kd fragment and not with the 120-kd fragment or intact alpha-fodrin, indicating that the epitope recognized became exposed after alpha-fodrin cleavage. Analysis of a large panel of SS patients (defined by the strict San Diego diagnostic criteria) showed that 25% of SS sera exhibited this 150-kd alpha-fodrin specificity. The hmAbs stained human cultured salivary acinar cells and the staining was redistributed to surface blebs during apoptosis. They also stained inflamed acinar/ductal epithelial cells in SS salivary tissue biopsies, and only partially co-localized with monoclonal Abs recognizing the full-length alpha-fodrin. Our study shows that in SS patients, neoepitopes on the 150-kd cleaved product of alpha-fodrin become exposed to the immune system, frequently eliciting anti-150-kd alpha-fodrin Abs in addition to the previously reported anti-120-kd Abs. The anti-150-kd alpha-fodrin hmAbs may serve as valuable reagents for the study of SS pathogenesis and diagnostic analyses of SS salivary gland tissue.