To examine the role of apoptosis in the pathogenesis of Sjögren syndrome (SS), a chronic autoimmune disease characterized by the infiltration of mononuclear cells in the salivary and lacrimal glands, leading to the destruction of the parenchymal tissue.
A detailed search via MEDLINE (PubMed) and Biosis, covering the period from January 1994 to July 2002, was accomplished, combining the key terms SS and apoptosis. A qualitative review of the articles was undertaken and the obtained information was summarized.
Apoptosis of the acinar and ductal epithelial cells of the salivary and lacrimal glands has been proposed as a possible mechanism responsible for the impairment of secretory function. Apoptotic cell death may be induced by either cytotoxic T cells through the release of proteases, such as perforin and granzyme B, or the interaction of Fas ligand (FasL/CD95L), expressed by T lymphocytes, with Fas (Apo-1/CD95) on epithelial cells. The increased rate of apoptosis of epithelial cells in SS may result from either the imbalance between the down-regulated apoptosis-inhibitor Bcl-2 and the up-regulated apoptosis-inducer Bax, or the autocrine and/or paracrine Fas/FasL interaction. Lymphocytes infiltrating the salivary glands are blocked in their ability to commit to apoptosis, despite the expression of the apoptosis-inducer Fas. The expression of Bcl-2 in these cells may explain their resistance to apoptosis, resulting in a prolonged production of proinflammatory cytokines and autoantibodies, as well as in their longer survival that may result in the late development of lymphoma in some SS patients. Studies of the SS-like sialoadenitis of nonobese diabetic (NOD) mice with severe combined immunodeficiency (NOD.scid) suggest that the primary defect responsible for the initiation of SS resides in the epithelial cells that undergo apoptosis mediated by the autocrine Fas/FasL interaction. This first not-immune-mediated phase of target cell destruction is followed by a lymphocyte-dependent autoimmune aggression, which leads to extensive tissue damage and subsequent loss of secretory function. Apoptosis of the salivary epithelial cells has been shown in other animal models of SS and in cell lines in vitro. Apoptosis may also play a role in the pathogenesis of some extraglandular manifestations of SS, such as interstitial nephritis and peripheral CD4(+) lymphocytopenia.
The possible role of apoptosis in SS is suggested from the literature review. A better understanding of the mechanisms responsible for the epithelial cell apoptosis may allow the discovery of new therapeutic strategies. By inhibiting programmed cell death, the glandular damage and the subsequent impairment of the secretory function should be reduced.