Pathogenesis of Sjögren's syndrome: what we know and what we should learn.

Sjögren's syndrome (SS) or autoimmune epithelitis is a prototype autoimmune disorder with unique features: a broad clinical spectrum that extends from local exocrinopathy to systemic disease and lymphoma development, and an easy access to the inflamed tissues (minor salivary glands; MSG), which enables the investigators to study the autoimmune processes. The autoimmune lesion consists of lymphocytic infiltrates that develop around the ducts and vary in severity and composition. T cells (mainly CD4(+)) are the dominant lymphocytes in mild MSG lesions, whereas B cells in severe ones. Th1 cytokines predominate in SS infiltrates, albeit Th2 and Th17 responses have been also reported. Notably, increased infiltration by IL-18(+) cells has been associated with parotid gland enlargement and C4-hypocomplementemia, which are adverse prognostic factors for lymphoma development. Even though SS pathogenesis has not been fully revealed, several aspects have been delineated. Among them, the key role of MSG epithelia in the initiation and perpetuation of local autoimmune responses is well-established and involves the capacity of epithelial cells to mediate the recruitment, homing, activation, proliferation and differentiation of immunocytes. In addition, genetic features, including certain HLA phenotypes and polymorphisms in genes encoding cytokines or factors implicated in cytokine signaling, environmental (such as viruses) and hormonal factors are thought to participate in disease pathogenesis. Herein, the known aspects of SS pathogenesis, as well as unmet issues are discussed.