Pathogenetic mechanisms in the initiation and perpetuation of Sjögren's syndrome

Sjögren's syndrome (SS), a chronic autoimmune disorder, particularly compromises the function of exocrine glands. The involvement of these glands is characterized by focal, mononuclear cell infiltrates that surround the ducts and replace the secretory units. The pathogenetic mechanisms of this autoimmune exocrinopathy have not been fully elucidated. Immunologically-activated or apoptotic glandular epithelial cells that expose autoantigens in genetically predisposed individuals might drive autoimmune-mediated tissue injury. Alterations in several immune mediators, such as upregulation of type I interferon-regulated genes, abnormal expression of B-cell-activating factor and activation of the interleukin-23-type 17 T-helper cell pathway, have been reported. Extension of the pathological process that affects the exocrine glands into periepithelial and extraepithelial tissue can cause a considerable percentage of patients to exhibit systemic findings that involve the lungs, liver or kidneys. These manifestations develop as a result of lymphocytic invasion or an immune-complex-mediated process, or both, and present as skin vasculitis coupled with peripheral neuropathy or glomerulonephritis (or both). Patients with systemic extraepithelial manifestations display low serum levels of the complement component C4 and mixed type II cryoglobulins, and show an increased risk of developing non-Hodgkin lymphoma, thereby reflecting an overall worse prognosis with higher mortality rates than those without extraepithelial manifestations.