Cytokine-Induced Injury of the Lacrimal and Salivary Glands

Damages to the lacrimal and salivary glands that accompany various autoimmune diseases are categorized as secondary Sjögren syndrome. Cytokines and free radicals are thought to be responsible for the pathologic changes, but the precise mechanisms are not clear. We evaluated whether cytokines alone can cause the damages in these exocrine tissues, and whether gaseous molecules such as nitric oxide (NO) play a role in these injuries. Various knockout (KO) mice as well as wild-type mice were injected intraperitoneally (i.p.) with the proinflammatory cytokines, IL-12 and IL-18, singly or in combination. Concurrent administration of IL-12 and IL-18 to mice caused serious atrophy in the lacrimal and salivary glands, which was spared when each cytokine was singly administered. Microscopically, there were apparently no infiltrating cells; nonetheless, numerous apoptotic cells were observed in the epithelium, which was confirmed by DNA ladder formation on gel electrophoresis. Serum levels of IFN-gamma and NO2/NO3 were markedly elevated. Combined injections of IL-12 and IL-18 caused the same changes in Fas-deficient and Fas-ligand deficient mice, as well as in perforin-KO mice, but the same changes were not detected in inducible NO synthase-KO mice or IFN-gamma KO mice. Thus, the synergistic effect of IL-12 and IL-18 was dependent on production of IFN-gamma and NO, but independent of Fas/Fas ligand system and perforin-dependent cytotoxic T cells. IL-18 together with IL-12 caused destructive changes in the glandular tissues without apparent lymphocyte infiltration. It is suggested that these cytokines can mediate apoptosis in glandular epithelial cells and that the elevated NO production is responsible for the change.