TLR9 and autoimmune disease: Upon endocytosis of extracellular DNA, engagement of endosomal and intracellular DNA sensors such as Toll-like receptor 9 (TLR9) and the cyclic-di-GMP-AMP synthetase (cGAS) results in the activation of the stimulator of interferon genes (STING) adaptor protein, which stimulates the release of proinflammatory cytokines that may contribute the pathogenesis of autoimmune diseases (31).