CpG-ODN-induced inflammation is sufficient to cause T-cell-mediated autoaggression against hepatocytes

Autoimmune diseases are often associated with microbial infections. Molecular mimicry between microbial antigens and self-epitopes has been suggested as a mechanism for breaking self-tolerance and induction of autoimmunity. Since infections also cause inflammatory responses we explored the role of local inflammation in organ-specific autoimmunity. For this purpose, transgenic mice were used expressing the MHC class I molecule K^b exclusively on hepatocytes. These mice exhibit K^b-specific tolerance as exemplified by the acceptance of K^b+ grafts. Inflammatory reactions were induced by injection of immunostimulatory cytosine-phosphorothioate-guanine (CpG)-rich oligodeoxynucleotides (ODN). Application of CpG-ODN is sufficient to break tolerance in vivo, and to cause activation of K^b-specific CD8⁺ T cells and subsequent autoaggression against hepatocytes. The CpG-ODN-induced inflammation appears to have two major effects. First, it causes infiltration of T cells into the liver parenchyma. Second, adhesion and costimulatory molecules are up-regulated on hepatocytes so that the infiltrating CD8⁺ T cells encounter K^b on hepatocytes, which display an APC-like phenotype, resulting in activation and tissue damage. Autoimmune hepatitis can be maintained for at least eight weeks by repeated application of CpG-ODN but subsides after termination of the inflammatory stimulus, suggesting the requirement of additional factors for a self-perpetuation of autoimmunity. These observations describe an additional pathway for the induction of autoimmunity, i.e. in the absence of microbial antigens inflammatory reactions alone can lead to infiltration of T cells into organs, resulting in breaking of tolerance and autoaggression. Moreover, the results provide evidence that T cell activation can take place not only in draining lymph nodes but also directly on parenchymal cells.