Although events responsible for the activation of lupus-antigen reactive T cells are not known, the strong environmental influence on SLE is suggestive of microbial involvement. Microbes can influence T cell activation through molecular mimicry. To test this hypothesis, we selected Ro60 as the candidate antigen and HLA-DR3 transgenic mice as the experimental mouse model system. Autoantibodies against Ro60 are present in lupus patients and HLA-DR3 has the strongest genetic linkage to lupus. Using a panel of T cell hybridomas, 3 T cell epitopes on Ro60 were characterized. Amino acids critical for T cell activation were identified and pattern search analysis was carried out. This strategy identified hundreds of mimicry peptides. Several mimicry peptides originated from microbes residing in the human oral cavity, gut and skin, and were able to activate the Ro60 reactive T cell hybridomas. In addition, immunization of HLA-DR3 transgenic mice with some of the mimicry peptides induced autoantibody responses. Our data suggests that T cells reactive against Ro60 can be activated by exposure to multiple peptides originating from different microbes that are present in the human microbiome. Thus, we would like to suggest that repeated exposure to different microbes shapes the self-reactive T cell repertoire in SLE and a dysregulated immune response against normal microbiota can initiate autoimmune responses in lupus.