The etiology of human autoimmune diseases in general remains largely unknown, although the genetic and environmental interplay may be relevant. This applies to the autoimmune diseases of the skin such as the pemphigus phenotypes and others. In this group, there is an endemic form of pemphigus foliaceus (also known as fogo selvagem [FS]) in which the pathogenic IgG4 autoantibody response to the self-antigen desmoglein 1 (Dsg1) cross-reacts with the LJM11 sand fly salivary gland Ag. In this investigation, we dissected the IgG4 autoantibody repertoires used by FS patients in response to endogenous self-Dsg1 and exogenous LJM11 sand fly Ag. Based on analyses of the genetic clonal signatures of these Abs, our results indicate that there is a significant overlap between these two responses, as all identified IgG4 mAbs cross-react to both Dsg1 and LJM11 Ags. Germline H-and L-chain V gene Abs generated according to mutated cross-reactive mAbs preserved their reactivity to both Ags. Our findings suggest that both Dsg1 autoantigen and LJM11 environmental Ag could be the initial antigenic stimulants for the IgG4 autoimmune responses in FS. These results support our hypothesis that LJM11 Ag plays a substantial role in triggering the IgG4 autoantibody development in FS and provide new insights on how noninfectious environmental Ag (s) may drive the generation of autoantibodies in IgG4-related autoimmune diseases.