The influence of the normal microbial flora on the susceptibility of rats to experimental autoimmune thyroiditis.

Female PVG/c strain rats maintained under specific pathogen-free conditions until weaning were found to be significantly less susceptible to the induction of autoimmune thyroiditis by thymectomy and irradiation than conventionally reared rats of the same strain. Other SPF-derived strains had a similarly low susceptibility, thus pointing to an important external factor influencing the induction of autoimmunity by this procedure. It was found that the oral administration of antibiotics followed by fresh, homogenized, intestinal contents obtained from conventionally reared rats to newly weaned SPF PVG/c rats significantly augmented their autoimmune susceptibility. The offspring of female SPF rats which had been treated in late pregnancy with oral antibiotics and conventional intestinal contents were similarly more susceptible than the offspring of non-treated counterparts. It is proposed that the composition of the normal gastrointestinal flora of conventionally reared rats profoundly influences susceptibility to the induction of autoimmune thyroiditis in this particular experimental model, possibly by antigenic cross-reactivity with thyroid tissue.