Thyroiditis in T cell-depleted rats. Influence of strain, radiation dose, adjuvants and antilymphocyte serum

Abstract

Strain differences in susceptibility to the development of thyroiditis following adult thymectomy and repeated sublethal X-irradiation (4 X 200 rad) were investigated in the rat. Marked strain variation was noted, with AUG and PVG/c rats having the highest incidence and severity of lesion (100 and 80% respectively) whilst CAM rats had a low incidence (9%). WAG and LIS rats occupied an intermediate position. The incidence of autoantibodies to thyroglobulin correlated closely with the strain incidence of thyroiditis (r = 0-87). Fractionation of the sera from thymectomized and irradiated rats by gel filtration chromatography indicated that the bulk of the antibodies to thyroglobulin were of the IgG class. Freund's complete adjuvant, but not lipopolysaccharide, enhanced the rate of development of thyroiditis in thymectomized rats given a shortened series of irradiations. It was not found possible to induce thyroiditis in thymectomized rats by substituting antilymphocyte serum for irradiation as a T cell-depleting agent, despite the fact that the treated rats had markedly reduced responses to phytohaemagglutinin. A combination of thymectomy, three doses of 200 rad, and a development period of 8 weeks were found to be the optimum conditions for induction of severe thyroiditis together with high antibody titres to thyroglobulin. These findings add support for the role of thymus-derived cells in the regulation of autoimmune responses and further suggest that thymectomy, combined with a series of sublethal irradiations, causes a selective depletion of those T cells specifically involved in the suppression of autoimmune reactivity to thyroid components, whilst leaving autoreactive helper T cells unimpaired.