Rat thyroid epithelial cells provide limited accessory function but fail to present allo antigens in vitro

Human and mouse thyroid epithelial cells (TEC) have been reported to exhibit a number of immunological activities including partial, but not total, antigen presenting capability. In the studies described here, the thyroid stimulating hormone-dependent rat thyroid epithelial cell line FRTL-5 was tested for its abilities to provide accessory and antigen presenting cell (APC) activity. FRTL-5 cells alone were not able to function as accessory cells for ConA induced polyclonal proliferation of purified T-cells and were not capable of inducing alloreactive T-cell proliferation in mixed lymphocyte reaction (MLR) cultures. Furthermore, even following the induction and enhanced expression of MHC class II and class I MHC products, respectively by IFN-gamma, FRTL-5, cells could not function as accessory cells or induce alloreactive T-cell proliferation as the singular stimulatory population in MLR cultures. However, these epithelial cells could effectively synergize with either low numbers of spleen cells, or with supernatants from activated T-cells, to provide accessory function in ConA stimulated cultures. These findings suggest that hormone-dependent and functional rat TEC cannot directly activate resting syngeneic or unprimed allogeneic T-cells, although they can provide at least one accessory cell signal involved in T-cell activation by mitogen. Thus, the results presented here do not support contentions that such a nonhemopoietic resident cell population is capable of directly triggering the initial activation of anti-thyroid specific T-cells.