Effect of thyroxine on the immune response of mice in vivo and in vitro

The effect of thyroxine on the immune response of BALB/c mice to sheep erythrocytes was investigated. In mice rendered hyperthyroid by subcutaneous injections of T4 the primary immune response to an injection of SRBC in vivo did not show a consistent increase in splenic anti-SRBC plaque-forming cells. However, the total number of splenic cells in T4-treated mice was generally decreased, and thus, the number of PFC per 10(6) splenic cells in T4-treated mice was higher than those of saline and buffer controls. In in-vitro primary response to SRBC PFC per culture (3x10(7) splenic cells) increased significantly in T4-injected animals as compared with controls. The calculated PFC per spleen also increased significantly. The addition of T4 to normal splenic cell cultures enhanced the primary immune response to SRBC in vitro. The optimum concentration of T4 was found to be 10(-8) M. These results indicate a direct enhancing effect of T4 on the immune response of lymphoid cells. This enhancing effect, however, may be attenuated in vivo by the alteration of the number and/or composition of lymphoid cells brought about directly or indirectly by injections of exogenous thyroxine.