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 hypertiyroid 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 10⁶ 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 (3×10⁷ 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.