PROPERTIES OF ANTIGEN-SPECIFIC SUPPRESSIVE T-CELL FACTOR IN THE REGULATION OF ANTIBODY RESPONSE OF THE MOUSE I . In Vivo Activity and Immunochemical Characterizations* BY TOSHITADA TAKEMORI AND TOMIO TADA

Our previous publications (1, 2) indicated that thymocytes and spleen cells of mice that had been immunized with a high dose of carrier antigen, when transferred into syngeneic host, could suppress the antibody response against a hapten coupled to the homologous carrier . This suppressive effect was found to be mediated by a population of thymus-derived lymphocytes, whose activity was completely abrogated by the treatment with anti-B and complement . Moreover, the specificity of the observed suppression has been established by the fact that neither normal T cells nor those obtained from mice immunized with an unrelated antigen could suppress the antibody response . These results indicate that immunization with a relatively high dose of carrier antigen generated a subpopulation of T cells that specifically inhibits the antibody response against hapten on the same carrier. Similar antigen-specific T-cell-mediated suppression has recently been demonstrated in a number of other experimental systems (3-10), and is now considered to be an important regulatory mechanism in various forms of immune responses (reviewed in 11). A possible molecular mechanism of the antigen-specific suppression is that certain primed T cells liberate a suppressive factor(s) which then gives rise to an "off" signal to other cell types via combination with antigen. There are several examples of soluble T-cell factors that influence the magnitude and quality of the immune response (12-24). Thus, the above findings prompted us to explore the subcellular component of T cells which can mediate the antigen-specific suppression of the antibody response . We have attempted to separate such a soluble component by simple sonication followed by ultracentrifugation of the thymocytes and spleen cells possessing the suppressor activity, since we were successful in obtaining an antigen-specific suppressive factor from sonicated Tcells of the rat, which had shown a strong inhibitory effect on an ongoing IgE antibody response of the same species (19, 25) . This paper will describe some of the properties of the antigen-specific suppressive T-cell factor obtained in mice, which was found to suppress mainly IgG antibody response of syngeneic mice in an in vivo experimental system .