Generation of T helper cells in vitro. III. Helper cell culture-derived factors are related to alloantigens coded for the I region of the H-2 major histocompatibility complex.

Thymocyte-macrophage cultures primed with carrier protein release an alloantigen-related factor that enhances the anti-hapten plaque-forming cell response of hapten-primed spleen cells in vitro. Use of immunoadsorbant columns made with a variety of alloantisera indicates that the relevant antigens in this system are coded for the the H-2 major histocompatibility locus in mice. The data indicate that in H-2d and H-2k strains the important genetic regions are in the I region between the K region and the I-C subregion and suggest, based on current understanding of Ia specificities, that the I-A subregion codes for these antigens.