T cell-produced immunoglobulin binding factor (IBF) bears determinants coded for by the I region of the major histocompatibility complex and lacks allogeneic restriction.

An immunoglobulin binding factor (IBF) produced by in vivo alloantigen-activated T cells suppresses in vitro antibody production to sheep red blood cells (SRBC) and probably represents the soluble form of the T cell Fc receptor. To study the relationship between IBF and the major histocompatibility complex (MHC), two types of experiments were undertaken. First, different IBF were prepared by injecting thymocytes from mice of H-2k, H-2b and H-2d haplotypes into irradiated, allogeneic recipient mice. Cells recovered in the spleens five days later were incubated for 2 h at 37 degrees C, and IBF was purified by affinity chromatography on insolubilized IgG. Purified IBF was then applied to immunosorbents prepared with anti-H-2 and anti-Ia anti sera. Acid eluates were then tested for their suppressive activity on in vitro antibody production to SRBC. Under these conditions, IBF was found to react with antisera raised against products of the whole MHC and of the I region of the MHC, but not with anti-H-2 D antisera. These data indicate that IBF bears, or is associated with Ia determinants. Second, IBF preparations from mice of H-2k, H-2b, H-2d haplotypes were tested for their capacity to suppress the in vitro antibody production to SRBC by spleen cells of a series of mouse strains. The data indicate that IBF is not genetically restricted in its function.