Antigen density on target cells determines the immunosuppressive potential of rat IgG2b monoclonal antibodies.

The current studies were designed to determine the relevance of T cell antigen density, besides antibody isotype, with regard to the success of antibody serotherapy. We compared the immunosuppressive effects of two rat IgG2b monoclonal anti-Thy-1 antibodies, RmT1 and 30-H12, with distinct binding sites in a graft-vs.-host disease (GVHD) model of fully H-2 and I-A region-mismatched bone marrow transplantation, making use of the difference in Thy-1.2 antigen density between homozygous (BALB/c) and heterozygous (BALB/c X AKR/J)F1 GVHD-promoting donor cells. Antibodies RmT1 (directed against a monomorphic determinant on mouse Thy-1) and 30-H12 (reactive with the Thy-1.2 allele-specific determinant) did not differ in their anti-GVHD activity with regard to Thy-1.2 homozygous grafts. However, in the region of a critical number of binding sites a small difference in the amounts of the two antibodies bound (about 8 X 10(3) IgG molecules/cell) obviously accounts for a great difference in anti-GVHD activity. This is shown in a two haplotype host-graft disparity between C57BL/6 recipients treated with either RmT1 or 30-H12 before challenging them with (BALB/c X AKR/J)F1 grafts, where the Thy-1.2 antigen concentration is approximately 50% compared to the density on BALB/c lymphocytes. Here, mAb 30-H12 loses its remarkable in vivo immunosuppressive quality, whereas RmT1 treatment protects mice against lethal GVHD. Binding sites were quantitated using a computerized approach for the analysis of data from ligand binding experiments of the respective mAb, RmT1 and 30-H12, coated to LN cells of BALB/c and F1 hybrid origin. Furthermore, the in vivo immunosuppressive activity of rat IgG2b antibodies directed against Thy-1 was found to correlate with their ability to generate stable antibody-C1q complexes on the cell surface of immunocompetent T cells.