Prevention of graft-versus-host disease using antibody to Thy 1. A role for complement in vivo.

An in vitro assay was used to determine whether mouse complement (C') was able to lyse T cells coated with a monoclonal IgM anti-Thy-1 antibody. Using lymph node cells (LNC) as targets, it was found that antibody-coated mature T cells were efficiently lysed by serum from C57BL/6, but inefficiently lysed by serum from BALB/c mice. Serum from both strains lysed antibody-coated thymocytes, demonstrating that BALB/c serum is not deficient in one of the C' components. The lytic activity of BALB/c serum could be improved by increasing the concentration of monoclonal anti-Thy-1 used to coat the target LNC. In contrast, serum from C5-deficient A/HeJ mice was unable to lyse either of the antibody-coated target cells in vitro. These in vitro results were confirmed in vivo using a model of lethal graft-versus-host disease (GVHD) induced to minor histocompatibility antigens. In this model, GVHD occurs when lethally irradiated mice are transplanted with bone marrow plus spleen cells from H-2-identical donor mice. GVHD is prevented when T cells are removed from the cells by in vitro treatment with anti-Thy-1 antibody plus rabbit C'. As predicted by the in vitro data, C57BL/6 mice were protected from GVHD when transplanted with LP cells treated with a 1:100 dilution of anti-Thy-1 antibody, but without complement, but BALB/c recipients of antibody-coated B10.D2/nSN cells were not. When the concentration of anti-Thy-1 used to coat the donor cells was increased to 1:50, 8 of 9 recipients were protected from GVHD. In contrast, none of the C5-deficient DBA/2 recipients of anti-Thy-1 (1:50 dilution) treated B10.D2 cells were protected from GVHD. Complement-mediated lysis appears to be the primary mechanism by which T cells coated with a monoclonal IgM anti-Thy-1 antibody are functionally inactivated in vivo.