Suppression of kidney allograft rejection across full MHC barriers by recipient-specific antibodies to class II MHC antigens.

The aim of these studies was to see if recipient-specific antibodies to class II MHC antigens might be effective in suppressing kidney graft rejection in rats. For these experiments, the polymorphic BMAC-4 mouse IgG1 monoclonal antibody to RT1-D class II MHC antigens was raised. This antibody reacts with the DA, LEW, PVG, and SHR strains, but not the BN or WAG strains, and is therefore recipient-specific in the WAG to PVG combination. Initial in vivo titrations demonstrated that 1 ml doses of the BMAC-4 and also of the MRC OX6 (monomorphic mouse IgG1 anti-RT1-B class II) antibody resulted in the maintenance of free antibody levels in blood for greater than 24 hr. Treatment of PVG recipients of WAG kidney allografts with the BMAC-4 antibody, but not the MRC OX6 antibody, resulted in greatly prolonged graft survival. To examine possible mechanisms, several experiments were performed. After intravenous injection, the antibody was found to have ready access to the connective tissues of nonlymphoid organs, to the red and white pulp of the spleen, and to the medulla of lymph nodes. However, there was poor early access to the cortex and paracortex of lymph nodes. Both MRC OX6 and BMAC-4 could completely suppress PVG anti-WAG and WAG anti-PVG mixed lymphocyte culture reactions. Both antibodies were also equally effective for opsonisation of class II-positive cells from the blood circulation. However, only the recipient-specific, anti-RT1-D BMAC-4 antibody suppressed graft rejection. Thus, while the BMAC-4 antibody is likely to have had a variety of different effects on RT1-D positive recipient cells, the locus specificity of the immunosuppression is consistent with an important component of those effects being the blocking of presentation of WAG donor alloantigens by PVG RT1-D class II antigens on PVG antigen-presenting cells.