Immune Modulation of Antibody Responses Induced Through Allogeneic Cell Transplantation

Combined renal and hematopoietic cell transplantation (HCT) protocols have successfully induced allograft tolerance despite loss of chimerism in patients; however, the mechanism remains unclear.  Using a miniature swine model with demonstrated clinical relevance, we assessed immune responses following HCT in ten recipients that lost chimerism.  All animals received 30 days of cyclosporine (CyA) with taper until day 45; low-dose total body irradiation (100cGy TBI) and T cell depletion using a CD3 immunotoxin (pCD3-IT).  Six animals received mobilized donor cells and 4 animals received unmobilized cells. Control animals received allogeneic cells without any immunosuppression.  Cellular responses were assessed by mixed lymphocyte reactivity and cell mediated lympholysis assays.  Donor specific antibody was assessed by flow cytometry and complement mediated cytotoxicity assays.

Following loss of chimerism, anti-donor cellular proliferative and cytotoxic responses returned without alloantibody.  Alloantibody responses were not induced even after a second exposure to donor cells intravenously without immunosuppression (n=10) or following donor skin graft rejection (n=3).  Attempts to further immunize some of these animals (n=6) with multiple subcutaneous injections of donor cells with or without complete Freund’s adjuvant also failed to induce donor specific antibody.  In contrast, control animals exposed to donor cells without conditioning had sustained alloantibody responses detectable within two weeks following either intravenous (n=2) or subcutaneous (n=2) injection of cells as well as following skin graft rejection. 

These results suggest that exposure to donor cells following reduced intensity conditioning can result in robust immune modulation of antibody responses to allogeneic cells.  B cell unresponsiveness is stable and does not depend on persistence of unresponsiveness at the T cell level or the persistence of donor cells.  We speculate that this mechanism of immune modulation of B cell responses by allogeneic cells may play an important role in facilitating induction of transplantation tolerance through HCT.