CONRIBUTION OF THE INDIRECT RECOGNITION PATHWAY TO CARDIAC GRAFT REJECTION IN THE RAT-to-MOUSE XENOGENIC MODEL

Abstract

49 The T cell response to transplanted xenogenic organs/tissues is mediated via direct (donor MHC plus peptides) and indirect (recipient MHC plus donor-derived peptides) xenorecognition pathways. The role of the indirect pathway in xenoantigen recognition and xenograft rejection has not been fully investigated. In this study, we, therefore, used cardiac grafts in the rat-to-mouse (PVG.R8-to-C57BL/B10) species combination as a model system to determine relative contribution of indirect recognition to the rejection of vascularized xenografts. The T cell response to xenoantigens was determined using lymph node cells or splenocytes harvested from naive or xenograft recipients at rejection in a standard mixed lymphocyte reaction assay. The responses to auto and allogenic antigens were used as negative and positive controls, respectively. We obtained a higher (30,000-35,000 c.p.m.) T cell response to xenoantigens when compared with that for alloantigens (18,000-21,000 c.p.m.) at all time points of cell proliferation examined. The proliferative response of immune T cells was more robust and vigorous than that of naive T cells (45,000-50,000 vs 30,000-35,000 c.p.m.). We used monoclonal antibodies to mouse CD80 and CD86 costimulatory molecules at various doses (1-25 μg/ml) to determine the contribution of indirect recognition to the observed T cell responses. There was a dose-dependent inhibition of the proliferative response of both naïve and immune T cells when these antibodies used either individually or in combination. The maximum inhibition (95-98%), however, was observed when both antibodies used in combination at a dose of 25 μg/ml. The suppression of cell proliferation by antibodies against mouse CD80 and CD86 costimulatory molecules provide clear evidence that the indirect recognition pathway plays a critical role in xenoreactive immunity in this model as these antibodies can only block antigen presentation by the recipient (mouse) antigen presenting cells (indirect recognition). The dominant role of indirect recognition in T cell responses to xenoantigens suggests that the modulation of this pathway will be critical for prevention of cellular immunity and long-term graft survival in a xenosetting.