ABSTRACT
BACKGROUND: Since the future of clinical transplantation will undoubtedly include xenotransplantation, there is a need to examine human anti-pig cellular reactions. The objective of this study is to use human anti-porcine mixed lymphocyte endothelial cell culture (MLEC) to investigate cell interactions, cross-species molecular compatibilities, and the induction of human cytokines and porcine activation markers. METHODS: Human peripheral blood mononuclear cells or enriched CD4+ T cells depleted of professional antigen-presenting cells were cultured with resting pig aortic endothelial cells in the absence of exogenous cytokines. T-cell proliferative responses were measured and PAEC were monitored for cell surface markers by flow cytometry. Culture supernatants were assayed for human TNF-alpha and IFN-gamma by ELISA. RESULTS: Human T cells proliferated strongly in response to PAEC (median stimulation index = 75), even in serum-free cultures. High levels of the human Th1 cytokines TNF-alpha (20-350 pg/ml) and IFN-gamma (200-3800 pg/ml) were detected only in cultures containing PAEC, with levels peaking on Day 4. CD4+ T-cell-enriched, APC-depleted responders maintained proliferative anti-PAEC responses and cytokine release. By Day 3, MHC Class II and VCAM expression was induced in 92-96% PAEC: mean fluorescence intensity (MFI) increased from 5 to 83 +/- 12 and 166 +/- 74, respectively, and MHC Class I was increased from MFI 31 to 965 +/- 269. CONCLUSIONS: These results indicate that MLEC is an excellent in vitro model in which to study human anti-porcine cellular responses. Human T cells are activated in response to direct antigen presentation by PAEC, which are also activated in this system. Specific cytokines, receptors, and adhesion molecules appear to cross the xenograft barrier and play a critical role in T-cell - PAEC interactions. Such interactions are likely to affect VEC activation and immune responses to porcine xenografts in vivo.
