48 Rejection of a mouse-to-rat cardiac transplant is dependent on antibody deposition, complement activation and cellular infiltration. Endothelial cell activation is hypothesized to play a central role in the pathogenesis of delayed xenograft rejection (DXR), mediated in part by the effects of proinflamatory cytokines causing phenotypic changes in the endothelium including upregulation of adhesion molecules. We tested this hypothesis by examining graft survival and cellular infiltration of hearts from mice deficient in adhesion molecules or receptors for proinflamatory cytokines transplanted into complement depleted rats. Heterotopic heart transplants were performed from mice deficient in TNFα receptor-1 (TNF-αR), IL-1 receptor-1 (IL-1R) and both TNF-αR and IL-1R (TNF-αR/IL-1R) into cobra venom factor (CVF)-treated normocomplementemic PVG rats, and from mice deficient in ICAM-1, P-selectin, E-selectin and ICAM-1/P-selectin into CVF-treated and untreated C6-deficient PVG rats. Graft survival was assessed by palpation, and cellular infiltration determined by immunofluorescence staining for rat neutrophils (PMN), macrophages (MAC), natural killer (NK) cells and T cells. Graft survival of hearts from TNF-αR/IL-1R deficient donors was modestly prolonged compared to controls (6.0±0.6 days vs. 5.1±0.6 days, p=0.03) but not those from donors with isolated deficiency of TNF-αR, IL-1R, ICAM-1, P-selectin or E-selectin or with combined ICAM-1/P-selectin deficiency. Cellular infiltration however was markedly decreased in rejected hearts from all TNF-αR and IL-1R deficient strains as shown in the table. The rejected TNF-αR/IL-1R deficient hearts had large areas with no infiltrating cells.TableThere were no significant differences in the number or distribution of infiltrating cells in any of the rejected adhesion molecule deficient hearts. In conclusion, rejected cardiac xenografts from mice deficient in TNF-αR, IL-1R and TNF-αR/IL-1R have markedly reduced infiltration of macrophages and neutrophils suggesting that endothelial cell activation by TNF-α or IL-1 greatly amplifies recruitment of recipient leukocytes into the xenograft; however, suppression of this response interfered with rejection only marginally. No benefit on xenograft survival or cellular infiltration was seen with hearts from mice deficient in adhesion molecules.