Background.Data supporting the differential activation of T helper (Th) 2 cells in transplantation acceptance/tolerance in rodents have been presented by several investigators. However, the differential activation of Th2 cells may be simply the result of allograft acceptance/tolerance induction instead of a contribution to the maintenance of grafts.Methods.Therefore, we established interleukin (IL)-4 transgenic mice under the control of a cardiac α-myosin heavy chain promotor and transplanted IL-4-expressing heart allografts into unmodified recipients to determine the actual contribution of the Th2 bias to allograft acceptance.Results.Among 16 newborn C57BL/6J (B6) mice, three were positive for the IL-4 transgene. Serum IL-4 levels of transgenic versus control B6×C3H F1mice were not statistically different. Reverse-transcriptase polymerase chain reaction showed that the transgenic B6×C3H F1mice expressed IL-4 mRNA in the heart and in the lung, whereas control mice did not express IL-4 in any organ. IL-4 mRNA expression in the transgenic but not in the control heart was also confirmed by RNAse protection assay and fluorescence in situ hybridization. The survival of IL-4 transgenic B6×C3H heart grafts heterotopically placed in C3H recipients was prolonged compared with that of nontransgenic grafts (19.0±9.1 vs. 6.8±2.2 days,P=0.003). Interferon-γ mRNA expression in IL-4 transgenic heart grafts on the fifth posttransplant day as assessed by Northern blotting was suppressed compared with that in control grafts. Reverse-transcriptase polymerase chain reaction analysis showed that IL-2 mRNA was suppressed in the IL-4 transgenic grafts compared with that in control grafts, while IL-4 mRNA was observed only in IL-4 transgenic grafts. IL-10 mRNA was detected at similar levels in both transgenic and control grafts.Conclusions.A Th2 bias may contribute to allograft acceptance in part by inducing the down-regulation of Th1 -cytokine mRNAs, but it may not be sufficient to induce indefinite graft survival.