Blockade of the CD40-CD154 pathway can inhibit CD4+ T cell activation but is unable to prevent immune responses mediated by CD8+ T cells. However, even in the absence of CD8+ T cells, inhibition of the CD40-CD154 pathway is insufficient to prevent the development of transplant arteriosclerosis. This study investigated the mechanisms of transplant arteriosclerosis in the absence of the CD40 pathway. C57BL/6 CD40-/- (H2b) recipients were transplanted with MHC-mismatched BALB/c (H2d) aortas. Transplant arteriosclerosis was evident in both CD40-/- and CD40+/- mice (intimal proliferation was 59 ± 5% for CD40-/- mice vs 58 ± 4% for CD40+/- mice) in the presence or absence of CD8+ T cells (intimal proliferation was 46 ± 7% for CD40-/- anti-CD8-treated mice vs 50 ± 10% for CD40+/- anti-CD8-treated mice), confirming that CD8+ T cells are not essential effector cells for the development of this disease. In CD40-/- recipients depleted of CD8+ T cells, the number of eosinophils infiltrating the graft was markedly increased (109 ± 24 eosinophils/grid for CD40-/- anti-CD8-treated mice vs 28 ± 7 for CD40+/- anti-CD8-treated mice). The increased presence of eosinophils correlated with augmented intragraft production of IL-4. To test the hypothesis that IL-4 was responsible for the intimal proliferation, CD8 T cell-depleted CD40-/- recipients were treated with anti-IL-4 mAb. This resulted in significantly reduced eosinophil infiltration into the graft (12 ± 5 eosinophils/grid for CD40-/- anti-CD8+, anti-IL-4-treated mice vs 109 ± 24 for CD40-/- anti-CD8-treated mice), intragraft eotaxin, CCR3 mRNA production, and the level of intimal proliferation (18 ± 5% for CD40-/- anti-CD8+-, anti-IL-4-treated mice vs 46 ± 7% for CD40-/- anti-CD8-treated mice). In conclusion, elevated intragraft IL-4 production results in an eosinophil infiltrate and is an important mechanism for CD8+ T cell-independent transplant arteriosclerosis in the absence of CD40-CD154 costimulation.