Background: It has recently been shown that treatment of animals with antibodies to CD154 (CD40L), allows for prolongation of cardiac allograft survival, but does not inhibit development of graft vasculopathy. CD8+ T cells have been implicated in this effect. In this study we assess the role of CD40-CD154 interactions and CD40-independent CD8+ T cells in the permanent and complete absence of CD40 by using donors and recipients genetically deficient in CD40. Methods: Hearts from BALB/c CD40-/- donors were transplanted into C57BL/6 CD40-/- recipients in the presence or absence of CD8+ T-cell depletion. At Day 60, hearts were examined for vasculopathy using quantitative morphometry and numbers of infiltrating T cells were counted. The intragraft expression of interferon-γ (IFN-γ), transforming growth factor-β1 (TGF-β1), interleukin-4 (IL-4), eotaxin and CCR3 was assessed using competitive reverse transcription-polymerase chain reaction (RT-PCR). Results: In the absence of CD8+ T-cell depletion, the mean percent intimal occlusion was 28% (with 50% of vessels showing no intimal occlusion). This figure was reduced significantly to 12% and 80% of vessels showing no intimal occlusion in mice receiving anti-CD8 antibody. Depletion of CD8+ T cells was associated with significantly reduced intragraft IFN-γ, TGF-β1 and CCR3 expression, whereas mRNA production of IL-4 and eotaxin was increased. Conclusion: Vascular intimal occlusion progresses in the complete absence of CD40-CD154 interactions, albeit to quite a small degree. The residual disease is significantly reduced by anti CD8+ T-cell treatment, confirming the importance of CD40-CD154-independent CD8+ T cells in the genesis of this disease.