BACKGROUND: Indirect allorecognition has been implicated in the initiation of chronic allograft dysfunction. Our aim was to develop an animal model that allowed the contribution of the direct and indirect pathway of allorecognition in the evolution of transplant arteriosclerosis, the main feature of chronic allograft rejection, to be evaluated. METHODS: Aortic allografts mismatched for a single MHC class I antigen were transplanted into athymic NUDE or RAG (-/-) mice. Immunodeficient mice were reconstituted with either CD4(+) (indirect) or CD8(+) (direct + indirect) T cells in the presence or absence of depleting antibodies specific for the opposite T-cell subset. Aortic grafts were analyzed by performing morphometry, immunohistochemistry, and quantitative reverse transcriptase-polymerase chain reaction for the detection of cytokine mRNA production. Donor-specific alloantibody production was measured by fluorescence-activated cell sorter analysis. RESULTS: Reconstitution of athymic nude mice with 4 x 10(7) purified CD4(+) T cells resulted in vascular rejection of MHC class I mismatched aortic grafts. Intimal proliferation was 24+/-8% and did not decrease when nude-derived endogenous CD8(+) T cells were depleted from the nude recipients (intimal proliferation, 21+/-7%). Transplant arteriosclerosis initiated by CD4+ T cells was associated with the presence of intragraft mRNA for interferon-gamma, tumor necrosis factor-alpha, inducible nitric oxide synthase, and interleukin 12. Reconstitution of RAG-1(-/-) mice with 4 x 10(7) purified CD4(+) T cells resulted in a similar degree of transplant arteriosclerosis (intimal proliferation, 20+/-9%) in MHC class I mismatched aortic grafts in the absence of alloantibody production. CONCLUSION: Indirect recognition of donor MHC class I molecules by CD4(+) T cells can play an important role in the process of transplant arteriosclerosis. CD8(+) T-cell effector function and alloantibody production in this model are dependent on CD4(+) T-cell help after indirect allorecognition.