BACKGROUND: Chronic vascular rejection (CVR) is characterized by an intimal thickening in the arteries of allografts due to immunoinflammatory reactions and smooth muscle cell proliferation. Interleukin 6 (IL-6) levels are increased in patients with graft rejection, however the role of IL-6 in CVR is not known. We investigated if IL-6 deficiency in the recipient could prevent CVR after an aortic allograft. METHODS: Donor aortas from wild-type DBA/2 mice were transplanted into C57BL/6 recipients, either wild-type mice or mice deficient for IL-6 (IL-6(-/-)), apolipoprotein E (ApoE(-/-)), or both (IL-6(-/-)ApoE(-/-)). Alloantibody titers were determined at Day 30, 60, or 90 after grafting. The grafts were examined for CVR lesions by morphometry and immunohistology. RESULTS: All recipient allografts displayed lesions typical for CVR. The lesions were larger in IL-6-deficient strains, and significantly so in IL-6(-/-)ApoE(-/-) recipients. Early immunoglobulin (Ig) G1 alloantibody deposits were observed in the grafts of ApoE-deficient strains and late IgG2a deposits in the grafts of IL-6-deficient strains. A rapid and sustained type 1 helper T cell (Th1; IgG2a) alloresponse in IL-6(-/-) mice, and a strong type 2 helper T cell (Th2; IgG1) response in ApoE(-/-) mice were observed. IL-6(-/-)ApoE(-/-) mice displayed the highest alloantibody titer, with a Th1 dominance. CONCLUSIONS: Unexpectedly, IL-6 deficiency in the recipient mice did not prevent CVR lesions but even aggravated them in IL-6(-/-)ApoE(-/-) recipients. This was associated with increased local and systemic alloresponses.