A20-Mediated Modulation of Inflammatory and Immune Responses in Aortic Allografts and Development of Transplant Arteriosclerosis

Transplant arteriosclerosis (TA) is the pathognomonic feature of chronic rejection, the primary cause of allograft failure. We have shown that the NF-κB inhibitory protein A20 exerts vasculoprotective effects in endothelial and smooth muscle cells (SMC), and hence is a candidate to prevent TA. We sought direct proof for this hypothesis. Fully mismatched, C57BL/6 (H2) into BALB/c (H2), aorta to carotid allografts were preperfused with saline, recombinant A20 adenovirus (rAd.A20) or rAd.β-galactosidase (β-gal), implanted, harvested 4 weeks after transplantation, and analyzed by histology, immunohistochemistry, and immunofluorescence staining. We measured indoleamine 2,3-dioxygenase, interleukin-6, and transforming growth factor-β mRNA and protein levels in nontransduced, and rAd.A20 or rAd.β-gal-transduced human SMC cultures after cytokine treatment. Vascular overexpression of A20 significantly reduced TA lesions. This correlated with decreased graft inflammation and increased apoptosis of neointimal SMC. Paradoxically, T-cell infiltrates increased in A20-expressing allografts, including the immunoprivileged media, which related to A20 preventing indoleamine 2,3-dioxygenase upregulation in SMC. However, infiltrating T cells were predominantly T-regulatory cells (CD25+/Forkhead Box P3 [FoxP3+]). This agrees with A20 inhibiting interleukin-6 and promoting transforming growth factor-β production by medial SMC and in SMC cultures exposed to cytokines, which favors differentiation of regulatory over pathogenic T cells. In summary, A20 prevents immune-mediated remodeling of vascular allografts, therefore reduces TA lesions by affecting apoptotic and inflammatory signals and modifying the local cytokine milieu to promote an immunoregulatory response within the vessel wall. This highlights a novel function for A20 in local immunosurveillance, which added to its vasculoprotective effects, supports its therapeutic promise in TA.