Interleukin-6 and TGFbeta promote connective tissue growth factor in chronic cardiac allograft rejection

Cardiac transplantation is an effective treatment for multiple types of heart failure refractive to therapy. Although immunosuppressive therapeutics have increased survival rates within the first year post-transplant, chronic rejection (CR) remains a significant barrier to long term graft survival. Indicators of CR include patchy interstitial fibrosis, vascular occlusion, and progressive loss of graft function. However, the functional and anatomical changes associated with CR, as well as the factors responsible for the induction and progression of the disease are not understood. These experiments utilized serial echocardiography to assess the progression of chronic rejection in vascularized mouse cardiac allografts. Cardiac allografts in mice transiently depleted of CD4+ cells that develop chronic rejection were compared with those receiving anti-CD4OL therapy that do not develop chronic rejection. Echocardiography revealed the development of hypertrophy in grafts undergoing chronic rejection which was confirmed by histologic analysis and coincided with graft fibrosis and elevated intragraft expression of IL-6. To elucidate the role of IL-6 in chronic rejection, cardiac allograft recipients depleted of CD4+ cells were treated with neutralizing anti-IL-6 mAb. IL-6 neutralization ameliorated cardiomyocyte hypertrophy, graft fibrosis, and prevented deterioration of graft contractility associated with chronic rejection. The association of IL-6 with CR prompted us to investigate the relationship between IL-6 and two other factors known to be associated with CR, namely TGFbeta and connective tissue growth factor (CTGF). To this end, we utilized forced expression and neutralizing antibody approaches. Transduction of allografts with CTGF significantly increased fibrotic tissue development, though not to levels observed with TGFbeta transduction. Further, intragraft CTGF expression was inhibited by IL-6 neutralization while TGFbeta expression remained unchanged, indicating that IL-6 effects may potentiate TGFbeta-mediated induction of CTGF. Finally, neutralizing CTGF significantly reduced graft fibrosis without reducing TGFbeta and IL-6 expression levels. These findings indicate that IL-6 and TGFbeta function as promoters of CR while CTGF functions as a downstream mediator of fibrosis in CR. Further, therapeutics targeting IL-6 or CTGF may hold promise in the treatment of CR of cardiac grafts.