Cardiac allograft hypertrophy: a new target for therapy, a surrogate marker for survival?

Recipient-derived EDA fibronectin promotes cardiac allograft fibrosis

Advances in donor matching and immunosuppressive therapies have decreased the prevalence of acute rejection of cardiac grafts; however, chronic rejection remains a significant obstacle for long-term allograft survival. While initiating elements of anti-allograft immune responses have been identified, the linkage between these factors and the ultimate development of cardiac fibrosis is not well understood. Tissue fibrosis resembles an exaggerated wound healing response, in which extracellular matrix (ECM) molecules are central. One such ECM molecule is an alternatively spliced isoform of the ubiquitous glycoprotein fibronectin (FN), termed extra domain A-containing cellular fibronectin (EDA cFN). EDA cFN is instrumental in fibrogenesis; thus, we hypothesized that it might also regulate fibrotic remodelling associated with chronic rejection. We compared the development of acute and chronic cardiac allograft rejection in EDA cFN-deficient (EDA(-/-)) and wild-type (WT) mice. While EDA(-/-) mice developed acute cardiac rejection in a manner indistinguishable from WT controls, cardiac allografts in EDA(-/-) mice were protected from fibrosis associated with chronic rejection. Decreased fibrosis was not associated with differences in cardiomyocyte hypertrophy or intra-graft expression of pro-fibrotic mediators. Further, we examined expression of EDA cFN and total FN by whole splenocytes under conditions promoting various T-helper lineages. Conditions supporting regulatory T-cell (Treg) development were characterized by greatest production of total FN and EDA cFN, though EDA cFN to total FN ratios were highest in Th1 cultures. These findings indicate that recipient-derived EDA cFN is dispensable for acute allograft rejection responses but that it promotes the development of fibrosis associated with chronic rejection. Further, conditions favouring the development of regulatory T cells, widely considered graft-protective, may drive production of ECM molecules which enhance deleterious remodelling responses. Thus, EDA cFN may be a therapeutic target for ameliorating fibrosis associated with chronic cardiac allograft rejection.

Cardiac allograft hypertrophy is associated with impaired exercise tolerance after heart transplantation

BACKGROUND

Exercise performance, an important aspect of quality of life, remains limited after heart transplantation (HTx). This study examines the effect of cardiac allograft remodeling on functional capacity after HTx.

METHODS

The total cohort of 117 HTx recipients, based on echocardiographic determination of left ventricle mass and relative wall thickness at 1 year after HTx, was divided into 3 groups: (1) NG, normal geometry; (2) CR, concentric remodeling; and (3) CH, concentric hypertrophy. Cardiopulmonary exercise testing was performed 5.03 ± 3.08 years after HTx in all patients. Patients with acute rejection or significant graft vasculopathy were excluded.

RESULTS

At 1 year post-HTx, 30% of patients had CH, 55% had CR and 15% had NG. Exercise tolerance, measured by maximum achieved metabolic equivalents (4.62 ± 1.44 vs 5.52 ± 0.96 kcal/kg/h), normalized peak Vo(2) (52 ± 14% vs 63 ± 12%) and Ve/Vco(2) (41 ± 17 vs 34 ± 6), was impaired in the CH group compared with the NG group. A peak Vo(2) ≤14 ml/kg/min was found in 6%, 22% and 48% of patients in the NG, CR and CH groups, respectively (p = 0.01). The CH pattern was associated with a 7.4-fold increase in relative risk for a peak Vo(2) ≤14 ml/kg/min compared with NG patients (95% confidence interval 1.1 to 51.9, p = 0.001). After multivariate analysis, a 1-year CH pattern was independently associated with a reduced normalized peak Vo(2) (p = 0.018) and an elevated Ve/Vco(2) (p = 0.035).

CONCLUSIONS

The presence of CH at 1 year after HTx is independently associated with decreased normalized peak Vo(2) and increased ventilatory response in stable heart transplant recipients. The identification of CH, a potentially reversible mechanism of impairment in exercise capacity after HTx, may have major clinical implications.

TGF-beta, IL-6, IL-17 and CTGF direct multiple pathologies of chronic cardiac allograft rejection

Cardiac transplantation is an effective treatment for heart failure refractive to therapy. Although immunosuppressive therapeutics have increased first year survival rates, chronic rejection remains a significant barrier to long-term graft survival. Chronic rejection manifests as patchy interstitial fibrosis, vascular occlusion and progressive loss of graft function. Recent evidence from experimental and patient studies suggests that the development of cardiomyocyte hypertrophy is another hallmark of chronic cardiac allograft rejection. This pathologic hypertrophy is tightly linked to the immune cytokine IL-6, which promotes facets of chronic rejection in concert with TGF-beta and IL-17. These factors potentiate downstream mediators, such as CTGF, which promote the fibrosis associated with the disease. In this article, we summarize contemporary findings that have revealed several elements involved in the induction and progression of chronic rejection of cardiac allografts. Further efforts to elucidate the interplay between these factors may direct the development of targeted therapies for this disease.

Connective tissue growth factor promotes fibrosis downstream of TGFbeta and IL-6 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 posttransplant, 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. Multiple factors have been implicated in the onset and progression of CR, including TGFbeta, IL-6 and connective tissue growth factor (CTGF). While associated with CR, the role of CTGF in CR and the factors necessary for CTGF induction in vivo are not understood. 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 whereas 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 CTGF functions as a downstream mediator of fibrosis in CR, and that CTGF neutralization may ameliorate fibrosis and hypertrophy associated with CR.

TGFbeta neutralization within cardiac allografts by decorin gene transfer attenuates chronic rejection

Chronic allograft rejection (CR) is the leading cause of late graft failure following organ transplantation. CR is a progressive disease, characterized by deteriorating graft function, interstitial fibrosis, cardiac hypertrophy, and occlusive neointima development. TGFbeta, known for its immunosuppressive qualities, plays a beneficial role in the transplant setting by maintaining alloreactive T cells in a hyporesponsive state, but has also been implicated in promoting graft fibrosis and CR. In the mouse vascularized cardiac allograft model, transient depletion of CD4(+) cells promotes graft survival but leads to CR, which is associated with intragraft TGFbeta expression. Decorin, an extracellular matrix protein, inhibits both TGFbeta bioactivity and gene expression. In this study, gene transfer of decorin into cardiac allografts was used to assess the impact of intragraft TGFbeta neutralization on CR, systemic donor-reactive T cell responses, and allograft acceptance. Decorin gene transfer and neutralization of TGFbeta in cardiac allografts significantly attenuated interstitial fibrosis, cardiac hypertrophy, and improved graft function, but did not result in systemic donor-reactive T cell responses. Thus, donor-reactive T and B cells remained in a hyporesponsive state. These findings indicate that neutralizing intragraft TGFbeta inhibits the cytokine's fibrotic activities, but does not reverse its beneficial systemic immunosuppressive qualities.

Critical role for IL-6 in hypertrophy and fibrosis in chronic cardiac allograft rejection

Chronic cardiac allograft rejection is the major barrier to long term graft survival. There is currently no effective treatment for chronic rejection except re-transplantation. Though neointimal development, fibrosis, and progressive deterioration of graft function are hallmarks of chronic rejection, the immunologic mechanisms driving this process are poorly understood. These experiments tested a functional role for IL-6 in chronic rejection by utilizing 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-CD40L therapy that do not develop chronic rejection. Echocardiography revealed the development of hypertrophy in grafts undergoing chronic rejection. Histologic analysis confirmed hypertrophy that 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. These observations reveal a new paradigm in which IL-6 drives development of pathologic hypertrophy and fibrosis in chronic cardiac allograft rejection and suggest that IL-6 could be a therapeutic target to prevent this disease.