Cardiac allograft vasculopathy and dysregulation of the NO synthase pathway

Homocysteine-lowering therapy and early progression of transplant vasculopathy: a prospective, randomized, IVUS-based study

Although observational studies suggest that hyperhomocysteinemia may be a risk factor for coronary allograft vasculopathy (CAV), prospective data on homocysteine-lowering interventions and CAV development are lacking. We, therefore, randomized 44 de novo heart transplant (HT) recipients to 15 mg/day of 5-methyl-tetrahydrofolate (n=22), or standard therapy (control group, n=22) to investigate the effect of homocysteine lowering on the change in coronary intimal hyperplasia during the first 12 months after transplant, as detected by intra-vascular ultrasound (IVUS). Although 12 months after HT, homocysteinemia was lower in folate-treated patients (p<0.001), coronary intimal area increased similarly in the two groups (p>0.4). Conversely, hypercholesterolemia and cytomegalovirus infection were both associated with increased intimal hyperplasia (p<0.04), independently from folate intake. Sub-group analysis revealed that folate therapy reduced intimal hyperplasia in patients with hyperhomocysteinemia before randomization (n=19; p=0.02), but increased intimal hyperplasia in patients with normal homocysteine plasma concentrations (p=0.02). This bimodal effect of folate therapy persisted significantly after adjusting for cytomegalovirus infection and hypercholesterolemia. Despite effective in prevent hyperhomocysteinemia after heart transplantation, folate therapy does not seem to affect early CAV onset. However, sub-group analysis suggests that folate therapy may delay CAV development only in patients with baseline hyperhomocysteinemia, while may favor CAV progression in recipients with normal baseline homocysteinemia.

Early Inhibition of Caspase-3 Activity Lessens the Development of Graft Coronary Artery Disease

BACKGROUND

The role of apoptosis in the development of graft coronary artery disease (GCAD) is poorly understood. We have previously shown that early overexpression of the anti-apoptotic protein Bcl-2 lessens the development of GCAD. We hypothesized that early inhibition of apoptosis with a caspase-3 inhibitor would also lessen the development of GCAD.

METHODS

Heterotopic heart transplantation was performed in 4 groups of rats. Donor hearts were pretreated with 50 microg DEVD-CHO, a cell-permeable caspase-3 inhibitor, or vehicle. Recipient animals were pretreated with 1.7 mg/kg intraperitoneal DEVD-CHO or vehicle. Animals were treated with 7.5 mg/kg/d cyclosporine for 10 days to prevent acute rejection. On post-operative day 90, the animals were sacrificed and the transplanted hearts were assessed morphometrically for evidence of GCAD.

RESULTS

At 90 days, intimal proliferation was significantly higher in vehicle treated animals than in inhibitor treated animals. Moreover, the percentage of vessels with high-grade occlusion (>50%) was also lower in inhibitor treated animals.

CONCLUSIONS

Early inhibition of caspase-3 activity with cell-permeable DEVD-CHO lessens the development of GCAD. Caspase-3 inhibition may be a useful strategy for prevention of GCAD in clinical transplantation.

Cytomegalovirus infection in heart transplant recipients is associated with impaired endothelial function

BACKGROUND

Cardiac allograft vasculopathy (CAV) is initiated by allograft endothelial injury. We hypothesized that a major mechanism by which cytomegalovirus (CMV) could contribute to CAV is by dysregulation of the endothelial vasomotor response.

METHODS

Coronary endothelial vasomotor function was determined in 183 consecutive patients (24+/-33 months after transplantation), and was correlated with recipient and donor CMV serological status before transplantation and with documented CMV infection episodes (CMVpp65Ag+). Serial endothelial function measurements were performed in a subgroup of 53 transplant recipients (1 month and 12 months after transplantation). The composite endpoint of cardiovascular related events and death during a follow-up of 66+/-41 months was analyzed based on the CMV serological status before transplantation.

RESULTS

The medium event-free time for CMV-negative recipients of CMV-positive hearts was 8.1 years compared with 13.3 years for the other groups (P<0.05). Distal epicardial but not microvascular endothelial function was significantly impaired in CMV seronegative recipients of seropositive donor hearts (n=48) compared with all other groups (P<0.01 versus seronegative recipient/seronegative donor; P<0.05 versus seropositive recipient/seronegative donor; P<0.05 versus seropositive recipient/seropositive donor). Distal epicardial endothelial dysfunction was more pronounced in heart transplant recipients with a history of documented CMV infection compared with patients without any documented CMV infection (P<0.01). In a longitudinal subgroup analysis, distal epicardial and microcirculatory endothelial vasomotor response deteriorated significantly in recipients with documented CMV infection (P<0.05 versus baseline) but not in patients without previous CMV infection.

CONCLUSIONS

Documented CMV infection episodes in heart transplant recipients are associated with impaired coronary endothelial function. CMV-negative recipients of CMV-positive donor hearts have an impaired distal epicardial endothelial function and an increased incidence of cardiovascular-related events and death during follow-up. CMV infection may contribute to allograft failure by accelerating coronary endothelial dysfunction.

T cell-mediated vascular dysfunction of human allografts results from IFN-gamma dysregulation of NO synthase

Allograft vascular dysfunction predisposes to arteriosclerosis and graft loss. We examined how dysfunction develops in transplanted human arteries in response to circulating allogeneic T cells in vivo using immunodeficient murine hosts. Within 7-9 days, transplanted arteries developed endothelial cell (EC) dysfunction but remained sensitive to exogenous NO. By 2 weeks, the grafts developed impaired contractility and desensitization to NO, both signs of VSMC dysfunction. These T cell-dependent changes correlated with loss of eNOS and expression of iNOS--the latter predominantly within infiltrating T cells. Neutralizing IFN-gamma completely prevented both vascular dysfunction and changes in NOS expression; neutralizing TNF reduced IFN-gamma production and partially prevented dysfunction. Inhibiting iNOS partially preserved responses to NO at 2 weeks and reduced graft intimal expansion after 4 weeks in vivo. In vitro, memory CD4+ T cells acted on allogeneic cultured ECs to reduce eNOS activity and expression of protein and mRNA. These effects required T cell activation by class II MHC antigens and costimulators (principally lymphocyte function-associated antigen-3, or LFA-3) on the ECs and were mediated by production of soluble mediators including IFN-gamma and TNF. We conclude that IFN-gamma is a central mediator of vascular dysfunction and, through dysregulation of NOS expression, links early dysfunction with late arteriosclerosis.

The role of viruses in cardiac allograft vasculopathy

Considerable evidence suggests a role for viruses in transplant arteriosclerosis (TA), including observational data, experimental models and therapeutic trials implicating human cytomegalovirus (HCMV) in the progression to TA. In pediatric heart transplant patients, adenoviral genome in endomyocardial biopsies (EMB) is an important predictor of TA and graft loss. During CMV viremia, EMBs from adult patients demonstrate endothelialitis and vascular smooth muscle cell proliferation. These changes are predictors of subsequent diffuse TA. HCMV immediate early proteins (IE-1 and IE-2) increase the constitutive expression of intercellular adhesion molecule-1 (ICAM-1) independent of other intracellular cytokines. Likewise, viral chemokines such as US28 have been implicated in vascular disease because of their ability to induce smooth muscle cell migration. Recent data suggests that CMV might accelerate TA through its ability to abrogate the vascular protective effects of the endothelium-derived nitric oxide system (eNOS). Confirmation of causality requires clinical trials demonstrating that antiviral agents such as ganciclovir inhibit TA. Such studies in patients though limited to retrospective analyses, suggest that ganciclovir prophylaxis early after heart transplantation reduces the risk of TA. These observations emphasize the need for randomized controlled clinical trials to confirm a causal role for CMV (and other viruses) in TA.