Histomorphometric comparison of cardiac allograft vasculopathy in miniature swine

Beneficial effects of perioperative low-dose inhaled carbon monoxide on pulmonary allograft survival in MHC-inbred CLAWN miniature swine

BACKGROUND

We have recently reported that perioperative low-dose carbon monoxide (CO) inhalation decreases lung ischemia-reperfusion injury in miniature swine. The aims of this study were to establish a large animal model of pulmonary allograft rejection using polymerase chain reaction-typed major histocompatibility complex (MHC)-inbred CLAWN miniature swine and to examine the effects of CO on allograft survival.

METHODS

Eleven CLAWN miniature swines received fully MHC-mismatched lungs followed by 12 days of tacrolimus (days 0-11; blood level 35-45 ng/mL). Six recipients received tacrolimus alone (control group). Five recipients were additionally treated with inhaled CO (180 min for donors until graft harvest; 390 min for recipients until 2 hr after reperfusion).

RESULTS

All recipients treated with tacrolimus alone uniformly rejected their grafts by postoperative day 63 with development of cytotoxic antidonor antibodies. CO treatment was effective in prolonging allograft survival from a mean of 47±7 to 82±13 days (P=0.017), with one CO-treated animal maintaining function until postoperative day 120. Development of antidonor antibodies and donor-specific responsiveness by cell-mediated lympholysis and mixed lymphocyte reaction assays was delayed in animals that received CO therapy. Furthermore, serum concentrations of proinflammatory cytokines (interleukin-1β and -6) 1 day after transplant were significantly decreased in the CO-treated group.

CONCLUSIONS

Fully MHC-mismatched lungs in CLAWN miniature swine were consistently rejected within 63 days, suggesting that this is a robust large animal model ideal for investigating mechanisms and treatment of lung rejection. Perioperative low-dose CO inhalation prolonged graft survival and inhibited antidonor antibody production and was associated with decreased proinflammatory mediators in this model.

Intracellular MHC class II controls regulatory tolerance to allogeneic transplants

MHC class II (MHCII) genes have been implicated in the regulation of T lymphocyte responses. However, the mechanism of MHCII-driven regulation remains unknown. Matching for MHCII between donors and recipients of allografts favors regulatory T cell tolerance to transplants and provides a unique opportunity to study this regulation. In this study, we investigated MHCII regulation using transfer of donor MHCII genes in recipients of cardiac allografts. Transfer of MHCII IA(b) genes in the bone marrow of CBA mice (H-2(k)) prior to the grafting of IA(b+) fully allogeneic C57BL/6 (B6, H-2(b)) heart transplants resulted in donor-specific tolerance associated with long-term survival of B6, but not third-party, allografts without sustained immunosuppression. Strikingly, the majority of accepted heart transplants (>170 d) were devoid of allograft vasculopathy. Further studies indicated that intracellular IA(b) initiated the tolerogenic process, which was mediated by regulatory T cells (Tregs) that polarized antigraft responses to Th2 cytokine producers. This mechanism seems to be unique to MHCII genes, because previous MHC class I gene-based therapies failed to produce Tregs. These results demonstrate the key role of MHCII in the induction of Tregs. They also underscore a potential mechanism of specific inactivation of T cells in this model; when activated by IA(b+) grafts, IA(b)-specific Tregs repress the entire alloresponse to C57BL/6 transplants (including MHC I and minor Ags), thus mediating T cell tolerance.

Cardiac allograft vasculopathy: real or a normal morphologic variant?

BACKGROUND

Naive coronary vessels may appear to have intimal thickening histologically characteristic of cardiac allograft vasculopathy (CAV). This study appraises the experimental and clinical impact of this observation.

METHODS

Tissue sections from 12 naive hearts of miniature swine, 13 native porcine hearts of recipients of heterotopic cardiac allografts, 3 native human hearts and 3 human hearts with CAV were compared with light microscopy and morphometric analysis. Results were also compared with morphometric data previously gathered from 3 grafts in a standard experimental model of CAV (rejectors) and 3 grafts harvested from swine rendered tolerant to their donor hearts (chimeras).

RESULTS

In the naive and native porcine hearts, the prevalence of CAV "mimics" was 0% to 6.94% (mean +/- SD: 1.99 +/- 1.97%) and 0% to 7.57% (2.97 +/- 2.20%), respectively (p = 0.12). The prevalence of CAV in the grafts of porcine rejectors and chimeras was 9.9% to 14.8% (12.4 +/- 2.5%) and 0.6% to 4.5% (2.6 +/- 2.0%), respectively (p < 0.05). CAV in the chimeras was similar in prevalence to that of the naive and native hearts. In native human hearts and human grafts, the prevalence was 1.86% to 2.00% (1.95 +/- 0.08%) and 9.09% to 17.50% (12.80 +/- 4.29%), respectively (p = 0.01).

CONCLUSIONS

Smooth muscle bundles inside the internal elastic laminae are similarly prevalent in human and porcine coronary vasculature. Their histologic similarity to intimal thickening of CAV could lead to an inaccurate distinction between graft tolerance and CAV in both clinical and experimental studies of heart transplantation.

Effects of Mycophenolate Mofetil on Cardiac Allograft Survival and Cardiac Allograft Vasculopathy in Miniature Swine

BACKGROUND

Chronic rejection, as manifested by cardiac allograft vasculopathy, remains the leading cause of late graft failure in heart transplant recipients. Despite recent clinical trials, the efficacy of mycophenolate mofetil in preventing human cardiac allograft vasculopathy remains controversial. We investigated whether mycophenolate mofetil would prevent cardiac allograft vasculopathy and prolong cardiac allograft survival in our well-established miniature swine model of heart transplantation.

METHODS

Hearts disparate at the major histocompatibility complex class I locus were heterotopically transplanted into miniature swine recipients treated with a 12-day course of mycophenolate mofetil (n = 3) or cyclosporine A (n = 3). Allograft survival, acute rejection, and chronic rejection were monitored in the two groups.

RESULTS

Hearts transplanted with 12 days of cyclosporine were rejected between 46 and 61 days, whereas two of the three hearts transplanted with mycophenolate mofetil remained beating beyond 120 days (p = 0.02). At necropsy, there was a 4.9% mean prevalence of cardiac allograft vasculopathy in the mycophenolate mofetil group as compared with 16.6% in the cyclosporine group (p = 0.03). Cardiac allograft rejection and vasculopathy in the cyclosporine-treated group was associated with prominent myocardial interferon-gamma gene expression, a finding absent in two thirds of the mycophenolate mofetil-treated swine. Moreover, the mycophenolate mofetil-treated swine failed to develop IgM or IgG alloantibodies.

CONCLUSIONS

A short course of mycophenolate mofetil resulted in a longer allograft survival than a similar course of cyclosporine. Moreover, mycophenolate mofetil reduced the prevalence of cardiac allograft vasculopathy as compared with cyclosporine-treated controls. The salutary effect of mycophenolate mofetil may be related to its ability to decrease interferon-gamma expression in the myocardium and prevent the generation of alloantibodies.