James F. George

Report from a consensus conference on the sensitized patient awaiting heart transplantation.

A consensus conference took place on April 8, 2008 to assess the current status of sensitization in the pre–heart transplant patient, the use and efficacy of desensitization therapies, and the outcome of desensitized patients after heart transplantation. The conference had 71 participants (transplant cardiologists, surgeons, immunologists and pathologists; see Appendix) representing 51 heart transplant centers from North America, Europe, Asia and Australia. Prior to the conference, survey data (regarding the sensitized patient) were submitted by 23 of the 51 centers participating in the conference (Table 1).

Suppression by CD4+CD25+ regulatory T cells is dependent on expression of heme oxygenase-1 in antigen-presenting cells.

Heme oxygenase-1 (HO-1) has been viewed as a cytoprotective protein, ameliorating the effects of inflammatory cellular damage, and as beneficial in allograft protection from acute and chronic rejection, suggesting important functions in both innate and adaptive immune responses. Mice deficient in HO-1 exhibit defective immune regulation characterized by a proinflammatory phenotype. We examined if impaired regulatory T cell (Treg) function contributes to the immunoregulatory defects observed in HO-1(-/-) mice. HO-1(-/-) mice exhibited a significantly higher proportion of Foxp3-expressing cells among total CD4(+) and CD4(+)CD25(+) cells in comparison to HO-1(+/+) mice, and HO-1(-/-) Treg cells were at least as effective as HO-1(+/+) Treg cells in suppressing proliferation of effector T cells in vitro from either HO-1(+/+) or HO-1(-/-) mice. However, the absence of HO-1 in antigen-presenting cells abolished the suppressive activity of Treg cells on effector T cells. These findings demonstrate that HO-1 activity in antigen-presenting cells is important for Treg-mediated suppression, providing an explanation for the apparent defect in immune regulation in HO-1(-/-) mice.

Allosensitization and outcomes in pediatric heart transplantation

BACKGROUND Allosensitization among children being considered for heart transplantation remains a great challenge. Controversy exists as to the best approach for those with elevated panel-reactive antibody (PRA) titers. We sought to define the association between elevated PRA and outcomes using data from the multi-institutional Pediatric Heart Transplant Study Group. METHODS Between January 1993 and December 2008, 3,016 patients (>1 month of age) were listed for heart transplantation. PRA data at listing were available for 2,500 (83%) patients, and 2,237 underwent transplantation with PRA data being available for 1,904 (85%). Because various PRA assays were employed (e.g., cell-based and solid phase) we entered the highest value regardless of methodology. RESULTS Among the factors associated with high PRA at transplant were Status 1 at listing, previous sternotomy and prior Norwood procedure. An elevated PRA at listing was associated with higher risk of death while waiting. Of subjects with PRA ≥ 50% only 57% were transplanted by 1 year on the waitlist, as compared with 76% of those with PRA <10%. Waitlist mortality for the highly allosensitized subjects (≥ PRA 50%) was 19% by 12 months. Survival at 1 year after transplantation was significantly lower in those with PRA ≥ 50% versus those with PRA <10% (73% vs 90%, respectively, p < 0.0001). Those with elevated PRA who had a negative prospective crossmatch had no difference in survival compared with those without allosensitization. There was no significant association between PRA levels and time to first rejection or development of coronary allograft vasculopathy. CONCLUSIONS Significant allosensitization is associated with more than a 2-fold increased risk of death within the first transplant year. Although prospective crossmatching abrogates the risk of post-transplant mortality, it may contribute to higher pre-transplant attrition due to longer waitlist times. There is a critical need for strategies to minimize the impact of allosensitization and antibody-mediated rejection immediately after transplantation.

Carbon monoxide rescues heme oxygenase-1-deficient mice from arterial thrombosis in allogeneic aortic transplantation.

Heme oxygenase-1 (HO-1) catalyzes the conversion of heme into carbon monoxide (CO), iron, and biliverdin. In preliminary studies, we observed that the absence of HO-1 in aortic allograft recipients resulted in 100% mortality within 4 days due to arterial thrombosis. In contrast, recipients normally expressing HO-1 showed 100% graft patency and survival for more than 56 days. Abdominal aortic transplants were performed using Balb/cJ mice as donors and either HO-1(+/+) or HO-1(-/-) (C57BL/6xFVB) mice as recipients. Light and electron microscopy revealed extensive platelet-rich thrombi along the entire length of the graft in HO-1(-/-) recipients at 24 hours. Treatment of recipients with CORM-2, a CO-releasing molecule (10 mg/kg of body weight intravenously), 1 hour prior and 1, 3, and 6 days after transplantation, significantly improved survival (62% at >56 days, P < 0.001) compared with HO-1(-/-) recipients treated with inactive CORM-2 (median survival 1 day). Histological analyses revealed that CO treatment markedly reduced platelet aggregation within the graft. Adoptive transfer of wild-type platelets to HO-1(-/-) recipients also conferred protection and increased survival. Aortic transplants from either HO-1(-/-) or HO-1(+/+) C57BL/6 donors into HO-1(+/+) (Balb/cJ) mice did not develop arterial thrombosis, surviving more than 56 days. These studies demonstrate an important role for systemic HO-1/CO for protection against vascular arterial thrombosis in murine aortic allotransplantation.

Carbon monoxide releasing molecule-2 increases the velocity of thrombus growth and strength in human plasma

Carbon monoxide derived from degradation of heme by heme oxygenase or carbon monoxide releasing molecules (CORMs) has been demonstrated to decrease thrombosis in vivo and to weakly inhibit platelet aggregation. We tested the hypothesis that carbon monoxide released from tricarbonyldichlororuthenium (II) dimer (CORM-2) would diminish the velocity of formation and strength of plasma thrombi as determined by thrombelastography. Normal plasma was exposed to 0 or 100 μmol/l CORM-2 or inactivated CORM-2 (iCORM-2), with coagulation initiated with tissue factor or celite (n = 8 per condition). Additional experiments utilized factor XIII (FXIII) deficient plasma activated with celite. Coagulation kinetics was monitored with thrombelastography for 15 min. CORM-2, and to a lesser extent, iCORM-2, significantly (P < 0.05) increased the velocity of formation (122 and 56%, respectively) and strength (66 and 57%, respectively) of plasma clots initiated with either tissue factor or celite compared with thrombi not exposed to CORM-2 or iCORM. In FXIII deficient plasma CORM-2 significantly increased the velocity of clot formation (264%) and strength (240%). Carbon monoxide and iCORM-2 derived from CORM-2 markedly enhance the velocity of clot growth and strength. These findings serve as the rationale for further investigations to determine if CORMs could be utilized as hemostatic agents.

Role for CD4(+)CD25(+) T cells in inhibition of graft rejection by extracorporeal photopheresis.

BACKGROUND Extracorporeal photopheresis (ECP) is used to treat recurrent severe rejection in clinical heart and lung recipients. The mechanisms of the salutary effects of ECP are poorly understood, but appear to involve regulation of T-cell-mediated alloreactive responses, possibly by induction of regulatory T cells. We created a mouse model of ECP to determine the effects of ECP on T-cell responses in vivo and the contribution of CD4(+)CD25(+) T cells. METHODS In this study, 1 x 10(7) splenocytes were treated with 8-methoxypsoralen (8-MOP, 200 ng/ml), followed by ultraviolet A (UVA) irradiation (2 J/cm(2), 350 nm), and then injected intravenously into syngeneic mice. Thirty minutes later, the treated animals received heterotopic cardiac allografts with no immunosuppression. Treated graft recipients were analyzed to determine the effect of ECP on graft survival, deletion of allospecific T cells, and the frequency and in vivo suppressive activity of CD4(+)CD25(+) T cells. RESULTS ECP extends cardiac allograft survival in at least two different strain combinations. For CBA/Ca recipients of C57BL/6 allografts, median survival time (MST) in ECP-treated animals was 16 days vs 10 days in graft recipients treated with cells exposed only to 8-MOP (p = 0.04). The frequency of splenic CD4(+)CD25(+) cells expressing FoxP3(+) increased 2-fold in ECP-treated CBA/Ca mice (82.6 +/- 5.2%, n = 4) relative to untreated mice (44.9 +/- 4.5%, n = 4, p < 0.001). Adoptive transfer of 3 x 10(5) sorted CD4(+)CD25(+) splenocytes from ECP-treated graft recipients to untreated cardiac allograft recipients 30 minutes after transplantation resulted in extended graft survival compared with animals that received the same number of CD4(+)CD25(+) splenocytes from cardiac allograft recipients not treated with ECP (MST: 24 days vs 13 days, respectively, p = 0.001). Analyses of 5,6-carboxy-fluorescein-succinimidyl-ester (CFSE)-labeled H-2K(b)-specific T cells in the spleen and lymph node showed no evidence of peripheral deletion after ECP treatment. CONCLUSIONS ECP extends graft survival even in fully histoincompatible strain combinations with no immunosuppression. It increases the frequency of FoxP3(+)CD4(+)CD25(+) splenic T cells, and its effects can be transferred to untreated recipients using minimal numbers of CD4(+)CD25(+) T cells, indicating that CD4(+)CD25(+) T cells may play a key role in the immunomodulatory effects of ECP.

Gene Polymorphisms for Plasminogen Activator Inhibitor-1/Tissue Plasminogen Activator and Development of Allograft Coronary Artery Disease

BACKGROUND Impaired fibrinolytic activity has been linked to the presence and severity of allograft vasculopathy (Tx CAD). This impairment may be associated with the presence of certain fibrinolytic protein gene polymorphisms. METHODS AND RESULTS To investigate the relation between donor-specific fibrinolytic protein genotypes and Tx CAD, we identified donor plasminogen activator inhibitor-1 (PAI-1) HindIII and tissue plasminogen activator (TPA) EcoRI restriction fragment length polymorphisms-based genotypes by Southern blot analysis in 48 recipients of cardiac allografts and correlated these genotypes with the development of CAD. No association was found between donor TPA genotypes and the presence of Tx CAD. Among the 48 patients, 17% were homozygous for the 1/1 PAI-1 genotype, 51% for the 2/2 PAI-1 genotype, and 32% for the 1/2 PAI-1 genotype. The actuarial freedom from any CAD for the recipients with each respective donor PAI-1 genotype at 12 and 24 months was 100% and 100% for the 1/1 PAI-1 genotype, 92% and 92% for the 1/2 PAI-1 genotype, and 75% and 45% for the 2/2 PAI-1 genotype (P=0.03). Recipients with a diseased 2/2 PAI-1 genotyped allograft had longer ischemic times (P=0.02) than those recipients with a Tx CAD-free allograft. CONCLUSIONS These data suggest that recipients with a 2/2 PAI-1 genotype are at a significant risk of developing Tx CAD. This genotype may serve as a useful screening tool for predicting the future development of Tx CAD.

A 16-Year Multi-Institutional Study of the Role of Age and EBV Status on PTLD Incidence Among Pediatric Heart Transplant Recipients

The objective was to determine the incidence and hazard for posttransplant lymphoproliferative disease (PTLD) in a study of 3170 pediatric primary heart transplants between 1993 and 2009 at 35 institutions in the Pediatric Heart Transplant Study. 147 of 151 reported malignancy events were classified as PTLD. Overall freedom from PTLD was 98.5% at 1 year, 94% at 5 years and 90% at 10 years. Freedom from PTLD was lowest in children (ages 1 to < 10 years) versus infants (<1 year) and adolescents (10 to < 18 years) with children at highest risk for PTLD with a relative risk of 2.4 compared to infants and 1.7 compared to adolescents. Positive donor EBV status was a strong risk factor for PTLD in the seronegative recipient, but risk magnitude was dependent on recipient age at the time of transplantation. Nearly 25% of EBV seronegative recipients of EBV+ donors at ages 4–7 at transplantation developed some form of PTLD. The overall risk for PTLD declined in the most recent transplant era (2001–2009, p = 0.003). These findings indicate that EBV status and the age of the recipient at the time of transplantation are important variables in the development of PTLD in the pediatric heart transplant recipient.

Carbon monoxide-releasing molecule-2 decreases fibrinolysis in human plasma.

Carbon monoxide, derived from carbon monoxide-releasing molecules, has been recently demonstrated to enhance the velocity of formation and strength of plasma thrombi. We tested the hypothesis that carbon monoxide-releasing molecule-2 would modulate fibrinolysis of plasma thrombi. Normal plasma was exposed to 0, 25, 50, 100 or 200 μmol/l carbon monoxide-releasing molecule-2, with coagulation activated with tissue factor and fibrinolysis initiated with tissue-type plasminogen activator. Additional experiments utilized factor XIII, plasminogen activator inhibitor-1, thrombin activatable fibrinolysis inhibitor or α2-antiplasmin-deficient plasmas. Thrombus growth/disintegration kinetics was monitored with thrombelastography. Carbon monoxide-releasing molecule-2, in a concentration-dependent fashion, increased the velocity of thrombus formation and strength, and markedly attenuated fibrinolysis in normal plasma. In factor XIII-deficient plasma, carbon monoxide-releasing molecule-2 mediated effects on thrombus growth/disintegration kinetics were similar to that seen with normal plasma; however, carbon monoxide-releasing molecule-2 had a less marked effect on thrombus growth/disintegration in both plasminogen activator inhibitor-1 and thrombin activatable fibrinolysis inhibitor-deficient plasma, with even less carbon monoxide-releasing molecule-2-mediated effects noted in α2-antiplasmin-deficient plasma. Carbon monoxide-releasing molecule-2 attenuated fibrinolysis by enhancing the velocity of clot growth and strength while augmenting the effects of plasminogen activator inhibitor-1, thrombin activatable fibrinolysis inhibitor and α2-antiplasmin. These findings serve as the rationale for further investigations to determine if carbon monoxide-releasing molecules could be utilized as hemostatic agents.

Immunoregulatory role of CD8alpha in the veto effect.

BACKGROUND Allogeneic bone marrow cell (allo-BMC) infusion induces tolerance to incompatible renal allografts in rhesus macaques after depletion of peripheral T lymphocytes with cytolytic anti-T cell antibodies. The tolerogenic effect of allo-BMC, ascribed to a veto mechanism, associates with specific functional deletion of antidonor cytotoxic T lymphocyte precursor (CTLp), and is dependent on a CD8+ donor BMC subset. In previous studies, the CD8 molecule was implicated by loss of suppression after blocking interaction between CD8 on allo-BMC and major histocompatibility complex class Ialpha3 domain on CTLp. CD8 cross-linking on BMC induced secretion of active transforming growth factor-beta1 (TGF-beta1), suggesting a regulatory mechanism(s) operating via a CD8-mediated signaling pathway. METHODS CD8 on rhesus cells was cross-linked using IgG-conjugated beads, and TGF-beta1 mRNA and protein were quantified. CD8+ cells were tested for veto activity by mixed lymphocyte reaction (MLR)-induced cell-mediated lymphocytotoxicity (CML) assay. Activated rhesus T cells exposed to TGF-beta1 were examined for apoptosis by TdT-mediated end-labeling and annexin staining. RESULTS CD8 cross-linking induces accumulation of TGF-beta1 mRNA and protein. Both CD3- CD8+CD16+ and CD3+ CD8+CD16- subsets of allo-BMC up-regulate TGF-beta1 mRNA after CD8 cross-linking, and exhibit veto activity. The CD3-CD8+CD16+ subset expresses more TGF-beta1 mRNA and increased veto activity at low BMC/CTLp ratios. Exposure of activated T cells to TGF-beta1 induces apoptosis. CONCLUSIONS CD8+ allo-BMC are enriched for veto activity and activation via CD8 induces TGF-beta1 mRNA and protein accumulation. These results agree with the hypothesis that paracrine TGF-beta1 may be involved in peripheral deletion of alloreactive CTLp by CD8+ allo-BMC. We suggest that TGF-beta1 overexpression by donor lymphohematopoietic cells may enhance tolerance induction.