Inducible nitric oxide-mediated myocardial apoptosis contributes to graft failure during acute cardiac allograft rejection in mice

Has the 2004 revision of the International Society of Heart and Lung Transplantation grading system improved the reproducibility of the diagnosis and grading of cardiac transplant rejection?

We compared the interobserver reproducibility of the 1990 and 2004 International Society for Heart and Lung Transplantation (ISHLT) grading system for cardiac rejection. The 2004 ISHLT grading system for cardiac allograft rejection did not improve reproducibility partly due to pathologists' disagreement in diagnosing Grades 1B/1R and 3A/2R rejection. To achieve better reproducibility, better criteria for defining 1B/1R vs. 3A/2R rejection and markers of myocyte injury are needed.

The complex role of iNOS in acutely rejecting cardiac transplants

This review summarizes the evidence for a detrimental role of nitric oxide (NO) derived from inducible NO synthase (iNOS) and/or reactive nitrogen species such as peroxynitrite in acutely rejecting cardiac transplants. In chronic cardiac transplant rejection, iNOS may have an opposing beneficial component. The purpose of this review is primarily to address issues related to acute rejection, which is a recognized risk factor for chronic rejection. The evidence for a detrimental role is based upon strategies involving nonselective NOS inhibitors, NO neutralizers, selective iNOS inhibitors, and iNOS gene deletion in rodent models of cardiac rejection. The review is presented in the context of the impact on various components, including graft survival, histological rejection, and cardiac function, which may contribute to the process of graft rejection in toto. Possible limitations of each strategy are discussed in order to understand better the variance in published findings, including issues related to the potential importance of cell localization of iNOS expression. Finally, the concept of a dual role for NO and its downstream product, peroxynitrite, in rejection vs immune regulation is discussed.

Immunosuppression and transplant vascular disease: benefits and adverse effects

Cardiac allograft vasculopathy (CAV) occurs within 5 years of transplantation surgery and represents the main cause of death in long-term heart transplant survivors. The detailed pathogenesis of CAV is unknown, but there are strong indications that immunologic mechanisms, which are regulated by nonimmunologic factors, are the major cause of this phenomenon. Cyclosporine A (CsA) is a frequently used immunosuppressive agent in transplant medicine to prevent rejection. The mechanism of action of CsA involves initial binding to cyclophilin to form a complex that then inhibits calcineurin (CN), leading to reduced interleukin (IL)-2 production as part of the signal transduction pathway for the activation of B-lymphocytes and T-lymphocytes. Based on this proposed mechanism, it was expected that CsA should be an effective strategy in attenuating the host immune response against transplanted allograft tissue; however, CsA has not changed the outcome of CAV. Several mechanisms have been suggested for the ineffectiveness of CsA in long-term prevention of CAV. For example, routine therapeutic doses of CsA may block CN incompletely (50%), whereas complete blockade requires doses that are not clinically tolerable. Another explanation is the possible activation of T-cell receptors directly (CN independent) by the immune response, which induces protein kinase C theta (PKCtheta) and leads to IL-2 production and immune rejection. Moreover, there may be a role for nonimmunologic mechanisms, such as complement, which cannot be controlled by CsA, or CsA may cause hypercholesterolemia or induce overexpression of transforming growth factor-beta (TGF-beta). This review also compares the effect of CsA with other immunosuppressants in allograft artery preservation and their clinical efficacy.

Variable efficacy ofN6-(1-iminoethyl)-L-lysine in acute cardiac transplant rejection

We examined the efficacy and mechanism of action of N6-(1-iminoethyl)-l-lysine (l-NIL), a highly selective inhibitor of inducible nitric oxide (NO) synthase (iNOS), on acute cardiac transplant rejection. l-NIL produced a concentration-dependent attenuation of plasma NO by-products and a decrease in nitrosylation of heme protein without altering protein levels of iNOS. At postoperative day 4, l-NIL did not alter the increased binding activities for transcription factors nuclear factor-κB and activator protein-1. Whereas l-NIL decreased inflammatory cell infiltration, graft survival was only prolonged at the dose of 1.0 μg/ml that incompletely blocked NO production. Higher l-NIL concentrations (30 and 60 μg/ml) ablated the increased NO production but failed to improve graft survival and even potentiated NF-κB binding activity examined at day 6. Alloimmune activation indicated by increased cytokine gene expression for interferon-γ, interleukin-6, and interleukin-10 was inhibited in grafts only by treatment with 1.0 μg/ml l-NIL. These findings suggest a complex role of NO in acute cardiac allograft rejection. Partial inhibition of iNOS is beneficial to graft survival, whereas total ablation may oppose any benefits to graft survival. These studies have important implications in understanding the dual role of NO in acute rejection and help to reconcile discrepancies in the literature.

Prolonging organ allograft survival: potential role of nitric oxide scavengers

A growing number of studies suggest a key role of nitric oxide (NO) derived from the inducible NO synthase (iNOS) isoform as a signalling molecule leading to acute organ transplant rejection. Current theory suggests that NO targets certain tissue proteins for nitrosylation or nitration leading to inhibition of enzyme/protein function and to cell death via apoptosis. Gene expression of iNOS and formation of nitrotyrosine residues have been confirmed in biopsies of rejecting grafts in humans. Experimental attempts to delay graft rejection by treatment with iNOS enzyme inhibitors have yielded conflicting results. An alternative strategy to alter rejection mediated by NO is to scavenge and/or neutralise the actions of excess NO, thereby by-passing the inhibition of iNOS enzyme activity. This review summarises recent laboratory evidence that new experimental NO scavengers/neutralisers have potential value to prolong graft survival. To date, various metal-based NO scavenging/neutralising compounds have been shown to enhance cardiac allograft survival in the absence of immunosuppression. When used in combination with low-dose cyclosporin, these agents produce a synergistic action to enhance graft survival or even to produce "permanent graft survival" under certain prolonged drug regimens. A portion of this benefit may be accounted for by the property of some of these compounds to display immunosuppressant and anti-inflammatory activity in vivo. These properties are based on findings including the following: (i) attenuating cell infiltration into the graft; (ii) attenuating activation of NFkappaB (a transcription factor important for upregulation of various inflammatory genes); (iii) attenuating cyclin D3 gene expression (a marker of cell proliferation; (iv) antagonising autoimmune activation (as determined by attenuated cytokine gene expression in splenocytes isolated from treated animals but stimulated for several days ex vivo in mixed lymphocyte cultures).