Pathologic markers of allograft arteriopathy: insight into the pathophysiology of cardiac allograft chronic rejection

Antibody-mediated rejection in the cardiac allograft: diagnosis, treatment and future considerations

PURPOSE OF REVIEW

This review summarizes the latest publications dealing with antibody-mediated rejection (AMR) and defines areas of controversy and future steps that may improve the outcome for patients with this virulent form of rejection.

RECENT FINDINGS

Recent progress includes publication of standardized pathologic criteria for acute AMR by the International Society for Heart and Lung Transplantation (ISHLT) and guidelines for treatment of acute AMR by the American Heart Association, endorsed by ISHLT as well. Recently published review articles emphasize the important role of innate immune mechanisms, clarify the role of viral infection and provide insights into vascular biology and the role of innate effector populations, macrophages and dendritic cells.

SUMMARY

Strategies for future studies are discussed in the context of these new findings and similar efforts undertaken by renal and liver allograft investigators.

Complement-mediated microvascular injury leads to chronic rejection

Microvascular loss may be an unappreciated root cause of chronic rejection for all solid organ transplants. As the only solid organ transplant that does not undergo primary systemic arterial revascularization at the time of surgery, lung transplants rely on the establishment of a microcirculation and are especially vulnerable to the effects of microvascular loss. Microangiopathy, with its attendant ischemia, can lead to tissue infarction and airway fibrosis. Maintaining healthy vasculature in lung allografts may be critical for preventing terminal airway fibrosis, also known as the bronchiolitis obliterans syndrome (BOS). BOS is the major obstacle to lung transplant success and affects up to 60% of patients surviving 5 years. The role of complement in causing acute microvascular loss and ischemia during rejection has recently been examined using the mouse orthotopic tracheal transplantation; this is an ideal model for parsing the role of airway vasculature in rejection. Prior to the development of airway fibrosis in rejecting tracheal allografts, C3 deposits on the vascular endothelium just as tissue hypoxia is first detected. With the eventual destruction of vessels, microvascular blood flow to the graft stops altogether for several days. Complement deficiency and complement inhibition lead to markedly improved tissue oxygenation in transplants, diminished airway remodeling, and accelerated vascular repair. CD4+ T cells and antibody-dependent complement activity independently mediate vascular destruction and sustained tissue ischemia during acute rejection. Consequently, interceding against complement-mediated microvascular injury with adjunctive therapy during acute rejection episodes, in addition to standard immunosuppression which targets CD4+ T cells, may help prevent the subsequent development of chronic rejection.

CD4+ T cells and complement independently mediate graft ischemia in the rejection of mouse orthotopic tracheal transplants

RATIONALE

While microvascular injury is associated with chronic rejection, the cause of tissue ischemia during alloimmune injury is not yet elucidated.

OBJECTIVE

We investigated the contribution of T lymphocytes and complement to microvascular injury-associated ischemia during acute rejection of mouse tracheal transplants.

METHODS AND RESULTS

Using novel techniques to assess microvascular integrity and function, we evaluated how lymphocyte subsets and complement specifically affect microvascular perfusion and tissue oxygenation in MHC-mismatched transplants. To characterize T cell effects on microvessel loss and recovery, we transplanted functional airway grafts in the presence and absence of CD4(+) and CD8(+) T cells. To establish the contribution of complement-mediated injury to the allograft microcirculation, we transplanted C3-deficient and C3-inhibited recipients. We demonstrated that CD4(+) T cells and complement are independently sufficient to cause graft ischemia. CD8(+) T cells were required for airway neovascularization to occur following CD4-mediated rejection. Activation of antibody-dependent complement pathways mediated tissue ischemia even in the absence of cellular rejection. Complement inhibition by CR2-Crry attenuated graft hypoxia, complement/antibody deposition on vascular endothelium and promoted vascular perfusion by enhanced angiogenesis. Finally, there was a clear relationship between the burden of tissue hypoxia (ischemia×time duration) and the development of subsequent airway remodeling.

CONCLUSIONS

These studies demonstrated that CD4(+) T cells and complement operate independently to cause transplant ischemia during acute rejection and that sustained ischemia is a precursor to chronic rejection.

Lung transplant airway hypoxia: a diathesis to fibrosis?

RATIONALE

Chronic rejection, manifested pathologically as airway fibrosis, is the major problem limiting long-term survival in lung transplant recipients. Airway hypoxia and ischemia, resulting from a failure to restore the bronchial artery (BA) circulation at the time of transplantation, may predispose patients to chronic rejection. To address this possibility, clinical information is needed describing the status of lung perfusion and airway oxygenation after transplantation.

OBJECTIVES

To determine the relative pulmonary arterial blood flow, airway tissue oxygenation and BA anatomy in the transplanted lung was compared with the contralateral native lung in lung allograft recipients.

METHODS

Routine perfusion scans were evaluated at 3 and 12 months after transplantation in 15 single transplant recipients. Next, airway tissue oximetry was performed in 12 patients during surveillance bronchoscopies in the first year after transplant and in 4 control subjects. Finally, computed tomography (CT)-angiography studies on 11 recipients were reconstructed to evaluate the post-transplant anatomy of the BAs.

MEASUREMENTS AND MAIN RESULTS

By 3 months after transplantation, deoxygenated pulmonary arterial blood is shunted away from the native lung to the transplanted lung. In the first year, healthy lung transplant recipients exhibit significant airway hypoxia distal to the graft anastomosis. CT-angiography studies demonstrate that BAs are abbreviated, generally stopping at or before the anastomosis, in transplant airways.

CONCLUSIONS

Despite pulmonary artery blood being shunted to transplanted lungs after transplantation, grafts are hypoxic compared with both native (diseased) and control airways. Airway hypoxia may be due to the lack of radiologically demonstrable BAs after lung transplantation.

Influence of mast cells on outcome after heterotopic cardiac transplantation in rats

Correlative data suggest that mast cells adversely affect cardiac transplantation. This study uses a mast cell-deficient rat model to directly address the role of mast cells in cardiac allotransplantation. Standardized cardiac heterotopic transplantation with cyclosporine immunosuppression was performed in mast cell-deficient and mast cell-competent rats. Rejection, ischemia, fibrosis, fibrin deposition, numbers of T-cell receptor alpha/beta positive cells, expression of transforming growth factor-beta (TGF-beta), and of endothelin-1 (ET-1) and its receptors ETA and ETB were assessed. Differences in baseline cardiac gene expression were quantified by real-time PCR in a separate group of untransplanted animals. Baseline cardiac gene expression levels of all investigated growth factors, cytokines, ET-1, ETA, and ETB were similar in mast cell-deficient and mast cell-competent rats. Surprisingly, upon heterotopic transplantation, donor heart survival was significantly reduced in mast cell-deficient rats. Moreover, in mast cell-deficient donor hearts rejection was more severe, although nonsignificant, and extracellular matrix associated TGF-beta immunoreactivity was significantly lower than in mast cell-competent donor hearts. Fibrin immunoreactive area, on the other hand, was only increased in mast cell-deficient donor hearts, but not in mast cell-competent donor hearts. Histopathological changes in all donor hearts were accompanied by increased immunoreactivity for ET-1. In conclusion, this study shows that mast cells play a protective role after cardiac transplantation.

Treatment of allograft vasculopathy in heart transplantation

Cardiac allograft vasculopathy remains one of the main causes of morbidity and mortality after heart transplantation, although its impact is becoming somewhat smaller as prophylactic measures are implemented. Advances in the understanding of the molecular and cellular mechanisms involved in the genesis and development of cardiac allograft vasculopathy are opening ways for new diagnostic and therapeutic strategies. Successful prophylaxis of the early stages of the disease has been demonstrated with the use of newer immunosuppressive agents, such as sirolimus and everolimus, that will probably be included in future protocols. For most patients with established cardiac allograft vasculopathy, currently available revascularisation methods and retransplantation are not appropriate options. Antiproliferative agents could provide significant improvement in terms of symptom relief and prognosis, but their definite value must be proven in well-designed trials.

Value of soluble adhesion molecules and plasma coagulation markers in assessing transplant coronary artery disease in pediatric heart transplant recipients

BACKGROUND

With an increasing number of heart transplantations (HTx) performed in children and an extended long-term survival of these patients, the importance of transplant coronary artery disease (TCAD) rises in this group of transplant recipients. Reliable serum markers for diagnosis or non-invasive monitoring of this disease in pediatric transplant recipients are still missing. We studied the systemic expression of adhesion molecules as well as plasma coagulation markers and the occurrence of TCAD and/or rejection in pediatric heart transplant recipients.

METHODS AND RESULTS

The systemic plasma levels of soluble forms of sVCAM-1 and sICAM-1, d-dimer, tissue factor (TF), prothombin fragments F(1+2), and tissue factor pathway inhibitor (TFPI) were assessed in serial venous blood samples (2-4 per patient) in 50 pediatric transplant recipients children and 63 age- and sex-matched non-transplanted controls. TCAD and rejection were diagnosed angiographically or by combined histological, echocardiographic, or clinical signs, respectively. Plasma levels of sICAM-1 and sVCAM-1, d-dimers and prothrombin fragment F(1+2) but not TF and TFPI were significantly increased in children following HTx compared with non-transplanted controls (p<0.001). Among the transplanted patients, sICAM-1 levels were significantly higher in patients with angiographically detectable TCAD than in patients without evidence of TCAD (p<0.005). Plasma sICAM-1 levels above a cutoff value of 1500 ng/mL (95.5 percentile of control values) were indicative of the presence of TCAD (odds ratio 2.7; 95% confidence interval, 1.34-5.56, p = 0.022; Fisher's exact test). Only d-dimers were found to be significantly elevated in children with signs of myocardial rejection compared with those without rejection.

CONCLUSIONS

Our results suggest that plasma sICAM-1 and d-dimer levels may be potentially useful to non-invasively assess TCAD and rejection, respectively, in pediatric heart transplant recipients.

Cardiac allograft vasculopathy: The achilles’ heel of long-term survival after cardiac transplantation

Over the past 40 years, cardiac transplantation has evolved as the single best long-term option for eligible candidates with end-stage heart failure. Approximately 2000 transplants are performed annually in the United States, and with the institution of calcineurin-based immunotherapy, surveillance biopsies, and programmatic-based patient care, life expectancy at 1 and 12 years is 85% and 50%, respectively. Cardiac allograft vasculopathy (CAV) is the number one cause of death after the first year of transplantation. The incidence of CAV remains as high as 50% at 5 years, with life expectancy significantly abbreviated once it is recognized. Although current immunotherapy has reduced the likelihood of cellular rejection, it has not impacted CAV substantially. Better treatment of established risk factors and the advent of newer antiproliferative immunotherapy may hold promise in treating CAV. However, future therapies must address the multitude of mechanisms underlying CAV. This manuscript reviews the pathophysiology, clinical manifestations, screening, and diagnostic strategies for cardiac allograft vasculopathy while emphasizing current treatment paradigms designed to stave off or retard the progression of CAV.

Syncytiotrophoblast intercellular adhesion molecule-1 expression in placental villitis of unknown cause

OBJECTIVE

The purpose of this study was to determine syncytiotrophoblast intercellular adhesion molecule-1 expression in villitis and in normal chorionic villi from term (37-42 weeks of gestation) placentas with or without villitis.

STUDY DESIGN

A cross-sectional study was conducted to determine syncytiotrophoblast intercellular adhesion molecule-1 expression in villitis (n = 16) and in normal villi from placentas with or without villitis (n = 16). Villitis was diagnosed with antibodies to human leukocyte antigen-DR and CD3 and hematoxylin and eosin staining of serial sections; intercellular adhesion molecule-1 reactivity in syncytiotrophoblast was confirmed with antibodies to intercellular adhesion molecule-1 and cytokeratin.

RESULTS

Villitis lesions had higher syncytiotrophoblast intercellular adhesion molecule-1 expression than normal chorionic villi from placentas with (19.9% vs 3.5% villi; P < .001) or without (19.9% vs 0.31% villi; P < .001) villitis. Normal villi from placentas with villitis had higher syncytiotrophoblast intercellular adhesion molecule-1 than villi from placentas without villitis (3.5% vs 0.31% villi; P < .001).

CONCLUSION

Placentas with villitis have significantly more syncytiotrophoblast intercellular adhesion molecule-1 expression than placentas without villitis. The finding that normal villi from placentas with villitis have more syncytiotrophoblast intercellular adhesion molecule-1 than normal villi from placentas without villitis suggests that syncytiotrophoblast intercellular adhesion molecule-1 could be the first step in villitis development.

Small vessel disease after heart transplantation: impact of immunologic and nonimmunologic risk factors*

To determine risk factors for small vessel disease after heart transplantation (HTx), characteristics of donors and organ harvesting were evaluated in 246 HTx patients (205 male, 41 female, mean survival 5.4 years). In right ventricular endomyocardial biopsies (EMB, n = 5421) evidence of microvascular disease [endothelial cell (EC) swelling/vessel wall thickening] was evaluated by light microscopy (hematoxylin and eosin staining, x 200). Mild EC swelling/vessel wall thickening were found in 204 and 213 patients, severe EC swelling/vessel wall thickening were present in 23 and 142 patients respectively. Evidences of mild and severe acute cellular rejection were found in 2064 and 421 EMB respectively. Microvascular disease was positively correlated with mild acute rejection episodes (P < 0.05). EC swelling was more frequent in patients with donors dying of craniocerebral trauma. No correlations were present to further demographical data. Microvascular alterations after HTx seem to be the result of an immunologic conflict rather than to depend on nonimmunologic factors.

Induction therapy after cardiac transplantation: A comparison of anti-thymocyte globulin and daclizumab in the prevention of acute rejection

BACKGROUND

Induction therapy with antibodies decreases and delays early allograft rejection. We compared the safety and efficacy of daclizumab and anti-thymocyte globulin (ATG) with respect to the frequency and severity of acute cardiac allograft rejection in heart transplant recipients.

METHODS

Forty sequential adult patients were retrospectively studied. In the first 20 patients ATG (2.5 mg/kg daily for 3 to 5 days peri-/and post-operatively) was used as induction therapy and, in the remaining 20 patients, daclizumab (1 mg/kg peri-operatively and every 2 weeks thereafter for a total of 5 doses) was used. A standard triple-drug immunosuppression regimen was administered to all patients.

RESULTS

Baseline characteristics and trough levels of cyclosporine in the 2 groups were similar. During the induction period, defined as the first 3 months, 12 acute rejection episodes requiring treatment (ISHLT Grade > or =2) occurred in the ATG group and 9 in the daclizumab group (p > 0.05). However, the number of biopsies with Grade 1 rejection was increased >2-fold in the daclizumab group (n = 35) compared with the ATG group (n = 17; p = 0.04). The total number of biopsies performed within the first 3 months increased by 26% in the daclizumab group. The number and severity of rejection episodes after 3 months was similar in the 2 groups. The overall occurrence of bacterial infections was significantly higher in the ATG group than in the daclizumab group (p = 0.05).

CONCLUSIONS

ATG and daclizumab are equally effective in preventing acute rejections requiring treatment (ISHLT Grade > or =2). Due to the significantly greater frequency of Grade 1 rejections, daclizumab was found to be associated with an increased number of additional biopsies for monitoring rejection status. This implies additional costs to the transplant program, and the long-term implications of the increased number of low-grade rejection episodes remains to be determined.

FTY720: sphingosine 1-phosphate receptor-1 in the control of lymphocyte egress and endothelial barrier function

The novel immunomodulator FTY720 is effective in experimental models of transplantation and autoimmunity, and is currently undergoing Phase III clinical trials for prevention of kidney graft rejection. In contrast to conventional immunosuppressants, FTY720 does not impair T- and B-cell activation, proliferation and effector function, but interferes with cell traffic between lymphoid organs and blood. The molecular basis for the mode of action of the drug has only recently been established. FTY720, after phosphorylation, acts as a high-affinity agonist at the G protein-coupled sphingosine 1-phosphate receptor-1 (S1P(1)) on thymocytes and lymphocytes, thereby inducing aberrant internalization of the receptor. This renders the cells unresponsive to the serum lipid sphingosine 1-phosphate (S1P), depriving them from an obligatory signal to egress from lymphoid organs. As a consequence, lymphocytes are unable to recirculate to peripheral inflammatory tissues and graft sites but remain functional in the lymphoid compartment. In addition to the effects on lymphocyte recirculation, the drug acts on endothelial cells and preserves vascular integrity by enhancing adherens junction assembly and endothelial barrier function. The available data establish S1P(1) as a key target for FTY720, and further point to therapeutically relevant effects of the drug on lymphocytes and vascular endothelium.

Proteomics Strategies in Cardiovascular Research

Cardiovascular research of the past decades dealt with classical pathophysiological descriptions, then shifted toward the identification of relevant receptors, and then proceeded to the analysis of signal transduction pathways. Most recently, hand in hand with the achievements of the human genome project, the research has gone down the road toward molecular biological "disease gene(s) mapping". The application of proteome research will attempt to close the gap between genomic (and genetic) analysis and the physiological research. The rich source of heart surgery specimens represents an excellent starting point in data acquisition of proteomic context. Furthermore, animal models of cardiovascular diseases and deficiencies are considered, and will be explored. Examples of results from feasibility studies are given, with the emphasis on quantitative evaluation of proteomic components, hoping to discover co-regulated sets of proteins that are involved in any particular disease state. Identification of new, not yet discovered proteins will be pursued, though the emphasis of this work will be on the definition of characteristic sets of expressed proteins, which in turn might be able to delimit the state of disease and prognosis of therapy outcome. Besides the systematic issues, this paper refers to a number of methodological questions, like the comparison of the proteins detected by staining procedures and proteins detected in models in which biosynthetic labeling is applicable.

Coronary artery vasculopathy in pediatric cardiac transplant patients: the therapeutic potential of immunomodulators

The single largest cause of late graft loss in pediatric cardiac transplantation is transplant coronary artery vasculopathy (CAV). The mechanism of CAV remains unknown; it appears to have both immune and non-immune causes. The final common pathway of these mechanisms is endothelial activation, a prothrombotic environment, and endothelial damage with subsequent diffuse intimal proliferation. The disease process has largely been thought to be progressive and unresponsive to treatment. Re-transplantation has been advocated as the only definitive treatment. The appropriate management is largely unknown; intervention or surgical management has had limited utility, while medical management appears to have the most promise. Improvement in outcome can be achieved by optimizing non-immune factors and aggressive management of the immune mechanisms. Long-term survival of transplant patients after diagnosis with CAV is now being reported.

Association between Gly241Arg ICAM-1 gene polymorphism and serum sICAM-1 concentration in the Stanislas cohort

Intracellular adhesion molecule-1 (ICAM-1), a cellular adhesion molecule that mediates the interaction of activated endothelial cells with leukocytes, is involved in various inflammatory and cardiovascular disorders. The relation between these markers and genetic polymorphism, however, remains to be elucidated. The aim of this study is to estimate the effect of a single-base polymorphism at codon 241 in exon 4 of ICAM-1 gene on serum sICAM-1 concentration in a healthy population, taking into account other biological determinants of sICAM-1 level. Serum sICAM-1 levels and the G/R241 polymorphism of the ICAM-1 gene were measured in a large healthy population consisting of 413 children aged 6-21 years and 363 adults aged 38-55 years extracted from the Stanislas cohort. The R241 allele was significantly associated with lower sICAM-1 levels and explained 3.4 and 1.9% of the sICAM-1 variability in children and adults, respectively. A codominant pattern contributed better to the model after adjustment for covariates as the RR homozygote effect was higher than that of the GR heterozygote. Moreover, significant independent associations were found between sICAM-1 and smoking, insulin resistance index (HOMA IR), interleukin-6 level, and alkaline phosphatase and aspartate aminotransferase activities. In conclusion, this study revealed a significant association between the G/R241 ICAM-1 polymorphism and serum sICAM-1 levels, probably due to the impairment in binding of ICAM-1 to leukocyte integrin Mac-1 protein.

Soluble inflammatory markers in coronary sinus and peripheral blood of heart transplant recipients

Cardiac allograft rejection is a focal inflammation and soluble markers are released into coronary sinus (CS). We investigated whether plasma-soluble markers in CS is better to predict the clinical status of transplant recipients than in peripheral blood (PB). Between February 1998 and January 2001, 51 patients admitted for endomyocardial biopsy were included. The clinical events of the transplant recipient were recorded as: early post-transplant, long-term uneventful status, infection, acute rejection and transplant coronary artery disease. The plasma levels of interleukin-2 (IL-2), tumor necrosis factor-alpha (TNF-alpha), ICAM-1, P-selectin, high-sensitive C-reactive protein (CRP) and troponin-I of CS and PB were determined. There were 71 blood samples. In patients within 1 month after heart transplant, there was a higher level of P-selectin, ICAM-1, CRP and troponin-I in CS and PB. In patients with infection, there was a higher level of all soluble markers except IL-2 in CS and PB. Patients with a long-term uneventful status had a lower level of CRP in PB but not in CS. Patients with acute rejection had a higher level of IL-2 in PB but not in CS. Patients with transplant coronary artery disease had a higher level of TNF-alpha in PB but not in CS. Soluble markers in CS failed to predict the occurrence of acute or chronic rejections.

Pediatric heart transplantation

Heart transplantation is now a treatment option with good outcome for infants and children with end-stage heart failure or complex, inoperable congenital cardiac defects. One-year and 5-year actuarial survival rates are high, approximately 75% and 65%, respectively, with overall patient survival half-life greater than 10 years. To date, survival has been improving as a result of reducing early mortality. Further reductions in late mortality, in part because of graft coronary artery disease and rejection, will allow achievement of the goal of decades-long survival. Quality of life in surviving children, as judged by activity, is usually "normal." Somatic growth is usually at the low normal range but linear growth can be reduced. Of infant recipients, 85% evaluated at 6 years of age or older were in an age-appropriate grade level. Long-term management of childhood heart recipients requires the collaboration of transplant physicians, given the increasing number of immunosuppressive agents and the balance between rejection and infection. Currently, recipients are maintained on immunosuppressive medications that target calcineurin (eg, cyclosporine, tacrolimus), lymphocyte proliferation (eg, azathioprine, mycophenolate mofetil [MMF], sirolimus) and, in some instances antiinflammatory corticosteroids. Emerging evidence now suggests a favorable immunologic opportunity for transplantation in childhood and, conversely, a higher mortality rate in children who have had prior cardiac surgery. Further studies are needed to define age-dependent factors that are likely to play a role in graft survival and possible graft-specific tolerance (eg, optimal conditions for tolerance induction and how immunosuppressive regimens should be changed with maturation of the immune system). As late outcomes continue to improve, the need for donor organs likely will increase, as transplantation affords a better quality and duration of life for children with complex congenital heart disease, otherwise facing a future of multiple palliative operations and chronic heart failure.

Microvascular prothrombogenicity and transplant coronary artery disease

A major impediment for the long-term success of heart transplantation is the development of transplant coronary artery disease (CAD). Several risk factors for the development of transplant CAD are associated with the transformation of a normal thromboresistant microvasculature into a prothrombogenic microvasculature. Prothrombogenicity is characterized by loss of anticoagulation (i.e. loss of antithrombin), loss of fibrinolytic activity (i.e., loss of tissue plasminogen activator) and presence of endothelial activation (i.e. upregulation of endothelial intercellular adhesion molecule-1 and major histocompatibility class II antigen human leukocyte antigen-DR) in the arterial allograft microvasculature. Microvascular prothrombogenicity during the first trimester after transplantation is directly associated with subsequent development of transplant CAD. Although the mechanisms responsible for the loss of thromboresistant endothelium are unclear, the fact that changes in the anticoagulant, fibrinolytic, and activational status of endothelial cells may occur early after transplantation suggests a peritransplant phenomenon as an initiating event. Reducing prothrombogenicity of the cardiac microvasculature early after transplantation could slow the development of transplant CAD and significantly improve allograft survival.