Progenitor cells and vascular disease.

Vascular progenitor cells have been the focus of much attention in recent years; both from the point of view of their pathophysiological roles and their potential as therapeutic agents. However, there is as yet no definitive description of either endothelial or vascular smooth muscle progenitor cells. Cells with the ability to differentiate into mature endothelial and vascular smooth muscle reportedly reside within a number of different tissues, including bone marrow, spleen, cardiac muscle, skeletal muscle and adipose tissue. Within these niches, vascular progenitor cells remain quiescent, until mobilized in response to injury or disease. Once mobilized, these progenitor cells enter the circulation and migrate to sites of damage, where they contribute to the remodelling process. It is generally perceived that endothelial progenitors are reparative, acting to restore vascular homeostasis, while smooth muscle progenitors contribute to pathological changes. Indeed, the number of circulating endothelial progenitor cells inversely correlates with exposure to cardiovascular risk factors and numbers of animal models and human studies have demonstrated therapeutic roles for endothelial progenitor cells, which can be enhanced by manipulating them to overexpress vasculo-protective genes. It remains to be determined whether smooth muscle progenitor cells, which are less well studied than their endothelial counterparts, can likewise be manipulated to achieve therapeutic benefit. This review outlines our current understanding of endothelial and smooth muscle progenitor cell biology, their roles in vascular disease and their potential as therapeutic agents.

Smooth muscle cell proliferation but not neointimal formation is dependent on alloantibody in a murine model of intimal hyperplasia.

Transplant coronary artery disease is the pre-eminent cause of late cardiac allograft failure. It is primarily characterized by a concentric intimal hyperplasia, which we designate transplant intimal hyperplasia (TIH). Although the pathogenesis of TIH is predominately immune driven, the specific role of alloantibodies in the disease process remains undefined. In this study we investigated the contribution of alloantibodies to the development of TIH in a murine model. Orthotopic, carotid artery transplantation was performed between B10A(2R) (H-2(h2)) donor mice and B-cell deficient muMT(-/-) knockout or wild-type C57BL/6 (H-2(b)) recipients in the absence of immunosuppression. Grafts were harvested at 35 days and subjected to planimetry and immunohistochemistry. Alloantibodies were detectable in wild-type recipients within 7 days of transplantation and recipients developed marked TIH at 35 days. Allografts harvested from B-cell deficient recipient mice also developed TIH, which was comparable in severity with wild-type recipients. However, whereas allografts from wild-type recipients showed marked intimal smooth muscle cell (SMC) proliferation, the neointima in B-cell deficient recipients lacked SMCs. Post-transplantation administration of anti-donor serum to muMT(-/-) recipients restored neointimal SMC population but did not influence the severity of TIH. Significant neointimal formation occurs in the absence of alloantibodies but lacks a SMC component. Therefore, SMC migration and proliferation is antibody dependent.

Role of alloantibodies in the pathogenesis of graft arteriosclerosis in cardiac transplantation.

Graft arteriosclerosis (GA) remains the leading obstacle to long-term survival of cardiac allografts. The pathogenesis of this chronic disease, though perceived to be multifactorial, is most likely immune-driven. Based on clinical and experimental observations, the humoral arm of the immune system has long been suspected to play a pivotal role in the disease process. In this article, we shall review the evidence generated from key clinical and experimental studies on the role of alloantibodies in GA. We will argue that although the strong correlation between the presence of anti-donor antibodies in clinical and experimental GA is highly suggestive of a pathogenic role for alloantibodies, a direct causal link between GA and the humoral arm of the alloresponse cannot yet be established based on the currently available evidence, and may in fact be one of a number of pathogenic processes that potentiate this vasculopathy. Finally, in this article, we shall discuss some of the potential mechanisms by which alloantibodies may exert their pathogenic effect in GA.

Coronary artery bypass grafting in non-dialysis-dependent mild-to-moderate renal dysfunction.

OBJECTIVES: The effect of mild-to-moderate elevation of preoperative serum creatinine levels on morbidity and mortality from coronary artery bypass grafting has not been investigated in a large multivariable model incorporating preoperative and intraoperative variables. Our first objective was to ascertain the effect of a mild-to-moderate elevation in the preoperative serum creatinine level on the need for mechanical renal support; the duration of special care and total postoperative stay; the occurrence of infective, respiratory, and neurologic complications; and hospital mortality. Our second objective was to ascertain which patient variables contributed to an increase in the serum creatinine level in association with coronary artery bypass grafting. METHODS: A total of 1427 patients who had no known pre-existing renal disease and who were undergoing first-time coronary artery bypass grafting with cardiopulmonary bypass were recruited for the study. Patients were divided, on the basis of preoperative serum creatinine level, into 3 groups as follows: creatinine level of less than 130 micromol. L(-1); creatinine level of 130 to 149 micromol. L(-1); and creatinine level of 150 micromol. L(-1) or greater. A multivariable stepwise logistic regression analysis was used, and variables significant at the 5% level were included when developing the final multivariable models. RESULTS: Multivariable analysis showed that elevation of the preoperative serum creatinine level to 130 micromol. L(-1) or greater increased the likelihood of needing mechanical renal support postoperatively (P <.001), as well as the need for postoperative special care (P <.001) and total hospital stay (P <.001). In-hospital mortality was also significantly elevated as the preoperative creatinine level rose to 130 to 149 micromol. L(-1) (P =.045) and to 150 micromol. L(-1) or greater (P <.001). It was further observed that patients with preoperative serum creatinine levels of 130 to 149 micromol. L(-1) (P =.02), patients with preoperative serum creatinine levels of 150 micromol. L(-1) or greater (P =.001), hypertensive patients (P =.007), patients with angina of New York Heart Association class III or greater (P =.001), patients having a nonelective operation (P =.002), and patients having a prolonged cardiopulmonary bypass time (P =.008) had a significantly greater increase in the serum creatinine level as a result of coronary artery bypass grafting. Of particular note was the finding that the method of myocardial protection (cardioplegia or crossclamp fibrillation) did not significantly influence in-hospital mortality, need for mechanical renal support, or special care or total postoperative hospital stay. CONCLUSIONS: A mild elevation (130-149 micromol. L(-1)) in the preoperative serum creatinine level significantly increases the need for mechanical renal support, the duration of special care and total postoperative stay, and the in-hospital mortality. As the preoperative serum creatinine level increases further (> or =150 micromol. L(-1)), this effect is more pronounced. No significant difference in outcome was observed between the use of cardioplegia or crossclamp fibrillation for myocardial protection.

Significant frequencies of T cells with indirect anti-donor specificity in heart graft recipients with chronic rejection.

BACKGROUND: The purpose of this study was to determine whether T cells with indirect allospecificity could be detected in heart transplant recipients with chronic rejection. METHOD AND RESULTS: Human T-cell clones were used to determine the most effective way to deliver major histocompatibility complex alloantigens for indirect presentation. Seven allograft recipients with evidence of progressive, chronic rejection were selected. Four heart graft recipients with no evidence of chronic rejection were used as controls. Peripheral blood T cells and antigen-presenting cells from the recipients were cultured with frozen/thawed stored donor cells or major histocompatibility complex class I-derived synthetic peptides in limiting dilution cultures and then compared with controls using tetanus toxoid and frozen/thawed third-party cells with no human leukocyte antigens in common with the donor. In 5 of 7 patients analyzed who had chronic rejection, elevated frequencies of T cells with indirect, anti-donor specificity (iHTLf) were detected. No such elevated iHTLf were detected in recipients without chronic rejection. DISCUSSION: iHTLf can be obtained from human transplant recipients, which supports the contention that the indirect pathway is involved in chronic transplant rejection.

Role of donor and recipient antigen-presenting cells in priming and maintaining T cells with indirect allospecificity.

BACKGROUND: It has been suggested that the sensitization of recipient T lymphocytes against peptides derived from allogeneic major histocompatibility complex (MHC) antigens in the context of self-MHC molecules may contribute to the pathogenesis of chronic allograft rejection. The purpose of this study was to quantitate and characterize the indirect alloresponse in renal transplantation. METHODS: An HLA-A2-negative patient whose A2-positive kidney transplant failed as a result of chronic rejection was selected for this study. T-cell clones were raised using a cocktail of peptides corresponding to polymorphic regions of the A2 sequence and studied by measuring their proliferation using [3H]thymidine incorporation. The presence in vivo of HLA-A2-specific T cells was assessed using limiting dilution analysis. RESULTS: T-cell clones were specific for a single peptide of HLA-A2, residues 92-120, and restricted by HLA-DRB1*1502. The frequency of interleukin-2-secreting T cells specific for this A2 peptide was 1:86,000, only 2-fold lower than that measured against the recall antigen tetanus toxoid. Capitalizing on the similarity of the donor and recipient DR15 alleles (DRB1*1501 and 1502), the question was addressed as to how these T cells had been primed in vivo. Although the large majority of clones responded to A2 synthetic peptide presented by both DR15 alleles, only 3 of 10 clones responded to cells co-expressing DRB1*1501 and A2. CONCLUSION: These data suggest that antigen presentation by recipient APCs is responsible for maintaining T cells with indirect allospecificity in vivo and that, in the context of partial DR matching, indirect presentation by the parenchymal cells of the graft may serve to induce tolerance in T cells with indirect allospecificity.

Assessment of the contribution that direct allorecognition makes to the progression of chronic cardiac transplant rejection in humans.

BACKGROUND: Two populations of T cells contribute to allograft rejection. T cells with direct allospecificity are activated after recognition of intact MHC alloantigens displayed at the surface of donor passenger leukocytes carried within the graft. In contrast, T cells with indirect allospecificity recognize donor alloantigens as processed peptides associated with self (recipient)-MHC class II molecules. In small animal models of transplantation, direct pathway T cells dominate the acute rejection process and are rendered tolerant to the graft after the loss of donor passenger leukocytes. It has been argued that indirect pathway T cells contribute substantially to continual graft damage after passenger cell loss. The purpose of this study was to determine whether donor-specific tolerance could be detected in T cells with direct anti-donor allospecificity in human heart transplant recipients after prolonged graft residence. METHODS AND RESULTS: Alloreactive helper (HTLf) and cytotoxic (CTLf) T cells were enumerated by use of limiting dilution analysis. These assay systems were refined to make them specific for the direct pathway of allorecognition and more sensitive in the case of the HTLf assay. Recipient:anti-donor frequencies were generated in 10 long-term recipients of heart grafts with progressive chronic rejection and compared with those against equivalently HLA mismatched recipient:third-party controls. For HTLf, direct pathway donor-specific hyporesponsiveness was detected in 5 of the 10 recipients (HTLf<1:100,000). Of these 5 recipients, 4 also had low anti-donor CTLf (<1:100,000). In the 5th recipient, although the CTLf was >1:100,000, it was significantly lower than that estimated against the third-party control. CONCLUSIONS: Donor-specific hyporesponsiveness is demonstrated in 50% of recipients in both the HTLf and CTLf compartments of the direct alloresponse. Direct allorecognition therefore appears unlikely to be responsible for the progression of chronic rejection, implicating indirect allorecognition as the predominant immunological driving force. Furthermore, these data have potential implications for graft outcome, adjustment of immunosuppression, and recipient monitoring.

Effect of HLA mismatching and antibody status on "homovital" aortic valve homograft performance.

BACKGROUND: Recipients of "homovital" aortic valve homografts are known to produce specific antibodies to human leukocyte antigen (HLA) determinants present on the cellular compartment of the valve tissue; however, the clinical significance of these antibodies is unknown. Data from 182 patients receiving homovital aortic valve homografts has been analyzed to determine the impact of HLA disparity and HLA antibody production on survival and function of the homograft. METHODS: Human leukocyte antigen mismatch data were available for 127 patients (mean follow-up, 6.02+/-0.26 years). Two patients were considered well matched for HLA A+B antigens (zero or one mismatch) compared with 125 poorly matched (two to four mismatches). Nine patients had a zero HLA-DR mismatch compared with 52 with one mismatch and 59 patients completely mismatched for DR antigens. RESULTS: There was no significant association between the degree of HLA mismatch for either class I or class II antigens whether the loci were considered alone or in combination (ie, A, B, DR, AB, or ABDR mismatching) with markers of long-term valve function including patient mortality, reoperation, valve degeneration, valve stenosis, presence of regurgitation, and postoperative New York Heart Association class. One hundred thirty-six of 167 (82%) were found to have produced antibodies after operation (mean time after operation, 6.42+/-0.58 years). In 61 cases both antibody specificity and donor HLA typing was available. In 92% of these, the antibodies were of the IgG subclass and were specific for the HLA class I molecules of the donor. The presence of HLA antibodies was associated with an increase in the frequency of mild valve stenosis (not significant) compared with those patients who did not develop HLA antibodies (antibody negative = 9.7%; panel reactive antibodies <50% = 29.1%; and panel reactive antibodies >50% = 22.2%; not significant). There was also an increased prevalence of valve degeneration associated with HLA antibodies. The actuarial freedom from valve degeneration for the 35 HLA antibody-negative patients was 100% at 1, 5, and 10 years compared with 100% at 1 year, 97% at 5 years, and 92% at 10 years for 55 patients with panel reactivity less than 50%, and 98% at 1 year, 94% at 5 years, and 88% at 10 years for the 77 patients who were highly sensitized (not significant). There was no correlation with other markers of long-term valve function. CONCLUSIONS: The influence of the immune response on valve function requires further studies involving large numbers of patients followed for a longer period of time. We believe prospective matching for HLA antigens is warranted to produce a well-matched cohort of patients for analysis and to reduce antibody sensitization, which would help to clarify this issue.

Rational approach to combined carotid and ischaemic heart disease.

BACKGROUND: The management of patients with concomitant coronary and carotid artery disease remains a controversial subject. The aim of this review was to develop a rational plan for the management of such patients based on a review of the literature. METHOD AND RESULTS: A retrospective review was carried out of relevant papers derived from the Medline database from 1964 to 1996. CONCLUSION: The management of patients with concomitant coronary and carotid artery disease has not yet been put to the test in a properly designed and randomized multicentre trial. It is suggested that, until the results of such a trial are available, the rational approach to combined symptomatic disease is combined carotid endarterectomy and coronary artery bypass grafting (CABG). Combined surgery is also appropriate for patients with symptomatic carotid artery disease and significant but asymptomatic cardiac disease. At present there is inadequate evidence to promote carotid endarterectomy for asymptomatic disease in combination with CABG.