Effect of antibodies on endothelium

HLA class I molecules partner with integrin β4 to stimulate endothelial cell proliferation and migration

Among transplant recipients, those who produce antibodies against the donor's human leukocyte antigens (HLAs) are at higher risk for antibody-mediated rejection and transplant vasculopathy, which is a progressive, vasculo-occlusive disease that results in ischemic injury and deterioration of organ function. Antibodies against HLA class I (HLA-I) molecules are thought to contribute to transplant vasculopathy by triggering signals that elicit the activation and proliferation of endothelial cells. Here, we demonstrate a molecular association between HLA-I and the integrin β(4) subunit after the stimulation of endothelial cells with HLA-I-specific antibodies. Knockdown of integrin β(4) in these cells abrogated the ability of HLA-I to stimulate the phosphorylation of the kinases Akt, extracellular signal-regulated kinase (ERK), and Src, as well as cellular proliferation. Similarly, reducing the abundance of HLA-I suppressed integrin β(4)-mediated phosphorylation of ERK and the migration of endothelial cells on laminin-5, a component of the extracellular matrix. These results indicate a mutual dependency between HLA-I and the integrin β(4) subunit to stimulate the proliferation and migration of endothelial cells, which may be important in promoting transplant vasculopathy and tumor angiogenesis.

Pre- and posttransplantation allosensitization in heart allograft recipients: major impact of de novo alloantibody production on allograft survival

The involvement of humoral response in allograft rejection has been suggested by both immunologic and histochemistry studies. In the present study, we explored the role of alloantibodies in a large cohort of heart allograft recipients followed for 15 years. Sequential samples of sera were obtained from 950 recipients of heart allografts before and after transplantation at the time when protocol endomyocardial biopsies were performed. The presence of anti-human leukocyte antigen (HLA) antibodies was investigated using complement mediated cytotoxicity and solid phase assay (SPA). Our data reveal an inverse correlation between the development of alloantibodies after transplantation and heart allograft survival. The 15-year graft survival was highest in patients who never developed alloantibodies (70%) or who displayed them only before transplantation (71%); graft survival in recipients who showed antibodies both before and after transplantation (56%), or only after transplantation (47%), was lower. The deleterious effect of antibodies on graft survival started 8 years after transplantation, suggesting that the production of de novo antibodies may have been triggered by some later event. We found that patients with de novo antibodies appearing more than 1 year after transplantation had the poorest survival. Furthermore, the development of de novo antibodies was preceded in 76% of these patients by cellular rejection grade 3 or higher, according to the International Society for Heart Transplantation (ISHT) grading criteria. Development of antibody-mediated rejection (AMR) had a significant negative impact on graft survival (16% in AMR(+) vs 63% in AMR(-) patients, p = 0.0008). Of the 23 patients with AMR, 21 displayed cytotoxic donor-specific antibodies (DSA) at the time of diagnosis, and in 18 of these cases SPA showed that they were directed against the donors' HLA. The data demonstrate that the detection of alloantibodies permits a better definition of AMR in heart allograft recipients. Identification of patients at risk for developing AMR is of great importance for early treatment of rejection episodes.

Humoral autoimmunity and transplant vasculopathy: when allo is not enough

For several decades, allograft rejection was believed to be mediated almost exclusively by cellular immune responses, but it is now realized that humoral responses also play a major role. Although directed typically against donor human leukocyte antigen, it is becoming increasingly evident that the antibody response can also target autoantigens that are shared between donor and recipient and that this autoantibody may contribute to graft rejection. Many aspects of transplant-induced humoral autoimmunity remain poorly understood and key questions persist; not least what triggers the response and how autoantibody causes graft damage. Here, we collate results from recent clinical and experimental studies in transplantation and autoimmune diseases to propose answers to these questions.

NK cell transcripts and NK cells in kidney biopsies from patients with donor-specific antibodies: evidence for NK cell involvement in antibody-mediated rejection

To explore the mechanisms of antibody-mediated rejection (ABMR) in kidney transplants, we studied the transcripts expressed in clinically indicated biopsies from patients with donor-specific antibody (DSA). Comparison of biopsies from DSA-positive versus DSA-negative patients revealed 132 differentially expressed transcripts: all were associated with class II DSA but none with class I DSA. Many transcripts were expressed in DSA-positive ABMR but were also expressed in T-cell-mediated rejection (TCMR), reflecting shared molecular features. Removal of shared transcripts created 23 DSA selective transcripts (DSASTs). Some DSASTs (6/23) showed selective high expression in NK cells, whereas others (8/23) were expressed in endothelium or in endothelium plus other cell types (7/23). Of 145 biopsies ranked by DSAST expression, the 25 with highest DSAST expression primarily consisted of ABMR (22/25, 88%), either C4d-positive or C4d-negative. By immunostaining, CD56+ and CD68+ cells in peritubular capillaries, but not CD3+ cells, were increased in ABMR compared to TCMR, compatible with a role for NK cells, as well as macrophages, as effectors in endothelial injury during ABMR. Thus, the strategy of using DSASTs in the biopsy to identify mechanism-related transcripts in biopsies from patients with clinical phenotypes indicates the selective involvement of NK cells in ABMR.

Antibody-mediated rejection: emergence of animal models to answer clinical questions

Decades of experiments in small animals had tipped the balance of opinion away from antibodies as a cause of transplant rejection. However, clinical experience, especially with sensitized patients, has convinced basic immunologists of the need to develop models to investigate mechanisms underlying antibody-mediated rejection (AMR). This resurgent interest has resulted in several new rodent models to investigate antibody-mediated mechanisms of heart and renal allograft injury, but satisfactory models of chronic AMR remain more elusive. Nevertheless, these new studies have begun to reveal many insights into the molecular and pathological sequelae of antibody binding to the allograft endothelium. In addition, complement-independent and complement-dependent effects of antibodies on endothelial cells have been identified in vitro. As small animal models become better defined, it is anticipated that they will be more widely used to answer further questions concerning mechanisms of antibody-mediated tissue injury as well as to design therapeutic interventions.