Anti-HLA class I antibody-mediated activation of the PI3K/Akt signaling pathway and induction of Bcl-2 and Bcl-xL expression in endothelial cells

Anti-human leukocyte antigen (HLA) antibodies (Ab) have long been implicated in the process of acute and chronic allograft rejection, yet their mechanism(s) of action is not well understood. The aim of this study was to determine whether ligation of HLA class I molecules by anti-HLA Ab on the surface of human endothelial cells (EC) activates the PI3 Kinase (PI3K)/Akt signaling pathway and downstream target proteins of the cell death apparatus. We report that Ab ligation of major histocompatibility complex (MHC) class I molecules on the surface of EC triggers phosphorylation of Akt, PI3K, and recruitment of PI3K and Akt into a signaling unit with focal adhesion kinase. Signaling through class I also stimulated phosphorylation of Bad and upregulated expression of Bcl-2 and Bcl-xL. Pretreatment of EC with the PI3K inhibitor wortmannin blocked class I-mediated expression of Bcl-2, but not Bcl-xL, suggesting a role for the PI3K/Akt signaling pathway in regulation of class I-induced Bcl-2 expression. The intracellular events initiated by class I ligation were influenced by the concentration of the anti-HLA Ab with the lowest tested concentrations of Ab stimulating the highest level of Akt phosphorylation, Bcl-xL and Bcl-2 expression. Consistent with the in vitro experiments, analysis of biopsy samples from heart transplant recipients with evidence of Ab-mediated rejection exhibited increased Bcl-2 expression on the vascular endothelium. These results suggest that exposure of the graft endothelium to low concentrations of anti-HLA Ab may promote cell survival by transducing signals resulting in upregulation of cell survival genes.

Chronic heart allograft rejection in rats demonstrates a dynamic interplay between IFN-γ and IL-10 producing T cells

The relative roles of different types of T cells and their modes of alloantigen recognition and cytokine production in chronic rejection have been controversial. This may be due to the use of models involving various immune interventions. We recently reported a rat heart allograft model (PVG.1U-to-PVG.R8) that consistently develops chronic rejection without any immune manipulations. Using this model, we investigated the frequency of indirectly activated alloreactive CD4+ T cells and directly/indirectly induced CD8+ T cells and characterized their cytokine profiles at various times posttransplantation. In vitro quantitative-proliferative and intracellular cytokine assays were performed using recipient lymphocytes as responders against donor and third party stimulator cells. Intragraft transcript levels for IFN-gamma were measured using competitive RT-PCR. We observed a steady increase in the frequency of donor reactive CD4+ T cells, peaking on day 18-22 posttransplantation, followed by a decline to background levels on day 60. These cells produced high levels of IL-10 that steadily increased throughout the 100-day experimental period. In contrast, the frequency of donor reactive CD8+ T cells was similar to that of naïve cells and remained unchanged throughout the study. CD8+ T cells produced high amounts of IFN-gamma only on days 39-45 posttransplantation when chronic rejection was apparent. There was a sharp increase in intragraft IFN-gamma transcripts on days 8-12 that peaked on days 39-45 followed by a steady decrease thereafter. Our results demonstrate that chronic rejection in this model involves a dynamic interplay between alloreactive CD8+IFN-gamma+ and CD4+IL-10+ T cells, both of which may have differential effects on the pathogenesis of the disease.

Reduction of human anti-tetanus toxoid antibody in hu-PBL-SCID mice by immunodominant peptides of tetanus toxoid

Immunotherapy of murine autoimmune and allergic diseases by administration of peptides corresponding to the dominant T cell epitope is a reality. However, problems remain in applying this therapy to reduce antibody responses in humans. To overcome these difficulties, a preclinical system was developed to test the effect of immunodominant peptides from a common antigen, tetanus toxoid (TT), on the long-term human anti-TT response. Individuals whose T cells proliferated against dominant TT peptides were identified. Peripheral blood leucocytes (PBL) from these donors were injected intraperitoneally (i.p.) into mice with severe combined immunodeficiency (SCID) that had been depleted of murine natural killer (NK) cells (hu-PBL-SCID mice). Peptides or PBS were injected i.p. before a further injection of PBL and immunization with TT. The concentration of human IgG and anti-TT in murine plasma was followed for 10 weeks. The total IgG was similar in both groups. By contrast, there was a statistically significant reduction in IgG anti-TT from eight weeks onwards. It is considered that the hu-PBL-SCID model system may provide a means by which the efficacy of peptide immunotherapy for reduction of pathological antibodies in humans can be examined.

Variable cyclosporine exposure: a risk factor for chronic allograft nephropathy and graft loss?

BACKGROUND

Following the introduction of ciclosporine (CsA), the 2-year survival of transplanted kidneys improved from less than 60% to over 80%. Though the introduction of this drug resulted in a marked improvement in graft survival, its use was not without problems. Variable absorption and a narrow therapeutic index resulted in the need for measurements of CsA blood concentrations to tailor the drug dose to maximize therapeutic efficacy while minimizing toxicity.

METHODS

Data were available from the LOTESS study of 4948 transplant patients receiving Neoral with at least 5 years' follow-up. Potential risk factors associated with outcome in renal transplant recipients treated with CsA were explored: the primary outcome variable was graft loss. A stepwise binary logistic regression analysis was used to identify donor, recipient, and treatment variables related to outcome.

RESULTS

In the initial analysis, chronic rejection was the only significant predictor of graft loss. The relative risk (RR) of graft loss was 16.9 (95% CI = 13.9-20.4). Further analysis identified four independent risk factors for chronic rejection cadaveric donor (RR, 1.50; 95% CI = 1.05-2.15), older donor (RR, 1.02; 95% CI = 1.01-1.02), younger recipient (RR, 1.02; 95% CI = 1.02-1.03), and variable predose CsA concentration (RR, 1.25; 95% CI = 1.06-1.48).

CONCLUSION

With the UK kidney transplant waiting list at about 5000 patients and only 1658 transplants performed during 2002, it is important maximize graft survival. For example, perhaps marginal donors (age > 55) can be matched to older recipients without increasing the risk of chronic allograft nephropathy and therefore graft loss. Variable predose CsA concentrations may arise from at least three different sources: adherence to treatment, drug formulation, and individual variation in absorption. Therefore, it is important to emphaze to patients that erratic compliance may increase their risk of graft loss. Second, although only one CsA formulation is marketed in the UK, when generic forms of CsA are introduced it will be important to demonstrate consistent delivery of CsA from these new formulations. Third, improved monitoring of CsA using a C2 rather than a predose blood concentration measurement may be used to reduce intra-individual variations in drug exposure.

The pathobiology of chronic allograft nephropathy: Immune-mediated damage and accelerated aging

Chronic allograft nephropathy includes chronic calcineurin nephrotoxicity, recurrent and de novo glomerulonephritis and a group of disorders with graft dysfunction of unknown etiology designated chronic rejection. Review of risk factors of the latter category show that the chronic rejection lesions emerge in organs that have undergone injury. Despite the relevance of nonalloantigen-dependent progression factors in the tissue injury, alloantigen-dependent factors predominate in the pathogenesis. Lately, B cell responses have received increasing interest in transplant rejection and include responses against both major histocompatibility complex (MHC) and tissue-specific antigens, mainly on the endothelium and in the glomeruli. These humoral responses are thought to be involved in the development of vascular and glomerular lesions. Furthermore, at the tissue level, markers of senescence are found in the tubular epithelium contributing to the lesions of tubular atrophy and interstitial fibrosis.

Immunosuppression for Long-Term Maintenance of Renal Allograft Function

The incidence and severity of acute rejection episodes was markedly reduced by the introduction of new immunosuppressive drug regimens for renal transplantation, resulting in improved graft survival at 1 year. However, only modest improvement has been shown in long-term graft function rates. This overview evaluates the efficacy of currently used immunosuppressive drugs and drug combinations for long-term maintenance therapy. Prospective controlled trials rarely extend beyond 5 years; therefore, registry data and retrospective reports have also been employed. From currently available data it may be concluded that the initial beneficial effect of ciclosporin (cyclosporin) is lost 10 years after transplantation. Tacrolimus is an alternative to ciclosporin with a different profile of adverse effects and a higher efficacy in acute rejection treatment. For long-term maintenance, projected half-lives of kidney graft function are in favour of tacrolimus. Mycophenolate mofetil (MMF) has been shown to significantly reduce the incidence of early rejections. However, the improved long-term graft survival reported in retrospective studies has still to be confirmed in controlled trials. There is no convincing evidence for superiority of triple therapy including prednisone (or prednisolone), calcineurin inhibitors and azathioprine/MMF over dual therapy without azathioprine/MMF with respect to long-term outcome. Withdrawal of corticosteroids or calcineurin inhibitors clearly reduces adverse drug effects but carries the risk of acute rejection episodes. Avoidance of corticosteroids by using new immunosuppressive drug combinations may be an option to minimise toxic adverse effects in the future. At present, it seems unjustified to convert renal transplant recipients with stable graft function and tolerable adverse effects from one drug to another solely in expectation of future benefits. Acute early or late rejection episodes and intolerable adverse effects are good reasons for conversions between calcineurin inhibitors or cytotoxic agents. Chronic allograft nephropathy with slowly deteriorating graft function remains an unresolved problem.