The importance of the indirect pathway of allorecognition in clinical transplantation

The immune system mounts a response to non-self transplanted tissue through a number of mechanisms. The indirect pathway of allorecognition, in which cells of the adaptive immune system recognize MHC alloantigen-derived peptide on self-MHC molecules, has emerged as a potent inducer of allograft rejection. In particular, recent evidence convincingly connects the indirect pathway with chronic rejection, including antibody-mediated and CD8(+) T cell-mediated rejection. However, the indirect pathway can also promote the generation of regulatory T cells, which have emerged as crucial suppressors of the alloresponse, and hold much promise in the quest for clinical tolerance. An improved understanding of the indirect pathway is likely to bring important benefits to transplant recipients.     

Indirect allorecognition in solid organ transplantation

Recipient T cell recognition of donor major histocompatibility complex (MHC) alloantigens plays a central role in both acute and chronic rejection of human organ allografts. Two different pathways of T cell recognition of donor MHC alloantigens have been described. The direct pathway involves T cell recognition of intact MHC molecules expressed by donor antigen-presenting cells (APCs). The second, or indirect pathway, operates via T helper cell recognition of peptides derived from the processing and presentation of allogeneic MHC molecules on self-APCs. At the onset of primary acute rejection, recipient CD4+ T cell responses to donor HLA-DR alloantigens are limited to a single dominant determinant present on one of the disparate alloantigens and restricted by one of the responder's HLA-DR molecules. In allograft recipients with recurring episodes of rejection, and/or at the onset of chronic rejection, recipient T cell reactivity may spread to other epitopes within the allogeneic MHC molecule, as well as to other alloantigens expressed by graft tissue. Both quantitative and qualitative alterations in T cell allopeptide reactivity are associated with increased risk of cellular and/or humoral rejection. These studies provide a basis for the design of new therapeutic strategies and for immunologic monitoring of transplant recipients.     

MD-1 expression regulates direct and indirect allorecognition

Expression of the molecule MD-1 was previously described to regulate allogeneic and xenogeneic skin graft survival, as documented by the decrease in rejection seen following functional blockade of MD-1 expression in vivo, using antisense oligodeoxynucleotides (ODNs) or anti-MD-1 antibodies. It was unclear from these data whether blockade of expression of MD-1 on donor or recipient cells was crucial. We have investigated the effect on allorecognition of treating skin graft donors, and/or recipients, of either fully major histocompatibility complex (MHC)-mismatched allogeneic skin grafts (C3H with C57BL/6 grafts and vice versa) or grafts differing at only multiple minor alloantigens (C3H with B10.BR grafts; C57BL/6 with C3H.SW), with antisense ODNs to MD-1, or in some cases, following transplantation of class II-deficient cells into class I-deficient mice. Graft-specific cytotoxic T lymphocytes (CTLs) were measured in spleen cells recovered at sacrifice of recipients and following donor-specific restimulation in vitro. In the latter case, we also measured cell proliferation and (by enzyme-linked immunosorbent assay) production of interleukin-2 (IL-2)/interferon-gamma (IFN-gamma) or IL-4/IL-10 in vitro (nominal type-1 vs type-2 cytokines). CTL responses to minor-incompatible grafts were diminished, only if graft recipients were treated with ODNs. However, treatment of graft donor and/or recipient of MHC-incompatible grafts produced inhibition of CTL production. Optimal inhibition came from treating both. Specific suppression of CTL production coincided with inhibition of proliferation and preferential production of IL-4 and IL-10 at the expense of IL-2 and IFN-gamma. Our data are consistent with the hypothesis that MD-1 expression regulates both the direct and indirect pathways of allorecognition and that regulation of MD-1 expression may thus help regulate clinical graft rejection.     

Allorecognition of isolated, denatured chains of class I and class II major histocompatibility complex molecules. Evidence for an important role for indirect allorecognition in transplantation.

Classical RT1-A class I and RT1-B class II major histocompatibility complex (MHC) molecules were purified from DA (RT1avl) spleens, and the individual chains separated and purified by preparative polyacrylamide gel electrophoresis in sodium dodecyl sulfate. LEW (RT1l) rats were immunized with the pure class I heavy chain, the RT1-B alpha chain and the RT1-B beta chain with the aim of priming to indirect allorecognition (i.e. after processing and presentation of DA MHC chains on LEW antigen-presenting cells) in the absence of any priming to direct allorecognition (i.e. to whole, undenatured, dimeric DA MHC molecules). LEW rats immunized with each of the three DA MHC chains produced alloantibodies to these chains, suggesting that indirect allorecognition did occur, because of the requirement for cognate recognition of B cells by T helper cells. This also demonstrated polymorphism of all three chains between the DA and LEW strains. The antibodies to the isolated, denatured MHC chains did not react to the whole MHC molecules on DA cells, with the possible exception of very weak reactions in some class I heavy chain-immunized rats. DA skin grafts placed on LEW recipients immunized with each of the DA MHC chains were rejected in an accelerated fashion. Following DA skin grafting, there was an accelerated production of antibodies to whole, undenatured class I MHC molecules, even in the LEW rats preimmunized with RT1-B alpha and RT1-B beta chains. These data suggest that indirect allorecognition can play an important role in the effector mechanisms of allograft rejection, and demonstrate T helper priming as one possible mechanism whereby this might be effective.     

Peritoneal infections in patients on long-term peritoneal dialysis before and after human cadaveric renal transplantation

Long-term peritoneal dialysis was carried out on 38 patients awaiting human cadaveric renal transplantation. Fifty-eight per cent of patients developed infection before transplantation and 41% of those requiring dialysis after transplantation became infected.Infection did not usually occur before the third day of dialysis and 26% of patients developed their first episode of infection in the first week. Almost 50% of patients had only a single episode of infection.The overall rate of infection per 100 patient days of dialysis was 1.23 before transplantation and 4.17 after transplantation.The commonest infecting organisms were Staph aureus and Staph albus and these organisms were frequently isolated from the skin around the dialysis catheter before the infection occurred. Gram-negative infections were more commonly seen after transplantation.Sixty-five per cent of the infections responded to a single course of antibiotic therapy administered in the dialysis fluid. After transplantation Gram-negative infections were usually associated with severe complications and were a major cause of death.Peritoneal dialysis is a convenient way of maintaining patients with chronic renal failure before human cadaveric renal transplantation, and infection, although common is not inevitable, provided precautions are taken to ensure that the dialysis procedure is carried out using aseptic techniques and prophylactic therapy is applied to the skin around the dialysis catheter.     

CD4+CD25+Foxp3+ indirect alloreactive T cells from renal transplant patients suppress both the direct and indirect pathways of allorecognition

Alloreactive T cells recognize donor antigens by two routes: direct and indirect pathways of allorecognition. Although the direct pathway is reported to be dominant in allograft rejection, indirect allorecognition also plays an important role. Indirect alloreactivity is also observed in renal transplant patients irrespective of rejection. Previously we showed a predominance of interleukin (IL)-10 induced by indirect allorecognition of donor human leucocyte antigen (HLA)-DR peptides, suggesting the existence of indirect alloreactive T cells displaying regulatory activity. In the present work, our objective was to characterize these regulatory T cells. We detected indirect alloproliferation of peripheral blood mononuclear cells (PBMC) from renal transplant patients, induced by donor HLA-DR peptides, dependent on IL-4 or IL-10, suggesting regulatory activity as part of the alloreactive T-cell repertoire. PBMC-derived indirect alloreactive T-cell lines were established and produced both inflammatory and regulatory cytokines. We showed that two of these T-cell lines which were able to inhibit both direct and indirect alloproliferation of another T-cell line from the same patient presented a CD4(+)CD25(+)Foxp3(+) T-cell population. These data support the idea that indirect alloreactive T cells may also have regulatory activity and may contribute to the maintenance of the human renal allograft.     

Pathologic pulmonary alterations in long-term human heart-lung transplantation

Twenty-one patients with end-stage pulmonary hypertension underwent combined allograft heart-lung transplantation after 1980. Almost 80 per cent of these patients survived beyond the immediate postoperative period, with the longest survival period more than 3 1/2 years at the time of this report. Five patients died in the perioperative or immediate postoperative period, and 11 returned to normal lives with essentially normal pulmonary function. In the remaining five allograft recipients recurrent respiratory infections and progressive obstructive airway disease developed, with superimposed restrictive deficits in three of them. Two open lung biopsies, two autopsies, and one retransplantation were performed in these recipients. Morphologically, these allograft recipients showed extensive bronchiolitis obliterans, interstitial and pleural fibrosis, and accelerated arterial and venous arteriosclerosis. Bronchiolitis obliterans may prove to be a significant complication of heart-lung transplantation.     

Indirect T-cell allorecognition: perspectives for peptide-based therapy in transplantation l

Indirect allorecognition is an important component of allotransplant rejection. Although the initial indirect alloresponse is limited to a few dominant determinants on donor major histocompatibility complex (MHC) molecules, subsequent spreading to additional determinants on recipient and donor antigens is common. Gilles Benichou and colleagues discuss the mechanisms by which immunodominance is acquired or disrupted in indirect alloresponses, and examine the implications for the design of peptide-based selective immunotherapy in transplantation.     

Parathyroidectomy after kidney transplantation: short-and long-term impact on renal function

INTRODUCTION:

Kidney transplantation corrects endocrine imbalances. Nevertheless, these early favorable events are not always followed by rapid normalization of parathyroid hormone secretion. A possible deleterious effect of parathyroidectomy on kidney transplant function has been reported. This study aimed to compare acute and long-term renal changes after total parathyroidectomy with those occurring after general surgery.

MATERIALS AND METHODS:

This was a retrospective case-controlled study. Nineteen patients with persistent hyperparathyroidism underwent parathyroidectomy due to hypercalcemia. The control group included 19 patients undergoing various general and urological operations.

RESULTS:

In the parathyroidectomy group, a significant increase in serum creatinine from 1.58 to 2.29 mg/dl (P < 0.05) was noted within the first 5 days after parathyroidectomy. In the control group, a statistically insignificant increase in serum creatinine from 1.49 to 1.65 mg/dl occurred over the same time period. The long-term mean serum creatinine level was not statistically different from baseline either in the parathyroidectomy group (final follow-up creatinine = 1.91 mg/dL) or in the non-parathyroidectomy group (final follow-up creatinine = 1.72 mg/dL).

CONCLUSION:

Although renal function deteriorates in the acute period following parathyroidectomy, long-term stabilization occurs, with renal function similar to both preoperative function and to a control group of kidney-transplanted patients who underwent other general surgical operations by the final follow up.     

Fresh human orthotopic ovarian cortex transplantation: long-term results

BACKGROUND: Ovarian orthotopic transplantation in patients with premature ovarian failure is reported to result in full-term pregnancies. Ischaemia and freezing/thawing are potentially injurious for tissues. This study was designed to analyse the effect of ischaemia on long-term ovarian function in humans. METHODS: Prospective case-control study. Subjects were 12 premenopausal women undergoing hysterectomy and fresh orthotopic transplantation of the entire ovarian cortex plus a control group of five patients undergoing hysterectomy only. Follow-up lasted 2 years. Serum FSH and anti-Müllerian hormone (AMH) were recorded, and ovulatory cycles were determined by vaginal ultrasound and serum progesterone levels. RESULTS: Follow-up showed that ovulation was restored in 11 of the 12 patients who received grafts over the duration of the study (9.3 +/- 1.73 ovulations versus 12.0 +/- 0.86 in controls, NS), and 9 of 12 patients remained ovulatory after 2 years. We identified four patterns of FSH secretion during the study, 5 of 12 (41.7%) women having the same pattern as controls. There was a trend for serum AMH levels 7 days after surgery (0.16 +/- 0.02 microg/l) to be lower than pre-surgery levels (0.38 +/- 0.09 microg/l, P = 0.07) and higher in women whose FSH patterns suggested normal ovarian function, but the results did not reach significance. After transplantation, FSH correlated more closely (r = -0.639, P = 0.02) with normal ovarian function than AMH (r = 0.465, P = 0.12). CONCLUSIONS: Fresh orthotopic ovarian cortex transplantation is a viable procedure. It maintains normal ovarian function after 2 years in 75% of cases and preserves ovarian function against ischaemia in 41.7% of patients.     

New spectrum of allorecognition pathways: implications for graft rejection and transplantation tolerance

It has long been appreciated that MHC alloantigens can be recognized via two pathways; direct and indirect. The relative contributions of these two pathways to transplant rejection are partially understood. In studies of transplantation tolerance it appears that regulatory T cells (Trs) with indirect allospecificity, particularly the CD4+CD25+ population, play a key role and can regulate responder cells with direct allospecificity for the same alloantigens. One of the conundrums that remains is how helper T and Tr cells with indirect allospecificity regulate T cells with direct allospecificity. At face value, this appears to break the rules of linkage that require interacting T cells to make contact with the same antigen-presenting cell. A third, 'semi-direct' pathway involving MHC exchange may help to resolve this conundrum. Insights into how these pathways interact in transplant immunity and tolerance will assist the pursuit of clinical tolerance.     

Dendritic cell modification as a route to inhibiting corneal graft rejection by the indirect pathway of allorecognition

Dendritic cell (DC) modification is a potential strategy to induce clinical transplantation tolerance. We compared two DC modification strategies to inhibit allogeneic T‐cell proliferation. In the first strategy, murine DCs were transduced with a lentiviral vector expressing CTLA4‐KDEL, a fusion protein that prevents surface CD80/86 expression by retaining the co‐stimulatory molecules within the ER. In the second approach, DCs were transduced to express the tryptophan‐catabolising enzyme IDO. CTLA4‐KDEL‐expressing DCs induced anergy in alloreactive T cells and generated both CD4⁺CD25⁺ and CD4⁺CD25^? Treg cells (with direct and indirect donor allospecificity and capacity for linked suppression) both in vitro and in vivo. In contrast, T‐cell unresponsiveness induced by IDO⁺ DCs lacked donor specificity. In the absence of any immunosuppressive treatment, i.v. administration of CTLA4‐KDEL‐expressing DCs resulted in long‐term survival of corneal allografts only when the DCs were capable of indirect presentation of alloantigen. This study demonstrates the therapeutic potential of CTLA4‐KDEL‐expressing DCs in tolerance induction.     

Leflunomide in renal transplantation

Leflunomide is a synthetic isoxazole-derivative drug that possesses both immunosuppressive and antiviral properties. Although its only US FDA-approved indication is for the treatment of rheumatoid arthritis, accumulating clinical experience in addition to animal study data makes it an appealing option for patients who are in need of reduction of immunosuppression in the setting of resistant cytomegalovirus infection or BK virus nephropathy, or in renal transplant recipients with chronic allograft dysfunction. While concern over adverse effects such as hepatotoxicity and hemolytic anemia cannot be ignored and there has yet to be a prospective randomized trial for its use in transplantation, its careful usage under close monitoring may provide the best chance for patients who risk allograft rejection during the time of immunosuppressive reduction as they attempt to eradicate BK virus or cytomegalovirus. At the present time, its use as a first-line agent in lieu of mycophenolate mofetil or sirolimus cannot be recommended.     

Leflunomide in renal transplantation

Leflunomide is a synthetic isoxazole-derivative drug that possesses both immunosuppressive and antiviral properties. Although its only US FDA-approved indication is for the treatment of rheumatoid arthritis, accumulating clinical experience in addition to animal study data makes it an appealing option for patients who are in need of reduction of immunosuppression in the setting of resistant cytomegalovirus infection or BK virus nephropathy, or in renal transplant recipients with chronic allograft dysfunction. While concern over adverse effects such as hepatotoxicity and hemolytic anemia cannot be ignored and there has yet to be a prospective randomized trial for its use in transplantation, its careful usage under close monitoring may provide the best chance for patients who risk allograft rejection during the time of immunosuppressive reduction as they attempt to eradicate BK virus or cytomegalovirus. At the present time, its use as a first-line agent in lieu of mycophenolate mofetil or sirolimus cannot be recommended.     

The direct and indirect allogeneic presentation pathway during acute rejection after human cardiac transplantation

Alloreactive T cells may be activated via a direct or an indirect antigen presentation pathway. We questioned whether the frequency of interferon (IFN)-gamma producing cells determined by enzyme-linked immunospot (ELISPOT) assay is an effective tool to monitor the direct and/or indirect presentation pathway. Secondly, we wondered whether early and late acute rejection (AR) are associated with both pathways. Before (n = 15), during (n = 18) and after (n = 16) a period of AR, peripheral blood mononuclear cell (PBMC) samples were tested from 13 heart transplant recipients. The direct presentation pathway was always present. The number of IFN-gamma producing cells reactive to this pathway increased significantly (P = 0.04) during AR and the number decreased (P = 0.005) after AR therapy. In contrast, the indirect allogeneic presentation pathway was present in only eight of 18 AR samples. When the indirect presentation pathway was detectable, it increased significantly during AR. Five of eight of these AR occurred more than 6 months after transplantation. The ELISPOT assay, enumerating alloreactive IFN-gamma producing cells, is a valuable tool to determine the reactivity via both the direct and the indirect presentation pathway. The direct presentation pathway always plays a role in AR, while the indirect pathway contributes especially to late AR.     

Psychiatric morbidity in renal transplantation

51 patients on chronic dialysis, awaiting transplantation, were assessed using a standardised psychiatric interview, together with measures of anxiety, depression, locus of control and social adjustment. 33% of the patients warranted a psychiatric diagnosis. During the study period, 30 patients received a transplanted kidney and 18 could be followed up 12 months after transplantation, when only 3 had a psychiatric disorder (17%). Previous psychiatric history was the main predictor of psychiatric symptomatology and of psychiatric referral during follow-up. However, symptomatology at follow-up also correlated with affective symptoms and selected aspects of social adjustment at initial interview.