Prolonged survival of allogeneic islets in cynomolgus monkeys after short-term triple therapy

Preclinical studies in nonhuman primates (NHP) are particularly useful to evaluate the safety and efficacy of new therapeutic proteins developed for use in clinical transplantation. We hypothesized that a treatment that selectively destroys activated cytopathic donor reactive T cells while sparing resting and immunoregulatory T cells in a mouse model might also produce long-term drug-free engraftment and tolerance without the hazards of lymphopenia in the challenging nonhuman primate islet allograft model. Short-term treatment with a regimen consisting of rapamycin, and IL-2.Ig plus mutant antagonist-type IL-15.Ig cytolytic fusion proteins (triple therapy) posttransplantation results in prolonged, drug-free engraftment of cynomolgus islet allografts. Moreover slow progressive loss of islet function in some recipients was not associated with obvious pathologic evidence of rejection.

Beta 3 integrins regulate lymphocyte migration and cytokine responses in heart transplant rejection

Integrin alpha v beta 3 is important for cell survival, signaling and migration, particularly during angiogenesis and tumorigenesis, where it has been proposed as a therapeutic target. alpha v beta 3 is up-regulated following transplantation and beta 3 polymorphisms are associated with increased acute kidney rejection, suggesting that alpha v beta 3 may also play a role in transplant rejection. Here, using a model of allogeneic heart transplantation, we show that allograft survival is prolonged in beta 3 integrin-deficient (beta 3(-/-)) mice. This is associated with Th2-type immune responses and reduced T-cell infiltration into grafts and T cells from beta 3(-/-) mice show impaired adhesion and migration, consistent with a role for alpha v beta 3 in transmigration. These studies provide evidence that targeting beta 3 integrins impairs recruitment of effector cells and alters cytokine production, so prolonging graft survival. We also show that low doses of blocking antibodies against leukocyte function associated antigen-1 (LFA-1)/alpha L beta 2 and very late antigen-4 (VLA-4)/alpha 4 beta 1, when combined with deletion of beta 3, lead to long-term survival of allografts with no evidence of chronic rejection. Hence we provide strong mechanistic evidence supporting previous genetic studies, demonstrate the involvement of beta 3 integrins in both acute and chronic rejection and identify beta 3 as a new target for immunosuppressive therapy.

Prolonged survival of allogeneic islets in cynomolgus monkeys after short-term anti-CD154-based therapy: nonimmunologic graft failure?

Conventional drug therapy and several anti-CD154 mAb-based regimens were tested in the nonhuman primate (NHP) islet allograft model and found to be inadequate because islets were lost to rejection. Short-term therapy with an optimized donor-specific transfusion (DST) + rapamycin (RPM) + anti-CD154 mAb regimen enables immunosuppression drug-free islet allograft function for months following cessation of therapy in the NHP islet allograft model. After a substantial period of drug-free graft function, these allografts slowly and progressively lost function. Pathologic studies failed to identify islet allograft rejection as a destructive islet invasive lymphocytic infiltration of the allograft was not detected. To evaluate the mechanism, immunologic versus nonimmunologic, of the late islet allograft loss in hosts receiving the optimized therapeutic regimen, we performed experiments with islet autografts and studied islet function in NHPs with partial pancreatectomy. The results in both experiments utilizing autologous islet allografts and partially pancreatectomized hosts reinforce the view that the presence of a marginal islet mass leads to slowly progressive nonimmunological islet loss. Long-term clinically successful islet cell transplantation cannot be realized in the absence of parallel improvements in tolerizing regimens and in the preparation of adequate numbers of islets.

Effects of streptozotocin on autoimmune diabetes in NOD mice

Non-obese diabetic (NOD) mice develop autoimmunity that destroys their native beta cells causing diabetes. Their autoimmunity will also destroy syngeneic transplanted islets and transfer both autoimmunity and diabetes via spleen cells to non-diabetic mice. In this report, we studied the effects of streptozotocin (STZ) on the autoimmune diabetes in NOD mice. We transplanted NOD.SCID islets into three groups of NOD mice: (1) spontaneously diabetic NOD mice (NOD-sp.); (2) prediabetic NOD mice made diabetic by streptozotocin (NOD-stz); and (3) diabetic NOD mice also treated with streptozotocin (NOD-sp./stz). In the first group, the transplants were rejected within 3 weeks. In the second and third groups, the transplants survived indefinitely. Alloxan, a drug similar to streptozotocin, did not have the same effect as streptozotocin. The ability of streptozotocin to prevent diabetes in young NOD mice was reversed by anti-CD8 antibody treatment but not by anti-CD4 treatment. Streptozotocin also made spleen cells from diabetic NOD mice less effective transferring diabetes. These results indicate that streptozotocin treatment both prevents and reverses the islet destructive autoimmunity in NOD mice. We postulate that the effects of streptozotocin treatment may be mediated in part by regulatory T cells.

The effect of low versus high dose of streptozotocin in cynomolgus monkeys (Macaca fascilularis)

Streptozotocin (STZ) is often used to induce diabetes in animal models. However, morbidity associated with STZ and its ability to induce diabetes vary with different dosages among different animal species, including nonhuman primates. To find an optimal dose of STZ that would cause diabetes with minimal toxicity, we compared low and high doses of STZ. Male cynomolgus monkeys (3-6 years old) were given a single dose of 100 mg/kg (high dose, 4 animals) or 55 mg/kg (low dose, 20 animals) of STZ. Blood glucose levels, intravenous glucose tolerance test (IVGTT), pancreatic biopsies, liver function tests (LFTs), liver biopsies, kidney function tests, and kidney biopsies were performed periodically. Animals from both groups developed diabetes within 24 h after administration of STZ. Serum C-peptide levels in both groups decreased from 2 to 8 ng/mL before STZ to between 0.01 and 0.6 ng/mL after STZ. Animals with the high dose of STZ developed transient vomiting within minutes after injection. During the first week after STZ injection, high-dose animals developed elevated LFTs, BUN and creatinine. In contrast, low-dose animals had normal liver and kidney function tests. Histological analysis showed that animals given the high dose of STZ developed marked steatosis of the liver and tubular injury in the kidneys, whereas animals given the low dose of STZ had normal-looking liver and kidney histology. The pancreatic islets in both groups were indistinguishable by immunoperoxidase staining for insulin, and showed either no insulin-positive cells or rare insulin-positive cells. Glucagon staining was normal. Over time, low-dose diabetic monkeys remained persistently hyperglycemic with negligible C-peptide stimulation by intravenous glucose. We conclude that low-dose STZ at 55 mg/mL successfully induces diabetes in cynomolgus monkeys with minimal liver and kidney toxicity.

The kidney transplant program at the Massachusetts general hospital

Between February 1963 and December 2000, 1,627 kidney transplants were performed at the Massachusetts General Hospital. The majority (62%) were from cadaveric donors, although in recent years (1996-2000) 52% have been allografts from living donors, with an increase in living unrelated donors. The introduction of CsA and OKT3 in 1984 was associated with a significant improvement in actuarial renal allograft survival, although a persistent late attrition of allografts continues beyond the first year after transplantation. As reported in other centers, current actuarial survival for living unrelated allografts is superior to that of cadaveric allografts, and is quite similar to that observed in recipients of non-HLA identical living-related transplants. Our preliminary laparoscopic donor nephrectomy experience is encouraging as excellent allograft survival and function has been observed, with minimal morbidity associated with the procedure and a low rate of conversion to open nephrectomy. Recent changes in immunosuppressive protocols have resulted in lower early acute rejection rates, however the incidence of delayed graft function remains unchanged in cadaveric renal transplantation. The role of humoral immunity in allograft rejection has been progressively clarified and new approaches to control donor specific alloantibody production have been shown to be effective. Current clinical studies are ongoing to determine the optimal type and dose of calcineurin inhibitors beyond the first year after transplantation and to study whether avoidance of steroids is safe and feasible. Finally, an innovative tolerance induction protocol using the mixed chimerism approach has been successfully accomplished in selected patients with end-stage renal disease secondary to multiple myeloma. These encouraging observations emphasize that major changes from current immunosuppressive regimens are likely to occur over the next few years as more approaches to tolerance induction are explored clinically.

Recipient MHC class II expression is required to achieve long-term survival of murine cardiac allografts after costimulatory blockade

To study the role of the direct and indirect pathways in achieving tolerance, we used genetically altered mouse strains in two ways: 1) MHC class II-deficient mice were used as donors of skin and cardiac grafts to eliminate the direct CD4(+) T cell response, and 2) B6 II(-)4(+) mice, which are MHC class II-deficient mice expressing an MHC class II transgene only on thymic epithelium, were used as recipients of normal grafts. These mice cannot mount an indirect response. Eliminating the indirect pathway actually made it more difficult to achieve prolonged allograft survival when we used costimulatory blockade than when both pathways were available. Costimulatory blockade was ineffective even when CD4(+) T cells from normal animals were transferred into recipients that lacked MHC class II molecules. These results suggest that an active CD4(+) response through the indirect pathway is necessary for costimulatory blockade to be effective in prolonging allograft survival.

Involvement of the direct and indirect pathways of allorecognition in tolerance induction

It is generally accepted that there are two pathways of allorecognition, direct and indirect, that together contribute to allograft rejection. Although it has been suggested that the direct pathway predominates during early acute rejection and that the indirect pathway provides a continuous supply of alloantigen responsible for chronic rejection, the true relative contribution of each pathway to the overall rejection process is still not entirely known. It is clear, however, that any strategies designed to achieve the ultimate goal in transplantation, the induction of tolerance, will need to take into account both pathways. This review seeks to explore the involvement of the direct and indirect pathways of allorecognition on a mechanistic level as it relates to the induction of tolerance. A brief historical perspective is included for each pathway as well as a comprehensive review of the mechanisms felt to be active during tolerance induction.

Rejection of mouse cardiac allografts by costimulation in trans

The activation of T cells by B7 costimulation in trans has been demonstrated in vitro, but the in vivo relevance is unknown. To study costimulation in trans of CD4(+) T cells in vivo, we performed cardiac transplants from B7-1/B7-2-deficient mice to recipients that do not express MHC class II molecules on peripheral APCs, but do have functional CD4(+) T cells (II(-)/4(+) mice). This model restricts the B7-dependent activation of CD4(+) T cells to costimulation in trans and excludes any contribution from indirect Ag presentation. We find that II(-)/4(+) recipients reject B7-deficient grafts as rapidly as wild-type grafts, suggesting that costimulation in trans can mediate rejection as potently as costimulation in cis. Treatment of II(-)/4(+) recipients of B7-deficient grafts with depleting Abs to CD4 or CD8 demonstrates that indirect Ag presentation to CD8(+) cells does not significantly contribute to rejection. This is the first demonstration that costimulation in trans can mediate an immune response in vivo and has important therapeutic implications.

CD28-independent costimulation of T cells in alloimmune responses

T cell costimulation by B7 molecules plays an important role in the regulation of alloimmune responses. Although both B7-1 and B7-2 bind CD28 and CTLA-4 on T cells, the role of B7-1 and B7-2 signaling through CTLA-4 in regulating alloimmune responses is incompletely understood. To address this question, we transplanted CD28-deficient mice with fully allogeneic vascularized cardiac allografts and studied the effect of selective blockade of B7-1 or B7-2. These mice reject their grafts by a mechanism that involves both CD4(+) and CD8(+) T cells. Blockade of CTLA-4 or B7-1 significantly accelerated graft rejection. In contrast, B7-2 blockade significantly prolonged allograft survival and, unexpectedly, reversed the acceleration of graft rejection caused by CTLA-4 blockade. Furthermore, B7-2 blockade prolonged graft survival in recipients that were both CD28 and CTLA-4 deficient. Our data indicate that B7-1 is the dominant ligand for CTLA-4-mediated down-regulation of alloimmune responses in vivo and suggest that B7-2 has an additional receptor other than CD28 and CTLA-4 to provide a positive costimulatory signal for T cells.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

The role of CD154-CD40 versus CD28-B7 costimulatory pathways in regulating allogeneic Th1 and Th2 responses in vivo

We used signal transducer and activator of transcription 4 (STAT4) and STAT6 gene knockout (-/-) mice as recipients of fully mismatched cardiac allografts to study the role of T-cell costimulatory pathways in regulating allogeneic T-helper 1 (Th1) versus Th2 responses in vivo. STAT4(-/-) mice have impaired Th1 responses, whereas STAT6(-/-) mice do not generate normal Th2 responses. Cardiac allografts from C57BL/6 mice were transplanted into normal wild-type (WT), STAT4(-/-), and STAT6(-/-) BALB/c recipients. STAT4(-/-) and STAT6(-/-) mice rejected their grafts with the same tempo as untreated WT recipients. CD28-B7 blockade by a single injection of CTLA4Ig induced long-term engraftment and donor-specific tolerance in all three groups of recipients. CD154 blockade by a single injection of MR1 was effective in prolonging allograft survival and inducing tolerance in STAT4(-/-) mice but was only marginally effective in STAT6(-/-) recipients and WT controls. In addition, a similar protocol of MR1 was ineffective in prolonging graft survival in CD28(-/-) BALB/c recipients, suggesting that the lack of efficacy seen in WT and STAT6(-/-) mice is not due to the presence of a functional CD28-B7 pathway. Furthermore, there was a similar differential effect of CD28-B7 versus CD154-CD40 blockade in inhibiting immune responses in animals immunized with ovalbumin and complete Freund's adjuvant. These novel data indicate that Th1 and Th2 cells are differentially regulated by CD28-B7 versus CD154-CD40 costimulation pathways in vivo and may have potential implications for the development of therapeutic strategies such as T-cell costimulatory blockade in humans.

Direct and indirect recognition: the role of MHC antigens in graft rejection

In graft rejection, T-cell stimulation by donor APCs and self-APCs (presenting peptides of donor origin) has been called 'direct' and 'indirect' recognition, respectively. Here, Dina Gould and Hugh Auchincloss consider the traditional arguments favoring direct recognition and highlight recent findings suggesting the importance of indirect responses, thereby questioning some of our basic concepts of transplantation immunology.

The strength of cell-mediated xenograft rejection in the mouse is due to the CD4+ indirect response

Previous studies have shown that CD4+ T cells are responsible for the great strength of cell-mediated xenograft rejection in the mouse. In vitro studies have suggested that this CD4+ response is to xenogeneic antigens that are presented indirectly. The present studies were carried out in order to determine whether the strength of cell-mediated xenograft rejection in vivo is dependent on the CD4+ indirect response. We grafted pig skin onto mice that express class II MHC antigens only on their thymic epithelial cells (II-4+ mice). These mice have normal numbers of functional peripheral CD4+ T cells; however they lack class II MHC expression on their antigen presenting cells and are thus incapable of mounting a CD4+ T cell-mediated indirect response. Xenograft survival was prolonged on these mice. Furthermore, administration of cyclosporine and anti-CD8 monoclonal antibodies to II-4+ recipients prolonged xenograft survival to at least the same extent as allograft survival, demonstrating that the strength of cell-mediated xenograft rejection resides in the CD4+ indirect response. Despite the increased survival time, xenograft rejection still occurred in the absence of the indirect pathway. Depletion of the II-4+ recipients of their CD4+ T cell population prolonged xenograft survival even further, suggesting the presence of a weaker CD4+ direct mechanism which was virtually undetectable in vitro.

Delayed rejection of soluble tumor necrosis factor receptor-secreting tumor allografts

BACKGROUND

Exogenous soluble tumor necrosis factor receptor (TNFR) has been shown to be an effective immunosuppressant. It has yet to be tested whether tissues secreting soluble TNFR, when transplanted into a foreign host, could locally generate immunosuppression and therefore manifest prolonged survival.

METHODS

A murine tumor line was transfected with the gene encoding a chimeric protein consisting of the extracellular domain of the human 75-kDa TNFR fused to the Fc region of the human IgG1 heavy chain. This tumor line was then injected into allogeneic recipients.

RESULTS

Transfected tumor cells were shown to secrete soluble TNFR. When transplanted into minor histocompatibility antigen-disparate allogeneic recipients, these tumor cells grew as a solid tumor and resisted rejection, whereas untransfected tumors and interleukin-4 receptor transfectant controls were rejected within 4 weeks. The resistance to rejection could be reversed by coadministration of an anti-TNFR monoclonal antibody.

CONCLUSIONS

Prolongation of graft survival can be achieved by genetically altering transplanted tissue to secrete soluble cytokine receptors.

Contribution of native kidney function to total glomerular filtration rate after combined kidney-pancreas transplantation

BACKGROUND

Combined kidney-pancreas transplantation (CKPT) with its associated euglycemia has been shown to prevent or reduce recurrent diabetic nephropathy in the renal allograft. There has been no evaluation of residual native kidney function after CKPT. The purpose of this study was to determine whether native kidney function may be present in diabetic recipients years after CKPT.

METHODS

Between 1986 and 1992, 37 patients with type 1 insulin-dependent diabetes mellitus with renal failure underwent CKPT. In each case, a single native nephrectomy was performed. We studied 16 patients who had continuing renal and pancreas function more than 4 years after CKPT. Fourteen diabetics with a functioning renal allograft but no pancreas function were used as a control group. Simultaneous renal scans (technetium-99m diethylenetriamine pentaacetic acid) of the native and transplanted kidneys were obtained with a dual-head scintillation camera. Total glomerular filtration rate (GFR) was determined from the rate of clearance of the tracer from the extracellular space measured for 2 hr with an ambulatory renal monitor.

RESULTS

The study groups had similar pretransplant characteristics. At the time of the study, the mean serum creatinine level was not significantly different in the CKPT and control groups (1.7+/-0.7 vs. 1.5+/-0.3 mg/dl, respectively). In the CKPT and control groups, total GFRs were 70.1+/-33 vs. 72.1+/-16.5 ml/min (NS), allograft GFRs were 63+/-34.2 vs. 70.4+/-16 ml/min (NS), and native kidney GFRs were 7.1+/-7.2 vs. 1.7+/-1.9 ml/min (P < 0.05), respectively. In both groups, there was a significant correlation between total GFR and allograft GFR (P < 0.001), but not between total GFR and native kidney GFR. Significant single native kidney GFR (more than 8 ml/min) was found in 7/16 (44%) patients in the CKPT group, but in none of the controls.

CONCLUSIONS

These results suggest that residual native kidney function can be present and contribute moderately to total GFR after CKPT. Euglycemia after CKPT may have a protective role in native kidneys.

CD8+ effector cells responding to residual class I antigens, with help from CD4+ cells stimulated indirectly, cause rejection of "major histocompatibility complex-deficient" skin grafts

BACKGROUND

Skin grafts from mice that are deficient in the expression of both class I and class II major histocompatibility complex (MHC) antigens are rejected rapidly by normal recipients.

METHODS

To determine the mechanism of this rejection, MHC-deficient skin grafts were placed on recipients with different degrees of antigenic disparity and on recipients depleted of selected T cell subpopulations. In addition, the recipient's T cells were examined in vitro for their responses before and after graft rejection.

RESULTS

The results indicate that (1) CD4+ cells provide help for this rejection by recognizing donor antigens presented by recipient class II antigens, and (2) CD8+ cells can participate as effector cells, recognizing residual class I antigens expressed by the MHC-deficient grafts.

CONCLUSIONS

The primary conclusion from these studies is that the supposedly MHC-deficient mice actually do have sufficient class I antigen expression to cause skin graft rejection. This finding prevents the use of these mice to answer definitively the question of whether grafts entirely lacking MHC antigens would be rejected. However, these studies do illustrate two important (although previously recognized) features of allogeneic skin graft rejection: (1) that rejection can be initiated by help provided entirely through the indirect pathway, and (2) that help provided through the indirect pathway is available for effector T cells sensitized directly by donor cells. However, the results from these and other studies suggest that indirect effector mechanisms would probably be able to destroy truly MHC-deficient grafts under some circumstances.

Studies of transplantation immunology with major histocompatibility complex knockout mice

Mice deficient in the expression of either class I or class II major histocompatibility complex (MHC) antigens have been generated by use of the technique of gene disruption by homologous recombination. These animals have subsequently been mated to generate mice that are deficient in the expression of both classes of MHC antigens. Class I MHC-deficient animals have a greater than 90% reduction in cell surface expression of MHC I molecules; however, they do express low levels of class I heavy chains on their cells. Furthermore, class I-deficient mice have very few CD8S+R T cells. Class II MHC-deficient animals have no detectable expression of class II MHC molecules and a reduction in the CD4+ T cell population. Mice deficient in both MHC antigens share the characteristics of the two founder animals: low levels of class I heavy chain expression, no detectable class II expression and reduced levels of CD4+ and CD8+ T cells. Allotransplantation experiments with these animals have suggested that different mechanisms of graft rejection predominate depending on the target organ and have provided evidence for the role of the indirect pathway of antigen recognition in graft rejection. Xenotransplantation experiments involving these animals have revealed that donor MHC deficiency offers no protection to the graft, suggesting that strategies to eliminate MHC antigen expression will not be successful in generating "universal donors."

Antigen processing and presentation in transplantation

For many years the direct stimulation of T cells in response to donor MHC antigens expressed on donor antigen-presenting cells has been the focus of transplantation immunology. Indirect recognition in response to peptides of donor antigens presented by self MHC molecules on recipient antigen-presenting cells has not generally been considered an important feature of graft rejection. Recent evidence suggests that indirect responses may be more important than previously considered and the new emphasis on indirect pathways in allograft rejection has raised new issues, many of which are unresolved.

T cell recognition of xeno-MHC peptides during concordant xenograft rejection

T cell recognition of xenoantigens is likely to play a key role in rejection of xenografts surviving hyperacute and delayed xenograft rejection, but the mechanisms of how this might occur are unknown. We used synthetic rat class II MHC peptides to study the role of the indirect pathway, where processed xenogeneic MHC antigens are presented in the context of self MHC, in a concordant xenograft rejection model in vivo. Mice of four different strains, BALB/c, B1O.A, CBA/ca, and C57BL/6j, were immunized with a mixture of rat class II MHC 25-mer xenopeptides representing the full-length sequence of the beta chain hypervariable domain of either RT1.Du (DR and I-E like) or RT1.Bu (DQ and I-A like) of the Wistar-Furth (WF) (RT1u) rat. Draining lymph node cells were capable of recognizing and proliferating to specific class II xeno-MHC peptides. The immunogenicity of the different peptides varied with the responder mouse strain. Responder T cells were CD4+, and were inhibited by anti-I-A and anti-I-E antibodies. We then examined the proliferative response of T cells from B1O.A primed by WF skin or vascularized cardiac xenografts to the class II MHC xenopeptides, when presented by naive B1O.A splenic antigen-presenting cells. These T cells were capable of proliferating to the same xeno-MHC peptides shown to be immunogenic by immunization. These data confirm the occurrence of self-restricted T cell recognition of xeno-MHC peptides in xenograft rejection, and provide the rationale for further investigating the role of the indirect pathway of recognition in xenotransplantation.

Selective use of veno-venous bypass in orthotopic liver transplantation

The use of veno-venous bypass (VVB) during the anhepatic phase of orthotopic liver transplantation (OLT) remains controversial. We employ VVB on a selective basis: patients who tolerate intra-operative supra-hepatic IVC test cross-clamping undergo OLT without VVB while patients who, despite maximal volume resuscitation, develop hemodynamic instability during test cross-clamping, undergo OLT with VVB. The records of 150 adult orthotopic liver allograft recipients transplanted at the Massachusetts General Hospital from January 1984 to December 1994 were reviewed to identify any potential adverse affects on peri-operative, 6 months, 1 year outcomes in recipients foregoing VVB during liver transplantation. Thirty-eight patients (25%) underwent OLT without VVB with actuarial survivals of 78.4% and 69% at 6 months and 1 year. 112 patients (75%) underwent OLT with VVB with actuarial survivals at 6 months and 1 year of 73% and 72%. Demographic data, UNOS status, and diagnoses were similar in each group. There were no significant differences in intra-operative PRBC requirements; lengths of hospital stay; retransplantation rates; or 30 day, 6 months and 1 year survivals between these two groups. There was no significant difference in renal function as determined by preoperative, peak post-operative, discharge serum creatinine levels, or number of patients requiring HD between these two groups. There were two major complications (1.8%) possibly resulting from VVB. In conclusion, patients who tolerate IVC test cross-clamping can safely undergo orthotopic liver transplantation without veno-venous bypass. In our experience, there were no significant differences in peri-operative parameters, post-operative renal function, or short-term survival when compared to patients who, due to hemodynamic instability during IVC cross-clamping, underwent OLT with VVB. Given the potential complications associated with VVB, we feel that in those patients who tolerate intra-operative IVC cross-clamping, it is better to proceed without the use of VVB.

Two levels of help for B cell alloantibody production

We have examined whether T cell stimulation by direct or indirect pathways contributes to alloantibody production by B cells after major histocompatibility complex (MHC)-disparate skin graft rejection in mice. Experiments were performed using normal mice, MHC class II-deficient mice, MHC class II-deficient mice with an intact peripheral CD4+ cell population (due to expression of class II antigens only on thymic epithelium), mice lacking the cytoplasmic tail of their MHC class II antigens, and mice depleted of CD4+ cells by anti-CD4 monoclonal antibody treatment. Depletion of recipient CD4+ cells reduced alloantibody production to barely detectable levels. Absence of donor MHC class II antigens did not affect the production of either immunoglobulin (Ig)M or IgG antibodies directed at class I alloantigens. Absence of recipient MHC class II antigens, however, led to production of only IgM but not IgG antibodies, even if the recipients had an intact CD4+ cell population. Absence of the cytoplasmic tail of the recipient's MHC class II antigens led to the production of slightly reduced amounts of IgG antibody. These findings indicate that (a) CD4+ cells are essential helper cells for B cell alloantibody production; (b) production of IgM alloantibody can occur with help from CD4+ cells, which recognize either donor class II antigens or modified recipient class II antigens; (c) isotype switching from IgM to IgG alloantibody requires help from CD4+ cells activated by antigens presented by recipient MHC class II molecules; and (d) the cytoplasmic domain of the recipient MHC class II molecules may be involved in the mechanism that leads to isotype switching by B cells. Thus, there are two levels of CD4-mediated help available for B cells responding to alloantigens: one (involving a noncognate interaction) can produce B cell activation, and a second (involving a cognate interaction) is required for differentiation and IgG alloantibody production.

MHC class I expression and CD8+ T cell development in TAP1/beta 2-microglobulin double mutant mice

We have bred to homozygosity gene disruptions for the transporter associated with antigen processing 1 (TAP1) and beta 2-microglobulin (beta 2m), each of which plays a distinct role in providing class I MHC subunits. Surface expression of H-2Kb or Db on cells derived from TAP1/beta 2m -/- mice was undetectable by immunofluorescence or immunoprecipitation, unlike the situation observed for TAP1 -/- and beta 2m -/- single mutant mice. Yet, TAP1/beta 2m -/- cells were able to elicit a CD8+ cytotoxic T cell (CTL) response in mice of different H-2 haplotypes and could be killed by anti-H-2b specific CTL. Furthermore, TAP1/beta 2m -/- skin grafts were rejected by bm1 mutant mice. This suggests that very low levels of conformed class I heavy chains can reach the cell surface even in the complete absence of TAP1 and beta 2m gene products, and that these molecules may select a functional CD8+ T cell repertoire. Indeed, CD4-CD8+ T cells were detected in TAP1/beta 2m -/- mice, but in numbers lower than in either of the single mutant mice. Nonetheless, it was possible to elicit a CD8+ allospecific and H-2b reactive CTL response in TAP1/beta 2m -/- mice. In line with this, TAP1/beta 2m -/- mice rapidly rejected TAP1/beta 2m +/- skin grafts. Our results suggest that some MHC class I heavy chains in TAP1/beta 2m -/- cells can reach the cell surface in a form that allows recognition by allospecific CTL and positive selection of CD8+ T cells.

Xenotransplantation

The need for an alternative source of donor organs, together with the expansion of scientific data in this field, has focused attention on xenotransplantion as a possible alternative to allotransplantation in the treatment of patients with end-stage disease of vital organs. However, xenotransplantation is rarely successful. Not only are the immunological barriers to the acceptance of xenogeneic tissue more powerful than those seen in allotransplantation, but the potential for the transmission of xenograft-associated zoonoses to the human host at the time of transplantation is also present. In addition, data on the physiological performance of the xenograft in the human environment are lacking, although a few functioning xenografts have been shown to be capable of supporting human life. Although progress has been made in clarifying some of the barriers to xenotransplantation and in defining appropriate therapeutic interventions, including interventions aimed at the removal of natural antibody and at the limitation of complement activation, xenotransplantation is not yet a viable alternative to allotransplantation in the clinical setting.

Cell-mediated xenoresponses: strong or weak?

Cell-mediated responses to xenoantigens are different from those to alloantigens. CD4+ T cells are especially important in xenoresponses and depletion of CD4+ T cells can prolong survival of xenografts better than allografts in some circumstances. The difference between the two responses is due in part to the relative weakness of direct T-cell responses stimulated by xenogeneic antigen-presenting cells. Instead, T cells tend to require that donor antigens be processed and presented indirectly in association with the MHC antigens on their own APCs. The defect in direct stimulation is due to failure of some of the several T cell/APC interactions when the receptors and ligands of these interactions come from different species. Although the cell-mediated response to xenografts is different, it remains very strong. The mechanism by which CD4+ cells cause xenograft destruction remains to be determined, but a better understanding of this mechanism may help to select the most useful types of immunosuppression for xenogeneic transplantation.

Second renal transplantations. Ethical issues clarified by outcome; outcome enhanced by a reliable crossmatch

OBJECTIVE

To determine whether the appropriate use of scarce donor resources has been accomplished by renal retransplantation by reviewing the initial and long-term outcomes of second-renal transplant recipients at the Massachusetts General Hospital, Boston.

PATIENTS AND RESULTS

With a mean follow-up of nearly 5 years following transplantation, 54 (68%) of 80 second-transplant recipients had functioning allografts (allograft failure was defined by patient death or a return to dialysis). Rejection was the most common cause of failure (14 [54%] of 26 patients). The 1-, 3-, and 5-year actuarial allograft survival rates were 86%, 78%, and 69%, respectively, which were not significantly different from the survival rates of primary allografts at this center. These results support the continued approach of providing both cadaver-donor and living-donor renal allografts for recipients whose primary renal allograft has failed. The antiglobulin crossmatch may have contributed to the successful outcome by accurately determining compatibility and by averting early rejection failures.

CONCLUSIONS

Health care policy reviewers should clearly distinguish the prospects for successful second renal transplants from the outcomes of extrarenal retransplantation. Moreover, because excellent second-renal allograft survival is attainable and comparable to primary-renal allograft survival and because the costs are comparable, restricting suitable patients to subsequent lifelong dialysis becomes unethical.

Mechanisms of transplantation tolerance

Transplantation tolerance, the long-term acceptance of grafted tissue in the absence of continuous immunosuppression, remains an elusive goal in humans, but it has been achieved in animal models using numerous approaches. The mechanisms behind graft acceptance vary according to the means used to create the state of acceptance. Several major mechanisms can now be recognized. While thymic deletion of T cells appears to be a mainstay of self-tolerance, its role in transplantation tolerance now seems to be less significant. In contrast, extrathymic mechanisms of transplantation tolerance seem to be major factors in long-term graft acceptance. If donor antigens are presented in a nonimmunogenic manner on the graft, e.g. due to modification of graft tissue by culture, peripheral T cells of the recipient may ignore the graft. Alternatively, nonstimulatory presentation of donor antigens on graft tissue can induce a state of unresponsiveness in recipient T cells, i.e. anergy, rather than activating them to destroy the graft. Suppression mechanisms also operate to control graft rejection and may be specific or nonspecific in nature. Specific suppression mechanisms might act in an idiotype or antigen-specific fashion, and evidence is accumulating that this may be mediated through the elaboration of cytokines. Donor antigen-specific T cells may be activated to produce "protective" cytokines which then regulate the generation of destructive T cells. Future therapies will be aimed at affecting graft acceptance through these peripheral mechanisms.

Indirect recognition by helper cells can induce donor-specific cytotoxic T lymphocytes in vivo

In vitro studies have revealed that help for cytotoxic T lymphocyte (CTL) induction can be mediated through several pathways, including direct recognition of allogenic class II antigens by CD4+ cells, direct recognition of allogeneic class I antigens by "CD4-independent" CD8+ cells, and "indirect" recognition of peptides of alloantigens presented in association with self class II molecules. Whereas good evidence for the two direct pathways is available in vivo, there is relatively little evidence to show that indirect recognition can initiate graft rejection. This study examined the role of indirect allorecognition during the generation of CTLs in mice as they rejected major histocompatibility complex (MHC) class II-deficient skin after depletion of CD8+ T cells in vivo. Recipients were depleted of CD8+ T cells by in vivo treatment with anti-CD8 monoclonal antibody and then grafted with allogeneic skin lacking MHC class II antigens. The mice rejected the skin grafts rapidly. Although flow cytometry showed marked depletion of CD8+ T cells in these mice, we found that (a) CD8+ CTLs were generated and sensitized to MHC class I antigens of the donor; (b) the generation of the CD8+ CTLs required the help in vivo of CD4+ cells, as well as priming with the allogeneic skin graft; and (c) the CD4+ T helper cells were sensitized indirectly to donor peptides presented in association with class II antigens on recipient antigen-presenting cells. These results provide evidence that indirect recognition can provide effective help for CTL induction during graft rejection, even when the cytotoxic T cells are sensitized by determinants expressed only on the donor graft.

Management of the renal allograft recipient: immunosuppressive protocols for long-term success

To determine the benefits of long-term cyclosporine (CsA) immunosuppression, renal allograft recipients were randomly assigned to a protocol of either: CsA+azathioprine (Aza)+prednisone (TD), or to a protocol in which CsA was discontinued from the regimen of Aza+prednisone (CsA D/C). With a mean follow-up of nearly 7 years since transplantation, 30/47 (64%) CsA D/C and 27/45 (60%) TD had functioning allografts. Although long-term survivals were similar, hazards of the CsA D/C protocol were evident (40% rate of acute rejection following CsA D/C). Conversely, continued CsA in the TD protocol provided the opportunity for prednisone reduction, or even complete prednisone withdrawal in selected patients. A TD protocol which can provide equivalent long-term success, and eventually lower or omit prednisone, is preferable to a protocol of CsA D/C.

Mice lacking major histocompatibility complex class I and class II molecules

Mice lacking major histocompatibility complex (MHC) antigens were generated by mating beta 2-microglobulin-deficient, and therefore class I-deficient, animals with MHC class II-deficient animals. When housed under sterile conditions, the resulting MHC-deficient mice appear healthy, survive for many months, and breed successfully. Phenotypically, MHC-deficient mice are depleted of CD4+ and CD8+ T cells in peripheral lymphoid organs due to a lack of appropriate restricting elements. In contrast, the B-cell compartment of these animals appears intact, and MHC-deficient mice can mount specific antibody responses when challenged with a T-independent antigen. Spleen cells from MHC-deficient animals are poor stimulators and responders in a mixed lymphocyte reaction. Despite their relatively weak cellular immune responses in vitro, MHC-deficient mice reject allogeneic skin grafts with little delay, and grafts from MHC-deficient animals are rapidly rejected by normal allogeneic recipients. Taken together, these results emphasize the plasticity of the immune system and suggest that MHC-deficient mice may be useful for examining compensatory mechanisms in severely immunocompromised animals.

The role of "indirect" recognition in initiating rejection of skin grafts from major histocompatibility complex class II-deficient mice

In vitro studies have revealed several pathways by which T cells can respond to alloantigens, including CD4+ direct responses to allogeneic class II antigens, CD8+ direct responses to allogeneic class I antigens, and CD4+ "indirect" responses to peptides of alloantigens presented in association with responder class II molecules. In vivo studies of skin graft rejection, however, have so far provided clear evidence for the contribution of only the two direct pathways and not for indirect recognition. We have used major histocompatibility complex class II-deficient mice as donors to test the role of indirect recognition in rejection of skin grafts. Class II-deficient skin was always rejected without delay by normal recipients. Removal of recipient CD8+ cells (to leave the animals dependent on CD4+ function) or depletion of recipient CD4+ cells revealed that CD4+ cells were usually involved and sometimes absolutely required in this rapid rejection. Since the donor grafts lacked class II antigens, the CD4+ cells must have recognized donor antigens presented in association with recipient class II molecules. These results therefore indicate that indirect recognition can initiate rapid skin graft rejection.