Development of tolerogenic strategies in the clinic

TNX-1500, a crystallizable fragment-modified anti-CD154 antibody, prolongs nonhuman primate cardiac allograft survival

Blockade of the CD40/CD154 T cell costimulation pathway is a promising approach to supplement or replace current clinical immunosuppression in solid organ transplantation. We evaluated the tolerability and activity of a novel humanized anti-CD154 monoclonal antibody, TNX-1500 (TNX), in a nonhuman primate heterotopic cardiac allogeneic (allo) transplant model. TNX-1500 contains a rupluzimab fragment antigen-binding region and an immunoglobin G4 crystallizable fragment region engineered to reduce binding to the crystallizable fragment gamma receptor IIa and associated risks of thrombosis. Recipients were treated for 6 months with standard-dose TNX (sTNX) monotherapy, low-dose TNX monotherapy (loTNX), or loTNX with mycophenolate mofetil (MMF) (loTNX + MMF). Results were compared with historical data using chimeric humanized 5c8 monotherapy dosed as for loTNX but discontinued at 3 months. Median survival time was similar for humanized 5c8 and both loTNX groups, but significantly longer with sTNX (>265 days) than with loTNX (99 days) or loTNX + MMF (88 days) (P < 0.05 for both comparisons against sTNX). Standard-dose TNX prevented antidonor alloantibody elaboration, inhibited chronic rejection, and was associated with a significantly reduced effector T cells/regulatory T cells ratio relative to loTNX with MMF. No thrombotic complications were observed. This study demonstrated that TNX was well tolerated, prolongs allograft survival, and prevents alloantibody production and cardiac allograft vasculopathy in a stringent preclinical nonhuman primate heart allotransplant model.

Microarrays: monitoring for transplant tolerance and mechanistic insights

With recent advances in immunology and a growing understanding of transplantation biology, the development of reliable assays that may be used for identification and prediction of the current state of an immune response (rejection and tolerance) are urgently needed to allow us to predict the development of immunologic graft injury, individualize immunosuppression, rationally minimize immunosuppressive drug toxicity, promote a better understanding of the mechanisms underlying stable graft acceptance, and aid in the design of tolerance-inducing clinical transplantation trials. Microarrays can provide nonbiased, simultaneous global expression patterns for more than 40,000 human genes across different experiments. High throughput microarray technology offers a means to study disease-specific transcriptional changes in tissue biopsy, peripheral blood, and biofluids.

Hematopoietic stem cell engraftment and seeding permits multi-lymphoid chimerism in vascularized bone marrow transplants

Vascularized bone marrow transplantation (VBMT) across a MHC barrier under a 7-day alphabeta-TCR mAb and CsA protocol facilitated multiple hematolymphoid chimerism via trafficking of the immature (CD90) bone marrow cells (BMC) between donor and recipient compartments. Early engraftment of donor BMC [BN(RT1(n))] into the recipient BM compartment [LEW(RT1(l))] was achieved at 1 week posttransplant and this was associated with active hematopoiesis within allografted bone and correlated with high chimerism in the hematolymphoid organs. Two-way trafficking between donor and recipient BM compartments was confirmed by the presence of recipient MHC class I cells (RT1(l)) within the allografted bone up to 3 weeks posttransplant. At 10 weeks posttransplant, decline of BMC viability in allografted bone corresponded with bone fibrosis and lack of hematopoiesis. In contrast, active hematopoiesis was present in the recipient bone as evidenced by the presence of donor-specific immature (CD90/RT1(n)) cells, which correlated with chimerism maintenance. Clonogenic activity of donor-origin cells (RT1(n)) engrafted into the host BM compartment was confirmed by colony-forming units (CFU) assay. These results confirm that hematolymphoid chimerism is developed early post-VBMT by T-cell lineage and despite allografted bone fibrosis chimerism maintenance is supported by B-cell linage and active hematopoiesis of donor-origin cells in the host BM compartment.

Homeostatic repopulation by CD28-CD8+ T cells in alemtuzumab-depleted kidney transplant recipients treated with reduced immunosuppression

Alemtuzumab (CAMPATH-1H) is a depleting agent introduced recently in transplantation and often used with reduced maintenance immunosuppression. In the current study we investigated the immune response of 13 kidney allograft recipients treated with alemtuzumab followed by weaned immunosuppression with reduced dose of mycophenolate mofetil (MMF) and tacrolimus. Tacrolimus was switched to sirolimus at 6 months and MMF withdrawn at 12 months after transplantation. We found that after alemtuzumab induction the recovery of CD8(+) T cells was much faster than that of CD4(+) T cells. It was complete 6 months posttransplant while CD4(+) T cells did not fully recover even 15 months posttransplant. Repopulating CD8(+) T cells were mainly of immunosenescent CD28(-)CD8(+) phenotype. In a series of in vitro experiments we showed that CD28(-)CD8(+) T cells might suppress proliferation of CD4(+) T cells. There were three successfully treated acute rejections during the study (first at +70 day, two others +12 months) that occurred in patients with the lowest level of CD28(-)CD8(+) T cells. We hypothesize that expanded CD28(-)CD8(+) T cells might compete for 'immune space' with CD4(+) T cells suppressing their proliferation and therefore delaying CD4(+) T-cells recovery. This delay might be associated with the clinical outcome as CD4(+) T cells, notably CD4(+) T effector memory cells, were shown to be associated with rejection.

Evidence for functional inter-relationships between FOXP3, leukaemia inhibitory factor, and axotrophin/MARCH-7 in transplantation tolerance

In an ex vivo mouse model, regulatory transplantation tolerance is not only linked to Foxp3, but also to release of leukaemia inhibitory factor (LIF) and to expression of axotrophin (also known as MARCH-7), a putative ubiquitin E3 ligase associated with feedback control of T cell activation and of T cell-derived LIF. Given this coordinate correlation with tolerance, we now ask if Foxp3 expression is influenced by LIF or by axotrophin. In spleen cells from allo-rejected mice we found that exogenous LIF reduced interferon gamma release in response to donor antigen by 50%, but LIF had no direct effect on levels of Foxp3 protein in allo-primed cells that were either tolerant, or aggressive, for donor antigen. However, we did find an effect of axotrophin on Foxp3: in the axotrophin null mouse, thymic Foxp3 transcripts were reduced compared to axotrophin wildtype littermates. To test whether these findings in the mouse were of potential significance in man we measured transcript levels of axotrophin and LIF in peripheral blood cell samples collected for a recently published clinical study concerning haematopoietic stem cell recipients. In controls, human peripheral blood CD4+CD25+cells contained significantly more FOXP3 and axotrophin than CD4+CD25-cells. In bone marrow autograft recipients, where peripheral blood cell samples directly represent both the grafted tissue and the immune response, both FOXP3 and axotrophin negatively correlated with graft versus host disease (GVHD). These data suggest that (i) thymic Foxp3+T cell development is influenced by axotrophin; and (ii) clinical auto-GVHD inversely correlates with axotrophin transcript expression as has been previously reported for FOXP3.

Undifferentiated Murine Embryonic Stem Cells Cannot Induce Portal Tolerance but May Possess Immune Privilege Secondary to Reduced Major Histocompatibility Complex Antigen Expression

Induction of donor-specific tolerance using embryonic stem (ES) cells followed by transplantation of ES cell-derived tissues from the same allogeneic strain could theoretically engender successful transplantation without immunosuppression. We sought to induce tolerance using bona fide murine ES cells in immunocompetent mice. ES cells were evaluated for the expression of markers restricted to undifferentiated cells [stage-specific embryonic antigen-1 (SSEA-1) and OCT-4] and the ability to form teratomas in immunodeficient mice. BALB/cByJ mice underwent intraportal inoculation with YC5-EYFP ES cells (129 strain; R1-derived) or saline followed by transplantation with 129X1/SvJ, CBA/J, or BALB/cByJ nonvascularized, neonatal cardiac grafts. Mice were sacrificed at graft failure and underwent histologic evaluation of transplanted grafts and lymphoid organs. ES cells and early differentiated progeny underwent real time (RT)-PCR and fluorescence-activated cell sorting (FACS) analysis to detect major histocompatibility complex (MHC) gene transcription and antigen expression. ES cells expressed markers restricted to undifferentiated cells while maintaining the ability to form teratomas in immunodeficient mice. No prolongation of allograft survival or evidence of lymphoid chimerism was observed in immunocompetent recipient mice despite hepatic teratoma formation. MHC class I, class II, and nonclassical antigens were undetectable on ES cells and early differentiated progeny despite the presence of mRNA transcripts. Class I expression was strongly upregulated upon exposure to gamma-interferon. Intraportal inoculation with murine ES cells does not produce lymphoid chimerism or induce donor-specific unresponsiveness to neonatal cardiac grafts in unmanipulated immunocompetent hosts. However, specific differentiated cell types such as ES cellderived dendritic cells, or alternate routes of ES cell administration, may be effective. ES cells appear to have immune privilege, allowing them to form teratomas in immunocompetent mice.

Clinical transplantation: current problems, possible solutions

I have attempted to summarize the progress that has been made in organ transplantation in the past 50 years since the first identical twin transplant. For those who have worked long in this area its success has been remarkable. We currently expect patients to survive the operation and more than 90% of the graft to be functioning at a year with the half-life of the graft beyond 10 years, with some patients surviving into the fifth decade after kidney transplantation with grafts from unrelated donors and the fourth decade for liver transplants. Now the main stumbling block is shortage of organ donors and this is unlikely to be solved easily. There has been a considerable increase in donations from living volunteers and also the worry of immoral and illegal practices. In the future, we can expect considerable advances in immunosuppression with more effective, less toxic drugs and in some patients induction therapy that may approach tolerance so that no maintenance therapy will eventually be needed. Cell transplantation is likely to be developed as treatment for the clinic in the next 5-10 years, but developments of transplantation from animal to man still remains unsolved and unlikely to be successful in the clinic in the near future.