Successful tolerance induction under CD40 ligation in a rodent small bowel transplant model: first report of a study with the novel antibody AH.F5

Macrophages in Organ Transplantation

Current immunosuppressive therapy has led to excellent short-term survival rates in organ transplantation. However, long-term graft survival rates are suboptimal, and a vast number of allografts are gradually lost in the clinic. An increasing number of animal and clinical studies have demonstrated that monocytes and macrophages play a pivotal role in graft rejection, as these mononuclear phagocytic cells recognize alloantigens and trigger an inflammatory cascade that activate the adaptive immune response. Moreover, recent studies suggest that monocytes acquire a feature of memory recall response that is associated with a potent immune response. This form of memory is called “trained immunity,” and it is retained by mechanisms of epigenetic and metabolic changes in innate immune cells after exposure to particular ligands, which have a direct impact in allograft rejection. In this review article, we highlight the role of monocytes and macrophages in organ transplantation and summarize therapeutic approaches to promote tolerance through manipulation of monocytes and macrophages. These strategies may open new therapeutic opportunities to increase long-term transplant survival rates in the clinic.

IFN-γ production by memory helper T cells is required for CD40-independent alloantibody responses

Cognate T-B cell interactions and CD40-CD154 costimulation are essential for productive humoral immunity against T-dependent Ags. We reported that memory CD4 T cells can deliver help to B cells and induce pathogenic IgG alloantibodies in the absence of CD40-CD154 interactions. To determine cytokine requirements for CD40-independent help, we used CD40(-/-) mice containing differentiated subsets of donor-reactive memory Th cells as heart allograft recipients. Th1 and Th17, but not Th2, memory CD4 T cells elicited high titers of anti-donor Ab. Abs induced by Th17 memory CD4 T cells had decreased reactivity against donor MHC class I molecules and inferior ability to cause complement deposition in heart allografts compared with Abs induced by Th1 cells, suggesting a requirement for IFN-γ during CD40-independent help. IFN-γ neutralization inhibited helper functions of memory CD4 T cells in both CD40(-/-) recipients and wild type recipients treated with anti-CD154 mAb. Our results suggest that IFN-γ secreted by pre-existing memory helper cells determines both isotype and specificity of donor-reactive alloantibodies and can thus affect allograft pathology. This information may be valuable for identifying transplant patients at risk for de novo development of pathogenic alloantibodies and for preventing alloantibody production in T cell-sensitized recipients.

Generation and characterization of a novel anti-rat CD40L antibody with inhibitory activities in vitro and in vivo

The CD40-CD40L interaction plays a critical role in cell-mediated immune responses. Blocking this interaction has been shown to be beneficial in the treatment of various diseases studied in murine models. Although rats are widely used to test therapeutic strategies in several disease models, a monoclonal antibody (mAb) to block the CD40-CD40L interaction in rats is not broadly available. In the present study we generated Armenian hamster fibroblasts expressing rat CD40L and used these to generate a novel anti-rat CD40L mAb (AS1). In vitro studies showed that AS1 was able to block CD40L-induced DC maturation and B cell proliferation. Most importantly, in vivo, AS1 inhibited B cell responses in a dose-dependent fashion, as measured by the production of OVA specific antibodies after subcutaneous immunization with OVA. AS1 was shown to be a powerful tool to modulate Ag presentation in vitro and in vivo. Elucidating the effect of AS1 in various rat models for human diseases will provide more insight into blocking the CD40-CD40L interaction as a therapeutic strategy to prevent human diseases.

Orthotopic small bowel transplantation in mice

On the basis of our experience with rat small bowel transplantation (SBT), we established a model of orthotopic SBT in mice. The immediate success rate was approximately 50%. Indefinite survival was obtained in the syngeneic group. The effect of different immunosuppressive treatments was also investigated: mean survival was 10.2+/-1.2 days in untreated mice receiving allogeneic grafts, 14.2+/-1.6 days with cyclosporine, 24+/-5.1 days in the rapamycin group, and 73.6+/-29.1 days with tacrolimus-treated animals. From this study, we conclude that although the model is technically difficult, it may provide an excellent tool to further investigate the physiology and immunology of small-intestinal transplantation.

Blockade of the CD154-CD40 costimulatory pathway prevents the development of experimental autoimmune glomerulonephritis

BACKGROUND

Experimental autoimmune glomerulonephritis (EAG) was induced in Wistar-Kyoto (WKY) rats by immunization with rat glomerular basement membrane (GBM) in adjuvant. This model is characterized by anti-GBM antibody production, accompanied by focal necrotizing glomerulonephritis with crescent formation. There is also glomerular infiltration by T cells and macrophages. Our hypothesis was that blocking the interaction between CD154 (CD40L) on Th cells and CD40 on antigen-presenting cells should inhibit T-cell activation, and thus the development of EAG.

METHODS

The in vivo effects of a hamster anti-rat monoclonal antibody to CD154 (AH.F5) were examined in EAG starting at day -1 prior to immunization, day +7 after immunization, or day +14 after immunization.

RESULTS

When administered from day -1 at a dose of 10 mg/kg intraperitoneally three times per week for the duration of the study (4 weeks), AH.F5 resulted in a marked reduction in circulating anti-alpha3(IV)NC1 antibodies, deposits of IgG on the GBM, albuminuria, deposits of fibrin in the glomeruli, severity of glomerular abnormalities, and numbers of glomerular T cells and macrophages. When administered from day +7 at the same dose, AH.F5 resulted in a moderate reduction in the severity of disease, while administration from day +14 had no significant effect.

CONCLUSION

These studies demonstrate for the first time that early blockade of the CD154-CD40 T-cell costimulatory pathway can prevent the development of crescentic nephritis, and that delayed treatment can reduce the severity of disease. This confirms the importance of T cell mediated immunity in the pathogenesis of EAG, and suggests that strategies targeting T-cell costimulation may provide a novel approach in the treatment of human glomerulonephritis.

Human growth hormone presented by K14hGH‐transgenic skin grafts induces a strong immune response but no graft rejection

Although immune responses leading to rejection of transplantable tumours have been well studied, requirements for epithelial tumour rejection are unclear. Here, we use human growth hormone (hGH) expressed in epithelial cells (skin keratinocytes) as a model neo-self antigen to investigate the consequences of antigen presentation from epithelial cells. Mice transgenic for hGH driven from the keratin 14 promoter express hGH in skin keratinocytes. This hGH-transgenic skin is not rejected by syngeneic non-transgenic recipients, although an antibody response to hGH develops in grafted animals. Systemic immunization of graft recipients with hGH peptides, or local administration of stimulatory anti-CD40 antibody, induces temporary macroscopic graft inflammation, and an obvious dermal infiltrate of inflammatory cells, but not graft rejection. These results suggest that a neo-self antigen expressed in somatic cells in skin can induce an immune response that can be enhanced further by induction of specific immunity systemically or non-specific immunity locally. However, immune responses do not always lead to rejection, despite induction of local inflammatory changes. Therefore, in vitro immune responses and in vivo delayed type hypersensitivity are not surrogate markers for immune responses effective against epithelial cells expressing neoantigens.

Increased apoptosis is specific for acute rejection in rat small bowel transplant

Apoptosis has been associated with several events in solid organ transplantation, including ischemia/reperfusion (IR) injury and acute rejection. To determine whether apoptosis-profiles may distinguish these two conditions, we analyzed apoptosis rates in a rat orthotopic small bowel transplant (SBT) model. SBT was performed in Lewis rats with either freshly harvested or preserved (4 h, in UW at 4 degrees C) syngeneic and allogeneic (Brown-Norway) grafts. Bowel samples were collected 2 h after reperfusion and on small bowel transplant postoperative days (POD) 1, 4, and 7. Apoptosis was detected by measuring levels of histone-associated DNA fragments and caspase 3 expression, and by determining apoptotic body counts. All markers measured 2 h after reperfusion increased profoundly in association with preservation. After a significant decrease on POD 1, apoptosis rates rose again between POD 4 and 7 only in allogeneic grafts. This distinct second increase in apoptosis may be an early and specific sign of acute rejection.

Tolerance induction by costimulator blockade in 100 cGy treated hosts with varying degrees of genetic disparity

Long-term multilineage allochimerism can be obtained in H2-mismatched B6.SJL to BALB/c transplants with host irradiation of 100 cGy, donor spleen cell pre-exposure and costimulator blockade with anti-CD40 ligand (CD40L) antibody. We evaluated this allochimerism approach in murine marrow transplants with different degrees of major histocompatibility complexe (MHC) mismatching; these include: (1) H2-mismatched transplant H2Kk to H2Kb, (2) full haplo-identical transplant H2Kbd to H2Kbk, (3) a partial haplo-identical transplant H2Kd to H2Kbd and (4) an MHC class II mismatch. Levels of chimerism increased up to 12 weeks and then stayed relatively stable up to 1 year after transplant. At 18 weeks post-transplant, the H2-mismatched, haplo-identical, partial haplo-identical and class II-mismatch transplants evidenced 17.9+/-4.4, 40.7+/-0.9, 25.1+/-4.19 and 33.7+/-3.5% donor chimerism, respectively. Dropping the anti-CD40 antibody treatment and spleen cells or changing the schedule of antibody to one injection, in haplo-identical or full-mismatched transplants resulted in no donor-derived chimerism. On the other hand, these still resulted in minor chimerism in class II-mismatched transplants. Lineage analysis of peripheral blood at 6 and 12 months post-transplant demonstrated a significant shift toward increased chimeric lymphocytes and decreased chimeric granulocytes in the full H2 as compared with haplo-identical or class II transplants. Transplantation with anti-CD40L antibody eliminated both graft-versus-leukemia and graft-versus-host disease (GVHD) and delayed lymphocyte infusion did not rescue animals from fatal leukemia. In conclusion, under the conditions of our tolerization regimen, a haplo transplant gives higher engraftment levels than a full H2 mismatch, and despite lower engraftment levels, a class II-mismatched transplant can be successfully accomplished with only 100 cGy and no CD40L blockade.