Asialo GM1(+) CD8(+) T cells play a critical role in costimulation blockade-resistant allograft rejection

Inability to induce tolerance through direct antigen presentation

Both the direct and indirect antigen presentation pathways are important mechanisms for T cell-mediated allograft rejection. Studies using knockout mice and monoclonal antibodies have demonstrated that CD4+ T cells are both necessary and sufficient for the rejection of allogeneic tissues, including skin, heart, and islet. Furthermore, combined blockade of the CD28/B7 and CD154/CD40 costimulatory pathways induces tolerance in multiple CD4+ T-cell dependent allograft models. In this study, we addressed the T-cell requirement for costimulation in direct antigen presentation. We demonstrated that class II-specific alloreactive T-cell receptor transgenic T cells were sufficient to mediate allograft rejection independent of costimulatory blockade. Analysis of the costimulatory capacity of different antigen presenting cell (APC) populations demonstrated that APCs resident within the donor skin, Langerhans cells, are potent stimulators not requiring CD28- or CD154-dependent costimulation for direct major histocompatibility complex (MHC) antigen presentation. These results complement previous work examining the role of costimulation on CD8+ T cells, supporting a model in which the effectiveness of costimulatory blockade in the setting of transplantation may be selective for the indirect pathway of MHC alloantigen presentation.

Viral abrogation of stem cell transplantation tolerance causes graft rejection and host death by different mechanisms

Tolerance-based stem cell transplantation using sublethal conditioning is being considered for the treatment of human disease, but safety and efficacy remain to be established. We have shown that mouse bone marrow recipients treated with sublethal irradiation plus transient blockade of the CD40-CD154 costimulatory pathway develop permanent hematopoietic chimerism across allogeneic barriers. We now report that infection with lymphocytic choriomeningitis virus at the time of transplantation prevented engraftment of allogeneic, but not syngeneic, bone marrow in similarly treated mice. Infected allograft recipients also failed to clear the virus and died. Postmortem study revealed hypoplastic bone marrow and spleens. The cause of death was virus-induced IFN-alphabeta. The rejection of allogeneic bone marrow was mediated by a radioresistant CD8(+)TCR-alphabeta(+)NK1.1(-) T cell population. We conclude that a noncytopathic viral infection at the time of transplantation can prevent engraftment of allogeneic bone marrow and result in the death of sublethally irradiated mice treated with costimulation blockade. Clinical application of stem cell transplantation protocols based on costimulation blockade and tolerance induction may require patient isolation to facilitate the procedure and to protect recipients.

Th1 cytokines, programmed cell death, and alloreactive T cell clone size in transplant tolerance

The Th1 cytokines IL-2 and IFN-gamma, which inhibit T cell proliferation and promote activation induced cell death, may be required to diminish alloreactive T cell numbers and to foster tolerance across full allogeneic barriers. However, we hypothesized that these cytokines might be dispensable when the alloreactive T cell clone size is relatively small, as is seen in recipients of minor-mismatched grafts. We show that alloreactive T cell clone size of C57BL/6 mice against multiple minor-mismatched 129X1/sv mice was approximately 4-9-fold smaller than that against MHC-mismatched BALB/c mice. In the MHC-mismatched combination, CD28-B7 blockade by CTLA4Ig induced long-term graft survival in wild-type recipients, but this treatment was ineffective in IFNgamma(-/-) or IL-2(-/-) recipients. In contrast, in the minor-mismatched combination, CTLA4Ig induced long-term allograft survival in wild-type, IFNgamma(-/-), and IL-2(-/-) recipients. Bcl-x(L) transgenic animals, which are defective in "passive" T cell death, are likewise sensitive to the effects of CTLA4Ig only in the setting of the minor-mismatch grafts. Therefore, the alloreactive T cell clone size is an important determinant affecting the need for Th1 cytokines and T cell death in tolerance induction. These data have implications for the design of tolerance strategies in transplant recipients with varying degrees of MHC mismatching.

Tolerance through bone marrow transplantation with costimulation blockade

The routine induction of tolerance in organ transplant recipients remains an unattained goal. The creation of a state of mixed chimerism through allogeneic bone marrow transplantation leads to robust donor-specific tolerance in several experimental models and this approach has several features making it attractive for clinical development. One of its major drawbacks, however, has been the toxicity of the required host conditioning. The use of costimulation blocking reagents (anti-CD 154 monoclonal antibodies and the fusion protein CTLA4Ig) has led to much less toxic models of mixed chimerism in which global T cell depletion of the host is no longer necessary and which has even allowed the elimination of all cytoreductive treatment when combined with the injection of very high doses of bone marrow cells. In this overview we will briefly discuss general features of tolerance induction through bone marrow transplantation, will then describe recent models using costimulation blockade to induce mixed chimerism and will review the mechanisms of tolerance found with these regimens. Finally we will attempt to identify issues related to the clinical introduction of bone marrow transplantation with costimulation blockade which remain unresolved.

On CD28/CD40 ligand costimulation, common gamma-chain signals, and the alloimmune response

Activation and robust expansion of naive T cells often require T cell costimulatory signals and T cell growth factors. However, the precise growth and costimulation requirements for activation and expansion of CD4(+) and CD8(+) T cells in vivo in allograft response are still not clearly defined. In the present study, we critically examined the role of CD28/CD40 ligand (CD40L) costimulation and the common gamma-chain (gamma(c)) signals, a shared signaling component by receptors for all known T cell growth factors (i.e., IL-2, IL-4, IL-7, IL-9, IL-15, IL-21), in activation and expansion of CD4(+) and CD8(+) T cells in the allogeneic hosts. We found that CD28/CD40L costimulation and the gamma(c) signals are differentially involved in proliferation and clonal expansion of CD4(+) and CD8(+) T cells in response to alloantigen stimulation. CD8(+) T cells are highly dependent on the gamma(c) signals for survival, expansion, and functional maturation, whereas in vivo expansion of alloreactive CD4(+) T cells is largely gamma(c) independent. T cell costimulation via CD28 and CD40L, however, is necessary and sufficient for activation and expansion of CD4(+) T cells in vivo. In a skin transplant model, blocking both CD28/CD40L and the gamma(c) pathways induced prolonged skin allograft survival. Our study provides critical insights that the CD4 and CD8 compartments are most likely governed by distinct mechanisms in vivo, and targeting both costimulatory and gamma(c) signals may be highly effective in certain cytopathic conditions involving activation of both CD4(+) and CD8(+) T cells.

CD154-dependent priming of diabetogenic CD4(+) T cells dissociated from activation of antigen-presenting cells

We followed the fate of K(d)- or I-A(g7)-restricted beta cell-autoreactive T cells in monoclonal TCR-transgenic NOD mice expressing or lacking CD154. 8.3-NOD.RAG-2(-/-)/CD154(-/-) mice, which bear autoreactive CD8(+) T cells, developed diabetes with the same incidence and tempo as 8.3-NOD.RAG-2(-/-)/CD154(+) mice. Recruitment of CD154(-/-) 8.3-CD8(+) CTL was accelerated by CD154(+)CD4(+) T cells, by expression of a B7.1 transgene in beta cells or by treatment of the mice with CpG-DNA or an agonistic anti-CD40 antibody. In contrast, the autoreactive CD4(+) T cells maturing in 4.1-NOD.RAG-2(-/-) mice lost their diabetogenic potential if they lacked CD154, even in the presence of CD154(+)CD4(+) T cells, B7.1 molecules on beta cells, CpG-DNA treatment, or systemic CD40 ligation. These results demonstrate the existence of a novel, CD154-dependent pathway of CD4(+) T cell activation that is independent of CD40-mediated activation of APCs.

A rendezvous before rejection: where do T cells meet transplant antigens?

Interactions between recipient T cells and donor endothelial graft cells may be an important mechanism for both acute and chronic rejection of vascularized allografts. This finding provides a starting point for investigations to develop novel ways of inducing long-lasting immunologic tolerance to donor antigens.

Non-hematopoietic allograft cells directly activate CD8+ T cells and trigger acute rejection: an alternative mechanism of allorecognition

Despite evidence that human non-hematopoietic cells, such as vascular endothelium, can activate allogeneic T lymphocytes in vitro, the prevailing view has been that hematopoietic antigen-presenting cells are required to trigger alloimmune responses in vivo. Here we report that mouse non-hematopoietic cells activate alloreactive CD8+ T lymphocytes in vitro and in vivo. We also show that vascularized cardiac allografts are acutely rejected via CD8+ direct allorecognition even if the alloantigen is not presented by hematopoietic professional antigen-presenting cells. Because activation of alloreactive CD8+ T cells by donor-type non-hematopoietic cells can continue for the life of the allograft, these findings present a new clinically relevant mechanism of allorecognition and should be taken into consideration when developing strategies to prevent allograft vasculopathy or to induce tolerance.

Prolonged blockade of CD40-CD40 ligand interactions by gene transfer of CD40Ig results in long-term heart allograft survival and donor-specific hyporesponsiveness, but does not prevent chronic rejection

Previous work on blockade of CD40-CD40 ligand interaction in mice and primates with anti-CD40 ligand mAbs has resulted in a moderate prolongation of allograft survival without the development of true allograft tolerance. In this study, we show in rats that adenovirus-mediated gene transfer of CD40Ig sequences into the graft resulted in prolonged (>200 days) expression of CD40Ig and in long-term (>300 days) survival. Recipients expressing CD40Ig displayed strongly (>90%) inhibited mixed leukocyte reactions and alloantibody production at early (days 5 and 17) and late time points (>100 day) after transplantation, but showed limited inhibition of leukocyte infiltration and cytokine production as evaluated by immunohistology at early time points (day 5). Recipients of long-surviving hearts showed donor-specific hyporesponsiveness since acceptance of second cardiac allografts was donor specific. Nevertheless, long-term allografts (>100 days) displayed signs of chronic rejection vasculopathy. Occluded vessels showed leukocyte infiltration, mainly composed of CD4(+) and CD8(+) cells, macrophages, and mast cells. These recipients also showed antidonor CTL activity. Recipients expressing CD40Ig did not show nonspecific immunosuppression, as they were able to mount anticognate immune responses that were partially inhibited at early time points and were normal thereafter. We conclude that gene transfer-mediated expression of CD40Ig resulted in a highly efficient inhibition of acute heart allograft rejection in rats. This treatment induced donor-specific inhibition of certain alloreactive mechanisms in the short-, but not the long-term, which resulted in long-term survival of allografts concomitant with the development of chronic rejection.

The role of the IL-2 pathway in costimulation blockade-resistant rejection of allografts

Blockade of the CD40 and CD28 costimulatory pathways significantly prolongs allograft survival; however, certain strains of mice (i.e., C57BL/6) are relatively resistant to the effects of combined CD40/CD28 blockade. We have previously shown that the costimulation blockade-resistant phenotype can be attributed to a subset of CD8+ T cells and is independent of CD4+ T cell-mediated help. Here we explore the role of the IL-2 pathway in this process using mAbs against the high affinity IL-2R, CD25, and IL-2 in prolonging skin allograft survival in mice receiving combined CD40/CD28 blockade. We have also investigated the effects of treatment on effector function by assessment of cytotoxicity and the generation of IFN-gamma-producing cells in response to allogeneic stimulators as well as proliferation in an in vivo graft-vs-host disease model. We find that additional blockade of either CD25 or IL-2 significantly extends allograft survival beyond that in mice receiving costimulation blockade alone. This correlates with diminished frequencies of IFN-gamma-producing allospecific T cells and reduced CTL activity. Anti-CD25 therapy also synergizes with CD40/CD28 blockade in suppressing proliferative responses. Interestingly, depletion of CD4+ T cells, but not CD8+ cells, prevents prolongation in allograft survival, suggesting an IL-2-independent role for regulation in extended survival.

CTLA4ig induces long-term graft survival of allogeneic skin grafts and totally inhibits T-cell proliferation in LFA-1-deficient mice

BACKGROUND

It was recently shown that some strains of mice are capable of rejecting transplants independently of B7 and CD40L signaling and that this rejection is mediated by CD8(+) T cells. LFA-1 is known to be important for CD8(+) T cell activation and cytotoxicity. Therefore, blockade of LFA-1 could be important in overcoming costimulation blockade, CD8(+) T-cell-mediated, resistant rejection. The purpose of this study was to define the effect of combined blockade of the LFA-1 and B7 costimulation pathways on the alloimmune response in mice.

METHODS

Allogeneic skin transplantation was performed using BALB/c mice as donors and C57BL/6J wild-type or LFA-1-deficient (CD11a(-/-)) mice as recipients. CTLA4Ig or anti-LFA-1 was administered either as an induction or a prolonged therapy. Mixed lymphocyte reactions were conducted to study the effect of CTLA4Ig on T-cell proliferation in CD11a(-/-) mice.

RESULTS AND CONCLUSIONS

Administration of CTLA4Ig completely inhibits CD11a(-/-) T-cell proliferation in response to alloantigens and significantly improved skin allograft survival in CD11a(-/-) mice. Prolonged treatment of wild-type recipient mice with CTLA4Ig and anti-LFA-1 increased median survival time to 45.5 days compared with 16 days after induction therapy, but it was not sufficient to induce indefinite allograft survival in this model.

The CD154-CD40 costimulatory pathway in transplantation

The CD154-CD40 pathway is one of the critical costimulatory pathways that are required for full activation of T cells during alloimmune responses. Blockade of this pathway with anti-CD154 antibodies has been reported to prolong allograft survival in experimental transplantation models and to induce tolerance in some instances. However, anti-CD154 monotherapy cannot induce tolerance in "stringent" models such as skin and islet transplantation and is not sufficient to prevent chronic graft vasculopathy in vascularized organ transplantation. Therefore, combined therapies of anti-CD154 antibodies plus donor-specific transfusion, bone marrow infusion, or B7 blockade by CTLA4-Ig have been tried, and synergistic effects for tolerance induction have been reported. Furthermore, the efficacy of CD154 blockade in primate models has been confirmed for islet and kidney transplantation. The mechanisms of CD154 blockade in vivo include CTLA4-dependent anergy or regulation, T-cell apoptosis, and induction of regulatory cells. Finally, anti-CD154 antibody therapy has been reported to result in unexpected thromboembolic complications in both primates and humans, although the etiology of these conditions remains unclear. In addition, not all antibodies cause this side effect. Clinical trials with humanized anti-CD154 monoclonal antibodies are underway in severe autoimmune diseases, but its development in transplantation is unclear at this time.

The role of TNF receptor and TNF superfamily molecules in organ transplantation

The rapid increase in the number of molecules demonstrated to regulate immune responses has provided new opportunities for manipulation of the recipient immune response to transplanted organs. Molecules belonging to the TNF receptors and TNF superfamily are increasingly recognized as playing a major role in the regulation of immune responses to tumor, viral, and autoantigens. The mechanisms by which these molecules regulate immune responses are diverse. TNF receptor-related molecules have been shown to control the development of secondary lymphoid organs, affect the activation and survival of T cells and antigen presenting cells, and alter cytokine and chemokine production. An increasing amount of data suggest that some TNFR superfamily members are particularly important for the function of CD8+ T cells. Based on our current understanding of these molecules it seems highly likely that strategies that target selected TNFR/TNF superfamily molecules will be useful for controlling or preventing the rejection of transplanted organs and tissues.

CTLA-41g in combination with anti-CD40L prolongs xenograft survival and inhibits anti-gal ab production in GT-Ko mice

The generation of GT-Ko mice has provided unique opportunities to study allograft and xenograft rejection in the context of anti-alpha1,3-Gal antibody (anti-Gal Ab) responses. In this study we used the allotransplantation model of C3H hearts into galactosyltransferase-deficient (GT-Ko) mice and the xenotransplantation model of baby Lewis rat hearts into GT-Ko mice to investigate the ability of CTLA-41g in combination with anti-CD40L mAb to control graft rejection and anti-Gal Ab production. Murine CTLA-41g or anti-CD40L monotherapy prolonged allograft survival, and the combination of these reagents was most immunosuppressive. However short-term treatment with murine cytotoxic T lymphocyte associated antigen-4 (muCTLA-41g) and/or CD40 ligand (CD154) monoclonal antibodies (anti-CD40L mAbs) was unable to induce indefinite allograft survival. CTLA-4-immunoglobulin fusion protein (CTLA-41g) or anti-CD40L monotherapy only marginally prolonged xenograft survival; the combination of human CTLA-41g and anti-CD40L significantly prolonged xenograft survival (74days), while the combination of murine CTLA-41g and anti-CD40L resulted in graft survival of >120days. CTLA-41g or anti-CD40L monotherapy or the combination of these agents inhibited the production of alloAbs, including anti-Gal Abs. CTLA-41g or anti-CD40L monotherapy partially controlled xenoAb and anti-Gal Ab production, while the combination was more effective. These observations corroborate our previous observations that humoral, including anti-Gal Ab, responses and rejection following allograft or concordant xenograft transplantation in GT-Ko mice are T-cell dependent and can be controlled by costimulation blockade.

Simultaneous inhibition of B7 and LFA-1 signaling prevents rejection of discordant neural xenografts in mice lacking CD40L

Transplantation of embryonic human neural tissue can restore dopamine neurotransmission and improve neurological function in patients with Parkinson's disease. Logistical and ethical factors limit the availability of human embryonic allogeneic tissue. Embryonic xenogeneic neural tissue from porcine donors is an alternative form of donor tissue, but effective immunomodulatory techniques are warranted for neural xenotransplantation to become clinically feasible. We transplanted embryonic porcine ventral mesencephalic tissue into the brains of adult untreated C57BL/6 mice, untreated CD40L-/-mice and CD40L-/-mice that received injections of anti-LFA-1, CTLA41g or both compounds. Double-treated CD40L-/-mice had large grafts with high numbers of dopaminergic neurons 4 wk after transplantation. The grafts were completely devoid of lymphocytes, macrophages and activated microglia. Untreated C57BL/6 mice had rejected their grafts. Untreated CD40L-/-mice and CD40L-/-mice treated with monotherapy of anti-LFA-1 or CTLA41g had smaller grafts and more microglial and lymphocytic infiltration than double-treated CD40L-/-mice. We conclude that immunomodulation with concomitant inhibition of LFA-1 and B7 signaling in the perioperative period in CD40L-/-mice prevented the rejection of discordant neural xenografts. The treatment most likely reduced antigen presenting capacity and interfered with the costimulatory signaling needed for T cell activation to occur.

Skin allograft maintenance in a new synchimeric model system of tolerance

Treatment of mice with a single donor-specific transfusion plus a brief course of anti-CD154 mAb uniformly induces donor-specific transplantation tolerance characterized by the deletion of alloreactive CD8+ T cells. Survival of islet allografts in treated mice is permanent, but skin grafts eventually fail unless recipients are thymectomized. To analyze the mechanisms underlying tolerance induction, maintenance, and failure in euthymic mice we created a new analytical system based on allo-TCR-transgenic hemopoietic chimeric graft recipients. Chimeras were CBA (H-2(k)) mice engrafted with small numbers of syngeneic TCR-transgenic KB5 bone marrow cells. These mice subsequently circulated a self-renewing trace population of anti-H-2(b)-alloreactive CD8+ T cells maturing in a normal microenvironment. With this system, we studied the maintenance of H-2(b) allografts in tolerized mice. We documented that alloreactive CD8+ T cells deleted during tolerance induction slowly returned toward pretreatment levels. Skin allograft rejection in this system occurred in the context of 1) increasing numbers of alloreactive CD8+ cells; 2) a decline in anti-CD154 mAb concentration to levels too low to inhibit costimulatory functions; and 3) activation of the alloreactive CD8+ T cells during graft rejection following deliberate depletion of regulatory CD4+ T cells. Rejection of healed-in allografts in tolerized mice appears to be a dynamic process dependent on the level of residual costimulation blockade, CD4+ regulatory cells, and activated alloreactive CD8+ thymic emigrants that have repopulated the periphery after tolerization.

Mechanisms involved in the establishment of tolerance through costimulatory blockade and BMT: lack of requirement for CD40L-mediated signaling for tolerance or deletion of donor-reactive CD4+ cells

We have previously shown that high levels of multiline-age mixed hematopoietic chimerism and systemic T-cell tolerance can be achieved in mice without myeloablation through the use of anti-CD40L and costimulatory blockade alone (plus CTLA4Ig) or with recipient CD8 depletion and allogeneic bone marrow transplantation. Chimeric mice permanently accept donor skin grafts (> 100 days), and rapidly reject third-party grafts. The mechanisms by which costimulatory blockade facilitates the engraftment of allogeneic hematopoietic cells have not been defined. To further understand the in vivo mechanisms by which the administration of anti-CD40L mAb facilitates the engraftment of donor bone marrow and rapidly tolerizes CD4+ T cells, we analyzed the establishment of chimerism and tolerance in CD40L -/- mice. We demonstrate here that anti-CD40L mAb treatment is required only to prevent CD40L/CD40 interactions, and that no signal to the T cell through CD40L is necessary for the induction of CD4+ tolerance. Peripheral deletion of donor-reactive CD4+ T cells occurs rapidly in CD40L -/- mice receiving bone marrow transplantation (BMT), indicating that this deletion in the presence of anti-CD40L is not due to targeting of activated CD4+ cells by the antibody. Complete CD4+ cell tolerance is observed by both skin graft acceptance and in vitro assays before deletion is complete, indicating that additional mechanisms play a role in inducing CD4+ T-cell tolerance as the result of BMT in the presence of CD40/CD40L blockade.

Cutting edge: membrane lymphotoxin regulates CD8(+) T cell-mediated intestinal allograft rejection

Blocking the CD28/B7 and/or CD154/CD40 costimulatory pathways promotes long-term allograft survival in many transplant models where CD4(+) T cells are necessary for rejection. When CD8(+) T cells are sufficient to mediate rejection, these approaches fail, resulting in costimulation blockade-resistant rejection. To address this problem we examined the role of lymphotoxin-related molecules in CD8(+) T cell-mediated rejection of murine intestinal allografts. Targeting membrane lymphotoxin by means of a fusion protein, mAb, or genetic mutation inhibited rejection of intestinal allografts by CD8(+) T cells. This effect was associated with decreased monokine induced by IFN-gamma (Mig) and secondary lymphoid chemokine (SLC) gene expression within allografts and spleens respectively. Blocking membrane lymphotoxin did not inhibit rejection mediated by CD4(+) T cells. Combining disruption of membrane lymphotoxin and treatment with CTLA4-Ig inhibited rejection in wild-type mice. These data demonstrate that membrane lymphotoxin is an important regulatory molecule for CD8(+) T cells mediating rejection and suggest a strategy to avoid costimulation blockade-resistant rejection.

Analysis of the CD40 and CD28 pathways on alloimmune responses by CD4+ T cells in vivo

BACKGROUND

Blockade of the CD40 and CD28 pathways is a powerful strategy to inhibit CD4-mediated alloimmune responses. In this study, we examine the relative roles of the CD40 and CD28 pathways on CD4-mediated allograft rejection responses, and further characterize the role of these pathways on CD4+ T-cell activation, priming for cytokine production, and cell proliferation in response to alloantigen in vivo.

METHODS

BALB/c skin allografts were transplanted onto C57BL/6 Rag 1-/- recipients reconstituted with CD4 cells from CD28-/- or CD40L-/- donors. The popliteal lymph node assay was used to study the role of these pathways on CD4-cell activation and priming in vivo. To investigate the role of CD40 and CD28 blockade on CD4-cell proliferation, the fluorescein dye carboxyfluorescein diacetate succinimidyl ester was used. We performed heterotopic cardiac transplantation using CD40-/- mice to evaluate the role of CD40 on donor versus recipient cells in CD4-mediated rejection.

RESULTS

B6 Rag 1-/- recipients reconstituted with CD28-/- CD4+ T cells acutely rejected allografts (median survival time 15 days), whereas recipients reconstituted with CD40L-/- CD4+ T cells had significantly prolonged survival of BALB/c skin grafts (MST 71 days). CD40L blockade was equivalent to or inferior to CD28 blockade in inhibition of in vivo CD4-cell activation, priming for cytokine production, and proliferation responses to alloantigen. BALB/c recipients depleted of CD8 cells promptly rejected donor B6 CD40-/- cardiac allografts, whereas B6 CD40-/- recipients depleted of CD8 cells had significantly prolonged survival of BALB/c wild-type cardiac allografts.

CONCLUSIONS

The CD40/CD40L pathway, but not the CD28/B7 pathway, is critical for CD4-mediated rejection responses, however, the responsible mechanisms remain unclear.

An antagonist IL-15/Fc protein prevents costimulation blockade-resistant rejection

IL-15 is a powerful T cell growth factor (TCGF) with particular importance for the maintenance of CD8(+) T cells. Because costimulation blockade does not result in universal tolerance, we hypothesized that "escape" from costimulation blockade might represent a CD8(+) and IL-15/IL-15R(+)-dependent process. For this analysis, we have used an IL-15 mutant/Fcgamma2a protein, a potentially cytolytic protein that is also a high-affinity receptor site specific antagonist for the IL-15Ralpha receptor protein, as a therapeutic agent. The IL-15-related fusion protein was used as monotherapy or in combination with CTLA4/Fc in murine islet allograft models. As monotherapies, CTLA4/Fc and an IL-15 mutant/Fcgamma2a were comparably effective in a semiallogeneic model system, and combined treatment with IL-15 mutant/Fcgamma2a plus CTLA4/Fc produced universal permanent engraftment. In a fully MHC-mismatched strain combination known to be refractory to costimulation blockade treatment, combined treatment with both fusion proteins proved to be highly effective; >70% of recipients were tolerized. The analysis revealed that the IL-15 mutant/Fc treatment confers partial protection from both CD4(+) and CD8(+) T cell graft infiltration. In rejections occurring despite CTLA4/Fc treatment, concomitant treatment with the IL-15 mutant/Fcgamma2a protein blocked a CD8(+) T cell-dominated rejection processes. This protection was linked to a blunted proliferative response of alloreactive T cells as well silencing of CTL-related gene expression events. Hence, we have demonstrated that targeting the IL-15/IL-15R pathway represents a new and potent strategy to prevent costimulation blockade-resistant CD8(+) T cell-driven rejection.

The role of CD154 in organ transplant rejection and acceptance

CD154 plays a critical role in determining the outcome of a transplanted organ. This simple statement is amply supported by experimental evidence demonstrating that anti-CD154 antibodies are potent inhibitors of allograft rejection in many rigorous transplant models. Unfortunately, despite intensive investigation over the past ten years, the precise mechanisms by which antibodies against CD154 exert their anti-rejection effects have remained less obvious. Though originally classified with reference to B-cell function, CD154-CD40 interactions have also been shown to be important in T cell-antigen-presenting cell interactions. Accordingly, CD154 has been classified as a T-cell co-stimulatory molecule. However, mounting data suggest that treatment with anti-CD154 antibodies does not simply block costimulatory signals, but rather that the antibodies appear to induce signalling in receptor-bearing T cells. Other data suggest that anti-CD154 effects may be mediated by endothelial cells and possibly even platelets. In fact, the current literature suggests that CD154 can either stimulate or attenuate an immune response, depending upon the model system under study. CD154 has secured a fundamental place in transplant biology and general immunology that will no doubt be the source of considerable investigation and therapeutic manipulation in the coming decade.

Appropriate targets for monoclonal antibodies in the induction of transplantation tolerance

There are many routes to exploiting tolerance processes to ensure long-term graft survival. Complete tolerance although attractive as a goal, may not be the most practical in the clinic. Instead simple and low-impact procedures that harness tolerance processes used in conjunction with low doses of immunosuppressive drugs may prove the most reliable and user-friendly of approaches.

Critical role for IL-4 in the development of transplant arteriosclerosis in the absence of CD40-CD154 costimulation

Blockade of the CD40-CD154 pathway can inhibit CD4(+) T cell activation but is unable to prevent immune responses mediated by CD8(+) T cells. However, even in the absence of CD8(+) T cells, inhibition of the CD40-CD154 pathway is insufficient to prevent the development of transplant arteriosclerosis. This study investigated the mechanisms of transplant arteriosclerosis in the absence of the CD40 pathway. C57BL/6 CD40(-/-) (H2(b)) recipients were transplanted with MHC-mismatched BALB/c (H2(d)) aortas. Transplant arteriosclerosis was evident in both CD40(-/-) and CD40(+/-) mice (intimal proliferation was 59 +/- 5% for CD40(-/-) mice vs 58 +/- 4% for CD40(+/-) mice) in the presence or absence of CD8(+) T cells (intimal proliferation was 46 +/- 7% for CD40(-/-) anti-CD8-treated mice vs 50 +/- 10% for CD40(+/-) anti-CD8-treated mice), confirming that CD8(+) T cells are not essential effector cells for the development of this disease. In CD40(-/-) recipients depleted of CD8(+) T cells, the number of eosinophils infiltrating the graft was markedly increased (109 +/- 24 eosinophils/grid for CD40(-/-) anti-CD8-treated mice vs 28 +/- 7 for CD40(+/-) anti-CD8-treated mice). The increased presence of eosinophils correlated with augmented intragraft production of IL-4. To test the hypothesis that IL-4 was responsible for the intimal proliferation, CD8 T cell-depleted CD40(-/-) recipients were treated with anti-IL-4 mAb. This resulted in significantly reduced eosinophil infiltration into the graft (12 +/- 5 eosinophils/grid for CD40(-/-) anti-CD8(+), anti-IL-4-treated mice vs 109 +/- 24 for CD40(-/-) anti-CD8-treated mice), intragraft eotaxin, CCR3 mRNA production, and the level of intimal proliferation (18 +/- 5% for CD40(-/-) anti-CD8(+)-, anti-IL-4-treated mice vs 46 +/- 7% for CD40(-/-) anti-CD8-treated mice). In conclusion, elevated intragraft IL-4 production results in an eosinophil infiltrate and is an important mechanism for CD8(+) T cell-independent transplant arteriosclerosis in the absence of CD40-CD154 costimulation.

CD8 T cell-mediated rejection of intestinal allografts is resistant to inhibition of the CD40/CD154 costimulatory pathway

BACKGROUND

Disruption of the CD40/CD154 pathway inhibits rejection in numerous models. The importance of this pathway on intestinal allograft rejection was examined in this study.

METHODS

Intestinal grafts from B6C3F1 mice transplanted into C57BL/6 recipients were assessed histologically for rejection.

RESULTS

The monoclonal antibody to CD154, MR1, failed to inhibit rejection in wild-type mice. Similarly, CD154-/- recipient mice rejected intestinal allografts. MR1 did inhibit early rejection in CD8-/- mice, but had no effect in CD4-/- recipients. All MR1-treated CD8-/- recipients eventually developed rejection. No benefit was observed when blockade of the CD40/CD154 pathway by MR1 was combined with blockade of the CD28/B7 pathway by mCTLA4Ig.

CONCLUSIONS

These data suggest that CD4+ T cells mediating intestinal allograft rejection may be more dependent upon the CD40/CD154 pathway than CD8+ T cells. This finding highlights the importance of identifying agents that suppress CD8+ T cell-mediated rejection.

Different mechanisms of cardiac allograft rejection in wildtype and CD28-deficient mice

Although CD28 blockade results in long-term cardiac allograft survival in wildtype mice, CD28-deficient mice effectively reject heart allografts. This study compared the mechanisms of allogeneic responses in wildtype and CD28-deficient mice. Adoptive transfer of purified CD28-deficient T cells into transplanted nude mice resulted in graft rejection. However, this model demonstrated that the allogeneic T cell function was severely impaired when compared with wildtype T cells, despite similar survival kinetics. Cardiac allograft rejection depended on both CD4+ and CD8+ T cell subsets in CD28-deficient mice, whereas only CD4+ T cells were necessary in wildtype recipients. These results suggested that CD8+ T cells were more important in CD28-deficient than wildtype mice. In addition to the CD8+ T cell requirement, allograft rejection in CD28-deficient mice was dependent on a sustained presence of CD4+ T cells, whereas it only required the initial presence of CD4+ T cells in wildtype mice. Taken together, these data suggest that CD4+ T cells from CD28-deficient mice have impaired responses to alloantigen in vivo, thus requiring long-lasting cooperation with CD8+ T cell responses to facilitate graft rejection. These results may help to explain the failure to promote graft tolerance in some preclinical and clinical settings.

Variation in adenovirus transgene expression between BALB/c and C57BL/6 mice is associated with differences in interleukin-12 and gamma interferon production and NK cell activation

The innate immune response against replication-defective adenoviruses (Ad) is poorly defined. We and others have previously observed striking differences in the rate at which the Ad vector itself or the virus encoding a variety of transgenes is eliminated in different mouse strains. Here, we report that Ad infection of BALB/ mice is associated with sixfold-higher levels of serum alanine aminotransferase and that Ad transgenes induce two- to threefold-higher levels of intrahepatic NK cells and NK activity compared to C57BL/6 mice. The increase in NK activation in BALB/c mice was associated with approximately 4-fold higher level of mRNA expression of a newly described NKG2 receptor activator, H-60, as well as increased expression of interleukin-12 and gamma interferon mRNAs in BALB/c mice compared to C57BL/6 mice. NK depletion in BALB/c mice or defective NK function in C3H beige mice extended transgene expression compared to their appropriate controls, and attenuation of NK together with CD8 T-cell function had a synergistic effect. These findings indicate that there are intrinsic differences in the innate immune responses of different mouse strains to Ad and Ad transgenes and that NK cells, in cooperation with CD8 T cells, play a pivotal role in the early extinction of transgene expression in BALB/c mice.

Inhibition of natural killer cells results in acceptance of cardiac allografts in CD28-/- mice

Successful transplantation of allogeneic organs is an important objective in modern medicine. However, sophisticated immune defense mechanisms, primarily evolved to combat infections, often work against medical transplantation. To investigate the roles of natural and adaptive immune responses in transplant rejection, we functionally inactivated key effector systems of the innate (NK cells) and the adaptive immune system (CD28-mediated costimulation of T cells) in mice. Neither of these interventions alone led to acceptance of allogeneic vascularized cardiac grafts. In contrast, inhibition of NK-receptor-bearing cells combined with CD28-costimulation blockade established long-term graft acceptance. These results indicate a concerted interplay between innate and adaptive immune surveillance for graft rejection. Thus we suggest that inactivation of NK-receptor-bearing cells could be a new strategy for successful survival of solid-organ transplants.

The effect of CD28/B7 blockade on alloreactive T and B cells after liver cell transplantation

BACKGROUND

Hepatocyte cell lines are beginning to be developed as universal donors for isolated liver cell transplantation, which is a less invasive method than orthotopic liver transplantation for treatment of metabolic liver disease. The immune response to isolated liver cell transplantation and its modification by costimulatory blockade are as yet not well delineated.

METHODS

Adenovirus expressing CTLA4Ig was used to study blockade of the costimulatory CD28/B7 pathway in murine models of hepatocyte transplantation, and the effects on alloreactive T and B cells were studied.

RESULTS

CTLA4Ig delayed rejection of subcutaneously administered C57L-derived murine hepatoma cells in CBA/J recipients for >50 days. Activation and cytokine secretion by allospecific CD4+ and CD8+ T cells were initially blocked by CTLA4Ig; delayed rejection was associated with tumor infiltration by CD8+ T cells that did not secrete interferon-gamma. CTLA4Ig failed to block transplant rejection in primed mice, indicating that memory effector T cells were resistant to its action. In contrast, CTLA4Ig suppressed both naive and memory alloreactive B cells. High levels of CTLA4Ig mediated acceptance of hepatoma cells delivered directly into the spleen. However, isolated primary C57BL/6 mouse hepatocytes delivered into the spleen were rejected with only moderately delayed kinetics.

CONCLUSIONS

Transplant antigenicity, transplant site, and CTLA4Ig dose all affected the survival of transplanted liver cells. CD8+ T cells are significant mediators of hepatocyte transplant rejection and are relatively resistant to costimulatory blockade with CTLA4Ig. Strategies to specifically antagonize CD8+ T cells or to modulate MHC class I expression in association with costimulatory blockade by CTLA4Ig may enhance the clinical feasibility of transplanting allogeneic hepatocytes.

Maturation of antigen-presenting cells is compromised in HLA-G transgenic mice

The human MHC class Ib antigen HLA-G is thought to regulate maternal immune responses during pregnancy. Here we show that expression of HLA-G in transgenic mice diminished cellular immunity by inhibiting maturation of myelomonocytic cells into functional antigen-presenting cells (APC). Skin allografts applied to HLA-G transgenic mice survived longer and resultant T cell responses were less potent compared to control mice. T cells from HLA-G mice responded normally to allogeneic APC and immunohistological analyses of spleen revealed no marked abnormalities. However, spontaneous outgrowths of myeloid cells were observed when bone marrow or splenocytes from HLA-G mice were cultured in vitro, but functionally competent APC did not develop spontaneously in bone marrow cultures supplemented with granulocyte macrophage colony stimulating factor (GM-CSF). Addition of lipopolysaccharide (LPS) to GM-CSF-derived bone marrow cultures rescued APC maturation. Studies using HLA-G tetrameric reagents revealed that HLA-G-specific binding activity was associated with CD11c(+) myelomonocytic cells, while binding to lymphoid and NK cell subsets was undetectable. These data show that spontaneous maturation of functionally competent dendritic cells (DC) is compromised in HLA-G mice. We hypothesize that HLA-G inhibits maturation of DC via receptor-mediated interactions with myelomonocytic precursors, which render immature DC precursors unable to receive signals from activated T cells.

Role of CD8(+) cells in the progression of murine adriamycin nephropathy

BACKGROUND

Many studies have shown that interstitial inflammation in human and experimental renal disease is characterized by T-cell infiltration, but published data on the involvement of inflammatory cell subsets in progressive tubulointerstitial lesions are often conflicting. A previous study suggested a role for cytotoxic T lymphocytes in the damaging effect of CD4(+) T-cell depletion in murine adriamycin (ADR) nephropathy, a model of focal segmental glomerulosclerosis (FSGS), and tubulointerstitial inflammation. The aim of this study was to investigate the role of CD8(+) cells in this model.

METHODS

Male BALB/c mice were treated with five intraperitoneal injections of anti-CD8 monoclonal antibody (mAb), beginning from five days after ADR treatment, when overt proteinuria was established. Seven mice in each of groups A (ADR + mAb), B (ADR only), and C (saline treated, age matched) were sacrificed at week 6. Changes in renal function and histopathological features were assessed. Tubulointerstitial inflammation and glomerular inflammation were examined immunohistochemically.

RESULTS

mAb treatment reduced CD8(+) cell levels to <2% of normal in spleen. Proteinuria in group A was no different from that in group B at week 6, but was markedly higher than in group C. Creatinine clearance was significantly ameliorated by anti-CD8 treatment (71.8 +/- 4.9 microL/min vs. 29.2 +/- 2.8 in group B and 81.9 +/- 3.7 in group C). Morphometric analysis showed less FSGS in group A compared with group B (6.5 +/- 1.9 vs. 13.0 +/- 2.8, P < 0.001), as well as less tubular atrophy (indicated by increased ratio of tubule cell height to tubular diameter, 0.25 +/- 0.24 in group A vs. 0.04 +/- 0.02 in group B, P < 0.05). CD8 depletion also reduced interstitial expansion (6.3 +/- 2.2% vs. 16.4 +/- 3.1 in group B, P < 0.001) and fibrosis (P < 0.01). Macrophage infiltration in tubulointerstitium was less in group A than in group B (P = 0.052). The number of interstitial CD4(+) cells appeared to increase after anti-CD8 treatment, but was not statistically different between groups A and B.

CONCLUSION

Anti-CD8 treatment protects against renal functional and structural injury in this murine model of chronic proteinuric renal disease.

CD4 T cell-mediated alloresistance to fully MHC-mismatched allogeneic bone marrow engraftment is dependent on CD40-CD40 ligand interactions, and lasting T cell tolerance is induced by bone marrow transplantation with initial blockade of this pathway

Costimulatory blockade can be used to promote allogeneic marrow engraftment and tolerance induction, but on its own is not 100% reliable. We sought to determine whether one or the other of the CD4 or CD8 T cell subsets of the recipient was primarily responsible for resistance to allogeneic marrow engraftment in mice receiving costimulatory blockade, and to use this information to develop a more reliable, minimal conditioning regimen for induction of mixed chimerism and transplantation tolerance. We demonstrate that a single anti-CD40 ligand mAb treatment is sufficient to completely overcome CD4 cell-mediated resistance to allogeneic marrow engraftment and rapidly induce CD4 cell tolerance, but does not reliably overcome CD8 CTL-mediated alloresistance. The data suggest that costimulation, which activates alloreactive CTL, is insufficient to activate alloreactive CD4 cells when the CD40 pathway is blocked. The addition of host CD8 T cell depletion to anti-CD40 ligand treatment reliably allows the induction of mixed chimerism and donor-specific skin graft tolerance in 3 Gy-irradiated mice receiving fully MHC-mismatched bone marrow grafts. Thus, despite the existence of multiple costimulatory pathways and pathways of APC activation, our studies demonstrate an absolute dependence on CD40-mediated events for CD4 cell-mediated rejection of allogeneic marrow. Exposure to donor bone marrow allows rapid tolerization of alloreactive CD4 cells when the CD40 pathway is blocked, leading to permanent marrow engraftment and intrathymic tolerization of T cells that develop subsequently.

Long-term acceptance of allografts by in vivo gene transfer of regulatable adenovirus vector containing CTLA4IgG and loxP

CTLA4IgG was shown to inhibit the costimulatory signal for T cell activation by interfering with the ligation of CD28 and B7-1 or B7-2. To inhibit various immune responses including acute cellular rejection of allografts, a certain level of serum CTLA4IgG should be maintained for an appropriate period. We previously reported on an adenovirus vector containing CTLA4IgG, which we designated Adex1CACTLA4IgG. Adex1CACTLA4IgG was able to maintain a significant level of serum CTLA4IgG for a long period on intravenous injection, which in turn inhibited various immune responses including protective immunity against infectious agents. To overcome the inhibitory effect, we constructed a new adenovirus vector, Adex1CALoxCTLA4IgGLox, by cloning CTLA4IgG cDNA between two loxP sequences under the control of the CAG promoter. We demonstrated that the administration of adenovirus vector containing Cre recombinase gene (Adex1CACre) at the desired time induced Cre-mediated recombination within a gene derived from Adex1CALoxCTLA4IgGLox vector, and the cDNA of CTLA4IgG was excised from the transduced gene and terminated the expression of CTLA4IgG in vitro and in vivo. More importantly, we also demonstrated that the long-term acceptance of allografts was achieved after the termination of CTLA4IgG expression, while the immune response against adenovirus was restored.

FasL is important in costimulation blockade-resistant skin graft rejection

BACKGROUND

Simultaneous blockade of the CD40 and CD28 costimulatory pathways is effective in prolonging allograft survival in murine and primate models. Recent data suggest that intact apoptotic pathways are crucial for the induction of hyporesponsiveness by costimulation blockade. We have studied the impact of fas/fasL signaling, an important T cell apoptotic pathway, on the effects of costimulation blockade. Methods. Wild type, lpr (fas deficient), and gld (fasL deficient), mice were used as donors and recipients in the murine skin graft model. Allograft survival was compared in untreated and costimulation blockade (500 microg anti-CD40L and 500 microg CTLA4-Ig, days 0, 2, 4, 6) treated recipients. In some recipients, CD4+ T cells were depleted using rat anti-murine CD4 (100 microg day -3, -2, -1, and weekly).

RESULTS

gld mice treated with costimulation blockade enjoy a significantly greater increase in skin allograft survival than do wild-type mice. This effect is not replicated using lpr donors or recipients. Experiments in which CD4+ cells were depleted demonstrate that fasL is not necessary for CD8-mediated allograft rejection, and that depletion of CD4+ cells eliminates some of the survival advantage induced by costimulation blockade.

CONCLUSIONS

FasL is not required for the establishment of costimulation blockade induced hyporesponsiveness, but rather appears to be required for normal costimulation blockade resistant rejection. Fas expression is not critical for costimulation blockade resistant rejection, suggesting that fasL may be interacting with other receptors. Further, it appears that CD4+ cells are important in the maintenance of allograft protection induced by costimulation blockade in this model.

Targeting signal 1 through CD45RB synergizes with CD40 ligand blockade and promotes long term engraftment and tolerance in stringent transplant models

The induction and maintenance of allograft tolerance is a daunting challenge. Although combined blockade of CD28 and CD40 ligand (CD40L)-costimulatory pathways prevents allograft rejection in some murine models, this strategy is unable to sustain engraftment in the most immunogenic allograft and strain combinations. By targeting T cell activation signals 1 and 2 with the novel combination of anti-CD45RB and anti-CD40L, we now demonstrate potent enhancement of engraftment in C57BL/6 recipients that are relatively resistant to costimulatory blockade. This combination significantly augments the induction of tolerance to islet allografts and dramatically prolongs primary skin allograft survival. Compared with either agent alone, anti-CD45RB plus anti-CD40L inhibits periislet infiltration by CD8 cells, B cells, and monocytes; inhibits Th1 cytokines; and increases Th2 cytokine expression within the graft. These data indicate that interference with activation signals one and two may provide synergy essential for prolonged engraftment in situations where costimulatory blockade is only partially effective.

Genetic characterization of strain differences in the ability to mediate CD40/CD28-independent rejection of skin allografts

Simultaneous blockade of the CD40 and CD28 T cell costimulatory pathways effectively promotes skin allograft survival in C3H/HeJ mice, extending median survival times (MSTs) beyond 100 days. This strategy is markedly less effective in C57BL/6 mice, with MSTs ranging between 20 and 30 days. In this study, we investigate the underlying genetic causes of these distinct phenotypes. Using H-2 congenic mice, we show that the genetic basis for the varied responses between these two strains is independent of the H-2 locus and T cell precursor frequency. C57BL/6 mice treated with costimulation blockade are able to generate allospecific CTL- and IFN-gamma-producing T cells within 3-4 wk posttransplant, whereas mice with a C3H background generate neither CTL- nor IFN-gamma-producing cells. Thus, differences appear to be in the generation of the immune response and not T cell homing. Strain differences in costimulation blockade-induced hyporesponsiveness persist in the absence of CD4(+) T cells, implying a direct effect on CD8(+) T cells. We demonstrate that genetic differences are important in cells of hemopoietic origin and that the costimulation blockade-resistant phenotype is dominant. Analysis of BXH recombinant inbred strains indicates that multiple loci contribute to the phenotype, and that the blockade resistance loci are preliminarily linked to 17 markers on four chromosomes. We conclude that strain variation in allograft MSTs following CD40/CD28 blockade results from the ability of CD8(+) T cells in some strains to use alternative modes of costimulation to mount an effective alloresponse.

Intragraft interleukin-4 mRNA expression after short-term CD154 blockade may trigger delayed development of transplant arteriosclerosis in the absence of CD8+ T cells

BACKGROUND

It has recently been shown that, although anti-CD154 induces CD4+ T-cell tolerance, it is unable to prevent allograft rejection mediated by CD8+ T cells. We have also shown that anti-CD154 monotherapy does not protect the graft from the development of transplant arteriosclerosis even in the absence of CD8+ T cells. This study was designed to investigate and characterize possible mechanisms responsible for the development of transplant arteriosclerosis after CD154 blockade in the absence of CD8+ T cells.

METHODS

C57BL/6 (H2b) recipients received a fully MHC-mismatched BALB/c donor aorta (H2d). Animals were either treated with anti-CD154 monoclonal antibody (mAb) in the presence or absence of CD8 T cells. Histology, morphometric measurements, immunohistochemistry, and the production of alloantibodies (IgM, IgG1, IgG2a) were analyzed on days 14, 30, and 50 after transplantation. Cytokine production within the graft was investigated by competitive reverse transcription-polymerase chain reaction on day 14.

RESULTS

Combined treatment with anti-CD154 and a depleting CD8 mAb resulted in a delay in the development of transplant arteriosclerosis (intimal proliferation: 33+/-10% vs. 67+/-11% untreated control, day 30) but ultimately did not prevent its progression (intimal proliferation: 55+/-10% vs. 78+/-9% untreated control, day 50). Although there was a significant decrease in the number of CD4+, CD11b+, and CD40+ graft-infiltrating cells and a reduction in the formation of donor-specific IgG1 alloantibodies in recipients treated with anti-CD154 and anti-CD8 mAbs, mRNA for interleukin (IL)-4 was increased, suggesting a shift in the intragraft cytokine profile towards a Th2-like pattern.

CONCLUSIONS

Our data provide evidence that short-term CD154 blockade is insufficient to prevent transplant arteriosclerosis, even in combination with CD8+ T-cell depletion. Moreover, the increased expression of the Th2 cytokine interleukin-4 within the graft may be responsible for the development of transplant arteriosclerosis in the long term.

CD40-CD40 ligand-independent activation of CD8+ T cells can trigger allograft rejection

In experimental transplantation, blockade of CD40-CD40 ligand (CD40L) interactions has proved effective at permitting long-term graft survival and has recently been approved for clinical evaluation. We show that CD4+ T cell-mediated rejection is prevented by anti-CD40L mAb therapy but that CD8+ T cells remain fully functional. Furthermore, blocking CD40L interactions has no effect on CD8+ T cell activation, proliferation, differentiation, homing to the target allograft, or cytokine production. We conclude that CD40L is not an important costimulatory molecule for CD8+ T cell activation and that following transplantation donor APC can activate recipient CD8+ T cells directly without first being primed by CD4+ T cells.

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.

Allogeneic bone marrow transplantation with co-stimulatory blockade induces macrochimerism and tolerance without cytoreductive host treatment

Allogeneic bone marrow transplantation (in immunocompetent adults) has always required cytoreductive treatment of recipients with irradiation or cytotoxic drugs to achieve lasting engraftment at levels detectable by non-PCR-based techniques ('macrochimerism' or 'mixed chimerism'). Only syngeneic marrow engraftment at such levels has been achieved in unconditioned hosts. This requirement for potentially toxic myelosuppressive host pre-conditioning has precluded the clinical use of allogeneic bone marrow transplantation for many indications other than malignancies, including tolerance induction. We demonstrate here that treatment of naive mice with a high dose of fully major histocompatibility complex-mismatched allogeneic bone marrow, followed by one injection each of monoclonal antibody against CD154 and cytotoxic T-lymphocyte antigen 4 immunoglobulin, resulted in multi-lineage hematopoietic macrochimerism (of about 15%) that persisted for up to 34 weeks. Long-term chimeras developed donor-specific tolerance (donor skin graft survival of more than 145 days) and demonstrated ongoing intrathymic deletion of donor-reactive T cells. A protocol of high-dose bone marrow transplantation and co-stimulatory blockade can thus achieve allogeneic bone marrow engraftment without cytoreduction or T-cell depletion of the host, and eliminates a principal barrier to the more widespread use of allogeneic bone marrow transplantation. Although efforts have been made to minimize host pre-treatment for allogeneic bone marrow transplantation for tolerance induction, so far none have succeeded in eliminating pre-treatment completely. Our demonstration that this can be achieved provides the rationale for a safe approach for inducing robust transplantation tolerance in large animals and humans.

Anti-LFA-1 therapy induces long-term islet allograft acceptance in the absence of IFN-gamma or IL-4

mAb therapy directed against a variety of cell surface accessory molecules has been effectively utilized to prolong allograft acceptance in various models of tissue and organ transplantation. The purpose of this study was to determine whether transient therapy directed against the adhesion molecule LFA-1 (CD11a) was sufficient to induce donor-specific tolerance to pancreatic islet allografts. Anti-LFA-1 monotherapy was found to be efficacious in inducing long-term islet allograft acceptance in multiple donor-recipient strain combinations. Graft acceptance following anti-LFA-1 therapy was not simply due to clonal ignorance of donor Ags in that the majority of recipients bearing established islet allografts resisted rejection induced by immunization with donor-type APCs. Furthermore, donor-specific tolerance from anti-LFA-1-treated animals could be transferred to secondary immune-deficient animals. Taken together, these results indicated that transient anti-LFA-1 monotherapy resulted in donor-specific tolerance. In vitro, functionally tolerant animals retained normal anti-donor reactivity as assessed by proliferative, cytotoxic, and cytokine release assays that demonstrated that tolerance was not secondary to general clonal deletion or anergy of donor-reactive T cells. Finally, anti-LFA-1 treatment was effective in both IL-4-deficient and IFN-gamma-deficient recipients, indicating that neither of these cytokines are universally required for allograft acceptance. These results suggest that anti-adhesion-based therapy can induce a nondeletional form of tolerance that is not overtly dependent on the prototypic Th1 and Th2 cytokines, IFN-gamma and IL-4, respectively, in contrast to results in other transplantation models.