Cutting edge: CD4+CD25+ alloantigen-specific immunoregulatory cells that can prevent CD8+ T cell-mediated graft rejection: implications for anti-CD154 immunotherapy

Transplantation tolerance: lessons from experimental rodent models

Immunological tolerance or functional unresponsiveness to a transplant is arguably the only approach that is likely to provide long-term graft survival without the problems associated with life-long global immunosuppression. Over the past 50 years, rodent models have become an invaluable tool for elucidating the mechanisms of tolerance to alloantigens. Importantly, rodent models can be adapted to ensure that they reflect more accurately the immune status of human transplant recipients. More recently, the development of genetically modified mice has enabled specific insights into the cellular and molecular mechanisms that play a key role in both the induction and maintenance of tolerance to be obtained and more complex questions to be addressed. This review highlights strategies designed to induce alloantigen specific immunological unresponsiveness leading to transplantation tolerance that have been developed through the use of experimental models.

Dependency of the trans vivo delayed type hypersensitivity response on the action of regulatory T cells: implications for monitoring transplant tolerance

BACKGROUND

The trans vivo delayed-type hypersensitivity (DTH) assay has been used for monitoring the immune status of clinical transplant recipients. Here we tested the hypothesis that the assay can reveal control of allograft rejection by CD25CD4 regulatory T cells (Treg).

METHODS

CBA.Ca (H2) recipients of heterotopic C57BL/10 (H2, B10) heart transplants were untreated or pretreated with anti-CD4 antibody and donor-specific blood. This protocol has been shown previously to induce operational tolerance to alloantigens that is dependent on CD25CD4 Treg. Four weeks after transplantation leukocytes were harvested and used for the trans vivo DTH assay. Cells were stimulated with irradiated B10 leukocytes or subcellular antigen and injected into ear pinnae of immune deficient CB17.SCID.beige hosts.

RESULTS

Stimulation of leukocytes from recipients rejecting B10 cardiac allografts with recall alloantigen caused a "strong" swelling response, whereas similar stimulation of leukocytes from operationally tolerant mice resulted in significantly less swelling (n=17; P=0.003). When CD25 T cells were depleted from "tolerant" leukocytes, the swelling response triggered was similar to that obtained using cells from rejecting animals (P<0.001), while coinjection of purified CD25CD4 cells from tolerant recipients with rejecting cells suppressed the swelling response (P=0.007) to the level observed with cells from tolerant mice. Both the effector and the regulatory response were triggered by indirect allorecognition and blocking experiments showed that the regulation observed was dependent on cytoxic T lymphocyte antigen (CTLA) 4.

CONCLUSION

The trans vivo DTH assay can be used to reveal regulation mediated by CD25CD4 T cells and is CTLA4 dependent.

An investigation to assess the potential of CD25highCD4+ T cells to regulate responses to donor alloantigens in clinically stable renal transplant recipients

Regulatory T cells are enriched within CD25(high)CD4(+) leukocytes, however their role in renal transplant recipients with stable function vs. recipients with biopsy-proven chronic allograft dysfunction remains unclear. We therefore studied the number, phenotype, and function of CD25(high)CD4(+) cells in the peripheral blood of 30 renal transplant recipients of living-related grafts, comprising 15 rejection-free recipients with stable graft function (Group A) and 15 with biopsy-proven chronic graft dysfunction (Group B). A higher absolute number of CD25(high)CD4(+) cells were present in the peripheral blood of rejection-free recipients (Group A) vs. those recipients with chronic graft dysfunction (Group B) (P = 0.019); but there was no significant difference with healthy volunteers (P = 0.084). In carboxyfluorescein diacetate succinimidyl ester-mixed leukocyte culture assays, depletion of CD25(high)CD4(+) revealed active regulation in 11 (74%) of 15 rejection-free recipient samples (Group A) in response to donor- but not third party-leukocytes, whereas no regulatory activity was observed in any samples from recipients with chronic graft dysfunction (Group B). In conclusion, these data provide evidence for the presence of an increased number of CD25(high)CD4(+) T cells with donor-specific regulatory activity in the peripheral blood of renal transplant recipients with stable graft function compared with recipients with chronic graft dysfunction.

Rapamycin enhances the number of alloantigen-induced human CD103+CD8+ regulatory T cells in vitro

BACKGROUND

Regulatory T cells (T(reg) cells) may be operational in both the induction and maintenance of transplantation tolerance. We recently showed that alloantigen-induced CD103+ CD8+ T cells strongly suppressed T-cell proliferation in mixed lymphocyte culture (MLC) via a contact-dependent mechanism. CD103 directs T lymphocytes to their ligand E-cadherin, which is expressed on renal tubular epithelial cells, and CD103+ CD8+ T cells have been described to be present in late renal allograft rejection.

METHODS

We studied the influence of prednisolone, cyclosporin, tacrolimus, CD25 monoclonal antibodies, rapamycin, and mycophenolate mofetil (MMF) on the development and functional activity of alloantigen-activated CD103+ CD8+ T cells in MLC.

RESULTS

Calcineurin inhibitors, MMF, and CD25mAb did not influence the number of CD103 expressing CD8+ T cells. In contrast, corticosteroids diminished CD103 expression on alloactivated CD8+ T cells, which appeared to be caused by their inhibitory action on myeloid dendritic cells. Addition of rapamycin to allocultures led to an increased percentage of CD103+ CD8+ alloreactive T cells. Moreover, in the presence of rapamycin, these cells tended to show higher suppressive capacity.

CONCLUSIONS

Alloreactive CD103+ CD8+ T(reg) cells may expand and exert their suppressive function during immunosuppressive treatment with rapamycin. These data are relevant in the design of immunosuppressive drug regimens intended to induce and/or maintain transplantation tolerance.

Allosuppressive Donor CD4+CD25+ Regulatory T Cells Detach from the Graft and Circulate in Recipients after Liver Transplantation

Organ transplantation (Tx) results in a transfer of donor leukocytes from the graft to the recipient, which can lead to chimerism and may promote tolerance. It remains unclear whether this tolerance involves donor-derived regulatory T cells (Tregs). In this study, we examined the presence and allosuppressive activity of CD4+CD25+Foxp3+ Tregs in perfusates of human liver grafts and monitored the cells presence in the circulation of recipients after liver Tx. Vascular perfusions of 22 liver grafts were performed with University of Wisconsin preservation and albumin solutions. Flow cytometric analysis revealed that perfusate T cells had high LFA-1 integrin expression and had a reversed CD4 to CD8 ratio compared with control blood of healthy individuals. These findings indicate that perfusate cells are of liver origin and not derived from residual donor blood. Further characterization of perfusate mononuclear cells showed an increased proportion of CD4+CD25+CTLA4+ T cells compared with healthy control blood. Increased percentages of Foxp3+ cells, which were negative for CD127, confirmed the enrichment of Tregs in perfusates. In MLR, CD4+CD25+ T cells from perfusates suppressed proliferation and IFN-gamma production of donor and recipient T cells. In vivo within the first weeks after Tx, up to 5% of CD4+CD25+CTLA4+ T cells in recipient blood were derived from the donor liver. In conclusion, a substantial number of donor Tregs detach from the liver graft during perfusion and continue to migrate into the recipient after Tx. These donor Tregs suppress the direct pathway alloresponses and may in vivo contribute to chimerism-associated tolerance early after liver Tx.

Transfer of Allograft Specific Tolerance Requires CD4+CD25+T Cells but Not Interleukin-4 or Transforming Growth Factor–β and Cannot Induce Tolerance to Linked Antigens

BACKGROUND

The mechanisms by which CD4+T cells, especially CD4+ CD25+T cells, transfer allograft specific tolerance are poorly defined. The role of cytokines and the effect on antigen-presenting cells is not resolved.

METHODS

Anti-CD3 monoclonal antibody (mAb) therapy induced tolerance to PVG heterotopic cardiac transplantation in DA rats. Peripheral CD4+T cells or CD4+ CD25+ and CD4+ CD25-T cell subsets were adoptively transferred to irradiated DA hosts grafted with PVG heart grafts. For specificity studies, tolerant CD4+T cells were transferred to hosts with Lewis or (PVGxLewis)F1 heart grafts. Cytokine mRNA induction and the requirement for interleukin (IL)-4 and transforming growth factor (TGF)-beta in the transfer of tolerance was assessed.

RESULTS

CD4+T cells transferred specific tolerance and suppressed naïve CD4+T cells capacity to effect rejection of PVG but not Lewis grafts. (PVGxLewis)F1 grafts had a major rejection episode but recovered. Later these hosts accepted PVG but not Lewis skin grafts. Adoptive hosts restored with tolerant or naïve cells had similar levels of mRNA expression for all Th1 and Th2 cytokines and effector molecules assayed. Transfer of tolerance by CD4+T cells was not blocked by mAb to IL-4 or TGF-beta. CD4+ CD25-T cells from either naïve or tolerant hosts effected rejection. In contrast neither tolerant nor naïve CD4+ CD25+T cells restored rejection.

CONCLUSIONS

Specific tolerance transfer required CD4+ containing CD4+ CD25+T cells. An inflammatory response with induction of mRNA for Th1 and Th2 cytokines plus cytotoxic effector molecules occurred, but IL-4 and TGF-beta were not essential. Inhibition of antigen presenting cells was not the sole mechanism as there was no linked tolerance.

Resting B Cells Suppress Tumor Immunity via an MHC Class-II Dependent Mechanism

Resting B cells have been variously shown to induce direct tolerance of antigen specific CD8+ T cells, induce T-cell anergy via transforming growth factor-beta production, down-regulate interleukin-12 production by dendritic cells (DC) and influence TH1/TH2 differentiation via the production of regulatory cytokines. Through these mechanisms, B cells can exert a regulatory function in in vivo models of T-cell immunity including, experimental autoimmune encephalitis and rheumatoid arthritis. Here, we show that the resting B cells inhibit the ability of DC vaccination to provide protection from tumor growth. Inhibition of DC induced immunity by B cells was independent of presentation of major histocompatibility molecule (MHC) class-I bound tumor antigen but dependent on B-cell expression of MHC class-II. Administration of B cells did not alter the ability of DC to migrate from the injection site or impair DC-T cell interactions within the draining lymph node. The inhibitory effect of B cells was lost when they were activated by CD40L and partially reversed by the depletion of CD4+/CD25+ regulatory T cells. Together our findings indicate that the resting B cells are capable of limiting CD8+ T-cell effector function induced by DC vaccination via a mechanism that is dependent on the expression of MHC class-II molecules.

Generation of HSP60-specific regulatory T cell and effect on atherosclerosis

Although CD4(+)CD25(+) regulatory T cells are pivotal in the suppression of autoimmunity, little is known about the effect of antigen-specific regulatory T cells on the formation of atheromatous plaques. Here, we describe the induction of heat-shock protein 60 (HSP60)-specific CD4(+)CD25(high) T cells by rapamycin (RPM)-treated immature dendritic cells in vitro and explore their effect on plaques in apolipoprotein E-deficient mice. Rapamycin-treated bone marrow-derived dendritic cells (DC) were immature, expressing a low level of co-stimulation factors CD86 and CD80. Naive CD4(+) T cells expressed high levels of CD25 and forkhead box P3 (Foxp3) after incubation with rapamycin-treated and HSP60-loaded DC and displayed moderate antigen-specific, IL-10-independent inhibitory function in vitro. After adoptive transfer, HSP60-specific CD4(+)CD25(high) T cells inhibited the formation of plaques, while ovalbumin-specific cells did not. These findings suggest that RPM-treated DC can induce antigen-specific CD4(+)CD25(high) Treg cells that have inhibitory activity in vitro and prevent the development of plaques in vivo.

Diversity of regulatory T cells to control arthritis

During follow-up of the suppressive functions of CD4+ T helper (Th) 2 cells in recent years, the suppressive capacities of newly recognised CD4+ Th cells with more widespread suppressive potential have been extensively investigated. These Th cells, collectively termed regulatory T cells, are characterised by the secretion of specific cytokines, such as interleukin (IL)-10 (Tr1 Th cells), transforming growth factor (TGF)beta (Th3 cells) or the constitutive expression of CD25 (naturally occurring T regulatory cells, nTregs). The balance of these regulatory T cells with pro-inflammatory effector T cells, such as Th1 (interferon (IFN)gamma secreting), Th17 (IL-17 secreting) and CD25- Th cells, has been shown to be of pivotal importance for the development and persistence of autoimmune diseases. The high potential of regulatory T cells (in particular nTregs), to efficiently suppress several arthritic responses both in humans and in animal models of arthritis, make them therapeutic targets of interest in arthritic conditions such as rheumatoid arthritis.

TGF-beta: a mobile purveyor of immune privilege

Functionally barricaded immune responses or sites of immune privilege are no longer considered dependent on specific anatomical considerations, but rather, they can develop in any location where immunoregulatory cells congregate and express or release products capable of deviating the host response to foreign antigens. Among the pivotal molecules involved in orchestrating these ectopic sites of immune suppression is transforming growth factor-beta (TGF-beta), a secreted and cell-associated polypeptide with a multiplicity of actions in innate and adaptive immunity. While beneficial in initiating and controlling immune responses and maintaining immune homeostasis, immunosuppressive pathways mediated by TGF-beta may obscure immune surveillance mechanisms, resulting in failure to recognize or respond adequately to self, foreign, or tumor-associated antigens. CD4+CD25+Foxp3+ regulatory T cells represent a dominant purveyor of TGF-beta-mediated suppression and are found in infiltrating tumors and other sites of immune privilege, where they influence CD8+ T cells; CD4+ T-helper (Th)1, Th2, and Th17 cells; natural killer cells; and cells of myeloid lineage to choreograph and/or muck up host defense. Defining the cellular sources, mechanisms of action, and networking that distinguish the dynamic establishment of localized immune privilege is vital for developing strategic approaches to diminish or to embellish these tolerogenic events for therapeutic benefit.

Costimulatory Blockade Induces Hyporesponsiveness in T Cells that Recognize Alloantigen via Indirect Antigen Presentation

BACKGROUND

Blockade of T cell costimulation by treatment with donor-specific transfusion (DST) and anti-CD154 monoclonal antibody (mAb) induces prolonged allograft survival in mice. This effect is due in part to deletion of host CD8 and CD4 T cells that recognize alloantigen by direct presentation. The fate of host CD4 T cells that recognize alloantigen by indirect presentation, however, is unclear.

METHODS

We studied Tg361 TCR transgenic CD4 T cells that recognize alloantigen by indirect presentation. Carboxyfluorescein diacetate, succinimidyl ester-labeled Tg361 cells were adoptively transferred into syngeneic nontransgenic recipients and their fate in the peripheral blood, spleen, and lymph nodes following treatment with DST and anti-CD154 was analyzed.

RESULTS

Treatment of mice with DST plus anti-CD154 mAb does not delete Tg361 CD4 T cells, but instead renders them hyporesponsive to rechallenge with alloantigen. Mice circulating hyporesponsive CD4 T cells also fail to reject skin allografts. The hyporesponsive state of the T cells is not reversed by the addition of interleukin-2, anti-CD28 mAb, or an agonistic anti-CD134 mAb in the presence of antigen. These T cells are capable of activation, however, as evidenced by in vitro proliferation in response to anti-CD3 mAb.

CONCLUSIONS

These results demonstrate that costimulation blockade can induce hyporesponsiveness of host CD4 T cells recognizing alloantigens by indirect presentation, thus prolonging graft survival by a mechanism that does not involve deletion of alloreactive T cells.

Dendritic cells expand antigen-specific Foxp3+ CD25+ CD4+ regulatory T cells including suppressors of alloreactivity

Thymic derived naturally occurring CD25+ CD4+ T regulatory cells (Tregs) suppress immune responses, including transplantation. Here we discuss the capacity of dendritic cells (DCs) to expand antigen-specific Tregs, particularly polyclonal Tregs directed to alloantigens. Initial studies have shown that mature DCs are specialized antigen-presenting cells (APCs) for expanding antigen-specific CD25+ CD4+ Tregs from TCR transgenic mice. When triggered by specific antigen, these Tregs act back on immature DCs to block the upregulation of CD80 and CD86 costimulatory molecules. More recently, DCs have been used to expand alloantigen-specific CD25+ CD4+ Tregs from the polyclonal repertoire in the presence of interleukin-2 (IL-2). Allogeneic DCs are much more effective than allogeneic spleen cells for expanding CD25+ CD4+ Tregs. The DC-expanded Tregs continue to express high levels of Foxp3, even without supplemental IL-2, whereas spleen cells poorly sustain Foxp3 expression. When suppressive activity is tested, relatively small numbers of DC-expanded CD25+ CD4+ Tregs exert antigen-specific suppression in the mixed leukocyte reaction (MLR), blocking immune responses to the original stimulating strain 10 times more effectively than to third party stimulating cells. DC-expanded Tregs also retard graft versus host disease (GVHD) across full major histocompatibility complex (MHC) barriers. In vitro and in vivo, the alloantigen-specific CD25+ CD4+ Tregs are much more effective suppressors of transplantation reactions than polyclonal populations. We suggest that the expansion of Tregs from a polyclonal repertoire via antigen-presenting DCs will provide a means for antigen-specific control of unwanted immune reactions.

Role and mechanisms of CD4+CD25+ regulatory T cells in the induction and maintenance of transplantation tolerance

To gain transplantation tolerance between donor organs and hosts is the ultimate goal of all sorts of organ transplantations. Induction of regulatory T cells has been demonstrated to lead to transplantation tolerance. This paper will review subsets of regulatory T cells, the role and mechanisms of CD4(+)CD25(+) regulatory T cells (Tregs) in graft rejection and tolerance, pathway used by Tregs to recognized alloantigens, pathways of Tregs homing into the graft and effects of immunosuppression on Tregs. It was well known that Tregs play a pivotal role in transplantation tolerance. The mechanisms by which Tregs exert their regulatory effect in the induction and maintenance of transplantation tolerance, anthropogenically, consist of physical cell-to-cell contact with potential target cells, autocrine and paracrine properties. ICAM-1, TGF-beta, CTLA-4, GITR and OX40 (CD134), etc. are involved in the regulatory function of Tregs through cell-to-cell contact mechanism. IL-10 and TGF-beta are two important soluble mediators involved in the autocrine mechanism by which Tregs exert their regulatory function. Paracrine properties refer to re-educate potentially destructive alloresponsive T cells to gain regulatory function. All that discussed above could illustrate, at least partially, how naturally occurring Tregs exert their regulatory function in vivo as they constitute only 5-10% of peripheral CD4(+) T cells.

In vitro–expanded donor alloantigen–specific CD4+CD25+ regulatory T cells promote experimental transplantation tolerance

CD4+CD25+ regulatory T (Treg) cells play a critical role in the induction and maintenance of peripheral immune tolerance. In experimental transplantation models in which tolerance was induced, donor-specific Treg cells could be identified that were capable of transferring the tolerant state to naive animals. Furthermore, these cells appeared to have indirect allospecificity for donor antigens. Here we show that in vivo alloresponses can be regulated by donor alloantigen-specific Treg cells selected and expanded in vitro. Using autologous dendritic cells pulsed with an allopeptide from H2-Kb, we generated and expanded T-cell lines from purified Treg cells of CBA mice (H2k). Compared with fresh Treg cells, the cell lines maintained their characteristic phenotype, suppressive function, and homing capacities in vivo. When cotransferred with naive CD4+CD25− effector T cells after thymectomy and T-cell depletion in CBA mice that received CBK (H2k+Kb) skin grafts, the expanded Treg cells preferentially accumulated in the graft-draining lymph nodes and within the graft while preventing CBK but not third-party B10.A (H2k+Dd) skin graft rejection. In wild-type CBA, these donor-specific Treg cells significantly delayed CBK skin graft rejection without any other immunosuppression. Taken together, these data suggest that in vitro–generated tailored Treg cells could be considered a therapeutic tool to promote donor-specific transplant tolerance.

The impact of memory T cells on rejection and the induction of tolerance

Accumulating evidence suggests that alloreactive memory T cells (Tm) may be generated in transplant recipients that have not previously been exposed to alloantigen through mechanisms such as cross-reactivity and homeostatic proliferation. The presence of Tm correlates with both acute and chronic rejection episodes and, furthermore, may be responsible for the failure to induce tolerance in large animal and clinical settings. A clearer understanding of how Tm function and their requirements to mount an effective response to alloantigen will be key to further attempts to translate tolerance induction protocols from the experimental setting to the clinic.

Anergic lymphocytes generated by blocking CD28 and ICOS pathways in vitro prolong rat cardiac graft survival

Regulatory cells may play a pivotal role in inducing and maintaining transplantation tolerance. We investigated the mechanism of anergic lymphocytes with regulatory cell potential generated in vitro by ICOS and CD 28 co-stimulatory blockades as a source of cellular therapy for treating allograft rejection. Anergic lymphocytes were generated by a mixed lymphocyte reaction consisting of DA splenocytes as the stimulator and Lewis splenocytes as the responder in the presence of anti-ICOS mAb and rCTLA-4I g. Immunoregulatory effects of these lymphocytes were evaluated by secondary MLR and using other various stimulations. DA heart was transplanted into 7.5 Gy-irradiated Lewis rat after intravenous administration of these cells and/or Lewis spleen lymphocytes. We observed that these lymphocytes were not only anergic to alloantigen and polyclonal stimulations but also exhibited regulative activity to inhibit the alloreactive T-cell response. Our adoptive transfer studies revealed that irradiated recipients that received both anergic lymphocytes and naIve Lewis lymphocytes had significantly prolonged DA cardiac graft survival (mean 17.5 days) compared with a group that received Lewis lymphocytes alone (mean 10.8 days). Furthermore, some of the recipients accepted the graft indefinitely after receiving anergic lymphocytes alone (>100 days). These results demonstrated that anergic lymphocytes with regulatory activities can be generated through blocking co-stimulatory signals, CD 28 and ICOS, simultaneously in vitro, and may advance a new immunomodulatory strategy for preventing allorejection in organ transplantation.

Regulatory T cell-mediated transplantation tolerance

The existence of naturally occurring regulatory T cells in normal hosts and their pivotal role in maintaining both auto- and allo-tolerance have direct implications on the therapy of autoimmune disorders and for achieving immunosuppression-free allotransplantation. Among the various forms of regulatory T cells described, CD4(+)CD25(+) T cells have emerged as one of the most potent tolerogenic subsets. In this review, we discuss the molecular basis of development and function of these regulatory T cells and their potential role in the context of chronic lung allograft rejection.

Negative costimulatory molecules: the proximal of regulatory T cells?

Regulatory T cells (Tregs) are a central mechanism of immune regulation. It is well known that their regulatory effect is antigen-specific and depends on cell-cell contact. In addition, some immunological phenomenon such as linked suppression and iDC-induced tolerance are related with Tregs. But the surface markers, which reliably distinguish Treg from other T cell populations, and the regulatory mechanism still remain to be further revealed. Negative costimulatory molecule (NCM) family is one natural intrinsic mechanism that delivers the negative signal into cytoplasma to modulate immunoresponse and its expression can be induced not only on the immune cells but also on the parenchymal cells. Based on the present knowledge, we hypothesize NCMs are the specific surface markers to define Tregs. Tregs are one kind of activated T cells with high expression of NCM receptor and have the capability to induce NCM ligands expression on the membrane of APCs and the target cells of the activated cells. The NCM receptor-ligand complexes deliver negative signal into lymphocytes to regulate the immune response. This hypothesis remains to be fully elucidated.

CD4+CD25+ regulatory T cells inhibit natural killer cell functions in a transforming growth factor-beta-dependent manner

Tumor growth promotes the expansion of CD4⁺CD25⁺ regulatory T (T reg) cells that counteract T cell–mediated immune responses. An inverse correlation between natural killer (NK) cell activation and T reg cell expansion in tumor-bearing patients, shown here, prompted us to address the role of T reg cells in controlling innate antitumor immunity. Our experiments indicate that human T reg cells expressed membrane-bound transforming growth factor (TGF)–β, which directly inhibited NK cell effector functions and down-regulated NKG2D receptors on the NK cell surface. Adoptive transfer of wild-type T reg cells but not TGF-β^−/− T reg cells into nude mice suppressed NK cell–mediated cytotoxicity, reduced NKG2D receptor expression, and accelerated the growth of tumors that are normally controlled by NK cells. Conversely, the depletion of mouse T reg cells exacerbated NK cell proliferation and cytotoxicity in vivo. Human NK cell–mediated tumor recognition could also be restored by depletion of T reg cells from tumor-infiltrating lymphocytes. These findings support a role for T reg cells in blunting the NK cell arm of the innate immune system.

CTLA-4 Engagement and Regulatory CD4+CD25+T Cells Independently Control CD8+-Mediated Responses under Costimulation Blockade

Blockade of costimulatory signals is a promising therapeutic target to prevent allograft rejection. In this study, we sought to characterize to what extent CTLA-4 engagement contributes to the development of transplantation tolerance under the cover of CD40/CD40L and CD28/CD86 blockade. In vitro, we found that inhibition of the primary alloresponse and induction of alloantigen hyporesponsiveness by costimulation blockade was abrogated by anti-CTLA-4 mAb. In addition, regulatory CD4(+)CD25(+) T cells (T(REG)) were confirmed to play a critical role in the induction of hyporesponsiveness by anti-CD40L and anti-CD86 mAb. Our data indicated that CTLA-4 engagement is not required for activation or suppressor function of T(REG). Instead, in the absence of either CTLA-4 signaling or T(REG), CD8(+) T cell division was enhanced, whereas the inhibition of CD4(+) T cell division by costimulation blockade remained largely unaffected. In vivo, the administration of additional anti-CTLA-4 mAb abrogated anti-CD40L- and anti-CD86 mAb-induced cardiac allograft survival. Correspondingly, rejection was accompanied by enhanced allograft infiltration of CD8(+) cells. We conclude that CTLA-4 signaling and T(REG) independently cooperate in the inhibition of CD8(+) T cell expansion under costimulation blockade.

CD4(+) CD25(+) T cells prevent arthritis associated with Borrelia vaccination and infection

CD4(+) CD25(+) T cells are a population of regulatory T cells associated with control of arthritis in anti-interleukin-17 antibody-treated Borrelia-vaccinated and challenged gamma interferon-deficient mice. Here, we present direct evidence that adoptive transfer of enriched CD4(+) CD25(+) T cells from these mice can prevent the development of arthritis in Borrelia-vaccinated and challenged mice. These findings establish a major role for CD4(+) CD25(+) T cells in the prevention of arthritis in Borrelia-vaccinated and challenged animals.

Targeting LFA-1 and cd154 suppresses the in vivo activation and development of cytolytic (cd4-Independent) CD8+ T cells

Short-term immunotherapy targeting both LFA-1 and CD40/CD154 costimulation produces synergistic effects such that long-term allograft survival is achieved in the majority of recipients. This immunotherapeutic strategy has been reported to induce the development of CD4+ regulatory T cells. In the current study, the mechanisms by which this immunotherapeutic strategy prevents CD8+ T cell-dependent hepatocyte rejection in CD4 knockout mice were examined. Combined blockade of LFA-1 and CD40/CD154 costimulation did not influence the overall number or composition of inflammatory cells infiltrating the liver where transplanted hepatocytes engraft. Expression of T cell activation markers CD43, CD69, and adhesion molecule CD103 by liver-infiltrating cells was suppressed in treated mice with long-term hepatocellular allograft survival compared to liver-infiltrating cells of untreated rejector mice. Short-term immunotherapy with anti-LFA-1 and anti-CD154 mAb also abrogated the in vivo development of alloreactive CD8+ cytotoxic T cell effectors. Treated mice with long-term hepatocyte allograft survival did not reject hepatocellular allografts despite adoptive transfer of naive CD8+ T cells. Unexpectedly, treated mice with long-term hepatocellular allograft survival demonstrated prominent donor-reactive delayed-type hypersensitivity responses, which were increased in comparison to untreated hepatocyte rejectors. Collectively, these findings support the conclusion that short-term immunotherapy with anti-LFA-1 and anti-CD154 mAbs induces long-term survival of hepatocellular allografts by interfering with CD8+ T cell activation and development of CTL effector function. In addition, these recipients with long-term hepatocellular allograft acceptance show evidence of immunoregulation which is not due to immune deletion or ignorance and is associated with early development of a novel CD8+CD25high cell population in the liver.

Peripheral blood mobilization of different lymphocyte and dendritic cell subsets with the use of intermediate doses of G-CSF in patients with non-Hodgkin's lymphoma and multiple myeloma

Between June 2003 and November 2004, we collected mobilized peripheral blood units from 29 patients with non-Hodgkin's lymphoma and multiple myeloma for autologous peripheral blood stem cell transplantation. They received granulocyte colony-stimulating factor (G-CSF) (16 micro g/kg/day) for a total of 5 days. Immediately before and 3 h after the fourth and fifth dose of G-CSF, we performed flow cytometry analysis to quantify: T cells (CD3+CD4+, CD3+CD8+), B cells (CD19+), NK cells (CD3-CD16+CD56+), NKT cells (CD3+CD16+CD56+), type 1 dendritic cells (DC1) (lin-HLA-DR+CD11c+), type 2 dendritic cells (DC2) (lin-HLA-DR+CD123+), regulatory T cells (Tregs) (CD4+CD25+), and activated T cells (CD3+HLA-DR+). All cell subsets were mobilized after G-CSF treatment with the exception of B, NK, and NKT lymphocytes. The median number of Treg cells before and after G-CSF was statistically different (29+/-14.9x10(6)/l vs 70.1+/-46.1x10(6)/l, P<0.02). DCs were mobilized significantly with a 5.9-fold increase in DC2 (15.1+/-30.3x10(6)/l vs 89.8+/-81.0x10(6)/l, P<0.02) and a 2.6-fold increase for DC1 (41+/-42.5x10(6)/l vs 109.5+/-58.0x10(6)/l, P<0.04). Patients received a mean of 3.1+/-1.2x10(7)/kg NK cells, 1.3+/-0.9x10(7)/kg NKT cells, 0.41+/-0.29x10(7)/kg DC1, 0.2+/-0.22x10(7)/kg DC2, and 1.8+/-1.9x10(7)/kg Tregs. In conclusion, intermediate doses of G-CSF induce mobilization of different lymphocyte subsets, with the exception of B, NK, and NKT cells. The mobilization of certain suppressive populations (DC2 and Treg) could be in theory deleterious, at least in patients with cancer.

Effector and Memory CD8+ T Cells Can Be Generated in Response to Alloantigen Independently of CD4+ T Cell Help

There is now considerable evidence suggesting that CD8(+) T cells are able to generate effector but not functional memory T cells following pathogenic infections in the absence of CD4(+) T cells. We show that following transplantation of allogeneic skin, in the absence of CD4(+) T cells, CD8(+) T cells become activated, proliferate, and expand exclusively in the draining lymph nodes and are able to infiltrate and reject skin allografts. CD44(+)CD8(+) T cells isolated 100 days after transplantation rapidly produce IFN-gamma following restimulation with alloantigen in vitro. In vivo CD44(+)CD8(+) T cells rejected donor-type skin allografts more rapidly than naive CD8(+) T cells demonstrating the ability of these putative memory T cells to mount an effective recall response in vivo. These data form the first direct demonstration that CD8(+) T cells are able to generate memory as well as effector cells in response to alloantigen during rejection in the complete absence of CD4(+) T cells. These data have important implications for the design of therapies to combat rejection and serve to reinforce the view that CD8(+) T cell responses to allografts require manipulation in addition to CD4(+) T cell responses to completely prevent the rejection of foreign organ transplants.

Transfer of regulatory T cells generated ex vivo modifies graft rejection through induction of tolerogenic CD4+CD25+ cells in the recipient

Certain CD4+CD25+ T cells can induce and maintain T-cell non-responsiveness to donor alloantigens and have therapeutic potential in solid organ transplantation. Peripheral CD4+CD25- cells alloactivated with IL-2 and transforming growth factor beta (TGF-beta) ex vivo express the transcription factor FoxP3, and become potent antigen-specific CD4+CD25- suppressor cells. Here we report that the transfer of TGF-beta-induced regulatory CD4+ and CD8+ T cells (Tregs) co-incident with transplantation of a histoincompatible heart resulted in extended allograft survival. To account for this result, we injected non-transplanted mice with a single dose of CD4+ and CD8+ Tregs and transferred donor cells every 2 weeks to mimic the continuous stimulation of a transplant. We observed increased splenic CD4+CD25+ cells that were of recipient origin. These cells rendered the animals non-responsive to donor alloantigens by an antigen-specific and cytokine-dependent mechanism of action. Both the increased number of CD4+CD25+ cells and their tolerogenic effect were dependent on continued donor antigen boosting. Thus, Tregs generated ex vivo can act like a vaccine that generates host suppressor cells with the potential to protect MHC-mismatched organ grafts from rejection.

Regulatory T-Cells and Autoimmunity

Approximately 20% of the population is affected by autoimmune or inflammatory diseases mediated by an abnormal immune response. A characteristic feature of autoimmune disease is the selective targeting of a single cell type, organ or tissue by certain populations of autoreactive T-cells. Examples of such diseases include rheumatoid arthritis, insulin-dependent diabetes mellitus, and systemic lupus erythematosus (SLE), all of which are characterized by chronic inflammation, tissue destruction and target organ malfunction. Although strong evidence links most autoimmune diseases to specific genes, considerable controversy prevails regarding the role of regulatory T-cell populations in the disease process. These cells are now also believed to play a key role in mediating transplantation tolerance and inhibiting the induction of tumor immunity. Though the concept of therapeutic immune regulation aimed at treating autoimmune pathology has been validated in many animal models, the development of strategies for the treatment of human autoimmune disorders remains in its infancy. The main obstacles to this include the conflicting findings of different model systems, as well as the contrasting functions of regulatory T-cells and cytokines involved in the development of such disorders. This review examines the role of regulatory T-cells in the pathogenesis of autoimmunity and describes the therapeutic potential of these cells for the prevention of immune-mediated pathologies in the future. Although much remains to be learned about such pathologies, a clearer understanding of the mechanisms by which regulatory T-cells function will undoubtedly lead to exciting new possibilities for immunotherapeutics.

Functional Dynamics of Naturally Occurring Regulatory T Cells in Health and Autoimmunity

A network of regulatory T (Treg) cells exists to downregulate immune responses in various inflammatory circumstances and ultimately assure peripheral T cell tolerance. Naturally occurring CD4(+)CD25(+) Treg cell represents a major lymphocyte population engaged in the dominant control of self-reactive T responses and maintenance of tolerance within this network. CD4(+)CD25(+) Treg cells differentiate in the normal thymus as a functionally distinct subpopulation of T cells bearing a broad T cell receptor repertoire endowing these cells with the capacity to recognize a wide spectrum of self-Ag and non-self-Ag specificities. The development of CD4(+)CD25(+) Treg cells is genetically determined, influenced by Ag-specific and nonspecific signals, costimulation, and cytokines that control their activation, expansion, and suppressive activity. Functional abrogation of these cells in vivo, or genetic defects that affect their development or function, unequivocally predisposes animals and humans to the onset of autoimmune and other inflammatory diseases. Studies have shed light in our understanding of the cellular and molecular basis of CD4(+)CD25(+) Treg cell-mediated immune regulation. In this chapter, we discuss the contribution of naturally occurring CD4(+)CD25(+) Treg cells in the induction of immunologic self-tolerance in animal models and humans and attempt to provide a comprehensive overview of recent findings regarding the phenotype, functional dynamics, and effector mechanism of these cells in autoimmune diseases.

Combined donor specific transfusion and anti-CD154 therapy achieves airway allograft tolerance

BACKGROUND

The state of tolerance allows long term graft survival without immunosuppressants. Lung transplantation tolerance has not been consistently achieved in either small or large animal models.

METHODS

The mechanisms and effectiveness of a tolerance induction protocol consisting of donor specific transfusion (DST; day 0) and a short course of co-stimulatory blockade (anti-CD154 antibody; days -7, -4, 0 and +4) were studied in the mouse heterotopic tracheal transplant model of chronic lung rejection. C57BL/6 mice received BALB/c tracheal grafts (day 0) and were treated with DST alone, anti-CD154 alone, the combination (DST/anti-CD154), or no treatment. No non-specific immunosuppressants were used.

RESULTS

DST/anti-CD154 in combination, but neither treatment alone, markedly prolonged the lumen patency and survival (>100 days) of fully histo-incompatible allografts (p<0.05 versus control allografts at every time point studied up to 16 weeks) without immunosuppression. This protocol was donor antigen specific as third party grafts (C3H) were promptly rejected. In addition, DST/anti-CD154 did not result in mixed chimerism but induced transplantation tolerance via a peripheral mechanism(s), which included significantly reduced cytotoxic T cell activity (p<0.001) and a significantly increased percentage of CD4+CD25+ cells (p = 0.03).

CONCLUSIONS

The DST/anti-CD154 protocol successfully induced and maintained long term, donor specific tolerance in the mouse heterotopic airway graft model of chronic lung rejection. This finding may lead us closer to successful tolerance induction in lung transplantation.

Early regulation of CD8 T cell alloreactivity by CD4+CD25- T cells in recipients of anti-CD154 antibody and allogeneic BMT is followed by rapid peripheral deletion of donor-reactive CD8+ T cells, precluding a role for sustained regulation

While acquisition of regulatory function by CD4+CD25- T cells has been reported following antigenic stimulation, "naturally occurring" regulatory CD4+ T cells (Treg) are believed to express CD25. We examined the mechanisms involved in peripheral CD8 T cell tolerance by induction of mixed chimerism using non-myeloablative conditioning with low-dose (3 Gy) total body irradiation and anti-CD154 antibody. Recipient CD4+ T cells were initially required for the induction of CD8 cell tolerance, but were not needed beyond 2 weeks. Depletion of CD25+ Treg prior to bone marrow transplantation and blockade of IL-2 with neutralizing antibody did not impede tolerance induction. Tolerance was dependent on CTLA4, but not on IFN-gamma. In C57BL/6 mice containing a fraction of 2C TCR transgenic CD8+ T cells, which recognize the MHC class I alloantigen Ld, induction of chimerism with L(d+), but not Ld-, bone marrow cells led to deletion of peripheral 2C+ CD8+ cells within 1 week in peripheral blood and spleen. Complete deletion required the presence of recipient CD4+ T cells. Thus, a novel, rapid form of regulation by CD4+CD25- T cells permits initial CD8 T cell tolerance in this model. Rapid peripheral deletion of donor-specific CD8 T cells precludes an ongoing requirement for CD4 T cell-mediated regulation.

Transforming growth factor-beta1 gene transfer is associated with the development of regulatory cells

Adenovirus-mediated transfection of mouse cardiac allografts with active human transforming growth factor-beta 1 (TGF-beta1) prolongs transplant survival provided that recipients are initially depleted of CD8+ T cells. To test if graft survival was prolonged by persistent TGF-beta1 transgene expression, long-term transfected allografts were re-transplanted into naïve mice that were transiently depleted of CD8+ T cells. Re-transplanted allografts were acutely rejected, indicating that TGF-beta1 transgene expression did not suppress effector cell function. We next asked whether TGF-beta1 gene transfer was associated with the development of regulatory cells. When splenocytes obtained from mice bearing long-term TGF-beta1-transfected allografts were adoptively transferred into recipients of non-transfected cardiac allografts, prolonged allograft survival was observed, and increased levels of the regulatory T cell transcription factor Foxp3 were present. To further test for regulation, differentiated effector cells were obtained from mice that had rejected cardiac allografts and were adoptively transferred into mice bearing long-term TGF-beta1 transfected cardiac allografts. The effector cells failed to mediate rejection in mice bearing TGF-beta1-transfected allografts and we observed a significant increase in intra-graft Foxp3 expression. These findings indicate that TGF-beta1 gene transfer allows for the development of regulatory cells that control graft-reactive T cell responses once therapeutic levels of the transgene product are no longer produced.

Analysis of the underlying cellular mechanisms of anti-CD154-induced graft tolerance: the interplay of clonal anergy and immune regulation

Although it has been shown that CD4(+)CD25(+) regulatory T cells (T(reg)) contribute to long-term graft acceptance, their impact on the effector compartment and the mechanism by which they exert suppression in vivo remain unresolved. Using a CD4(+) TCR transgenic model for graft tolerance, we have unveiled the independent contributions of anergy and active suppression to the fate of immune and tolerant alloreactive T cells in vivo. First, it is shown that anti-CD154-induced tolerance resulted in the abortive expansion of the alloreactive, effector T cell pool. Second, commensurate with reduced expansion, there was a loss of cytokine production, activation marker expression, and absence of memory T cell markers. All these parameters defined the tolerant alloreactive T cells and correlated with the inability to mediate graft rejection. Third, the tolerant alloreactive T cell phenotype that is induced by CD154 was reversed by the in vivo depletion of T(reg). Reversal of the tolerant phenotype was followed by rapid rejection of the allograft. Fourth, in addition to T(reg) depletion, costimulation of the tolerant alloreactive T cells or activation of the APC compartment also reverted alloreactive T cell tolerance and restored an activated phenotype. Finally, it is shown that the suppression is long-lived, and in the absence of anti-CD154 and donor-specific transfusion, these T(reg) can chronically suppress effector cell responses, allowing long-lived graft acceptance.

IL-10-dependent infectious tolerance after the treatment of experimental allergic encephalomyelitis with redirected CD4+CD25+ T lymphocytes

How small numbers of CD4+CD25+ regulatory T cells suppress autoimmune responses in vivo is unclear. In this report we analyze the immunomodulatory activity of CD4+CD25+ T cells that are antigen-specifically redirected against myelin basic protein (MBP)89-101-specific autoreactive T cells by a MBP89-101-IA(s)-zeta chimeric receptor. We have previously shown that these redirected regulatory T cells are highly potent in treating a model autoimmune disease, experimental allergic encephalomyelitis. We show here that they have only limited effect in vivo on autoreactive T cell proliferation and therefore do not act by deleting or suppressing the expansion of pathologic effector cells. Rather, the redirected CD4+CD25+ T cells divert the pathologic T helper 1 self-specific T cell response to one characterized by high IL-10 and lower IL-4 production. Significantly, when isolated from the inducing CD4+CD25+ regulatory T cells, these self-specific T cells can independently suppress the autoreactive T cell response and experimental allergic encephalomyelitis development in an IL-10-dependent manner. These results provide evidence that CD4+CD25+ regulatory T cells can manipulate the adaptive immune response in vivo through the infectious induction of tolerance, specifically by promoting the formation of antigen-specific, IL-10-secreting regulatory T cells.

Adoptive transfer of in vitro-stimulated CD4+CD25+ regulatory T cells increases bacterial clearance and improves survival in polymicrobial sepsis

Regulatory CD4(+)CD25(+) T cells (Tregs) suppress autoimmune and inflammatory diseases through mechanisms that are only partly understood. Previous studies suggest that Tregs can suppress bacterially triggered intestinal inflammation and respond to LPS through TLRs with enhanced suppressive activity. In this study, we have used murine cecal ligation and puncture as a model of polymicrobial sepsis to explore the effects of adoptive transfer of Tregs on septic outcome. Adoptive transfer of in vitro-stimulated Tregs in both prevention and therapeutic modes significantly improved survival of cecal ligation and puncture mice. Furthermore, the effect was dependent on both the number of Tregs adoptively transferred and the presence of host T cells. Animals that received stimulated Tregs had significantly increased peritoneal mast cells and peritoneal TNF-alpha production. More importantly, adoptive transfer of in vitro-stimulated Tregs significantly improved bacterial clearance, which resulted in improved survival. Our results suggest a novel role for Tregs in sepsis.

The role of non-deletional tolerance mechanisms in a murine model of mixed chimerism with costimulation blockade

Peripheral and central clonal deletion are important tolerance mechanisms in models using bone marrow transplantation (BMT) with costimulation blockade (CB). However, since tolerance can be found before peripheral deletion is complete and since elimination of recipient CD4(+) cells at the time of BMT prevents tolerance induction, we investigated the potential roles of regulation and anergy in such a murine model. We found that transient elimination of CD25(+) cells or neutralization of IL2 immediately after BMT and CB prevented the induction of skin graft tolerance. Cotransfer into SCID mice of CD4(+) cells taken from chimeras early after BMT, together with naive recipient-type CD4(+) cells significantly prolonged donor skin graft survival. In contrast, cotransfer of CD4(+) cells harvested from chimeras late after BMT did not prolong donor skin graft survival. Besides, depletion of CD25(+) cells in established chimeras several months post-BMT did not break tolerance. In vivo administration of recombinant IL2 inhibited chimerism and tolerance neither early nor late post-BMT, arguing against a decisive role for classical anergy. Thus, CD4 cell-mediated regulation contributes significantly to tolerance induction early after BMT, but appears to have no critical role in the maintenance of tolerance.

CD4+CD25+Lymphocyte and Dendritic Cell Mobilization with Intermediate Doses of Recombinant Human Granulocyte Colony-Stimulating Factor in Healthy Donors

We prospectively conducted a quantitative and phenotypic analysis of T, B, natural killer (NK), NKT, type 1 and 2 dendritic cells (DC), and regulatory T cells, before and after mobilization with intermediate doses of granulocyte colony-stimulating factor (G-CSF) (16 microg/kg per day). Between November, 2003, and December, 2004, we collected stem cells from 25 HLA identical sibling donors for allogeneic hematopoietic stem cell transplantation. Before mobilization and 3 h after the fourth and fifth doses of G-CSF, blood samples were taken for blood counts and flow cytometry. The median number of regulatory T cells before and after G-CSF was statistically different (69 +/- 41 x 10(6)/L versus 161 +/- 159 x 10(6)/L, p < 0.01). We observed a 1.7-fold increase in NK and NKT cells (p < 0.009 and p < 0.02, respectively). DC were mobilized with a 11.5-fold increase in type 2 (p < 0.004) and a 8.5-fold increase in type 1 DC (p < 0.003). The patients received a mean of: 2.2 x 10(7)/kg +/- 1.4 x 10(7)/kg of NK cells, 0.95 x 10(7)/kg +/- 0.81 x 107/kg of NKT cells, 0.43 x 107/kg +/- 0.53 x 10(7)/kg of type 1 DC, 0.3 v 10(7)/kg +/- 0.45 x 10(7)/kg of type 2 DC and 1.4 x 10(7)/kg +/- 1.2 x 10(7)/kg of regulatory T cells. Using intermediate doses of G-CSF, we have demonstrated the mobilization of different lymphocyte subsets, in particular regulatory T cells and DC, which can be expanded later and used in the treatment of cancer and autoimmune diseases.

CD8 memory T cells: cross-reactivity and heterologous immunity

Virus-specific memory T cell populations demonstrate plasticity in antigen recognition and in their ability to accommodate new memory T cell populations. The degeneracy of T cell antigen recognition and the flexibility of diverse antigen-specific repertoires allow the host to respond to a multitude of pathogens while accommodating these numerous large memory pools in a finite immune system. These cross-reactive memory T cells can be employed in immune responses and mediate protective immunity, but they can also induce life-threatening immunopathology or impede transplantation tolerance and graft survival. Here we discuss examples of altered viral pathogenesis occurring as a consequence of heterologous T cell immunity and propose models for the maintenance of a dynamic pool of memory cells.

The Generation of CD25+CD4+Regulatory T Cells That Prevent Allograft Rejection Does Not Compromise Immunity to a Viral Pathogen

In all but a small minority of cases, continued survival of solid organ grafts after transplantation depends on lifelong, nonselective immunosuppression that, although effective, results in increased rates of infection, cancer, and vascular disease. Therapeutic strategies that engage or mimic self-tolerance may allow prolonged allograft survival without the disadvantages of nonspecific immunotherapy. Pretreatment of recipient mice with donor alloantigen combined with transient modulation of the peripheral T cell pool with anti-CD4 Ab leads to the indefinite survival of MHC-incompatible cardiac allografts without further therapy. Tolerance is dependent on CD25+ CD4+ regulatory T cells that arise from naive CD25- precursors and regulate rejection via both IL-10 and CTLA-4. Although these cells are clearly effective at controlling rejection, the proven ability of recently activated CD25+ cells to mediate bystander regulation raises the possibility that tolerized individuals might also have a reduced capacity to respond to environmental pathogens. We have examined anti-influenza responses in tolerized primary heart recipients, secondary recipients following adoptive transfer of regulatory populations, and tolerized mice in which bystander regulation has been deliberately induced. Neither virus-specific CTL activity in vitro nor the clearance of virus in vivo was significantly diminished in any of these treatment groups compared with infected unmanipulated controls. The data suggest that the induction of dominant allograft tolerance dependent on regulatory T cells does not necessarily result in attenuated responses to pathogens providing further support for the development of tolerance induction protocols in clinical transplantation.

Everolimus and basiliximab permit suppression by human CD4+CD25+ cells in vitro

Immunosuppressive drugs are essential for the prevention of acute transplant rejection but some may not promote long-term tolerance. Tolerance is dependent on the presence and regulatory function of CD4(+)CD25(+) T cells in a number of animal models. The direct effects of immunosuppressive drugs on CD4(+)CD25(+) cells, particularly those that interfere with IL-2 signaling are uncertain. We studied the effects of the rapamycin derivative everolimus and the anti-CD25 monoclonal antibody basiliximab on the regulatory capacity of human CD4(+)CD25(+) cells in vitro. Both drugs permitted the suppression of proliferation and IFN-gamma secretion by CD4(+)CD25(-) cells responding to allogeneic and other polyclonal stimuli; CTLA-4 expression was abolished on CD4(+)CD25(+) cells without compromising their suppressive ability. Everolimus reduced IFN-gamma secretion by CD4(+)CD25(-) cells before the anti-proliferative effect: this is a novel finding. Exogenous IL-2 and IL-15 could prevent the suppression of proliferation by CD4(+)CD25(+) cells and the drugs could not restore suppression. By contrast, suppression of IFN-gamma secretion was only slightly impeded with the exogenous cytokines. Finally, CD4(+)CD25(+) cells were more resistant than CD4(+)CD25(-) cells to the pro-apoptotic action of the drugs. Together these data suggest that CD4(+)CD25(+) cells may still exert their effects in transplant patients taking immunosuppression that interferes with IL-2 signaling.

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

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

CD25+CD4+ regulatory T cells generated by exposure to a model protein antigen prevent allograft rejection: antigen-specific reactivation in vivo is critical for bystander regulation

The importance of CD25(+)CD4(+) regulatory T (Treg) cells in the control of immune responses is established, but their antigen specificity in vivo remains unclear. Understanding Treg-cell specificity requirements will be important if their potential is to be developed for immunotherapy. Pretreatment of recipient mice with donor alloantigen plus anti-CD4 antibody generates CD25(+)CD4(+) Treg cells with the capacity to prevent skin allograft rejection in adoptive transfer recipients. Here we demonstrate that, although this regulation can be antigen-specific, reactivation with the original tolerizing alloantigen allows the Treg cells to suppress rejection of third-party allografts. Aware of the limitations of alloantigen pretreatment, we asked whether graft-protective Treg cells could be generated against unrelated, nongraft antigens. We demonstrate that bystander regulation also extends to CD25(+)CD4(+) Treg cells generated in vivo by exposure to nominal antigens under anti-CD4 antibody cover. Providing these Treg cells are reexposed to the tolerizing antigens before adoptive transfer, they prevent the rejection of fully allogeneic skin grafts. That this might form the basis of a clinically relevant tolerance induction strategy is demonstrated by the fact that, when combined with subtherapeutic anti-CD8 antibody, Treg cells generated in response to nongraft antigens facilitate the acceptance of cardiac allografts in primary recipients.

Combinations of anti-LFA-1, everolimus, anti-CD40 ligand, and allogeneic bone marrow induce central transplantation tolerance through hemopoietic chimerism, including protection from chronic heart allograft rejection

Central transplantation tolerance through hemopoietic chimerism initially requires inhibition of allogeneic stem cell or bone marrow (BM) rejection, as previously achieved in murine models by combinations of T cell costimulation blockade. We have evaluated LFA-1 blockade as part of regimens to support mixed hemopoietic chimerism development upon fully allogeneic BALB/c BM transfer to nonirradiated busulfan-treated B6 recipient mice. Combining anti-LFA-1 with anti-CD40 ligand (CD40L) induced high incidences and levels of stable multilineage hemopoietic chimerism comparable to chimerism achieved with anti-CD40L and everolimus (40-O-(2-hydroxyethyl)-rapamycin) under conditions where neither Ab alone was effective. The combination of anti-LFA-1 with everolimus also resulted in high levels of chimerism, albeit with a lower incidence of stability. Inhibition of acute allograft rejection critically depended on chimerism stability, even if maintained at very low levels around 1%, as was the case for some recipients without busulfan conditioning. Chimerism stability correlated with a significant donor BM-dependent loss of host-derived Vbeta11(+) T cells 3 mo after BM transplantation (Tx). Combinations of anti-CD40L with anti-LFA-1 or everolimus also prevented acute rejection of skin allografts transplanted before established chimerism, albeit not independently of allospecific BMTx. All skin and heart allografts transplanted to stable chimeras 3 and 5 mo after BMTx, respectively, were protected from acute rejection. Moreover, this included prevention of heart allograft vascular intimal thickening ("chronic rejection").

Depletion of donor-reactive cells as a new concept for improvement of mismatched bone marrow engraftment using reduced-intensity conditioning

OBJECTIVE

New nonmyeloablative strategies to improve acceptance of mismatched bone marrow (BM) may compensate for the inadequate supply of compatible grafts. Recently we proposed to facilitate engraftment of mismatched BM by selective depletion of activated donor-reactive host cells with cyclophosphamide (CY). Here we have compared engraftment of allogeneic BM after depletion of antigen-activated host lymphocytes by CY, with BM engraftment following general immunosuppression by the same CY dose.

MATERIALS AND METHODS

Naive or mildly irradiated BALB/c mice were primed with C57BL/6 BM cells (day 0), treated with CY in order to deplete activated T cells (day 1), and transplanted with a second C57BL/6 BM inoculum (day 2) in order to achieve BM engraftment. Alternatively, mice received an equal dose of donor BM cells in a single injection one day after the same CY dose. Treated animals were repeatedly tested for persistence of donor cells in the blood.

RESULTS

Depletion of alloantigen-primed lymphocytes by 200 mg/kg CY provided stable GVHD-free engraftment of allogeneic BM in nonirradiated mice, while immunosuppressive treatment with the same CY dose alone resulted in BM rejection. Low-dose irradiation before priming with donor BM allowed the tolerance-inducing CY dose to be reduced to 100 mg/kg. Alloantigen-primed lymphocyte depletion (APLD) by a reduced CY dose resulted in engraftment of donor BM after a significantly lower irradiation dose than treatment with irradiation and CY alone.

CONCLUSION

Our results demonstrate that conditioning that focuses on APLD has a definite advantage over general immunosuppression with CY and radiation therapy.

Mechanisms of tolerance induced by donor-specific transfusion and ICOS-B7h blockade in a model of CD4+ T-cell-mediated allograft rejection

The inducible co-stimulatory molecule (ICOS) has been shown to play a critical role in T-cell activation and differentiation, and the regulation of alloimmune responses in vivo. Using an MHC class II mismatched model of CD4(+) T-cell-mediated rejection, we found that treatment of mice with DST and ICOS-B7h blockade induced long-term skin allograft survival and donor-specific transplantation tolerance. ICOS blockade, either during antigen priming or during the effector phase, previously shown to alter the outcome of the immune response, had a similar effect on graft survival. DST and anti-B7h mAb reduced the frequency of IFN-gamma-producing allospecific cells but did not produce deviation to a T(H)2 phenotype. In an adoptive transfer model using ABM TCR transgenic mice directly reactive to I-A(bm12), DST and anti-B7h mAb reduced the number of allospecific CD4(+) T cells and increased CD4(+) T-cell apoptosis. These data demonstrate that DST and anti-B7h mAb induces transplantation tolerance to MHC class II mismatched skin grafts by a reduction of the alloreactive clone size that is, at least in part, dependent on apoptosis of host alloantigen-specific CD4(+) T cells.

Regulation of skin and islet allograft survival in mice treated with costimulation blockade is mediated by different CD4+ cell subsets and different mechanisms

BACKGROUND

Donor-specific transfusion (DST) and a brief course of anti-CD154 monoclonal antibody (mAb) induces permanent islet and prolonged skin allograft survival in mice. Induction of skin allograft survival requires the presence of CD4 cells and deletion of alloreactive CD8 cells. The specific roles of CD4 and CD4CD25 cells and the mechanism(s) by which they act are not fully understood.

METHODS

We used skin and islet allografts, a CD8 T cell receptor (TCR) transgenic model system, and in vivo depleting antibodies to analyze the role of CD4 cell subsets in regulating allograft survival in mice treated with DST and anti-CD154 mAb.

RESULTS

Deletion of CD4 or CD25 cells during costimulation blockade induced rapid rejection of skin but only minimally shortened islet allograft survival. Deletion of CD4 or CD25 cells had no effect upon survival of healed-in islet allografts, and CD25 cell deletion had no effect upon healed-in skin allograft survival. In the TCR transgenic model, DST plus anti-CD154 mAb treatment deleted alloreactive CD8 T cells, and anti-CD4 mAb treatment prevented that deletion. In contrast, injection of anti-CD25 mAb did not prevent alloreactive CD8 T cell deletion.

CONCLUSIONS

These data document that (1) both CD4CD25 and CD4CD25 cells are required for induction of skin allograft survival, (2) CD4CD25 T cells are not required for alloreactive CD8 T cell deletion, and (3) CD4CD25 regulatory cells are not critical for islet allograft tolerance. It appears that skin and islet transplantation tolerance are mediated by different CD4 cell subsets and different mechanisms.

Kidney Transplant Patients with Long-Term Graft Survival Have Altered Expression of Molecules Associated with T-Cell Activation

BACKGROUND

Patients with long-term functioning organ allografts may have developed different mechanisms that explain the lack of graft rejection. However, it is not known how these mechanisms interplay or whether one of them predominates in such situations.

METHODS

The authors analyzed the expression of T-cell surface molecules involved in alloantigen recognition, signal transduction, co-stimulation, and activation markers on circulating T cells from patients with normal kidney function 10 or more years after transplantation, short-term survival (1-4 years), and chronic rejection and from healthy adults. The percentage and the median fluorescent intensity of each marker were determined by flow cytometry. Proliferative response against specific and third-party donors and mitogenic stimulation were also determined.

RESULTS

Peripheral blood lymphocytes from patients with long-term surviving kidneys had decreased expression of T-cell receptor (TCR)-alphabeta (P < 0.01), CD3epsilon (P < 0.05), and CD3 zeta-chains (P < 0.001); diminished percentages of CD4(+)CD28(+) (P < 0.001) cells; and increased expression of CTLA-4(+) (P < 0.01) on CD3(+) cells. CD4(+)CD25(+)CD69(+) cells were also increased in long-term surviving patients. Long-term surviving patients had decreased donor-specific proliferative responses.

CONCLUSIONS

The decreased expression of TCR-alphabeta and epsilon- and zeta-chains on circulating T cells of long-term surviving patients suggests that these cells may have defects in alloantigen recognition or signal transduction that may result in decreased numbers of T cells expressing co-stimulatory molecules and activation markers as well as a decreased specific proliferative response. The decrease in the percentage of CD28(+) cells and the increase in CD4(+)CD25(+)CD69(+) cells suggest that regulatory mechanisms of the immune response are still active in such patients.