Alloreactive CD4 T cell activation in vivo: an autonomous function of the indirect pathway of alloantigen presentation

The composition of the microbiota modulates allograft rejection

Transplantation is the only cure for end-stage organ failure, but without immunosuppression, T cells rapidly reject allografts. While genetic disparities between donor and recipient are major determinants of the kinetics of transplant rejection, little is known about the contribution of environmental factors. Because colonized organs have worse transplant outcome than sterile organs, we tested the influence of host and donor microbiota on skin transplant rejection. Compared with untreated conventional mice, pretreatment of donors and recipients with broad-spectrum antibiotics (Abx) or use of germ-free (GF) donors and recipients resulted in prolonged survival of minor antigen-mismatched skin grafts. Increased graft survival correlated with reduced type I IFN signaling in antigen-presenting cells (APCs) and decreased priming of alloreactive T cells. Colonization of GF mice with fecal material from untreated conventional mice, but not from Abx-pretreated mice, enhanced the ability of APCs to prime alloreactive T cells and accelerated graft rejection, suggesting that alloimmunity is modulated by the composition of microbiota rather than the quantity of bacteria. Abx pretreatment of conventional mice also delayed rejection of major antigen-mismatched skin and MHC class II-mismatched cardiac allografts. This study demonstrates that Abx pretreatment prolongs graft survival, suggesting that targeting microbial constituents is a potential therapeutic strategy for enhancing graft acceptance.

Spontaneous restoration of transplantation tolerance after acute rejection

Transplantation is a cure for end-stage organ failure but, in the absence of pharmacological immunosuppression, allogeneic organs are acutely rejected. Such rejection invariably results in allosensitization and accelerated rejection of secondary donor-matched grafts. Transplantation tolerance can be induced in animals and a subset of humans, and enables long-term acceptance of allografts without maintenance immunosuppression. However, graft rejection can occur long after a state of transplantation tolerance has been acquired. When such an allograft is rejected, it has been assumed that the same rules of allosensitization apply as to non-tolerant hosts and that immunological tolerance is permanently lost. Using a mouse model of cardiac transplantation, we show that when Listeria monocytogenes infection precipitates acute rejection, thus abrogating transplantation tolerance, the donor-specific tolerant state re-emerges, allowing spontaneous acceptance of a donor-matched second transplant. These data demonstrate a setting in which the memory of allograft tolerance dominates over the memory of transplant rejection.

Distinct modes of antigen presentation promote the formation, differentiation, and activity of foxp3+ regulatory T cells in vivo

How the formation and activity of CD4(+)Foxp3(+) regulatory T cells (Tregs) are shaped by TCR recognition of the diverse array of peptide:MHC complexes that can be generated from self-antigens and/or foreign Ags in vivo remains poorly understood. We show that a self-peptide with low (but not high) stimulatory potency promotes thymic Treg formation and can induce conventional CD4(+) T cells in the periphery to become Tregs that express different levels of the transcription factor Helios according to anatomical location. When Tregs generated in response to this self-peptide subsequently encountered the same peptide derived instead from influenza virus in the lung-draining lymph nodes of infected mice, they proliferated, acquired a T-bet(+)CXCR3(+) phenotype, and suppressed the antiviral effector T cell response in the lungs. However, these self-antigen-selected Tregs were unable to suppress the antiviral immune response based on recognition of the peptide as a self-antigen rather than a viral Ag. Notably, when expressed in a more immunostimulatory form, the self-peptide inhibited the formation of T-bet(+)CXCR3(+) Tregs in response to viral Ag, and Ag-expressing B cells from these mice induced Treg division without upregulation of CXCR3. These studies show that a weakly immunostimulatory self-peptide can induce thymic and peripheral Foxp3(+) Treg formation but is unable to activate self-antigen-selected Tregs to modulate an antiviral immune response. Moreover, a strongly immunostimulatory self-peptide expressed by B cells induced Tregs to proliferate without acquiring an effector phenotype that allows trafficking from the draining lymph node to the lungs and, thereby, prevented the Tregs from suppressing the antiviral immune response.

The degree of CD4+ T cell autoreactivity determines cellular pathways underlying inflammatory arthritis

Although therapies targeting distinct cellular pathways (e.g., anticytokine versus anti-B cell therapy) have been found to be an effective strategy for at least some patients with inflammatory arthritis, the mechanisms that determine which pathways promote arthritis development are poorly understood. We have used a transgenic mouse model to examine how variations in the CD4(+) T cell response to a surrogate self-peptide can affect the cellular pathways that are required for arthritis development. CD4(+) T cells that are highly reactive with the self-peptide induce inflammatory arthritis that affects male and female mice equally. Arthritis develops by a B cell-independent mechanism, although it can be suppressed by an anti-TNF treatment, which prevented the accumulation of effector CD4(+) Th17 cells in the joints of treated mice. By contrast, arthritis develops with a significant female bias in the context of a more weakly autoreactive CD4(+) T cell response, and B cells play a prominent role in disease pathogenesis. In this setting of lower CD4(+) T cell autoreactivity, B cells promote the formation of autoreactive CD4(+) effector T cells (including Th17 cells), and IL-17 is required for arthritis development. These studies show that the degree of CD4(+) T cell reactivity for a self-peptide can play a prominent role in determining whether distinct cellular pathways can be targeted to prevent the development of inflammatory arthritis.

Sculpting MHC class II-restricted self and non-self peptidome by the class I Ag-processing machinery and its impact on Th-cell responses

It is generally assumed that the MHC class I antigen (Ag)-processing (CAP) machinery - which supplies peptides for presentation by class I molecules - plays no role in class II-restricted presentation of cytoplasmic Ags. In striking contrast to this assumption, we previously reported that proteasome inhibition, TAP deficiency or ERAAP deficiency led to dramatically altered T helper (Th)-cell responses to allograft (HY) and microbial (Listeria monocytogenes) Ags. Herein, we tested whether altered Ag processing and presentation, altered CD4(+) T-cell repertoire, or both underlay the above finding. We found that TAP deficiency and ERAAP deficiency dramatically altered the quality of class II-associated self peptides suggesting that the CAP machinery impacts class II-restricted Ag processing and presentation. Consistent with altered self peptidomes, the CD4(+) T-cell receptor repertoire of mice deficient in the CAP machinery substantially differed from that of WT animals resulting in altered CD4(+) T-cell Ag recognition patterns. These data suggest that TAP and ERAAP sculpt the class II-restricted peptidome, impacting the CD4(+) T-cell repertoire, and ultimately altering Th-cell responses. Together with our previous findings, these data suggest multiple CAP machinery components sequester or degrade MHC class II-restricted epitopes that would otherwise be capable of eliciting functional Th-cell responses.

Requirement for diverse TCR specificities determines regulatory T cell activity in a mouse model of autoimmune arthritis

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) are required to restrain the immune system from mounting an autoaggressive systemic inflammatory response, but why their activity can prevent (or allow) organ-specific autoimmunity remains poorly understood. We have examined how TCR specificity contributes to Treg activity using a mouse model of spontaneous autoimmune arthritis, in which CD4(+) T cells expressing a clonotypic TCR induce disease by an IL-17-dependent mechanism. Administration of polyclonal Tregs suppressed Th17 cell formation and prevented arthritis development; notably, Tregs expressing the clonotypic TCR did not. These clonotypic Tregs exerted Ag-specific suppression of effector CD4(+) T cells using the clonotypic TCR in vivo, but failed to mediate bystander suppression and did not prevent Th17 cells using nonclonotypic TCRs from accumulating in joint-draining lymph nodes of arthritic mice. These studies indicate that the availability of Tregs with diverse TCR specificities can be crucial to their activity in autoimmune arthritis.

Lung function early after lung transplantation is correlated with the frequency of regulatory T cells

PURPOSES

Outcomes following lung transplantation are limited by bronchiolitis obliterans syndrome (BOS). As the number of circulating regulatory T cells (Treg) is lower in lung recipients with BOS than in stable lung recipients, we hypothesized that Treg is also correlated with lung function in the early post-transplantation period.

METHODS

This prospective study included 18 consecutive patients whose lung function parameters were recorded 3 weeks and 3 months after transplantation, between February and July 2007. Peripheral blood mononuclear cells were stained with anti-CD3, -CD4, -CD8, -CD19, -CD25, -CD28, -CD45RA, -CD45RO, -CD69, -CD127, -CTLA4, and -Foxp3 antibodies and FACS assays were performed. In addition, intracellular cytokines were stained for FACS.

RESULTS

Treg-specific markers (Foxp3, CD127(lo), and CTLA4) in the CD25+ CD4+ population were correlated with both forced expiratory volume in 1 s and forced vital capacity. Th1-cytokine secretion was more dominant in CD4+ CD25+ T cells than in CD4+ CD25- T cells. In contrast, Th2 and Treg cytokine secretion was the dominant response in stable recipients.

CONCLUSIONS

The frequency of Treg cells was positively correlated with good lung function in the early period after lung transplantation.

CD4⁺CD25⁺Foxp3⁺ regulatory T cell formation requires more specific recognition of a self-peptide than thymocyte deletion

CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells are generated during thymocyte development and play a crucial role in preventing the immune system from attacking the body's cells and tissues. However, how the formation of these cells is directed by T-cell receptor (TCR) recognition of self-peptide:major histocompatibility complex (MHC) ligands remains poorly understood. We show that an agonist self-peptide with which a TCR is strongly reactive can induce a combination of thymocyte deletion and CD4(+)CD25(+)Foxp3(+) Treg cell formation in vivo. A weakly cross-reactive partial agonist self-peptide could similarly induce thymocyte deletion, but failed to induce Treg cell formation. These studies indicate that CD4(+)CD25(+)Foxp3(+) Treg cell formation can require highly stringent recognition of an agonist self-peptide by developing thymocytes. They also refine the "avidity" model of thymocyte selection by demonstrating that the quality of the signal mediated by agonist self-peptides, rather than the overall intensity of TCR signaling, can be a critical factor in directing autoreactive thymocytes to undergo CD4(+)CD25(+)Foxp3(+) Treg cell formation and/or deletion during their development.

Proteasomes, TAP, and endoplasmic reticulum-associated aminopeptidase associated with antigen processing control CD4+ Th cell responses by regulating indirect presentation of MHC class II-restricted cytoplasmic antigens

Cytoplasmic Ags derived from viruses, cytosolic bacteria, tumors, and allografts are presented to T cells by MHC class I or class II molecules. In the case of class II-restricted Ags, professional APCs acquire them during uptake of dead class II-negative cells and present them via a process called indirect presentation. It is generally assumed that the cytosolic Ag-processing machinery, which supplies peptides for presentation by class I molecules, plays very little role in indirect presentation of class II-restricted cytoplasmic Ags. Remarkably, upon testing this assumption, we found that proteasomes, TAP, and endoplasmic reticulum-associated aminopeptidase associated with Ag processing, but not tapasin, partially destroyed or removed cytoplasmic class II-restricted Ags, such that their inhibition or deficiency led to dramatically increased Th cell responses to allograft (HY) and microbial (Listeria monocytogenes) Ags, both of which are indirectly presented. This effect was neither due to enhanced endoplasmic reticulum-associated degradation nor competition for Ag between class I and class II molecules. From these findings, a novel model emerged in which the cytosolic Ag-processing machinery regulates the quantity of cytoplasmic peptides available for presentation by class II molecules and, hence, modulates Th cell responses.

T-cell alloimmunity and chronic allograft dysfunction

Solid organ transplantation is the standard treatment to improve both the quality of life and survival in patients with various end-stage organ diseases. The primary barrier against successful transplantation is recipient alloimmunity and the need to be maintained on immunosuppressive therapies with associated side effects. Despite such treatments in renal transplantation, after death with a functioning graft, chronic allograft dysfunction (CAD) is the most common cause of late allograft loss. Recipient recognition of donor histocompatibility antigens, via direct, indirect, and semidirect pathways, is critically dependent on the antigen-presenting cell (APC) and elicits effector responses dominated by recipient T cells. In allograft rejection, the engagement of recipient and donor cells results in recruitment of T-helper (Th) cells of the Th1 and Th17 lineage to the graft. In cases in which the alloresponse is dominated by regulatory T cells (Tregs), rejection can be prevented and the allograft tolerated with minimum or no immunosuppression. Here, we review the pathways of allorecognition that underlie CAD and the T-cell effector phenotypes elicited as part of the alloresponse. Future therapies including depletion of donor-reactive lymphocytes, costimulation blockade, negative vaccination using dendritic cell subtypes, and Treg therapy are inferred from an understanding of these mechanisms of allograft rejection.

Mesenchymal stem cells together with mycophenolate mofetil inhibit antigen presenting cell and T cell infiltration into allogeneic heart grafts

Donor-derived mesenchymal stem cells (MSC) can induce long-term acceptance in a rat heart transplantation model when injected prior to transplantation in combination with mycophenolate mofetil (MMF). In contrast, MSC alone cause accelerated graft rejection. To better understand these conflicting data we studied the effects of MSC and MMF on lymphocyte populations in heart allografts and secondary lymphatic organs. Allogeneic MSC injected prior to transplantation are immunogenic in this model because activated CD4+ and CD8+ cells emerged earlier in secondary lymphatic organs of MSC- and MSC/MMF-treated animals, compared to animals not treated with MSC. Consequently T cells infiltrated the grafts of MSC-only treated animals promptly causing accelerated graft rejection. However, few T cells or antigen-presenting cells (APC) infiltrated the grafts of animals treated with MSC and MMF. Consistent with this finding, intercellular adhesion molecule 1 (ICAM-1) and E-selectin was down-regulated exclusively in MSC/MMF-treated grafts, indicating that MSC together with MMF interfere with endothelial activation. Additionally, the presence of interferon-gamma (IFN-γ) enhanced MSC capabilities to suppress T cell proliferation in vitro. Interestingly, MMF did not influence serum IFN-γ levels in vivo. Together, our data indicate that MSC pre-activate T cells, but co-treatment with MMF eliminates these T cells, decreases intragraft APC and T cell trafficking by inhibiting endothelial activation, and allows IFN-γ stimulation of suppressive MSC.

Minor histocompatibility antigens: presentation principles, recognition logic and the potential for a healing hand

PURPOSE OF REVIEW

There is ample evidence indicating a pathologic role for minor histocompatibility antigens in inciting graft-versus-host disease in major histocompatibility complex (MHC)-matched bone marrow transplantation and rejection of solid organ allografts. Here we review the current knowledge of the genetic and biochemical bases for the cause of minor histoincompatibility and the structural basis for the recognition of the resulting alloantigens by the T-cell receptor.

RECENT FINDINGS

Recent evidence indicates that we as independently conceived individuals are genetically unique, thus, offering a mechanism for minor histoincompatibility between MHC-identical donor-recipient pairs. Furthermore, advances in delineating the mechanisms underlying antigen cross-presentation by MHC class I molecules and a critical role for autophagy in presenting cytoplasmic antigens by MHC class II molecules have been made. These new insights coupled with the X-ray crystallographic solution of several peptide/MHC-T-cell receptor structures have revealed mechanisms of histoincompatibility.

SUMMARY

On the basis of these new insights, ways to test for allograft compatibility and concoction of immunotherapies are discussed.

Immunotherapy with myeloid cells for tolerance induction

PURPOSE OF REVIEW

Understanding the interplay between myeloid dendritic cells and T cells under tolerogenic conditions, and whether their interactions induce the development of antigen-specific regulatory T cells (Tregs) is critical to uncover the mechanisms involved in the induction of indefinite allograft survival.

RECENT FINDINGS

Myeloid dendritic cell-T-cell interactions are seminal events that determine the outcome of the immune response, and multiple in-vitro protocols suggest the generation of tolerogenic myeloid dendritic cells that modulate T-cell responses, and determine the outcome of the immune response to an allograft following adoptive transfer. We believe that identifying specific conditions that lead to the generation of tolerogenic myeloid dendritic cells and Tregs are critical for the manipulation of the immune response towards the development of transplantation tolerance.

SUMMARY

We summarize recent findings regarding specific culture conditions that generate tolerogenic myeloid dendritic cells that induce T-cell hyporesponsiveness and Treg development, which represents a novel immunotherapeutic approach to promote the induction of indefinite graft survival prolongation. The interpretations presented here illustrate that different mechanisms govern the generation of tolerogenic myeloid dendritic cells, and we discuss the concomitant therapeutic implications.

CD4+CD25+ regulatory T cells in autoimmune arthritis

CD4(+)CD25(+) regulatory T (Treg) cells can play a critical role in the prevention of autoimmunity, as evidenced by the cataclysmic autoimmune disease that develops in mice and humans lacking the key transcription factor forkhead box protein 3 (Foxp3). At present, however, how and whether Treg cells participate in the development of rheumatoid arthritis (RA), which has both systemic manifestations and a joint-targeted pathology that characterizes the disease, remains unclear. In this review, we describe work that has been carried out aimed at determining the role of Treg cells in disease development in RA patients and in mouse models of inflammatory arthritis. We also describe studies in a new model of spontaneous autoimmune arthritis (TS1 x HACII mice), in which disease is caused by CD4(+) T cells recognizing a neo-self-antigen expressed by systemically distributed antigen-presenting cells. We show that TS1 x HACII mice develop arthritis despite the presence of CD4(+)CD25(+)Foxp3(+) Treg cells that recognize this target autoantigen, and we outline steps in the development of arthritis at which Treg cells might potentially act, or fail to act, in the development of inflammatory arthritis.

ICOS expression by effector T cells influences the ability of regulatory T cells to inhibit anti-chromatin B cell responses in recipient mice

T regulatory cells are critical for the prevention of autoimmunity. Specifically, Treg cells can control anti-chromatin antibody production in vivo, and this correlates with decreased ICOS expression on CD4(+) T helper cells. Here we test the significance of high ICOS expression by T effector cells, firstly in terms of the anti-chromatin B cell response, and secondly on the ability of Treg cells to suppress T cell help. We bred CD4(+) T cell receptor transgenic mice with mice that carry the Roquin(san/san) mutation. The Roquin gene functions to limit ICOS mRNA such that CD4 T cells from mutant mice express elevated ICOS. Using an in vivo model, TS1.Roquin(san/san) Th cells were compared with wild-type TS1 Th cells with regard to their ability to help anti-chromatin B cells in the presence or absence of Treg cells. Both TS1 and TS1.Roquin(san/san) Th cells induced anti-chromatin IgM(a) antibodies, but the TS1.Roquin(san/san) Th cells resulted in the recovery of more class-switched and germinal center B cells. Neither source of Th cells were capable of inducing long-lived autoantibodies. Treg cells completely suppressed anti-chromatin IgM(a) antibody production and reduced anti-chromatin B cell recovery induced by TS1 Th cells. Importantly, this suppression was less effective when TS1.Roquin(san/san) Th cells were used. Thus, high ICOS levels on effector T cells results in autoimmunity by augmenting the autoreactive B cell response and by dampening the effect of Treg cell suppression.

CD4 T cell-mediated rejection of cardiac allografts in B cell-deficient mice

CD4 T cell-dependent mechanisms promoting allograft rejection include expression of inflammatory functions within the graft and the provision of help for donor-reactive CD8 T cell and Ab responses. These studies tested CD4 T cell-mediated rejection of MHC-mismatched cardiac allografts in the absence of both CD8 T and B lymphocytes. Whereas wild-type C57BL/6 recipients depleted of CD8 T cells rejected A/J cardiac grafts within 10 days, allografts were not rejected in B cell-deficient B6.muMT(-/-) recipients depleted of CD8 T cells. Isolated wild-type C57BL/6 and B6.muMT(-/-) CD4 T cells had nearly equivalent in vivo alloreactive proliferative responses. CD4 T cell numbers in B6.muMT(-/-) spleens were 10% of that in wild-type mice but were only slightly decreased in peripheral lymph nodes. CD8 T cell depletion did not abrogate B6.muMT(-/-) mice rejection of A/J skin allografts and this rejection rendered these recipients able to reject A/J cardiac allografts. Redirection of the alloimmune response to the lymph nodes by splenectomy conferred the ability of B6.muMT(-/-) CD4 T cells to reject cardiac allografts. These results indicate that the low number of splenic CD4 T cells in B6.muMT(-/-) mice underlies the inability to reject cardiac allografts and this inability is overcome by diverting the CD4 T cell response to the peripheral lymph nodes.

Regulatory T cells after organ transplantation: where does their action take place?

Regulatory T cells are considered to be pivotal for the induction of tolerance to donor antigens. In the past decades, several regulatory T-cell subsets have been identified, such as CD4(+)CD25(+) regulatory T cells and the CD8(+)CD28(-) suppressor T cells. Although many studies have investigated the role of these regulators in transplant tolerance, relatively little attention has focused on the exact place where these cells suppress immune responses directed to donor antigens. The localization of regulatory T cells may influence their effect on allogeneic immune responses. More insight into the localization and migration of regulatory T cells in transplant recipients is therefore important, especially when these cells are to be used for monitoring purposes and for cellular immune therapy. In the present review we summarize current knowledge about the presence of functional donor-directed regulatory T cells in the secondary lymphoid organs, peripheral blood, and the transplanted organ itself. In addition, we discuss the importance of the appropriate localization for the control of anti-donor immune reactivity.

Langerhans cells are not required for efficient skin graft rejection

The mechanism of skin allograft rejection has been thought to require presentation of graft antigen by resident epidermal Langerhans cells (LCs). We have previously engineered mice that have a selective and constitutive absence of epidermal LCs. By using donor skin from these LC-deficient mice, we show that LCs are not required for rejection of major (FVB --> B6) or minor (H-Y, male --> female on B6 background) antigen-mismatched skin grafts. On the FVB background, where H-Y mismatched grafts are normally maintained indefinitely, grafts lacking LCs are efficiently rejected. Thus, LCs in the donor graft are required for long-term skin engraftment, which supports a regulatory role for LCs in skin graft acceptance.

CD4+ T cells recognizing a single self-peptide expressed by APCs induce spontaneous autoimmune arthritis

We have examined processes leading to the spontaneous development of autoimmune inflammatory arthritis in transgenic mice containing CD4+ T cells targeted to a nominal Ag (hemagglutinin (HA)) and coexpressing HA driven by a MHC class II promoter. Despite being subjected to multiple tolerance mechanisms, autoreactive CD4+ T cells accumulate in the periphery of these mice and promote systemic proinflammatory cytokine production. The majority of mice spontaneously develop inflammatory arthritis, which is accompanied by an enhanced regional immune response in lymph nodes draining major joints. Arthritis development is accompanied by systemic B cell activation; however, neither B cells nor Ab is required for arthritis development, since disease develops in a B cell-deficient background. Moreover, arthritis also develops in a recombinase activating gene-deficient background, indicating that the disease process is driven by CD4+ T cells recognizing the neo-self HA Ag. These findings show that autoreactive CD4+ T cells recognizing a single self-Ag, expressed by systemically distributed APCs, can induce arthritis via a mechanism that is independent of their ability to provide help for autoantibody production.

Epidermal Langerhans cells promote skin allograft rejection in mice with NF-kappa B-impaired T cells

T cells play a major role in the acute rejection of transplanted organs. Using mice transgenic for a T-cell-restricted NF-kappaB super-repressor (IkappaBalphaDeltaN-Tg mice), we have previously shown that T-cell-NF-kappaB is essential for the acute rejection of cardiac but not skin allografts. In this study, we investigated the mechanism by which skin grafts activate IkappaBalphaDeltaN-Tg T cells. Rejection was not due to residual T-cell-NF-kappaB activity as mice with p50/p52(-/-) T cells successfully rejected skin grafts. Rather, skin but not cardiac allografts effectively induced proliferation of graft-specific IkappaBalphaDeltaN-Tg T cells. Rejection of skin grafts by IkappaBalphaDeltaN-Tg mice was in part dependent on the presence of donor Langerhans cells (LC), a type of epidermal dendritic cells (DC), as lack of LC in donor skin grafts resulted in prolongation of skin allograft survival and injection of LC at the time of cardiac transplantation was sufficient to promote cardiac allograft rejection by IkappaBalphaDeltaN-Tg mice. Our results suggest that LC allow NF-kappaB-impaired T cells to reach an activation threshold sufficient for transplant rejection. The combined blockade of T-cell-NF-kappaB with that of alternative pathways allowing activation of NF-kappaB-impaired T cells may be an effective strategy for tolerance induction to highly immunogenic organs.

Allopeptide-specific CD4(+) T cells facilitate the differentiation of directly alloreactive graft-infiltrating CD8(+) T Cells

To investigate the mechanism of CD4(+) T-cell help during the activation and differentiation of directly alloreactive CD8(+) T cells, we examined the development of obliterative airways disease (OAD) following transplantation of airways into fully mismatched recipient mice deficient in CD4(+) T cells. BALB/c trachea allografts became fibrosed significantly less frequently in B6 CD4(-/-) recipients as compared to wildtype controls. Furthermore, class I-directed cytotoxicity failed to develop in the absence of CD4(+) T cells. The infiltration of graft tissue by primed L(d)-specific directly alloreactive 2C CD8(+) T cells was not found to depend on the presence of CD4(+) T cells. Nevertheless, graft-infiltrating 2C CD8(+) T cells failed to express CD69 and granzyme B when CD4(+) T-cell help was unavailable. Importantly, reconstitution of B6 CD4(-/-) recipient mice with graft peptide-specific TCR-Tg CD4(+) T cells (OT-II or TEa) capable of recognizing antigen only on recipient APC allowed for full expression of CD69 and granzyme B by the directly alloreactive CD8(+) T cells and restored the capacity of recipients to reject their allografts. These results demonstrate that indirectly alloreactive CD4(+) T cells ensure the optimal activation and differentiation of graft-infiltrating directly alloreactive CD8(+) T cells independent of donor APC recognition.

Dendritic cell homeostasis and trafficking in transplantation

Hematopoietic cell transplantation and solid organ transplantation are definitive therapies for several otherwise fatal conditions. Post-transplant immune reactions are the major cause of morbidity after transplantation and limit the extended use of these critical therapies. Post-transplant immune complications include graft rejection by the host and injury to the host mediated by the graft. Dendritic cells (DCs), a population of professional antigen-presenting cells, are thought to be crucial in triggering primary immune responses against both the graft and the host. Here, we review studies on DC homeostasis and trafficking after transplantation, and examine the role of the host and graft DC in post-transplant immune responses. We also discuss the therapeutic implications of these studies.

B Cell-Mediated Antigen Presentation Is Required for the Pathogenesis of Acute Cardiac Allograft Rejection

Acute allograft rejection requires the activation of alloreactive CD4 T cells. Despite the capacity of B cells to act as potent APCs capable of activating CD4 T cells in vivo, their role in the progression of acute allograft rejection was unclear. To determine the contribution of B cell APC function in alloimmunity, we engineered mice with a targeted deficiency of MHC class II-mediated Ag presentation confined to the B cell compartment. Cardiac allograft survival was markedly prolonged in these mice as compared to control counterparts (median survival time, >70 vs 9.5 days). Mechanistically, deficient B cell-mediated Ag presentation disrupted both alloantibody production and the progression of CD4 T cell activation following heart transplantation. These findings demonstrate that indirect alloantigen presentation by recipients' B cells plays an important role in the efficient progression of acute vascularized allograft rejection.

Direct versus indirect allorecognition: Visualization of dendritic cell distribution and interactions during rejection and tolerization

Interactions of donor and recipient dendritic cells (DCs) with CD4+ T cells determine the alloantigenic response in organ transplantation, where recipient T cells respond either directly to donor MHC, or indirectly to processed donor MHC allopeptides in the context of recipient MHC molecules. The present study evaluates donor and recipient alloantigen-presenting DC trafficking and their interactions with CD4+ T cells in the lymph nodes (LNs) and the spleen under tolerogenic treatment with anti-CD2 plus anti-CD3 mAb compared with untreated rejecting conditions. CX3CR1(GFP) BALB/c (I-A(d)) donor hearts were transplanted into C57BL/6 (I-A(b)) mice and quantification of donor DC direct (GFP+ or I-A(d+)) and recipient DC indirect (YAe+) trafficking and interactions with host CD4+ T cells was performed by fluorescent microscopy. Our data indicate that although both direct and indirect interactions between CD4+ T cells and donor and recipient DCs occur shortly after engraftment, only indirect presentation persists in the LN, but not the spleen, of tolerized recipients. These data suggest that distinct anatomic lymphoid compartments play a critical role in peripheral tolerance induction and maintenance, and persistent indirect presentation to CD4+ T cells within the LNs is an important process during tolerization.

Impact of effector cell differentiation on CD4+ T cells that evade negative selection by a self-peptide

We have used a transgenic mouse system to examine how differing reactivities of TCRs expressed by naive versus effector cells can shape the functional potential of autoreactive CD4+ T cells. Transgenic mice expressing TCRs that exhibit either high (TS1) or low [TS1(SW)] reactivity toward the I-Ed-restricted determinant S1 from the influenza virus PR8 hemagglutinin (HA) were mated with transgenic mice expressing HA under the control of different promoters. HACII mice express HA driven by an MHC class II promoter, and both the TS1 and TS1(SW) TCRs underwent substantial deletion in this background. HA104 mice express HA driven by an SV40 promoter, and the highly reactive TS1 TCR was substantially deleted. By contrast, the less reactive TS1(SW) TCR underwent little or no deletion in TS1(SW) x HA104 mice, although CD5 up-regulation indicated that they had interacted with the S1 self-peptide. In adoptive transfer studies, naive CD4+ T cells expressing the TS1(SW) TCR failed to proliferate in response to the S1 peptide in HA104 mice, and were inefficient at providing help for HA-specific antibody responses. However, effector CD4+ T cells generated from TS1(SW) x HA104 mice acquired the ability to proliferate in response to the S1 peptide in HA104 mice, and were as efficient as CD4+ T cells expressing the high reactivity TS1 TCR in helping HA-specific antibody responses. Collectively, these studies demonstrate a basis by which CD4+ T cells expressing TCRs with low reactivity toward self-peptides can evade negative selection and acquire enhanced autoreactivity following activation by a cross-reactive antigen.

Alloantigen-presenting plasmacytoid dendritic cells mediate tolerance to vascularized grafts

The induction of alloantigen-specific unresponsiveness remains an elusive goal in organ transplantation. Here we identify plasmacytoid dendritic cells (pDCs) as phagocytic antigen-presenting cells essential for tolerance to vascularized cardiac allografts. Tolerizing pDCs acquired alloantigen in the allograft and then moved through the blood to home to peripheral lymph nodes. In the lymph node, alloantigen-presenting pDCs induced the generation of CCR4+ CD4+ CD25+ Foxp3+ regulatory T cells (Treg cells). Depletion of pDCs or prevention of pDC lymph node homing inhibited peripheral Treg cell development and tolerance induction, whereas adoptive transfer of tolerized pDCs induced Treg cell development and prolonged graft survival. Thus, alloantigen-presenting pDCs home to the lymph nodes in tolerogenic conditions, where they mediate alloantigen-specific Treg cell development and allograft tolerance.

Pluripotent allospecific CD8+ effector T cells traffic to lung in murine obliterative airway disease

Long-term success in lung transplantation is limited by obliterative bronchiolitis, whereas T cell effector mechanisms in this process remain incompletely understood. Using the mouse heterotopic allogeneic airway transplant model, we studied T cell effector responses during obliterative airways disease (OAD). Allospecific CD8+ IFN-gamma+ T cells were detected in airway allografts, with significant coexpression of TNF-alpha and granzyme B. Therefore, using IFN-gamma as a surrogate marker, we assessed the distribution and kinetics of extragraft allo-specific T cells during OAD. Robust allospecific IFN-gamma was produced by draining the lymph nodes, spleen, and lung mononuclear cells from allograft, but not isograft recipients by Day 14, and significantly decreased by Day 28. Although the majority of allospecific T cells were CD8+, allospecific CD4+ T cells were also detected in these compartments, with each employing distinct allorecognition pathways. An influx of pluripotent CD8+ effector cells with a memory phenotype were detected in the lung during OAD similar to those seen in the allografts and secondary lymphoid tissue. Antibody depletion of CD8+ T cells markedly reduced airway lumen obliteration and fibrosis at Day 28. Together, these data demonstrate that allospecific CD8+ effector T cells play an important role in OAD and traffic to the lung after heterotopic airway transplant, suggesting that the lung is an important immunologic site, and perhaps a reservoir, for effector cells during the rejection process.

Exhaustive Depletion of Graft Resident Dendritic Cells: Marginally Delayed Rejection but Strong Alteration of Graft Infiltration

BACKGROUND

Donor dendritic cells (DDC) are believed to sustain direct recognition leading to acute allograft rejection. However, DDC are also required for tolerance induction in various models.

METHODS

We studied the effect of DDC depletion on major histocompatibility complex (MHC) mismatched rat heart allografts in a strain combination characterized by a DDC-dependant tolerance induction. Grafts were depleted of DDC either by pretreating donors with cyclophosphamide (CyP) or by being parked in an intermediate recipient treated with cyclosporine A (CsA).

RESULTS

CyP depleted 95% of resident DC and no specific donor MHC class II staining was observed in parked grafts. Parked grafts survived significantly but only moderately longer than untreated grafts (10.8+/-1.9 days vs. 6.5+/-0.5 days; P<0.05). Compared to unmodified grafts, on day 5 after transplantation, the magnitude of the graft infiltrate was dramatically decreased in DDC-depleted grafts, with IgG deposition within the grafts at the time of rejection. In parallel, the cytokine transcript levels were also lower in these grafts on day 5, but reached levels similar to those of unmodified grafts by day 7, indicating a delayed pattern of rejection.

CONCLUSIONS

Taken collectively, these data suggest that DDC depletion has a greater effect on the capacity of tolerance induction than the rejection process.

Lymph Node Occupancy Is Required for the Peripheral Development of Alloantigen-Specific Foxp3+ Regulatory T Cells

We previously demonstrated that L-selectin (CD62L)-dependent T cell homing to lymph nodes (LN) is required for tolerance induction to alloantigen. To explore the mechanisms of this observation, we analyzed the development and distribution of regulatory T cells (Treg), which play an important protective role against allograft rejection in transplantation tolerance. Alloantigen-specific tolerance was induced using either anti-CD2 plus anti-CD3 mAbs, or anti-CD40L mAbs plus donor-specific transfusion, in fully mismatched (BALB/c donor, C57BL/6 recipient) vascularized cardiac allografts. An expansion of CD4(+)CD25(+)CD62L(high) T cells was observed specifically within the LN of tolerant animals, but not in other anatomic sites or under nontolerizing conditions. These cells exhibited a substantial up-regulation of Foxp3 expression as measured by real-time PCR and by fluorescent immunohistochemistry, and possessed alloantigen-specific suppressor activity. Neither LN nor other lymphoid cells expressed the regulatory phenotype if recipients were treated with anti-CD62L mAbs, which both prevented LN homing and caused early allograft rejection. However, administration of FTY720, a sphingosine 1-phosphate receptor modulator that induces CD62L-independent T cell accumulation in the LNs, restored CD4(+)CD25(+) Treg in the LNs along with graft survival. These data suggest that alloantigen-specific Foxp3(+)CD4(+)CD25(+) Treg develop and are required within the LNs during tolerization, and provide compelling evidence that distinct lymphoid compartments play critical roles in transplantation tolerance.

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.

Evidence for cooperativity in the rejection of cardiac grafts mediated by CD4 TCR Tg T cells specific for a defined allopeptide

Understanding the mechanisms of rejection of organs transplanted between unrelated individuals is confounded by the complexity of the alloantigens and the diversity of T cells responding to these alloantigens. To circumvent these problems, we developed a transgenic (Tg) C57BL/6 model system in which the T-cell receptor (TCR) expressed by CD4 T cells is specific for a defined allogeneic H-2Kd peptide and the cardiac donor expressed H-2Kd as a transgene on the C57BL/6 background (B6.Kd). These TCR Tg T cells were previously shown to mediate rapid rejection of a B10.D2 cardiac allograft when transferred to Rag1 recipients, demonstrating that the "indirect" pathway of allorecognition is sufficient for complete rejection in the absence of other T cells or antibody. Here, we report that B6.Kd hearts were rejected in an accelerated fashion by Rag1(-/-) TCR Tg T cells adoptively transferred to normal B6 recipients. Rejection in this model was associated with large myocardial infarcts and significant coronary artery inflammation. Moreover, transferred TCR Tg CD4+ cells mediated allograft injury without the requirement for cytotoxic function from recipient-derived CD8 T cells. A non-linear relationship was observed between the initial precursor frequency of the antigen-specific TCR Tg cells and the ultimate tempo of acute rejection, which is taken as evidence for cooperativity between components of the system.

Specificity-Based Negative Selection of Autoreactive B Cells during Memory Formation

Autoreactive B cells are not completely purged from the primary B cell repertoire, and whether they can be prevented from maturation into memory B cells has been uncertain. We show here that a population of B cells that dominates primary immune responses of BALB/c mice to influenza virus A/PR/8/34 hemagglutinin (HA) are negatively selected in transgenic mice expressing PR8 HA as an abundant membrane-bound Ag (HACII mice). However, a separate population of B cells that contains precursors of memory B cells is activated by PR8 virus immunization and is subsequently negatively selected during the formation of the memory response. Negative selection of PR8 HA-specific B cells altered the specificity of the memory B cell response to a mutant virus containing a single amino acid substitution in a B cell epitope. Strikingly, this skewed reactivity resulted from an increase in the formation of memory B cells directed to non-self-epitopes on the mutant virus, which increased 8-fold in HACII mice relative to nontransgenic mice and precisely compensated for the absence of autoreactive PR8 HA-specific memory B cells. Negative selection of PR8 HA-specific B cells was a dominant process, since B cells from HACII mice could induce negative selection of PR8 HA-specific B cells from BALB/c mice. Lastly, HA-specific memory responses were unaffected by self-tolerance in another lineage of HA-transgenic mice (HA104 mice), indicating that the amount and/or cell type in which self-Ags are expressed can determine their ability to prevent autoreactive memory B cell formation.

Therapeutic manipulation of T cell chemotaxis in transplantation

T cell migration and trafficking are regulated by the well defined cellular processes of rolling, activation, tight adhesion, arrest and diapedesis. These processes are, in turn, controlled by molecular events involving integrins, selectins, chemokines and chemokine receptors. Recent studies have shown that sphingosine 1-phosphate receptors and their ligands are also important molecular modulators of migration and trafficking. Many of these molecules are appropriate targets for preventing allograft rejection or for achieving tolerance. Studies of migration and trafficking have also shown that the anatomic choreography of alloantigen presentation and T cell encounter with alloantigen and immunosuppression, are over-riding determinants of T cell priming versus tolerization.

Activation and migration of allo-peptide specific TCR transgenic T cells in cardiac allograft rejection

T cells from TCR transgenic mice, expressing receptors specific for an allogeneic MHC class I peptide, were used to track T cell activation and migration in normal adoptive recipients that were subsequently transplanted with heterotopic hearts that were syngeneic except for a transgenic MHC class I antigen. T cells rapidly disappeared from the blood into the lymphoid tissues where they were activated within one day after transplantation. T cells initially formed discrete clusters in the spleen and lymph nodes. After proliferating for 2-4 days in lymphoid tissues, T cells reappeared in the blood and migrated to the heart and the intestines. The T cells underwent another round of proliferation in the heart, but not the intestines, and induced cardiac rejection uniformly on 6 day.

Indirect Minor Histocompatibility Antigen Presentation by Allograft Recipient Cells in the Draining Lymph Node Leads to the Activation and Clonal Expansion of CD4+T Cells That Cause Obliterative Airways Disease

Ag recognition by OVA-reactive OT-II (I-Ab restricted) and DO11.10 (I-Ad restricted) TCR-Tg CD4+ T cells after heterotopic transplantation of OVA transgene-expressing tracheal grafts was examined as a model of minor histocompatibility Ag (mHAg)-induced chronic allograft rejection. In response to airway allotransplantation with grafts expressing the OVA transgene, these TCR-Tg CD4+ T cells expressed the activation markers CD69 and CD44, demonstrated evidence of blastogenesis, underwent multiple rounds of cell division leading to their clonal expansion in the draining lymph node, and proceeded to differentiate to a effector/memory T cell phenotype based on a reduction in the expression of CD45RB. These mHAg-specific TCR-Tg CD4+ T cells responded equally well to fully MHC-mismatched tracheas and to class II-deficient allografts, demonstrating that donor mHAg recognition by recipient CD4+ T cells does not rely on Ag presentation by donor-derived APC. The activation of mHAg-specific TCR-Tg CD4+ T cells after their adoptive transfer into recipient mice given MHC-matched, but mHAg-disparate, airway allografts was associated with their movement into the allograft and the near uniform destruction of the transplanted airway tissue secondary to the development of obliterative airways disease. These results demonstrate that an activation of mHAg-reactive CD4+ T cells in the draining lymph node by recipient APC that indirectly express graft mHAg-derived peptide/class II MHC complexes precedes responder T cell proliferation and differentiation, and leads to the eventual migration of these alloreactive T cells to the transplanted airway tissue and the promotion of chronic graft rejection.