Reciprocal differentiation and tissue-specific pathogenesis of Th1, Th2, and Th17 cells in graft-versus-host disease

Plasticity of human Th17 cells and iTregs is orchestrated by different subsets of myeloid cells

CD4+ T helper cell differentiation is essential for mounting robust immune responses without compromising unresponsiveness toward self-tissue. Here, we show that different subsets of myeloid cells isolated from human peripheral blood modulate TGF-β-dependent CD4+ T-cell developmental programs ex vivo. Human CD14+HLA-DR(-/low) myeloid-derived suppressor cells (MDSCs) induce Foxp3+ regulatory T cells, whereas CD14+HLA-DR+ monocytes promote generation of IL-17-secreting RORc+ Th17 cells when cocultured with naive CD4+ T cells. More importantly, not only do these 2 subsets modulate the de novo induction of Tregs and Th17 cells from CD4+ T cells, but MDSCs also catalyze the transdifferentiation of Foxp3+ regulatory T cells from monocyte-induced Th17 cells. The mechanism of such Th17 plasticity is dependent on MDSC-derived TGF-β and retinoic acid. Our results identify a previously unknown feature of the different subsets of CD14+ myeloid cells namely their pivotal role in immune response regulation and plasticity of CD4+ T helper cells. We propose that different subsets of myeloid cells in humans can orchestrate the differentiation of naive CD4+ T cells into effector/regulatory T-cell subsets. The balance between these 2 subsets can impact the outcome of immune reaction from inflammation to tolerance.

Effects of Pre-conditioning Dose on the Immune Kinetics and Cytokine Production in the Leukocytes Infiltrating GVHD Tissues after MHC-matched Transplantation

Background

Graft-versus-host disease (GVHD) is a huddle for success of hematopoietic stem cell transplantation. In this study, effects of irradiation dose on immune kinetics of GVHD were investigated using B6 → BALB.B system, a mouse model for GVHD after MHC-matched allogeneic transplantation.

Methods

BALB.B mice were transplanted with bone marrow and spleen cells from C57BL/6 mice after irradiation with different doses. Leukocytes residing in the peripheral blood and target organs were collected periodically from the GVHD hosts for analysis of chimerism formation and immune kinetics along the GVHD development via flow cytometry. Myeloid cells were tested for production of IL-17 via flow cytometry.

Results

Pre-conditioning of BALB.B hosts with 900 cGy and 400 cGy resulted in different chimerism of leukocytes from the blood and affected survival of GVHD hosts. Profiles of leukocytes infiltrating GVHD target organs, rather than profiles of peripheral blood leukocytes (PBLs), were significantly influenced by irradiation dose. Proportions of IL-17 producing cells in the infiltrating Gr-1⁺ or Mac-1⁺ cells were higher in the GVHD hosts with high does irradiation than those with low dose irradiation.

Conclusion

Pre-conditioning dose affected tissue infiltration of leukocytes and cytokine production by myeloid cells in the target organs.

Host APCs augment in vivo expansion of donor natural regulatory T cells via B7H1/B7.1 in allogeneic recipients

Foxp3(+) regulatory T (Treg) cells include thymic-derived natural Treg and conventional T-derived adaptive Treg cells. Both are proposed to play important roles in downregulating inflammatory immune responses. However, the mechanisms of Treg expansion in inflammatory environments remain unclear. In this study, we report that, in an autoimmune-like graft-versus-host disease model of DBA/2 (H-2(d)) donor to BALB/c (H-2(d)) recipients, donor Treg cells in the recipients predominantly originated from expansion of natural Treg cells and few originated from adaptive Treg cells. In vivo neutralization of IFN-γ resulted in a marked reduction of donor natural Treg expansion and exacerbation of graft-versus-host disease, which was associated with downregulation of host APC expression of B7H1. Furthermore, host APC expression of B7H1 was shown to augment donor Treg survival and expansion. Finally, donor Treg interactions with host APCs via B7.1/B7H1 but not PD-1/B7H1 were demonstrated to be critical in augmenting donor Treg survival and expansion. These studies have revealed a new immune regulation loop consisting of T cell-derived IFN-γ, B7H1 expression by APCs, and B7.1 expression by Treg cells.

Blockade of IL-6-signaling inhibits the pathogenesis of CD4+ T cell-mediated lethal graft-versus-host reaction against minor histocompatibility antigen

Graft-versus-host reaction (GVHR) is considered as a problem in hematopoietic cell transplantation. We found that CD45RB(high) CD62L(+) naïve CD4(+) T cells from wild-type B10D2 (H-2d MMTV6(-)) mice immediately differentiated into effector T cells producing high-levels of various cytokines after the transfer into BALB/c RAG2(-/-) (H-2d MMTV6(+)) mice. The expanded CD4(+) T cells, which have almost TCR Vβ3 chain, recognized the minor antigen of recipient mice and brought typical severe GVHR symptoms such as eyelid irritation, diarrhea, and liver failure. Eventually, all of the recipient mice transferred CD4(+) T cells was dead within 10 days. We demonstrated here that blockade of IL-6 signaling by administration of anti-IL-6 receptor (IL-6R) monoclonal antibody (mAb) remarkably inhibited the CD4(+) T cell-mediated lethal GVHR. In addition, we confirmed that the in vivo injection of anti-IL-6R mAb prevented the generation of effector CD4(+) T cells which produce the inflammatory cytokines such as IFN-γ, TNF-α, and IL-17. These findings indicated that IL-6 was a critical factor in the CD4(+) T cell-dependent acute GVHR induced by a minor-antigen, suggesting that IL-6-mediated signaling pathway would be a strong therapeutic target in T cell-mediated GVHR as well as other diseases including autoimmune and inflammation.

IL-17 enhances tumor development in carcinogen-induced skin cancer

Inflammatory conditions elicited by extrinsic environmental factors promote malignant cell transformation, tumor growth, and metastasis. Although most attention has been focused on innate immune mechanisms of inflammatory carcinogenesis, more recently the role of T cells in cancer promotion has been examined. Although IFN-dependent Th1 responses that promote Stat1 signaling inhibit tumor growth, the role of T helper type 17 responses, and interleukin-17 (IL-17) in particular, has been controversial. Indeed, IL-17 has been reported to either enhance or inhibit the growth of transplantable tumors, depending on the system. Little is known about the role of IL-17 in de novo carcinogenesis. Using IL-17 knockout mice, we examined the role of IL-17 in the classic DMBA/TPA-induced skin carcinogenesis model. Disruption of IL-17 dramatically reduced tumorigenesis in this model in a manner correlated with diminished Stat3 activation in the tumor microenvironment. IL-17 loss reduced Stat3-associated proliferative and antiapoptotic gene expression along with epidermal cell proliferation and hyperplasia. In addition, IL-17 loss was associated with reduced expression of Stat3-regulated chemokines that attract myeloid cells and a decreased infiltration of myeloid cells into the local tumor microenvironment. Together, our findings point to a critical role of the IL-17-Stat3 pathway in supporting cancer-associated inflammation in the tumor microenvironment. Therapeutic approaches that target this pathway may therefore be effective to inhibit carcinogenesis.

Effector CD4+ T cells, the cytokines they generate, and GVHD: something old and something new

GVHD is a syndrome that results from minor and major histocompatibility complex incompatibilities between the donor and recipient. More than 50 years after its initial description, the pathophysiology of GVHD remains poorly understood. Nonetheless, donor T cells have been shown to be critical to the pathophysiology of acute and chronic GVHD, yet precisely how they function remains unclear. The effector mechanisms by which donor T cells mediate tissue inflammation is even less well understood. Identification of several new lineages of CD4(+) T cells made in the past decade and their roles in the pathophysiology of T cell-mediated diseases has shed new light on these effector mechanisms. In this review, we summarize the recent descriptions of these T-cell lineages and the current data supporting their role in acute and to a lesser extent chronic GVHD. Investigations into the activity of these new T-cell lineages may provide more rationale approaches to the treatment or prevention of GVHD.

IL-17-producing T cells contribute to acute graft-versus-host disease in patients undergoing unmanipulated blood and marrow transplantation

The aim of this study was to investigate the effects of IL-17-producing T cells, including Th17 and Tc17 cells, on acute graft-versus-host disease (aGVHD) in patients who had undergone granulocyte colony-stimulating factor (G-CSF)-mobilised peripheral blood progenitor cell (PBPC) and G-CSF-primed bone marrow (G-BM) transplantation. Allografts from forty-one patients were analysed for IL-17-producing T cells with respect to aGVHD. Furthermore, ten patients with aGVHD onset were monitored for the presence of Th17 cells in the peripheral blood by flow cytometry. Patients who received a higher dose of Th17 cells in the G-BM (>8.5 × 10(4) /kg, p=0.005) or a higher dose of Tc17 cells in PBPC (>16.8 × 10(4) /kg, p=0.001) exhibited a higher incidence of aGVHD. An increased Th17 population (up to 4.99% CD4(+) T lymphocytes) was observed in patients with aGVHD onset. In contrast, the percentage of Th17 population decreased drastically in aGVHD patients following treatment to achieve partial and complete remission (p=0.013 and p=0.008, respectively). All percentages of Th17 and Tc17 cells were significantly reduced after in vivo G-CSF application. Our results suggested that IL-17-producing T cells contributed to aGVHD. The application of G-CSF in vivo aided in reducing the occurrence of aGVHD through a decrease in IL-17 secretion by T cells.

Blockade of osteopontin reduces alloreactive CD8+ T cell-mediated graft-versus-host disease

Graft-versus-host disease (GVHD), a life-threatening complication after allogeneic hematopoietic stem cell transplantation, is caused by alloreactive donor T cells that trigger host tissue damage. The inflammatory environment inside recipients is critical for GVHD pathogenesis, but the underpinning mechanisms remain elusive. Using mouse model of human GVHD, we demonstrate osteopontin (OPN), a potent proinflammatory cytokine, plays an important role in regulating activation, migration, and survival of alloreactive T cells during GVHD. OPN was significantly elevated after irradiation and persisted throughout the course of GVHD. Blockade of OPN attenuated GVHD with reduced accumulation of donor T cells in recipient organs. Amelioration was the result of migration and survival suppression caused by anti-OPN treatment on donor-derived T cells for 2 reasons. First, OPN promoted the migration and infiltration of naive and alloreactive CD8(+) T cells into host organs. Second, it also facilitated activation and viability of donor-derived CD8(+) T cells via synergizing with T-cell receptor/CD3 signaling. Finally, anti-OPN treatment retained graft-versus-leukemia effect of alloreactive CD8(+) T cells. This study demonstrates, to our knowledge for the first time, the critical effect of OPN in the initiation and persistence of CD8(+) T cell-mediated GVHD and validates OPN as a potential target in GVHD prevention.

SMAD3 prevents graft-versus-host disease by restraining Th1 differentiation and granulocyte-mediated tissue damage

Gene expression profiling of human donor T cells before allogeneic hematopoietic cell transplantation revealed that expression of selected genes correlated with the occurrence of graft-versus-host disease (GVHD) in recipients. The gene with the best GVHD predictive accuracy was SMAD3, a core component of the transforming growth factor-β signaling pathway, whose expression levels vary more than a 6-fold range in humans. The putative role of SMAD3 in the establishment of graft-host tolerance remained elusive. We report that SMAD3-KO mice present ostensibly normal lymphoid and myeloid cell subsets. However, the lack of SMAD3 dramatically increased the frequency and severity of GVHD after allogeneic hematopoietic cell transplantation into major histocompatibility complex-identical recipients. Lethal GVHD induced by SMAD3-KO donors affected mainly the intestine and resulted from massive tissue infiltration by T-bet(+) CD4 T cells and granulocytes that caused tissue damage by in situ release of Th1 cytokines and oxidative-nitrosative mediators, respectively. Our report reveals the nonredundant roles of SMAD3 in the development of tolerance to the host. Furthermore, our data support the concept that SMAD3 levels in donor cells dictate the risk of GVHD and that SMAD3 agonists would be attractive for prevention of GVHD.

STAT1-activating cytokines limit Th17 responses through both T-bet-dependent and -independent mechanisms

Given the association with autoimmune disease, there is great interest in defining cellular factors that limit overactive or misdirected Th17-type inflammation. Using in vivo and in vitro models, we investigated the molecular mechanisms for cytokine-mediated inhibition of Th17 responses, focusing on the role of STAT1 and T-bet in this process. These studies demonstrate that, during systemic inflammation, STAT1- and T-bet-deficient T cells each exhibit a hyper-Th17 phenotype relative to wild-type controls. However, IL-17 production was greater in the absence of T-bet, and when both STAT1 and T-bet were deleted, there was no further increase, with the double-deficient cells instead behaving more like STAT1-deficient counterparts. Similar trends were observed during in vitro priming, with production of Th17-type cytokines greater in T-bet(-/-) T cells than in either STAT1(-/-) or STAT1(-/-) T-bet(-/-) counterparts. The ability of IFN-γ and IL-27 to suppress Th17 responses was reduced in T-bet-deficient cells, and most importantly, ectopic T-bet could suppress signature Th17 gene products, including IL-17A, IL-17F, IL-22, and retinoic acid-related orphan receptor γT, even in STAT1-deficient T cells. Taken together, these studies formally establish that, downstream of IFN-γ, IL-27, and likely all STAT1-activating cytokines, there are both STAT1 and T-bet-dependent pathways capable of suppressing Th17 responses.

Notch signaling is a critical regulator of allogeneic CD4+ T-cell responses mediating graft-versus-host disease

Graft-versus-host disease (GVHD) remains the major barrier to the success of allogeneic hematopoietic stem cell transplantation (HSCT). GVHD is caused by donor T cells that mediate host tissue injury through multiple inflammatory mechanisms. Blockade of individual effector molecules has limited efficacy in controlling GVHD. Here, we report that Notch signaling is a potent regulator of T-cell activation, differentiation, and function during acute GVHD. Inhibition of canonical Notch signaling in donor T cells markedly reduced GVHD severity and mortality in mouse models of allogeneic HSCT. Although Notch-deprived T cells proliferated and expanded in response to alloantigens in vivo, their ability to produce interleukin-2 and inflammatory cytokines was defective, and both CD4(+) and CD8(+) T cells failed to up-regulate selected effector molecules. Notch inhibition decreased the accumulation of alloreactive T cells in the intestine, a key GVHD target organ. However, Notch-deprived alloreactive CD4(+) T cells retained significant cytotoxic potential and antileukemic activity, leading to improved overall survival of the recipients. These results identify Notch as a novel essential regulator of pathogenic CD4(+) T-cell responses during acute GVHD and suggest that Notch signaling in T cells should be investigated as a therapeutic target after allogeneic HSCT.

Cutaneous GVHD is associated with the expansion of tissue-localized Th1 and not Th17 cells

Studies in mice have shown that proinflammatory Th17 cells can cause acute graft-versus-host disease (aGVHD) related tissue damage; however, whether they play a role in human aGVHD remains unclear. In a prospective study, we measured the proportion of Th17 cells in the blood and skin of patients at the onset of aGVHD. We found no difference in the proportion or amount of IL-17 produced by T cells in the blood of patients with aGVHD (n = 20) compared with time-matched patients without GVHD (n = 14). Moreover, Th17 cells were not increased in the skin of patients with cutaneous aGVHD (n = 7) compared with healthy controls (n = 10). In contrast, we found significantly more interferon-γ-producing T cells in the skin of patients with aGVHD compared with controls. These data support the long-standing paradigm that tissue localized interferon-γ-producing cells are the perpetrators of aGVHD.

GVHD after haploidentical transplantation: a novel, MHC-defined rhesus macaque model identifies CD28- CD8+ T cells as a reservoir of breakthrough T-cell proliferation during costimulation blockade and sirolimus-based immunosuppression

We have developed a major histocompatibility complex-defined primate model of graft-versus-host disease (GVHD) and have determined the effect that CD28/CD40-directed costimulation blockade and sirolimus have on this disease. Severe GVHD developed after haploidentical transplantation without prophylaxis, characterized by rapid clinical decline and widespread T-cell infiltration and organ damage. Mechanistic analysis showed activation and possible counter-regulation, with rapid T-cell expansion and accumulation of CD8(+) and CD4(+) granzyme B(+) effector cells and FoxP3(pos)/CD27(high)/CD25(pos)/CD127(low) CD4(+) T cells. CD8(+) cells down-regulated CD127 and BCl-2 and up-regulated Ki-67, consistent with a highly activated, proliferative profile. A cytokine storm also occurred, with GVHD-specific secretion of interleukin-1 receptor antagonist (IL-1Ra), IL-18, and CCL4. Costimulation Blockade and Sirolimus (CoBS) resulted in striking protection against GVHD. At the 30-day primary endpoint, CoBS-treated recipients showed 100% survival compared with no survival in untreated recipients. CoBS treatment resulted in survival, increasing from 11.6 to 62 days (P < .01) with blunting of T-cell expansion and activation. Some CoBS-treated animals did eventually develop GVHD, with both clinical and histopathologic evidence of smoldering disease. The reservoir of CoBS-resistant breakthrough immune activation included secretion of interferon-γ, IL-2, monocyte chemotactic protein-1, and IL-12/IL-23 and proliferation of cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin-resistant CD28(-) CD8(+) T cells, suggesting adjuvant treatments targeting this subpopulation will be needed for full disease control.

Immune regulatory cells in umbilical cord blood and their potential roles in transplantation tolerance

Umbilical cord blood (UCB) is a source of primitive hematopoietic stem (HSC) and progenitor cells, that served as an alternative to bone marrow (BM) for effective transplantation therapy. Success of HSC transplantation (HSCT) is limited in part by graft-versus-host disease (GVHD), graft rejection and delayed immune reconstitution, which all relate to immunological complications. GVHD after UCB transplantation is lower compared to that of BM HSCT. This may relate to the tolerogenic nature of T cells, mononuclear cells (MNCs) and especially immune regulatory cells existing in UCB. UCB contains limiting numbers of HSC or CD34(+) cell dose for adult patients resulting in delayed engraftment after UCB transplantation (UCBT). This needs to be improved for optimal transplantation outcomes. Approaches have been undertaken to promote HSC engraftment, including co-infusion of multiple units of UCB cells. These new methods however added additional immunological complications. Herein, we describe current knowledge on features of UCB immune cells, including regulatory T cells (Tregs) and mesenchymal stem/stromal cells (MSCs) and their potential future usage to reduce GVHD.

Indoleamine 2,3-dioxygenase in human hematopoietic stem cell transplantation

In recent years tryptophan metabolism and its rate limiting enzyme indoleamine 2,3-dioxygenase (IDO) have attracted increasing attention for their potential to modulate immune responses including the regulation of transplantation tolerance. The focus of this review is to discuss some features of IDO activity which particularly relate to hematopoietic stem cell transplantation (HSCT). HSCT invariably involves the establishment of some degree of a donor-derived immune system in the recipient. Thus, the outstanding feature of tolerance in HSCT is that in this type of transplantation it is not rejection, which causes the most severe problems to HSCT recipients, but the reverse, graft-versus-host (GvH) directed immune responses. We will discuss the peculiar role of IDO activity and accelerated tryptophan metabolism at the interface between immune activation and immune suppression and delineate from theoretical and experimental evidence the potential significance of IDO in mediating tolerance in HSCT. Finally, we will examine therapeutic options for exploitation of IDO activity in the generation of allo-antigen-specific tolerance, i.e. avoiding allo-reactivity while maintaining immunocompetence, in HSCT.

Th17/Treg ratio in human graft-versus-host disease

Th17 cells have never been explored in human graft-versus-host disease (GVHD). We studied the correlation between the presence of Th17 cells with histologic and clinical parameters. We first analyzed a cohort of 40 patients with GVHD of the gastrointestinal tract. Tumor necrosis factor (TNF), TNF receptors, and Fas expression, and apoptotic cells, CD4(+)IL-17(+) cells (Th17), and CD4(+)Foxp3(+) cells (Treg) were quantified. A Th17/Treg ratio less than 1 correlated both with the clinical diagnosis (P < .001) and more than 2 pathologic grades (P < .001). A Th17/Treg ratio less than 1 also correlated with the intensity of apoptosis of epithelial cells (P = .03), Fas expression in the cellular infiltrate (P = .003), TNF, and TNF receptor expression (P < .001). We then assessed Th17/Treg ratio in 2 other independent cohorts; a second cohort of 30 patients and confirmed that Th17/Treg ratio less than 1 correlated with the pathologic grade of GI GVHD. Finally, 15 patients with skin GVHD and 11 patients with skin rash but without pathologic GVHD were studied. Results in this third cohort of patients with skin disease confirmed those found in patients with GI GVHD. These analyses in 96 patients suggest that Th17/Treg ratio could be a sensitive and specific pathologic in situ biomarker of GVHD.

Constitutively active Stat5b in CD4+ T cells inhibits graft-versus-host disease lethality associated with increased regulatory T-cell potency and decreased T effector cell responses

Overexpression of a constitutively active form of Stat5b (Stat5b-CA) increases regulatory T cells (Tregs). We show that Stat5b-CA transgenic (TG) CD4(+) T cells had a markedly reduced graft-versus-host disease (GVHD) capacity versus wild-type (WT) T cells. Stat5b-CA TG versus WT CD4(+) T cells had a higher proportion of Tregs, which were superior in suppressing alloresponses mediated by CD4(+)CD25(-) effector T cells (Teffs). By day 5 after transplantation, Stat5b-CA TG Tregs had expanded approximately 3-fold more than WT Tregs. Purified Stat5b-CA TG Tregs added to WT CD4(+)CD25(-) Teffs were superior on a per-cell basis for inhibiting GVHD versus WT Tregs. Surprisingly, rigorously Treg-depleted Stat5b-CA TG versus WT CD4(+)CD25(-) Teffs caused less GVHD lethality associated with diminished Teff proinflammatory and increased Th2 anti-inflammatory cytokine responses. Reduced GVHD by Stat5b-CA TG versus WT Teffs could not be explained by conversion into Tregs in day 10 posttransplantation spleen or small intestine. In addition, Stat5b-CA TG Teffs retained a graft-versus-leukemia response. These results indicate a major role for Stat5 in Treg expansion and potency along with a lesser but significant role in Teff activation and suggest a strategy of pharmacologic Stat5b up-regulation as a means of decreasing GVHD while retaining a graft-versus-leukemia effect.

SOCS3 regulates graft-versus-host disease

Suppressor of cytokine signaling-3 (SOCS3) is the main intracellular regulator of signaling by granulocyte colony-stimulating factor, an immune-modulatory cytokine used to mobilize stem cells for transplantation. We have therefore studied the contribution of SOCS3 to the spectrum of graft-versus-host disease (GVHD) after allogeneic stem cell transplantation (SCT). Grafts from SOCS3(-/Deltavav) donor mice in which SOCS3 deficiency is restricted to the hematopoietic compartment had an augmented capacity to induce acute GVHD. With the use of SOCS3(-/DeltaLysM) and SOCS3(-/Deltalck) donors in which SOCS3 deficiency was restricted to the myeloid or T-cell lineage, respectively, we confirmed SOCS3 deficiency promoted acute GVHD mortality and histopathology within the gastrointestinal tract by effects solely within the donor T cell. SOCS3(-/Deltalck) donor T cells underwent enhanced alloantigen-dependent proliferation and generation of interleukin-10 (IL-10), IL-17, and interferon-gamma (IFNgamma) after SCT. The enhanced capacity of the SOCS3(-/Deltalck) donor T cell to induce acute GVHD was dependent on IFNgamma but independent of IL-10 or IL-17. Surprisingly, SOCS3(-/Deltalck) donor T cells also induced severe, transforming growth factor beta- and IFNgamma-dependent, sclerodermatous GVHD. Thus, the delivery of small molecule SOCS3 mimetics may prove to be useful for the inhibition of both acute and chronic GVHD.

Stem cell mobilization with G-CSF induces type 17 differentiation and promotes scleroderma

The recent shift to the use of stem cells mobilized by granulocyte colony-stimulating factor (G-CSF) for hematopoietic transplantation has increased chronic graftversus-host disease (GVHD), although the mechanisms of this are unclear. We have found that G-CSF invokes potent type 17 rather than type 1 or type 2 differentiation. The amplification of interleukin-17 (IL-17) production by G-CSF occurs in both CD4 and CD8 conventional T cells and is dependent on, and downstream of, G-CSF-induced IL-21 signaling. Importantly, donor IL-17A controls the infiltration of macrophages into skin and cutaneous fibrosis, manifesting late after transplantation as scleroderma. Interestingly, donor CD8 T cells were the predominant source of IL-17A after transplantation and could mediate scleroderma independently of CD4 T cells. This study provides a logical explanation for the propensity of allogeneic stem cell transplantation to invoke sclerodermatous GVHD and suggests a therapeutic strategy for intervention.

Premetastatic milieu explained by TLR4 agonist-mediated homeostatic inflammation

Accumulating evidence suggests that Toll-like receptor 4 (TLR4), a sensor for danger signals, is expressed not only in immune cells, but also in resident epithelial cells, and appears to participate in tissue homeostasis. To explain the premetastatic microenvironment created by the newly discovered endogenous TLR4 ligands, I propose a hypothesis of homeostatic inflammation that includes the classical danger hypothesis.

Homing in on acute graft vs. host disease: tissue-specific T regulatory and Th17 cells

Acute graft vs. host disease (aGVHD) is a major limitation of hematopoietic stem cell transplantation (HSCT), and it causes significant morbidity and mortality for this patient population. This immune-mediated injury occurs unpredictably and is caused by donor-derived T cells reacting to recipient alloantigens. Although donor Th1 cells play a critical role in aGVHD generation, numerous arms of both the innate and the adaptive immune systems along with determinants of lymphocyte trafficking are likely involved in the multifaceted cascade of immunological events that culminates in clinical aGVHD. T regulatory and Th17 cells are T cell subsets distinct from Th1 cells that are likely involved with aGVHD. Regulatory T cells (Tregs) have been implicated in the prevention of aGVHD in both mouse and man, while Th17 cells may modulate early inflammatory responses associated with aGVHD, especially those involving the skin and the lungs. Interestingly, these two lymphocyte subsets appear to be reciprocally regulated in part through retinoic acid, through cytokines such as IL-6, and via interactions with dendritic cells. Another area under tight regulation appears to be the homing of lymphocytes to lymph nodes, skin, and gut. Adhesion molecules including chemokine receptors, selectins, and integrins may identify specific T cell subsets with unique migratory functional properties during HSCT. Controlling the migration patterns of Th17 cells and Tregs represents a potential therapeutic target. A major goal of HSCT research will be to develop approaches to pharmacologically manipulate T cell subsets in vivo or to select, expand, and infuse T cell subsets that will maximize the targeted graft vs. tumor effect while minimizing the potentially fatal side effects of aGVHD. A better understanding of Tregs and their tissue specificity should lead to improvement in the success of HSCT.

T helper17 cells are sufficient but not necessary to induce acute graft-versus-host disease

T helper (Th)1 cells were considered responsible for the induction of graft-versus-host disease (GVHD), but recently the concept has been challenged. Th17 cells play a critical role in mediating autoimmune diseases, but their role in the pathogenesis of GVHD remains unclear. Herein we compare the ability of in vitro generated Th1 and Th17 cells from C57BL/6 mice to induce GVHD in lethally irradiated BALB/c recipients. Allogeneic Th17 cells had superior expansion and infiltration capabilities in GVHD target organs, which correlated with their increased pathogenicity when compared with naïve or Th1 controls. Th17 cells caused no pathology in the syngeneic recipients, indicating that antigen-activation was required for their pathogenicity. Polarized Th17 cells could not maintain their phenotype in vivo as they produced a significant amount of interferon (IFN)-gamma after being transplanted into allogeneic recipients; however, IFN-gamma was not required for Th17 cell-induced GVHD. Further, we evaluated the pathogenesis of Th17 cells in GVHD by using polyclonal nonprimed CD4T cells in a clinically relevant allogeneic bone marrow transplantation (BMT) setting. We found that disruption of Th17-differentiation alone by targeting RORgammat (Th17-specific transcription factor) had no significant effect on GVHD development. We conclude that Th17 cells are sufficient but not necessary to induce GVHD.

T cells helping GVHD: take-away lessons

Using double cytokine knockout donor CD4 cells, in this issue of Blood, Yi and colleagues help clarify the complex role of IFN-γ and the tissue specificity of Th1, Th17, and Th2 subsets in murine GVHD.1