gammadelta T cells: an important source of IL-17

Interleukin-17-producing gammadelta T cells selectively expand in response to pathogen products and environmental signals

Gammadelta T cells are an innate source of interleukin-17 (IL-17), preceding the development of the adaptive T helper 17 (Th17) cell response. Here we show that IL-17-producing T cell receptor gammadelta (TCRgammadelta) T cells share characteristic features with Th17 cells, such as expression of chemokine receptor 6 (CCR6), retinoid orphan receptor (RORgammat), aryl hydrocarbon receptor (AhR), and IL-23 receptor. AhR expression in gammadelta T cells was essential for the production of IL-22 but not for optimal IL-17 production. In contrast to Th17 cells, CCR6(+)IL-17-producing gammadelta T cells, but not other gammadelta T cells, express Toll-like receptors TLR1 and TLR2, as well as dectin-1, but not TLR4 and could directly interact with certain pathogens. This process was amplified by IL-23 and resulted in expansion, increased IL-17 production, and recruitment of neutrophils. Thus, innate receptor expression linked with IL-17 production characterizes TCRgammadelta T cells as an efficient first line of defense that can orchestrate an inflammatory response to pathogen-derived as well as environmental signals long before Th17 cells have sensed bacterial invasion.

Interleukin-17-producing gammadelta+ T cells protect NOD mice from type 1 diabetes through a mechanism involving transforming growth factor-beta

Whether interleukin (IL)-17 promotes a diabetogenic response remains unclear. Here we examined the effects of neutralization of IL-17 on the progress of adoptively transferred diabetes. IL-17-producing cells in non-obese diabetic (NOD) mice were identified and their role in the pathogenesis of diabetes examined using transfer and co-transfer assays. Unexpectedly, we found that in vivo neutralization of IL-17 did not protect NOD-severe combined immunodeficiency (SCID) mice against diabetes transferred by diabetic splenocytes. In NOD mice, gammadelta(+) T cells were dominated by IL-17-producing cells and were found to be the major source of IL-17. Interestingly, these IL-17-producing gammadelta T cells did not exacerbate diabetes in an adoptive transfer model, but had a regulatory effect, protecting NOD mice from diabetes by up-regulating transforming growth factor (TGF)-beta production. Our data suggest that the presence of IL-17 did not increase the chance of the development of diabetes; gammadelta T cells protected NOD mice from diabetes in a TGF-beta-dependent manner, irrespective of their role as major IL-17 producers.

Separating graft-versus-leukemia from graft-versus-host disease in allogeneic hematopoietic stem cell transplantation

Routine methods to maximize the graft-versus-leukemia (GvL) activity of allogeneic hematopoietic stem cell transplantation (HSCT) without the detrimental effects of graft-versus-host disease (GvHD) are lacking. Depletion or inhibition of alloreactive T cells is partially effective in preventing GvHD, but usually leads to decreased GvL activity. The current model for the pathophysiology of acute GvHD describes a series of immune pathways that lead to activation of donor T cells and inflammatory cytokines responsible for tissue damage in acute GvHD. This model does not account for how allotransplant can lead to GvL effects without GvHD, or how the initial activation of donor immune cells may lead to counter-regulatory effects that limit GvHD. In this review, we will summarize new findings that support a more complex model for the initiation of GvHD and GvL activities in allogeneic HSCT, and discuss the potential of novel strategies to enhance GvL activity of the transplant.

Vdelta1 T lymphocytes producing IFN-gamma and IL-17 are expanded in HIV-1-infected patients and respond to Candida albicans

In early HIV-1 infection, Vdelta1 T lymphocytes are increased in peripheral blood and this is related to chemokine receptor expression, chemokine response, and recirculation. Herein we show that, at variance with healthy donors, in HIV-1-infected patients ex vivo-isolated Vdelta1 T cells display cytoplasmic interferon-gamma (IFN-gamma). Interestingly, these cells coexpress cytoplasmic interleukin-17 (IL-17), and bear the CD27 surface marker of the memory T-cell subset. Vdelta1 T cells, isolated from either patients or healthy donors, can proliferate and produce IFN-gamma and IL-17 in response to Candida albicans in vitro, whereas Vdelta2 T cells respond with proliferation and IFN-gamma/IL-17 production to mycobacterial or phosphate antigens. These IFN-gamma/IL-17 double-producer gammadelta T cells express the Th17 RORC and the Th1 TXB21 transcription factors and bear the CCR7 homing receptor and the CD161 molecule that are involved in gammadelta T-cell transendothelial migration. Moreover, Vdelta1 T cells responding to C albicans express the chemokine receptors CCR4 and CCR6. This specifically equipped circulating memory gammadelta T-cell population might play an important role in the control of HIV-1 spreading and in the defense against opportunistic infections, possibly contributing to compensate for the impairment of CD4(+) T cells.

Differential regulation of Foxp3 and IL-17 expression in CD4 T helper cells by IRAK-1

Host immune responses are finely regulated by the opposing effects of Th17 and T regulatory (Treg) cells. Treg cells help to dampen inflammatory processes and Th17 cells facilitate various aspects of immune activation. The differentiation of Th cells depends on a unique combination of stimulants and subsequent activation of diverse transcription factors. In particular, cooperative activation of NFAT and Smad3 leads to the induction of Treg cells, and cooperation among STAT3 and Smad3 switches to the induction of Th17 cells. We have previously shown that the IL-1 receptor associated kinase 1 (IRAK-1) selectively activates STAT3 and inactivates NFAT. Physiological studies have shown that IRAK-1(-/-) mice are protected from developing various inflammatory diseases, including experimental autoimmune encephalomyelitis and atherosclerosis with unknown mechanism. In this study, we demonstrate that IRAK-1 plays a critical modulatory role in the differentiation of Th17 and Treg cells. Following stimulation with TCR agonists and TGFbeta, IRAK-1(-/-) CD4 Th cells display elevated nuclear NFATc2 levels and increased interaction of NFATc2 and Smad3, resulting in increased expression of Foxp3, a key marker for Treg cells. IRAK-1(-/-) mice have constitutively higher populations of Treg cells. In contrast, when stimulated with TCR agonists together with IL-6 and TGF-beta, IRAK-1(-/-) CD4 Th cells exhibit attenuated STAT3 Ser727 phosphorylation and reduced expression of IL-17 and RORgamma t compared with wild-type cells. Correspondingly, IRAK-1 deletion results in decreased IL-17 expression and dampened inflammatory responses in acute and chronic inflammatory mice models. Our data provides mechanistic explanation for the anti-inflammatory phenotypes of IRAK-1(-/-) mice.

TLR2 deficiency leads to increased Th17 infiltrates in experimental brain abscesses

TLR2 plays a pivotal role in recognizing Staphylococcus aureus, a common etiologic agent of CNS parenchymal infections, such as brain abscess. We previously reported that brain abscesses of TLR2 knockout (KO) mice exhibited elevated IL-17 levels, suggesting the presence of an alternative pathway available to respond to S. aureus infection that may involve Th17 cells. Both CD4(+) and CD8(+) T cell infiltrates were elevated in brain abscesses of TLR2 KO mice at days 3, 7, and 14 postinfection compared with wild-type animals. Intracellular cytokine staining revealed a significant increase in the frequency of IL-17-producing Th17 cells in TLR2 KO mice with relatively few IFN-gamma-positive cells. gammadelta T cells were also a source of IL-17 in brain abscesses. Microglia, astrocytes, and macrophages were shown to express both IL-17RA and IL-17RC. Despite receptor expression, IL-17 was relatively ineffective at eliciting glial activation, whereas the cytokine augmented the ability of TNF-alpha to induce CXCL2 and CCL2 expression by macrophages. Based on the ability of IL-17 to elicit the release of chemokines and other proinflammatory mediators, we propose that the exaggerated IL-17 response that occurs in TLR2 KO mice functions in a compensatory manner to control brain abscess pathogenesis, with cells other than glia as targets for IL-17 action. This is supported by our findings in which innate immune infiltrates were not significantly different between TLR2 KO and wild-type mice in conjunction with the lack of prolonged alterations in the synthesis of other proinflammatory molecules during the course of infection.

Intestinal T cells: facing the mucosal immune dilemma with synergy and diversity

The epithelium of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a plethora of foreign and potentially harmful antigens. Consequently, the immune system of the gut faces the daunting task of distinguishing harmless dietary proteins and commensal bacteria from potentially dangerous pathogens, and of then responding accordingly. Mucosal T cells play a central role in maintaining barrier function and controlling the delicate balance between immune activation and immune tolerance. This review will focus on the unique features of mucosal T cell subsets that reside in the epithelium and lamina propria of the gut.

The IL-23/Th17 axis in the immunopathogenesis of psoriasis

Abnormal production of inflammatory mediators is believed to play an important role in the pathogenesis of psoriasis. Emerging data, both in mice and in humans, put the spotlight on a new subset of T helper (Th) cells, in part characterized by their production of IL-17 and accordingly named Th17 cells. Here, we review the development, characterization, and function of human Th17 cells as well as the crucial role of IL-23 in the context of Th17-cell-dependent chronic inflammation in psoriasis. We further discuss recent clinical trials targeting the IL-23/Th17 axis in psoriasis.

Barrier immunity and IL-17

CD4+ T(H)17 cells display a featured role in barrier immunity. This effector population of T cells is important for clearance of microorganisms but can also promote autoimmunity at barrier sites. Recent work has indicated that these effector cells share a pathway with CD4+ regulatory T cells (T(R) cells) that also have a critical function in barrier protection and immune regulation. The development and function of T(H)17 cells, and their relationship with T(R) cells are discussed.

Human memory FOXP3+ Tregs secrete IL-17 ex vivo and constitutively express the T(H)17 lineage-specific transcription factor RORgamma t

Recent studies have suggested a close relationship between CD4(+)FOXP3(+) regulatory T cells (Tregs) and proinflammatory IL-17-producing T helper cells (T(H)17) expressing the lineage-specific transcription factor RORgamma t. We report here the unexpected finding that human memory Tregs secrete IL-17 ex vivo and constitutively express RORgamma t. IL-17-secreting Tregs share some phenotypic and functional features with conventional T(H)17 cells, expressing high levels of CCR4 and CCR6 and low levels of CXCR3. However, unlike conventional T(H)17 cells, they express low levels of CD161 and mostly fail to cosecrete IL-22 and TNF-alpha ex vivo. Ex vivo secretion of IL-17 and constitutive expression of RORgamma t by human memory Tregs suggest that, in addition to their well-known suppressive functions, these cells likely play additional, as yet undescribed, proinflammatory functions.

Profiling of the early transcriptional response of murine gammadelta T cells following TCR stimulation

Gammadelta T cells represent one of the three lineages of lymphocytes, along with alphabeta T cells and B cells, which express antigen receptors. Since their discovery over two decades ago, considerable effort has been made to understand their antigen specificity and their contribution to the immune response. From these studies, we have learned that gammadelta T cells recognize a different set of antigens than alphabeta T cells, acquire effector functions faster than alphabeta T cells, regulate the response of other immune cells during infection, and play distinct roles in immunity. The molecular basis for how gammadelta T cells manifest their unique functions, however, remains unknown. To address this, we profiled the genes upregulated soon after TCR stimulation in order to identify which gene networks associated with T cell effector function are induced in gammadelta T cells. Interestingly, most of the genes in this transcriptional profile were not unique to activated gammadelta T cells, as they were also expressed in activated alphabeta T cells. However, many of the genes within this profile were upregulated with faster kinetics and/or greater magnitude in activated gammadelta T cells than in activated alphabeta T cells. In addition, we found that the genes in the transcriptional profile of activated wild-type gammadelta T cells can be used as a standard to screen activated gammadelta T cells from mice with potential signaling defects for alterations in gammadelta TCR signal transduction. Thus, by defining the early transcriptional response of activated wild-type gammadelta T cells and by comparing their transcriptional profile to that of activated wild-type alphabeta T cells as well as to that of activated gammadelta T cells from signaling defective mice, we are able to gain important insights into the molecular basis for gammadelta T cell function.

Differential IL-23 requirement for IL-22 and IL-17A production during innate immunity against Salmonella enterica serovar Enteritidis

Early activation of the IL-12/IFN-gamma axis has been shown following Salmonella enterica serovar Enteritidis (S. Enteritidis) infection. We were interested to study whether IL-22 and IL-17A production is initiated early in response to S. Enteritidis. We demonstrate here that IL-22 was strongly elevated in the peritoneal lavage fluid and in serum already 1 day post-intraperitoneal infection (d.p.i.) of mice; not only IL-22 but also IL-17A was produced ex vivo by activated peritoneal exudate cells (PEC). Peritoneal gammadelta T cells were identified as cellular source of IL-17A. The early IL-22 production was completely IL-23-dependent. In contrast, IL-17A production was only partially IL-23-dependent. To investigate the local production of upstream cytokines important for induction of IL-22, IL-17A and IFN-gamma during salmonellosis, the production of IL-23 and IL-12 was studied. Elevated p19 and p40 mRNA levels were found in PEC at 1 d.p.i., whereas p35 mRNA levels were not changed. Besides, the T(h)17-promoting cytokines IL-6, IL-1beta and transforming growth factor-beta were produced in response to S. Enteritidis. However, IL-6 was not required for IL-22 or IL-17A production by PEC. By ex vivo analysis of PEC at 1 d.p.i., we show that the major producers of early IL-12/23p40 in the peritoneal cavity were dendritic cells (DC), whereas macrophages notably contributed to IL-6 production. Taken together, these data suggest that DC initiate early IL-22 production at the site of infection which may contribute to resistance against salmonellosis. Furthermore, we provide evidence that production of IL-22 and IL-17A is differentially regulated during infection.

The atypical chemokine receptor D6 contributes to the development of experimental colitis

Proinflammatory CC chemokines control leukocyte recruitment and function during inflammation by engaging chemokine receptors expressed on circulating leukocytes. The D6 chemokine receptor can bind several of these chemokines, but appears unable to couple to signal transduction pathways or direct cell migration. Instead, D6 has been proposed to act as a chemokine scavenger, removing proinflammatory chemokines to dampen leukocyte responses. In this study, we have examined the role of D6 in the colon using the dextran sodium sulfate-induced model of colitis. We show that D6 is expressed in the resting colon, predominantly by stromal cells and B cells, and is up-regulated during colitis. Unexpectedly, D6-deficient mice showed reduced susceptibility to colitis and had less pronounced clinical symptoms associated with this model. D6 deletion had no impact on the level of proinflammatory CC chemokines released from cultured colon explants, or on the balance of leukocyte subsets recruited to the inflamed colon. However, late in colitis, inflamed D6-deficient colons showed enhanced production of several proinflammatory cytokines, including IFN-gamma and IL-17A, and there was a marked increase in IL-17A-secreting gammadelta T cells in the lamina propria. Moreover, Ab-mediated neutralization of IL-17A worsened the clinical symptoms of colitis at these later stages of the response in D6-deficient, but not wild-type, mice. Thus, D6 can contribute to the development of colitis by regulating IL-17A secretion by gammadelta T cells in the inflamed colon.

CD27 is a thymic determinant of the balance between interferon-gamma- and interleukin 17-producing gammadelta T cell subsets

The production of cytokines such as interferon-gamma and interleukin 17 by alphabeta and gammadelta T cells influences the outcome of immune responses. Here we show that most gammadelta T lymphocytes expressed the tumor necrosis factor receptor family member CD27 and secreted interferon-gamma, whereas interleukin 17 production was restricted to CD27(-) gammadelta T cells. In contrast to the apparent plasticity of alphabeta T cells, the cytokine profiles of these distinct gammadelta T cell subsets were essentially stable, even during infection. These phenotypes were established during thymic development, when CD27 functions as a regulator of the differentiation of gammadelta T cells at least in part by inducing expression of the lymphotoxin-beta receptor and genes associated with trans-conditioning and interferon-gamma production. Thus, the cytokine profiles of peripheral gammadelta T cells are predetermined mainly by a mechanism involving CD27.

Thymic maturation determines gammadelta T cell function, but not their antigen specificities

gammadelta T cells contribute uniquely to host immune defense, but how they do so remains unclear. Recent work suggests that thymic selection does little to constrain gammadelta T cell antigen specificities, but instead determines their effector fate. When triggered through the T cell receptor, ligand-experienced cells make IFNgamma, whereas ligand-naïve gammadelta T cells produce IL-17, a major initiator of inflammation. These advances warrant a fresh look at how gammadelta T cells may function in the immune system.

Development of a bovine ileal cannulation model to study the immune response and mechanisms of pathogenesis of paratuberculosis

An ileal cannulation model was developed in conjunction with a flow cytometric assay to gain a better understanding of the mechanisms of immunopathogenesis of Johne's disease caused by Mycobacterium avium subsp. paratuberculosis. Initial studies with calves showed that M. avium subsp. paratuberculosis DNA is detectable by PCR in ileal biopsies during the first months following experimental infection. Inflammatory lesions were not detected on endoscopic evaluation up to 8 months postexperimental infection. M. avium subsp. paratuberculosis DNA was detected in multiple tissues at necropsy 8 months postinfection. Examination of the activation status of epithelial lymphocytes from the jejunum and ileum from infected and control animals at necropsy revealed that none of the major subsets of lymphocytes (NK, CD2(+), and CD2(-) gammadelta T lymphocytes, or CD4 and CD8 alphabeta T lymphocytes) expressed activation molecules CD25, CD26, CD71, ACT1, or ACT16. Subsets of CD4 and CD8 T lymphocytes from control and infected animals expressed CD26. The majority of CD4 and CD8 T lymphocytes expressed CD45R0, the memory T-lymphocyte marker. An immune response to M. avium subsp. paratuberculosis was detected by 3 months postinfection, dominated by a strong proliferative response of CD4 memory T lymphocytes. The findings indicate an immune response develops following initial exposure to M. avium subsp. paratuberculosis that controls but does not eliminate the pathogen. This persistence of M. avium subsp. paratuberculosis possibly leads to erosion and dysregulation of protective immunity at later time points postinfection. Continuous access to the ileum offers an opportunity to elucidate the cellular and molecular events leading to immune dysregulation and development of chronic inflammatory ileitis.

[Helper (TH1, TH2, TH17) and regulatory cells (Treg, TH3, NKT) in rheumatoid arthritis]

The immune response foreign antigens require a perfect coordination of cells that participate in its different phases. The objective of the response is the rapid destruction of the microorganisms with a minimum repercussion on self-cells and tissues. The regulation of this process is carried out fundamentally by T lymphocytes. There are two main types of coordinator cells: helper cells, what organize the initial immune response, and regulatory cells, what avoid immune attack against self and once the infection is controlled, disassemble the response. There are three types of helper cells which coordinate answers to intracellular parasites (TH1), helmints (TH2) and extracellular bacteria and fungi (TH17). The hyperfunction of TH17 cells is associated with diseases as reumatoid arthritis, due to the hypersecretion of the proinflammatory citoquine IL17. The condition of helper or regulatory cell is the current object of review. TH1, TH2 and TH17 cells have helper and also regulatory functions. In addition, regulatory T cells play an important role in the coordination of the first moments of the response to viral infection in a direct and indirect way, inducing differentiation of TH17 cells.

Progress of gut associated lymphocyte homing in inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by chronic inflammation of intestinal tract, and it is classified into 2 subtypes traditionally, namely ulcerative colitis (UC) and Crohn's disease (CD). Many investigations have shown that intestinal lymphocyte homing (lymphocyte homing, LH) is closely related to IBD. This paper reviews the advances in the relationship between inflammatory bowel disease and lymphocyte homing.

IL-17-producing gammadelta T cells

IL-17 is produced not only by CD4(+) alphabeta T cells, but also CD8(+) alphabeta T cells, NKT cells, and gammadelta T cells, plus some non-T cells, including macrophages and neutrophils. The ability of IL-17 to deploy neutrophils to sites of inflammation imparts this cytokine with a key role in diseases of several types. Surprisingly, gammadelta T cells are responsible for much of the IL-17 produced in several disease models, particularly early on.

Allergic airway hyperresponsiveness-enhancing gammadelta T cells develop in normal untreated mice and fail to produce IL-4/13, unlike Th2 and NKT cells

Allergic airway hyperresponsiveness (AHR) in OVA-sensitized and challenged mice, mediated by allergen-specific Th2 cells and Th2-like invariant NKT (iNKT) cells, develops under the influence of enhancing and inhibitory gammadelta T cells. The AHR-enhancing cells belong to the Vgamma1(+) gammadelta T cell subset, cells that are capable of increasing IL-5 and IL-13 levels in the airways in a manner like Th2 cells. They also synergize with iNKT cells in mediating AHR. However, unlike Th2 cells, the AHR enhancers arise in untreated mice, and we show here that they exhibit their functional bias already as thymocytes, at an HSA(high) maturational stage. In further contrast to Th2 cells and also unlike iNKT cells, they could not be stimulated to produce IL-4 and IL-13, consistent with their synergistic dependence on iNKT cells in mediating AHR. Mice deficient in IFN-gamma, TNFRp75, or IL-4 did not produce these AHR-enhancing gammadelta T cells, but in the absence of IFN-gamma, spontaneous development of these cells was restored by adoptive transfer of IFN-gamma-competent dendritic cells from untreated donors. The i.p. injection of OVA/aluminum hydroxide restored development of the AHR enhancers in all of the mutant strains, indicating that the enhancers still can be induced when they fail to develop spontaneously, and that they themselves need not express TNFRp75, IFN-gamma, or IL-4 to exert their function. We conclude that both the development and the cytokine potential of the AHR-enhancing gammadelta T cells differs critically from that of Th2 cells and NKT cells, despite similar influences of these cell populations on AHR.

Role of γδ T Cells in Lung Inflammation

The resident population of γδ T cells in the normal lung is small but during lung inflammation, γδ T cells can increase dramatically. Histological analysis reveals diverse interactions between γδ T cells and other pulmonary leukocytes. Studies in animal models show that γδ T cells play a role in allergic lung inflammation where they can protect normal lung function, that they also are capable of resolving infection-induced pulmonary inflammation, and that they can help preventing pulmonary fibrosis. Lung inflammation threatens vital lung functions. Protection of the lung tissues and their functions during inflammation is the net-effect of opposing influences of specialized subsets of γδ T cells as well as interactions of these cells with other pulmonary leukocytes.

Exploring the full spectrum of macrophage activation

Macrophages display remarkable plasticity and can change their physiology in response to environmental cues. These changes can give rise to different populations of cells with distinct functions. In this Review we suggest a new grouping of macrophage populations based on three different homeostatic activities - host defence, wound healing and immune regulation. We propose that similarly to primary colours, these three basic macrophage populations can blend into various other 'shades' of activation. We characterize each population and provide examples of macrophages from specific disease states that have the characteristics of one or more of these populations.

gammadelta T cell selection: is anyone useless?

In this issue of Immunity, a study by Jensen et al. (2008) suggests that T cell-receptor engagement during development affects gammadelta T cell polarization toward either interferon-gamma or interleukin-17 production. This might underlie their unique innate ability to regulate inflammation.

Response of gammadelta T Cells to plant-derived tannins

Many pharmaceutical drugs arc isolated from plants used in traditional medicines, and new plant-derived pharmaceutical drugs continue to be identified. Relevant to this review, different plant-derived agonists for gammadelta T cells are described that impart effector functions upon distinct subsets of these cells. Recently, plant tannins have been defined as one class of gammadelta T cell agonist and appear to preferentially activate the mucosal population. Mucosal gammadelta T cells function to modulate tissue immune responses and induce epithelium repair. Select tannins, isolated from apple peel, rapidly induce immune gene transcription in gammadelta T cells, leading to cytokinc production and increased responsiveness to secondary signals. Activity of these tannin preparations tracks to the procyanidin fraction, with the procyanidin trimer (C1) having the most robust activity defined to date. The response to the procyanidins is evolutionarily conserved in that responses are seen with human, bovine, and murine gammadelta T cells, although human cells show less selectivity. Procyanidin-induced responses described in this review likely account for the expansion of mucosal gammadelta T cells seen in mice and rats fed soluble extracts of tannins. Use of procyanidins to activate gammadelta T cells may represent a novel approach for the treatment of tissue damage and autoimmune diseases.