Protein kinase C-theta mediates negative feedback on regulatory T cell function

The role of protein kinase cη in T cell biology

Protein kinase Cη (PKCη) is a member of the novel PKC subfamily, which also includes δ, ε, and θ isoforms. Compared to the other novel PKCs, the function of PKCη in the immune system is largely unknown. Several studies have started to reveal the role of PKCη, particularly in T cells. PKCη is highly expressed in T cells, and is upregulated during thymocyte positive selection. Interestingly, like the θ isoform, PKCη is also recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell. However, unlike PKCθ, which becomes concentrated to the central region of the synapse, PKCη remains in a diffuse pattern over the whole area of the synapse, suggesting distinctive roles of these two isoforms in signal transduction. Although PKCη is dispensable for thymocyte development, further analysis of PKCη- or PKCθ-deficient and double-knockout mice revealed the redundancy of these two isoforms in thymocyte development. In contrast, PKCη rather than PKCθ, plays an important role for T cell homeostatic proliferation, which requires recognition of self-antigen. Another piece of evidence demonstrating that PKCη and PKCθ have isoform-specific as well as redundant roles come from the analysis of CD4 to CD8 T cell ratios in the periphery of these knockout mice. Deficiency in PKCη or PKCθ had opposing effects as PKCη knockout mice had a higher ratio of CD4 to CD8 T cells compared to that of wild-type mice, whereas PKCθ-deficient mice had a lower ratio. Biochemical studies showed that calcium flux and NFκB translocation is impaired in PKCη-deficient T cells upon TCR crosslinking stimulation, a character shared with PKCθ-deficient T cells. However, unlike the case with PKCθ, the mechanistic study of PKCη is at early stage and the signaling pathways involving PKCη, at least in T cells, are essentially unknown. In this review, we will cover the topics mentioned above as well as provide some perspectives for further investigations regarding PKCη.

Polarity and lymphocyte fate determination

Polarity within lymphocytes has been recognized to regulate a variety of processes, including migration, signaling, and the execution of effector function. It has been recently proposed, however, that this polarized behavior may also serve a different purpose in lymphocytes that have not yet encountered their foreign antigen-to coordinate asymmetric cell division. Asymmetric division is an evolutionarily conserved mechanism allowing a single cell to give rise to two distinct daughter cells from inception. In this review, recent findings in polarity and asymmetric division in lymphocytes are discussed.

Synapse-directed delivery of immunomodulators using T-cell-conjugated nanoparticles

Regulating molecular interactions in the T-cell synapse to prevent autoimmunity or, conversely, to boost anti-tumor immunity has long been a goal in immunotherapy. However, delivering therapeutically meaningful doses of immune-modulating compounds into the synapse represents a major challenge. Here, we report that covalent coupling of maleimide-functionlized nanoparticles (NPs) to free thiol groups on T-cell membrane proteins enables efficient delivery of compounds into the T-cell synapse. We demonstrate that surface-linked NPs are rapidly polarized toward the nascent immunological synapse (IS) at the T-cell/APC contact zone during antigen recognition. To translate these findings into a therapeutic application we tested the NP delivery of NSC-87877, a dual inhibitor of Shp1 and Shp2, key phosphatases that downregulate T-cell receptor activation in the synapse, in the context of adoptive T cell therapy of cancer. Conjugating NSC-87877-loaded NPs to the surface of tumor-specific T cells just prior to adoptive transfer into mice with advanced prostate cancer promoted a much greater T-cell expansion at the tumor site, relative to co-infusing the same drug dose systemically, leading to enhanced survival of treated animals. In summary, our studies support the application of T-cell-linked synthetic NPs as efficient drug delivery vehicles into the IS, as well as the broad applicability of this new paradigm for therapeutically modulating signaling events at the T-cell/APC interface.

Advances in graft-versus-host disease biology and therapy

Allogeneic haematopoietic stem cell transplantation is used to treat a variety of disorders, but its efficacy is limited by the occurrence of graft-versus-host disease (GVHD). The past decade has brought impressive advances in our understanding of the role of stimulatory and suppressive elements of the adaptive and innate immune systems from both the donor and the host in GVHD pathogenesis. New insights from basic immunology, preclinical models and clinical studies have led to novel approaches for prevention and treatment. This Review highlights the recent advances in understanding the pathophysiology of GVHD and its treatment, with a focus on manipulations of the immune system that are amenable to clinical application.

Protein kinase C-θ inhibits inducible regulatory T cell differentiation via an AKT-Foxo1/3a-dependent pathway

Protein kinase C (PKC)-θ has been shown to be a critical TCR signaling molecule that promotes the activation and differentiation of naive T cells into inflammatory effector T cells. In this study, we demonstrate that PKC-θ-mediated signals inhibit inducible regulatory T cell (iTreg) differentiation via an AKT-Foxo1/3A pathway. TGF-β-induced iTreg differentiation was enhanced in PKC-θ(-/-) T cells or wild-type cells treated with a specific PKC-θ inhibitor, but was inhibited by the PKC-θ activator PMA, or by CD28 crosslinking, which enhances PKC-θ activation. PKC-θ(-/-) T cells had reduced activity of the AKT kinase, and the expression of a constitutively active form of AKT in PKC-θ(-/-) T cells restored the ability to inhibit iTreg differentiation. Furthermore, knockdown or overexpression of the AKT downstream targets Foxo1 and Foxo3a was found to inhibit or promote iTreg differentiation in PKC-θ(-/-) T cells accordingly, indicating that the AKT-Foxo1/3A pathway is responsible for the inhibition of iTreg differentiation of iTregs downstream of PKC-θ. We conclude that PKC-θ is able to control T cell-mediated immune responses by shifting the balance between the differentiation of effector T cells and inhibitory Tregs.

Integrin signalling and function in immune cells

Integrins not only mediate cell-cell and cell-extracellular matrix adhesion, but also affect the multitude of signal transduction cascades in control of cell survival, proliferation, differentiation and organ development. Mutations in integrins or the major effectors of integrin signalling pathways cause defective organ development, immunodeficiency, cancer or autoimmune disease. Understanding of the signalling events that drive integrin activation and signalling is therefore crucial to uncover the molecular mechanisms of these diseases. This review discusses the key signalling complexes regulating integrin activation and function in both 'inside-out' and 'outside-in' pathways in T lymphocytes, including kinases, SLP-76, VAV1, ADAP, SKAP-55, RapL, RIAM, Rap1, Talin and Kindlin.

Embryonic stem cell-derived factors inhibit T effector activation and induce T regulatory cells by suppressing PKC-θ activation

Embryonic stem cells (ESCs) possess immune privileged properties and have the capacity to modulate immune activation. However, the mechanisms by which ESCs inhibit immune activation remain mostly unknown. We have previously shown that ESC-derived factors block dendritic cell maturation, thereby indirectly affecting T cell activation. Here, we show that ESC-derived factors also directly affect T cell activation. We provide the first demonstration that ESC-derived factors significantly down-regulated the expressions of IL-2 and IFN-γ, while markedly up-regulating the expression of IL-10, TGF-β, and Treg transcription factor Foxp3 in CD4+ CD25+ T cells. Furthermore, ESC-derived factors robustly suppressed T cell proliferation in response to the protein kinase C-θ (PKC-θ) activator phorbol 12-myristate 13-acetate (PMA). Western blot analysis indicated that ESC-derived factors prevented PKC-θ phosphorylation without influencing total PKC-θ levels. Moreover, IκB-α degradation was abrogated, confirming absence of PKC-θ activity. The impact of ESC-derived factors on PKC-θ activation appeared to be specific since other upstream T cell signaling components were not affected. In conclusion, ESCs appear to directly impact T cell activation and polarization by negatively regulating the PKC-θ pathway.

Harnessing regulatory T cells for transplant tolerance in the clinic through mTOR inhibition: myth or reality?

PURPOSE OF REVIEW

The inhibition of mTOR promotes immune tolerance in mouse models of transplantation, by favoring the expansion of regulatory T cells over effector T cells. However, attempts at inducing immune tolerance with the mTOR inhibitor (mTOR-I) in humans have so far failed. We herein review the immunological obstacles that need to be overcome in order to translate mTOR-I-related tolerogenic properties into the clinic.

RECENT FINDINGS

Our knowledge of regulatory T-cell biology has exploded over the past few years, providing clues to explain the complex impact of prolonged mTOR inhibition on the biology of regulatory T cells. Furthermore, recent data have shed light on the unexpected pro-inflammatory burst observed in some transplant recipients treated with mTOR-I. We propose that the exposure of an organism to pathogens determines the immunodominant effect of mTOR-I, altering the immune system from a state of tolerance in inbred animals to a state of infection-triggered enhanced inflammation in humans.

SUMMARY

Recent advances in the understanding of the pleiotropic effects of mTOR-I on the immune system are paving the way to new therapeutic avenues. Future mTOR-I-based tolerogenic protocols should counter the mTOR-I-related inflammation in order to selectively promote expansion of stable regulatory T cells. We herein envisage promising therapeutic perspectives.

PKC theta ablation improves healing in a mouse model of muscular dystrophy

Inflammation is a key pathological characteristic of dystrophic muscle lesion formation, limiting muscle regeneration and resulting in fibrotic and fatty tissue replacement of muscle, which exacerbates the wasting process in dystrophic muscles. Limiting immune response is thus one of the therapeutic options to improve healing, as well as to improve the efficacy of gene- or cell-mediated strategies to restore dystrophin expression. Protein kinase C θ (PKCθ) is a member of the PKCs family highly expressed in both immune cells and skeletal muscle; given its crucial role in adaptive, but also innate, immunity, it is being proposed as a valuable pharmacological target for immune disorders. In our study we asked whether targeting PKCθ could represent a valuable approach to efficiently prevent inflammatory response and disease progression in a mouse model of muscular dystrophy. We generated the bi-genetic mouse model mdx/θ(-/-), where PKCθ expression is lacking in mdx mice, the mouse model of Duchenne muscular dystrophy. We found that muscle wasting in mdx/θ(-/-) mice was greatly prevented, while muscle regeneration, maintenance and performance was significantly improved, as compared to mdx mice. This phenotype was associated to reduction in inflammatory infiltrate, pro-inflammatory gene expression and pro-fibrotic markers activity, as compared to mdx mice. Moreover, BM transplantation experiments demonstrated that the phenotype observed was primarily dependent on lack of PKCθ expression in hematopoietic cells.These results demonstrate a hitherto unrecognized role of immune-cell intrinsic PKCθ activity in the development of DMD. Although the immune cell population(s) involved remain unidentified, our findings reveal that PKCθ can be proposed as a new pharmacological target to counteract the disease, as well as to improve the efficacy of gene- or cell- therapy approaches.

Scaffold protein Disc large homolog 1 is required for T-cell receptor-induced activation of regulatory T-cell function

Foxp3(+)CD4(+)CD25(high) regulatory T cell (Treg) suppression of inflammation depends on T-cell receptor-mediated Nuclear Factor of Activated T cells c1 (NFATc1) activation with reduced Akt activity. We investigated the role of the scaffold protein Disc large homolog 1 (Dlgh1) in linking the T-cell receptor to this unique signaling outcome. The Treg immunological synapse (IS) recruited fourfold more Dlgh1 than conventional CD4(+) T-cell IS. Tregs isolated from patients with active rheumatoid arthritis, or treated with tumor necrosis factor-α, displayed reduced function and diminished Dlgh1 recruitment to the IS. Furthermore, Dlgh1 silencing abrogated Treg function, impaired NFATc1 activation, reduced phosphatase and tensin homolog levels, and increased Akt activation. Dlgh1 operates independently of the negative feedback pathway mediated by the related adapter protein Carma1 and thus presents an array of unique targets to selectively manipulate Treg function.

Itk controls the spatiotemporal organization of T cell activation

During T cell activation by antigen-presenting cells (APCs), the diverse spatiotemporal organization of components of T cell signaling pathways modulates the efficiency of activation. Here, we found that loss of the tyrosine kinase interleukin-2 (IL-2)-inducible T cell kinase (Itk) in mice altered the spatiotemporal distributions of 14 of 16 sensors of T cell signaling molecules in the region of the interface between the T cell and the APC, which reduced the segregation of signaling intermediates into distinct spatiotemporal patterns. Activation of the Rho family guanosine triphosphatase Cdc42 at the center of the cell-cell interface was impaired, although the total cellular amount of active Cdc42 remained intact. The defect in Cdc42 localization resulted in impaired actin accumulation at the T cell-APC interface in Itk-deficient T cells. Reconstitution of cells with active Cdc42 that was specifically directed to the center of the interface restored actin accumulation in Itk-deficient T cells. Itk also controlled the central localization of the guanine nucleotide exchange factor SLAT [Switch-associated protein 70 (SWAP-70)-like adaptor of T cells], which may contribute to the activation of Cdc42 at the center of the interface. Together, these data illustrate how control of the spatiotemporal organization of T cell signaling controls critical aspects of T cell function.

Autoimmune arthritis: the interface between the immune system and joints

Rheumatoid arthritis (RA) is an autoimmune disease, characterized by chronic inflammation and synovial hyperplasia in the joints that ultimately lead to cartilage and bone destruction. A wealth of research has shown that CD4(+) T cells, especially IL-17 producing helper T (Th17) cells, play an important role in RA development. However, it still remains to be clarified how the systemic immune response results in the local joint disorders. Studies on animal models of RA have shed light on the importance of the interaction between immune cells and joint-specific mesenchymal cells. In particular, joint-specific mesenchymal cells contribute to the Th17-mediated augmentation of the inflammatory phase in RA by promoting the migration of Th17 cells to the inflammatory joint and then homeostatic proliferation with increase in IL-17 production. In addition, recent progress in osteoimmunology has provided new insights into the pathogenesis of the bone destruction phase in RA. Of note, Th17 cells have been shown to enhance the differentiation of osteoclasts via joint-specific mesenchymal cells. Thus, the interaction of CD4(+) T cells and nonhematopoietic mesenchymal cells in joints plays a key role in RA pathogenesis during both the inflammatory and bone destruction phases. Focusing on this interaction will lead to a better understanding of the mechanism by which the systemic immune response results in local joint disorders and also helps provide a molecular basis for novel therapeutic strategies.

Clinical perspectives for regulatory T cells in transplantation tolerance

Three main types of CD4+ regulatory T cells can be distinguished based upon whether they express Foxp3 and differentiate naturally in the thymus (natural Tregs) or are induced in the periphery (inducible Tregs); or whether they are FoxP3 negative but secrete IL-10 in response to antigen (Tregulatory type 1, Tr1 cells). Adoptive transfer of each cell type has proven highly effective in mouse models at preventing graft vs. host disease (GVHD) and autoimmunity. Although clinical application was initially hampered by low Treg frequency and unfavorable ex vivo expansion properties, several phase I trials are now being conducted to assess their effect on GVHD following hematopoietic stem cell transplantation (HSCT) and in type I diabetes. Human Treg trials for HSCT recipients have preceded other indications because GVHD onset is precisely known, the time period needed for prevention relatively short, initial efficacy is likely to provide life-long protection, and complications of GVHD can be lethal. This review will summarize the clinical trials conducted to date that have employed Tregs to prevent GVHD following HSCT and discuss recent advances in Treg cellular therapy.

Blocking IL-21 signaling ameliorates xenogeneic GVHD induced by human lymphocytes

In rodent graft-versus-host disease (GVHD) models, anti-IL-21 neutralizing mAb treatment ameliorates lethality and is associated with decreases in Th1 cytokine production and gastrointestinal tract injury. GVHD prevention was dependent on the in vivo generation of donor-inducible regulatory T cells (Tregs). To determine whether the IL-21 pathway might be targeted for GVHD prevention, skin and colon samples obtained from patients with no GVHD or grade 2 to 4 GVHD were analyzed for IL-21 protein expression. By immunohistochemistry staining, IL-21 protein-producing cells were present in all gastrointestinal tract samples and 54% of skin samples obtained from GVHD patients but not GVHD-free controls. In a human xenogeneic GVHD model, human IL-21-secreting cells were present in the colon of GVHD recipients and were associated with elevated serum IL-21 levels. A neutralizing anti-human IL-21 mAb given prophylactically significantly reduced GVHD-associated weight loss and mortality, resulting in a concomitant increase in Tregs and a decrease in T cells secreting IFN-γ or granzyme B. Based on these findings, anti-IL-21 mAb could be considered for GVHD prevention in the clinic.

Role of T cell-nuclear factor κB in transplantation

Nuclear factor (NF) κB is a pleiotropic transcription factor that is ubiquitously expressed. After transplantation of solid organs, NF-κB in the graft is activated within a few hours as a consequence of ischemia/reperfusion and then again after a few days in intragraft infiltrating cells during the process of acute allograft rejection. In the present article, we review the components of the NF-κB pathway, their mechanisms of activation, and their role in T cell and antigen-presenting cell activation and differentiation and in solid organ allograft rejection. Targeted inhibition of NF-κB in selected cell types may promote graft survival with fewer adverse effects compared with global immunosuppressive therapies.

Dendritic cells in tissues: in situ stimulation of immunity and immunopathology

Dendritic cells (DCs) prime and orchestrate naïve T cell immunity in lymphoid organs, but recent data also highlight the importance of DC-effector T cell interactions in tissues. These studies suggest that effector T cells require a second activating step in situ from tissue DCs to become fully competent for effector functions and/or proliferation and survival. DC stimulation of effector T cells within tissues has evolved as a mechanism to ensure that T cells are activated to their full potential only at the site of ongoing infection. Here, we propose that under conditions of uncontrolled inflammation and release of tissue antigens, the same DC-dependent checkpoint perpetuates a destructive response and immunopathology.

The kinase GLK controls autoimmunity and NF-κB signaling by activating the kinase PKC-θ in T cells

Protein kinase C-θ (PKC-θ) is required for activation of the transcription factor NF-κB induced by signaling via the T cell antigen receptor (TCR); however, the direct activator of PKC-θ is unknown. We report that the kinase GLK (MAP4K3) directly activated PKC-θ during TCR signaling. TCR signaling activated GLK by inducing its direct interaction with the upstream adaptor SLP-76. GLK-deficient mice had impaired immune responses and were resistant to experimental autoimmune encephalomyelitis. Consistent with that, people with systemic lupus erythematosus had considerable enhanced GLK expression and activation of PKC-θ and the kinase IKK in T cells, and the frequency of GLK-overexpressing T cells was directly correlated with disease severity. Thus, GLK is a direct activator of PKC-θ, and activation of GLK-PKC-θ-IKK could be used as new diagnostic biomarkers and therapeutic targets for systemic lupus erythematosus.

Progranulin: a growth factor, a novel TNFR ligand and a drug target

Progranulin (PGRN) is abundantly expressed in epithelial cells, immune cells, neurons, and chondrocytes, and reportedly contributes to tumorigenesis. PGRN is a crucial mediator of wound healing and tissue repair. PGRN also functions as a neurotrophic factor and mutations in the PGRN gene resulting in partial loss of the PGRN protein cause frontotemporal dementia. PGRN has been found to be a novel chondrogenic growth factor and to play an important role in cartilage development and inflammatory arthritis. Although research has shown that PGRN exhibits anti-inflammatory properties, the details about the exact molecular pathway of such effects, and, in particular, the PGRN binding receptor, have not been identified so far. Recently, researchers have shown that PGRN binds to tumor necrosis factor (TNF)-receptors (TNFR), interfering with the interaction between TNFα and TNFR. They further demonstrated that mice deficient in PGRN are susceptible to collagen-induced arthritis, an experimental model of rheumatoid arthritis, and that administration of PGRN reversed the arthritic process. An engineered protein made of three PGRN fragments (Atsttrin), displayed selective TNFR binding and was more active than natural PGRN. Both PGRN and Atsttrin prevented inflammation in various arthritis mouse models and inhibited TNFα-induced intracellular signaling pathways. Thus, PGRN is a key regulator of inflammation and it may mediate its anti-inflammatory effects, at least in part, by blocking TNF binding to its receptors. As we discuss here, TNFR-based interventions may both stimulate and suppress the growth of cancer cells, and the same may be true in analogy for Atsttrin as a new player.

Highlights of 10 years of immunology in Nature Reviews Immunology

As Nature Reviews Immunology reaches its 10(th) anniversary, the authors of one of the top-cited articles from each year take a trip down memory lane. We've asked them to look back on the state of research at the time their Review was published, to consider why the article has had the impact it has and to discuss the future directions of their field. This Viewpoint article provides an interesting snapshot of some of the fundamental advances in immunology over the past 10 years. Highlights include our improved understanding of Toll-like receptor signalling, and of immune regulation mediated by regulatory T cells, indoleamine 2,3-dioxygenase, myeloid-derived suppressor cells and interleukin-10. The complexities in the development and heterogeneity of macrophages, dendritic cells and T helper cells continue to engage immunologists, as do the immune processes involved in diseases such as atherosclerosis. We look forward to what the next 10 years of immunology research may bring.

New insights into the T cell synapse from single molecule techniques

T cell activation depends on extracellular ligation of the T cell receptor (TCR) by peptide-MHC complexes in a synapse between the T cell and an antigen-presenting cell. The process then requires the assembly of signalling complexes between the TCR and the adaptor protein linker for activation of T cells (LAT), and subsequent filamentous actin (F-actin)-dependent TCR cluster formation. Recent progress in each of these areas, made possible by the emergence of new techniques, has forced us to rethink our assumptions and consider some radical new models. These describe the receptor interaction parameters that control T cell responses and the mechanism by which LAT is recruited to the TCR signalling machinery. This is an exciting time in T cell biology, and further innovation in imaging and genomics is likely to lead to a greater understanding of how T cells are activated.

Therapeutic potential of Tregs to treat rheumatoid arthritis

There is accumulating evidence for regulatory T cell defects in rheumatoid arthritis and that some biologic interventions, in particular anti-TNF, can target this population. Despite the challenges in defining regulatory T cells in patients, there are a number of approaches currently being developed to utilise their potent immunosuppressive properties. Through genetic manipulation Tregs can be generated ex vivo or in vivo that target antigens present in the inflamed joint. Here we discuss these approaches, their refinement to restore tolerance in patients with rheumatoid arthritis, and strategies to prevent their conversion towards a Th17 phenotype.

The microbiome and rheumatoid arthritis

Humans are not (and have never been) alone. From the moment we are born, millions of micro-organisms populate our bodies and coexist with us rather peacefully for the rest of our lives. This microbiome represents the totality of micro-organisms (and their genomes) that we necessarily acquire from the environment. Micro-organisms living in or on us have evolved to extract the energy they require to survive, and in exchange they support the physiological, metabolic and immune capacities that have contributed to our evolutionary success. Although currently categorized as an autoimmune disorder and regarded as a complex genetic disease, the ultimate cause of rheumatoid arthritis (RA) remains elusive. It seems that interplay between predisposing genetic factors and environmental triggers is required for disease manifestation. New insights from DNA sequence-based analyses of gut microbial communities and a renewed interest in mucosal immunology suggest that the microbiome represents an important environmental factor that can influence autoimmune disease manifestation. This Review summarizes the historical clues that suggest a possible role for the microbiota in the pathogenesis of RA, and will focus on new technologies that might provide scientific evidence to support this hypothesis.

PKC-θ function at the immunological synapse: prospects for therapeutic targeting

Protein kinase C (PKC)-θ regulates conventional effector T (Teff) cell function. Since this initial finding, it has become clear that the role of PKC-θ in T cells is complex. PKC-θ plays a central role in Teff cell activation and survival, and negatively regulates stability of the immunological synapse (IS). Recent studies demonstrated that PKC-θ is required for the development of natural CD4(+)Foxp3(+) regulatory T (Treg) cells, and mediates negative regulation of Treg cell function. Here, we examine the role of PKC-θ in the IS, evidence for its distinct localization in Treg cells and the therapeutic implications of inhibiting PKC-θ in Teff cells, to reduce effector function, and in Treg cells, to increase suppressor function, for the prevention and treatment of autoimmune and alloimmune disease states.

Intraflagellar transport: a new player at the immune synapse

The assembly and maintenance of primary cilia, which orchestrate signaling pathways centrally implicated in cell proliferation, differentiation and migration, are ensured by multimeric protein particles in a process known as intraflagellar transport (IFT). It has recently been demonstrated that a number of IFT components are expressed in hematopoietic cells, which have no cilia. Here, we summarize data for an unexpected role of IFT proteins in immune synapse assembly and intracellular membrane trafficking in T lymphocytes, and discuss the hypothesis that the immune synapse could represent the functional homolog of the primary cilium in these cells.

Progranulin: a promising therapeutic target for rheumatoid arthritis

Progranulin (PGRN) is an autocrine growth factor with multiple functions. This review provides updates about the interplays of PGRN with extracellular matrix proteins, proteolytic enzymes, inflammatory cytokines, and cell surface receptors in cartilage and arthritis, with a special focus on the interaction between PGRN and TNF receptors (TNFR) and its implications in inflammatory arthritis. The paper also highlights Atsttrin, an engineered protein composed of three PGRN fragments that prevents inflammation in several inflammatory arthritis models. Identification of PGRN as a ligand of TNFR and an antagonist of TNFα signaling, together with the discovery of Atsttrin, not only betters our understanding of the pathogenesis of arthritis, but also provides new therapeutic interventions for various TNFα-mediated pathologies and conditions, including rheumatoid arthritis.

The growth factor progranulin binds to TNF receptors and is therapeutic against inflammatory arthritis in mice

The growth factor progranulin (PGRN) has been implicated in embryonic development, tissue repair, tumorigenesis, and inflammation, but its receptors remain unidentified. We report that PGRN bound directly to tumor necrosis factor receptors (TNFRs) and disturbed the TNFα-TNFR interaction. PGRN-deficient mice were susceptible to collagen-induced arthritis, and administration of PGRN reversed inflammatory arthritis. Atsttrin, an engineered protein composed of three PGRN fragments, exhibited selective TNFR binding. PGRN and Atsttrin prevented inflammation in multiple arthritis mouse models and inhibited TNFα-activated intracellular signaling. Collectively, these findings demonstrate that PGRN is a ligand of TNFR, an antagonist of TNFα signaling, and plays a critical role in the pathogenesis of inflammatory arthritis in mice. They also suggest new potential therapeutic interventions for various TNFα-mediated pathologies and conditions, including rheumatoid arthritis.

Primary human CD4+ T cells have diverse levels of membrane lipid order that correlate with their function

Membrane lipid microdomains (lipid rafts) play an important role in T cell function by forming areas of high lipid order that facilitate activation. However, their role in regulating T cell differentiation and function remains controversial. In this study, by applying a new approach involving microscopy and flow cytometry, we characterize membrane lipid order in ex vivo primary human CD4(+) T cells. We reveal that differential membrane lipid order dictates the response to TCR stimulation. T cells with high membrane order formed stable immune synapses and proliferated robustly, intermediate order cells had reduced proliferative ability accompanied by unstable immune synapse formation, whereas low order T cells were profoundly unresponsive to TCR activation. We also observed that T cells from patients with autoimmune rheumatic disease had expanded intermediate order populations compared with healthy volunteers. This may be important in dictating the nature of the immune response since most IFN-γ(+)CD4(+) T cells were confined within intermediate membrane order populations, whereas IL-4(+)CD4(+) T cells were contained within the high order populations. Importantly, we were able to alter T cell function by pharmacologically manipulating membrane order. Thus, the results presented from this study identify that ex vivo CD4(+) T cells sustain a gradient of plasma membrane lipid order that influences their function in terms of proliferation and cytokine production. This could represent a new mechanism to control T cell functional plasticity, raising the possibility that therapeutic targeting of membrane lipid order could direct altered immune cell activation in pathology.

Immune suppression and immune activation in depression

Depression has been characterized as a disorder of both immune suppression and immune activation. Markers of impaired cellular immunity (decreased natural killer cell cytotoxicity) and inflammation (elevated IL-6, TNFα, and CRP) have been associated with depression. These immunological markers have been associated with other medical illnesses, suggesting that immune dysregulation may be a central feature common to both depression and to its frequent medical comorbidities. Yet the significant associations of findings of both immune suppression and immune activation with depression raise questions concerning the relationship between these two classes of immunological observations. Depressed populations are heterogeneous groups, and there may be differences in the immune profiles of populations that are more narrowly defined in terms of symptom profile and/or demographic features. There have been few reports concurrently investigating markers of immune suppression and immune activation in the same depressed individuals. An emerging pre-clinical literature suggests that chronic inflammation may directly contribute to the pathophysiology of immune suppression in the context of illnesses such as cancer and rheumatoid arthritis. This literature provides us with specific immunoregulatory mechanisms mediating these relationships that could also explain differences in immune disturbances between subsets of depressed individuals We propose a research agenda emphasizing the assessment of these immunoregulatory mechanisms in large samples of depressed subjects as a means to define the relationships among immune findings (suppression and/or activation) within the same depressed individuals and to characterize subsets of depressed subjects based on shared immune profiles. Such a program of research, building on and integrating our knowledge of the psychoneuroimmunology of depression, could lead to innovation in the assessment and treatment of depression and its medical comorbidities.

Can TNF-α boost regulatory T cells?

Deleterious immune responses that cause autoimmune diseases such as type 1 diabetes are normally kept in check by a myriad of mechanisms. Among these, protection mediated by CD4+Foxp3+ Tregs constitutes an essential pathway. Much work over the past decade aimed to understand how Tregs affect immune responses triggered by effector T cells (Teffs), but less is known about how Teffs affect Tregs. In this issue of the JCI, Grinberg-Bleyer et al. report the clearest example thus far regarding this important aspect of Treg biology. They find that in mice, sustained protection from diabetes by Tregs is dependent on Teffs and partially dependent on TNF-α, a cytokine traditionally considered proinflammatory.

Endoplasmic reticulum stress and inflammation in obesity and diabetes

Obesity is a major problem worldwide that increases risk for a wide range of diseases, including diabetes and heart disease. As such, it is increasingly important to understand how excess adiposity can perturb normal metabolic functions. It is now clear that this disruption involves not only pathways controlling lipid and glucose homeostasis but also integration of metabolic and immune response pathways. Under conditions of nutritional excess, this integration can result in a metabolically driven, low-grade, chronic inflammatory state, referred to as "metaflammation," that targets metabolically critical organs and tissues to adversely affect systemic homeostasis. Endoplasmic reticulum dysfunction is another important feature of chronic metabolic disease that is also linked to both metabolic and immune regulation. A thorough understanding of how these pathways intersect to maintain metabolic homeostasis, as well as how this integration is altered under conditions of nutrient excess, is important to fully understand, and subsequently treat, chronic metabolic diseases.

The yin and yang of regulatory T cells and inflammation in RA

Rheumatoid arthritis (RA) is characterized by chronic inflammation leading to joint destruction. Regulatory T (T(REG)) cells are potent suppressors of autoimmunity, but are not capable of controlling every aspect of the inflammatory reaction. We have found that T(REG)-cell function is abnormal in patients with RA, and that a distinct population of T(REG) cells with potent suppressive properties is induced after therapy with inhibitors of tumor necrosis factor. In this Review, we discuss the mutual interactions between the opposing forces of T(REG) cells and inflammation in the context of RA. Therapeutic approaches that enhance T(REG)-cell function whilst controlling inflammation are likely to be the most effective strategies for restoring immune tolerance in patients with this disease.