TNF is essential to control Mycobacterium tuberculosis infection and cannot be replaced by other proinflammatory cytokines. Overproduction of TNF may cause immunopathology, while defective TNF production results in uncontrolled infection. The critical role of TNF in the control of tuberculosis has been illustrated recently by primary and reactivation of latent infection in some patients under pharmacological anti-TNF therapy for rheumatoid arthritis or Crohn's disease. In this review, we discuss results of recent studies aimed at better understanding of molecular, cellular and kinetic aspects of TNF-mediated regulation of host-mycobacteria interactions. In particular, recent data using either mutant mice expressing solely membrane TNF or specific inhibitor sparing membrane TNF demonstrated that membrane TNF is sufficient to control acute M. tuberculosis infection. This is opening the way to selective TNF neutralization that might retain the desired anti-inflammatory effect but reduce the infectious risk.
TNF is a pleiotropic cytokine produced by many cell types upon different stimuli and in various physiological and pathological conditions. In this review, we focus on the role of TNF in T cell responses as demonstrated by in vitro and in vivo observations in mice and humans. TNF has an impact on all aspects of T cell biology such as development in the thymus, peripheral homeostasis, primary antigenic responses, apoptosis, effector functions, memory cell formation and tolerance induction and maintenance. In most cases, TNF has an immunostimulatory role in T cell responses; however, under certain conditions, TNF can exert immunomodulatory effects on T cells. We also review how T cell-derived TNF is an important component of T cell immunity as exemplified by many studies involving intracellular pathogens and tumors. Finally, we summarize how TNF T cells interplay contributes to pathology in autoimmune disorders and what is known about the effect of widely used TNF blockers on T cell differentiation/function.
TNF-alpha (TNF) is a pleiotropic cytokine which can have proinflammatory or immunosuppressive effects, depending on the context, duration of exposure and disease state. The basis for the opposing actions of TNF remains elusive. The growing appreciation of CD4+FoxP3+ regulatory T cells (Tregs), which comprise approximately 10% of peripheral CD4 cells, as pivotal regulators of immune responses has provided a new framework to define the cellular and molecular basis underlying the contrasting action of TNF. TNF by itself can overcome the profound anergic state of T cell receptor-stimulated Tregs. Furthermore, in concert with IL-2, TNF selectively activates Tregs, resulting in proliferation, upregulation of FoxP3 expression and increases in their suppressive activity. Both human and mouse Tregs predominantly express TNFR2, making it possible for TNF to enhance Treg activity, which helps limit the collateral damage caused by excessive immune responses and eventually terminates immune response. TNFR2-expressing CD4+FoxP3+ Tregs comprise approximately 40% of peripheral Tregs in normal mice and present the maximally suppressive subset of Tregs. In this review, studies describing the action of TNF on Treg function will be discussed. The role of Tregs in the autoimmune disorders and cancer as well as the effect of anti-TNF therapy on Tregs, especially in rheumatoid arthritis, will also be considered.
Results from clinical trials of biologic anti-TNF drugs performed in the late 1990s confirmed the biological relevance of TNF function in the pathogenesis of chronic noninfectious inflammation of joints, skin and gut, which collectively affects 2-3% of the population. Up to April 2009, more than two million patients worldwide have received the first marketed drugs, namely the monoclonal anti-TNF antibodies infliximab and adalimumab and the soluble TNF receptor etanercept. All three are equally effective in rheumatoid arthritis, ankylosing spondylitis, psoriasis and psoriatic arthritis, but, for not clearly defined reasons, only the monoclonal antibodies are effective in inflammatory bowel disease. About 60% of patients who do not benefit from standard nonbiologic treatments for these diseases respond to TNF antagonists. Less than half of responding patients achieve complete remission of disease. Importantly, some of those patients with rheumatoid arthritis in whom long-term anti-TNF therapy induced disease remission remain disease-free after discontinuation of any kind of treatment. There are not yet reliable predictors of which patients will or will not respond on anti-TNF therapy, whereas subsequent loss of an initial clinical response occurs frequently. The spectrum of efficacy anti-TNF therapies widens to include diseases such as systemic vasculitis and sight-threatening uveitis. While paradoxical new adverse effects are recognized, i.e. exacerbation or development of new onset psoriasis, reactivation of latent tuberculosis remains the most important safety issue of anti-TNF therapies. Clinical practice guidelines and consensus statements on the criteria of introduction, duration of treatment and cessation of TNF antagonists, including safety issues, are under constant revision as data from longer periods of patient exposure accumulate. It is hoped that more efficacious drugs that will ideally target the deleterious proinflammatory properties of TNF without compromising its protective role in host defense and (auto)immunity will be available in the near future.
TNF and type I interferons (IFNs) are induced by microbial stimuli and mediate innate immune responses. They are also involved in the pathogenesis of chronic inflammatory diseases, such as rheumatoid arthritis and systemic lupus erythematosus. Activated macrophages are an important driving force of inflammatory reactions and one of the major producers of TNF in innate immunity and chronic inflammation. Despite the fact that cells at sites of damage are continuously exposed to both cytokines, little is known about mechanisms regulating TNF and type I IFN interactions during inflammation. In this review, we discuss the role of an IFN-beta-mediated autocrine loop in the regulation of gene expression program induced by TNF in myeloid cells.
The TNF/TNF receptor (TNFR) system has a prominent role in the pathogenesis of chronic inflammatory and autoimmune disorders. Extensive research in animal models with deregulated TNF expression has documented that TNF may initiate or sustain inflammatory pathology, while at the same time may exert immunomodulatory or disease-suppressive activities. The TNF/TNFR system encompassing both the soluble and the transmembrane form of TNF with differential biological activities, as well as the differential usage of its receptors, mediating distinct functions, appears to confer complexity but also specificity in the action of TNF. The inherent complexity in TNF-mediated pathophysiology highlights the requirement to address the role of TNF taking into account both proinflammatory tissue-damaging and immunomodulatory functions in a cellular and receptor-specific manner. In this review, we discuss our current understanding of the involvement of TNF in chronic inflammation and autoimmunity, focusing on TNF-mediated cellular pathways leading to the pathogenesis or progression of joint and intestinal inflammatory pathology. Knowledge of the mechanisms by which TNF either initiates or contributes to disease pathology is fundamentally required for the design of safe and effective anti-TNF/TNFR therapies for human inflammatory and autoimmune disorders.
The cytokine TNF is a critical mediator of immune and inflammatory responses. The TNF gene is an immediate early gene, rapidly transcribed in a variety of cell types following exposure to a broad range of pathogens and signals of inflammation and stress. Regulation of TNF gene expression at the transcriptional level is cell type- and stimulus-specific, involving the recruitment of distinct sets of transcription factors to a compact and modular promoter region. In this review, we describe our current understanding of the mechanisms through which TNF transcription is specifically activated by a variety of extracellular stimuli in multiple cell types, including T cells, B cells, macrophages, mast cells, dendritic cells, and fibroblasts. We discuss the role of nuclear factor of activated T cells and other transcription factors and coactivators in enhanceosome formation, as well as the contradictory evidence for a role for nuclear factor kappaB as a classical activator of the TNF gene. We describe the impact of evolutionarily conserved cis-regulatory DNA motifs in the TNF locus upon TNF gene transcription, in contrast to the neutral effect of single nucleotide polymorphisms. We also assess the regulatory role of chromatin organization, epigenetic modifications, and long-range chromosomal interactions at the TNF locus.
The relationship between TNF and immune pathology forced an intense research into the regulation of its biosynthesis that extends to multiple mechanisms controlling the utilization of its mRNA. These posttranscriptional mechanisms gradually and variably impose a series of flexible rate-limiting controls to modify the abundance of the TNF mRNA and the rate of its translation in response to environmental signals. Mechanistically, these controls consist of signaling networks converging to RNA-binding proteins and microRNAs, which in turn target a code of secondary or tertiary ribonucleotide structures located on the TNF mRNA. The outcome of these interactions is the stringent control of this mRNA's maturation, localization, turnover and translation. A wealth of molecular and genetic data highlighted that if these posttranscriptional interactions fail, they perturb cellular responses to provide the impetus for TNF-mediated inflammatory disease. Here, we highlight the parameters guiding the posttranscriptional regulation of TNF mRNA and their relevance to homeostasis and pathology.
Bone is subject to permanent remodeling during development and through life. This activity is essential for (a) proper shaping and growth of each bone during development; (b) maintenance of bone mass as well as structural integrity of the micro architecture of bone through adult life, and (c) tissue repair needed for healing of fracture as well as of micro-damage. In addition to genetically linked rare developmental diseases, disturbances in bone remodeling are causing common bone pathologies, which severely impair the quality of life of patients. Among them are postmenopausal osteoporosis and local as well as systemic bone loss observed in chronic inflammatory diseases such as rheumatoid arthritis. The role of TNF-alpha in mediating bone remodeling will be presented and discussed in this chapter.
Obesity, an epidemic of our times with rates rising to alarming levels, is associated with comorbidities including cardiovascular diseases, arthritis, certain cancers, and degenerative diseases of the brain and other organs. Importantly, obesity is a leading cause of insulin resistance and type 2 diabetes. As emerging evidence has shown over the last decade, inflammation is one of the critical processes associated with the development of insulin resistance, diabetes and related diseases, and obesity is now considered as a state of chronic low-grade inflammation. Adipose tissue, apart from its classical role as an energy storage depot, is also a major endocrine organ secreting many factors, whose local and circulating levels are affected by the degree of adiposity. Obesity leads to infiltration of the expanded adipose tissue by macrophages and increased levels in proinflammatory cytokines. The first indication for increased cytokine release in obesity was provided by the identification of increased expression of TNF-alpha, a proinflammatory cytokine, in the adipose tissue of obese mice in the early 1990s. TNF-alpha is expressed in and secreted by adipose tissue, its levels correlating with the degree of adiposity and the associated insulin resistance. Targeting TNF-alpha and/or its receptors has been suggested as a promising treatment for insulin resistance and type 2 diabetes. This review will summarize the available knowledge on the role of TNF-alpha in obesity and related processes and the potential implications of the above in the development of new therapeutic approaches for obesity and insulin resistance. Recent data from clinical studies will also be described together with late findings on the pathogenesis of obesity and insulin resistance. Copyright (C) 2010 S. Karger AG, Basel
TNF is a potent cytokine with an important role in the regulation of a multitude of cellular responses and in coordinating immune and inflammatory reactions. TNF exerts its effects by binding to the TNFR1- and TNFR2-specific cell surface receptors, which activate a number of intracellular signaling cascades including the nuclear factor kappaB (NF-kappaB) and mitogen-activated protein kinase pathways. Activation of NF-kappaB mediates many of the functions of TNF by transmitting information from the cell surface TNF receptors to the nucleus, where it coordinates a gene expression program that allows the cell to survive and elicit its responses. The intimate interplay of TNF with the NF-kappaB signaling pathway is highlighted by results obtained in transgenic and knockout mice with defects in NF-kappaB signaling components, where TNF has been shown to contribute to different pathologies observed in these mice. This chapter focuses on the function of TNF in pathologies induced by NF-kappaB deficiency and discusses the implications of these findings for our understanding of inflammatory diseases.
Dermatomyositis (DM) is a chronic inflammatory disorder of the skin and muscles. Although thought to be autoimmune in origin, many questions remain as to the etiopathogenesis of this disease. DM has classically been considered a humorally mediated disease. Current evidence, however, seems to increasingly support alternative (though not mutually exclusive) mechanisms of pathogenesis, including cell-mediated and innate immune system dysfunction. Pathologic findings of DM in muscle include infarcts, perifascicular atrophy, endothelial cell swelling and necrosis, vessel wall membrane attack complex deposition, and myocyte-specific MHC I upregulation. As for the skin, histopathologic findings include hyperkeratosis, epidermal basal cell vacuolar degeneration and apoptosis, increased dermal mucin deposition, and a cell-poor interface dermatitis. Autoantibodies, particularly those that bind nuclear or cytoplasmic ribonucleoprotein antigens, are also commonly found in DM, although their importance in pathogenesis remains unclear. Defective cellular clearance, genetic predilection and environmental exposures, such as viral infection, may also play an important role in the pathogenesis of DM. The seminal work regarding the pathogenesis of DM is reviewed and an update on the recent basic and molecular advances in the field is provided.
This chapter summarizes the evidence that defined compartments of the hair follicle (HF) and nail epithelium maintain an area of relative immune privilege (IP). HF and nail IP is chiefly characterized by absent or very low level of expression of major histocompatibility complex class Ia antigens, complemented by a number of factors, such as the local production of potent immunosuppressive agents, dysfunction of professional antigen-presenting cells and inhibition of natural killer cell activities. In the hair bulb, IP is seen only in the anagen stage of HF cycling, while the nail apparatus continuously maintains an IP site in its proximal nail matrix, since the nail apparatus does not cycle. Possibly, the (non-cycling) bulge area of human scalp HFs also enjoys some relative, stably maintained IP, even though it is not as pronounced as the IP of the anagen hair bulb. A collapse of HF and nail IP likely plays a key role in the pathogenesis of one of the most common organ-specific autoimmune diseases, alopecia areata. Therefore, the therapeutic restoration of IP collapse promises to be a particularly effective future strategy for the treatment of alopecia areata.
Dermatomyositis (DM) is a chronic inflammatory disorder of the skin and muscles. Although thought to be autoimmune in origin, many questions remain as to the etiopathogenesis of this disease. DM has classically been considered a humorally mediated disease. Current evidence, however, seems to increasingly support alternative (though not mutually exclusive) mechanisms of pathogenesis, including cell-mediated and innate immune system dysfunction. Pathologic findings of DM in muscle include infarcts, perifascicular atrophy, endothelial cell swelling and necrosis, vessel wall membrane attack complex deposition, and myocyte-specific MHC I upregulation. As for the skin, histopathologic findings include hyperkeratosis, epidermal basal cell vacuolar degeneration and apoptosis, increased dermal mucin deposition, and a cell-poor interface dermatitis. Autoantibodies, particularly those that bind nuclear or cytoplasmic ribonucleoprotein antigens, are also commonly found in DM, although their importance in pathogenesis remains unclear. Defective cellular clearance, genetic predilection and environmental exposures, such as viral infection, may also play an important role in the pathogenesis of DM. The seminal work regarding the pathogenesis of DM is reviewed and an update on the recent basic and molecular advances in the field is provided.
Autoimmune diseases present with varied and broad-ranging cutaneous manifestations. Connective tissue disorders have a plethora of skin manifestations such as rheumatoid nodules in rheumatoid arthritis, psoriatic plaques in psoriatic arthritis, acne and pustulosis in SAPHO syndrome, livedo reticularis and ulceration in antiphospholipid antibody syndrome and xerosis in Sjögren syndrome. Cutaneous manifestations of autoimmune vasculitides such as polyarteritis nodosa, Kawasaki disease, Henoch-Schönlein purpura, cryoglobulinemic vasculitis, Behcet disease, Wegener granulomatosis, microscopic polyangiitis and Churg-Strauss syndrome range from papules, subcutaneous nodules and livedo reticularis, to palpable purpura, hemorrhagic bulla and ulcerating lesions. Pathological skin manifestations in autoimmune endocrinopathies include pretibial myxedema/dermopathy in Graves' disease, diabetic dermopathy and necrobiosis lipoidica in type I autoimmune diabetes mellitus, candidiasis, ectodermal dysplasia, vitiligo and alopecia areata in APECED and uniform hyperpigmentation of the skin in Addison's disease. Autoimmune gastrointestinal disorders such as inflammatory bowel disease (with erythema nodosum), gluten-sensitive enteropathy (with dermatitis herpetiformis), autoimmune hepatitis and primary biliary cirrhosis (with jaundice and pruritus), hematologic/oncologic disorders such as acute and chronic graft-versus-host disease (with skin manifestations ranging from pruritic maculopapular eruptions and lichen planus-like lesions to generalized scleroderma), and paraneoplastic autoimmune dermatoses are discussed as well.
Psoriasis is one of the most common chronic inflammatory disorders with a strong genetic background. Recent progress in the understanding of both the immunological as well as the genetic basis has provided an unprecedented opportunity to move scientific insights from the bench to bedside. Based on insights from laboratory research, targeted immunotherapies are now available for the benefit of patients suffering from psoriasis. The success of these therapies has validated insights into disease pathogenesis and also provides the opportunity to increase our understanding about the pathways underpinning autoimmune-type inflammation in the skin.
Epidermolysis bullosa acquisita (EBA) is an acquired blistering skin disease characterized by the presence of IgG autoantibodies to type VII collagen. EBA autoantibodies recognize four major immunodominant epitopes localized within the amino-terminal, noncollagenous (NC1) domain, In this study, we developed a rapid, quantitative enzyme-linked immunosorbent assay (ELISA) to detect autoantibody activity against the complete NC1 domain of type VII collagen with the use of an eukaryotic-expressed, recombinant human NC1 antigen, With the ELISA, we tested serum from patients with EBA (n = 24), bullous systemic lupus erythematosus (BSLE) (n = 3), bullous pemphigoid (n = 16), pemphigus (n = 11), and normal controls (n = 12). All EBA and BSLE serum, including four sera that were negative by indirect immunofluorescence, demonstrated reactivity with immobilized NC1 in the ELISA, In contrast, none of the sera from healthy control subjects or patients with unrelated blistering skin diseases reacted with NC1. The EBA sera also reacted with recombinant NC1 by immunoblot analysis but with less sensitivity, Thus, the newly developed ELISA using recombinant NC1 is a sensitive, specific assay and a useful tool for rapidly screening EBA and BSLE serum.
Vitiligo is characterized by progressive skin depigmentation resulting from an autoimmune response targeting epidermal melanocytes. Melanocytes are particularly immunogenic by virtue of the contents of their melanosomes, generating the complex radical scavenging molecule melanin in a process that involves melanogenic enzymes and structural components, including tyrosinase, MART-1, gp100, TRP-2 and TRP-1. These molecules are also prime targets of the immune response in both vitiligo and melanoma. The immunogenicity of melanosomal proteins can partly be explained by the dual role of melanosomes, involved both in melanin synthesis and processing of exogenous antigens. Melanocytes are capable of presenting antigens in the context of MHC class II, providing HLA-DR+ melanocytes in perilesional vitiligo skin the option of presenting melanosomal antigens in response to trauma and local inflammation. Type I cytokine-mediated immunity to melanocytes in vitiligo involves T cells reactive with melanosomal antigens, similar to T cells observed in melanoma. In vitiligo, however, T cell tuning allows T cells with higher affinity for melanocyte differentiation antigens to enter the circulation after escaping clonal deletion in primary lymphoid organs. The resulting efficacious and progressive autoimmune response to melanocytes provides a roadmap for melanoma therapy.
Atopic dermatitis (also termed atopic eczema and infantile eczema), a chronic, itchy, inflammatory skin disease that sets on at infancy or early childhood, is observed with increasing prevalence around the world, particularly in developed nations. Although sufficient evidences are not yet available to define it as a classical autoimmune disease, autoantigens have been identified. Investigations of atopic dermatitis in human patients and animal models suggest that this disease is initiated, maintained and perpetuated by the actions of cytokines, chemokines, T cells, antigen-presenting cells and other inflammatory cells; there is also evidence of skin barrier defect and angiogenesis. Recent identification of mutations of the epidermal barrier protein filaggrin (encoded by FLG), present in about 9% of people of European origin, with 70% of individuals homozygous or compound heterozygous for FLG null alleles developing atopic dermatitis, provides a strong link between a defect of the epidermal barrier that allows easy penetration of pathogen/allergen through the skin and a systemic hyperactive immune response to the penetrated pathogen/allergen. The newly introduced concept of 'intrinsic' and 'extrinsic' atopic dermatitis has fueled the debate among academic dermatologists as to how 'atopic' atopic dermatitis should be defined. Some recent advancements on the management options for atopic dermatitis are also discussed.
