Polyarteritis Nodosa, a vanishing vasculitis since its main cause has been identified

The polyarteritis nodosa is a vasculitis for which the main cause had been identified as hepatitis B, a chronic infectious disease inducing an overshooting immune reaction. Thanks to this discovery, the treatment of polyarteritis nodosa has changed from a symptomatic immunosuppressive therapy to a treatment strategy focusing on the disease inducing infectious agent. Vaccination against hepatitis B- virus has been instrumental in the reduction of the prevalence of polyarteritis nodosa. In case of the rare entity of polyarteritis nodosa without hepatitis B, only a symptomatic immunosuppressive therapy can be prescribed. The history of polyarteritis nodosa clearly illustrates that future research in the field of so called autoimmune diseases should focus on the causes rather than on the immunologic effector mechanisms and/or the symptomatic immunosuppressive therapy.

Polyclonal and specific antibodies mediate protective immunity against enteric helminth infection

Anti-helminth immunity involves CD4+ T cells, yet the precise effector mechanisms responsible for parasite killing or expulsion remain elusive. We now report an essential role for antibodies in mediating immunity against the enteric helminth Heligmosomoides polygyrus (Hp), a natural murine parasite that establishes chronic infection. Polyclonal IgG antibodies, present in naive mice and produced following Hp infection, functioned to limit egg production by adult parasites. Comparatively, affinity-matured parasite-specific IgG and IgA antibodies that developed only after multiple infections were required to prevent adult worm development. These data reveal complementary roles for polyclonal and affinity-matured parasite-specific antibodies in preventing enteric helminth infection by limiting parasite fecundity and providing immune protection against reinfection, respectively. We propose that parasite-induced polyclonal antibodies play a dual role, whereby the parasite is allowed to establish chronicity, while parasite load and spread are limited, likely reflecting the long coevolution of helminth parasites with their hosts.

Immune cell-mediated liver injury

Liver diseases represent an important cause of morbidity and mortality in the world. Death of hepatocytes and other hepatic cell types is a characteristic feature of several forms of liver injury such as cholestasis, viral hepatitis, drug- or toxin-induced injury, and alcohol-induced liver damage. Moreover, irrespectively of the reason, liver injury seems to be facilitated by similar immune effector mechanisms common to these various liver diseases. Indeed, common immune effector mechanisms may explain the high prevalence of cirrhosis and cancer development in most forms of liver disease. Improved understanding of the immune cell-mediated mechanisms involved in hepatocyte cell death could be beneficial for the development of common therapeutic strategies against different forms of liver diseases. In this review, we will discuss novel findings on the role of different immune cells in liver disease and immune cell-induced death executioner mechanisms involved in hepatocyte cell death.

Distinct requirements for activation-induced cell surface expression of preformed Fas/CD95 ligand and cytolytic granule markers in T cells

Fas (CD95/Apo-1) ligand is a potent inducer of apoptosis and one of the major killing effector mechanisms of cytotoxic T cells. Thus, Fas ligand activity has to be tightly regulated, involving various transcriptional and post-transcriptional processes. For example, preformed Fas ligand is stored in secretory lysosomes of activated T cells, and rapidly released by degranulation upon reactivation. In this study, we analyzed the minimal requirements for activation-induced degranulation of Fas ligand. T cell receptor activation can be mimicked by calcium ionophore and phorbol ester. Unexpectedly, we found that stimulation with phorbol ester alone is sufficient to trigger Fas ligand release, whereas calcium ionophore is neither sufficient nor necessary. The relevance of this process was confirmed in primary CD4(+) and CD8(+) T cells and NK cells. Although the activation of protein kinase(s) was absolutely required for Fas ligand degranulation, protein kinase C or A were not involved. Previous reports have shown that preformed Fas ligand co-localizes with other markers of cytolytic granules. We found, however, that the activation-induced degranulation of Fas ligand has distinct requirements and involves different mechanisms than those of the granule markers CD63 and CD107a/Lamp-1. We conclude that activation-induced degranulation of Fas ligand in cytotoxic lymphocytes is differently regulated than other classical cytotoxic granule proteins.

T cell-mediated immunoregulation in the gastrointestinal tract

In the intestinal tract, only a single layer of epithelial cells separates innate and adaptive immune effector cells from a vast amount of antigens. Here, the immune system faces a considerable challenge in tolerating commensal flora and dietary antigens while preventing the dissemination of potential pathogens. Failure to tightly control immune reactions may result in detrimental inflammation. In this respect, 'conventional' regulatory CD4(+) T cells, including naturally occurring and adaptive CD4(+) CD25(+) Foxp3(+) T cells, Th3 and Tr1 cells, have recently been the focus of considerable attention. However, regulatory mechanisms in the intestinal mucosa are highly complex, including adaptations of nonhaematopoietic cells and innate immune cells as well as the presence of unconventional T cells with regulatory properties such as resident TCRgammadelta or TCRalphabeta CD8(+) intraepithelial lymphocytes. This review aims to summarize the currently available knowledge on conventional and unconventional regulatory T cell subsets (Tregs), with special emphasis on clinical data and the potential role or malfunctioning of Tregs in four major human gastrointestinal diseases, i.e. inflammatory bowel diseases, coeliac disease, food allergy and colorectal cancer. We conclude that the clinical data confirms some but not all of the findings derived from experimental animal models.

Sphingosine kinase 1 is pivotal for Fc epsilon RI-mediated mast cell signaling and functional responses in vitro and in vivo

Mast cell degranulation is pivotal to allergic diseases; investigating novel pathways triggering mast cell degranulation would undoubtedly have important therapeutic potential. FcepsilonRI-mediated degranulation has contradictorily been shown to require SphK1 or SphK2, depending on the reports. We investigated the in vitro and in vivo specific role(s) of SphK1 and SphK2 in FcepsilonRI-mediated responses, using specific small interfering RNA-gene silencing. The small interfering RNA-knockdown of SphK1 in mast cells inhibited several signaling mechanisms and effector functions, triggered by FcepsilonRI stimulation including: Ca(2+) signals, NFkappaB activation, degranulation, cytokine/chemokine, and eicosanoid production, whereas silencing SphK2 had no effect at all. Moreover, silencing SPHK1 in vivo, in different strains of mice, strongly inhibited mast cell-mediated anaphylaxis, including inhibition of vascular permeability, tissue mast cell degranulation, changes in temperature, and serum histamine and cytokine levels, whereas silencing SPHK2 had no effect and the mice developed anaphylaxis. Our data differ from a recent report using SPHK1(-/-) and SPHK2(-/-) mice, which showed that SphK2 was required for FcepsilonRI-mediated mast cell responses. We performed experiments in mast cells derived from SPHK1(-/-) and SPHK2(-/-) mice and show that the calcium response and degranulation, triggered by FcepsilonRI-cross-linking, is not different from that triggered in wild-type cells. Moreover, IgE-mediated anaphylaxis in the knockout mice showed similar levels in temperature changes and serum histamine to that from wild-type mice, indicating that there was no protection from anaphylaxis for either knockout mice. Thus, our data strongly suggest a previously unrecognized compensatory mechanism in the knockout mice, and establishes a role for SphK1 in IgE-mediated mast cell responses.

XIAP discriminates between type I and type II FAS-induced apoptosis

FAS (also called APO-1 and CD95) and its physiological ligand, FASL, regulate apoptosis of unwanted or dangerous cells, functioning as a guardian against autoimmunity and cancer development. Distinct cell types differ in the mechanisms by which the 'death receptor' FAS triggers their apoptosis. In type I cells, such as lymphocytes, activation of 'effector caspases' by FAS-induced activation of caspase-8 suffices for cell killing, whereas in type II cells, including hepatocytes and pancreatic beta-cells, caspase cascade amplification through caspase-8-mediated activation of the pro-apoptotic BCL-2 family member BID (BH3 interacting domain death agonist) is essential. Here we show that loss of XIAP (X-chromosome linked inhibitor of apoptosis protein) function by gene targeting or treatment with a second mitochondria-derived activator of caspases (SMAC, also called DIABLO; direct IAP-binding protein with low pI) mimetic drug in mice rendered hepatocytes and beta-cells independent of BID for FAS-induced apoptosis. These results show that XIAP is the critical discriminator between type I and type II apoptosis signalling and suggest that IAP inhibitors should be used with caution in cancer patients with underlying liver conditions.

Fatal hepatitis mediated by tumor necrosis factor TNFalpha requires caspase-8 and involves the BH3-only proteins Bid and Bim

Apoptotic death of hepatocytes, a contributor to many chronic and acute liver diseases, can be a consequence of overactivation of the immune system and is often mediated by TNFalpha. Injection with lipopolysaccharide (LPS) plus the transcriptional inhibitor D(+)-galactosamine (GalN) or mitogenic T cell activation causes fatal hepatocyte apoptosis in mice, which is mediated by TNFalpha, but the effector mechanisms remain unclear. Our analysis of gene-targeted mice showed that caspase-8 is essential for hepatocyte killing in both settings. Loss of Bid, the proapoptotic BH3-only protein activated by caspase-8 and essential for Fas ligand-induced hepatocyte killing, resulted only in a minor reduction of liver damage. However, combined loss of Bid and another BH3-only protein, Bim, activated by c-Jun N-terminal kinase (JNK), protected mice from LPS+GalN-induced hepatitis. These observations identify caspase-8 and the BH3-only proteins Bid and Bim as potential therapeutic targets for treatment of inflammatory liver diseases.

Cathepsin W expressed exclusively in CD8+ T cells and NK cells, is secreted during target cell killing but is not essential for cytotoxicity in human CTLs

OBJECTIVE: Cathepsin W (CatW, lymphopain) is a putative cysteine protease with restricted expression to natural killer (NK) cells and CD8(+) T cells and so far unknown function and properties. Here, we characterize in detail, the regulation of human CatW during T-cell development in response to different stimuli and its functional involvement in cytotoxic lymphocyte effector function. MATERIALS AND METHODS: Western blots and real time polymerase chain reaction of sorted, unstimulated, and stimulated cell subsets (thymocytes, T cells, NK cells) and their culture supernatants were used to study regulation and expression of CatW. Primary CD8(+) T cells and short-term T-cell lines were transfected with small interfering RNA to study the involvement of CatW in effector function such as target cell killing and interferon-gamma production. RESULTS: Levels of CatW expression correlate closely with cytotoxic capacity both during development and in response to factors influencing cytotoxicity. Furthermore, CatW is secreted during specific target cell killing. However, knockdown of CatW expression by small interfering RNA neither influences target cell killing nor interferon-gamma production. CONCLUSION: Despite being expressed in the effector subset of CD8(+) and NK cells and of being released during target cell killing, our functional inhibition studies exclude an essential role of CatW in the process of cytotoxicity.

Antigen Processing and Presentation in Multiple Sclerosis

CD4(+) T cells play a central role in the pathogenesis of multiple sclerosis (MS). Generation, activation and effector function of these cells crucially depends on their interaction with MHC II-peptide complexes displayed by antigen presenting cells (APC). Processing and presentation of self antigens by different APC therefore influences the disease course at all stages. Selection by thymic APC leads to the generation of autoreactive T cells, which can be activated by peripheral APC. Reactivation by central nervous system APC leads to the initiation of the inflammatory response resulting in demyelination. In this review we will focus on how MHC class II antigenic epitopes are created by different APC from the thymus, the periphery and from the brain, and will discuss the relevance of the balance between creation and destruction of such epitopes in the context of MS. A solid understanding of these processes offers the possibility for designing future therapeutic strategies.

Pregnancy in patients with ankylosing spondylitis: do regulatory T cells play a role?

OBJECTIVE: To investigate the numerical and functional changes of CD4+CD25(high) regulatory T (Treg) cells during pregnancy and postpartum in patients with ankylosing spondylitis (AS). METHODS: The frequency of CD4+CD25(high) T cells was determined by flow cytometry in 10 pregnant and 5 nonpregnant patients with AS as well as in 14 pregnant and 4 nonpregnant healthy controls. Pregnant individuals were investigated at the third trimester and 8 weeks postpartum. Treg cells and CD4+CD25- effector T (Teff) cells separated by fluorescence-activated cell sorting were stimulated with anti-CD3 and anti-CD28 monoclonal antibodies, alone or in coculture, to investigate proliferation and cytokine secretion. RESULTS: The frequency of CD4+CD25(high) Treg cells was significantly higher during pregnancy than postpartum in both healthy control subjects and patients with AS. In contrast to Treg cells in healthy pregnant women, Treg cells in pregnant women with AS secreted only small amounts of interleukin-10 and showed lower suppression of tumor necrosis factor alpha and interferon-gamma secretion by CD4+CD25- Teff cells. At the postpartum time point, proinflammatory cytokine levels in the Treg/Teff cell cocultures and Teff cell monocultures were significantly higher in patients with AS than in healthy controls. CONCLUSION: Pregnancy influenced the expansion and cytokine secretion of Treg cells in both patients with AS and control subjects. However, the Treg cells of pregnant patients with AS failed to support an antiinflammatory cytokine milieu, thereby possibly contributing to the persistent disease activity of AS during pregnancy.

Intravenous immunoglobulins--understanding properties and mechanisms

High-dose intravenous immunoglobulin (IVIg) preparations are used currently for the treatment of autoimmune or inflammatory diseases. Despite numerous studies demonstrating efficacy, the precise mode of action of IVIg remains unclear. Paradoxically, IgG can exert both pro- and anti-inflammatory activities, depending on its concentration. The proinflammatory activity of low-dose IVIg requires complement activation or binding of the Fc fragment of IgG to IgG-specific receptors (FcgammaR) on innate immune effector cells. In contrast, when administered in high concentrations, IVIg has anti-inflammatory properties. How this anti-inflammatory effect is mediated has not yet been elucidated fully, and several mutually non-exclusive mechanisms have been proposed. This paper represents the proceedings of a session entitled 'IVIg--Understanding properties and mechanisms' at the 6th International Immunoglobulin Symposium that was held in Interlaken on 26-28 March 2009. The presentations addressed how IgG may affect the cellular compartment, evidence for IVIg-mediated scavenging of complement fragments, the role of the dimeric fraction of IVIg, the anti-inflammatory properties of the minor fraction of sialylated IgG molecules, and the genetic organization and variation in FcgammaRs. These findings demonstrate the considerable progress that has been made in understanding the mechanisms of action of IVIgs, and may influence future perspectives in the field of Ig therapy.

C-C chemokine receptor 6-regulated entry of TH-17 cells into the CNS through the choroid plexus is required for the initiation of EAE

Interleukin 17-producing T helper cells (T(H)-17 cells) are important in experimental autoimmune encephalomyelitis, but their route of entry into the central nervous system (CNS) and their contribution relative to that of other effector T cells remain to be determined. Here we found that mice lacking CCR6, a chemokine receptor characteristic of T(H)-17 cells, developed T(H)-17 responses but were highly resistant to the induction of experimental autoimmune encephalomyelitis. Disease susceptibility was reconstituted by transfer of wild-type T cells that entered into the CNS before disease onset and triggered massive CCR6-independent recruitment of effector T cells across activated parenchymal vessels. The CCR6 ligand CCL20 was constitutively expressed in epithelial cells of choroid plexus in mice and humans. Our results identify distinct molecular requirements and ports of lymphocyte entry into uninflamed versus inflamed CNS and suggest that the CCR6-CCL20 axis in the choroid plexus controls immune surveillance of the CNS.

Mechanistic insights from the crystal structures of a feast/famine regulatory protein from Mycobacterium tuberculosis H37Rv

Rv3291c gene from Mycobacterium tuberculosis codes for a transcriptional regulator belonging to the (leucine responsive regulatory protein/regulator of asparigine synthase C gene product) Lrp/AsnC-family. We have identified a novel effectorbinding site from crystal structures of the apo protein, complexes with a variety of amino acid effectors, X-ray based ligand screening and qualitative fluorescence spectroscopy experiments. The new effector site is in addition to the structural characterization of another distinct site in the protein conserved in the related AsnC-family of regulators. The structures reveal that the ligandbinding loops of two crystallographically ndependent subunits adopt different conformations to generate two distinct effector-binding sites. A change in the conformation of the binding site loop 100–106 in the B subunit is apparently necessary for octameric association and also allows the loop to interact with a bound ligand in the newly identified effector-binding site. There are four sites of each kind in the octamer and the protein preferentially binds to aromatic amino acids. While amino acids like Phe, Tyr and Trp exhibit binding to only one site, His exhibits binding to both sites. Binding of Phe is accompanied by a conformational change of 3.7A ° in the 75–83 loop, which is advantageously positioned to control formation of higher oligomers. Taken together, the present studies suggest an elegant control mechanism for global transcription regulation involving binding of ligands to the two sites, individually or collectively.

Human bronchial epithelium controls TH2 responses by TH1-induced, nitric oxide-mediated STAT5 dephosphorylation: implications for the pathogenesis of asthma

Increased levels of NO in exhaled air in association with increased NO synthetase (NOS)2 expression in bronchial epithelial are hallmark features of asthma. It has been suggested that NO contributes to asthma pathogenesis by selective down-regulation of TH1 responses. We demonstrate, however, that NO can reversibly limit in vitro expansion of both human TH1 and TH2 CD4+ T cells. Mechanistically, NO induces cGMP-mediated reversible STAT5 dephosphorylation and therefore interferes with the IL-2R activation cascade. Human bronchial epithelial cells (HBEC) up-regulate NOS2 after stimulation with IFN-gamma secreted by TH1 CD4+ T cells and release NO, which inhibits both TH1 and TH2 cell proliferation. This reversible T cell growth arrest depends on NO because T cell proliferation is completely restored after in vitro blocking of NOS2 on HBEC. HBEC thus drive the effector end of a TH1-controlled feedback loop, which protects airway mucosal tissues at the potential lesional site in asthma from overwhelming CD4+ TH2 (and potentially TH1) responses following allergen exposure. Variations in the efficiency of this feedback loop provides a plausible mechanism to explain why only a subset of atopics sensitized to ubiquitous aeroallergens progress to expression of clinically relevant levels of airways inflammation.