Modulation of Mycoplasma arthritidis-derived superantigen-induced cytokine gene expression by dexamethasone and interleukin-4

Multiplicity of Glucocorticoid Action in Inhibiting Allograft Rejection

Glucocorticoids (GCs) are used as immunosuppressive and antiinflammatory agents in organ transplantation and in treating autoimmune diseases and inflammatory disorders. GCs were shown to exert their antiproliferative effects directly through blockade of certain elements of an early membrane-associated signal transduction pathway, modulation of the expression of select adhesion molecules, and by suppression of cytokine synthesis and action. GCs may act indirectly by inducing lipocortin synthesis, which in turn, inhibits arachidonic acid release from membrane-bound stores, and also by inducing transforming growth factor (TGF)-β expression that subsequently blocks cytokine synthesis and T cell activation. Furthermore, by preferentially inhibiting the production of Th1 cytokines, GCs may enhance Th2 cell activity and, hence, precipitate a long-lasting state of tolerance through a preferential promotion of a Th2 cytokine-secreting profile. In exerting their antiproliferative effects, GCs influence both transcriptional and posttranscriptional events by binding their cytosolic receptor (GR), which subsequently binds the promoter region of cytokine genes on select DNA sites compatible with the GCs responsible elements (GRE) motif. In addition to direct DNA binding, GCs may also directly bind to, and hence antagonize, nuclear factors required for efficient gene expression, thereby markedly reducing transcriptional rate. The pleiotrophy of the GCs action, coupled with the diverse experimental conditions employed in assessing the GCs effects, indicate that GCs may utilize more than one mechanism in inhibiting T cell activation, and warrant careful scrutiny in assigning a mechanism by which GCs exert their antiproliferative effects. © 1998 Elsevier Science Inc.

Coexpression of TLR2 or TLR4 with HLA-DR potentiates the superantigenic activities of Mycoplasma arthritidis-derived mitogen

Mycoplasma arthritidis-derived mitogen (MAM) is a member of the superantigen family that structurally differs from other members while still capable of initiating cognate APC/T cell interaction. In addition to the critical role of MHC class II molecules, it has been suggested that TLR2 and TLR4 may cooperate with MHC class II during MAM-induced responses. In this study, we investigated the direct involvement of TLR2 and TLR4 in MAM binding and presentation to T cells. Our results showed that MAM fails to bind to TLR2- and TLR4-transfected cells. However, coexpression of TLR2 or TLR4 with HLA-DR significantly increases MAM binding and the subsequent T cell activation compared with cells expressing HLA-DR alone. The upregulated MAM binding and activity in HLA-DR/TLR-transfected cells is abrogated by an anti-HLA-DR Ab. Interestingly, we also found that MAM complexed with soluble HLA-DR is capable of binding to both TLR2 and TLR4. The enhancing effect of TLR2 or TLR4 on MAM-induced T cell proliferation was not due to TLR ligand contamination in the MAM preparation. Taken together, these results strongly suggest that binding of MAM to HLA-DR leads to a conformational change in MAM structure allowing its interaction with TLR2 and TLR4 and a better recognition by T cells.

A comprehensive molecular interaction map for rheumatoid arthritis

Background

Computational biology contributes to a variety of areas related to life sciences and, due to the growing impact of translational medicine - the scientific approach to medicine in tight relation with basic science -, it is becoming an important player in clinical-related areas. In this study, we use computation methods in order to improve our understanding of the complex interactions that occur between molecules related to Rheumatoid Arthritis (RA).

Methodology

Due to the complexity of the disease and the numerous molecular players involved, we devised a method to construct a systemic network of interactions of the processes ongoing in patients affected by RA. The network is based on high-throughput data, refined semi-automatically with carefully curated literature-based information. This global network has then been topologically analysed, as a whole and tissue-specifically, in order to translate the experimental molecular connections into topological motifs meaningful in the identification of tissue-specific markers and targets in the diagnosis, and possibly in the therapy, of RA.

Significance

We find that some nodes in the network that prove to be topologically important, in particular AKT2, IL6, MAPK1 and TP53, are also known to be associated with drugs used for the treatment of RA. Importantly, based on topological consideration, we are also able to suggest CRKL as a novel potentially relevant molecule for the diagnosis or treatment of RA. This type of finding proves the potential of in silico analyses able to produce highly refined hypotheses, based on vast experimental data, to be tested further and more efficiently. As research on RA is ongoing, the present map is in fieri, despite being -at the moment- a reflection of the state of the art. For this reason we make the network freely available in the standardised and easily exportable .xml CellDesigner format at ‘www.picb.ac.cn/ClinicalGenomicNTW/temp.html’ and ‘www.celldesigner.org’.

The superantigen staphylococcal enterotoxin A (SEA) and monoclonal antibody L243 share a common epitope but differ in their ability to induce apoptosis via MHC-II

Crosslinking of MHC class II (MHC-II) molecules by antibodies or by superantigens (SAg) induces a variety of functional responses in the antigen presenting cell. We were able to allocate K39 as the residue that is essential for binding of antibody L243 to the alpha chain of HLA-DR. K39 is also essential for binding of staphylococcal enterotoxin A (SEA). However, the functional responses of the two ligands differ considerably exemplified by the ability of L243 to induce apoptosis in monocytic cells and in B cells, whereas SEA is unable to activate the apoptosis pathway. Despite the differences in functional responses, both ligands induce cell aggregation in MonoMac-1 cells. The SEA molecule with its two different binding sites associates one MHC alpha chain with one beta chain as opposed to two alpha chains that are brought into close proximity by the two identical antigen binding sites of L243. We therefore conclude that the spatial orientation of dimerized MHC-II and their associated proteins is an important factor for the nature of the transduced signal and consequently the outcome of functional responses.

Lipid raft-dependent and -independent signaling through HLA-DR molecules

Lipid rafts are plasma membrane microdomains that are highly enriched in signaling molecules and that act as signal transduction platforms for many immune receptors. The involvement of these microdomains in HLA-DR-induced signaling is less well defined. We examined the constitutive presence of HLA-DR molecules in lipid rafts, their possible recruitment into these microdomains, and the role of these microdomains in HLA-DR-induced responses. We detected significant amounts of HLA-DR molecules in the lipid rafts of EBV(+) and EBV(-) B cell lines, monocytic cell lines, transfected HeLa cells, tonsillar B cells, and human monocytes. Localization of HLA-DR in these microdomains was unaffected by the deletion of the cytoplasmic domain of both the alpha and beta chains. Ligation of HLA-DR with a bivalent, but not a monovalent, ligand resulted in rapid tyrosine phosphorylation of many substrates, especially Lyn, and activation of ERK1/2 MAP kinase. However, the treatment failed to induce further recruitment of HLA-DR molecules into lipid rafts. The HLA-DR-induced signaling events were accompanied by the induction of cell-cell adhesion that could be inhibited by PTK and Lyn but not ERK1/2 inhibitors. Disruption of lipid rafts by methyl-beta-cyclodextrin (MbetaCD) resulted in the loss of membrane raft association with HLA-DR molecules, inhibition of HLA-DR-mediated protein tyrosine phosphorylation and cell-cell adhesion. MbetaCD did not affect the activation of ERK1/2, which was absent from lipid rafts. These results indicate that although all the HLA-DR-induced events studied are dependent on HLA-DR dimerization, some require the presence of HLA-DR molecules in lipid rafts, whereas others do not.

Transcriptional and post-transcriptional mechanisms of glucocorticoid antiproliferative effects

Glucocorticoids (GCs) are used as immunosuppressive and anti-inflammatory agents in treating organ transplantation rejection, autoimmune diseases, (hematological) cancers, and inflammatory disorders. GCs exert their effects through a multitude of mechanisms, the most significant of which is inhibition of cytokine production, and for some cytokines their effects on target cells. Paradoxically, GCs also upregulate the expression of (pro-inflammatory) high-affinity cytokine receptors on target cells in the face of lost ligand (cytokine) stimulation. GC inhibition of cytokine expression occurs at both transcriptional and post-transcriptional levels. GCs acted transcriptionally by binding their cytosolic receptor (GR), thereby facilitating its nuclear translocation and subsequent binding to the promoter region of cytokine genes on sites compatible with GC response element (GRE) motifs, which in turn directly or indirectly regulated gene expression. In addition to direct DNA binding, GCs acted post-transcriptionally by: (1) antagonism of nuclear factors required for efficient gene expression either directly or through induction of the expression of specific transcription factor antagonists, (2) altered Th lineage development by favouring the generation of (anti-inflammatory) Th2 cells and suppressing the induction or the activity of established (pro-inflammatory) Th1 cells, and (3) stimulating the expression of transforming growth factor (TGF)-beta, an immunosuppressive cytokine which inhibited cytokine production. However, these mechanisms are not mutually exclusive, since GCs may utilize more than one mechanism in exerting their anti-proliferative effect.

Presence of Lps(d) mutation influences cytokine regulation in vivo by the Mycoplasma arthritidis mitogen superantigen and lethal toxicity in mice infected with M. arthritidis

The Mycoplasma arthritidis mitogen (MAM) superantigen (SAg) is a potent activator of human and murine cells and is produced by an organism that is a cause of acute and chronic arthritis of rodents. It is phylogenetically unrelated to other bacterial SAgs and exhibits a number of unique features. We recently demonstrated that MAM differentially regulates the cytokine responses of different mouse strains following in vivo administration. Here we show that the presence in inbred C3H/HeJ mice of the mutant Lps(d) gene, which is associated with a defect in Toll-like receptor 4 (TLR4), influences MAM regulation of cytokine profiles in vivo. Whereas the levels of type 1 cytokines (interleukin-2 [IL-2], gamma interferon, IL-12, and tumor necrosis factor alpha) were depressed in cells from MAM-injected wild-type C3H/HeSnJ mice, they were elevated in cells from C3H/HeJ mice. Furthermore, the levels of type 2 cytokines (IL-4, IL-6, and IL-10) were elevated in Lps(n) C3H/HeSnJ mice but depressed in Lps(d) C3H/HeJ mice. The transcript for IL-12 p40 was highly expressed in C3H/HeJ but not C3H/HeSnJ mice. F(1) mice exhibited the same cytokine profile as C3H/HeJ mice, indicating that the mutant gene exhibited dominant-negative inheritance. In addition, C3H/HeJ mice were highly susceptible to toxic death in comparison with C3H/HeSnJ mice after injection with live M. arthritidis organisms. Our results suggest that MAM interacts with the lipopolysaccharide signaling pathway, possibly involving TLR4 or a combinatorial Toll complex.

Molecular biology and pathogenicity of mycoplasmas

The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors' chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

Microbial superantigens stimulate T cells by the superantigen bridge and independently by a cytokine pathway

Superantigens cross-link the MHC II molecule on accessory cells with the Vbeta region of the T cell receptor (TCR). In this study, we compared the capacity of established superantigens for inducing cytokine release. The experimental protocol was generated to answer the question whether all superantigen effects are transmitted by the MHC/TCR cross-linkage and induce mainly a T cell response. We found that TSST-1, ExFTA, and SEC3 differed from all other superantigens tested because they stimulated a stronger monokine release. T cell proliferation after challenge with these superantigens was mainly mediated by a cytokine pathway and not by the cross-linkage of MHC and TCR. For the other superantigens, we were able to demonstrate that major immunomodulatory effect is mediated by the superantigen bridge. With the exception of these three superantigens, the proliferative response of superantigens correlated with their Vbeta specificity. Interleukin-1 (IL-1) and IL-6 were induced in monocytes by all superantigens, whereas tumor necrosis factor-alpha (TNF-alpha) was induced in T cells and by some superantigens, also in monocytes. IL-2 was always induced by the superantigen bridge, whereas interferon-gamma (IFN-gamma) was also induced indirectly by monokines. Collectively, our results indicate that not all superantigens are suitable for investigating superantigen-specific effects, as they show indirect (mitogenic) side effects. Observations for an individual superantigen are, therefore, not transferable to all other superantigens.

Cross-linking of major histocompatibility complex class II molecules by staphylococcal enterotoxin A superantigen is a requirement for inflammatory cytokine gene expression

Staphylococcal enterotoxin A (SEA) has two distinct binding sites for major histocompatibility complex (MHC) class II molecules. The aspartic acid located at position 227 (D227) in the COOH terminus of SEA is one of the three residues involved in its interaction with the DR beta chain, whereas the phenylalanine 47 (F47) of the NH2 terminus is critical for its binding to the DR alpha chain. Upon interaction with MHC class II molecules, SEA triggers several cellular events leading to cytokine gene expression. In the present study, we have demonstrated that, contrary to wild-type SEA, stimulation of the THP1 monocytic cell line with SEA mutated at position 47 (SEAF47A) or at position 227 (SEAD227A) failed to induce interleukin 1 beta and tumor necrosis factor-alpha messenger RNA expression. Pretreatment of the cells with a 10-fold excess of either SEAF47A or SEAD227A prevented the increase in cytokine messenger RNA induced by wild-type SEA. However, cross-linking of SEAF47A or SEAD227A bound to MHC class II molecules with F(ab')2 anti-SEA mAb leads to cytokine gene expression, whereas cross-linking with F(ab) fragments had no effect. Taken together, these results indicate that cross-linking of two MHC class II molecules by one single SEA molecule is a requirement for cytokine gene expression.