Cutting edge: Immune stimulation by neisserial porins is toll-like receptor 2 and MyD88 dependent

Injury, sepsis, and the regulation of Toll-like receptor responses

Although we tend to think that the immune system has evolved to protect the host from invading pathogens and to discriminate between self and nonself, there must also be an element of the immune system that has evolved to control the response to tissue injury. Moreover, these potential immune-regulatory pathways controlling the injury response have likely coevolved in concert with self and nonself discriminatory immune-regulatory networks with a similar level of complexity. From a clinical perspective, severe injury upsets normal immune function and can predispose the injured patient to developing life-threatening infectious complications. This remains a significant health care problem that has driven decades of basic and clinical research aimed at defining the functional effects of injury on the immune system. This review and update on our ongoing research efforts addressing the immunological response to injury will highlight some of the most recent advances in our understanding of the impact that severe injury has on the innate and adaptive immune system focusing on phenotypic changes in innate immune cell responses to Toll-like receptor stimulation.

Eukaryotic cell signaling and transcriptional activation induced by bacterial porins

The protein composition of the outer membrane of Gram-negative bacteria consists of about 20 immunochemically distinct proteins, termed outer membrane proteins (OMPs). Apart from their structural role, OMPs have been shown to have other functions, particularly with regard to transport, and have been classified as permeases and porins. Porins, during their interaction with the host, are immunogenic and also directly stimulate several cellular functions. Porins work both as molecules present on the bacterial surface and as molecules released by bacteria. Lipopolysaccharide and OMPs, the major structural macromolecular constituents of the outer membrane, carry out a fundamental role in the pathogenesis of Gram-negative infections. This brief review describes the multiple facets of the biological activities of porins both in vitro and in vivo, particularly focusing on their ability to induce the expression of cytokines and other factors that modulate cellular activities with either pathological or adaptive consequences. This brief discussion will focus on the signal transmission mechanisms induced by bacterial porins.

Toll-like receptors

The innate immune system in drosophila and mammals senses the invasion of microorganisms using the family of Toll receptors, stimulation of which initiates a range of host defense mechanisms. In drosophila antimicrobial responses rely on two signaling pathways: the Toll pathway and the IMD pathway. In mammals there are at least 10 members of the Toll-like receptor (TLR) family that recognize specific components conserved among microorganisms. Activation of the TLRs leads not only to the induction of inflammatory responses but also to the development of antigen-specific adaptive immunity. The TLR-induced inflammatory response is dependent on a common signaling pathway that is mediated by the adaptor molecule MyD88. However, there is evidence for additional pathways that mediate TLR ligand-specific biological responses.

TLR2 and TLR4 serve distinct roles in the host immune response against Mycobacterium bovis BCG

Toll-like receptor (TLR) proteins mediate cellular activation by microbes and microbial products. To delineate the role of TLR proteins in the development of host immune responses against mycobacteria, wild-type and TLR-deficient mice were infected with nonpathogenic Mycobacterium bovis bacillus Calmette-Guerin (BCG). Two weeks after intraperitoneal challenge with BCG, few bacilli were present in the lungs of wild-type and TLR4(-/-) mice, whereas bacterial loads were tenfold higher in the lungs of infected TLR2(-/-) mice. BCG challenge in vitro strongly induced proinflammatory cytokine secretion by macrophages from wild-type and TLR4(-/-) mice but not by TLR2(-/-) macrophages. In contrast, intracellular uptake, intracellular bacterial growth, and suppression of intracellular bacterial growth in vitro by interferon-gamma (IFN-gamma) were similar in macrophages from all three mouse strains, suggesting that BCG growth in the lungs of TLR2(-/-) mice was a consequence of defective adaptive immunity. Antigenic stimulation of splenocytes from infected wild-type and TLR4(-/-) mice induced T cell proliferation in vitro, whereas T cells from TLR2(-/-) mice failed to proliferate. Unexpectedly, activated CD4(+) T cells from both TLR-deficient mouse strains secreted little IFN-gamma in vitro compared with control T cells. A role for TLR4 in the control of bacterial growth and IFN-gamma production in vivo was observed only when mice were infected with higher numbers of BCG. Thus, TLR2 and TLR4 appear to regulate distinct aspects of the host immune response against BCG.

Activation of toll-like receptor 2 (TLR2) and TLR4/MD2 by Neisseria is independent of capsule and lipooligosaccharide (LOS) sialylation but varies widely among LOS from different strains

Lipooligosaccharide (LOS) structure and capsular polysaccharide of Neisseria meningitidis each greatly influence the virulence of the organism and the quality of host innate immune responses. In this study, we found that production of the proinflammatory cytokine tumor necrosis factor (TNF) by a human monocyte-derived cell line (THP-1) exposed to strains of N. meningitidis lacking capsule and/or with truncated LOS was similar to that elicited by the isogenic wild-type strain. These mutants also exhibited no difference in induction of the interleukin-8 (IL-8) promoter in a transfected HeLa cell system of Toll-like receptor 2 (TLR2) and TLR4/MD2 signaling. However, purified LOS from diverse strains of Neisseria (both N. meningitidis and N. gonorrhoeae) caused widely variant levels of IL-8 promoter induction in cells expressing MD2 that correlated with the production of TNF from THP-1 cells. These data suggest that although modification of the oligosaccharide chain of LOS and/or absence of capsule do not affect cell signaling mediated by TLR4/MD2, fine-structural differences in the LOS do influence signaling through TLR4/MD2 and, through this pathway, influence some of the proinflammatory responses elicited by Neisseria.

Receptors, mediators, and mechanisms involved in bacterial sepsis and septic shock

Bacterial sepsis and septic shock result from the overproduction of inflammatory mediators as a consequence of the interaction of the immune system with bacteria and bacterial wall constituents in the body. Bacterial cell wall constituents such as lipopolysaccharide, peptidoglycans, and lipoteichoic acid are particularly responsible for the deleterious effects of bacteria. These constituents interact in the body with a large number of proteins and receptors, and this interaction determines the eventual inflammatory effect of the compounds. Within the circulation bacterial constituents interact with proteins such as plasma lipoproteins and lipopolysaccharide binding protein. The interaction of the bacterial constituents with receptors on the surface of mononuclear cells is mainly responsible for the induction of proinflammatory mediators by the bacterial constituents. The role of individual receptors such as the toll-like receptors and CD14 in the induction of proinflammatory cytokines and adhesion molecules is discussed in detail. In addition, the roles of a number of other receptors that bind bacterial compounds such as scavenger receptors and their modulating role in inflammation are described. Finally, the therapies for the treatment of bacterial sepsis and septic shock are discussed in relation to the action of the aforementioned receptors and proteins.

A bioinformatic approach to the identification of bacterial proteins interacting with Toll-interleukin 1 receptor-resistance (TIR) homology domains

Members of the Toll-like receptor (TLR) family are currently under intense scrutiny for their role in the sampling and recognition of pathogens. It has already been reported that both vaccinia virus and Yersinia spp. express proteins that help them evade the TLR mediated immune response, acting through the Toll-interleukin-1 receptor-resistance (TIR) domain and leucine-rich repeat region of the host TLRs respectively. The TIR domain is involved in the dimerisation of the TLRs and their complexation with their adapter molecules. We tested here the hypothesis that bacteria have the ability to secrete proteins containing similar motifs to the intracellular TIR domains that are involved in the TIR-TIR interaction necessary for the subsequent signal transmission. Based upon their sequence homology, proteins expressing TIRs have been divided into three sub-classes, based around the TLRs, the TLR adapter proteins, and the interleukin-1 and -18 adapter proteins. The highly conserved regions from these separate sub-families were then used to identify similar bacterial proteins. The bacterial proteins identified were then included in an iterative MEME-BLAST process to broaden the search. Tollip, a known TLR antagonist and adapter protein, was included in this investigation although it does not fit into any of the three sub-classes outlined above. If suitable bacterial proteins had been identified, it would signify that certain bacteria had evolved a mechanism to aid them in avoiding detection by the innate immune system acting through the TIR domains. At this stage one has to conclude that there is no evidence currently available suggesting such a mechanism, when using the strategy applied here.

The immunopathogenesis of meningococcal disease

This review describes the mechanisms of the immune response to meningococcal disease, examining the extent to which individual variation of the immune response can determine susceptibility. It concludes by summarising the difficulties encountered by recent efforts to develop new immunomodulatory treatments.

The role of Toll-like receptor 2 in microbial disease and immunity

Cells expressing Toll-like receptor (TLR), TLR2 in association with TLR1, TLR6 or some other unknown co-receptor can respond upon interaction with a large variety of microbial ligands. The variety of TLR2 ligands is the greatest among all the TLRs and this is due to the heterodimerization needed for TLR2 mediated responses. Like other TLRs, TLR2 signaling induces antigen presenting cell activation, pro-inflammatory cytokine production and increased expression of co-stimulatory ligand expression. These events are important for induction of innate immune responses and improved acquired immunity. There is strong suggestive evidence that alteration or lack of TLR2 function in vivo may correlate to decreased immune protection from pathogens that contain TLR2 ligands, but more work needs to be performed to strengthen this correlation.

Interaction of Yersinia enterocolitica with epithelial cells: invasin beyond invasion

The chromosomally encoded inv gene product is an outer membrane protein that is functionally expressed in the enteropathogenic Yersinia species Yersinia enterocolitica and Yersinia pseudotuberculosis. Invasin protein is a high-affinity ligand for beta1 integrins and especially important in the early phase of intestinal infection for efficient translocation through the M cells located in the follicle-associated epithelium overlying the Peyer's patches. In addition to bacterial internalization, Yersinia invasin mediates proinflammatory epithelial cell reactions. Epithelial cells exhibit immunological functions including production of cytokines thereby signaling to the immune system the presence of invasive or pathogenic bacteria. Several other enteropathogenic bacteria also induce cytokine production in epithelial cells. However, the signaling pathways by which this reaction is accomplished differ for various pathogens. Binding of invasin-expressing Yersinia to beta1 integrin receptors of epithelial cells induces activation of a signal cascade involving Rac1, MAP kinases, activation of the transcription factor NF-kappaB, and the subsequent production of chemotactic cytokines. The Yersinia invasin-triggered inflammatory epithelial cell reaction may lead to the recruitment of phagocytes followed by tissue disruption which may be part of the strategy of the pathogen to promote its dissemination in the host tissue.

virK, somA and rcsC are important for systemic Salmonella enterica serovar Typhimurium infection and cationic peptide resistance

Salmonella must express and deploy a type III secretion system located in Salmonella pathogenicity island 2 (SPI-2) in order to survive in host phagocytic vacuoles and to cause systemic infection in mouse models of typhoid fever. A genome-wide approach to screening for Salmonella genes that are transcriptionally co-regulated in vitro with SPI-2 genes was used to identify bacterial loci that might function in a mouse model of systemic disease. Strains with mutations in three SPI-2 co-expressed genes were constructed and tested for their ability to cause disease in mice. We found that virK, a homologue of a Shigella virulence determinant, and rcsC, a sensor kinase, are important at late stages of infection. A second Salmonella gene that has VirK homology, somA, is also important for systemic infection in mice. We have shown that expression of both virK and somA requires the transcription factor PhoP, whereas rcsC does not. Additionally, rcsC expression does not require the transcription factor OmpR, but expression of one of the known targets of RcsC, the yojN rcsB putative operon, does require OmpR. virK, somA and rcsC are expressed in tissue culture macrophages and confer Salmonella resistance to the cationic peptide polymyxin B. We conclude that virK, somA and rcsC are important for late stages of Salmonella enteric fever, and that they probably contribute to the remodelling of the bacterial outer membrane in response to the host environment.

The role of porins in neisserial pathogenesis and immunity

Neisseria meningitidis and Neisseria gonorrhoeae are Gram-negative pathogenic bacteria responsible for bacterial meningitis and septicemia, and the sexually transmitted disease gonorrhea, respectively. Porins are the most represented outer membrane proteins in the pathogenic Neisseria species, functioning as pores for the exchange of ions, and are characterized by a trimeric beta-barrel structure. Neisserial porins have been shown to act as adjuvants in the immune response via activation of B cells and other antigen-presenting cells (APCs). Their effect on the immune response is mediated by upregulation of the costimulatory molecule B7-2 (CD86) on the surface of APCs, an effect that is Toll-like receptor 2- and MyD88-dependent. The effect of neisserial porins on the immune system also involves interaction with components of the complement cascade. Furthermore, neisserial porins co-localize with mitochondria of target cells, where they appear to modulate apoptosis.

Recognition of pathogen-associated molecular patterns by TLR family

Toll-like receptors (TLRs) are type I transmembrane proteins involved in innate immunity by recognizing microbial conserved structures. Recent studies have shown that TLR3 recognizes dsRNA, a viral product, whereas TLR9 recognizes unmethylated CpG motifs frequently found in the genome of bacteria and viruses, but not vertebrates. TLR7 recognizes small synthetic immune modifiers including imiquimod, R-848, loxoribine, and bropirimine, all of which are already applied or promising for clinical use against viral infections and cancers. Plasmacytoid dendritic cells express TLR7 and TLR9, and respond to TLR7 and TLR9 ligands by producing a large amount of interferon (IFN-alpha). These results indicate that TLR3, TLR7 and TLR9 may play an important role in detecting and combating viral infections.

Lipopolysaccharides from Legionella and Rhizobium stimulate mouse bone marrow granulocytes via Toll-like receptor 2

Lipopolysaccharide (LPS) derived from enterobacteria elicit in several cell types cellular responses that are restricted in the use of Toll-like receptor 4 (TLR4) as the principal signal-transducing molecule. A tendency to consider enterobacterial LPS as a prototypic LPS led some authors to present this mechanism as a paradigm accounting for all LPSs in all cell types. However, the structural diversity of LPS does not allow such a general statement. By using LPSs from bacteria that do not belong to the Enterobacteriaceae, we show that in bone marrow cells (BMCs) the LPS of Rhizobium species Sin-1 and of three strains of Legionella pneumophila require TLR2 rather than TLR4 to elicit the expression of CD14. In addition, exposure of BMCs from TLR4-deficient (C3H/HeJ) mice to the lipid A fragment of the Bordetella pertussis LPS inhibits their activation by the Legionella lipid A. The data show selective action of different LPSs via different TLRs, and suggest that TLR2 can interact with many lipid A structures, leading to either agonistic or specific antagonistic effects.

TLR2: cellular sensor for microbial and endogenous molecular patterns

Toll-like receptor (TLR) 2 is a member of the vertebrate protein family of TLRs that has been studied in substantial detail over the last years. The extracellular domain of the type I receptor molecule TLR2 contains 18 to 20 leucine rich repeat (LRR) and LRR like motives. The intracellular domain of TLR2 contains a Toll/IL-1 receptor/resistance protein typical TIR domain. After the first implication of TLR4 in immunity thereinafter followed by the discovery of the lipopolysaccharide signal transducer function of TLR4, TLR2 was the first of ten mammalian TLRs proven to be directly involved in recognition of pathogen associated molecular patterns (PAMPs). Among the TLR2 specific agonists are microbial products representing broad groups of species such as Gram-positive and Gram-negative bacteria, as well as mycobacteria, spirochetes, and mycoplasm. PAMP induced phagosomal localization of TLR2 and TLR2 dependent apoptosis have been shown. Complex formation with other molecules involved in pattern recognition such as CD14, MD2, TLR1, and TLR6 has been implicated for TLR2. Surprisingly even proteinaceous host material such as heat shock protein (HSP) 60 has been demonstrated to activate cells through TLR2. Thus, TLR2 may be a sensor and inductor of specific defense processes, including oxidative stress and cellular necrosis initially spurred by microbial compounds. Here we summarize the current knowledge on the structure and function of TLR2, which is far from being complete. Detailed understanding of the biology of TLR2 will probably contribute to the characterization of a number of infectious diseases and potentially help in the development of novel intervention strategies.

Chromatin-IgG complexes activate B cells by dual engagement of IgM and Toll-like receptors

Autoreactive B cells are present in the lymphoid tissues of healthy individuals, but typically remain quiescent. When this homeostasis is perturbed, the formation of self-reactive antibodies can have serious pathological consequences. B cells expressing an antigen receptor specific for self-immunoglobulin-gamma (IgG) make a class of autoantibodies known as rheumatoid factor (RF). Here we show that effective activation of RF+ B cells is mediated by IgG2a-chromatin immune complexes and requires the synergistic engagement of the antigen receptor and a member of the MyD88-dependent Toll-like receptor (TLR) family. Inhibitor studies implicate TLR9. These data establish a critical link between the innate and adaptive immune systems in the development of systemic autoimmune disease and explain the preponderance of autoantibodies reactive with nucleic acid-protein particles. The unique features of this dual-engagement pathway should facilitate the development of therapies that specifically target autoreactive B cells.