Staphylococcal Protein A Deletes B-1a and Marginal Zone B Lymphocytes Expressing Human Immunoglobulins: An Immune Evasion Mechanism

Inflammasome functional activities in B lymphocytes

Studies in animal models and human subjects have shown that, in addition to their implication in innate immunity, inflammasomes also can play a role in adaptive immunity. However, the contribution of the nucleotide-binding oligomerization domain-, leucine-rich repeat-, and pyrin domain-containing protein 3 (NLRP3) inflammasome pathway to adaptive immunity remains incompletely explored. Here, we show that NLRP3 plays an important role in different facets of B cell functions, including proliferation, antibody production, and secretion of inflammatory and anti-inflammatory cytokines. When exposed to B cell receptor engagement, Toll-like receptor activation, stimulation in conditions that mimic T cell-dependent responses, or NLRP3 activation, B cells manifest disparate responses and produce different cytokine patterns critical for modulating innate and adaptive immunity, indicating that the cytokines produced serve a critical link between the early innate immune response and the delayed adaptive immunity. Importantly, genetic ablation of nlrp3 reduced the inflammasome-mediated functions of B cells. We propose that, in the absence of other cell types, the potential of B lymphocytes to respond to NLRP3 engagement enables them to initiate inflammatory cascades through recruitment of other cell subsets, such as macrophages and neutrophils. Since NLRP3 activation of B cells is not followed by pyroptosis, even in the presence of a basal caspase-1 activity, this pathway acts as a bridge that optimizes interactions between the innate and adoptive branches of the immune response.

Linking S. aureus Immune Evasion Mechanisms to Staphylococcal Vaccine Failures

Vaccination arguably remains the only long-term strategy to limit the spread of S. aureus infections and its related antibiotic resistance. To date, however, all staphylococcal vaccines tested in clinical trials have failed. In this review, we propose that the failure of S. aureus vaccines is intricately linked to prior host exposure to S. aureus and the pathogen's capacity to evade adaptive immune defenses. We suggest that non-protective immune imprints created by previous exposure to S. aureus are preferentially recalled by SA vaccines, and IL-10 induced by S. aureus plays a unique role in shaping these non-protective anti-staphylococcal immune responses. We discuss how S. aureus modifies the host immune landscape, which thereby necessitates alternative approaches to develop successful staphylococcal vaccines.

Loss of TET2 increases B-1 cell number and IgM production while limiting CDR3 diversity

Recent studies have demonstrated a role for Ten-Eleven Translocation-2 (TET2), an epigenetic modulator, in regulating germinal center formation and plasma cell differentiation in B-2 cells, yet the role of TET2 in regulating B-1 cells is largely unknown. Here, B-1 cell subset numbers, IgM production, and gene expression were analyzed in mice with global knockout of TET2 compared to wildtype (WT) controls. Results revealed that TET2-KO mice had elevated numbers of B-1a and B-1b cells in their primary niche, the peritoneal cavity, as well as in the bone marrow (B-1a) and spleen (B-1b). Consistent with this finding, circulating IgM, but not IgG, was elevated in TET2-KO mice compared to WT. Analysis of bulk RNASeq of sort purified peritoneal B-1a and B-1b cells revealed reduced expression of heavy and light chain immunoglobulin genes, predominantly in B-1a cells from TET2-KO mice compared to WT controls. As expected, the expression of IgM transcripts was the most abundant isotype in B-1 cells. Yet, only in B-1a cells there was a significant increase in the proportion of IgM transcripts in TET2-KO mice compared to WT. Analysis of the CDR3 of the BCR revealed an increased abundance of replicated CDR3 sequences in B-1 cells from TET2-KO mice, which was more clearly pronounced in B-1a compared to B-1b cells. V-D-J usage and circos plot analysis of V-J combinations showed enhanced usage of VH11 and VH12 pairings. Taken together, our study is the first to demonstrate that global loss of TET2 increases B-1 cell number and IgM production and reduces CDR3 diversity, which could impact many biological processes and disease states that are regulated by IgM.

Molecular Characterization of Staphylococcus aureus Complex Isolated from Free-Ranging Long-Tailed Macaques at Kosumpee Forest Park, Maha Sarakham, Thailand

The Staphylococcus (S.) aureus complex, including methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA), and S. argenteus are bacterial pathogens that are responsible for both human and animal infection. However, insights into the molecular characteristics of MRSA, MSSA, and S. argenteus carriages in wildlife, especially in long-tailed macaques, rarely have been reported in Thailand. The objective of this study was to assess molecular characterization of MRSA, MSSA, and S. argenteus strains isolated from free-ranging long-tailed macaques (Macaca fascicularis) at Kosumpee Forest Park, Maha Sarakham, Thailand. A total of 21 secondary bacterial isolates (including 14 MRSA, 5 MSSA, and 2 S. argenteus) obtained from the buccal mucosa of 17 macaques were analysed by a Polymerase chain reaction (PCR) to identify several virulence genes, including pvl, tst, hla, hlb clfA, spa (x-region), spa (IgG biding region), and coa. The most prevalent virulence genes were clfA, coa, and the spa IgG biding region which presented in all isolates. These data indicated that MRSA, MSSA, and S. argenteus isolates from the wild macaques at Kosumpee Forest Park possess a unique molecular profile, harbouring high numbers of virulence genes. These findings suggest that wild macaques may potentially serve as carriers for distribution of virulent staphylococcal bacteria in the study area.

Inflammasome is a central player in B cell development and homing

Whereas the role of the nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing protein (NLRP) 3 pathway in innate immunity has been extensively studied, little attention has been paid to its contribution to adaptive immunity. Studies in animal models and human subjects have shown the contribution of NLRP3 to the T cell compartment, and its role in B lymphocyte functions has been proposed. Here, we report that ablation of nlrp3 in mice led to altered B cell development in the bone marrow, and distorted expression of B cell subsets that play innate-like functions, that is, marginal zone B cells in the spleen and B-1a cells in the peritoneal cavity. Mechanistically, in the absence of NLRP3 expression, the transcription factor IRF4, previously found to interact with NLRP3 in the nucleus of lymphocytes, was up-regulated. NLRP3 ablation reduced the expression of the chemokine receptors CXCR4 and CCR7 in an IRF4-dependent manner, indicating that the presence of NLRP3 is critical for optimal expression of chemokine receptors on B cells. We conclude that activation of the NLRP3 inflammasome plays a role in B cell development, homing, and retention in lymphoid organs.

Staphylococcal protein A modulates inflammation by inducing interferon signaling in human nasal epithelial cells

OBJECTIVE AND DESIGN

Staphylococcus aureus (S. aureus) is one of the leading causes of human respiratory tract infections. The function of Staphylococcal protein A (SpA), expressed on the S. aureus bacterial membrane and released in the environment, on human nasal epithelial cells (HNECs) have not been fully elucidated. In this study, we tested the SpA expression in S. aureus from chronic rhinosinusitis patients and investigated the effects of SpA on HNECs inflammation through Interferon Gamma Receptor 1(IFNGR1)/phosphorylated Janus Kinase 2 (p-JAK2) pathway.

METHODS

RNA profiling was performed to investigate inflammatory activation in a S. aureus chronic rhinosinusitis (CRS) mouse model. SpA release by S. aureus clinical isolates was determined using ELISA. The effect of purified SpA and SpA enriched conditioned media from S. aureus clinical isolates on HNECs cytotoxicity, apoptosis and release of inflammatory cytokines was evaluated using lactate dehydrogenase assays, and flow cytometry. SpA dependent IFNGR1 and p-JAK2 expression were assessed by qPCR, immunofluorescence and western blot in HNECs.

RESULTS

49 genes were significantly induced in S. aureus CRS mice indicative of activation of interferon signaling. SpA release was significantly higher in S. aureus clinical isolates from chronic rhinosinusitis with nasal polyps (CRSwNP) patients. Purified SpA significantly increased IFNGR1 mRNA and protein expression in HNECs. SpA induced cytotoxic effects and induced the release of Interleukin-6 (IL-6) and IL-8 in an IFNGR1 dependent way.

CONCLUSION

SpA induces interferon signaling through activation of the IFNGR1-JAK-2 pathway, which provides an understanding of how S. aureus SpA affects the inflammatory process in the upper airways.

Mechanisms That Shape Human Antibody Repertoire Development in Mice Transgenic for Human Ig H and L Chain Loci

To determine the impact of the milieu on the development of the human B cell repertoire, we carried out a comprehensive analysis of productive and nonproductive Ig gene rearrangements from transgenic mice engineered to express single copies of the unrearranged human H chain and L chain Ig gene loci. By examining the nonproductive repertoire as an indication of the immediate product of the rearrangement machinery without an impact of selection, we discovered that the distribution of human rearrangements arising in the mouse was generally comparable to that seen in humans. However, differences between the distribution of nonproductive and productive rearrangements that reflect the impact of selection suggested species-specific selection played a role in shaping the respective repertoires. Although expression of some VH genes was similar in mouse and human (IGHV3-23, IGHV3-30, and IGHV4-59), other genes behaved differently (IGHV3-33, IGHV3-48, IGHV4-31, IGHV4-34, and IGHV1-18). Gene selection differences were also noted in L chains. Notably, nonproductive human VH rearrangements in the transgenic mice expressed shorter CDRH3 with less N addition. Even the CDRH3s in the productive rearrangements were shorter in length than those of the normal human productive repertoire. Amino acids in the CDRH3s in both species showed positive selection of tyrosines and glycines, and negative selection of leucines. The data indicate that the environment in which B cells develop can affect the expressed Ig repertoire by exerting influences on the distribution of expressed VH and VL genes and by influencing the amino acid composition of the Ag binding site.

Host-Pathogen Interface: Progress in Understanding the Pathogenesis of Infection Due to Multidrug-Resistant Bacteria in the Intensive Care Unit

The diverse responses of critically ill patients to infection with multi-drug resistant (MDR) bacteria are determined by many complex factors. These include the nature of the immune response activated by specific organisms. Properties unique to each organism such as adherence proteins, microvesicle formation, toxin production and the propensity to form biofilms are important factors in pathogenesis. Equally important is the variability in the host immune response, whether due to genetic or iatrogenic factors, including the presence of major comorbidities, treatment with immunomodulatory therapy and disruption of the microbiome. Future approaches in treating infections caused by MDR bacteria will be heavily influenced by a precision medicine approach, with rapid diagnostic techniques of both bacterial and host factors and high throughput screening of novel therapeutics becoming the mainstay of treatment.

HTJoinSolver: Human immunoglobulin VDJ partitioning using approximate dynamic programming constrained by conserved motifs

Background

Partitioning the human immunoglobulin variable region into variable (V), diversity (D), and joining (J) segments is a common sequence analysis step. We introduce a novel approximate dynamic programming method that uses conserved immunoglobulin gene motifs to improve performance of aligning V-segments of rearranged immunoglobulin (Ig) genes. Our new algorithm enhances the former JOINSOLVER algorithm by processing sequences with insertions and/or deletions (indels) and improves the efficiency for large datasets provided by high throughput sequencing.

Results

In our simulations, which include rearrangements with indels, the V-matching success rate improved from 61% for partial alignments of sequences with indels in the original algorithm to over 99% in the approximate algorithm. An improvement in the alignment of human VDJ rearrangements over the initial JOINSOLVER algorithm was also seen when compared to the Stanford.S22 human Ig dataset with an online VDJ partitioning software evaluation tool.

Conclusions

HTJoinSolver can rapidly identify V- and J-segments with indels to high accuracy for mutated sequences when the mutation probability is around 30% and 20% respectively. The D-segment is much harder to fit even at 20% mutation probability. For all segments, the probability of correctly matching V, D, and J increases with our alignment score.

Revisiting the B-cell compartment in mouse and humans: more than one B-cell subset exists in the marginal zone and beyond

The immunological roles of B-cells are being revealed as increasingly complex by functions that are largely beyond their commitment to differentiate into plasma cells and produce antibodies, the key molecular protagonists of innate immunity, and also by their compartmentalisation, a more recently acknowledged property of this immune cell category. For decades, B-cells have been recognised by their expression of an immunoglobulin that serves the function of an antigen receptor, which mediates intracellular signalling assisted by companion molecules. As such, B-cells were considered simple in their functioning compared to the other major type of immune cell, the T-lymphocytes, which comprise conventional T-lymphocyte subsets with seminal roles in homeostasis and pathology, and non-conventional T-lymphocyte subsets for which increasing knowledge is accumulating. Since the discovery that the B-cell family included two distinct categories - the non-conventional, or extrafollicular, B1 cells, that have mainly been characterised in the mouse; and the conventional, or lymph node type, B2 cells - plus the detailed description of the main B-cell regulator, FcγRIIb, and the function of CD40(+) antigen presenting cells as committed/memory B-cells, progress in B-cell physiology has been slower than in other areas of immunology. Cellular and molecular tools have enabled the revival of innate immunity by allowing almost all aspects of cellular immunology to be re-visited. As such, B-cells were found to express "Pathogen Recognition Receptors" such as TLRs, and use them in concert with B-cell signalling during innate and adaptive immunity. An era of B-cell phenotypic and functional analysis thus began that encompassed the study of B-cell microanatomy principally in the lymph nodes, spleen and mucosae. The novel discovery of the differential localisation of B-cells with distinct phenotypes and functions revealed the compartmentalisation of B-cells. This review thus aims to describe novel findings regarding the B-cell compartments found in the mouse as a model organism, and in human physiology and pathology. It must be emphasised that some differences are noticeable between the mouse and human systems, thus increasing the complexity of B-cell compartmentalisation. Special attention will be given to the (lymph node and spleen) marginal zones, which represent major crossroads for B-cell types and functions and a challenge for understanding better the role of B-cell specificities in innate and adaptive immunology.

Corruption of human follicular B-lymphocyte trafficking by a B-cell superantigen

Protein A (SpA) of Staphylococcus aureus is known to target the paratope of immunoglobulins expressing V(H)3 genes, and to delete marginal zone B cells and B-1a in vivo. We have discovered that SpA endows S. aureus with the potential to subvert B-cell trafficking in the host. We found that SpA, whose Fc-binding site has been inactivated, binds essentially to naïve B cells and induces a long-lasting decrease in CXCR4 expression and in B-cell chemotaxis to CXCL12. Competition experiments indicated that SpA does not interfere with binding of CXCR4 ligands and does not directly bind to CXCR4. This conclusion is strongly supported by the inability of SpA to modulate clathrin-mediated CXCR4 internalization, which contrasts with the potent effect of anti-immunoglobin M (IgM) antibodies. Microscopy and biochemical experiments confirmed that SpA binds to the surface IgM/IgD complex and induces its clathrin-dependent internalization. Concomitantly, the SpA-induced signaling leads to protein kinase C-dependent CXCR4 downmodulation, suggesting that SpA impairs the recycling of CXCR4, a postclathrin process that leads to either degradation into lysozomes or de novo expression at the cell surface. In addition to providing novel insight into disruption of B-cell trafficking by an infectious agent, our findings may have therapeutic implications. Because CXCR4 has been associated with cancer metastasis and with certain autoimmune diseases, SpA behaves as an evolutionary tailored highly specific, chemokine receptor inhibitor that may have value in addition to conventional cytotoxic therapy in patients with various malignancies and immune-mediated diseases.

Recurrent infections and immune evasion strategies of Staphylococcus aureus

Staphylococcus aureus causes purulent skin and soft tissue infections (SSTIs) that frequently reoccur. Staphylococal SSTIs can lead to invasive disease and sepsis, which are among the most significant causes of infectious disease mortality in both developed and developing countries. Human or animal infections with S. aureus do not elicit protective immunity against staphylococcal diseases. Here we review what is known about the immune evasive strategies of S. aureus that enable the pathogen's escape from protective immune responses. Three secreted products are discussed in detail, staphylococcal protein A (SpA), staphylococcal binder of immunoglobulin (Sbi) and adenosine synthase A (AdsA). By forming a complex with V(H)3-type IgM on the surface of B cells, SpA functions as a superantigen to modulate antibody responses to staphylococcal infection. SpA also captures pathogen-specific antibodies by binding their Fcγ portion. The latter activity of SpA is shared by Sbi, which also associates with complement factors 3d and factor H to promote the depletion of complement. AdsA synthesizes the immune signaling molecule adenosine, thereby dampening innate and adaptive immune responses during infection. We discuss strategies how the three secreted products of staphylococci may be exploited for the development of vaccines and therapeutics.

Marginal zone B-cells, a gatekeeper of innate immunity

To maintain the integrity of an organism constantly challenged by pathogens, the immune system is endowed with a variety of cell types. B lymphocytes were initially thought to only play a role in the adaptive branch of immunity. However, a number of converging observations revealed that two B-cell subsets, marginal zone (MZ) and B1 cells, exhibit unique developmental and functional characteristics, and can contribute to innate immune responses. In addition to their capacity to mount a local antibody response against type-2 T-cell-independent (TI-2) antigens, MZ B-cells can participate to T-cell-dependent (TD) immune responses through the capture and import of blood-borne antigens to follicular areas of the spleen. Here, we discuss the multiple roles of MZ B-cells in humans, non-human primates, and rodents. We also summarize studies – performed in transgenic mice expressing fully human antibodies on their B-cells and in macaques whose infection with Simian immunodeficiency virus (SIV) represents a suitable model for HIV-1 infection in humans – showing that infectious agents have developed strategies to subvert MZ B-cell functions. In these two experimental models, we observed that two microbial superantigens for B-cells (protein A from Staphylococcus aureus and protein L from Peptostreptococcus magnus) as well as inactivated AT-2 virions of HIV-1 and infectious SIV preferentially deplete innate-like B-cells – MZ B-cells and/or B1 B-cells – with different consequences on TI and TD antibody responses. These data revealed that viruses and bacteria have developed strategies to deplete innate-like B-cells during the acute phase of infection and to impair the antibody response. Unraveling the intimate mechanisms responsible for targeting MZ B-cells in humans will be important for understanding disease pathogenesis and for designing novel vaccine strategies.

Current understanding of the role of B cell subsets and intimal and adventitial B cells in atherosclerosis

PURPOSE OF REVIEW

Inflammation, in addition to high cholesterol is a major factor contributing to atherosclerosis-associated adverse cardiovascular events. Thus, there is a pressing need for additional therapeutic strategies to reduce inflammation, by targeting immune cells and cytokines. Here we review B cell subsets and adventitial and intimal B cells in atherosclerosis development and discuss potential B cell-targeted anti-inflammatory therapies for atherosclerosis.

RECENT FINDINGS

B cell subsets can have opposing proatherogenic and atheroprotective roles in atherosclerosis. CD-20-targeted B cell depletion has been shown to decrease murine atherosclerotic lesions. The accumulation of intimal and adventitial B cells associated with atherosclerotic lesions is consistent with their participation in local inflammatory responses. As B2 B cells are proatherogenic, blocking its survival factor B cell activating factor may selectively delete this proatherogenic subset.

SUMMARY

Both intimal and adventitial B cells appear important in atherosclerosis. B2 B cells are proatherogenic and other subsets such as regulatory B cells are antiatherogenic. Future B cell-targeted therapy for atherosclerosis should be customized to selectively deplete damaging B2 B cells while sparing or expanding protective B cell subsets.

Characterization of Virulence Factors of Staphylococcus aureus: Novel Function of Known Virulence Factors That Are Implicated in Activation of Airway Epithelial Proinflammatory Response

Airway epithelial cells play a major role in initiating inflammation in response to bacterial pathogens. S. aureus is an important pathogen associated with activation of diverse types of infection characterized by inflammation dominated by polymorphonuclear leukocytes. This bacterium frequently causes lung infection, which is attributed to virulence factors. Many of virulence determinants associated with S. aureus-mediated lung infection have been known for several years. In this paper, we discuss recent advances in our understanding of known virulence factors implicated in pneumonia. We anticipate that better understanding of novel functions of known virulence factors could open the way to regulate inflammatory reactions of the epithelium and to develop effective strategies to treat S. aureus-induced airway diseases.

Structural and functional role of Staphylococcus aureus surface components recognizing adhesive matrix molecules of the host

Staphylococcus aureus is a versatile and harmful human pathogen in both hospital- and community-acquired infections. S. aureus can initiate host infection by adhering to components of the extracellular matrix. Adherence is mediated by a variety of protein adhesins of the microbial surface component recognizing adhesive matrix molecule (MSCRAMM) family. In this article, we describe these MSCRAMMs in terms of structural organization and ligand-binding capacity and discuss their role as a possible target for immunotherapy.

Enhanced monocyte response and decreased central memory T cells in children with invasive Staphylococcus aureus infections

Staphylococcus aureus has emerged as a significant pathogen causing severe invasive disease in otherwise healthy people. Despite considerable advances in understanding the epidemiology, resistance mechanisms, and virulence factors produced by the bacteria, there is limited knowledge of the in vivo host immune response to acute, invasive S. aureus infections. Herein, we report that peripheral blood mononuclear cells from patients with severe S. aureus infections demonstrate a distinctive and robust gene expression profile which is validated in a distinct group of patients and on a different microarray platform. Application of a systems-wide modular analysis framework reveals significant over-expression of innate immunity genes and under-expression of genes related to adaptive immunity. Simultaneous flow cytometry analyses demonstrated marked alterations in immune cell numbers, with decreased central memory CD4 and CD8 T cells and increased numbers of monocytes. CD14+ monocyte numbers significantly correlated with the gene expression levels of genes related to the innate immune response. These results demonstrate the value of applying a systems biology approach that reveals the significant alterations in the components of circulating blood lymphocytes and monocytes in invasive S. aureus infections.

PEGylated liposomes elicit an anti-PEG IgM response in a T cell-independent manner

We recently reported that intravenous injections of "empty" PEGylated liposomes without encapsulated or surface coupled proteins elicit a PEG-specific IgM response in rats. In the present study, simultaneous weak anti-PEG IgG and strong anti-PEG IgM responses were detected following intravenous injections of "empty" PEGylated liposomes. The pattern of immune response appears to differ from the classic primary response against T cell-dependent (TD) antigens. The anti-PEG IgM response was detected in T-cell deficient nude BALB/c mice following intravenous injection of "empty" PEGylated liposomes, suggesting that "empty" PEGylated liposomes initiate the immune response against PEG in a T cell-independent manner. In vitro splenic lymphocytes-proliferation assay indicated that TNP-LPS, a typical type 1 T cell-independent (TI) antigen (TI-1 antigen), significantly primed the proliferation, while TNP-Ficoll, a typical type 2 TI antigen (TI-2 antigen), and "empty" PEGylated liposomes did not prime any proliferation under these experimental conditions. In addition, in splenic marginal zone (MZ) B-cell-depleted rats, the anti-PEG IgM response was diminished, while the immune reactions against TNP-BSA (a TD antigen) and TNP-LPS (TI-1 antigen) were not diminished. These results demonstrate that "empty" PEGylated liposomes may promote the immune response against PEG as a result of priming the activation of MZ B cells, as TI-2 antigen promotes a specific IgM response. In conclusion, although the mechanistic details behind the immune reaction against "empty" PEGylated liposomes are not yet clear, the liposomes elicit an anti-PEG IgM response in a T cell-independent manner and appear to be a TI-2 antigen, and splenic MZ B cells may be essential for the immune response against "empty" PEGylated liposomes.

Exploitation of host signaling pathways by B cell superantigens--potential strategies for developing targeted therapies in systemic autoimmunity

Some infectious agents produce molecules capable of interacting specifically with the immunoglobulin heavy- or light-chain variable regions, independently of the conventional-binding site. They are referred to as B cell superantigens (SAgs) and include protein A of Staphylococcus aureus (S. aureus), gp120 of HIV-1, and protein L of Peptostreptococcus magnus (P. magnus). In contrast to conventional antigens, B cell superantigens interact with conserved framework regions of immunoglobulins and can target a large proportion of B cells. In experimental models, they have been demonstrated to deplete B cell subsets responsible for innate functions, namely B-1a and marginal zone (MZ) B cells. As a result, the interactions of these superantigens with host cells impair the humoral immune response. In addition to providing clues toward understanding host-pathogen interactions and microbial pathogenesis, B cell superantigens represent potential therapeutic agents that could be used to specifically modulate expansion of B cell subsets in diseased subjects. In systemic autoimmune diseases, for example, there is activation and expansion of B cells that secrete pathogenic autoantibodies. Their depletion results in clinical improvement in both experimental animals and patients. Currently, attempts are being made to specifically deplete pathogenic autoantibody-producing B cells. Since B-1a and MZ B cells have been found to be expanded in autoimmune disorders, B cell superantigens, used alone or in combination with other biological agents, may have beneficial effects in autoimmune disease management.

Staphylococcus aureus protein A triggers T cell-independent B cell proliferation by sensitizing B cells for TLR2 ligands

B cells possess functional characteristics of innate immune cells, as they can present Ag to T cells and can be stimulated with microbial molecules such as TLR ligands. Because crude preparations of Staphylococcus aureus are frequently used as polyclonal B cell activators and contain potent TLR2 activity, the scope of this study was to analyze the impact of S. aureus-derived TLR2-active substances on human B cell activation. Peripheral B cells stimulated with chemically modified S. aureus cell wall preparations proliferated in response to stimulation with crude cell wall preparations but failed to be activated with pure peptidoglycan, indicating that cell wall molecules other than peptidoglycan are responsible for B cell proliferation. Subsequent analysis revealed that surface protein A (SpA), similar to BCR cross-linking with anti-human Ig, sensitizes B cells for the recognition of cell wall-associated TLR2-active lipopeptides (LP). In marked contrast to TLR7- and TLR9-triggered B cell stimulation, stimulation with TLR2-active LP and SpA or with crude cell wall preparations failed to induce IgM secretion, thereby revealing qualitative differences in TLR2 signaling compared with TLR7/9 signaling. Notably, combined stimulation with SpA plus TLR2 ligands induced vigorous proliferation of a defined B cell subset that expressed intracellular IgM in the presence of IL-2.

CONCLUSION

S. aureus triggers B cell activation via SpA-induced sensitization of B cells for TLR2-active LP. Combined SpA and TLR2-mediated B cell activation promotes B cell proliferation but fails to induce polyclonal IgM secretion as seen after TLR7 and TLR9 ligation.

Direct impact of inactivated HIV-1 virions on B lymphocyte subsets

Although there is no convincing evidence that HIV infects primary B cells, marked changes in B cell responses have been described in HIV-1-infected subjects, including B cell repertoire perturbations, depression of B cell memory and paucity of CD5(+) B cells. As it is hard to assess the consequences of these in vitro and ex vivo observations in patients, the pathogenic mechanisms responsible for the B cell deficit are unclear, and direct and indirect effects of HIV-1 remain possible. To gain further insight into the impact of HIV-1 on the B cell compartment in vivo, we used XenoMouse mice, mice genetically engineered to express human antibodies with an absence of mouse antibody expression. In these transgenic animals, B cells expressing a virtually full human Ig repertoire develop, which allows investigation of the in vivo consequences of confronting B cells expressing human immunoglobulins with HIV-1. We found that soluble gp120 induced an inversion in the B-1a/B-1b cell ratios, without impacting B-2 cells or affecting substantially the T cell compartment. Virion treatment specifically and dramatically depressed B-1a cells, which represent the majority of B-1 cells in normal mice. The observed B cell changes were associated with a functional alteration of the humoral response to tetanus toxoid. Thus, the results reveal a capacity of HIV-1 to specifically impact a highly specialized B cell subpopulation. Because there is evidence that human IgM memory B cells are functionally equivalent to murine B-1a cells, our findings suggest that gp120 may have a direct deleting activity on B cell memory.

Unscrambling the role of human parvovirus B19 signaling in systemic autoimmunity

Despite enormous progress in understanding how the immune system works, the pathogenesis of autoimmune diseases still remains unclear. Growing evidence indicates that infectious agents can be potent initial triggers, subverting and exploiting host cell signaling pathways. This role is exemplified by the association of parvovirus B19 (B19) with human autoimmune disease. Infection with this common virus exhibits striking similarities with systemic autoimmune diseases, and can be associated with elevated serum autoantibody titers. The B19 virus produces proline-rich, 11-kDa proteins that have been implicated in modulation of host signaling cascades involved in virulence and pathogenesis. Additionally, B19 produces a non-structural protein (NS1) that functions as a transcription regulator by directly binding the p6 promoter and the Sp1/Sp3 transcription factors. The protein is also involved in DNA replication, cell cycle arrest and initiation of apoptotic damage, particularly in erythroid cells. When transfected to non-permissive cells, NS1 recruits the mitochondria cell death pathway. It is even more remarkable that NS1 functions as a trans-acting transcription activator for the IL6 promoter, up-regulating IL6 expression in host cells. Hence, B19 infection may play a pivotal role in triggering inflammatory disorders. By promoting apoptotic damage and trans-activating pro-inflammatory cytokine promoters, B19 may break the delicate balance between cell survival and apoptosis, and may contribute to immune deregulation. Understanding the mechanisms used by B19 to alter the cell signaling machinery may provide further insight into the mechanism by which autoimmune diseases develop.

Evidence that marginal zone B cells possess an enhanced secretory apparatus and exhibit superior secretory activity

Marginal zone B (MZB) cells are the first splenic B cells to initiate Ab secretion against polysaccharide-encapsulated Ags in vivo. This swift MZB cell response can be reproduced in vitro as LPS treatment induces Ab secretion in as little as 12 h. Conversely, in vitro LPS treatment of splenic follicular B (FOB) cells results in Ab secretion after 2-3 days. The basis for these distinct response kinetics is not understood. We performed ex vivo analysis of resting and LPS-stimulated murine MZB and FOB cells and found that MZB cells express higher levels of the LPS TLR complex RP105/MD-1 and respond to much lower concentrations of LPS than do FOB cells. Furthermore, increasing doses of LPS do not accelerate the kinetics by which FOB cells transition into Ab secretion. Ultrastructural analysis of resting cells demonstrated that rough endoplasmic reticulum is more abundant in MZB cells than in FOB cells. Additionally, RT-PCR and immunoblot analyses revealed that numerous endoplasmic reticulum resident chaperones and folding enzymes are expressed at greater levels in resting MZB cells than in resting FOB cells. Although both LPS-stimulated MZB and FOB cells increase expression of these factors, MZB cells exhibit a more rapid increase that correlates with accelerated kinetics of Ab secretion and higher per cell output of secreted IgM. These data indicate that MZB cells are equipped for exquisite sensitivity to bacterial components like LPS and poised for rapid, robust Ab production, making MZB cells ideally suited as frontline defenders in humoral immunity.

Mapping the pathways to staphylococcal pathogenesis by comparative secretomics

The gram-positive bacterium Staphylococcus aureus is a frequent component of the human microbial flora that can turn into a dangerous pathogen. As such, this organism is capable of infecting almost every tissue and organ system in the human body. It does so by actively exporting a variety of virulence factors to the cell surface and extracellular milieu. Upon reaching their respective destinations, these virulence factors have pivotal roles in the colonization and subversion of the human host. It is therefore of major importance to obtain a clear understanding of the protein transport pathways that are active in S. aureus. The present review aims to provide a state-of-the-art roadmap of staphylococcal secretomes, which include both protein transport pathways and the extracytoplasmic proteins of these organisms. Specifically, an overview is presented of the exported virulence factors, pathways for protein transport, signals for cellular protein retention or secretion, and the exoproteomes of different S. aureus isolates. The focus is on S. aureus, but comparisons with Staphylococcus epidermidis and other gram-positive bacteria, such as Bacillus subtilis, are included where appropriate. Importantly, the results of genomic and proteomic studies on S. aureus secretomes are integrated through a comparative "secretomics" approach, resulting in the first definition of the core and variant secretomes of this bacterium. While the core secretome seems to be largely employed for general housekeeping functions which are necessary to thrive in particular niches provided by the human host, the variant secretome seems to contain the "gadgets" that S. aureus needs to conquer these well-protected niches.

Confounding B-cell defences: lessons from a staphylococcal superantigen

Studies of microbial superantigens that target large clonal sets of B cells through conserved antigen-receptor-variable-region sites are providing new insights into the mechanisms of B-cell activation-induced cell death. These investigations have shown differences between the clonal regulation of follicular B cells (B2 cells) and the innate-like marginal-zone B cells and B1 cells, and have also shown how B-cell superantigens can affect specialized host defences against infection. Agents designed to emulate the properties of B-cell superantigens might also provide new approaches for the treatment of B-cell-mediated autoimmune and neoplastic diseases.

Staphylococcus aureus protein A activates TNFR1 signaling through conserved IgG binding domains

Staphylococcus aureus continues to be a major cause of infection in normal as well as immunocompromised hosts, and the increasing prevalence of highly virulent community-acquired methicillin-resistant strains is a public health concern. A highly expressed surface component of S. aureus, protein A (SpA), contributes to its success as a pathogen by both activating inflammation and by interfering with immune clearance. SpA is known to bind to IgG Fc, which impedes phagocytosis. SpA is also a potent activator of tumor necrosis factor alpha (TNF-alpha) receptor 1 (TNFR1) signaling, inducing both chemokine expression and TNF-converting enzyme-dependent soluble TNFR1 (sTNFR1) shedding, which has anti-inflammatory consequences, particularly in the lung. Using a collection of glutathione S-transferase fusions to the intact IgG binding region of SpA and to each of the individual binding domains, we found that the SpA IgG binding domains also mediate binding to human airway cells. TNFR1-dependent CXCL8 production could be elicited by any one of the individual SpA IgG binding domains as efficiently as by either the entire SpA or the intact IgG binding region. SpA induction of sTNFR1 shedding required the entire IgG binding region and tolerated fewer substitutions in residues known to interact with IgG. Each of the repeated domains of the IgG binding domain can affect multiple immune responses independently, activating inflammation through TNFR1 and thwarting opsonization by trapping IgG Fc domains, while the intact IgG binding region can limit further signaling through sTNFR1 shedding.