Toll-like receptor 2-mediated human B cell differentiation

Control of adaptive immunity by pattern recognition receptors

One of the most significant conceptual advances in immunology in recent history is the recognition that signals from the innate immune system are required for induction of adaptive immune responses. Two breakthroughs were critical in establishing this paradigm: the identification of dendritic cells (DCs) as the cellular link between innate and adaptive immunity and the discovery of pattern recognition receptors (PRRs) as a molecular link that controls innate immune activation as well as DC function. Here, we recount the key events leading to these discoveries and discuss our current understanding of how PRRs shape adaptive immune responses, both indirectly through control of DC function and directly through control of lymphocyte function. In this context, we provide a conceptual framework for how variation in the signals generated by PRR activation, in DCs or other cell types, can influence T cell differentiation and shape the ensuing adaptive immune response.

Cell atlas of the Atlantic salmon spleen reveals immune cell heterogeneity and cell-specific responses to bacterial infection

The spleen is a conserved secondary lymphoid organ that emerged in parallel to adaptive immunity in early jawed vertebrates. Recent studies have applied single cell transcriptomics to reveal the cellular composition of spleen in several species, cataloguing diverse immune cell types and subpopulations. In this study, 51,119 spleen nuclei transcriptomes were comprehensively investigated in the commercially important teleost Atlantic salmon (Salmo salar L.), contrasting control animals with those challenged with the bacterial pathogen Aeromonas salmonicida. We identified clusters of nuclei representing the expected major cell types, namely T cells, B cells, natural killer-like cells, granulocytes, mononuclear phagocytes, endothelial cells, mesenchymal cells, erythrocytes and thrombocytes. We discovered heterogeneity within several immune lineages, providing evidence for resident macrophages and melanomacrophages, infiltrating monocytes, several candidate dendritic cell subpopulations, and B cells at distinct stages of differentiation, including plasma cells and an igt + subset. We provide evidence for twelve candidate T cell subsets, including cd4+ T helper and regulatory T cells, one cd8+ subset, three γδT subsets, and populations double negative for cd4 and cd8. The number of genes showing differential expression during the early stages of Aeromonas infection was highly variable across immune cell types, with the largest changes observed in macrophages and infiltrating monocytes, followed by resting mature B cells. Our analysis provides evidence for a local inflammatory response to infection alongside B cell maturation in the spleen, and upregulation of ccr9 genes in igt + B cells, T helper and cd8+ cells, and monocytes, consistent with the recruitment of immune cell populations to the gut to deal with Aeromonas infection. Overall, this study provides a new cell-resolved perspective of the immune actions of Atlantic salmon spleen, highlighting extensive heterogeneity hidden to bulk transcriptomics. We further provide a large catalogue of cell-specific marker genes that can be leveraged to further explore the function and structural organization of the salmonid immune system.

Characterizing infection of B cells with wild-type and vaccine strains of measles virus

Acute infection with measles virus (MeV) causes transient immunosuppression often leading to secondary infections. MeV infection of B lymphocytes results in changes in the antibody repertoire and memory B cell populations for which the mechanism is unknown. In this study, we characterize the infection of primary B cells with wild-type and vaccine strains of MeV. Vaccine-infected B cells were characterized by a higher percentage of cells positive for viral protein, a higher level of viral transcription and reduced cell death compared to wild-type infected cells, regardless of B cell subtype. Vaccine-infected cells showed more production of TNF-α and IL-10 but less production of IL-8 compared to wild-type infected cells. IL-4 and IL-6 levels detected were increased during both vaccine and wild-type infection. Despite evidence of replication, measles-infected B cells did not produce detectable viral progeny. This study furthers our understanding of the outcomes of MeV infection of human B cells.

Toll-like receptor-targeted anti-tumor therapies: Advances and challenges

Toll-like receptors (TLRs) are pattern recognition receptors, originally discovered to stimulate innate immune reactions against microbial infection. TLRs also play essential roles in bridging the innate and adaptive immune system, playing multiple roles in inflammation, autoimmune diseases, and cancer. Thanks to the immune stimulatory potential of TLRs, TLR-targeted strategies in cancer treatment have proved to be able to regulate the tumor microenvironment towards tumoricidal phenotypes. Quantities of pre-clinical studies and clinical trials using TLR-targeted strategies in treating cancer have been initiated, with some drugs already becoming part of standard care. Here we review the structure, ligand, signaling pathways, and expression of TLRs; we then provide an overview of the pre-clinical studies and an updated clinical trial watch targeting each TLR in cancer treatment; and finally, we discuss the challenges and prospects of TLR-targeted therapy.

The HMGB1 (C106A) mutation inhibits IL-10-producing CD19hiFcγRIIbhi B cell expansion by suppressing STAT3 activation in mice

Regulatory B cells have important roles in inflammation and autoimmune diseases. A newly discovered subpopulation of B cells with a CD19^hiFcγRIIb^hi phenotype inhibits the proliferation of CD4⁺ T cells by secreting interleukin (IL)-10. The expansion of CD19^hiFcγRIIb^hi B cells in mouse spleen can be induced by lipopolysaccharide (LPS) or CpG oligodeoxynucleotide stimulation. However, the mechanism of CD19^hiFcγRIIb^hi B cell expansion and its role in inflammatory diseases are unclear. Here, we report that, under inflammatory conditions, the proliferation and immunosuppressive function of CD19^hiFcγRIIb^hi B cells were decreased in high mobility group box1 (HMGB1) C106A mutant mice, compared with wild-type mice. The HMGB1 (C106A) mutation in B cells reduced STAT3 phosphorylation, restricting the expansion and suppressive function of CD19^hiFcγRIIb^hi B cells. Compared with CD19^hiFcγRIIb^hi B cells from wild-type mice, CD19^hiFcγRIIb^hi B cells from Hmgb1^(C106A) mice significantly reduced the survival of mice with sepsis. Recombinant HMGB1 promoted the expansion of IL-10-producing CD19^hiFcγRIIb^hi B cells among LPS-activated B cells in vitro. Furthermore, the percentage of CD19^hiFcγRIIb^hi regulatory B cells in the peripheral blood was increased in patients with sepsis, compared with healthy controls. These findings implicate the role of HMGB1 in the expansion and immunosuppressive function of CD19^hiFcγRIIb^hi B cells.

Lipidation of Haemophilus influenzae Antigens P6 and OMP26 Improves Immunogenicity and Protection against Nasopharyngeal Colonization and Ear Infection

Nontypeable Haemophilus influenzae (NTHi) causes respiratory infections that lead to high morbidity and mortality worldwide, encouraging development of effective vaccines. To achieve a protective impact on nasopharyngeal (NP) colonization by NTHi, enhanced immunogenicity beyond that achievable with recombinant-protein antigens is likely to be necessary. Adding a lipid moiety to a recombinant protein would enhance immunogenicity through Toll-like receptor 2 signaling of antigen-presenting cells and Th17 cell response in the nasal-associated lymphoid tissue (NALT). We investigated effects of lipidation (L) of recombinant proteins P6 and OMP26 compared to nonlipidated (NL) P6 and OMP26 and as fusion constructs (L-OMP26ϕNL-P6 and L-P6ϕNL-OMP26) in a mouse model. After intraperitoneal or intranasal vaccination, antibody responses were compared and protection from NP colonization and middle ear infection were assessed. L-P6 and L-OMP26 induced approximately 10- to 100-fold-higher IgG antibody levels than NL-P6 and NL-OMP26. Fusion constructs significantly increased IgG antibody to both target proteins, even though only one of the proteins was lipidated. NP colonization and middle ear bullae NTHi density was 1 to 4 logs lower following vaccination with L-P6 and L-OMP26 than with NL-P6 and NL-OMP26. Fusion constructs also resulted in a 1- to 3-log-lower NTHi density following vaccination. NALT cells from mice vaccinated with lipidated protein constructs had higher levels of interleukin-17 (IL-17), IL-22, and CD4⁺ T-cell memory. Passive transfer of sera from L-OMP26ϕNL-P6-vaccinated mice to recipient infant mice reduced NP colonization and ear bulla NTHi density. We conclude that L-P6, L-OMP26, and fusion constructs generate enhanced antibody responses and protection from NP colonization and middle ear infection by NTHi in mice.

Toll-Like Receptors in Adaptive Immunity

The immune (innate and adaptive) system has evolved to protect the host from any danger present in the surrounding outer environment (microbes and associated MAMPs or PAMPs, xenobiotics, and allergens) and dangers originated within the host called danger or damage-associated molecular patterns (DAMPs) and recognizing and clearing the cells dying due to apoptosis. It also helps to lower the tissue damage during trauma and initiates the healing process. The pattern recognition receptors (PRRs) play a crucial role in recognizing different PAMPs or MAMPs and DAMPs to initiate the pro-inflammatory immune response to clear them. Toll-like receptors (TLRs) are first recognized PRRs and their discovery proved milestone in the field of immunology as it filled the gap between the first recognition of the pathogen by the immune system and the initiation of the appropriate immune response required to clear the infection by innate immune cells (macrophages, neutrophils, dendritic cells or DCs, and mast cells). However, in addition to their expression by innate immune cells and controlling their function, TLRs are also expressed by adaptive immune cells. We have identified 10 TLRs (TLR1-TLR10) in humans and 12 TLRs (TLR1-TLR13) in laboratory mice till date as TLR10 in mice is present only as a defective pseudogene. The present chapter starts with the introduction of innate immunity, timing of TLR evolution, and the evolution of adaptive immune system and its receptors (T cell receptors or TCRs and B cell receptors or BCRs). The next section describes the role of TLRs in the innate immune function and signaling involved in the generation of inflammation. The subsequent sections describe the expression and function of different TLRs in murine and human adaptive immune cells (B cells and different types of T cells, including CD4⁺T cells, CD8⁺T cells, CD4⁺CD25⁺T_regs, and CD8⁺CD25⁺T_regs, etc.). The modulation of TLRs expressed on T and B cells has a great potential to develop different vaccine candidates, adjuvants, immunotherapies to target various microbial infections, including current COVID-19 pandemic, cancers, and autoimmune and autoinflammatory diseases.

Use of Toll-Like Receptor (TLR) Ligation to Characterize Human Regulatory B-Cells Subsets

Toll-like receptors (TLRs) are pattern recognition receptors (PRRs), which constitute key components in the recognition of pathogens, thereby initiating innate immune responses and promoting adaptive immune responses. In B cells, TLR ligation is important for their activation and, together with CD40, for their differentiation. TLR ligands are also strong promoters of regulatory B (Breg)-cell development, by enhancing the production of IL-10 and their capacity to induce tolerance. In inflammatory diseases, such as autoimmunity or allergies, Breg-cell function is often impaired, while in chronic infections, such as with helminths, or cancer, Breg-cell function is boosted. Following pathogen exposure, B cells can respond directly by producing cytokines and/or IgM (innate response) and develop into various memory B (Bmem)-cell subsets with class-switched immunoglobulin receptors. Depending on the disease state or chronic infection conditions, various Breg subsets can be recognized as well. Currently, a large array of surface markers is known to distinguish between these large range of B-cell subsets. In recent years, the development of mass cytometers and spectral flow cytometry has allowed for high-dimensional detection of up to 48 markers, including both surface and intracellular/intranuclear markers. Therefore, this novel technology is highly suitable to provide a comprehensive overview of Bmem/Breg-cell subsets in different disease states and/or in clinical intervention trials. Here, we provide detailed instructions of the steps necessary to obtain high-quality data for high-dimensional analysis of multiple human Breg-cell subsets using various TLR ligands.

Lipidation of Pneumococcal Antigens Leads to Improved Immunogenicity and Protection

Streptococcus pneumoniae infections lead to high morbidity and mortality rates worldwide. Pneumococcal polysaccharide conjugate vaccines significantly reduce the burden of disease but have a limited range of protection, which encourages the development of a broadly protective protein-based alternative. We and others have shown that immunization with pneumococcal lipoproteins that lack the lipid anchor protects against colonization. Since immunity against S. pneumoniae is mediated through Toll-like receptor 2 signaling induced by lipidated proteins, we investigated the effects of a lipid modification on the induced immune responses in either intranasally or subcutaneously vaccinated mice. Here, we demonstrate that lipidation of recombinant lipoproteins DacB and PnrA strongly improves their immunogenicity. Mice immunized with lipidated proteins showed enhanced antibody concentrations and different induction kinetics. The induced humoral immune response was modulated by lipidation, indicated by increased IgG2/IgG1 subclass ratios related to Th1-type immunity. In a mouse model of colonization, immunization with lipidated antigens led to a moderate but consistent reduction of pneumococcal colonization as compared to the non-lipidated proteins, indicating that protein lipidation can improve the protective capacity of the coupled antigen. Thus, protein lipidation represents a promising approach for the development of a serotype-independent pneumococcal vaccine.

Benzyl butyl phthalate decreases myogenic differentiation of endometrial mesenchymal stem/stromal cells through miR-137-mediated regulation of PITX2

Phthalate, an environmental toxin, has been considered as an endocrine-disrupting chemical. Growing evidence has demonstrated links between endocrine-disrupting chemicals, tissue development, and reproductive physiology, but the mechanisms of gene expression regulation by environmental factors that affect cell differentiation are unclear. Herein, we investigated the effects of butyl benzyl phthalate (BBP) on human endometrial mesenchymal stem/stromal cell (EN-MSC) differentiation and identified a novel signaling pathway. Differentiation of endometrial mesenchymal stem/stromal cells decreased after administration of BBP. We analyzed BBP regulation of gene expression in EN-MSC using cDNA microarrays and Ingenuity Pathway Analysis software to identify affected target genes and their biological functions. PITX2 emerged as a common gene hit from separate screens targeting skeletal and muscular disorders, cell morphology, and tissue development. BBP decreased transcription of PITX2 and elevated expression of the microRNA miR-137, the predicted upstream negative regulator of PITX2. These data indicated that BBP affects PITX2 expression through miR-137 targeting of the 3' untranslated region of PITX2 mRNA. PITX2 down-regulation also decreased MyoD transcript levels in EN-MSC. Our results demonstrate that BBP decreases EN-MSC myogenic differentiation through up-regulation of miR-137, contribute to our understanding of EN-MSC differentiation, and underline the hazardous potential of environmental hormones.

B cell responses in older adults with latent tuberculosis: Considerations for vaccine development

Reactivation of latent tuberculosis (LTBI) is more common among the aging population and may contribute to increased transmission in long-term health care facilities. Difficulties in detecting LTBI due to potential blunting of the tuberculin skin test (TST), and the lowered ability of the elderly to tolerate the course of antibiotics, underscore the need for an effective vaccine. Immuno-senescence reduces the capacity of vaccines to induce sufficient levels of protective immunity against many pathogens, further increasing the susceptibility of the elderly to infectious diseases. We sought to evaluate the response of B cells to Mycobacterium tuberculosis (Mtb) in residents of long-term care facilities to determine the feasibility of using a vaccine to control infection and transmission from reactivated LTBI. Our results demonstrate that although B cell responses were higher in subjects with LTBI, Mtb antigens could stimulate B cell activation and differentiation in vitro in TST negative subjects. B cells from elderly subjects expressed high basal levels of Toll-like receptor (TLR)2 and TLR4 and responded strongly to Mtb ligands with some activation pathways dependent on TLR2. B cells derived from blood, tonsil and spleen from younger subjects responded similarly and to the same magnitude. These results suggest that B cell responses are robust in the elderly and modifications to a TB vaccine, such as TLR2 ligand-based adjuvants, may help increase immune responses to a protective level.

Immunomodulation activity of alkali extract polysaccharide from Plantago asiatic L. seeds

The MAPK signaling pathway is greatly involved in PLP-induced macrophage cell response.

The role of toll-like receptors in B-cell development and immunopathogenesis of common variable immunodeficiency

Common variable immunodeficiency (CVID) is the most frequent symptomatic primary immune deficiency and is characterized by hypogammaglobulinemia, defect in specific antibody response and increased susceptibility to recurrent infections, malignancy and autoimmunity. Patients with CVID often have defects in post-antigenic B-cell differentiation, with fewer memory B cells and impaired isotype switching. Toll-like receptors (TLRs) are expressed on various immune cells as key elements of innate and adaptive immunity. TLR signaling in B cells plays multiple roles in cell differentiation and activation, class-switch recombination and cytokine and antibody production. Moreover, recent studies have shown functional alteration of TLRs responses in CVID patients including poor cell proliferation, impaired upregulation of co-stimulatory molecules and failure in cytokine and immunoglobulin production. The purpose of the present review is to discuss the role of TLRs in B-cell development and function as well as their role in the immunopathogenesis of CVID.

CD180 overexpression in follicular lymphoma is restricted to the lymph node compartment

BACKGROUND

Altered Toll-like receptor (TLR) expression levels and/or mutations in its signaling pathway (such as MyD88 mutation) contribute to the pathogenesis of lymphoproliferative disorders (LPD). CD180 is an orphan member of the TLR family that modulates the signaling of several TLRs, but only limited studies have evaluated its expression by flow cytometry (FCM) in LPD.

METHODS

Using a multiparameter FCM approach, we have assessed CD180 mean fluorescence intensity (MFI) in lymph nodes (LNs) and peripheral blood (PB) samples obtained from patients with follicular lymphoma (FL; LN/PB, n = 44/n = 15), chronic lymphocytic leukemia (CLL, n = 26/n = 21), mantle cell lymphoma (MCL, n = 13/n = 17), and marginal zone lymphoma (MZL, n = 16/n = 12). Specimens from non-tumoral PB and LN (n = 8/n = 12) were used as controls.

RESULTS

In the LN specimens, FL and control B-cells showed similar CD180 expression (MFI = 1,049 vs. 1,381, P > 0.05; Mann-Whitney U-test). This level was markedly lower in the other LPDs, MCL (MFI = 396, P < 0.05), or CLL (MFI = 502 P < 0.05), and similar to MZL (MFI = 858, P > 0.05). However, the CD180 expression of FL B-cells assessed in PB was dim and/or negative, in the same range as MCL and CLL (FL MFI = 453, MCL MFI = 305, CLL MFI = 420, P > 0.05) but lower than in MZL (MFI = 895, P < 0.05). Therefore, these results suggest a modulation of CD180 expression by neoplastic FL B-cells based on the anatomical compartment.

CONCLUSION

These FCM data confirm the usefulness of CD180 in the accurate diagnosis of LPDs and emphasize the need to interpret this marker according to the origin of the sample. © 2015 Clinical Cytometry Society.

Bacterial lysates improve the protective antibody response against respiratory viruses through Toll-like receptor 4

Respiratory viruses cause significant morbidity and mortality in infants and young children worldwide. Current strategies to modulate the immune system and prevent or treat respiratory viral infections in this age group have shown limited success. Here, we demonstrate that a lysate derived from Gram-positive and Gram-negative organisms positively modulates protective antibody responses against both respiratory syncytial virus (RSV) and influenza virus in murine models of infection. Interestingly, despite the complex mixture of Toll-like receptor (TLR) agonists present in the bacterial lysate, the modulatory effects were mostly dependent on TLR4 signaling. Our results indicate that the use of simple formulations of TLR-agonists can significantly improve the immune response against critical pediatric respiratory pathogens.

Toll-like receptors in the pathogenesis of human B cell malignancies

Toll-like receptors (TLRs) are important players in B-cell activation, maturation and memory and may be involved in the pathogenesis of B-cell lymphomas. Accumulating studies show differential expression in this heterogeneous group of cancers. Stimulation with TLR specific ligands, or agonists of their ligands, leads to aberrant responses in the malignant B-cells. According to current data, TLRs can be implicated in malignant transformation, tumor progression and immune evasion processes. Most of the studies focused on multiple myeloma and chronic lymphocytic leukemia, but in the last decade the putative role of TLRs in other types of B-cell lymphomas has gained much interest. The aim of this review is to discuss recent findings on the role of TLRs in normal B cell functioning and their role in the pathogenesis of B-cell malignancies.

Polysaccharide-specific memory B cells generated by conjugate vaccines in humans conform to the CD27+IgG+ isotype-switched memory B Cell phenotype and require contact-dependent signals from bystander T cells activated by bacterial proteins to differentiate into plasma cells

The polysaccharides (PS) surrounding encapsulated bacteria are generally unable to activate T cells and hence do not induce B cell memory (BMEM). PS conjugate vaccines recruit CD4(+) T cells via a carrier protein, such as tetanus toxoid (TT), resulting in the induction of PS-specific BMEM. However, the requirement for T cells in the subsequent activation of the BMEM at the time of bacterial encounter is poorly understood, despite having critical implications for protection. We demonstrate that the PS-specific BMEM induced in humans by a meningococcal serogroup C PS (Men C)-TT conjugate vaccine conform to the isotype-switched (IgG(+)CD27(+)) rather than the IgM memory (IgM(+)CD27(+)) phenotype. Both Men C and TT-specific BMEM require CD4(+) T cells to differentiate into plasma cells. However, noncognate bystander T cells provide such signals to PS-specific BMEM with comparable effect to the cognate T cells available to TT-specific BMEM. The interaction between the two populations is contact-dependent and is mediated in part through CD40. Meningococci drive the differentiation of the Men C-specific BMEM through the activation of bystander T cells by bacterial proteins, although these signals are enhanced by T cell-independent innate signals. An effect of the TT-specific T cells activated by the vaccine on unrelated BMEM in vivo is also demonstrated. These data highlight that any protection conferred by PS-specific BMEM at the time of bacterial encounter will depend on the effectiveness with which bacterial proteins are able to activate bystander T cells. Priming for T cell memory against bacterial proteins through their inclusion in vaccine preparations must continue to be pursued.

Toll-like receptor 2 and Toll-like receptor 4-dependent activation of B cells by a polysaccharide from marine fungus Phoma herbarum YS4108

Various natural polysaccharides are capable of activating the immune system and therefore can be employed as biological response modifiers in anti-tumor therapy. We previously found a homogenous polysaccharide from the mycelium of marine fungus Phoma herbarum YS4108, named YCP, exhibiting strong in vivo antitumor ability via enhancement of the host immune responses. To further elucidate the role of YCP as a biological response modifier, the immunomoduating activities of YCP in B cells was investigated in the current study. We demonstrated that stimulation of YCP with murine splenic B cells resulted in cell proliferation and generation of IgM antibody response. Binding of YCP to B cells was a direct, saturable and reversible event and required TLR2 and TLR4 involvement. TLR2 and TLR4 defunctionalization by either antibody blocking or allele-specific mutation significantly impaired the B-cell proliferative and IgM responses to YCP. YCP interaction with TLR2 and TLR4 led to the activation of intracellular p38, ERK and JNK, as well as the translocation of transcriptional factor NF-κB into nucleus. Furthermore, specific inhibitors of p38, ERK, JNK and NF-κB could attenuate the ability of YCP to induce B cell proliferation and IgM production. Taken together, this study has indicated for the first time the immunostimulating properties of YCP on B cells through a receptor-mediated mechanism, which involves TLR2 and TLR4 and resultant activation of MAPK and NF-κB signaling pathways, thereby highlighting the role of YCP as an efficacious biological response modifier in oncologic immunotherapy.

Improvement of immunogenicity of meningococcal lipooligosaccharide by coformulation with lipidated transferrin-binding protein B in liposomes: implications for vaccine development

Among various meningococcal antigens, lipooligosaccharide (LOS) and recombinant lipidated transferrin-binding protein B (rlip-TbpB) are considered to be putative vaccine candidates against group B Neisseria meningitidis. In the present work, we report the development of a new liposome-based vaccine formulation containing both rlip-TbpB and L8 LOS. The endotoxic activity of the liposomal LOS was evaluated in vitro using the Limulus Amebocyte Lysate assay and compared to the endotoxic activity of free LOS. Above a 250:1 lipid/LOS molar ratio, liposomes were shown to effectively detoxify the LOS as the endotoxic activity of the LOS was reduced by more than 99%. Immunogenicity studies in rabbits showed that the presence of rlip-TbpB dramatically increased the immunogenicity of the LOS. While the formulation raised a strong anti-TbpB response, it elicited a higher anti-LOS IgG level than the liposomal LOS alone. Sera from rabbits immunized with rlip-TbpB/liposomal LOS displayed increased ability to recognize LOS on live bacteria expressing the L8 immunotype and increased anti-LOS-specific bactericidal activity compared to sera from rabbits immunized with liposomal LOS alone. Measurement of interleukin-8 (IL-8) produced by HEK293 cells transfected with Toll-like receptor (TLR) after stimulation with rlip-TbpB showed that the protein is a TLR2 agonist, which is in accordance with the structure of its lipid. Furthermore, an in vivo study demonstrated that the lipid moiety is not only required for its adjuvant effect but also has to be linked to the protein. Overall, the rlip-TbpB/LOS liposomal formulation was demonstrated to induce an effective anti-LOS response due to the adjuvant effect of rlip-TbpB on LOS.

CD23b isoform expression in human schistosomiasis identifies a novel subset of activated B cells

Resistance to schistosomiasis is associated with increased levels of serum parasite-specific IgE. IgE exerts its functions through its cellular receptors, FcεRI and FcεRII/CD23; however, its functional significance in humans requires further characterization. We previously reported that increased levels of CD23(+) B cells correlate with resistance to schistosomiasis in hyperexposed populations and sought to define their potential function and relationship with IgE. We found that CD23(+) B cells are a heterogeneous cell population with functional and phenotypic differences. Circulating CD23(+) B cells are uniquely activated in schistosomiasis and express the CD23b isoform and CXCR5, the homing receptor for lymphoid follicles. High CXCR5 expression by CD23(+) B cells was associated with the capacity to home to the cognate ligand CXCL13. CD23-bound IgE cross-linking increased surface expression of CXCR5, suggesting that CD23(+) B cells home directly into the lymphoid follicles upon antigen capture. As human schistosomiasis is an intravascular parasitic infection associated with a high antigenic burden in the blood, circulating CD23(+) B cells may play a role in the capture and shuttling of antigens directly to splenic follicles, highlighting a new role for circulating B cells. This function likely plays an important role in the development of protective immunity to infection with schistosomes.

Outer membrane protein complex of Meningococcus enhances the antipolysaccharide antibody response to pneumococcal polysaccharide-CRM₁₉₇ conjugate vaccine

Bacterial polysaccharides (PS) are T cell-independent antigens that do not induce immunologic memory and are poor immunogens in infants. Conjugate vaccines in which the PS is covalently linked to a carrier protein have enhanced immunogenicity that resembles that of T cell-dependent antigens. The Haemophilus influenzae type b (Hib) conjugate vaccine, which uses the outer membrane protein complex (OMPC) from meningococcus as a carrier protein, elicits protective levels of anti-capsular PS antibody (Ab) after a single dose, in contrast to other conjugate vaccines, which require multiple doses. We have previously shown that OMPC robustly engages Toll-like receptor 2 (TLR2) and enhances the early anti-Hib PS Ab titer associated with an increase in TLR2-mediated induction of cytokines. We now show that the addition of OMPC to the 7-valent pneumococcal PS-CRM₁₉₇ conjugate vaccine during immunization significantly increases the anti-PS IgG and IgM responses to most serotypes of pneumococcus contained in the vaccine. The addition of OMPC also increased the likelihood of anti-PS IgG3 production against serotypes 4, 6B, 9V, 18C, 19F, and 23F. Splenocytes from mice who had received OMPC with the pneumococcal conjugate vaccine produced significantly more interleukin-2 (IL-2), IL-4, IL-6, IL-10, tumor necrosis factor alpha (TNF-α), and gamma interferon (IFN-γ) than splenocytes from mice who received phosphate-buffered saline (PBS) plus the conjugate vaccine. We conclude that OMPC enhances the anti-PS Ab response to pneumococcal PS-CRM₁₉₇ conjugate vaccine, an effect associated with a distinct change in cytokine profile. It may be possible to reduce the number of conjugate vaccine doses required to achieve protective Ab levels by priming with adjuvants that are TLR2 ligands.

State of the union between metabolism and the immune system in type 2 diabetes

Lymphocytes and myeloid cells (monocyte/macrophages) have important roles in multiple types of diseases characterized by unresolved inflammation. The relatively recent appreciation of obesity, insulin resistance and type 2 diabetes (T2D) as chronic inflammatory diseases has stimulated interest in understanding the role of immune cells in metabolic imbalance. Myeloid cells regulate inflammation through cytokine production and the adipose tissue remodeling that accompanies hyper-nutrition, thus are critical players in metabolic homeostasis. More recently, multiple studies have indicated a role for T cells in obesity-associated inflammation and insulin resistance in model organisms, with parallel work indicating that pro-inflammatory changes in T cells also associate with human T2D. Furthermore, the expansion of T cells with similar antigen-binding sites in obesity and T2D indicates these diseases share characteristics previously attributed to inflammatory autoimmune disorders. Parallel pro-inflammatory changes in the B-cell compartment of T2D patients have also been identified. Taken together, these studies indicate that in addition to accepted pro-inflammatory roles of myeloid cells in T2D, pro-inflammatory skewing of both major lymphocyte subsets has an important role in T2D disease pathogenesis. Basic immunological principles suggest that alterations in lymphocyte function in obesity and T2D patients are an integral part of a feed-forward pro-inflammatory loop involving additional cell types. Importantly, the pro-inflammatory loop almost inevitably includes adipocytes, known to respond to pro-inflammatory, pro-diabetogenic cytokines originating from the myeloid and lymphoid compartments. We propose a model for inflammation in T2D that functionally links lymphocyte, myeloid and adipocyte contributions, and importantly proposes that tools for B-cell ablation or regulation of T-cell subset balance may have a place in the endocrinologist's limited arsenal.

CD23-bound IgE augments and dominates recall responses through human naive B cells

Human peripheral blood BCRμ(+) B cells express high levels of CD23 and circulate preloaded with IgE. The Ag specificity of CD23-bound IgE presumably differs from the BCR and likely reflects the Ag-specific mix of free serum IgE. CD23-bound IgE is thought to enhance B cell Ag presentation to T cells raising the question of how a B cell might respond when presented with a broad mix of Ags and CD23-bound IgE specificities. We recently reported that an increase in CD23(+) B cells is associated with the development of resistance to schistosomiasis, highlighting the potential importance of CD23-bound IgE in mediating immunity. We sought to determine the relationship between BCR and CD23-bound IgE-mediated B cell activation in the context of schistosomiasis. We found that crude schistosome Ags downregulate basal B cell activation levels in individuals hyperexposed to infectious worms. Schistosome-specific IgE from resistant, occupationally exposed Kenyans recovered responses of B cells to schistosome Ag. Furthermore, cross-linking of CD23 overrode intracellular signals mediated via the BCR, illustrating its critical and dominating role in B cell activation. These results suggest that CD23-bound IgE augments and dominates recall responses through naive B cells.

Differential regulation of TLR4 expression in human B cells and monocytes

Toll-like receptor 4 (TLR4) is an innate immune receptor that is constitutively and inducibly activated in monocytes. Although TLR4 is expressed at very low levels on human B cells from healthy individuals, recent reports showed that TLR4 expression and function is elevated in B cells from inflammatory disease patients. New data showed that TLR4 expression on B cells is increased upon stimulation through surface Igμ and CD40 in combination with IL-4. In contrast, monocyte stimulation through CD40 and IL-4 receptors decreased TLR4 surface expression. Analysis of molecular signatures of TLR4 activation in stimulated B cells suggested that TLR4 is regulated by different mechanisms in B cells compared to monocytes. PU.1 and interferon regulatory factor association with the TLR4 promoter are sufficient for TLR4 transcription, but are not sufficient for surface TLR4 expression on B cells. In contrast, the PU.1/IRF combination is sufficient for surface TLR4 expression on monocytes. These data identify mechanisms that can activate B cell TLR4 expression in inflammatory disease patients, and demonstrate that B cells have additional layers of TLR4 regulation absent in monocytes.

Differential expression of Toll-like receptors in follicular lymphoma, diffuse large B-cell lymphoma and peripheral T-cell lymphoma

Although Toll-like receptors (TLRs) in mammals are well-known to play important roles in innate immunity, newer roles for the TLRs have suggested that cells with aberrant TLR expression may have a survival advantage over normal cells. Lymphocytes are one of a small number of cell types that express many of the TLRs, suggesting that abnormal TLR levels/signaling may potentially influence the progression of malignant lymphomas. Thus, frozen samples of 51 lymph nodes from patients with follicular lymphoma (FL), diffuse large B-cell lymphoma (DLBCL) and peripheral T-cell lymphoma (PTCL) were analyzed for the expression of TLR1 to 9 using quantitative real-time PCR, and compared to those in reactive lymphadenopathy (RL) samples. TLR2 was over-expressed in both DLBCL and PTCL but not in FL when compared to RL. TLR1 and TLR4 expression was up-regulated in PTCL, while TLR8 was highly expressed in DLBCL. Although TLR5 showed lower expression in FL, expression of TLR3, TLR6, TLR7 and TLR9 did not vary significantly between different lymphoma subtypes. Double immunostaining revealed an increase in the number of TLR2 and/or TLR8 expressing lymphoma cells in DLBCL. In PTCL, TLR2 and TLR4 expression was localized to neoplastic T cells. TLR expression is highly variable among lymphoma subtypes. However, despite this some significant differences exist that may prove useful in the development of novel therapeutic strategies.

Toll-like receptors regulate B cell cytokine production in patients with diabetes

AIMS/HYPOTHESIS

Understanding cellular and molecular events in diabetes mellitus will identify new approaches for therapy. Immune system cells are important modulators of chronic inflammation in diabetes mellitus, but the role of B cells is not adequately studied. The aim of this work was to define the function of B cells in diabetes mellitus patients through focus on B cell responses to pattern recognition receptors.

METHODS

We measured expression and function of Toll-like receptors (TLRs) on peripheral blood B cells from diabetes mellitus patients by flow cytometry and multiplexed cytokine analysis. We similarly analysed B cells from non-diabetic donors and periodontal disease patients as comparative cohorts.

RESULTS

B cells from diabetes mellitus patients secrete multiple pro-inflammatory cytokines, and IL-8 production is significantly elevated in B cells from diabetic patients compared with those from non-diabetic individuals. These data, plus modest elevation of TLR surface expression, suggest B cell IL-8 hyperproduction is a cytokine-specific outcome of altered TLR function in B cells from diabetes mellitus patients. Altered TLR function is further evidenced by demonstration of an unexpected, albeit modest 'anti-inflammatory' function for TLR4. Importantly, B cells from diabetes mellitus patients fail to secrete IL-10, an anti-inflammatory cytokine implicated in inflammatory disease resolution, under a variety of TLR-stimulating conditions. Comparative analyses of B cells from patients with a second chronic inflammatory disease, periodontal disease, indicated that some alterations in B cell TLR function associate specifically with diabetes mellitus.

CONCLUSIONS/INTERPRETATION

Altered TLR function in B cells from diabetes mellitus patients increases inflammation by two mechanisms: elevation of pro-inflammatory IL-8 and lack of anti-inflammatory/protective IL-10 production.

The expression and functions of toll-like receptors in atherosclerosis

Inflammation drives atherosclerosis. Both immune and resident vascular cell types are involved in the development of atherosclerotic lesions. The phenotype and function of these cells are key in determining the development of lesions. Toll-like receptors are the most characterised innate immune receptors and are responsible for the recognition of exogenous conserved motifs on pathogens, and, potentially, some endogenous molecules. Both endogenous and exogenous TLR agonists may be present in atherosclerotic plaques. Engagement of toll-like receptors on immune and resident vascular cells can affect atherogenesis as signalling downstream of these receptors can elicit proinflammatory cytokine release, lipid uptake, and foam cell formation and activate cells of the adaptive immune system. In this paper, we will describe the expression of TLRs on immune and resident vascular cells, highlight the TLR ligands that may act through TLRs on these cells, and discuss the consequences of TLR activation in atherosclerosis.

B cells as under-appreciated mediators of non-auto-immune inflammatory disease

B lymphocytes play roles in many auto-immune diseases characterized by unresolved inflammation, and B cell ablation is proving to be a relatively safe, effective treatment for such diseases. B cells function, in part, as important sources of regulatory cytokines in auto-immune disease, but B cell cytokines also play roles in other non-auto-immune inflammatory diseases. B cell ablation may therefore benefit inflammatory disease patients in addition to its demonstrated efficacy in auto-immune disease. Current ablation drugs clear both pro- and anti-inflammatory B cell subsets, which may unexpectedly exacerbate some pathologies. This possibility argues that a more thorough understanding of B cell function in human inflammatory disease is required to safely harness the clinical promise of B cell ablation. Type 2 diabetes (T2D) and periodontal disease (PD) are two inflammatory diseases characterized by little autoimmunity. These diseases are linked by coincident presentation and alterations in toll-like receptor (TLR)-dependent B cell cytokine production, which may identify B cell ablation as a new therapy for co-affected individuals. Further analysis of the role B cells and B cell cytokines play in T2D, PD and other inflammatory diseases is required to justify testing B cell depletion therapies on a broader range of patients.

Human TLRs 10 and 1 share common mechanisms of innate immune sensing but not signaling

TLRs are central receptors of the innate immune system that drive host inflammation and adaptive immune responses in response to invading microbes. Among human TLRs, TLR10 is the only family member without a defined agonist or function. Phylogenetic analysis reveals that TLR10 is most related to TLR1 and TLR6, both of which mediate immune responses to a variety of microbial and fungal components in cooperation with TLR2. The generation and analysis of chimeric receptors containing the extracellular recognition domain of TLR10 and the intracellular signaling domain of TLR1, revealed that TLR10 senses triacylated lipopeptides and a wide variety of other microbial-derived agonists shared by TLR1, but not TLR6. TLR10 requires TLR2 for innate immune recognition, and these receptors colocalize in the phagosome and physically interact in an agonist-dependent fashion. Computational modeling and mutational analysis of TLR10 showed preservation of the essential TLR2 dimer interface and lipopeptide-binding channel found in TLR1. Coimmunoprecipitation experiments indicate that, similar to TLR2/1, TLR2/10 complexes recruit the proximal adaptor MyD88 to the activated receptor complex. However, TLR10, alone or in cooperation with TLR2, fails to activate typical TLR-induced signaling, including NF-kappaB-, IL-8-, or IFN-beta-driven reporters. We conclude that human TLR10 cooperates with TLR2 in the sensing of microbes and fungi but possesses a signaling function distinct from that of other TLR2 subfamily members.

Toll-like receptors--sentries in the B-cell response

Toll-like receptors (TLR) play a central role in the initiation of the innate immune response to pathogens. Upon recognition of molecular motifs specific for microbial molecules TLR mediate pro-inflammatory cytokine secretion and enhance antigen presentation; in B cells they further promote expansion, class switch recombination and immunoglobulin secretion. As a result of their adjuvant properties, TLR ligands have become an integral component of antimicrobial vaccines. In spite of this, little is known of the direct effects of TLR engagement on B-lymphocyte function. The scope of this review is to outline the differences in TLR expression and reactivity in murine and human B-cell subsets and to provide an overview of the currently available literature. We will further discuss the possible roles of TLR in regulating B-cell effector functions and shaping antibody-mediated defence against microbial pathogens in vivo.

Toll-like receptor 2 gene polymorphisms, pulmonary tuberculosis, and natural killer cell counts

Background

To investigate whether the toll-like receptor 2 polymorphisms could influence susceptibility to pulmonary TB, its phenotypes, and blood lymphocyte subsets.

Methods

A total of 368 subjects, including 184 patients with pulmonary TB and 184 healthy controls, were examined for TLR2 polymorphisms over locus -100 (microsatellite guanine-thymine repeats), -16934 (T>A), -15607 (A>G), -196 to -174 (insertion>deletion), and 1350 (T>C). Eighty-six TB patients were examined to determine the peripheral blood lymphocyte subpopulations.

Results

We newly identified an association between the haplotype [A-G-(insertion)-T] and susceptibility to pulmonary TB (p = 0.006, false discovery rate q = 0.072). TB patients with systemic symptoms had a lower -196 to -174 deletion/deletion genotype frequency than those without systemic symptoms (5.7% vs. 17.7%; p = 0.01). TB patients with the deletion/deletion genotype had higher blood NK cell counts than those carrying the insertion allele (526 vs. 243.5 cells/μl, p = 0.009). TB patients with pleuritis had a higher 1350 CC genotype frequency than those without pleuritis (12.5% vs. 2.1%; p = 0.004). TB patients with the 1350 CC genotype had higher blood NK cell counts than those carrying the T allele (641 vs. 250 cells/μl, p = 0.004). TB patients carrying homozygous short alleles for GT repeats had higher blood NK cell counts than those carrying one or no short allele (641 vs. 250 cells/μl, p = 0.004).

Conclusions

TLR2 genetic polymorphisms influence susceptibility to pulmonary TB. TLR2 variants play a role in the development of TB phenotypes, probably by controlling the expansion of NK cells.

TLR cross-talk specifically regulates cytokine production by B cells from chronic inflammatory disease patients

Chronic systemic inflammation links periodontal disease and diabetes to increased incidence of serious comorbidities. Activation of TLRs, particularly TLR2 and TLR4, promotes chronic systemic inflammation. Human B cells have been generally thought to lack these TLRs. However, recent work showed that an increased percentage of circulating B cells from inflammatory disease patients express TLR2 and TLR4, and that TLR engagement on B cells resulted in unexpected changes in gene expression. New data show that B cells from inflammatory disease patients secrete multiple cytokines in response to different classes of TLR ligands. Furthermore, the B cell response to combinations of TLR ligands is cytokine- and ligand-specific. Some cytokines (IL-1beta and IL-10) are predominantly regulated by TLR4, but others (IL-8 and TNF-alpha) are predominantly regulated by TLR2, due in part to TLR-dictated changes in transcription factor/promoter association. TLR2 and TLR9 also regulate B cell TLR4 expression, demonstrating that TLR cross-talk controls B cell responses at multiple levels. Parallel examination of B cells from periodontal disease and diabetes patients suggested that outcomes of TLR cross-talk are influenced by disease pathology. We conclude that disease-associated alteration of B cell TLR responses specifically regulates cytokine production and may influence chronic inflammation.

Toll-like receptors and NOD-like receptors in rheumatic diseases

The past 10 years have seen the description of families of receptors that drive proinflammatory cytokine production in infection and tissue injury. Two major classes have been examined in the context of inflammatory joint disease--the Toll-like receptors (TLRs) and NOD-like receptors (NLRs). TLRs such as TLR2 and TLR4 are being implicated in the pathology of rheumatoid arthritis, ankylosing spondylitis, lyme arthritis and osteoarthritis. Nalp3 has been identified as a key NLR for IL-1beta production and has been shown to have a particular role in gout. These findings present new therapeutic opportunities, possibly allowing for the replacement of biologics with small molecule inhibitors.

Hyperactivated B cells in human inflammatory bowel disease

IBD is characterized by a chronic, dysregulated immune response to intestinal bacteria. Past work has focused on the role of T cells and myeloid cells in mediating chronic gastrointestinal and systemic inflammation. Here, we show that circulating and tissue B cells from CD patients demonstrate elevated basal levels of activation. CD patient B cells express surface TLR2, spontaneously secrete high levels of IL-8, and contain increased ex vivo levels of phosphorylated signaling proteins. CD clinical activity correlates directly with B cell expression of IL-8 and TLR2, suggesting a positive relationship between these B cell inflammatory mediators and disease pathogenesis. In contrast, B cells from UC patients express TLR2 but generally do not demonstrate spontaneous IL-8 secretion; however, significant IL-8 production is inducible via TLR2 stimulation. Furthermore, UC clinical activity correlates inversely with levels of circulating TLR2+ B cells, which is opposite to the association observed in CD. In conclusion, TLR2+ B cells are associated with clinical measures of disease activity and differentially associated with CD- and UC-specific patterns of inflammatory mediators, suggesting a formerly unappreciated role of B cells in the pathogenesis of IBD.

B cells from periodontal disease patients express surface Toll-like receptor 4

Chronic systemic inflammation links periodontal disease (PD) to increased incidence of cardiovascular disease. Activation of TLRs, particularly TLR4, promotes chronic inflammation in PD by stimulating myeloid cells. B cells from healthy individuals are generally refractory to TLR4 agonists as a result of low surface TLR4 expression. Unexpectedly, a significantly increased percentage of gingival and peripheral blood B cells from patients with PD expressed surface TLR4. Surface expression correlated with an active TLR4 promoter that mimicked the TLR4 promoter in neutrophils. B cells from PD patients were surface myeloid differentiation protein 2-positive and also packaged the enhancer of a proinflammatory cytokine, IL-1 beta, into an active structure, demonstrating that these cells harbor key characteristics of proinflammatory cell types. Furthermore, B cells lacked activating signatures of a natural IL-1 beta inhibitor, IL-1 receptor antagonist. Surprisingly, despite multiple signatures of proinflammatory cells, freshly isolated B cells from PD patients had decreased expression of TLR pathway genes compared with B cells from healthy individuals. Decreases in inflammatory gene expression were even more dramatic in B cells stimulated with a TLR4 ligand from a periodontal pathogen, Porphyromonas gingivalis LPS 1690. In contrast, B cell TLR4 was not activated by the prototypic TLR4 ligand Escherichia coli LPS. These findings raise the unexpected possibility that TLR4 engagement modulates B cell activation in PD patients.

Toll-like receptors: lessons to learn from normal and malignant human B cells

The humoral immune system senses microbes via recognition of specific microbial molecular motifs by Toll-like receptors (TLRs). These encounters promote plasma cell differentiation and antibody production. Recent studies have demonstrated the importance of the TLR system in enhancing antibody-mediated defense against infections and maintaining memory B cells. These results have led the way to the design of vaccines that target B cells by engaging TLRs. In hematologic malignancies, cells often retain B cell-specific receptors and associated functions. Among these, TLRs are currently exploited to target different subclasses of B-cell leukemia, and TLR agonists are currently being evaluated in clinical trials. However, accumulating evidence suggests that endogenous TLR ligands or chronic infections promote tumor growth, thus providing a need for further investigations to decipher the exact function of TLRs in the B-cell lineage and in neoplastic B cells. The aim of this review is to present and discuss the latest advances with regard to the expression and function of TLRs in both healthy and malignant B cells. Special attention will be focused on the growth-promoting effects of TLR ligands on leukemic B cells and their potential clinical impact.

E6020: a synthetic Toll-like receptor 4 agonist as a vaccine adjuvant

Safe and cost-effective adjuvants are a critical component to enhance the efficacy of subunit vaccines. Studies have demonstrated that modified natural lipid As derived from enterobacterial lipopolysaccharides, which are agonists of Toll-like receptor 4, are beneficial to vaccine performance. The synthetic phospholipid dimer, E6020, mimics the physicochemical and biological properties of many of the natural lipid As derived from gram-negative bacteria. Similar to its natural counterparts, E6020, which was discovered and developed by Eisai, agonizes Toll-like receptor 4, albeit in an attenuated fashion, eliciting an immunostimulatory response that is conducive to use as a vaccine adjuvant. The derivation of E6020, along with physicochemical properties and in vitro and in vivo studies of immunostimulation and adjuvant activity, are reviewed as a background to its imminent assessment in the clinic.

Induction of secretory immunity and memory at mucosal surfaces

Mucosal epithelia comprise an extensive vulnerable barrier which is reinforced by numerous innate defence mechanisms cooperating intimately with adaptive immunity. Local generation of secretory IgA (SIgA) constitutes the largest humoral immune system of the body. Secretory antibodies function both by performing antigen exclusion at mucosal surfaces and by virus and endotoxin neutralization within epithelial cells without causing tissue damage. SIgA is thus persistently containing commensal bacteria outside the epithelial barrier but can also target invasion of pathogens and penetration of harmful antigens. Resistance to toxin-producing bacteria such as Vibrio cholerae and enterotoxigenic Escherichia coli appears to depend largely on SIgA, and so does herd protection against horizontal faecal-oral spread of enteric pathogens under naïve or immunized conditions--with a substantial innate impact both on cross-reactivity and memory. Like natural infections, live mucosal vaccines or adequate combinations of non-replicating vaccines and mucosal adjuvants, give rise not only to SIgA antibodies but also to longstanding serum IgG and IgA responses. However, there is considerably disparity with regard to migration of memory/effector cells from mucosal inductive sites to secretory effector sites and systemic immune organs. Also, although immunological memory is generated after mucosal priming, this may be masked by a self-limiting response protecting the inductive lymphoid tissue in the gut. The intranasal route of vaccine application targeting nasopharynx-associated lymphoid tissue may be more advantageous for certain infections, but only if successful stimulation is achieved without the use of toxic adjuvants that might reach the central nervous system. The degree of protection obtained after mucosal vaccination ranges from reduction of symptoms to complete inhibition of re-infection. In this scenario, it is often difficult to determine the relative importance of SIgA versus serum antibodies, but infection models in knockout mice strongly support the notion that SIgA exerts a decisive role in protection and cross-protection against a variety of infectious agents. Nevertheless, relatively few mucosal vaccines have been approved for human use, and more basic work is needed in vaccine and adjuvant design, including particulate or live-vectored combinations.

BAFF and LPS cooperate to induce B cells to become susceptible to CD95/Fas-mediated cell death

Microorganisms with pathogen-associated molecular patterns (PAMP) activate B cells directly by binding to TLR and also indirectly by inducing APC to release cytokines such as BAFF that promote B cell survival. We found that murine B cells activated concomitantly with LPS (TLR-4 ligand) and BAFF are protected from spontaneous apoptosis, but are more susceptible to Fas/CD95-mediated cell death. This increased susceptibility to Fas-induced apoptosis is associated with a dramatic coordinated up-regulation of Fas/CD95 and IRF-4 expression through a mechanism mediated, at least in part, by inhibition of the MEK/ERK pathway. Up-regulation of Fas/CD95 by BAFF is restricted to B cells activated through TLR-4, but not through TLR-9, BCR or CD40. TLR ligands differ in the BAFF family receptors (R) they induce on B cells: BAFF-R is increased by the TLR4 ligand, LPS, but not by the TLR9 ligand, CpG-containing oligodeoxynucleotides, which, in contrast, strongly up-regulates transmembrane activator and CAML interactor (TACI). This suggests the up-regulation of Fas by BAFF is mediated by BAFF-R and not by TACI. Consistently, APRIL, which binds to TACI and B cell maturation antigen but not BAFF-R, did not enhance Fas expression on LPS-activated B cells. Increased susceptibility to Fas-mediated killing of B cells activated with LPS and BAFF may be a fail-safe mechanism to avoid overexpansion of nonspecific or autoreactive B cells.