Marginal Zone B Cells Are Necessary for the Formation of Anti-donor IgG After Allogeneic Sensitization

BACKGROUND

The formation of anti-major histocompatibility complex (MHC) antibodies is a significant barrier for many patients awaiting organ transplantation. Patients with preformed anti-MHC antibodies have limited options for suitable donors, and the formation of donor-specific anti-MHC antibodies after transplantation is a harbinger of graft rejection. Despite the recognized importance of anti-MHC antibodies, the mechanisms responsible for the differentiation of B cells after exposure to allogeneic antigens are poorly understood.

METHODS

To evaluate the differentiation of B cells in response to allogeneic antigen, we used a model of H-2 b C57Bl/6 sensitization with H-2 d antigen. We used a class I MHC tetramer-based approach to identify allogeneic B cells and flow cytometric crossmatch to identify allogeneic IgM and IgG.

RESULTS

We found that although the formation of anti-H-2 d IgG was robust, few class-switched B cells and germinal center B cells were formed. Antigen-specific B cells did not express classical memory B-cell markers after sensitization but had an IgM + CD21 + marginal zone B-cell phenotype. The frequency of marginal zone B cells increased after sensitization. Depletion of marginal zone B cells before sensitization or skin grafting resulted in a significant diminution of anti-H-2 d IgG and fewer germinal center B cells. Adoptive transfer experiments revealed that marginal zone B cells more efficiently differentiated into germinal center B cells and anti-donor IgG-producing cells than follicular B cells.

CONCLUSIONS

These results demonstrate an important role for marginal zone B cells as a reservoir of alloreactive B cells that are activated by allogeneic antigens.

The common variable immunodeficiency IgM repertoire narrowly recognizes erythrocyte and platelet glycans

BACKGROUND

Autoimmune cytopenias (AICs) regularly occur in profoundly IgG-deficient patients with common variable immunodeficiency (CVID). The isotypes, antigenic targets, and origin(s) of their disease-causing autoantibodies are unclear.

OBJECTIVE

We sought to determine reactivity, clonality, and provenance of AIC-associated IgM autoantibodies in patients with CVID.

METHODS

We used glycan arrays, patient erythrocytes, and platelets to determine targets of CVID IgM autoantibodies. Glycan-binding profiles were used to identify autoreactive clones across B-cell subsets, specifically circulating marginal zone (MZ) B cells, for sorting and IGH sequencing. The locations, transcriptomes, and responses of tonsillar MZ B cells to different TH- cell subsets were determined by confocal microscopy, RNA-sequencing, and cocultures, respectively.

RESULTS

Autoreactive IgM coated erythrocytes and platelets from many CVID patients with AICs (CVID+AIC). On glycan arrays, CVID+AIC plasma IgM narrowly recognized erythrocytic i antigens and platelet i-related antigens and failed to bind hundreds of pathogen- and tumor-associated carbohydrates. Polyclonal i antigen-recognizing B-cell receptors were highly enriched among CVID+AIC circulating MZ B cells. Within tonsillar tissues, MZ B cells secreted copious IgM when activated by the combination of IL-10 and IL-21 or when cultured with IL-10/IL-21-secreting FOXP3-CD25hi T follicular helper (Tfh) cells. In lymph nodes from immunocompetent controls, MZ B cells, plentiful FOXP3+ regulatory T cells, and rare FOXP3-CD25+ cells that represented likely CD25hi Tfh cells all localized outside of germinal centers. In CVID+AIC lymph nodes, cellular positions were similar but CD25hi Tfh cells greatly outnumbered regulatory cells.

CONCLUSIONS

Our findings indicate that glycan-reactive IgM autoantibodies produced outside of germinal centers may contribute to the autoimmune pathogenesis of CVID.

Two distinct subpopulations of marginal zone B cells exhibit differential antibody-producing capacities and radioresistance

Marginal zone (MZ) B cells, which are splenic innate-like B cells that rapidly secrete antibodies (Abs) against blood-borne pathogens, are composed of heterogeneous subpopulations. Here, we showed that MZ B cells can be divided into two distinct subpopulations according to their CD80 expression levels. CD80high MZ B cells exhibited greater Ab-producing, proliferative, and IL-10-secreting capacities than did CD80low MZ B cells. Notably, CD80high MZ B cells survived 2-Gy whole-body irradiation, whereas CD80low MZ B cells were depleted by irradiation and then repleted with one month after irradiation. Depletion of CD80low MZ B cells led to accelerated development of type II collagen (CII)-induced arthritis upon immunization with bovine CII. CD80high MZ B cells exhibited higher expression of genes involved in proliferation, plasma cell differentiation, and the antioxidant response. CD80high MZ B cells expressed more autoreactive B cell receptors (BCRs) that recognized double-stranded DNA or CII, expressed more immunoglobulin heavy chain sequences with shorter complementarity-determining region 3 sequences, and included more clonotypes with no N-nucleotides or with B-1a BCR sequences than CD80low MZ B cells. Adoptive transfer experiments showed that CD21+CD23+ transitional 2 MZ precursors preferentially generated CD80low MZ B cells and that a proportion of CD80low MZ B cells were converted into CD80high MZ B cells; in contrast, CD80high MZ B cells stably remained CD80high MZ B cells. In summary, MZ B cells can be divided into two subpopulations according to their CD80 expression levels, Ab-producing capacity, radioresistance, and autoreactivity, and these findings may suggest a hierarchical composition of MZ B cells with differential stability and BCR specificity.

The complement regulator CD55 modulates TLR9 signaling and supports survival in marginal zone B cells

Marginal zone (MZ) B cells bridge innate and adaptive immunity by sensing bloodborne antigens and producing rapid antibody and cytokine responses. CD55 is a membrane-bound complement regulator that interferes with complement activation, an important component of innate immunity. CD55 also regulates adaptive immunity-CD55 downregulation is critical for germinal center reactions. MZ B cells also express low CD55, but its role in MZ B cell function is unknown. Using germline knockout mice, we found that similar numbers of MZ B cells are initially established in 3-week-old CD55-deficient mice compared to wild-type (WT) mice. However, MZ B cells fail to accumulate as mice age and undergo increased apoptosis. Following ex vivo stimulation of MZ B cells through Toll-like receptor 9, we observed a proinflammatory phenotype with increased IL-6 expression. These findings demonstrate a critical role for CD55 in supporting MZ B cell survival while also regulating cellular function.

Altered expression of MZB1 in periodontitis: A possible link to disease pathogenesis

BACKGROUND

Our previous study explored the molecular signatures of generalized aggressive periodontitis (GAgP) using gingival tissues through omics-based-whole-genome transcriptomic analysis. This continuation study aimed to investigate the whole protein profiling of these gingival samples through liquid chromatography-mass spectroscopy/mass spectroscopy (LC-MS/MS) analysis and to validate the identified proteins through immunohistochemistry to provide further evidence for the quality of the results.

METHODS

In previous study, gene expression patterns were identified in gingival tissues from 23 GAgP and 25 control individuals. In the current study, comparative proteomic analysis was performed on isolated proteins from the same study groups using LC-MS/MS analysis. The data from the transcriptomics study published before and the proteomics data were integrated to reveal any common genes and proteins. Additionally, immunohistochemical analysis was conducted to further investigate the findings.

RESULTS

The most upregulated proteins in patients compared to controls were ITGAM, AZU1, MMP9, BPI, UGGG1, MZB1, TRFL, PDIA6, PRDX4, and PLG. The top six pathways associated with these proteins were involved in innate immune system, post-translational protein phosphorylation, interleukin-4 and -13 signaling, toll-like receptors cascades, and extracellular matrix organization. Based on the integration and validation analysis of transcriptomics and proteomics data, as well as immunohistochemical analysis, MZB1 was identified as a shared gene and protein that were upregulated in the patients.

CONCLUSIONS

MZB1 is a protein that is involved in the development of B cells and the production of antibodies. Its upregulation in periodontitis suggests that there may be a dysregulation of the immune response in this condition, and MZB1 may be a potent biomarker for periodontitis.

Exploring Perturbations in Peripheral B Cell Memory Subpopulations Early after Kidney Transplantation Using Unsupervised Machine Learning

BACKGROUND

B cells have a significant role in transplantation. We examined the distribution of memory subpopulations (MBCs) and naïve B cell (NBCs) phenotypes in patients soon after kidney transplantation. Unsupervised machine learning cluster analysis is used to determine the association between the cellular phenotypes and renal function.

METHODS

MBC subpopulations and NBCs from 47 stable renal transplant recipients were characterized by flow cytometry just before (T0) and 6 months after (T6) transplantation. T0 and T6 measurements were compared, and clusters of patients with similar cellular phenotypic profiles at T6 were identified. Two clusters, clusters 1 and 2, were formed, and the glomerular filtration rate was estimated (eGFR) for these clusters.

RESULTS

A significant increase in NBC frequency was observed between T0 and T6, with no statistically significant differences in the MBC subpopulations. Cluster 1 was characterized by a predominance of the NBC phenotype with a lower frequency of MBCs, whereas cluster 2 was characterized by a high frequency of MBCs and a lower frequency of NBCs. With regard to eGFR, cluster 1 showed a higher value compared to cluster 2.

CONCLUSIONS

Transplanted kidney patients can be stratified into clusters based on the combination of heterogeneity of MBC phenotype, NBCs and eGFR using unsupervised machine learning.

Marginal Zone B (MZB) Cells: Comparison of the Initial Identification of Immune Activity Leading to Dacryoadenitis and Sialadenitis in Experimental Sjögren's Syndrome

Although multiple mouse strains have been advanced as models for Sjögren's syndrome (SS), which is a human systemic autoimmune disease characterized primarily as the loss of lacrimal and salivary gland functions, the C57BL/6.NOD-Aec1Aec2 recombinant inbred (RI) mouse derived from the NOD/ShiLtJ line is considered one of the more appropriate models exhibiting virtually all the characteristics of the human disease. This mouse model, as well as other mouse models of SS, have shown that B lymphocytes are essential for the onset and development of observed clinical manifestations. Recently, studies carried out in the C57BL/6.IL14α transgenic mouse have provided clear evidence that the marginal zone B (MZB) cell population is directly involved in the early pathological events initiating the development of the clinical SS disease, as well as late-stage lymphomagenesis resulting in B-cell lymphomas. Since MZB cells are difficult to study in vivo and in vitro, we carried out a series of ex vivo investigations that utilize temporal global RNA transcriptomic analyses to profile differentially expressed genes exhibiting temporal upregulation during the initial onset and subsequent development of pathophysiological events within the lacrimal and salivary gland tissues per se or associated with the leukocyte cell migrations into these glands. The initial transcriptomic analyses revealed that while the upregulated gene expression profiles obtained from lacrimal and salivary glands overlap, multiple genetic differences exist between the defined activated pathways. In the current study, we present a concept suggesting that the initial pathological events differ between the two glands, yet the subsequent upregulated TLR4/TLR3 signal transduction pathway that activates the type-1 interferon signature appears to be identical in the two glands and indicates an autoimmune response against dsRNA, possibly a virus. Here, we attempt to put these findings into perspective and determine how they can impact the design of future therapeutic protocols.

B lymphocytes in COVID-19: a tale of harmony and discordance

B lymphocytes play a vital role in the human defense against viral infections by producing specific antibodies. They are also critical for the prevention of infectious diseases by vaccination, and their activation influences the efficacy of the vaccination. Since the beginning of coronavirus disease 2019 (COVID-19), which became the main concern of the world health system, many efforts have been made to treat and prevent the disease. However, for the development of successful therapeutics and vaccines, it is necessary to understand the interplay between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, and the immune system. The innate immune system provides primary and nonspecific defense against the virus, but within several days after infection, a virus-specific immune response is provided first by antibody-producing B cells, which are converted after the resolution of disease to memory B cells, which provide long-term immunity. Although a failure in B cell activation or B cell dysfunction can cause a severe form of the disease and also lead to vaccination inefficiency, some individuals with B cell immunodeficiency have shown less production of the cytokine IL-6, resulting in a better disease outcome. In this review, we present the latest findings on the interaction between SARS-CoV-2 and B lymphocytes during COVID-19 infection.

Beyond Adaptive Alloreactivity: Contribution of Innate B Cells to Allograft Inflammation and Rejection

Innate B cells are a heterogeneous group of cells that function in maintaining homeostatic levels of circulating natural antibodies and being the first line of defense against infections. Innate B-1 cells and marginal zone B cells may relocate to lymphoid follicles and differentiate into cytokine and antibody-secreting cells in T-independent and T-dependent manners. Although marginal zone B cells are widely described in humans, the presence of B-1 cells is more controversial. Here, we review the basic features of the innate B-cell subsets identified in mice and their equivalent in humans, as well as their potential roles in transplantation. We summarize the findings of Cascalho and colleagues on the unexpected protective role of tumor necrosis factor receptor superfamily member 13B in regulating circulating levels of protective natural immunoglobulin M, and the studies by Zorn and colleagues on the potential pathogenic role for polyreactive innate B cells infiltrating allograft explants. Finally, we discuss our studies that took a transcriptomic approach to identify innate B cells infiltrating kidney allografts with antibody-mediated rejection and to demonstrate that local antigens within the allograft together with inflammation may induce a loss of B-cell tolerance.

Metabolic requirements of Th17 cells and of B cells: Regulation and defects in health and in inflammatory diseases

The immune system protects from infections and cancer through complex cellular networks. For this purpose, immune cells require well-developed mechanisms of energy generation. However, the immune system itself can also cause diseases when defective regulation results in the emergence of autoreactive lymphocytes. Recent studies provide insights into how differential patterns of immune cell responses are associated with selective metabolic pathways. This review will examine the changing metabolic requirements of Th17 cells and of B cells at different stages of their development and activation. Both cells provide protection but can also mediate diseases through the production of autoantibodies and the production of proinflammatory mediators. In health, B cells produce antibodies and cytokines and present antigens to T cells to mount specific immunity. Th17 cells, on the other hand, provide protection against extra cellular pathogens at mucosal surfaces but can also drive chronic inflammation. The latter cells can also promote the differentiation of B cells to plasma cells to produce more autoantibodies. Metabolism-regulated checkpoints at different stages of their development ensure the that self-reactive B cells clones and needless production of interleukin (IL-)17 are limited. The metabolic regulation of the two cell types has some similarities, e.g. the utility of hypoxia induced factor (HIF)1α during low oxygen tension, to prevent autoimmunity and regulate inflammation. There are also clear differences, as Th17 cells only are vulnerable to the lack of certain amino acids. B cells, unlike Th17 cells, are also dependent of mechanistic target of rapamycin 2 (mTORC2) to function. Significant knowledge has recently been gained, particularly on Th17 cells, on how metabolism regulates these cells through influencing their epigenome. Metabolic dysregulation of Th17 cells and B cells can lead to chronic inflammation. Disease associated alterations in the genome can, in addition, cause dysregulation to metabolism and, thereby, result in epigenetic alterations in these cells. Recent studies highlight how pathology can result from the cooperation between the two cell types but only few have so far addressed the key metabolic alterations in such settings. Knowledge of the impact of metabolic dysfunction on chronic inflammation and pathology can reveal novel therapeutic targets to treat such diseases.

Pattern recognition receptor ligand-induced differentiation of human transitional B cells

B cells represent a critical component of the adaptive immune response whose development and differentiation are determined by antigen-dependent and antigen-independent interactions. In this study, we explored the effects of IL-4 and pattern-recognition receptor (PRR) ligands on B cell development and differentiation by investigating their capacity to drive the in vitro maturation of human transitional B cells. In the presence of IL-4, ligands for TLR7/8, TLR9, and NOD1 were effective in driving the in vitro maturation of cord blood transitional B cells into mature, naïve B cells as measured by CD23 expression, ABCB1 transporter activation and upregulation of sIgM and sIgD. In addition, several stimulation conditions, including TLR9 ligand alone, favored an expansion of CD27⁺ IgM memory B cells. Transitional B cells stimulated with TLR7/8 ligand + IL-4 or TLR9 ligand, with or without IL-4, induced a significant subpopulation of CD23⁺CD27⁺ B cells expressing high levels of sIgM and sIgD, a minor B cell subpopulation found in human peripheral blood. These studies illustrate the heterogeneity of the B cell populations induced by cytokine and PRR ligand stimulation. A comparison of transitional and mature, naïve B cells transcriptomes to identify novel genes involved in B cell maturation revealed that mature, naïve B cells were less transcriptionally active than transitional B cells. Nevertheless, a subset of differentially expressed genes in mature, naïve B cells was identified including genes associated with the IL-4 signaling pathway, PI3K signaling in B lymphocytes, the NF-κB signaling pathway, and the TNFR superfamily. When transitional B cells were stimulated in vitro with IL-4 and PRR ligands, gene expression was found to be dependent on the nature of the stimulants, suggesting that exposure to these stimulants may alter the developmental fate of transitional B cells. The influence of IL-4 and PRR signaling on transitional B cell maturation illustrates the potential synergy that may be achieved when certain PRR ligands are incorporated as adjuvants in vaccine formulations and presented to developing B cells in the context of an inflammatory cytokine environment. These studies demonstrate the potential of the PRR ligands to drive transitional B cell differentiation in the periphery during infection or vaccination independently of antigen mediated BCR signaling.

Marginal zone B cells: From housekeeping function to autoimmunity?

Marginal zone (MZ) B cells comprise a subset of innate-like B cells found predominantly in the spleen, but also in lymph nodes and blood. Their principal functions are participation in quick responses to blood-borne pathogens and secretion of natural antibodies. The latter is important for housekeeping functions such as clearance of apoptotic cell debris. MZ B cells have B cell receptors with low poly-/self-reactivity, but they are not pathogenic at steady state. However, if simultaneously stimulated with self-antigen and pathogen- and/or damage-associated molecular patterns (PAMPs/DAMPs), MZ B cells may participate in the initial steps towards breakage of immunological tolerance. This review summarizes what is known about the role of MZ B cells in autoimmunity, both in mouse models and human disease. We cover factors important for shaping the MZ B cell compartment, how the functional properties of MZ B cells may contribute to breaking tolerance, and how MZ B cells are being regulated.

Functional Role of B Cells in Atherosclerosis

Atherosclerosis is a lipid-driven inflammatory disease of blood vessels, and both innate and adaptive immune responses are involved in its development. The impact of B cells on atherosclerosis has been demonstrated in numerous studies and B cells have been found in close proximity to atherosclerotic plaques in humans and mice. B cells exert both atheroprotective and pro-atherogenic functions, which have been associated with their B cell subset attribution. While B1 cells and marginal zone B cells are considered to protect against atherosclerosis, follicular B cells and innate response activator B cells have been shown to promote atherosclerosis. In this review, we shed light on the role of B cells from a different, functional perspective and focus on the three major B cell functions: antibody production, antigen presentation/T cell interaction, and the release of cytokines. All of these functions have the potential to affect atherosclerosis by multiple ways and are dependent on the cellular milieu and the activation status of the B cell. Moreover, we discuss B cell receptor signaling and the mechanism of B cell activation under atherosclerosis-prone conditions. By summarizing current knowledge of B cells in and beyond atherosclerosis, we are pointing out open questions and enabling new perspectives.

Treatment with a Toll-like Receptor 7 ligand evokes protective immunity against atherosclerosis in hypercholesterolaemic mice

BACKGROUND

The interplay between innate and adaptive immunity is central in life-threatening clinical complications of atherosclerosis such as myocardial infarction and stroke. The specific mechanisms involved and their protective versus detrimental effects in the disease process remain poorly understood. We have previously shown that higher levels of Toll-like receptor 7 (TLR7) expression in human atherosclerotic lesions are correlated with better patient outcome.

OBJECTIVE

In this study, we explored whether TLR7 activation can ameliorate disease in experimental atherosclerosis in mice.

METHODS

Apolipoprotein E deficient mice (Apoe^-/- ) with established disease were injected for five weeks intraperitoneally with the TLR7 ligand R848. Local effects were evaluated by characterization of the lesion. Systemic effects of the treatment were investigated by immune composition analysis in the spleen and plasma measurements.

RESULTS

The in vivo treatment arrested lesion progression in the aorta. We also detected expansion of marginal zone B cells and T_reg in the spleen together with increased plasma IgM antibodies against oxidized low-density lipoprotein (oxLDL) and reduced plasma cholesterol levels. These changes were accompanied by increased accumulation of IgM antibodies, decreased necrosis and fewer apoptotic cells in atherosclerotic lesions.

CONCLUSIONS

Our findings show that TLR7 stimulation could ameliorate atherosclerotic lesion burden and reduce plasma cholesterol in Apoe^-/- mice. TLR7 stimulation was associated with an atheroprotective B-cell and T_reg response, which may have systemic and local effects within lesions that could prevent arterial lipid accumulation and inflammation.

Histone H2A-Reactive B Cells Are Functionally Anergic in Healthy Mice With Potential to Provide Humoral Protection Against HIV-1

Peripheral tolerance is essential for silencing weakly autoreactive B cells that have escaped central tolerance, but it is unclear why these potentially pathogenic B cells are retained rather than being eliminated entirely. Release from peripheral tolerance restraint can occur under certain circumstances (i.e., strong TLR stimulus), that are present during infection. In this regard, we hypothesized that autoreactive B cells could function as a reserve population that can be activated to contribute to the humoral immune response, particularly with pathogens, such as HIV-1, that exploit immune tolerance to avoid host defense. In this study, we identify a population of autoreactive B cells with the potential to neutralize HIV-1 and experimentally release them from the functional restrictions of peripheral tolerance. We have previously identified murine monoclonal antibodies that displayed autoreactivity against histone H2A and neutralized HIV-1 in vitro. Here, we identify additional H2A-reactive IgM monoclonal antibodies and demonstrate that they are both autoreactive and polyreactive with self and foreign antigens and are able to neutralize multiple clades of tier 2 HIV-1. Flow cytometric analysis of H2A-reactive B cells in naïve wildtype mice revealed that these B cells are present in peripheral B cell populations and we further document that murine H2A-reactive B cells are restrained by peripheral tolerance mechanisms. Specifically, we show endogenous H2A-reactive B cells display increased expression of the inhibitory mediators CD5 and phosphatase and tensin homolog (PTEN) phosphatase and fail to mobilize calcium upon immunoreceptor stimulation; all characterized markers of anergy. Moreover, we show that toll-like receptor stimulation or provision of CD4 T cell help induces the in vitro production of H2A-reactive antibodies, breaking tolerance. Thus, we have identified a novel poly/autoreactive B cell population that has the potential to neutralize HIV-1 but is silenced by immune tolerance.

Putative role of marginal zone B cells in pathophysiological processes

The maintenance of inner integrity of an organism is founded on the proper performance of two immunity branches, innate and adaptive immune responses. Recently, it became apparent that subset of splenic B cells named marginal zone B cells (MZB cells) exhibits unique developmental and functional features that bridge these two immunity branches. Strategically positioned at the site where blood and lymph are filtered, MZB cells represent a population of sentinels that rapidly proliferate and differentiate into IgM plasmablast cells when encountered with blood-borne, thymus-independent (TI) Ags. Moreover, MZB cells have intrinsic capability to induce potent CD4⁺ helper T cell response and cytokine production upon stimulation with soluble antigens. Due to their ability to overcome a time gap prior the establishment of the full adaptive response towards pathogens, MZB cells connect and direct innate and adaptive immunity. An additional interesting characteristic of MZB cells is capacity to function as regulatory cells in autoimmune processes. MZB cells may also contribute to the control of autoimmunity via the induction of tolerance by apoptotic cells. Importantly, in the clear association with inflammation and autoimmunity, MZB cells may transform into MALT lymphoma, representing a concurrence point for the infection, immunity and malignancy. This paper presents an insight into the complex biology of marginal zone B cells and their role in intertwining and directing innate and adaptive immune processes at the physiological and pathological level.

Molecular cues involved in the regulation of B cell dynamics: Assistants of antigen hunting

The ability of a cell to migrate, adhere, and change its morphology is determinant in developing its functions; these capacities reach their maximum relevance in immune cells. For an efficient immune response, immune cells must localize in the right place at the right time; that implies crossing tissue barriers and migrating in the interstitial space of the tissues at high velocities. The dependency on trafficking abilities is even higher for B cells, one of the arms of the adaptive immune system, considering that they must encounter specific antigens for their clonal receptor in the enormous tissue volume of the secondary lymphoid organs (spleen, lymph nodes, Peyer patches). The regulated interplay between cell motility and cell adhesion allows B cells to reach distinct lymphoid tissues and, within them, to explore the stromal cell networks where antigen might be exposed. In this meeting-invited review, I summarize the current knowledge on the molecular cues and mechanisms that shapes B cell dynamics at the initial phase of the humoral immune response, including homeostatic chemoattractants and innate/inflammatory stimuli. I also revised the B cell behavior alterations caused by BCR recognition of antigen and the molecular mechanisms involved.

Thioacetamide-induced liver damage and thrombocytopenia is associated with induction of antiplatelet autoantibody in mice

Thrombocytopenia is usually associated with liver injury, elevated plasma aspartate aminotransferase and alanine aminotransferase levels, and high antiplatelet immunoglobulin (Ig) titers, although the mechanism behind these effects remains elusive. Deciphering the mechanism behind acute liver disease–associated thrombocytopenia may help solve difficulties in routine patient care, such as liver biopsy, antiviral therapy, and surgery. To determine whether liver damage is sufficient per se to elicit thrombocytopenia, thioacetamide (TAA)-induced hepatitis rodent models were employed. The analysis results indicated that TAA treatment transiently induced an elevation of antiplatelet antibody titer in both rats and mice. B-cell-deficient (BCD) mice, which have loss of antibody expression, exhibited markedly less thrombocytopenia and liver damage than wild-type controls. Because TAA still induces liver damage in BCD mice, this suggests that antiplatelet Ig is one of the pathogenic factors, which play exacerbating role in the acute phase of TAA-induced hepatitis. TNF-α was differentially regulated in wild-type versus BCD mice during TAA treatment, and anti-TNF treatment drastically ameliorated antiplatelet Ig induction, thrombocytopenia, and liver injury, suggesting that the TNF pathway plays a critical role in the disease progression.

Identification of mechanisms conferring an enhanced immune response in mice induced by CVC1302-adjuvanted killed serotype O foot-and-mouth virus vaccine

The adjuvant CVC1302 was previously shown to efficiently enhance the immunogenicity of killed foot-and-mouth disease virus (FMDV) in mice and piglets. However, the underlining mechanism of action of CVC1302 remains unclear, especially at local injection sites and draining lymph nodes. Since the FMDV vaccine is administrated intramuscularly in field settings, we studied local immune responses to FMDV following intramuscular injection in mice, and found that CVC1302-adjuvanted killed FMDV (KV-CVC1302) induced secretion of several chemokines in murine muscle tissues, including MCP-1, MIP-1α, and MIP-1β. The number of monocytes recruited to the site of injection was significantly higher in mice immunized with KV-CVC1302 compared with mice immunized with killed FMDV alone (KV). iTAQ-based quantitative proteomic assays were additionally employed to explore the molecular mechanisms of CVC1302 action in the draining lymph nodes. A total of 35 proteins were identified as being differentially expressed among the control group, KV-immunized group and KV-CVC1302-immunized group at 10 days post immunization (dpi). Proteins exhibiting differential expression were mainly involved in signal transduction, apoptosis, endocytosis and innate immune responses. Pathway analysis demonstrated that AMPK, phospholipase D, cAMP, Rap1, and MAPK signaling pathways were potentially induced by the immunopotentiator CVC1302. Understanding the local mechanism of CVC1302 action at injection sites and draining lymph nodes will provide new insights into the development of FMDV vaccines.

LSD1 Cooperates with Noncanonical NF-κB Signaling to Regulate Marginal Zone B Cell Development

Marginal zone B cells (MZB) are a mature B cell subset that rapidly respond to blood-borne pathogens. Although the transcriptional changes that occur throughout MZB development are known, the corresponding epigenetic changes and epigenetic modifying proteins that facilitate these changes are poorly understood. The histone demethylase LSD1 is an epigenetic modifier that promotes plasmablast formation, but its role in B cell development has not been explored. In this study, a role for LSD1 in the development of B cell subsets was examined. B cell-conditional deletion of LSD1 in mice resulted in a decrease in MZB whereas follicular B cells and bone marrow B cell populations were minimally affected. LSD1 repressed genes in MZB that were normally upregulated in the myeloid and follicular B cell lineages. Correspondingly, LSD1 regulated chromatin accessibility at the motifs of transcription factors known to regulate splenic B cell development, including NF-κB motifs. The importance of NF-κB signaling was examined through an ex vivo MZB development assay, which showed that both LSD1-deficient and NF-κB-inhibited transitional B cells failed to undergo full MZB development. Gene expression and chromatin accessibility analyses of in vivo- and ex vivo-generated LSD1-deficient MZB indicated that LSD1 regulated the downstream target genes of noncanonical NF-κB signaling. Additionally LSD1 was found to interact with the noncanonical NF-κB transcription factor p52. Together, these data reveal that the epigenetic modulation of the noncanonical NF-κB signaling pathway by LSD1 is an essential process during the development of MZB.

Expression of Toll-like receptors and costimulatory molecules in splenic B cells in a normal and abortion-prone murine pregnancy model

PROBLEM

The regulatory role of B lymphocytes in the pregnancy-induced maternal immune response is not well recognized. B lymphocytes function as antigen-presenting cells (APCs) and regulate Toll-like receptors and costimulatory molecule expression in response to intrinsic and extrinsic signals. Therefore, the aim of this study was to determine the expression of TLR2, TLR4, TLR9, and MHC class II and the costimulatory molecules CD80, CD86, and CD40 in splenic B cells in a normal and abortion-prone murine pregnancy model.

METHODS OF STUDY

The expression level of these molecules on female splenic B cells was investigated using real-time PCR and flow cytometry. The analysis was performed on the 3rd and 14th day of normal (CBA/JxBALB/c) and abortion-prone (CBA/JxDBA/2J) murine pregnancy.

RESULTS

The expression of Tlr9, Cd86, and H2-Ab1 in splenic B cells on the 3rd day after mating was upregulated, whereas Tlr2 was downregulated in abortion-prone females. On day 14, we observed lower expression levels of Tlr4 and Cd80 and higher expression levels of Cd86 in CBA/J females mated with DBA/2J males. At the protein level, the differences were observed only on day 3 of pregnancy. TLR4 and CD40 molecules were upregulated in splenic B cells, while TLR9 and CD86 were downregulated in abortion-prone mice.

CONCLUSION

Differential expression of TLRs and costimulatory molecules in splenic B cells in abortion-prone and normal pregnancies suggests the involvement of these cells in the regulation of the immune response at the periphery in pregnant females.

Molecular regulation of peripheral B cells and their progeny in immunity

In this review, Boothby et al. summarize some salient advances toward elucidation of the molecular programming of the fate choices and function of B cells in the periphery. They also note unanswered questions that pertain to differences among subsets of B lymphocytes and plasma cells.

Mature B lymphocytes are crucial components of adaptive immunity, a system essential for the evolutionary fitness of mammals. Adaptive lymphocyte function requires an initially naïve cell to proliferate extensively and its progeny to have the capacity to assume a variety of fates. These include either terminal differentiation (the long-lived plasma cell) or metastable transcriptional reprogramming (germinal center and memory B cells). In this review, we focus principally on the regulation of differentiation and functional diversification of the “B2” subset. An overview is combined with an account of more recent advances, including initial work on mechanisms that eliminate DNA methylation and potential links between intracellular metabolites and chromatin editing.

B cell responses to apoptotic cells in MFG-E8-/- mice

Defective clearance of apoptotic cells in MFG-E8 deficient mice results in lupus-like disease in the mixed B6x129, but not pure B6 background. The lack of overt autoimmunity in MFG-E8-/- B6 mice suggests that accumulation of apoptotic cells is not sufficient to break central tolerance. However, the delayed clearance of apoptotic cells in the follicles of MFG-E8-/- B6 mice provides an excellent opportunity to investigate how B cells respond to excessive apoptotic cells in the periphery under relatively non-inflammatory conditions. In MFG-E8-/- B6 mice, we found increased IgG2c production against apoptotic cells and oxidized LDL. Apoptotic cell induced antibody responses depended on MyD88 signal and T cell help. In addition, MFG-E8-/- B6 mice had enlarged MZ B cell compartments as well as an enhanced antibody response to NP-Ficoll. Moreover, a significant percentage of MZ B cells in aged MFG-E8-/- B6 mice migrated into follicles. Injecting apoptotic cells or oxidized LDL into wild type mice as well as physiological accumulation of LDL in ApoE-/- mice recapitulated the translocation of MZ B cells. To determine how MFG-E8 deficiency affects the functions of autoreactive B cells specific for nucleic acids in the periphery under non-inflammatory conditions, we utilized BCR transgenic mice to bypass central selection and compared the differentiation of TLR9 dependent anti-dsDNA 56R B cells and TLR7 dependent anti-ssRNA H564 B cells in MFG-E8-/- mice. In MFG-E8-/- 56R mice, anti-dsDNA specific 56R/Vκ38c B cells differentiated into MZ B cells but not AFCs. On the contrary, in MFG-E8-/-H564 mice, anti-ssRNA specific H564 B cells further differentiated into GC B cells and AFCs. Adoptive transfer of activated autoreactive B cells confirmed that H564 B cells were more sensitive to apoptotic cell antigens than 56R B cells. Our observations provide new insights about the MZ B cell translocation in lupus patients as well as the dichotomy of TLR9 and TLR7 signals in the pathogenesis of lupus.

Pathogenic Correlates of Simian Immunodeficiency Virus-Associated B Cell Dysfunction

We compared and contrasted pathogenic (in pig-tailed macaques [PTMs]) and nonpathogenic (in African green monkeys [AGMs]) SIVsab infections to assess the significance of the B cell dysfunction observed in simian (SIV) and human immunodeficiency virus (HIV) infections. We report that the loss of B cells is specifically associated with the pathogenic SIV infection, while in the natural hosts, in which SIV is nonpathogenic, B cells rapidly increase in both lymph nodes (LNs) and intestine. SIV-associated B cell dysfunction associated with the pathogenic SIV infection is characterized by loss of naive B cells, loss of resting memory B cells due to their redistribution to the gut, increases of the activated B cells and circulating tissue-like memory B cells, and expansion of the B regulatory cells (Bregs). While circulating B cells are virtually restored to preinfection levels during the chronic pathogenic SIV infection, restoration is mainly due to an expansion of the "exhausted," virus-specific B cells, i.e., activated memory cells and tissue-like memory B cells. Despite of the B cell dysfunction, SIV-specific antibody (Ab) production was higher in the PTMs than in AGMs, with the caveat that rapid disease progression in PTMs was strongly associated with lack of anti-SIV Ab. Neutralization titers and the avidity and maturation of immune responses did not differ between pathogenic and nonpathogenic infections, with the exception of the conformational epitope recognition, which evolved from low to high conformations in the natural host. The patterns of humoral immune responses in the natural host are therefore more similar to those observed in HIV-infected subjects, suggesting that natural hosts may be more appropriate for modeling the immunization strategies aimed at preventing HIV disease progression. The numerous differences between the pathogenic and nonpathogenic infections with regard to dynamics of the memory B cell subsets point to their role in the pathogenesis of HIV/SIV infections and suggest that monitoring B cells may be a reliable approach for assessing disease progression.IMPORTANCE We report here that the HIV/SIV-associated B cell dysfunction (defined by loss of total and memory B cells, increased B regulatory cell [Breg] counts, and B cell activation and apoptosis) is specifically associated with pathogenic SIV infection and absent during the course of nonpathogenic SIV infection in natural nonhuman primate hosts. Alterations of the B cell population are not correlated with production of neutralizing antibodies, the levels of which are similar in the two species. Rapid progressive infections are associated with a severe impairment in SIV-specific antibody production. While we did not find major differences in avidity and maturation between the pathogenic and nonpathogenic SIV infections, we identified a major difference in conformational epitope recognition, with the nonpathogenic infection being characterized by an evolution from low to high conformations. B cell dysfunction should be considered in designing immunization strategies aimed at preventing HIV disease progression.

DNA-binding of the Tet-transactivator curtails antigen-induced lymphocyte activation in mice

The Tet-On/Off system for conditional transgene expression constitutes state-of-the-art technology to study gene function by facilitating inducible expression in a timed and reversible manner. Several studies documented the suitability and versatility of this system to trace lymphocyte fate and to conditionally express oncogenes or silence tumour suppressor genes in vivo. Here, we show that expression of the tetracycline/doxycycline-controlled Tet-transactivator, while tolerated well during development and in immunologically unchallenged animals, impairs the expansion of antigen-stimulated T and B cells and thereby curtails adaptive immune responses in vivo. Transactivator-mediated cytotoxicity depends on DNA binding, but can be overcome by BCL2 overexpression, suggesting that apoptosis induction upon lymphocyte activation limits cellular and humoral immune responses. Our findings suggest a possible system-intrinsic biological bias of the Tet-On/Off system in vivo that will favour the outgrowth of apoptosis resistant clones, thus possibly confounding data published using such systems.

Tet-transactivators are used for direct regulation of gene expression, RNA interference and for CRISPR/Cas9-based systems. Here the authors show that DNA-bound Tet-transactivators can induce cell death in antigen-activated lymphocytes in vivo, putting into question the use of, and in vivo data generated with, these molecular tools.

Lipopolysaccharides-Induced Suppression of Innate-Like B Cell Apoptosis Is Enhanced by CpG Oligodeoxynucleotide and Requires Toll-Like Receptors 2 and 4

Innate-like B lymphocytes play an important role in innate immunity in periodontal disease through Toll-like receptor (TLR) signaling. However, it is unknown how innate-like B cell apoptosis is affected by the periodontal infection-associated innate signals. This study is to determine the effects of two major TLR ligands, lipopolysaccharide (LPS) and CpG-oligodeoxynucleotides (CpG-ODN), on innate-like B cell apoptosis. Spleen B cells were isolated from wild type (WT), TLR2 knockout (KO) and TLR4 KO mice and cultured with E. coli LPS alone, P. gingivalis LPS alone, or combined with CpG-ODN for 2 days. B cell apoptosis and expressions of specific apoptosis-related genes were analyzed by flow cytometry and real-time PCR respectively. P. gingivalis LPS, but not E. coli LPS, reduced the percentage of AnnexinV⁺/7-AAD^- cells within IgM^highCD23^lowCD43^-CD93^- marginal zone (MZ) B cell sub-population and IgM^highCD23^lowCD43⁺CD93⁺ innate response activator (IRA) B cell sub-population in WT but not TLR2KO or TLR4KO mice. CpG-ODN combined with P. gingivalis LPS further reduced the percentage of AnnexinV⁺/7-AAD^- cells within MZ B cells and IRA B cells in WT but not TLR2 KO or TLR4 KO mice. Pro-apoptotic CASP4, CASP9 and Dapk1 were significantly down-regulated in P. gingivalis LPS- and CpG-ODN-treated B cells from WT but not TLR2 KO or TLR4 KO mice. Anti-apoptotic IL-10 was significantly up-regulated in P. gingivalis LPS- and CpG-ODN-treated B cells from WT and TLR2 KO but not TLR4 KO mice. These results suggested that both TLR2 and TLR4 signaling are required for P. gingivalis LPS-induced, CpG-ODN-enhanced suppression of innate-like B cell apoptosis.

Close Encounters of Lymphoid Cells and Bacteria

During infections, the first reaction of the host against microbial pathogens is carried out by innate immune cells, which recognize conserved structures on pathogens, called pathogen-associated molecular patterns. Afterward, some of these innate cells can phagocytose and destroy the pathogens, secreting cytokines that would modulate the immune response to the challenge. This rapid response is normally followed by the adaptive immunity, more specific and essential for a complete pathogen clearance in many cases. Some innate immune cells, usually named antigen-presenting cells, such as macrophages or dendritic cells, are able to process internalized invaders and present their antigens to lymphocytes, triggering the adaptive immune response. Nevertheless, the traditional boundary of separated roles between innate and adaptive immunity has been blurred by several studies, showing that very specialized populations of lymphocytes (cells of the adaptive immunity) behave similarly to cells of the innate immunity. These “innate-like” lymphocytes include γδ T cells, invariant NKT cells, B-1 cells, mucosal-associated invariant T cells, marginal zone B cells, and innate response activator cells, and together with the newly described innate lymphoid cells are able to rapidly respond to bacterial infections. Strikingly, our recent data suggest that conventional CD4⁺ T cells, the paradigm of cells of the adaptive immunity, also present innate-like behavior, capturing bacteria in a process called transinfection. Transinfected CD4⁺ T cells digest internalized bacteria like professional phagocytes and secrete large amounts of proinflammatory cytokines, protecting for further bacterial challenges. In the present review, we will focus on the data showing such innate-like behavior of lymphocytes following bacteria encounter.

The transcriptional repressor Gfi1 prevents lupus autoimmunity by restraining TLR7 signaling

The transcriptional repressor growth factor independence 1 (Gfi1) is important in myeloid and lymphoid differentiation. In the current study we evaluated the involvement of Gfi1 in systemic lupus erythematosus (SLE). We found that Genista mice, which carry a hypomorphic mutation in the gfi1 gene or Gfi1-deficient (Gfi1^-/- ) mice develop signs of spontaneous lupus autoimmunity, including increased serum levels of IgM and IgG2a, autoantibodies against RNA and DNA, glomerular immunodeposits and increased frequencies of plasmablasts, germinal center (GC) B cells and age-associated B cells (ABCs). On the contrary, Genista mice deprived of TLR7 did not show any of these phenotypes, suggesting that the observed lupus autoimmunity in Genista mice is TLR7-dependent. Moreover, Genista mice showed an increased activation of dendritic cells (DCs), B and T cells that was dependent on TLR7 for DCs and B cells, but not for T cells. Upon TLR7 or TLR4 stimulation Genista DCs produced increased amounts of TNF, IL-6 and IFN-β and showed increased NF-κB phosphorylation and IRF7 nuclear translocation, suggesting that Gfi1 controls the NF-κB and type I IFN signaling pathway downstream of TLRs. Our data reveal that Gfi1 plays a critical role in the prevention of spontaneous lupus autoimmunity by negatively regulating TLR7 signaling.

What rheumatologists need to know about innate lymphocytes

Many rheumatic diseases are characterized by having an autoimmune background. Determining the mechanisms underlying autoimmunity is, therefore, important to further understand these diseases and to inform future lines of research aimed at developing new treatments and cures. As fast responders, innate lymphocytes have protective or pathogenic roles in the initiation as well as the maintenance of immune responses in general, and they contribute to tissue homeostasis, among other functions. Innate lymphocytes also seem to be involved in autoimmunity in particular. Since 2010, accumulating evidence clearly shows that different populations of innate lymphocytes have roles in responding to antigen-specific autoantibody and autoreactive T cells, thereby amplifying or attenuating disease processes. Cytotoxicity is a cardinal feature of many innate lymphocytes and can contribute to inflammatory tissue damage. Finally, innate lymphocytes can respond to biologic therapies for autoimmune diseases. Consequently, like TNF and other effector molecules, certain innate lymphocyte subsets might be appropriate therapeutic targets to ameliorate various autoimmune diseases. In this Review, we summarize the main characteristics and functions of innate lymphocyte subsets, and describe their roles in autoimmune disease. We also discuss how biologic therapies influence innate lymphocyte function and consider the potential for these cell subsets to act as future therapeutic targets.

B cell autophagy mediates TLR7-dependent autoimmunity and inflammation

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease, defined by loss of B cell self-tolerance that results in production of antinuclear antibodies (ANA) and chronic inflammation. While the initiating events in lupus development are not well defined, overexpression of the RNA-recognizing toll-like receptor (TLR)7 has been linked to SLE in humans and mice. We postulated that autophagy plays an essential role in TLR7 activation of B cells for the induction of SLE by delivering RNA ligands to the endosomes, where this innate immune receptor resides. To test this hypothesis, we compared SLE development in Tlr7 transgenic (Tg) mice with or without B cell-specific ablation of autophagy (Cd19-Cre Atg5(f/f)). We observed that in the absence of B cell autophagy the 2 hallmarks of SLE, ANA and inflammation, were eliminated, thus curing these mice of lupus. This was also evident in the significantly extended survival of the autophagy-deficient mice compared to Tlr7.1 Tg mice. Furthermore, glomerulonephritis was ameliorated, and the serum levels of inflammatory cytokines in the knockout (KO) mice were indistinguishable from those of control mice. These data provide direct evidence that B cells require TLR7-dependent priming through an autophagy-dependent mechanism before autoimmunity is induced, thereafter involving many cell types. Surprisingly, hyper-IgM production persisted in Tlr7.1 Tg mice in the absence of autophagy, likely involving a different activation pathway than the production of autoantibodies. Furthermore, these mice still presented with anemia, but responded with a striking increase in extramedullary hematopoiesis (EMH), possibly due to the absence of pro-inflammatory cytokines.

Regulation of B lymphocyte responses to Toll-like receptor ligand binding during diabetes prevention in non-obese diabetic (NOD) mice

BACKGROUND

Interactions between genetic risk factors and the environment drive type 1 diabetes (T1D). The system of Toll-like receptors (TLR) detects these environmental triggers; however, the target cell that intermediates these interactions to drive T1D remains unknown.

METHODS

We investigated the effect of TLR pathway activation (myeloid differentiation primary response 88 [MyD88] vs TIR-domain-containing adapter-inducing interferon-β [TRIF]) on B cell subsets via flow cytometry, including their activation, survival, proliferation, and cytoskeletal mobilization. The effect of polyinosinic-polycytidylic acid (poly(I:C)) on diabetes development was addressed, including the B cell-dependent activation of diabetes-protective DX5+ cells, using genetic models and adoptive transfer.

RESULTS

B lymphocytes from non-obese diabetic (NOD) mice expressed enhanced levels of TLR-responsive proteins. Ex vivo analysis of B lymphocyte subsets demonstrated that TLR3 stimulation via TRIF deletes cells exhibiting a marginal zone phenotype, whereas MyD88-dependent ligands enhance their survival. In vivo, marginal zone B cells were activated by poly(I:C) and were unexpectedly retained in the spleen of NOD mice, in contrast with the mobilization of these cells in non-autoimmune mice, a phenotype we traced to defective actin cytoskeletal dynamics. These activated B cells mediated TLR3-induced diabetes protection.

CONCLUSIONS

Immunotherapies must account for both B cell location and activation, and these properties may differ in autoimmune and healthy settings.

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.

Caspase-1 is required for maintenance of marginal zone B cells in pristane-induced lupus

OBJECTIVE

Caspase-1 is required for nephritis and robust autoantibody development in the pristane model of murine lupus. The objective of this study was to evaluate the immune response and to study the splenic B and T cell populations in wild-type (WT) and caspase-1-/- mice following pristane injection in order to develop an understanding of why absence of caspase-1 is protective in pristane-induced lupus.

METHODS

Immunization responses to NP-Ficoll and NP-ovalbumin were assessed in WT and caspase-1-/- mice. In vitro IgM and IgG responses to R848 were measured by ELISA. Serum IgM anti-dsDNA and IL-1β were also measured by ELISA. B and T cell populations 2 weeks and 6 months following pristane injection were measured by flow cytometry in WT and caspase-1-/- mice.

RESULTS

Caspase-1-/- mice generate equivalent IgG responses to NP-Ficoll and NP-ova antigens when compared to wild-type mice. Additionally, they secrete IgM and IgG in response to TLR7 activation. Pristane injected WT and caspase-1-/- mice generate robust IgM anti-dsDNA responses. Caspase-1-/- mice have a significant reduction in marginal zone B cell populations compared to WT 6 months after pristane exposure whereas T cell responses are intact in these mice.

CONCLUSIONS

Caspase-1-/- mice have intact immune responses but do not develop an expanded marginal zone B cell population in response to pristane-induced lupus. This may be one explanation for reduced IgG autoantibody production in these mice.

B-cell intrinsic TLR7 signals promote depletion of the marginal zone in a murine model of Wiskott-Aldrich syndrome

Patients with Wiskott-Aldrich syndrome (WAS) exhibit prominent defects in splenic marginal zone (MZ), resulting in abnormal T-cell-independent antibody responses and increased bacterial infections. B-cell-intrinsic deletion of the affected gene WAS protein (WASp) markedly reduces splenic MZ B cells, without impacting the rate of MZ B-cell development, suggesting that abnormal B-cell retention within the MZ accounts for MZ defects in WAS. Since WASp regulates integrin-dependent actin cytoskeletal rearrangement, we previously hypothesized that defective B-cell integrin function promotes MZ depletion. In contrast, we now report that B-cell-intrinsic deletion of the TLR signaling adaptor MyD88 is sufficient to restore the MZ in WAS. We further identify TLR7, an endosomal single-stranded RNA (ssRNA) receptor, as the MyD88-dependent receptor responsible for WAS MZ depletion. These findings implicate spontaneous activation of MZ B cells by ssRNA-containing self-ligands (likely derived from circulating apoptotic material) as the mechanism underlying MZ depletion in WAS. Together, these data suggest a previously unappreciated role for B-cell intrinsic TLR signals in MZ homeostasis, of relevance to both pathogen responses and to the development of systemic autoimmunity.

Function and regulation of self-reactive marginal zone B cells in autoimmune arthritis

Polyreactive innate-type B cells account for many B cells expressing self-reactivity in the periphery. Improper regulation of these B cells may be an important factor that underlies autoimmune disease. Here we have explored the influence of self-reactive innate B cells in the development of collagen-induced arthritis (CIA), a mouse model of rheumatoid arthritis. We show that splenic marginal zone (MZ), but not B-1 B cells exhibit spontaneous IgM reactivity to autologous collagen II in naı¨ve mice. Upon immunization with heterologous collagen II in complete Freund's adjuvant the collagen-reactive MZ B cells expanded rapidly, while the B-1 B cells showed a modest anti-collagen response. The MZ B cells were easily activated by toll-like receptor (TLR) 4 and 9-ligands in vitro, inducing proliferation and cytokine secretion, implying that dual engagement of the B-cell receptor and TLRs may promote the immune response to self-antigen. Furthermore, collagen-primed MZ B cells showed significant antigen-presenting capacity as reflected by cognate T-cell proliferation in vitro and induction of IgG anti-collagen antibodies in vivo. MZ B cells that were deficient in complement receptors 1 and 2 demonstrated increased proliferation and cytokine production, while Fcγ receptor IIb deficiency of the cells lead to increased cytokine production and antigen presentation. In conclusion, our data highlight self-reactive MZ B cells as initiators of the autoimmune response in CIA, where complement and Fc receptors are relevant in controlling the self-reactivity in the cells.

B cell-intrinsic TLR7 signaling is essential for the development of spontaneous germinal centers

Spontaneous germinal center (Spt-GC) B cells and follicular helper T cells generate high-affinity autoantibodies that are involved in the development of systemic lupus erythematosus. TLRs play a pivotal role in systemic lupus erythematosus pathogenesis. Although previous studies focused on the B cell-intrinsic role of TLR-MyD88 signaling on immune activation, autoantibody repertoire, and systemic inflammation, the mechanisms by which TLRs control the formation of Spt-GCs remain unclear. Using nonautoimmune C57BL/6 (B6) mice deficient in MyD88, TLR2, TLR3, TLR4, TLR7, or TLR9, we identified B cell-intrinsic TLR7 signaling as a prerequisite to Spt-GC formation without the confounding effects of autoimmune susceptibility genes and the overexpression of TLRs. TLR7 deficiency also rendered autoimmune B6.Sle1b mice unable to form Spt-GCs, leading to markedly decreased autoantibodies. Conversely, B6.yaa and B6.Sle1b.yaa mice expressing an extra copy of TLR7 and B6.Sle1b mice treated with a TLR7 agonist had increased Spt-GCs and follicular helper T cells. Further, TLR7/MyD88 deficiency led to compromised B cell proliferation and survival after B cell stimulation both in vitro and in vivo. In contrast, TLR9 inhibited Spt-GC development. Our findings demonstrate an absolute requirement for TLR7 and a negative regulatory function for TLR9 in Spt-GC formation under nonautoimmune and autoimmune conditions. Our data suggest that, under nonautoimmune conditions, Spt-GCs initiated by TLR7 produce protective Abs. However, in the presence of autoimmune susceptibility genes, TLR7-dependent Spt-GCs produce pathogenic autoantibodies. Thus, a single copy of TLR7 in B cells is the minimal requirement for breaking the GC-tolerance checkpoint.

Immunogenicity and efficacy of a plasmid DNA rabies vaccine incorporating Myd88 as a genetic adjuvant

PURPOSE

Myeloid differentiation factor 88 (Myd88), a ubiquitous Toll-like receptor adaptor molecule, has been reported to play important roles in B cell responses to infections and vaccination. The present study evaluated the effects of genetic adjuvanting with Myd88 on the immune responses to a plasmid DNA rabies vaccine.

MATERIALS AND METHODS

Plasmids encoding rabies glycoprotein alone (pIRES-Rgp) or a fragment of Myd88 gene in addition (pIRES-Rgp-Myd) were constructed and administered intramuscularly or intrademally in Swiss albino mice (on days 0, 7, and 21). Rabies virus neutralizing antibody (RVNA) titres were estimated in the mice sera on days 14 and 28 by rapid fluorescent focus inhibition test. The protective efficacy of the constructs was evaluated by an intracerebral challenge with challenge virus standard virus on day 35.

RESULTS

Co-expression of Myd88 increased RVNA responses to pIRES-Rgp by 3- and 2-folds, following intramuscular and intradermal immunization, respectively. pIRES-Rgp protected 80% of the mice following intramuscular and intradermal immunizations, while pIRES-Rgp-Myd afforded 100% protection following similar administrations.

CONCLUSION

Genetic adjuvanting with Myd88 enhanced the RVNA responses and protective efficacy of a plasmid DNA rabies vaccine. This strategy might be useful for rabies vaccination of canines in the field, and needs further evaluation.

Hormonal milieu at time of B cell activation controls duration of autoantibody response

A strong gender bias is seen in many autoimmune diseases including systemic lupus erythematosus (SLE). To investigate the basis for the female preponderance in SLE, we have been studying BALB/c mice in which B cells express the R4A heavy chain of an anti-DNA antibody in association with an endogenous light chain repertoire (R4Atg mice). In unmanipulated mice, approximately 5% of B cells express the R4A transgene. R4Atg mice do not spontaneously develop elevated serum titers of anti-DNA antibodies. Administration of either estradiol (E2) or prolactin (Pr) results in escape from tolerance of autoreactive B cells, expressed as an increase in transgene-expressing B cells and elevated serum titers of anti-DNA antibodies. We previously demonstrated that autoreactive B cells maturing in an estrogenic milieu develop as marginal zone (MZ) B cells; when these same B cells mature in the presence of increased prolactin, they develop as follicular (Fo) B cells. To determine the long term consequence of this differential maturation of DNA-reactive B cells, we treated R4Atg BALB/c mice with E2 or Pr for 6 weeks until serum titers of anti-DNA antibody were high, at which time hormonal exposure was discontinued. In E2-treated mice, the anti-DNA titers remained high even 3 months after discontinuation of hormone exposure. Nascent B cells underwent normal tolerance induction, but existing autoreactive MZ B cells persisted and continued to secrete autoantibody. In contrast, Pr caused only a short-term increase in anti-DNA antibody titers. By 3 months after cessation of hormone treatment, serum anti-DNA antibody titers and B cell subsets were indistinguishable from those in placebo (P) treated mice. These findings suggest that autoantibody responses are sustained for variable lengths of time depending on the B cell subset producing the autoantibodies. This observation may be relevant to understanding the heterogeneous presentation of patients with SLE and to the design of therapies targeting specific B-cell populations in autoimmune disease.

Overexpression of TLR7 promotes cell-intrinsic expansion and autoantibody production by transitional T1 B cells

Transgenic expression of TLR7 results in the expansion and hyperactivation of T1 B cells in response to endogenous RNA complexes, leading to increased autoantibody production.

Toll-like receptor (TLR), a ligand for single-stranded RNA, has been implicated in the development of pathogenic anti-RNA autoantibodies both in systemic lupus erythematous (SLE) patients and in murine models of lupus. It is still unclear, however, where and how TLR7-mediated interactions affect the development of autoreactive B cells. We found that overexpression of TLR7 in transgenic mice (TLR7.1Tg) leads to marked alterations of transitional (T1) B cells, associated with their expansion and proliferation within the splenic red pulp (RP). This phenotype was intrinsic to the T1 subset of B cells and occurred independently of type 1 IFN signals. Overexpression of RNase in TLR7.1Tg mice significantly limited the expansion and proliferation of T1 cells, indicating that endogenous RNA complexes are driving their activation. TLR7.1Tg T1 cells were hyper-responsive to anti-IgM and TLR7 ligand stimulation in vitro and produced high concentrations of class-switched IgG2b and IgG2c, including anti-RNA antibodies. Our results demonstrate that initial TLR7 stimulation of B cells occurs at the T1 stage of differentiation in the splenic RP and suggest that dysregulation of TLR7 expression in T1 cells can result in production of autoantibodies.

The TACI receptor regulates T-cell-independent marginal zone B cell responses through innate activation-induced cell death

Activation-induced cell death (AICD) plays a critical role in immune homeostasis and tolerance. In T-cell-dependent humoral responses, AICD of B cells is initiated by Fas ligand (FasL) on T cells, stimulating the Fas receptor on B cells. In contrast, T-cell-independent B cell responses involve innate-type B lymphocytes, such as marginal zone (MZ) B cells, and little is known about the mechanisms that control AICD during innate B cell responses to Toll-like receptor (TLR) activation. Here, we show that MZ B cells undergo AICD in response to TLR4 activation in vivo. The transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI) receptor and TLR4 cooperate to upregulate expression of both FasL and Fas on MZ B cells and also to repress inhibitors of Fas-induced apoptosis signaling. These findings demonstrate an unappreciated role for TACI and its ligands in the regulation of AICD during T-cell-independent B cell responses.

n-3 PUFAs enhance the frequency of murine B-cell subsets and restore the impairment of antibody production to a T-independent antigen in obesity

The role of n-3 polyunsaturated fatty acids (PUFA) on in vivo B-cell immunity is unknown. We first investigated how n-3 PUFAs impacted in vivo B-cell phenotypes and antibody production in the absence and presence of antigen compared with a control diet. Lean mice consuming n-3 PUFAs for 4 weeks displayed increased percentage and frequency of splenic transitional 1 B cells. Upon stimulation with trinitrophenylated-lipopolysaccharide, n-3 PUFAs increased the number of splenic transitional 1/2, follicular, premarginal, and marginal zone B cells. n-3 PUFAs also increased surface, but not circulating, IgM. We next tested the effects of n-3 PUFAs in a model of obesity that is associated with suppressed humoral immunity. An obesogenic diet after ten weeks of feeding, relative to a lean control, had no effect on the frequency of B cells but lowered circulating IgM upon antigen stimulation. Administration of n-3 PUFAs to lean and obese mice increased the percentage and/or frequency of transitional 1 and marginal zone B cells. Furthermore, n-3 PUFAs in lean and obese mice increased circulating IgM relative to controls. Altogether, the data show n-3 PUFAs enhance B cell-mediated immunity in vivo, which has implications for immunocompromised populations, such as the obese.

Cannabinoid Receptor 2 (CB2) Plays a Role in the Generation of Germinal Center and Memory B Cells, but Not in the Production of Antigen-Specific IgG and IgM, in Response to T-dependent Antigens

The cannabinoid receptor 2 (CB2) has been reported to modulate B cell functions including migration, proliferation and isotype class switching. Since these processes are required for the generation of the germinal center (GC) and antigen-specific plasma and memory cells following immunization with a T-dependent antigen, CB2 has the capacity to alter the quality and magnitude of T-dependent immune responses. To address this question, we immunized WT and CB2^−/− mice with the T-dependent antigen 4-hydroxy-3-nitrophenylacetyl (NP)-chicken-gamma-globulin (CGG) and measured GC B cell formation and the generation of antigen-specific B cells and serum immunoglobulin (Ig). While there was a significant reduction in the number of splenic GC B cells in CB2^−/− mice early in the response there was no detectable difference in the number of NP-specific IgM and IgG₁ plasma cells. There was also no difference in NP-specific IgM and class switched IgG₁ in the serum. In addition, we found no defect in the homing of plasma cells to the bone marrow (BM) and affinity maturation, although memory B cell cells in the spleen were reduced in CB2^−/− mice. CB2-deficient mice also generated similar levels of antigen-specific IgM and IgG in the serum as WT following immunization with sheep red blood cells (sRBC). This study demonstrates that although CB2 plays a role in promoting GC and memory B cell formation/maintenance in the spleen, it is dispensable on all immune cell types required for the generation of antigen-specific IgM and IgG in T-dependent immune responses.

Division of labor during primary humoral immunity

B lymphocytes are often considered a homogenous population. However, B cells in both mouse and humans are comprised of distinct subpopulations that differ in development, phenotype, function, and microenvironmental niches. Much of our understanding about how these different B-cells populations mount antibody responses has been derived from experimental findings in mouse models and based on the use of model antigens. These reductionist studies performed over decades have been invaluable in defining the parameters of the B-cell antibody response to different types of antigens. However, these antigens also are now known to differ in a significant manner from bona fide physiological pathogens, and precisely how these different B-cell subsets divide labor in the primary humoral immune defense of pathogens is less well understood. While there are no absolutes in this area, there are recurring themes that divide the roles of B-cell subsets to different arms of the antibody response. This review provides an overview of rules that govern the B-cell labor roles, exceptions that break these rules, and models that have been used to define them.

Marginal zone B cells: virtues of innate-like antibody-producing lymphocytes

Protective responses to microorganisms involve the nonspecific but rapid defence mechanisms of the innate immune system, followed by the specific but slow defence mechanisms of the adaptive immune system. Located as sentinels at the interface between the circulation and lymphoid tissue, splenic marginal zone B cells rapidly respond to blood-borne antigens by adopting 'crossover' defensive strategies that blur the conventional boundaries of innate and adaptive immunity. This Review discusses how marginal zone B cells function as innate-like lymphocytes that mount rapid antibody responses to both T cell-dependent and T cell-independent antigens. These responses require the integration of activation signals from germline-encoded and somatically recombined receptors for microorganisms with helper signals from effector cells of the innate and adaptive immune systems.

FCRL5 exerts binary and compartment-specific influence on innate-like B-cell receptor signaling

Innate-like splenic marginal zone (MZ) and peritoneal cavity B1 B lymphocytes share critical responsibilities in humoral responses but have divergent B-cell receptor (BCR) signaling features. A discrete marker of these subsets with tyrosine-based dual regulatory potential termed "Fc receptor-like 5" (FCRL5) was investigated to explore this discrepancy. Although FCRL5 repressed the robust BCR activity that is characteristic of MZ B cells, it had no influence on antigen receptor stimulation that is blunted in peritoneal cavity-derived B1 B cells. The molecular basis for the receptor's inhibitory function derived from recruitment of the Src homology-2 domain-containing tyrosine phosphatase 1 (SHP-1) to a cytoplasmic immunoreceptor tyrosine-based inhibitory motif. Surprisingly, mutagenesis of this docking site unearthed coactivation properties for FCRL5 that were orchestrated by independent association of the Lyn Src-family kinase with an intracellular immunoreceptor tyrosine-based activation motif-like sequence. FCRL5's unique binary regulation directly correlated with SHP-1 and Lyn activity, which, like BCR function, differed between MZ and B1 B cells. These findings collectively imply a specialized counterregulatory role for FCRL molecules at the intersection of innate and adaptive immunity.

Mouse marginal zone B cells harbor specificities similar to human broadly neutralizing HIV antibodies

A series of potent, broadly neutralizing HIV antibodies have been isolated from B cells of HIV-infected individuals. VRC01 represents a subset of these antibodies that mediate neutralization with a restricted set of IGHV genes. The memory B cells expressing these antibodies were isolated years after infection; thus, the B-cell subpopulation from which they originated and the extent of participation in the initial HIV antibody response, if any, are unclear. Here we evaluated the frequency of anti-gp120 B cells in follicular (FO) and marginal zone (MZ) B-cell compartments of naïve WT mice and comparable human populations in uninfected individuals. We found that in non-HIV-exposed humans and mice, the majority of gp120-reactive B cells are of naïve and FO phenotype, respectively. Murine FO B cells express a diverse antibody repertoire to recognize gp120. In contrast, mouse MZ B cells recognize gp120 less frequently but preferentially use IGHV1-53 to encode gp120-specific antibodies. Notably, IGHV1-53 shows high identity to human IGHV1-2*02, which has been repeatedly found to encode broadly neutralizing mutated HIV antibodies, such as VRC01. Finally, we show that human MZ-like B cells express IGHV1-2*02, and that IGHV1-53 expression is enriched in mouse MZ B cells. These data suggest that efforts toward developing an HIV vaccine might consider eliciting protective HIV antibody responses selectively from alternative B-cell populations harboring IGHV gene segments capable of producing protective antibodies.

Sex bias in susceptibility to MCMV infection: implication of TLR9

Toll-like receptor (TLR)-dependent pathways control the activation of various immune cells and the production of cytokines and chemokines that are important in innate immune control of viruses, including mouse cytomegalovirus (MCMV). Here we report that upon MCMV infection wild-type and TLR7^−/− male mice were more resistant than their female counterparts, while TLR9^−/− male and female mice showed similar susceptibility. Interestingly, 36 h upon MCMV infection TLR9 mRNA expression was higher in male than in female mouse spleens. MCMV infection led to stronger reduction of marginal zone (MZ) B cells, and higher infiltration of plasmacytoid dendritic cells and neutrophils in wild-type male than female mice, while no such sex differences were observed in TLR9^−/− mice. In accordance, the serum levels of KC and MIP-2, major neutrophil chemoattractants, were higher in wild-type, but not in TLR9^−/−, male versus female mice. Wild-type MCMV-infected female mice showed more severe liver inflammation, necrosis and steatosis compared to infected male mice. Our data demonstrate sex differences in susceptibility to MCMV infection, accompanied by a lower activation of the innate immune system in female mice, and can be attributed, at least in a certain degree, to the lower expression of TLR9 in female than male mice.

Selective inhibitors of phosphoinositide 3-kinase delta: modulators of B-cell function with potential for treating autoimmune inflammatory diseases and B-cell malignancies

The delta isoform of the p110 catalytic subunit (p110δ) of phosphoinositide 3-kinase is expressed primarily in hematopoietic cells and plays an essential role in B-cell development and function. Studies employing mice lacking a functional p110δ protein, as well as the use of highly-selective chemical inhibitors of p110δ, have revealed that signaling via p110δ-containing PI3K complexes (PI3Kδ) is critical for B-cell survival, migration, and activation, functioning downstream of key receptors on B cells including the B-cell antigen receptor, chemokine receptors, pro-survival receptors such as BAFF-R and the IL-4 receptor, and co-stimulatory receptors such as CD40 and Toll-like receptors (TLRs). Similarly, this PI3K isoform plays a key role in the survival, proliferation, and dissemination of B-cell lymphomas. Herein we summarize studies showing that these processes can be inhibited in vitro and in vivo by small molecule inhibitors of p110δ enzymatic activity, and that these p110δ inhibitors have shown efficacy in clinical trials for the treatment of several types of B-cell malignancies including chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma (NHL). PI3Kδ also plays a critical role in the activation, proliferation, and tissue homing of self-reactive B cells that contribute to autoimmune diseases, in particular innate-like B-cell populations such as marginal zone (MZ) B cells and B-1 cells that have been strongly linked to autoimmunity. We discuss the potential utility of p110δ inhibitors, either alone or in combination with B-cell depletion, for treating autoimmune diseases such as lupus, rheumatoid arthritis, and type 1 diabetes. Because PI3Kδ plays a major role in both B-cell-mediated autoimmune inflammation and B-cell malignancies, PI3Kδ inhibitors may represent a promising therapeutic approach for treating these diseases.

Integration of B cell responses through Toll-like receptors and antigen receptors

Unlike other immune cells, B cells express both an antigen-specific B cell receptor (BCR) and Toll-like receptors (TLRs). Dual BCR and TLR engagement can fine-tune functional B cell responses, directly linking cell-intrinsic innate and adaptive immune programmes. Although most data regarding B cell-specific functions of the TLR signalling pathway have been obtained in mice, the discovery of patients with a deficiency in this pathway has recently provided an insight into human B cell responses. Here, we highlight the importance of the integration of signalling pathways downstream of BCRs and TLRs in modulating B cell function, focusing when possible on B cell-intrinsic roles.