The Fat Controller - an unexpected role of SELENOI in IgM responses

Discussion on the role of SELENOI-produced lipids in peripheral B cell differentiation

AKT activity orchestrates marginal zone B cell development in mice and humans

The signals controlling marginal zone (MZ) and follicular (FO) B cell development remain incompletely understood. Here, we show that AKT orchestrates MZ B cell formation in mice and humans. Genetic models that increase AKT signaling in B cells or abolish its impact on FoxO transcription factors highlight the AKT-FoxO axis as an on-off switch for MZ B cell formation in mice. In humans, splenic immunoglobulin (Ig) D+CD27+ B cells, proposed as an MZ B cell equivalent, display higher AKT signaling than naive IgD+CD27- and memory IgD-CD27+ B cells and develop in an AKT-dependent manner from their precursors in vitro, underlining the conservation of this developmental pathway. Consistently, CD148 is identified as a receptor indicative of the level of AKT signaling in B cells, expressed at a higher level in MZ B cells than FO B cells in mice as well as humans.

Expression of Homing Receptors in IgM+IgD+CD27+ B Cells and Their Frequencies in Appendectomized and/or Tonsillectomized Individuals

BACKGROUND

In humans, blood circulating IgM⁺IgD⁺CD27⁺ B cells are considered analogous to those described in the marginal zone of the spleen and are involved in important immunological processes. The homing receptors they express, and the organs involved in their development (for example, intestinal organs in rabbits) are only partially known. We recently reported that this population is heterogeneous and composed of at least two subsets: one expressing high levels of IgM - IgM^hi B cells - and another low levels - IgM^lo B cells.

OBJECTIVES

To evaluate the expression of homing receptors on IgD⁺CD27⁺ IgM^hi and IgM^lo B cells and quantify their frequencies in blood of control and appendectomized and/or tonsillectomized volunteers.

MATERIALS AND METHODS

Using spectral flow cytometry, the simultaneous expression of 12 previously reported markers that differentiate IgM^hi B cells and IgM^lo B cells and of α4β7, CCR9, CD22 and CCR10 were evaluated in blood circulating B cells of control and appendectomized and/or tonsillectomized volunteers.

RESULTS

The existence of phenotypically defined IgM^lo and IgM^hi B cell subsets was confirmed. They differentially expressed intestinal homing receptors, and the expression of α4β7 and CCR9 seems to determine new IgM subpopulations. IgM^lo and IgM^hi B cells were detected at lower frequencies in the appendectomized and/or tonsillectomized volunteers relative to controls.

CONCLUSIONS

Human blood circulating IgD⁺CD27⁺ IgM^lo and IgM^hi B cell subsets differentially express homing receptors, and it is necessary to investigate if mucosal organs are important in their development.

Current understanding of the immune potential of B-cell subsets in malarial pathogenesis

In the past several decades, our understanding of how B cells are generated and what function they perform has continued to advance. It is widely accepted that B-cell subsets play a critical role in mediating immune response. Surprisingly, human and murine malarial infections cause major alterations in the composition of B-cell subsets in both the spleen and periphery. Multiple B-cell subsets are well characterized in murine models following primary and secondary infection, although in human malarial infection, these subsets are not well defined. Furthermore, a rare known function of B cells includes the potential role of regulating the activities of other cells in the body as regulatory cells. Plasmodium infection strongly alters the frequency of these regulatory B cells indicating the immunoregulatory function of B cells in malarial. It is important to note that these subsets, taken together, form the cellular basis of humoral immune responses, allowing protection against a wide array of Plasmodium antigens to be achieved. However, it remains a challenge and an important area of investigation to understand how these B-cell subsets work together to provide protection against Plasmodium infection.

Epstein-Barr virus infection, B-cell dysfunction and other risk factors converge in gut-associated lymphoid tissue to drive the immunopathogenesis of multiple sclerosis: a hypothesis

Multiple sclerosis is associated with Epstein-Barr virus (EBV) infection, B-cell dysfunction, gut dysbiosis, and environmental and genetic risk factors, including female sex. A disease model incorporating all these factors remains elusive. Here, we hypothesise that EBV-infected memory B cells (MBCs) migrate to gut-associated lymphoid tissue (GALT) through EBV-induced expression of LPAM-1, where they are subsequently activated by gut microbes and/or their products resulting in EBV reactivation and compartmentalised anti-EBV immune responses. These responses involve marginal zone (MZ) B cells that activate CD4⁺ T-cell responses, via HLA-DRB1, which promote downstream B-cell differentiation towards CD11c⁺/T-bet⁺ MBCs, as well as conventional MBCs. Intrinsic expression of low-affinity B-cell receptors (BCRs) by MZ B cells and CD11c⁺/T-bet⁺ MBCs promotes polyreactive BCR/antibody responses against EBV proteins (e.g. EBNA-1) that cross-react with central nervous system (CNS) autoantigens (e.g. GlialCAM). EBV protein/autoantigen-specific CD11c⁺/T-bet⁺ MBCs migrate to the meningeal immune system and CNS, facilitated by their expression of CXCR3, and induce cytotoxic CD8⁺ T-cell responses against CNS autoantigens amplified by BAFF, released from EBV-infected MBCs. An increased abundance of circulating IgA⁺ MBCs, observed in MS patients, might also reflect GALT-derived immune responses, including disease-enhancing IgA antibody responses against EBV and gut microbiota-specific regulatory IgA⁺ plasma cells. Female sex increases MZ B-cell and CD11c⁺/T-bet⁺ MBC activity while environmental risk factors affect gut dysbiosis. Thus, EBV infection, B-cell dysfunction and other risk factors converge in GALT to generate aberrant B-cell responses that drive pathogenic T-cell responses in the CNS.

Receptor Interacting Protein Kinase Pathways Regulate Innate B Cell Developmental Checkpoints But Not Effector Function in Mice

Mutations in the scaffolding domain of Receptor Interacting Protein kinases (RIP) underlie the recently described human autoimmune syndrome, CRIA, characterized by lymphadenopathy, splenomegaly, and autoantibody production. While disease mechanisms for CRIA remain undescribed, RIP kinases work together with caspase-8 to regulate cell death, which is critical for normal differentiation of many cell types. Here, we describe a key role for RIP1 in facilitating innate B cell differentiation and subsequent activation. By comparing RIP1, RIP3, and caspase-8 triple deficient and RIP3, caspase-8 double deficient mice, we identified selective contributions of RIP1 to an accumulation of murine splenic Marginal Zone (MZ) B cells and B1-b cells. We used mixed bone-marrow chimeras to determine that innate B cell commitment required B cell-intrinsic RIP1, RIP3, and caspase-8 sufficiency. RIP1 regulated MZ B cell development rather than differentiation and RIP1 mediates its innate immune effects independent of the RIP1 kinase domain. NP-KLH/alum and NP-Ficoll vaccination of mice doubly deficient in both caspase-8 and RIP3 or deficient in all three proteins (RIP3, caspase-8, and RIP1) revealed uniquely delayed T-dependent and T-independent IgG responses, abnormal splenic germinal center architecture, and reduced extrafollicular plasmablast formation compared to WT mice. Thus, RIP kinases and caspase-8 jointly orchestrate B cell fate and delayed effector function through a B cell-intrinsic mechanism.

Kindlin-3 maintains marginal zone B cells but confines follicular B cell activation and differentiation

Integrin-mediated interactions between hematopoietic cells and their microenvironment are important for the development and function of immune cells. Here, the role of the integrin adaptor Kindlin-3 in B cell homeostasis is studied. Comparing the individual steps of B cell development in B cell-specific Kindlin-3 or alpha4 integrin knockout mice, we found in both conditions a phenotype of reduced late immature, mature, and recirculating B cells in the bone marrow. In the spleen, constitutive B cell-specific Kindlin-3 knockout caused a loss of marginal zone B cells and an unexpected expansion of follicular B cells. Alpha4 integrin deficiency did not induce this phenotype. In Kindlin-3 knockout B cells VLA-4 as well as LFA-1-mediated adhesion was abrogated, and short-term homing of these cells in vivo was redirected to the spleen. Upon inducible Kindlin-3 knockout, marginal zone B cells were lost due to defective retention within 2 weeks, while follicular B cell numbers were unaltered. Kindlin-3 deficient follicular B cells displayed higher IgD, CD40, CD44, CXCR5, and EBI2 levels, and elevated PI3K signaling upon CXCR5 stimulation. They also showed transcriptional signatures of spontaneous follicular B cell activation. This activation manifested in scattered germinal centers in situ, early plasmablasts differentiation, and signs of IgG class switch.

LncRNA00492 is required for marginal zone B-cell development

B-cell development undergoes a series of steps from the bone marrow to the secondary lymphoid organs. A defect in B-cell development can lead to immunodeficiency or malignant disorders, such as leukaemia or lymphoma. Long non-coding RNAs have been reported to act as important regulators of many pathological processes. However, very little is known regarding the role of lncRNAs during B-cell development and the regulation of their expression. In this study, we explored the expression and role of lncRNA Gme00492 in B-cell development. We observed that lnc00492 was highly expressed in B-cell development and primarily expressed in the nucleus. Lnc00492-deficient mice had fewer marginal zone B cells in the spleen, likely due to a developmental block. Importantly, lnc00492 interacts with CTBP1 and targets it for ubiquitination and degradation during B-cell development, whereas the transcriptional corepressor factor CTBP1 plays a critical role in Notch2 signalling. Thus, we identified a novel regulatory axis between lnc00492 and CTBP1 in B cells, suggesting that lnc00492 is essential for marginal zone B-cell development.

Homeostatic apoptosis prevents competition-induced atrophy in follicular B cells

While the intrinsic apoptosis pathway is thought to play a central role in shaping the B cell lineage, its precise role in mature B cell homeostasis remains elusive. Using mice in which mature B cells are unable to undergo apoptotic cell death, we show that apoptosis constrains follicular B (FoB) cell lifespan but plays no role in marginal zone B (MZB) cell homeostasis. In these mice, FoB cells accumulate abnormally. This intensifies intercellular competition for BAFF, resulting in a contraction of the MZB cell compartment, and reducing the growth, trafficking, and fitness of FoB cells. Diminished BAFF signaling dampens the non-canonical NF-κB pathway, undermining FoB cell growth despite the concurrent triggering of a protective p53 response. Thus, MZB and FoB cells exhibit a differential requirement for the intrinsic apoptosis pathway. Homeostatic apoptosis constrains the size of the FoB cell compartment, thereby preventing competition-induced FoB cell atrophy.

Upregulated Chemokine and Rho-GTPase Genes Define Immune Cell Emigration into Salivary Glands of Sjögren's Syndrome-Susceptible C57BL/6.NOD-Aec1Aec2 Mice

The C57BL/6.NOD-Aec1Aec2 mouse is considered a highly appropriate model of Sjögren's Syndrome (SS), a human systemic autoimmune disease characterized primarily as the loss of lacrimal and salivary gland functions. This mouse model, as well as other mouse models of SS, have shown that B lymphocytes are essential for the development and onset of observed clinical manifestations. More recently, studies carried out in the C57BL/6.IL14α transgenic mouse have indicated that the marginal zone B (MZB) cell population is responsible for development of SS disease, reflecting recent observations that MZB cells are present in the salivary glands of SS patients and most likely initiate the subsequent loss of exocrine functions. Although MZB cells are difficult to study in vivo and in vitro, we have carried out an ex vivo investigation that uses temporal global RNA transcriptomic analyses to profile differentially expressed genes known to be associated with cell migration. Results indicate a temporal upregulation of specific chemokine, chemokine receptor, and Rho-GTPase genes in the salivary glands of C57BL/6.NOD-Aec1Aec2 mice that correlate with the early appearance of periductal lymphocyte infiltrations. Using the power of transcriptomic analyses to better define the genetic profile of lymphocytic emigration into the salivary glands of SS mice, new insights into the underlying mechanisms of SS disease development and onset begin to come into focus, thereby establishing a foundation for further in-depth and novel investigations of the covert and early overt phases of SS disease at the cellular level.

Fab-dimerized glycan-reactive antibodies are a structural category of natural antibodies

Natural antibodies (Abs) can target host glycans on the surface of pathogens. We studied the evolution of glycan-reactive B cells of rhesus macaques and humans using glycosylated HIV-1 envelope (Env) as a model antigen. 2G12 is a broadly neutralizing Ab (bnAb) that targets a conserved glycan patch on Env of geographically diverse HIV-1 strains using a unique heavy-chain (VH) domain-swapped architecture that results in fragment antigen-binding (Fab) dimerization. Here, we describe HIV-1 Env Fab-dimerized glycan (FDG)-reactive bnAbs without VH-swapped domains from simian-human immunodeficiency virus (SHIV)-infected macaques. FDG Abs also recognized cell-surface glycans on diverse pathogens, including yeast and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike. FDG precursors were expanded by glycan-bearing immunogens in macaques and were abundant in HIV-1-naive humans. Moreover, FDG precursors were predominately mutated IgM+IgD+CD27+, thus suggesting that they originated from a pool of antigen-experienced IgM+ or marginal zone B cells.

Invariant natural killer T cells balance B cell immunity

Invariant natural killer T (iNKT) cells mediate rapid immune responses which bridge the gap between innate and adaptive responses to pathogens while also providing key regulation to maintain immune homeostasis. Both types of important iNKT immune responses are mediated through interactions with innate and adaptive B cells. As such, iNKT cells sit at the decision-making fulcrum between regulating inflammatory or autoreactive B cells and supporting protective or regulatory B cell populations. iNKT cells interpret the signals in their environment to set the tone for subsequent adaptive responses, with outcomes ranging from getting licensed to maintain homeostasis as an iNKT regulatory cell (iNKT_reg ) or being activated to become an iNKT follicular helper (iNKT_FH ) cell supporting pathogen-specific effector B cells. Here we review iNKT and B cell cooperation across the spectrum of immune outcomes, including during allergy and autoimmune disease, tumor surveillance and immunotherapy, or pathogen defense and vaccine responses. Because of their key role as influencers, iNKT cells provide a valuable target for therapeutic interventions. Understanding the nature of the interactions between iNKT and B cells will enable the development of clinical interventions to strategically target regulatory iNKT and B cell populations or inflammatory ones, depending on the circumstance.

GPR43 regulates marginal zone B-cell responses to foreign and endogenous antigens

Marginal zone (MZ) B cells are innate‐like B cells that produce polyreactive antibodies with an affinity for microbial molecular patterns and carbohydrate ligands. MZ B cells have been shown to be important in mediating immunity to various bacteria including Streptococcus pneumoniae and are also implicated in inflammatory syndromes including lupus erythematosus. The intestinal microbiota is responsible for producing short‐chain fatty acids, which can regulate immune cell function by several mechanisms including ligation of the G‐protein‐coupled receptor (GPR)43. Herein, we show that MZ B cells express Gpr43 messenger RNA and that the absence of this receptor impacts on MZ B‐cell surface marker expression and antibody production. In T‐cell‐independent responses to the hapten 4‐hydroxy‐3‐nitrophenylacetic acid (NP), mice deficient in GPR43 displayed higher serum titers of NP‐specific antibodies. Moreover, in response to a pneumococcal polysaccharide vaccine, GPR43‐deficient mice developed robust serum antibody responses and had markedly increased numbers of splenic antibody‐secreting cells, compared with control mice. Finally, serum immunoglobulin M autoantibodies to double‐stranded DNA and phosphatidylcholine were increased in resting 10–15‐week‐old mice lacking GPR43. Taken together, mice lacking GPR43 have heightened antibody responses to T‐cell‐independent antigens, which may be a result of impaired regulation of MZ B cells.

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.

Marginal zone SIGN-R1+ macrophages are essential for the maturation of germinal center B cells in the spleen

Germinal centers (GCs) are critical for the generation of memory B cells and high-affinity antibody-producing cells. This process is important for protection from acute infections and for establishment of long-lasting immunity in response to vaccination. The microanatomic organization of distinct niches within lymphoid tissues is fundamental for GC responses, as it provides the basis for coordinated interactions between rare antigen-specific cells and antigen-presenting cells. Here we reveal a role for a specialized resident macrophage subset in maintaining positional regulation of a key antigen-presenting cell type within functional niches in the spleen. Our study demonstrates the importance of this regulation to humoral immunity.

The mechanisms that regulate germinal center (GC) B cell responses in the spleen are not fully understood. Here we use a combination of pharmacologic and genetic approaches to delete SIGN-R1⁺ marginal zone (MZ) macrophages and reveal their specific contribution to the regulation of humoral immunity in the spleen. We find that while SIGN-R1⁺ macrophages were not essential for initial activation of B cells, they were required for maturation of the response and development of GC B cells. These defects could be corrected when follicular helper T (Tfh) cells were induced before macrophage ablation or when Tfh responses were enhanced. Moreover, we show that in the absence of SIGN-R1⁺ macrophages, DCIR2⁺ dendritic cells (DCs), which play a key role in priming Tfh responses, were unable to cluster to the interfollicular regions of the spleen and were instead displaced to the MZ. Restoring SIGN-R1⁺ macrophages to the spleen corrected positioning of DCIR2⁺ DCs and rescued the GC B cell response. Our study reveals a previously unappreciated role for SIGN-R1⁺ macrophages in regulation of the GC reaction and highlights the functional specification of macrophage subsets in the MZ compartment.

Differential Expression of IgM and IgD Discriminates Two Subpopulations of Human Circulating IgM+IgD+CD27+ B Cells That Differ Phenotypically, Functionally, and Genetically

The origin and function of blood IgM⁺IgD⁺CD27⁺ B cells is controversial, and they are considered a heterogeneous population. Previous staining of circulating B cells of healthy donors with rotavirus fluorescent virus-like particles allowed us to differentiate two subsets of IgM⁺IgD⁺CD27⁺: IgM^hi and IgM^lo B cells. Here, we confirmed this finding and compared the phenotype, transcriptome, in vitro function, and Ig gene repertoire of these two subsets. Eleven markers phenotypically discriminated both subsets (CD1c, CD69, IL21R, CD27, MTG, CD45RB, CD5, CD184, CD23, BAFFR, and CD38) with the IgM^hi phenotypically resembling previously reported marginal zone B cells and the IgM^lo resembling both naïve and memory B cells. Transcriptomic analysis showed that both subpopulations clustered close to germinal center-experienced IgM only B cells with a Principal Component Analysis, but differed in expression of 78 genes. Moreover, IgM^hi B cells expressed genes characteristic of previously reported marginal zone B cells. After stimulation with CpG and cytokines, significantly (p < 0.05) higher frequencies (62.5%) of IgM^hi B cells proliferated, compared with IgM^lo B cells (35.37%), and differentiated to antibody secreting cells (14.22% for IgM^hi and 7.19% for IgM^lo). IgM^hi B cells had significantly (p < 0.0007) higher frequencies of mutations in IGHV and IGKV regions, IgM^lo B cells had higher usage of IGHJ6 genes (p < 0.0001), and both subsets differed in their HCDR3 properties. IgM^hi B cells shared most of their shared IGH clonotypes with IgM only memory B cells, and IgM^lo B cells with IgM^hi B cells. These results support the notion that differential expression of IgM and IgD discriminates two subpopulations of human circulating IgM⁺IgD⁺CD27⁺ B cells, with the IgM^hi B cells having similarities with previously described marginal zone B cells that passed through germinal centers, and the IgM^lo B cells being the least differentiated amongst the IgM⁺CD27⁺ subsets.

Essential requirement for nicastrin in marginal zone and B-1 B cell development

γ-secretase is an intramembrane protease complex that catalyzes the proteolytic cleavage of amyloid precursor protein and Notch. Impaired γ-secretase function is associated with the development of Alzheimer's disease and familial acne inversa in humans. In a forward genetic screen of mice with N-ethyl-N-nitrosourea-induced mutations for defects in adaptive immunity, we identified animals within a single pedigree exhibiting both hypopigmentation of the fur and diminished T cell-independent (TI) antibody responses. The causative mutation was in Ncstn, an essential gene encoding the protein nicastrin (NCSTN), a member of the γ-secretase complex that functions to recruit substrates for proteolysis. The missense mutation severely limits the glycosylation of NCSTN to its mature form and impairs the integrity of the γ-secretase complex as well as its catalytic activity toward its substrate Notch, a critical regulator of B cell and T cell development. Strikingly, however, this missense mutation affects B cell development but not thymocyte or T cell development. The Ncstn allele uncovered in these studies reveals an essential requirement for NCSTN during the type 2 transitional-marginal zone precursor stage and peritoneal B-1 B cell development, the TI antibody response, fur pigmentation, and intestinal homeostasis in mice.

IL-10-Dependent Crosstalk between Murine Marginal Zone B Cells, Macrophages, and CD8α+ Dendritic Cells Promotes Listeria monocytogenes Infection

Type 1 CD8α⁺ conventional dendritic cells (cDC1s) are required for CD8⁺ T cell priming but, paradoxically, promote splenic Listeria monocytogenes infection. Using mice with impaired cDC2 function, we ruled out a role for cDC2s in this process and instead discovered an interleukin-10 (IL-10)-dependent cellular crosstalk in the marginal zone (MZ) that promoted bacterial infection. Mice lacking the guanine nucleotide exchange factor DOCK8 or CD19 lost IL-10-producing MZ B cells and were resistant to Listeria. IL-10 increased intracellular Listeria in cDC1s indirectly by reducing inducible nitric oxide synthase expression early after infection and increasing intracellular Listeria in MZ metallophilic macrophages (MMMs). These MMMs trans-infected cDC1s, which, in turn, transported Listeria into the white pulp to prime CD8⁺ T cells. However, this also facilitated bacterial expansion. Therefore, IL-10-mediated crosstalk between B cells, macrophages, and cDC1s in the MZ promotes both Listeria infection and CD8⁺ T cell activation.

Dysregulated Marginal Zone B Cell Compartment in a Mouse Model of Sjögren's Syndrome with Ocular Inflammation

The risk of developing lymphoma in patients with Sjögren’s syndrome (SS) is 44 times higher than in the normal population with the most common lymphomas derived from marginal zone B (MZB) cells. Current understanding of the role of MZB cells in SS is primarily based on salivary gland pathology, while their contextual association with lacrimal glands and ocular manifestations largely remains unknown. We examined this possibility using a SS mouse model (thrombospondin-1 deficient (TSP1^−/−)) with well-characterized ocular disease. We determined the frequency, localization, and cytokine profiles of MZB cells and their association with an antibody response in TSP1^−/− mice treated with a TSP-derived peptide. A significantly increased frequency of MZB cells was detected in the spleens and lacrimal glands of TSP1^−/− mice in comparison to wild-type tissues as detected by immunostaining. An altered cytokine profile of TSP1^−/− MZB cells was supportive of T helper 17 (Th17)-related pathogenesis. A significantly reduced antibody response and the splenic MZB compartment against an eye-derived antigen were noted in TSP-derived peptide-treated mice. These changes correspond with the previously reported ability of the peptide to ameliorate SS-related ocular manifestations. Collectively, our results demonstrate dysregulation of MZB cells in TSP1^−/− mice and highlight their role in the context of SS-related chronic ocular surface disease.

B cell contribution of the CD4+ T cell inflammatory phenotypes in systemic lupus erythematosus

Systemic lupus erythematosus is an autoimmune disease in which the effector molecules responsible for tissue damage are antibodies directed against a large number of self-antigens, among which nucleic acids complexed with proteins play a prominent role. These pathogenic autoantibodies are produced by plasma cells differentiated from activated autoreactive B cells, a process that requires complex interactions between multiple components of the immune systems. A key step in the activation of autoreactive B cells is provided by CD4⁺T cells through cytokines and cell-to-cell contact. Lupus CD4⁺T cells are autoreactive and they present an activated inflammatory phenotype that has been shown to contribute to disease. In addition to their role in antibody production, B cells have other effector functions, the most important ones being antigen presentation to and co-stimulation of CD4⁺T cells, as well as the secretion of cytokines. Here, we review what is known, largely based on mouse models, how these B cell effector functions contribute to the CD4⁺T cell inflammatory phenotypes in lupus. When possible, we compare CD4⁺T cell activation by B cells and by dendritic cells, and speculate how these interactions may contribute to the disease process.

Multiple Roles for B-Lymphocytes in Sjogren's Syndrome

Sjogren’s syndrome (SS) is a complex heterogeneous autoimmune disease resulting in loss of salivary gland and lacrimal gland function that may include multiple systemic manifestations including lymphoma. Multiple cell types participate in disease pathogenesis. This review discusses evidence for abnormal B cell subpopulations in patients with SS, critical roles of B cells in SS and the status of B cell–directed therapies in the management of patients with SS.

Immune regulation by Fcα/μ receptor (CD351) on marginal zone B cells and follicular dendritic cells

Although both Fcα/μ receptor (Fcα/μR) and polymeric Ig receptor (poly-IgR) are Fc receptors for IgA and IgM and are functionally and genetically related, the expression profile of Fcα/μR is unique. Unlike poly-IgR, Fcα/μR is expressed on marginal zone (MZ) B cells and follicular dendritic cells, suggesting that Fcα/μR plays an important role in humoral immune responses. Fcα/μR mediates endocytosis of the IgM immune complex (IC). Recent research demonstrated that Fcα/μR downregulated retention of the IgM IC with a T-independent (TI) antigen on MZ B cells and follicular dendritic cells due to endocytosis of the IgM IC, suppressing germinal center formation, affinity maturation, and memory B-cell generation in response to TI antigen challenge. In addition, Fcα/μR physically associates with Toll-like receptor 4 (TLR4) and augments TLR4 oligomerization and signaling in MZ B cells upon lipopolysaccharide (LPS) challenge, leading to increased proinflammatory cytokine production by MZ B cells. Thus, Fcα/μR is a unique Fc receptor that is involved in humoral immune responses and inflammation.

Analysis of subsets of B cells, Breg, CD4Treg and CD8Treg cells in adult patients with primary selective IgM deficiency

Primary selective IgM deficiency (SIGMD) is a rare and recently IUIS-recognized primary immunodeficiency disease with increased susceptibility to infections, allergy, and autoimmune diseases. The pathogenesis of selective IgM remains unclear. The objective of the study was to understand the pathogenesis of selective IgM deficiency via a comprehensive analysis of subsets of B cells, naïve and memory subsets of CD4+ and CD8+ T cells, and Breg, CD4Treg, and CD8Treg cells. Twenty adult patients with SIGMD (serum IgM 4 mg/dl-32 mg/dl) and age-and gender-matched healthy controls were studied. Naïve B cells, transitional B cells, marginal zone B cells, germinal center B cells, IgM memory B cells, switched memory B cells, plasmablasts, CD21(low) B cells, B1 cells, CXCR3+ naive and memory B cells; naïve, central memory, and effector memory subsets of CD4+ and CD8+ T cells, and CD4Treg, CD8Treg and Breg were phenotypically analyzed using multicolor flow cytometry. A significant increase in CD21(low), IgM memory B cells, Breg and CD8Treg, and a significant decreased in germinal center B cells, and CXCR3+ naïve and memory B cells were observed in SIGMD. These alterations in subsets of B cells, and Breg and CD8Treg cells may play a role in the pathogenesis of SIGMD.

Cognate interaction with iNKT cells expands IL-10-producing B regulatory cells

Successful induction of B-cell activation and memory depends on help from CD4+ T cells. Invariant natural killer T (iNKT) cells (glycolipid-specific, CD1d-restricted innate lymphocytes) provide both cognate (direct) and noncognate (indirect) helper signals to enhance B-cell responses. Both forms of iNKT-cell help induce primary humoral immune responses, but only noncognate iNKT-cell help drives humoral memory and plasma cells. Here, we show that iNKT cognate help for B cells is fundamentally different from the help provided by conventional CD4+ T cells. Cognate iNKT-cell help drives an early, unsustained germinal center B-cell expansion, less reduction of T follicular regulatory cells, an expansion of marginal zone B cells, and early increases in regulatory IL-10-producing B-cell numbers compared with noncognate activation. These results are consistent with a mechanism whereby iNKT cells preferentially provide an innate form of help that does not generate humoral memory and has important implications for the application of glycolipid molecules as vaccine adjuvants.

Cell extrinsic alterations in splenic B cell maturation in Flt3-ligand knockout mice

B lymphopoiesis in bone marrow (BM) is critical for maintaining a diverse peripheral B cell pool to fight infection and establish lifelong immunity. The generation of immature B cells is reduced in Flt3-ligand (FL-/-) mice leading to deficiencies in splenic B cells. Here, we sought to understand the cellular basis of the spleen B cell deficiency in FL-/- mice. Significant reductions in transitional (TS) and follicular (FO) B cells were found in FL-/- mice, and increased frequencies, but not absolute numbers, of marginal zone (MZ) B cells. BAFF-R expression on splenic B cells and serum levels of B cell activating factor (BAFF) was comparable to wildtype (WT) mice. Mixed BM chimeras revealed that the reductions in TS and FO B cells were cell extrinsic. FL administration into FL-/- mice restored the deficiency in TS B cells and normalized the MZ compartment. Ki67 analysis revealed a significant decrease in the proliferative capacity of TS B cells in FL-/- mice. A Bcl2 transgene did not rescue TS cells in FL-/- mice, uncoupling FL-deficiency to Bcl2-dependent survival pathways. Upregulation of CD1d expression and adoptive transfer experiments suggested MZ skewing in FL-/- mice. These findings support an integral role for Flt3 signaling in peripheral B cell maturation.

Negative Regulation of Humoral Immunity Due to Interplay between the SLAMF1, SLAMF5, and SLAMF6 Receptors

Whereas the SLAMF-associated protein (SAP) is involved in differentiation of T follicular helper (Tfh) cells and antibody responses, the precise requirements of SLAMF receptors in humoral immune responses are incompletely understood. By analyzing mice with targeted disruptions of the Slamf1, Slamf5, and Slamf6 genes, we found that both T-dependent and T-independent antibody responses were twofold higher compared to those in single knockout mice. These data suggest a suppressive synergy of SLAMF1, SLAMF5, and SLAMF6 in humoral immunity, which contrasts the decreased antibody responses resulting from a defective GC reaction in the absence of the adapter SAP. In adoptive co-transfer assays, both [Slamf1 + 5 + 6]^−/− B and T cells were capable of inducing enhanced antibody responses, but more pronounced enhancement was observed after adoptive transfer of [Slamf1 + 5 + 6]^−/− B cells compared to that of [Slamf1 + 5 + 6]^−/− T cells. In support of [Slamf1 + 5 + 6]^−/− B cell intrinsic activity, [Slamf1 + 5 + 6]^−/− mice also mounted significantly higher antibody responses to T-independent type 2 antigen. Furthermore, treatment of mice with anti-SLAMF6 monoclonal antibody results in severe inhibition of the development of Tfh cells and GC B cells, confirming a suppressive effect of SLAMF6. Taken together, these results establish SLAMF1, SLAMF5, and SLAMF6 as important negative regulators of humoral immune response, consistent with the notion that SLAM family receptors have dual functions in immune responses.

The first wave of B lymphopoiesis develops independently of stem cells in the murine embryo

In the developing mouse embryo, there are several waves of hematopoiesis. Primitive and definitive erythromyeloid lineages appear prior to hematopoietic stem cell (HSC) emergence, and these waves are considered to be transient and support embryonic homeostasis until HSC-derived hematopoiesis is established. However, recent evidence strongly suggests that HSC-independent immune cells, such as tissue macrophages and some innate lymphoid cells, develop in the mouse embryo and persist into postnatal life. Innate type B-1 cells have also been reported to emerge from hemogenic endothelial cells in the extraembryonic yolk sac and para-aortic splanchnopleura, and continue to develop in the fetal liver, even in HSC-deficient mouse embryos. Here, this review discusses B-1 cell development in the context of the layered immune system hypothesis of B lymphopoiesis and the emergence of B-1 cells independent of HSCs.

Enhanced auto-antibody production and Mott cell formation in FcμR-deficient autoimmune mice

The IgM-Fc receptor (FcμR) is involved in IgM homeostasis as evidenced by increased pre-immune serum IgM and natural auto-antibodies of both IgM and IgG isotypes in Fcmr-deficient C57BL/6 (B6) mice. To determine the impact of Fcmr-ablation on autoimmunity, we introduced the Fcmr null mutation onto the Fas-deficient autoimmune-prone B6.MRL Fas (lpr/lpr) mouse background (B6/lpr). Both IgM and IgG auto-antibodies against dsDNA or chromatin appeared earlier in FcμR(-) B6/lpr than FcμR(+) B6/lpr mice, but this difference became less pronounced with age. Splenic B2 cells, which were 2-fold elevated in FcμR(+) B6/lpr mice, were reduced to normal B6 levels in FcμR(-) B6/lpr mice, whereas splenic B1 cells were comparable in both groups of B6/lpr mice. By contrast, marginal zone (MZ) B cells were markedly reduced in FcμR(-) B6/lpr mice compared with either FcμR(+) B6/lpr or wild type (WT) B6 mice. This reduction appeared to result from rapid differentiation of MZ B cells into plasma cells in the absence of FcμR, as IgM antibody to a Smith (Sm) antigen, to which MZ B cells are known to preferentially respond, was greatly increased in both groups (B6/lpr and B6) of FcμR(-) mice compared with FcμR(+) B6/lpr or B6 mice. Mott cells, aberrant plasma cells with intra-cytoplasmic inclusions, were also increased in the absence of FcμR. Despite these abnormalities, the severity of renal pathology and function and survival were all indistinguishable between FcμR(-) and FcμR(+) B6/lpr mice. Collectively, these findings suggest that FcμR plays important roles in the regulation of auto-antibody production, Mott cell formation and the differentiation of MZ B cells into plasma cells in B6.MRL Fas (lpr/lpr) mice.

Reversible expression of CD138 on mature follicular B cells is downregulated by IL-4

CD138, known as a marker of plasma cells, was reported to be expressed to an intermediate level in the murine bone marrow precursor B cells. Here an intermediate level of CD138 expression was also noted in a subpopulation of splenic follicular B cells, which were distinguishable from CD138(high) plasma cells, whereas the majority of transitional or marginal zone B cells did not express CD138. These CD138(int) B cells were IgM(low)IgD(high) mature B cells, located within follicular B cell zone, and expressed a lower level of CD21 than CD138(-) follicular B cells. During in vitro culture of splenic cells, the proportion of CD138(int) B cells increased, which was noticeably reversed by the addition of IL-4 to the culture. The experiments with sorted CD138(int) cells showed that IL-4-mediated regulation of the CD138 expression was B cell-intrinsic and independent of in vitro B cell death. Our results demonstrate that mouse CD138(int) B cells characterize a subpopulation of IgM(low)IgD(high) mature follicular B cells. The CD138 expression on follicular B cells may represent a reversible status, reflecting a dynamic state probably influenced by IL-4.

The role of Bruton's tyrosine kinase in the development and BCR/TLR-dependent activation of AM14 rheumatoid factor B cells

The protein kinase Btk has been implicated in the development, differentiation, and activation of B cells through its role in the BCR and TLR signaling cascades. These receptors and in particular, the BCR and either TLR7 or TLR9 also play a critical role in the activation of autoreactive B cells by RNA- or DNA-associated autoantigens. To explore the role of Btk in the development of autoreactive B cells, as well as their responses to nucleic acid-associated autoantigens, we have now compared Btk-sufficient and Btk-deficient mice that express a prototypic RF BCR encoded by H- and L-chain sdTgs. These B cells bind autologous IgG2a with low affinity and only proliferate in response to IgG2a ICs that incorporate DNA or RNA. We found that Btk-sufficient RF(+) B cells mature into naïve FO B cells, all of which express the Tg BCR, despite circulating levels of IgG2a. By contrast, a significant proportion of Btk-deficient RF(+) B cells acquires a MZ or MZ precursor phenotype. Remarkably, despite the complete inability of RF(+) Xid/y B cells to respond to F(ab')2 anti-IgM, RF(+) Xid/y B cells could respond well to autoantigen-associated ICs. These data reveal unique features of the signaling cascades responsible for the activation of autoreactive B cells.

Cancer immunotherapy with exosomes requires B-cell activation

Exosomes derived from dendritic cells (dexosomes) induce potent antitumor immune responses in mice. We have shown that the efficacy of dexosome-elicited antitumor immunity relies on the presence of both T- and B-cell dexosome-associated epitopes. Hence, the inclusion of B-cell epitopes in anticancer vaccines is crucial for the success of this immunotherapeutic intervention.