Regulation of B1 cell migration by signals through Toll-like receptors

T-cell-independent immune responses do not require CXC ligand 13-mediated B1 cell migration

The dynamic movement of B cells increases the probability of encountering specific antigen and facilitates cell-cell interactions required for mounting a rapid antibody response. B1a and B1b cells are enriched in the coelomic cavity, contribute to T-cell-independent (TI) antibody responses, and increase in number upon antigen exposure. B1 cell movement is largely governed by Cxc ligand 13 (Cxcl13), and mice deficient in this chemokine have a severe reduction in peritoneal B1 cells. In this study, we examined the role of Cxcl13-dependent B cell migration using Borrelia hermsii infection or intraperitoneal immunization with pneumococcal polysaccharide or 4-hydroxy-3-nitrophenyl-acetyl (NP)-Ficoll, all of which induce robust antibody responses from B1b cells. Surprisingly, we found that antibody responses to B. hermsii or to FhbA, an antigenic target of B1b cells, and the resolution of bacteremia were indistinguishable between wild-type and Cxcl13-/- mice. Importantly, we did not observe an expansion of peritoneal B1b cell numbers in Cxcl13-/- mice. Nonetheless, mice that had resolved infection were resistant to reinfection, indicating that the peritoneal B1b cell reservoir is not required for controlling B. hermsii. Furthermore, despite a reduced peritoneal B1b compartment, immunization with pneumococcal polysaccharide vaccine yielded comparable antigen-specific antibody responses in wild-type and Cxcl13-/- mice and conferred protection against Streptococcus pneumoniae. Likewise, immunization with NP-Ficoll elicited similar antibody responses in wild-type and Cxcl13-/- mice. These data demonstrate that homing of B1 cells into the coelomic cavity is not a requirement for generating protective TI antibody responses, even when antigen is initially localized to this anatomical compartment.

Adaptive immune regulation in the gut: T cell-dependent and T cell-independent IgA synthesis

In mammals, the gastrointestinal tract harbors an extraordinarily dense and complex community of microorganisms. The gut microbiota provide strong selective pressure to the host to evolve adaptive immune responses required for the maintenance of local and systemic homeostasis. The continuous antigenic presence in the gut imposes a dynamic remodeling of gut-associated lymphoid tissues (GALT) and the selection of multiple layered strategies for immunoglobulin (Ig) A production. The composite and dynamic gut environment also necessitates heterogeneous, versatile, and convertible T cells, capable of inhibiting (Foxp3(+) T cells) or helping (T(FH) cells) local immune responses. In this review, we describe recent advances in our understanding of dynamic pathways that lead to IgA synthesis, in gut follicular structures and in extrafollicular sites, by T cell-dependent and T cell-independent mechanisms. We discuss the finely tuned regulatory mechanisms for IgA production and emphasize the role of mucosal IgA in the selection and maintenance of the appropriate microbial composition that is necessary for immune homeostasis.

Enhanced trafficking to the pancreatic lymph nodes and auto-antigen presentation capacity distinguishes peritoneal B lymphocytes in non-obese diabetic mice

AIMS/HYPOTHESIS

NOD.Igmicro ( null ) mice lacking mature B cells are highly resistant to diabetes and display poor CD4 T cell responses to autoantigens. Nevertheless, the degree to which different B cell subsets contribute to diabetes in NOD mice remains unresolved. Due to their role in the recognition of microbial and autoantigens, peritoneal B cell characteristics were examined in NOD mice to see if they differ developmentally, phenotypically or functionally in aspects relevant to diabetogenesis.

METHODS

The population dynamics, activation state, migratory behaviour and antigen presentation function were investigated in NOD peritoneal B cells.

RESULTS

NOD peritoneal B cells were found to express abnormally high levels of co-stimulatory molecules (CD40, CD86 and CD69). In contrast, the expression of L-selectin and integrin alpha4beta1 was markedly reduced in NOD mice compared with BALB/c and C57BL/6 mice. The number of B cells in the peritoneum was lower in NOD than in control mice throughout development; migration of B cells from the peritoneum to the pancreatic lymph nodes in NOD mice was enhanced tenfold. NOD B cells showed no chemotactic response to sphingosine-1-phosphate, which normally acts to retain B cells in the peritoneum. Peritoneal B cells of NOD mice also presented insulin autoantigen to CD4 T cells, inducing T cell proliferation.

CONCLUSIONS/INTERPRETATION

NOD peritoneal B cells are hyperactivated, migrate to the pancreatic lymph nodes and are capable of driving insulin-specific CD4 T cell activation. These characteristics could make them important for inducing or amplifying T cell responses against islet-antigens.

Phosphoinositide 3-kinase p110 delta regulates natural antibody production, marginal zone and B-1 B cell function, and autoantibody responses

B-1 and marginal zone (MZ) B cells produce natural Abs, make Ab responses to microbial pathogens, and contribute to autoimmunity. Although the delta isoform of the PI3K p110 catalytic subunit is essential for development of these innate-like B cells, its role in the localization, activation, and function of normal B-1 and MZ B cells is not known. Using IC87114, a highly selective inhibitor of p110delta enzymatic activity, we show that p110delta is important for murine B-1 and MZ B cells to respond to BCR clustering, the TLR ligands LPS and CpG DNA, and the chemoattractants CXCL13 and sphingosine 1-phosphate. In these innate-like B cells, p110delta activity mediates BCR-, TLR- and chemoattractant-induced activation of the Akt prosurvival kinase, chemoattractant-induced migration, and TLR-induced proliferation. Moreover, we found that TLR-stimulated Ab responses by B-1 and MZ B cells, as well as the localization of MZ B cells in the spleen, depend on p110delta activity. Finally, we show that the in vivo production of natural Abs requires p110delta and that p110delta inhibitors can reduce in vivo autoantibody responses. Thus, targeting p110delta may be a novel approach for regulating innate-like B cells and for treating Ab-mediated autoimmune diseases.

Chimeric NKG2D expressing T cells eliminate immunosuppression and activate immunity within the ovarian tumor microenvironment

Adoptive transfer of T cells expressing chimeric NKG2D (chNKG2D) receptors, a fusion of NKG2D and CD3zeta, can lead to long-term, tumor-free survival in a murine model of ovarian cancer. To determine the mechanisms of chNKG2D T cell antitumor efficacy, we analyzed how chNKG2D T cells altered the tumor microenvironment, including the tumor-infiltrating leukocyte populations. chNKG2D T cell treatment of mice bearing ID8 tumor cells increased the number and activation of NK cells and increased the activation of host CD8+ T cells within the tumor. Foxp3+ regulatory T cells at the tumor site decreased more than 300-fold after chNKG2D T cell treatment. Tumor-associated regulatory T cells expressed cell surface NKG2D ligands and were killed by chNKG2D T cells in a perforin-dependent manner. chNKG2D T cells also altered the function of myeloid cells at the tumor site, changing these cells from being immunosuppressive to enhancing T cell responses. Cells isolated from the tumor produced elevated amounts of IFN-gamma, NO, and other proinflammatory cytokines after chNKG2D T cell treatment. ChNKG2D T cells required perforin, IFN-gamma, and GM-CSF to induce a full response at the tumor site. In addition, transfer of chNKG2D T cells into mice bearing tumors that were established for 5 weeks led to long-term survival of the mice. Thus, chNKG2D T cells altered the ovarian tumor microenvironment to eliminate immunosuppressive cells and induce infiltration and activation of antitumor immune cells and production of inflammatory cytokines. This induction of an immune response likely contributes to chNKG2D T cells' ability to eliminate established tumors.

Differential preferences in serosal homing and distribution of peritoneal B-cell subsets revealed by in situ CFSE labeling

Peritoneal B cells represent a heterogeneous mixture of mature peripheral B lineage subsets with distinct developmental and functional characteristics. Here, we report that a single intraperitoneal injection of intracellular fluorescent dye carboxyfluorescein diacetate succinimidyl ester (CFSE) results in the stable fluorescent labeling of resident lymphocytes, without dissipation of the tracer compound into other peripheral lymphoid organs. Using this in situ labeling procedure followed by multicolor flow cytometry or tissue fluorescence at various periods for up to 4 weeks post-labeling, we demonstrate that the distinct peritoneal leukocyte sub-populations and, within the B lineage, B-1 and B-2 B-cell subsets have different exchange kinetics with extraperitoneal sites under steady-state conditions. The B cells labeled with CFSE showed only minimal localization to other peripheral lymphoid tissues. On the other hand, a substantial fraction of both B-1 and B-2 subsets labeled with CFSE accumulated within the pleural cavity, although at a lower frequency than in the peritoneum. We also show that exposure to LPS induces a rapid re-distribution of peritoneal B lymphocytes and an enhanced entry of B-1 cells in the pleural cavity, in addition to augmenting the egress and the division-linked reduction of CFSE fluorescence of both B-1 and B-2 cells. These data indicate that following their in situ labeling, peritoneal lymphocytes show preferential accumulation in serosal cavities, although with a differential efficiencies for T, B-1 and B-2 lymphocyte subsets.

How host-bacterial interactions lead to IgA synthesis in the gut

In mammals, the gut is populated with an extremely dense and diverse bacterial community. One response following intestinal colonization is the production of immunoglobulin (Ig) A by B cells present in the gut-associated lymphoid tissues (GALT). In this review, we summarize recent advances in our understanding of the sites, mechanisms, and functions of intestinal IgA synthesis. We discuss here the pathways leading to IgA production, in organized structures as well as nonorganized tissues, by T-dependent as well as T-independent mechanisms. In addition, we discuss new insights into the role of gut IgA in the regulation of bacterial communities and maintenance of immune homeostasis.

Omental milky spots develop in the absence of lymphoid tissue-inducer cells and support B and T cell responses to peritoneal antigens

The omentum is a site of B1 cell lymphopoiesis and immune responsiveness to T cell-independent antigens. However, it is unknown whether it supports immune responses independently of conventional lymphoid organs. We showed that the omentum collected antigens and cells from the peritoneal cavity and supported T cell-dependent B cell responses, including isotype switching, somatic hypermutation, and limited affinity maturation, despite the lack of identifiable follicular dendritic cells. The omentum also supported CD4+ and CD8+ T cell responses to peritoneal antigens and recruited effector T cells primed in other locations. Unlike conventional lymphoid organs, milky spots in the omentum developed in the absence of lymphoid tissue-inducer cells, but required the chemokine CXCL13. Although the lymphoid architecture of milky spots was disrupted in lymphotoxin-deficient mice, normal architecture was restored by reconstitution with lymphotoxin-sufficient hematopoietic cells. These results indicate that the milky spots of the omentum function as unique secondary lymphoid organs that promote immunity to peritoneal antigens.

Antigen-specific B-1a antibodies induced by Francisella tularensis LPS provide long-term protection against F. tularensis LVS challenge

Francisella tularensis (Ft), a gram-negative intracellular bacterium, is the etiologic agent of tularemia. Infection of mice with <10 Ft Live Vaccine Strain (Ft LVS) organisms i.p. causes a lethal infection that resembles human tularemia. Here, we show that immunization with as little as 0.1 ng Ft LVS lipopolysaccharide (Ft-LPS), but not Ft lipid A, generates a rapid antibody response that protects wild-type (WT) mice against lethal Ft LVS challenge. Protection is not induced in Ft-LPS-immunized B cell-deficient mice (muMT or JhD), male xid mice, or Ig transgenic mice that produce a single IgH (not reactive with Ft-LPS). Focusing on the cellular mechanisms that underlie this protective response, we show that Ft-LPS specifically stimulates proliferation of B-1a lymphocytes that bind fluorochrome-labeled Ft-LPS and the differentiation of these cells to plasma cells that secrete antibodies specific for Ft-LPS. This exclusively B-1a antibody response is equivalent in WT, T-deficient (TCRalphabeta(-/-), TCRgammadelta(-/-)), and Toll-like receptor 4 (TLR4)-deficient (TLR4(-/-)) mice and thus is not dependent on T cells or typical inflammatory processes. Serum antibody levels peak approximately 5 days after Ft-LPS immunization and persist at low levels for months. Thus, immunization with Ft-LPS activates a rare population of antigen-specific B-1a cells to produce a persistent T-independent antibody response that provides long-term protection against lethal Ft LVS infection. These data support the possibility of creating effective, minimally invasive vaccines that can provide effective protection against pathogen invasion.

Targeting of tetraspanin proteins--potential benefits and strategies

The tetraspanin transmembrane proteins have emerged as key players in malignancy, the immune system, during fertilization and infectious disease processes. Tetraspanins engage in a wide range of specific molecular interactions, occurring through the formation of tetraspanin-enriched microdomains (TEMs). TEMs therefore serve as a starting point for understanding how tetraspanins affect cell signalling, adhesion, morphology, motility, fusion and virus infection. An abundance of recent evidence suggests that targeting tetraspanins, for example, by monoclonal antibodies, soluble large-loop proteins or RNAi technology, should be therapeutically beneficial.

Dual role for B-1a cells in immunity to influenza virus infection

B-1 cells are known to contribute most of the “natural antibodies” that are secreted in the steady state, antibodies which are crucial for protection against many pathogens including influenza virus. Whether the CD5⁺ B-1a subset plays a role during an active immune response is incompletely understood. In contrast to recent data suggesting a passive role for B-1a cells, data provided here show strong highly localized activation of B-1 cells in the draining lymph nodes of the respiratory tract after influenza infection. B-1 cells are identified as a major source for both steady state and infection-induced local virus-neutralizing IgM. The CD5⁺ B-1a subset is the main B-1 cell subset generating this response. B-1a cell responses are generated by their increased local accumulation rather than by antigen-specific expansion. Our study reveals that during infection with influenza, CD5-expressing B-1a cells respond to and contribute to protection, presumably without the need for B cell receptor–mediated antigen-specific signals, which are known to induce the death of B-1a cells rather than activation. With that, our data reveal fundamental differences in the response regulation of B-1 and B-2 cells during an infection.

Role of retinoic acid in the imprinting of gut-homing IgA-secreting cells

Antibody-secreting cells (ASCs) lodging in the mucosa of the small intestine are derived from activated B cells that are thought to arise in gut-associated lymphoid tissues (GALT). Upon leaving the GALT, B cells return to the blood where they must express the gut-homing receptors alpha4beta7 and CCR9 in order to emigrate into the small bowel. Recent evidence indicates that gut-associated dendritic cells (DCs) in GALT induce gut-homing receptors on B cells via a mechanism that depends on the vitamin A metabolite retinoic acid (RA). In addition, although ASC associated with other mucosal tissues secrete IgA in an RA-independent fashion, the presence of high levels of RA in intestine and GALT can promote B cell class switching to IgA and thus, boost the production of IgA in the intestinal mucosa. Here, we discuss the role of RA in the imprinting of gut-homing ASC and the evidence linking RA with the generation of intestinal IgA-ASCs.

CD11c expression identifies a population of extrafollicular antigen-specific splenic plasmablasts responsible for CD4 T-independent antibody responses during intracellular bacterial infection

Although T-independent immunity is known to be generated against bacterial capsular and cell wall polysaccharides expressed by a number of bacterial pathogens, it has not been studied in depth during intracellular bacterial infections. Our previous study demonstrated that Ehrlichia muris, an obligate intracellular tick-borne pathogen, generates protective classical TI responses in CD4 T cell-deficient C57BL/6 mice. We found that E. muris T-independent immunity is accompanied by the expansion of a very large extrafollicular spleen population of CD11c(low)-expressing plasmablasts that exhibit characteristics of both B-1 and marginal zone B cells. The plasmablasts comprised up to 15% of the total spleen lymphocytes and approximately 70% of total spleen IgM(high)IgD(low) cells during peak infection in both wild-type and MHC class II-deficient mice. The CD11c(low) cells exhibited low surface expression of B220, CD19, and CD1d, high expression of CD11b, CD43, but did not express CD5. Approximately 50% of the CD11c(low) cells also expressed CD138. In addition to CD11b and CD11c, the plasmablasts expressed the beta(1) (CD29) and alpha4 (CD49d) integrins, as well as the chemokine receptor CXCR4, molecules which may play roles in localizing the B cells extrafollicular region of the spleen. During peak infection, the CD11c(low) cells accounted for the majority of the IgM-producing splenic B cells and nearly all of the E. muris outer membrane protein-specific IgM-secreting cells. Thus, during this intracellular bacterial infection, CD11c expression identifies a population of Ag-specific spleen plasmablasts responsible for T-independent Ab production.

The biology of intestinal immunoglobulin A responses

The gut mucosa is exposed to a large community of commensal bacteria that are required for the processing of nutrients and the education of the local immune system. Conversely, the gut immune system generates innate and adaptive responses that shape the composition of the local microbiota. One striking feature of intestinal adaptive immunity is its ability to generate massive amounts of noninflammatory immunoglobulin A (IgA) antibodies through multiple follicular and extrafollicular pathways that operate in the presence or absence of cognate T-B cell interactions. Here we discuss the role of intestinal IgA in host-commensal mutualism, immune protection, and tolerance and summarize recent advances on the role of innate immune cells in intestinal IgA production.

Evolution, development, mechanism and function of IgA in the gut

Since its discovery as the most abundant Ig produced at mucosal surfaces, IgA has been the subject of continuous studies. The concepts emerged were that the precursors for IgA plasma cells are efficiently generated in follicular organized structures in the gut with the help of CD4 T cells and that secretory IgA provides protection against mucosal pathogens. Novel conceptual advances have been made in the past few years in describing new sites, mechanisms and functions of mucosal IgA synthesis.

Sphingosine 1-phosphate-dependent trafficking of peritoneal B cells requires functional NFkappaB-inducing kinase in stromal cells

We previously reported that sphingosine 1-phosphate (S1P) regulates peritoneal B-cell trafficking and subsequent intestinal IgA production, but the underlying mechanisms remain obscure. We demonstrate here that nuclear factor kappaB-inducing kinase (NIK) is involved in the regulation of S1P-mediated trafficking of peritoneal B cells. Although peritoneal B cells from NIK-mutated alymphoplasia (aly) mice expressed type 1 S1P receptor (S1P(1)) at comparable levels and demonstrated normal migration toward S1P, aly peritoneal B cells showed decreased sensitivity to FTY720, an S1P(1) modulator. NIK-mutated stromal cells showed decreased levels of adhesion molecules (VCAM-1 and ICAM-1) and increased CXCL13 expressions, leading to impaired ability to support S1P-mediated emigration, but not immigration, of peritoneal B cells. Therefore, aly peritoneal B cells exhibited normal S1P-mediated peritoneal B-cell trafficking from peritoneum to intestine for IgA production when they were transferred into severe combined immunodeficient or wild-type mice. However, S1P-mediated emigration of wild-type B cells from the aly peritoneal cavity was impaired without affecting their immigration from the blood. Further, transfer of wild-type stromal cells into the peritoneum restored S1P-mediated trafficking of aly peritoneal B cells. These findings suggest that NIK in stromal cells has a specific role in the regulation of S1P-mediated trafficking of peritoneal B cells.

Peripheral B cell subsets

Our understanding of the origins and the biological functions of different peripheral B cell subsets continues to evolve. Some understanding has been obtained regarding the synergy between BCR-derived signals and other receptors and signaling pathways that drive the development of follicular, marginal zone, and B-1 B cells, but this remains a complex and poorly understood issue. More recent information regarding the origins of B-1 and B-2 B cells, the ability of follicular B cells to mature both in the bone marrow and the spleen, the existence of a definable precursor for MZ B cells, and the ability of follicular B cells to occupy two distinct niches are all highlighted in this review.

Differential molecular and anatomical basis for B cell migration into the peritoneal cavity and omental milky spots

The constitutive migration of B cells from the circulation into the peritoneal cavity and back is essential for peritoneal B cell homeostasis and function. However, the molecular machinery and the anatomical basis for these migratory processes have hardly been investigated. In this study, we analyze the role of integrins as well as the role of the omentum for B2 cell migration into and out of the peritoneal cavity of mice. We demonstrate that alpha(4)beta(7) integrin-mucosal addressin cell adhesion molecule 1 interaction enables B2 cell migration from the circulation into omental milky spots but not into the peritoneum. In contrast, alpha(4)beta(1) integrin mediates direct entry of B2 cells into the peritoneal cavity as well as their retention at that site, limiting B2 cell egress via the draining parathymic lymph nodes. Surgical removal of the omentum results in a 40% reduced immigration of B2 cells from the circulation into the peritoneum but does not impair B cell exit from this compartment. In conclusion, these data reveal the existence of alternative routes for B2 cell entry into the peritoneal cavity and identify integrins as key factors for peritoneal B2 cell homeostasis, mediating B2 cell migration into and out of the peritoneal cavity as well as their retention at this site.

Differentiation and homing of IgA-secreting cells

Most antibody-secreting cells (ASCs) in mucosal tissues produce immunoglobulin A (IgA), the most abundant immunoglobulin in the body and the main class of antibody found in secretions. IgA-ASCs differentiate in the mucosal-associated lymphoid tissues and are usually considered as a homogeneous population of cells. However, IgA-ASCs that travel to the small intestine have unique characteristics in terms of their migratory requirements. These IgA-ASCs require the homing molecules alpha4beta7 and CCR9 to interact with their ligands, mucosal addressin cell adhesion molecule-1 and CCL25, which are constitutively expressed in the small intestine. Indeed, recent work has shown that IgA-ASCs specific for the small bowel are generated under different conditions as compared with IgA-ASCs in other mucosal compartments. Moreover, the mechanisms inducing IgA class switching may also vary according to the tissue where IgA-ASCs differentiate. Here we describe the mechanisms involved in the differentiation of IgA-ASCs in mucosal compartments, in particular those involved in the generation of gut-homing IgA-ASCs.

Novel biphasic role for lymphocytes revealed during resolving inflammation

Acute inflammation is traditionally described as the influx of polymorphonuclear leukocytes (PMNs) followed by monocyte-derived macrophages, leading to resolution. This is a classic view, and despite subpopulations of lymphocytes possessing innate immune-regulatory properties, seldom is their role in acute inflammation and its resolution discussed. To redress this we show, using lymphocyte-deficient RAG1(-/-) mice, that peritoneal T/B lymphocytes control PMN trafficking by regulating cytokine synthesis. Once inflammation ensues in normal mice, lymphocytes disappear in response to DP1 receptor activation by prostaglandin D(2). However, upon resolution, lymphocytes repopulate the cavity comprising B1, natural killer (NK), gamma/delta T, CD4(+)/CD25(+), and B2 cells. Repopulating lymphocytes are dispensable for resolution, as inflammation in RAG1(-/-) and wild-type mice resolve uniformly. However, repopulating lymphocytes are critical for modulating responses to superinfection. Thus, in chronic granulomatous disease using gp91phox(-/-) mice, not only is resolution delayed compared with wild-type, but there is a failure of lymphocyte re-appearance predisposing to exaggerated immune responses upon secondary challenge that is rescued by resolution-phase lymphocytes. In conclusion, as lymphocyte repopulation is also evident in human peritonitis, we hereby describe a transition in T/B cells from acute inflammation to resolution, with a central role in modulating the severity of early onset and orchestrating responses to secondary infection.

A distinct role for B1b lymphocytes in T cell-independent immunity

Pathogenesis of infectious disease is not only determined by the virulence of the microbe but also by the immune status of the host. Vaccination is the most effective means to control infectious diseases. A hallmark of the adaptive immune system is the generation of B cell memory, which provides a long-lasting protective antibody response that is central to the concept of vaccination. Recent studies revealed a distinct function for B1b lymphocytes, a minor subset of mature B cells that closely resembles that of memory B cells in a number of aspects. In contrast to the development of conventional B cell memory, which requires the formation of germinal centers and T cells, the development of B1b cell-mediated long-lasting antibody responses occurs independent of T cell help. T cell-independent (TI) antigens are important virulence factors expressed by a number of bacterial pathogens, including those associated with biological threats. TI antigens cannot be processed and presented to T cells and therefore are known to possess restricted T cell-dependent (TD) immunogenicity. Nevertheless, specific recognition of TI antigens by B1b cells and the highly protective antibody responses mounted by them clearly indicate a crucial role for this subset of B cells. Understanding the mechanisms of long-term immunity conferred by B1b cells may lead to improved vaccine efficacy for a variety of TI antigens.

B cell lineage contributions to antiviral host responses

B cell responses are a major immune protective mechanism induced against a large variety of pathogens. Technical advances over the last decade, particularly in the isolation and characterization of B cell subsets by multicolor flow cytometry, have demonstrated the multifaceted nature of pathogen-induced B cell responses. In addition to participation by the major follicular B cell population, three B cell subsets are now recognized as key contributors to pathogen-induced host defenses: marginal zone (MZ) B cells, B-1a and B-1b cells. Each of these subsets seems to require unique activation signals and to react with distinct response patterns. Here we provide a brief review of the main developmental and functional features of these B cell subsets. Furthermore, we outline our current understanding of how each subset contributes to the humoral response to influenza virus infection and what regulates their differential responses. Understanding of the multilayered nature of the humoral responses to infectious agents and the complex innate immune signals that shape pathogen-specific humoral responses are likely at the heart of enhancing our ability to induce appropriate and long-lasting humoral responses for prophylaxis and therapy.

Siglec-G is a B1 cell–inhibitory receptor that controls expansion and calcium signaling of the B1 cell population

B1 cells are an important cell population for the production of natural antibodies and for antibacterial immunoglobulin responses. Here we identified the mouse protein Siglec-G as a B1 cell inhibitory receptor. Siglec-G was expressed in a B cell-restricted way, with large amounts present in B1 cells. When overexpressed, Siglec-G inhibited B cell receptor-mediated calcium signaling. Siglec-G-deficient mice had massive expansion of the B1a cell population, which began early in development and was B cell intrinsic. Siglec-G-deficient mice had higher titers of natural IgM antibodies but not a higher penetrance of IgG autoantibodies. Siglec-G-deficient B1 cells showed a strongly enhanced calcium signaling. Our results demonstrate that Siglec-G-dependent negative regulation exists in B1 cells, which may explain the naturally muted signaling response of B1 cells.

The peritoneal micromilieu commits B cells to home to body cavities and the small intestine

The distinct combination of homing receptors such as selectins, chemokine receptors, and integrins directs the migration of lymphocytes throughout the body. Upon activation lymphocytes irreversibly switch their set of homing receptors, now guiding them to entirely different destinations. Here we report that exposure of naive B cells to the microenvironment of the peritoneal cavity modulates their migration propensities in the absence of antigenic stimulation. B1 and B2 cells isolated from the peritoneal cavity reenter this compartment more efficiently compared with splenic follicular B cells. Moreover, when kept in the peritoneal cavity splenic follicular B cells gain such increased capability to reenter this compartment. These altered migratory capacities are reflected by an up-regulation of the chemokine receptors CXCR4 and CXCR5 and beta7 integrin by the peritoneum-experienced splenic B cells, among which CXCR5 is instrumental in directing B cells into the peritoneal cavity. Moreover, intraperitoneal transfer of plasma blasts favors their migration into the small intestine presumably before class switch recombination occurs, demonstrating that a reconfigured transient migration pattern is not restricted to naive cells. In conclusion, these data demonstrate a hitherto unrecognized role for tissue-specific cues, altering the migratory capacity of B1, naive B2, as well as antigen-experienced B2 cells.

Intestinal IgA synthesis: a primitive form of adaptive immunity that regulates microbial communities in the gut

Our intestine is colonized by an impressive community of bacteria, that has profound effects on the immune functions. The relationship between gut microbiota and the immune system is one of reciprocity: bacteria have important contribution in nutrient processing and education of the immune system and conversely, the immune system, particularly gut-associated lymphoid tissues (GALT) plays a key role in shaping the repertoire of gut microbiota. In this review we discuss new insights into the role of IgA in the maintenance of immune homeostasis and the reciprocal interactions between gut B cells and intestinal bacteria.