Montserrat Cols

B cell–helper neutrophils stimulate the diversification and production of immunoglobulin in the marginal zone of the spleen

Neutrophils utilize immunoglobulins (Igs) to clear antigen, but their role in Ig production is unknown. Here we identified neutrophils around the marginal zone (MZ) of the spleen, a B cell area specialized in T-independent Ig responses to circulating antigen. Neutrophils colonized peri-MZ areas after post-natal mucosal colonization by microbes and enhanced their B-helper function upon receiving reprogramming signals from splenic sinusoidal endothelial cells, including interleukin 10 (IL-10). Splenic neutrophils induced Ig class switching, somatic hypermutation and antibody production by activating MZ B cells through a mechanism involving the cytokines BAFF, APRIL and IL-21. Neutropenic patients had fewer and hypomutated MZ B cells and less preimmune Igs to T-independent antigens, which indicates that neutrophils generate an innate layer of antimicrobial Ig defense by interacting with MZ B cells.Neutrophils use immunoglobulins to clear antigen, but their role in immunoglobulin production is unknown. Here we identified neutrophils around the marginal zone (MZ) of the spleen, a B cell area specialized in T cell–independent immunoglobulin responses to circulating antigen. Neutrophils colonized peri-MZ areas after postnatal mucosal colonization by microbes and enhanced their B cell–helper function after receiving reprogramming signals, including interleukin 10 (IL-10), from splenic sinusoidal endothelial cells. Splenic neutrophils induced immunoglobulin class switching, somatic hypermutation and antibody production by activating MZ B cells through a mechanism that involved the cytokines BAFF, APRIL and IL-21. Neutropenic patients had fewer and hypomutated MZ B cells and a lower abundance of preimmune immunoglobulins to T cell–independent antigens, which indicates that neutrophils generate an innate layer of antimicrobial immunoglobulin defense by interacting with MZ B cells.

Mucus Enhances Gut Homeostasis and Oral Tolerance by Delivering Immunoregulatory Signals

Guardian of the Gut The intestine is able to tolerate continual exposure to large amounts of commensal bacteria and foreign food antigens without triggering an inappropriate inflammatory immune response. In the large intestine, this immunological tolerance is thought to occur via a physical separation between environment and host imposed by a continuous mucous layer built up from the secreted mucin protein, MUC2. However, in the small intestine, this mucous layer is porous, necessitating an additional layer of immune control. Shan et al. (p. 447, published online 26 September; see the Perspective by Belkaid and Grainger) now report that in the small intestine, MUC2 plays an active role in immunological tolerance by activating a transcription factor in resident dendritic cells, thereby selectively blocking their ability to launch an inflammatory response. This work identifies MUC2 as a central mediator of immune tolerance to maintain homeostasis in the gut and possibly at other mucosal surfaces in the body. Mucus not only forms a physical barrier in the intestine but also promotes immunological tolerance of bacteria and foods. [Also see Perspective by Belkaid and Grainger] A dense mucus layer in the large intestine prevents inflammation by shielding the underlying epithelium from luminal bacteria and food antigens. This mucus barrier is organized around the hyperglycosylated mucin MUC2. Here we show that the small intestine has a porous mucus layer, which permitted the uptake of MUC2 by antigen-sampling dendritic cells (DCs). Glycans associated with MUC2 imprinted DCs with anti-inflammatory properties by assembling a galectin-3–Dectin-1–FcγRIIB receptor complex that activated β-catenin. This transcription factor interfered with DC expression of inflammatory but not tolerogenic cytokines by inhibiting gene transcription through nuclear factor κB. MUC2 induced additional conditioning signals in intestinal epithelial cells. Thus, mucus does not merely form a nonspecific physical barrier, but also constrains the immunogenicity of gut antigens by delivering tolerogenic signals.

The transmembrane activator TACI triggers immunoglobulin class switching by activating B cells through the adaptor MyD88

BAFF and APRIL are innate immune mediators that trigger immunoglobulin (Ig) G and IgA class switch recombination (CSR) in B cells by engaging the receptor TACI. The mechanism underlying CSR signaling by TACI remains unknown. Here, we found that the cytoplasmic domain of TACI encompasses a conserved motif that bound MyD88, an adaptor protein that activates NF-κB signaling pathways via a Toll-interleukin-1 receptor (TIR) domain. TACI lacks a TIR domain, yet triggered CSR via the DNA-editing enzyme AID by activating NF-κB through a TLR-like MyD88–IRAK-1-IRAK-4–TRAF6–TAK1 pathway. TACI-induced CSR was impaired in mice and humans lacking MyD88 or IRAK-4, indicating that MyD88 controls a B cell-intrinsic, TIR-independent, TACI-dependent pathway for Ig diversification.BAFF and APRIL are innate immune mediators that trigger immunoglobulin G (IgG) and IgA class-switch recombination (CSR) in B cells by engaging the receptor TACI. The mechanism that underlies CSR signaling by TACI remains unknown. Here we found that the cytoplasmic domain of TACI encompasses a conserved motif that bound MyD88, an adaptor that activates transcription factor NF-κB signaling pathways via a Toll–interleukin 1 (IL-1) receptor (TIR) domain. TACI lacks a TIR domain, yet triggered CSR via the DNA-editing enzyme AID by activating NF-κB through a Toll-like receptor (TLR)-like MyD88-IRAK1-IRAK4-TRAF6-TAK1 pathway. TACI-induced CSR was impaired in mice and humans lacking MyD88 or the kinase IRAK4, which indicates that MyD88 controls a B cell–intrinsic, TIR-independent, TACI-dependent pathway for immunoglobulin diversification.

Innate lymphoid cells integrate stromal and immunological signals to enhance antibody production by splenic marginal zone B cells

Innate lymphoid cells (ILCs) regulate stromal cells, epithelial cells and cells of the immune system, but their effect on B cells remains unclear. Here we identified RORγt+ ILCs near the marginal zone (MZ), a splenic compartment that contains innate-like B cells highly responsive to circulating T cell–independent (TI) antigens. Splenic ILCs established bidirectional crosstalk with MAdCAM-1+ marginal reticular cells by providing tumor-necrosis factor (TNF) and lymphotoxin, and they stimulated MZ B cells via B cell–activation factor (BAFF), the ligand of the costimulatory receptor CD40 (CD40L) and the Notch ligand Delta-like 1 (DLL1). Splenic ILCs further helped MZ B cells and their plasma-cell progeny by coopting neutrophils through release of the cytokine GM-CSF. Consequently, depletion of ILCs impaired both pre- and post-immune TI antibody responses. Thus, ILCs integrate stromal and myeloid signals to orchestrate innate-like antibody production at the interface between the immune system and circulatory system.

Innate control of B cell responses.

Mature B cells generate protective immunity by undergoing immunoglobulin (Ig) class switching and somatic hypermutation, two Ig gene-diversifying processes that usually require cognate interactions with T cells that express CD40 ligand. This T cell-dependent pathway provides immunological memory but is relatively slow to occur. Thus, it must be integrated with a faster, T cell-independent pathway for B cell activation through CD40 ligand-like molecules that are released by innate immune cells in response to microbial products. Here, we discuss recent advances in our understanding of the interplay between the innate immune system and B cells, particularly at the mucosal interface. We also review the role of innate signals in the regulation of Ig diversification and production.

Stromal Endothelial Cells Establish a Bidirectional Crosstalk with Chronic Lymphocytic Leukemia Cells through the TNF-Related Factors BAFF, APRIL, and CD40L

Chronic lymphocytic leukemia (CLL) is a clonal B cell disorder of unknown origin. Accessory signals from the microenvironment are critical for the survival, expansion, and progression of malignant B cells. We found that the CLL stroma included microvascular endothelial cells (MVECs) expressing BAFF and APRIL, two TNF family members related to the T cell-associated B cell-stimulating molecule CD40L. Constitutive release of soluble BAFF and APRIL increased upon engagement of CD40 on MVECs by CD40L aberrantly expressed on CLL cells. In addition to enhancing MVEC expression of CD40, leukemic CD40L induced cleavases that elicited intracellular processing of pro-BAFF and pro-APRIL proteins in MVECs. The resulting soluble BAFF and APRIL proteins delivered survival, activation, Ig gene remodeling, and differentiation signals by stimulating CLL cells through TACI, BAFF-R, and BCMA receptors. BAFF and APRIL further amplified CLL cell survival by upregulating the expression of leukemic CD40L. Inhibition of TACI, BCMA, and BAFF-R expression on CLL cells; abrogation of CD40 expression in MVECs; or suppression of BAFF and APRIL cleavases in MVECs reduced the survival and diversification of malignant B cells. These data indicate that BAFF, APRIL, and CD40L form a CLL-enhancing bidirectional signaling network linking neoplastic B cells with the microvascular stroma.

New helping friends for B cells

Over the past decade, a growing recognition of the importance of neutralizing antibodies in host defense combined with the success of B‐cell depletion therapies in treating auto‐immune disorders has led to an increased focus on better understanding the pathways underpinning B‐cell antibody production. In general, B cells require cognate interaction with T helper cells in the germinal center of lymphoid follicles to generate protective antibodies. However, recent evidence shows that B cells receive additional help from invariant natural killer T cells, dendritic cells, and various granulocytes, including neutrophils, eosinophils, and basophils. These innate immune cells enhance T‐cell‐dependent antibody responses by delivering B‐cell helper signals both in the germinal center and at postgerminal center lymphoid sites such as the bone marrow. In addition to enhancing and complementing the B‐cell helper activity of canonical T cells, invariant natural killer T cells, dendritic cells, and granulocytes can deliver T cell‐independent B‐cell helper signals at the mucosal interface and in the marginal zone of the spleen to initiate rapid innate‐like antibody responses. Here, we discuss recent advances in the role of adaptive and innate B‐cell helper signals in antibody diversification and production.

Regulation of mucosal IgA responses: lessons from primary immunodeficiencies

Adaptive co‐evolution of mammals and bacteria has led to the establishment of complex commensal communities on mucosal surfaces. In spite of having available a wealth of immune‐sensing and effector mechanisms capable of triggering inflammation in response to microbial intrusion, mucosal immune cells establish an intimate dialogue with microbes to generate a state of hyporesponsiveness against commensals and active readiness against pathogens. A key component of this homeostatic balance is IgA, a noninflammatory antibody isotype produced by mucosal B cells through class switching. This process involves activation of B cells by IgA‐inducing signals originating from mucosal T cells, dendritic cells, and epithelial cells. Here, we review the mechanisms by which mucosal B cells undergo IgA diversification and production and discuss how the study of primary immunodeficiencies facilitates better understanding of mucosal IgA responses in humans.

Expansion of inflammatory innate lymphoid cells in patients with common variable immune deficiency.

BACKGROUND Common variable immunodeficiency (CVID) is an antibody deficiency treated with immunoglobulin; however, patients can have noninfectious inflammatory conditions that lead to heightened morbidity and mortality. OBJECTIVES Modular analyses of RNA transcripts in whole blood previously identified an upregulation of many interferon-responsive genes. In this study we sought the cell populations leading to this signature. METHODS Lymphoid cells were measured in peripheral blood of 55 patients with CVID (31 with and 24 without inflammatory/autoimmune complications) by using mass cytometry and flow cytometry. Surface markers, cytokines, and transcriptional characteristics of sorted innate lymphoid cells (ILCs) were defined by using quantitative PCR. Gastrointestinal and lung biopsy specimens of subjects with inflammatory disease were stained to seek ILCs in tissues. RESULTS The linage-negative, CD127(+), CD161(+) lymphoid population containing T-box transcription factor, retinoic acid-related orphan receptor (ROR) γt, IFN-γ, IL-17A, and IL-22, all hallmarks of type 3 innate lymphoid cells, were expanded in the blood of patients with CVID with inflammatory conditions (mean, 3.7% of PBMCs). ILCs contained detectable amounts of the transcription factors inhibitor of DNA binding 2, T-box transcription factor, and RORγt and increased mRNA transcripts for IL-23 receptor (IL-23R) and IL-26, demonstrating inflammatory potential. In gastrointestinal and lung biopsy tissues of patients with CVID, numerous IFN-γ(+)RORγt(+)CD3(-) cells were identified, suggesting a role in these mucosal inflammatory states. CONCLUSIONS An expansion of this highly inflammatory ILC population is a characteristic of patients with CVID with inflammatory disease; ILCs and the interferon signature are markers for the uncontrolled inflammatory state in these patients.

Activation of B cells by non-canonical helper signals.

Cognate interaction between T and B lymphocytes of the adaptive immune system is essential for the production of high‐affinity antibodies against microbes, and for the establishment of long‐term immunological memory. Growing evidence shows that—in addition to presenting antigens to T and B cells—macrophages, dendritic cells and other cells of the innate immune system provide activating signals to B cells, as well as survival signals to antibody‐secreting plasma cells. Here, we discuss how these innate immune cells contribute to the induction of highly diversified and temporally sustained antibody responses, both systemically and at mucosal sites of antigen entry.