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

Sphingosine-1-phosphate receptor type 4 is critically involved in the regulation of peritoneal B-1 cell trafficking and distribution in vivo

B-1 cells are crucially involved in immune defense and regulation of inflammation and autoimmunity. B-1 cells are predominantly located in the peritoneal and pleural cavities, although body cavity B-1 cells recirculate systemically under steady-state conditions. The chemokines CXCL12 and CXCL13 have been identified as the main regulators of peritoneal B-cell trafficking. In mice deficient for sphingosine-1-phosphate receptor 4 (S1PR4), B-1a and B-1b cell numbers are reduced in the peritoneal cavity by an unknown mechanism. In this study, we show that S1PR4-mediated S1P signaling modifies the chemotactic response of peritoneal B cells to CXCL13 and CXCL12 in vitro. In vivo, S1PR4-mediated S1P signaling affects both immigration into and emigration from the peritoneal cavity. Long-term reconstitution experiments of scid mice with wt or s1pr4 -/- peritoneal B cells revealed a distinct distributional pattern in secondary lymphoid organs. As a functional consequence, both plasmatic and mucosal IgM levels, the main product of B-1a cells, are reduced in mice reconstituted with s1pr4 -/- peritoneal cells. In summary, our data identify S1PR4 as the second S1P receptor (besides S1PR1), which is critically involved in the regulation of peritoneal B-1 cell function.

Microbiota and B-1 B cell repertoire development in mice

Microbiota-derived antigens play a critical role in the development of both the mucosal and systemic B cell repertoires; however, how glycan epitopes promote B cell repertoire selection is only recently being understood. The production of glycan-derived antigens by individual microbes within a host can be dynamic and influenced by interactions within other members of microbial communities, the composition of diet, and host-derived contents, including those of the mucosal immune system. The size and complexity of the emerging neonatal B cell repertoire are paralleled by the acquisition of a diverse microbiota from maternal and environmental sources, which is now appreciated to exert long-lasting influences on the nascent B cell repertoire.

Neutrophils disrupt B-1a cell homeostasis by targeting Siglec-G to exacerbate sepsis

B-1a cells, an innate-like cell population, are crucial for pathogen defense and the regulation of inflammation through their release of natural IgM and IL-10. In sepsis, B-1a cell numbers are decreased in the peritoneal cavity as they robustly migrate to the spleen. Within the spleen, migrating B-1a cells differentiate into plasma cells, leading to alterations in their original phenotype and functionality. We discovered a key player, sialic acid-binding immunoglobulin-like lectin-G (Siglec-G), which is expressed predominantly on B-1a cells and negatively regulates B-1a cell migration to maintain homeostasis. Siglec-G interacts with CXCR4/CXCL12 to modulate B-1a cell migration. Neutrophils aid B-1a cell migration via neutrophil elastase (NE)-mediated Siglec-G cleavage. Human studies revealed increased NE expression in septic patients. We identified an NE cleavage sequence in silico, leading to the discovery of a decoy peptide that protects Siglec-G, preserves peritoneal B-1a cells, reduces inflammation, and enhances sepsis survival. The role of Siglec-G in inhibiting B-1a cell migration to maintain their inherent phenotype and function is compromised by NE in sepsis, offering valuable insights into B-1a cell homeostasis. Employing a small decoy peptide to prevent NE-mediated Siglec-G cleavage has emerged as a promising strategy to sustain peritoneal B-1a cell homeostasis, alleviate inflammation, and ultimately improve outcomes in sepsis patients.

Noncanonical B Cells: Characteristics of Uncharacteristic B Cells

B lymphocytes were originally described as a cell type uniquely capable of secreting Abs. The importance of T cell help in Ab production was revealed soon afterward. Following these seminal findings, investigators made great strides in delineating steps in the conventional pathway that B cells follow to produce high-affinity Abs. These studies revealed generalized, or canonical, features of B cells that include their developmental origin and paths to maturation, activation, and differentiation into Ab-producing and memory cells. However, along the way, examples of nonconventional B cell populations with unique origins, age-dependent development, tissue localization, and effector functions have been revealed. In this brief review, features of B-1a, B-1b, marginal zone, regulatory, killer, NK-like, age-associated, and atypical B cells are discussed. Emerging work on these noncanonical B cells and functions, along with the study of their significance for human health and disease, represents an exciting frontier in B cell biology.

Humoral immune response and changes in peritoneal cell populations in rats immunized against two Leptospira serovars; serovar patoc and serovar pyrogenes

BACKGROUND

Leptospirosis is a zoonotic disease caused by Leptospira species. Variations in lipopolysaccharide (LPS) structure in Leptospira are known to be associated with the serovar diversity and antigenicity. Development of immunodiagnostics for early detection of leptospirosis based on immune responses against different pathogenic antigens as well as development of vaccines are important. Hence, this study has assessed the immune response generated against leptospiral LPS and whole antigen preparations of pathogenic and saprophytic Leptospira and specific changes in peritoneal cells was also studied to elucidate the cellular responses associated with immune response of Wistar rats.

METHODS

During the study, immune response induced by two types of Leptospira antigen preparations of two selected serovars was compared. Changes in the specific peritoneal cell subpopulations following immunizations of rats were analyzed using flow cytometry.

RESULTS

Of the two antigen preparations tested, the LPS extract induced a higher IgM immune response as opposed to the sonicated antigen preparation. Of the two serovars tested, L. interrogans serovar Pyrogenes had induced a higher IgM response compared to that by L. biflexa serovar Patoc. Considering the IgG titers, equivalent responses were observed with all four antigen preparations. Significant increases in lymphocytes were observed following immunization with LPS of both serovars. Interestingly, the B2 cell percentages increased significantly during the immunization period. Further, significant correlations were observed with both IgM and IgG responses and percentage of B2 cells in the peritoneal cavity (PC).

CONCLUSION

LPS extract of L. interrogans serovar Pyrogenes induced higher IgM response while the IgG response was equivalent among the four antigen preparations tested. Significant increase of B2 cell percentage in the peritoneal cavity during the immunization reflects the accumulation of B2 cells in the PC which may play considerable role in generating humoral response against Leptospira antigens.

Inhibiting DNA methylation and RNA editing upregulates immunogenic RNA to transform the tumor microenvironment and prolong survival in ovarian cancer

BACKGROUND

Novel therapies are urgently needed for ovarian cancer (OC), the fifth deadliest cancer in women. Preclinical work has shown that DNA methyltransferase inhibitors (DNMTis) can reverse the immunosuppressive tumor microenvironment in OC. Inhibiting DNA methyltransferases activate transcription of double-stranded (ds)RNA, including transposable elements. These dsRNAs activate sensors in the cytoplasm and trigger type I interferon (IFN) signaling, recruiting host immune cells to kill the tumor cells. Adenosine deaminase 1 (ADAR1) is induced by IFN signaling and edits mammalian dsRNA with an A-to-I nucleotide change, which is read as an A-to-G change in sequencing data. These edited dsRNAs cannot be sensed by dsRNA sensors, and thus ADAR1 inhibits the type I IFN response in a negative feedback loop. We hypothesized that decreasing ADAR1 editing would enhance the DNMTi-induced immune response.

METHODS

Human OC cell lines were treated in vitro with DNMTi and then RNA-sequenced to measure RNA editing. Adar1 was stably knocked down in ID8 Trp53^-/- mouse OC cells. Control cells (shGFP) or shAdar1 cells were tested with mock or DNMTi treatment. Tumor-infiltrating immune cells were immunophenotyped using flow cytometry and cell culture supernatants were analyzed for secreted chemokines/cytokines. Mice were injected with syngeneic shAdar1 ID8 Trp53^-/- cells and treated with tetrahydrouridine/DNMTi while given anti-interferon alpha and beta receptor 1, anti-CD8, or anti-NK1.1 antibodies every 3 days.

RESULTS

We show that ADAR1 edits transposable elements in human OC cell lines after DNMTi treatment in vitro. Combining ADAR1 knockdown with DNMTi significantly increases pro-inflammatory cytokine/chemokine production and sensitivity to IFN-β compared with either perturbation alone. Furthermore, DNMTi treatment and Adar1 loss reduces tumor burden and prolongs survival in an immunocompetent mouse model of OC. Combining Adar1 loss and DNMTi elicited the most robust antitumor response and transformed the immune microenvironment with increased recruitment and activation of CD8+ T cells.

CONCLUSION

In summary, we showed that the survival benefit from DNMTi plus ADAR1 inhibition is dependent on type I IFN signaling. Thus, epigenetically inducing transposable element transcription combined with inhibition of RNA editing is a novel therapeutic strategy to reverse immune evasion in OC, a disease that does not respond to current immunotherapies.

A role for the nuclear receptor NR2F6 in peritoneal B cell homeostasis

B cells are key mediators of humoral immunity. Mature B cells fall into various sub-classes that can be separated by their ontogeny, expression of cell surface markers, anatomical location, and function. B1 subsets play important roles in natural immunity and constitute the majority of B cells in newborns. In the adult, B1 cells predominate in the pleural and peritoneal cavities, while the mature B2 follicular subset makes up the major fraction of B cells in lymphoid tissue, although important subsets of antibody-secreting B1 cells are also present at these sites. B1 cells are the main producers of natural IgM but can also contribute to elimination of some pathogens, while B2 cells primarily mediate response to foreign antigens. The differential molecular underpinning of the B1 and B2 subsets remains incompletely understood. Here we demonstrate that germline-deficiency of the orphan nuclear receptor NR2F6 causes a partial loss of B1b and B2 B cells in the peritoneum while leaving peritoneal B1a cells unaltered. A competitive bone marrow chimera in Nr2f6+/+ host mice produced similar numbers of Nr2f6+/+ and Nr2f6-/- peritoneal B1b and B2 cells. The proliferation of Nr2f6-/- peritoneal B cells was not altered, while the migration marker CXCR5 was reduced on all subsets but Beta7-integrin was reduced only on peritoneal B1b and B2 cells. Similarly, B1b and B2 but not B1a cells, exhibited significantly reduced survival.

T-bet+ B cells Dominate the Peritoneal Cavity B Cell Response during Murine Intracellular Bacterial Infection

T-bet⁺ B cells have emerged as a major B cell subset associated with both protective immunity and immunopathogenesis. T-bet is a transcription factor associated with the type I adaptive immune response to intracellular pathogens, driving an effector program characterized by the production of IFN-γ. Murine infection with the intracellular bacterium, Ehrlichia muris, generates protective extrafollicular T cell-independent T-bet⁺ IgM-secreting plasmablasts, as well as T-bet⁺ IgM memory cells. Although T-bet is a signature transcription factor for this subset, it is dispensable for splenic CD11c⁺ memory B cell development, but not for class switching to IgG2c. In addition to the T-bet⁺ plasmablasts found in the spleen, we show that Ab-secreting cells can also be found within the mouse peritoneal cavity; these cells, as well as their CD138^- counterparts, also expressed T-bet. A large fraction of the T-bet⁺ peritoneal B cells detected during early infection were highly proliferative and expressed CXCR3 and CD11b, but, unlike in the spleen, they did not express CD11c. T-bet⁺ CD11b⁺ memory B cells were the dominant B cell population in the peritoneal cavity at 30 d postinfection, and although they expressed high levels of T-bet, they did not require B cell-intrinsic T-bet expression for their generation. Our data uncover a niche for T-bet⁺ B cells within the peritoneal cavity during intracellular bacterial infection, and they identify this site as a reservoir for T-bet⁺ B cell memory.

Supplying the trip to antibody production-nutrients, signaling, and the programming of cellular metabolism in the mature B lineage

The COVID pandemic has refreshed and expanded recognition of the vital role that sustained antibody (Ab) secretion plays in our immune defenses against microbes and of the importance of vaccines that elicit Ab protection against infection. With this backdrop, it is especially timely to review aspects of the molecular programming that govern how the cells that secrete Abs arise, persist, and meet the challenge of secreting vast amounts of these glycoproteins. Whereas plasmablasts and plasma cells (PCs) are the primary sources of secreted Abs, the process leading to the existence of these cell types starts with naive B lymphocytes that proliferate and differentiate toward several potential fates. At each step, cells reside in specific microenvironments in which they not only receive signals from cytokines and other cell surface receptors but also draw on the interstitium for nutrients. Nutrients in turn influence flux through intermediary metabolism and sensor enzymes that regulate gene transcription, translation, and metabolism. This review will focus on nutrient supply and how sensor mechanisms influence distinct cellular stages that lead to PCs and their adaptations as factories dedicated to Ab secretion. Salient findings of this group and others, sometimes exhibiting differences, will be summarized with regard to the journey to a distinctive metabolic program in PCs.

Sphingosine-1-Phosphate Receptor Type 4 (S1P4) Is Differentially Regulated in Peritoneal B1 B Cells upon TLR4 Stimulation and Facilitates the Egress of Peritoneal B1a B Cells and Subsequent Accumulation of Splenic IRA B Cells under Inflammatory Conditions

Background: Gram-negative infections of the peritoneal cavity result in profound modifications of peritoneal B cell populations and induce the migration of peritoneal B cells to distant secondary lymphoid organs. However, mechanisms controlling the egress of peritoneal B cells from the peritoneal cavity and their subsequent trafficking remain incompletely understood. Sphingosine-1-phosphate (S1P)-mediated signaling controls migratory processes in numerous immune cells. The present work investigates the role of S1P-mediated signaling in peritoneal B cell trafficking under inflammatory conditions. Methods: Differential S1P receptor expression after peritoneal B cell activation was assessed semi‑quantitatively using RT-PCR in vitro. The functional implications of differential S1P₁ and S1P₄ expression were assessed by transwell migration in vitro, by adoptive peritoneal B cell transfer in a model of sterile lipopolysaccharide (LPS)‑induced peritonitis and in the polymicrobial colon ascendens stent peritonitis (CASP) model. Results: The two sphingosine-1-phosphate receptors (S1PRs) expressed in peritoneal B cell subsets S1P₁ and S1P₄ are differentially regulated upon stimulation with the TLR4 agonist LPS, but not upon PMA/ionomycin or B cell receptor (BCR) crosslinking. S1P₄ deficiency affects both the trafficking of activated peritoneal B cells to secondary lymphoid organs and the positioning of these cells within the functional compartments of the targeted organ. S1P₄ deficiency in LPS-activated peritoneal B cells results in significantly reduced numbers of splenic innate response activator B cells. Conclusions: The S1P-S1PR system is implicated in the trafficking of LPS-activated peritoneal B cells. Given the protective role of peritoneal B1a B cells in peritoneal sepsis, further experiments to investigate the impact of S1P₄-mediated signaling on the severity and mortality of peritoneal sepsis are warranted.

Role of adipose-associated lymphoid tissues in the immunological homeostasis of the serosal surface

Although not typical lymphoid organs, analysis of the visceral adipose-associated lymphoid tissues has recently substantially expanded our knowledge about the immunological features of these elusive compartments. Recent data have highlighted their considerable complexity in cellular organization and interactions in several biological processes, including adaptive immune responses, tissue plasticity to accommodate mesenchymal stem cells and progenitors, and providing a suitable microenvironment for serosal tumor propagation. This review aims to present a comprehensive view of the adipose-associated lymphoid tissues in local and systemic immune responsiveness, with particular emphasis on the omental and mesenteric lymphoid tissues in the serosal defense of abdominal organs.

Specialized immune responses in the peritoneal cavity and omentum

The peritoneal cavity is a fluid filled space that holds most of the abdominal organs, including the omentum, a visceral adipose tissue that contains milky spots or clusters of leukocytes that are organized similar to those in conventional lymphoid tissues. A unique assortment of leukocytes patrol the peritoneal cavity and migrate in and out of the milky spots, where they encounter Ags or pathogens from the peritoneal fluid and respond accordingly. The principal role of leukocytes in the peritoneal cavity is to preserve tissue homeostasis and secure tissue repair. However, when peritoneal homeostasis is disturbed by inflammation, infection, obesity, or tumor metastasis, specialized fibroblastic stromal cells and mesothelial cells in the omentum regulate the recruitment of peritoneal leukocytes and steer their activation in unique ways. In this review, the types of cells that reside in the peritoneal cavity, the role of the omentum in their maintenance and activation, and how these processes function in response to pathogens and malignancy will be discussed.

Antibody Production in Murine Polymicrobial Sepsis-Kinetics and Key Players

Although antigen-specific priming of antibody responses is impaired during sepsis, there is nevertheless a strong increase in IgM and IgG serum concentrations. Using colon ascendens stent peritonitis (CASP), a mouse model of polymicrobial abdominal sepsis, we observed substantial increases in IgM as well as IgG of all subclasses, starting at day 3 and peaking 2 weeks after sepsis induction. The dominant source of antibody-secreting cells was by far the spleen, with a minor contribution of the mesenteric lymph nodes. Remarkably, sepsis induction in splenectomized mice did not change the dynamics of the serum IgM/IgG reaction, indicating that the marginal zone B cells, which almost exclusively reside in the spleen, are dispensable in such a setting. Hence, in systemic bacterial infection, the function of the spleen as dominant niche of antibody-producing cells can be compensated by extra-splenic B cell populations as well as other lymphoid organs. Depletion of CD4+ T cells did not affect the IgM response, while it impaired IgG generation of all subclasses with the exception of IgG3. Taken together, our data demonstrate that the robust class-switched antibody response in sepsis encompasses both T cell-dependent and -independent components.

Homeostasis and regulation of autoreactive B cells

In contrast to the previous belief that autoreactive B cells are eliminated from the normal repertoire of B cells, many autoreactive B cells actually escape clonal deletion and develop into mature B cells. These autoreactive B cells in healthy individuals perform some beneficial functions in the host and are homeostatically regulated by regulatory T and B cells or other mechanisms to prevent autoimmune diseases. Autoreactive B-1 cells constitutively produce polyreactive natural antibodies for tissue homeostasis. Recently, autoreactive follicular B cells were reported to participate actively in the germinal center reaction. Furthermore, the selection and usefulness of autoreactive marginal zone (MZ) B cells found in autoimmune diseases are not well understood, although the repertoire of MZ B-cell receptors (BCRs) is presumed to be biased to detect bacterial antigens. In this review, we discuss the autoreactive B-cell populations among all three major B-cell subsets and their regulation in immune responses and diseases.

Resident Memory B Cells

In mammals, adaptive immunity is mediated by a broadly diverse repertoire of naive B and T lymphocytes that recirculate between secondary lymphoid organs. Initial antigen exposure promotes lymphocyte clonal expansion and differentiation, including the formation of memory cells. Antigen-specific memory cells are maintained at higher frequencies than their naive counterparts and have different functional and homing abilities. Importantly, a subset of memory cells, known as tissue-resident memory cells, is maintained without recirculating in nonlymphoid tissues, often at barrier surfaces, where they can be reactivated by antigen and rapidly perform effector functions that help protect the tissue in which they reside. Although antigen-experienced B cells are abundant at many barrier surfaces, their characterization as tissue-resident memory B (BRM) cells is not well developed. In this study, we describe the characteristics of memory B cells in various locations and discuss their possible contributions to immunity and homeostasis as bona fide BRM cells.

2019 Russell Ross Memorial Lecture in Vascular Biology: B Lymphocyte-Mediated Protective Immunity in Atherosclerosis

Atherosclerosis-the major underlying pathology of cardiovascular disease-is characterized by accumulation and subsequent oxidative modification of lipoproteins within the artery wall, leading to inflammatory cell infiltration and lesion formation that can over time result in arterial stenosis, ischemia, and downstream adverse events. The contribution of innate and adaptive immunity to atherosclerosis development is well established, and B cells have emerged as important modulators of both pro- and anti-inflammatory effects in atherosclerosis. Murine B cells can broadly be divided into 2 subsets: (1) B-2 cells, which are bone marrow derived and include conventional follicular and marginal zone B cells, and (2) B-1 cells, which are largely fetal liver derived and persist in adults through self-renewal. B-cell subsets are developmentally, functionally, and phenotypically distinct with unique subset-specific contributions to atherosclerosis development. Mechanisms whereby B cells regulate vascular inflammation and atherosclerosis will be discussed with a particular emphasis on B-1 cells. B-1 cells have a protective role in atherosclerosis that is mediated in large part by IgM antibody production. Accumulating evidence over the last several years has pointed to a previously underappreciated heterogeneity in B-1 cell populations, which may have important implications for understanding atherosclerosis development and potential targeted therapeutic approaches. This heterogeneity within atheroprotective innate B-cell subsets will be highlighted.

Commensal bacteria contribute to insulin resistance in aging by activating innate B1a cells

Aging in humans is associated with increased hyperglycemia and insulin resistance (collectively termed IR) and dysregulation of the immune system. However, the causative factors underlying their association remain unknown. Here, using "healthy" aged mice and macaques, we found that IR was induced by activated innate 4-1BBL⁺ B1a cells. These cells (also known as 4BL cells) accumulated in aging in response to changes in gut commensals and a decrease in beneficial metabolites such as butyrate. We found evidence suggesting that loss of the commensal bacterium Akkermansia muciniphila impaired intestinal integrity, causing leakage of bacterial products such as endotoxin, which activated CCR2⁺ monocytes when butyrate was decreased. Upon infiltration into the omentum, CCR2⁺ monocytes converted B1a cells into 4BL cells, which, in turn, induced IR by expressing 4-1BBL, presumably to trigger 4-1BB receptor signaling as in obesity-induced metabolic disorders. This pathway and IR were reversible, as supplementation with either A. muciniphila or the antibiotic enrofloxacin, which increased the abundance of A. muciniphila, restored normal insulin response in aged mice and macaques. In addition, treatment with butyrate or antibodies that depleted CCR2⁺ monocytes or 4BL cells had the same effect on IR. These results underscore the pathological function of B1a cells and suggest that the microbiome-monocyte-B cell axis could potentially be targeted to reverse age-associated IR.

The Peritoneal Macrophages in Inflammatory Diseases and Abdominal Cancers

Peritoneal macrophages (PMs) are the major cell type of peritoneal cells that participate in multiple aspects of innate and acquired immunity in the peritoneal cavity. PMs have an ability to release a large amount of proinflammatory and anti-inflammatory cytokines and therefore play a critical role in regulating the differentiation of innate immune cells and inflammatory T cells. Accumulating studies demonstrate that the immunological reactions and inflammatory responses of PMs are strongly related to the pathogenic processes of various inflammatory diseases and abdominal cancers. Consequently, the regulation of PM activation has gradually emerged as a promising target for immunotherapy, and better understanding of the distinctly biological function of PMs in individual diseases is crucial for designing specific and effective therapeutic agents. This review covers the characterization and immunological function of PMs in hosts with inflammatory diseases and abdominal cancers.

Intraperitoneal injection (IP), Intravenous injection (IV) or anal injection (AI)? Best way for mesenchymal stem cells transplantation for colitis

Stem cell transplantation showed promising results in IBD management. However, the therapeutic impacts of cell delivery route that is critical for clinical translation are currently poorly understood. Here, three different MSCs delivery routes: intraperitoneal (IP), intravenous (IV), and anal injection (AI) were compared on DSS-induced colitic mice model. The overall therapeutic factors, MSCs migration and targeting as well as local immunomodulatory cytokines and FoxP3⁺ cells infiltration were analyzed. Colitis showed varying degrees of alleviation after three ways of MSCs transplantation, and the IP injection showed the highest survival rate of 87.5% and displayed the less weight loss and quick weight gain. The fecal occult blood test on the day 3 also showed nearly complete absence of occult blood in IP group. The fluorescence imaging disclosed higher intensity of engrafted cells in inflamed colon and the corresponding mesentery lymph nodes (MLNs) in IP and AI groups than the IV group. Real time-PCR and ELISA also demonstrate lower TNF-α and higher IL-10, TSG-6 levels in IP group. The immunohistochemistry indicated higher repair proliferation (Ki-67) and more FoxP3⁺ cells accumulation of IP group. IP showed better colitis recovery and might be the optimum MSCs delivery route for the treatment of DSS-induced colitis.

Aging Converts Innate B1a Cells into Potent CD8+ T Cell Inducers

B cell dysregulation in aging is thought to mostly occur in conventional B2 cells without affecting innate B1 cells. Elderly humans and mice also accumulate 4-1BBL(+)MHC class-I(Hi)CD86(Hi)B cells of unknown origin. In this article, we report that these cells, termed 4BL cells, are activated murine and possibly human B1a cells. The activation is mediated by aging human monocytes and murine peritoneal macrophages. They induce expression and activation of 4-1BBL and IFN-γR1 on B1a cells to subsequently upregulate membrane TNF-α and CD86. As a result, activated B1a/4BL cells induce expression of granzyme B in CD8(+)T cells by targeting TNFR2 via membrane TNF-α and providing costimulation with CD86. Thus, for the first time, to our knowledge, these results indicate that aging affects the function of B1a cells. Upon aging, these cells lose their tumor-supporting activity and become inducers of potentially antitumor and autoimmune CD8(+)T cells.

IL-17A Promotes Pulmonary B-1a Cell Differentiation via Induction of Blimp-1 Expression during Influenza Virus Infection

B-1 cells play a critical role in early protection during influenza infections by producing natural IgM antibodies. However, the underlying mechanisms involved in regulating this process are largely unknown. Here we found that during influenza infection pleural cavity B-1a cells rapidly infiltrated lungs, where they underwent plasmacytic differentiation with enhanced IgM production. This process was promoted by IL-17A signaling via induction of Blimp-1 expression and NF-κB activation in B-1a cells. Deficiency of IL-17A led to severely impaired B-1a-derived antibody production in the respiratory tract, resulting in a deficiency in viral clearance. Transfer of B-1a-derived natural antibodies rescued Il17a^-/- mice from otherwise lethal infections. Together, we identify a critical function of IL-17A in promoting the plasmacytic differentiation of B-1a cells. Our findings provide new insights into the mechanisms underlying the regulation of pulmonary B-1a cell response against influenza infection.

Influenza infection is highly localized in respiratory tract where immune response is triggered to provide protection from primary infection. Although natural IgM antibodies produced by B-1a cells have long been recognized as first-line protection against influenza, it remains unclear whether B-1a cell response occurs in the lung and what molecular mechanisms regulate this process. We show that airway exposure to influenza causes migration of B-1a cells to lungs for further differentiation into plasma cells with enhanced production of protective IgM antibodies. IL-17A critically regulates this process by driving differentiation of B-1a cells to high-rate IgM producing plasma cells in situ. Thus, IL-17A is a key factor in the local inflammatory milieu that modulates early humoral immunity afforded by B-1a cells.

Autophagy is dispensable for B-cell development but essential for humoral autoimmune responses

To gain new insight into the role of B-cell autophagy, we generated two novel mouse models deficient for the autophagy-related gene (Atg)5, one from the outset pro-B cell stage (Atg5(f/-) Mb1 cre) and the other in mature B cells only (Atg5(f/-) CD21 cre). We show that autophagy is dispensable for pro- to pre-B cell transition, but necessary at a basal level to maintain normal numbers of peripheral B cells. It appears non-essential for B-cell activation under B-cell receptor stimulation but required for their survival after lipopolysaccharide stimulation that drives plasmablast differentiation and for specific IgM production after immunization. Results obtained using Atg5(f/-) CD21 cre × C57BL/6(lpr/lpr) autoimmune-prone mice show that B-cell autophagy is involved in the maintenance of anti-nuclear antibody secretion, elevated number of long-lived plasma cells, and sustains IgG deposits in the kidneys. Thus, treatments specifically targeting autophagy might be beneficial in systemic autoimmune diseases.

Treatment of dextran sodium sulfate-induced experimental colitis by adoptive transfer of peritoneal cells

The adoptive transfer of the natural regulatory B cells and macrophages should be a useful treatment for inflammation and autoimmune disease. However, it is usually difficult to isolate these cells from the tissues and expand them. Here, we investigated the feasibility of adoptively transferring peritoneal cells (PCs) as a treatment for DSS-induced colitis. We found that peritoneal cavity can provide an easily accessible site for harvesting enough number of PCs, namely, two-dose PCs for the treatment from a mouse in one operation. Adoptive therapy of these cells from healthy mice or those with disease is effectively in reducing the disease activity score. The natural B cells and macrophages of the infused PCs can selectively migrate to lesion sites and regulate the expression of Stat3, NF−κB, Smad3 and Smad7. Additionally, PCs exert dual activity of IL-10 and TGF-β secreted spontaneously by both peritoneal B cells and macrophages, which in turn enhance the induction of regulatory B cells and Macrophages in microenvironment of inflammation. Moreover, PCs can re-establish immunological tolerance in the OVA-immunized mice. Thus, our findings provide a new strategy for colitis therapy and could be of importance in additional exploration of other inflammation and autoimmune diseases therapy.

The omentum is a site of protective IgM production during intracellular bacterial infection

Infection of mice with the bacterium Ehrlichia muris elicits a protective T cell-independent (TI) IgM response mediated primarily by a population of CD11c-expressing plasmablasts in the spleen. Although splenic marginal zone (MZ) B cells are considered to be important for TI responses to blood-borne pathogens, MZ B cells were not responsible for generating plasmablasts in response to Ehrlichia muris. Moreover, antigen-specific serum IgM was decreased only modestly in splenectomized mice and in mice that lacked spleen, lymph nodes, and Peyer's patches (SLP mice). Both splenectomized and SLP mice were protected against lethal ehrlichial challenge infection. Moreover, we found a high frequency of Ehrlichia-specific plasmablasts in the omentum of both conventional and SLP mice. Omental plasmablasts elicited during Ehrlichia infection lacked expression of CD138 but expressed CD11c in a manner similar to that of their splenic counterparts. Selective ablation of CD11c-expressing B cells nearly eliminated the omental Ehrlichia-specific plasmablasts and reduced antigen-specific serum IgM, identifying the omental B cells as a source of IgM production in the SLP mice. Generation of the omental plasmablasts was route dependent, as they were detected following peritoneal infection but not following intravenous infection. Our data identify the omentum as an important auxiliary site of IgM production during intracellular bacterial infection.

The development and function of mucosal lymphoid tissues: a balancing act with micro-organisms

Mucosal surfaces are constantly exposed to environmental antigens, colonized by commensal organisms and used by pathogens as points of entry. As a result, the immune system has devoted the bulk of its resources to mucosal sites to maintain symbiosis with commensal organisms, prevent pathogen entry, and avoid unnecessary inflammatory responses to innocuous antigens. These functions are facilitated by a variety of mucosal lymphoid organs that develop during embryogenesis in the absence of microbial stimulation as well as ectopic lymphoid tissues that develop in adults following microbial exposure or inflammation. Each of these lymphoid organs samples antigens from different mucosal sites and contributes to immune homeostasis, commensal containment, and immunity to pathogens. Here we discuss the mechanisms, mostly based on mouse studies, that control the development of mucosal lymphoid organs and how the various lymphoid tissues cooperate to maintain the integrity of the mucosal barrier.

The quinoline-3-carboxamide paquinimod (ABR-215757) reduces leukocyte recruitment during sterile inflammation: leukocyte- and context-specific effects

Quinoline-3-carboxamides (Q-compounds) are currently in clinical development for both autoimmune disease and cancer. We have previously shown that the Q-compound paquinimod (ABR-215757) significantly ameliorates disease symptoms in several mouse models of human inflammatory disease. Considering that recruitment of inflammatory cells into tissue is a common denominator of these models, we have in this report investigated whether paquinimod would interfere with cell accumulation during sterile peritoneal inflammation. To mimic the cell recruitment elicited by tissue injury, we used necrotic cells to induce the acute inflammatory response. We show that per oral treatment with paquinimod significantly reduced the accumulation of Ly6C(hi) inflammatory monocytes and eosinophils, but not neutrophils, in this model, and that this correlated with reduced number of such cells also in the omentum. Treatment also reduced the accumulation of these cell populations at a subcutaneous site of inflammation. In alum-induced inflammation, however, neutrophils were the dominant cell population and paquinimod failed to reduce the accumulation of inflammatory cells. Taken together, our results indicate that paquinimod selectively inhibits cell recruitment during acute sterile inflammation, but that this effect is context-dependent. These data have important implications for the understanding of the mechanism of action of Q-compounds in both pre-clinical and clinical settings.

Peritoneal cavity regulatory B cells (B10 cells) modulate IFN-γ+CD4+ T cell numbers during colitis development in mice

The spleen regulatory B cell subset with the functional capacity to express IL-10 (B10 cells) modulates both immune responses and autoimmune disease severity. However, the peritoneal cavity also contains relatively high frequencies of functionally defined IL-10-competent B10 cells. In this study, peritoneal cavity B10 cells shared similar cell surface phenotypes with their spleen counterparts. However, peritoneal cavity B10 cells were 10-fold more frequent among B cells than occurred within the spleen, intestinal tract, or mesenteric lymph nodes and were present at higher proportions among the phenotypically defined peritoneal B1a > B1b > B2 cell subpopulations. The development or localization of B10 cells within the peritoneal cavity was not dependent on the presence of commensal microbiota, T cells, IL-10 or B10 cell IL-10 production, or differences between their fetal liver or adult bone marrow progenitor cell origins. The BCR repertoire of peritoneal cavity B10 cells was diverse, as occurs in the spleen, and predominantly included germline-encoded VH and VL regions commonly found in either the conventional or B1 B cell compartments. Thereby, the capacity to produce IL-10 appears to be an intrinsic functional property acquired by clonally diverse B cells. Importantly, IL-10 production by peritoneal cavity B cells significantly reduced disease severity in spontaneous and induced models of colitis by regulating neutrophil infiltration, colitogenic CD4(+) T cell activation, and proinflammatory cytokine production during colitis onset. Thus, the numerically small B10 cell subset within the peritoneal cavity has regulatory function and is important for maintaining homeostasis within gastrointestinal tissues and the immune system.

New concepts in the generation and functions of IgA

The intestinal mucosa contains the largest population of antibody-secreting plasma cells in the body, and in humans several grams of secretory immunoglobulin A (SIgA) are released into the intestine each day. In the gut lumen, SIgA serves as a first-line barrier that protects the epithelium from pathogens and toxins. Recently, next-generation sequencing has revolutionized our understanding of the nature of the intestinal microbiota and has also shed new light on the important roles of SIgA in the regulation of host-commensal homeostasis. Here, I discuss pathways of IgA induction in the context of SIgA specificity and function.

Characterization of omental immune aggregates during establishment of a latent gammaherpesvirus infection

Herpesviruses are characterized by their ability to establish lifelong latent infection. The gammaherpesvirus subfamily is distinguished by lymphotropism, establishing and maintaining latent infection predominantly in B lymphocytes. Consequently, gammaherpesvirus pathogenesis is closely linked to normal B cell physiology. Murine gammaherpesvirus 68 (MHV68) pathogenesis in laboratory mice has been extensively studied as a model system to gain insights into the nature of gammaherpesvirus infection in B cells and their associated lymphoid compartments. In addition to B cells, MHV68 infection of macrophages contributes significantly to the frequency of viral genome-positive cells in the peritoneal cavity throughout latency. The omentum, a sheet of richly-vascularized adipose tissue, resides in the peritoneal cavity and contains clusters of immune cell aggregates termed milky spots. Although the value of the omentum in surgical wound-healing has long been appreciated, the unique properties of this tissue and its contribution to both innate and adaptive immunity have only recently been recognized. To determine whether the omentum plays a role in gammaherpesvirus pathogenesis we examined this site during early MHV68 infection and long-term latency. Following intraperitoneal infection, immune aggregates within the omentum expanded in size and number and contained virus-infected cells. Notably, a germinal-center B cell population appeared in the omentum of infected animals with earlier kinetics and greater magnitude than that observed in the spleen. Furthermore, the omentum harbored a stable frequency of viral genome-positive cells through early and into long-term latency, while removal of the omentum prior to infection resulted in a slight decrease in the establishment of splenic latency following intraperitoneal infection. These data provide the first evidence that the omentum is a site of chronic MHV68 infection that may contribute to the maintenance of chronic infection.

Key role of CXCL13/CXCR5 axis for cerebrospinal fluid B cell recruitment in pediatric OMS

To study aberrant B cell trafficking into the CSF in opsoclonus-myoclonus syndrome (OMS), chemoattractants CXCL13 and CXCL12, and B cell frequency and CXCR5 expression, were evaluated. CSF CXCL13 concentration and the CSF/serum ratio were higher in untreated OMS than controls, related directly to OMS severity and inversely to OMS duration, and correlated with CSF B cell frequency and oligoclonal bands. CXCL12 showed the opposite pattern. Selective accumulation of CXCR5+ memory B cells in CSF was found. In ACTH-treated OMS, CXCL13, but not CXCL12, was lower. These data implicate the chemokine/chemoreceptor pair CXCL13/CXR5 in B cell recruitment to the CNS in OMS. CXCL13 and CXCL12 may serve as reciprocal biomarkers of disease activity, but CXCL13 also had utility as a treatment biomarker.

Induction of gut IgA production through T cell-dependent and T cell-independent pathways

The gut immune system protects against mucosal pathogens, maintains a mutualistic relationship with the microbiota, and establishes tolerance against food antigens. This requires a balance between immune effector responses and induction of tolerance. Disturbances of this strictly regulated balance can lead to infections or the development inflammatory diseases and allergies. Production of secretory IgA is a unique effector function at mucosal surfaces, and basal mechanisms regulating IgA production have been the focus of much recent research. These investigations have aimed at understanding how long-term IgA-mediated mucosal immunity can best be achieved by oral or sublingual vaccination, or at analyzing the relationship between IgA production, the composition of the gut microbiota, and protection from allergies and autoimmunity. This research has lead to a better understanding of the IgA system; but at the same time seemingly conflicting data have been generated. Here, we discuss how gut IgA production is controlled, with special focus on how differences between T cell-dependent and T cell-independent IgA production may explain some of these discrepancies.

Peritoneal cavity is dominated by IFNγ-secreting CXCR3+ Th1 cells

The chemokine receptor CXCR3, which was shown to take part in many inflammatory processes, is considered as a Th1 specific marker. Here, we show in a mouse model that CXCR3 expressing CD4⁺ cells preferentially migrate to the peritoneal cavity under steady-state conditions. The peritoneal cavity milieu leads to an up-regulated expression of CXCR3. However, blocking of known ligands of this chemokine receptor did not alter the preferential migration. The peritoneal cavity environment also results in an increased percentage of memory cells producing cytokines. Up-regulation of IFNγ production occurs mostly in CXCR3⁺ cells considered as Th1, whereas the up-regulation of IL-4 affects mostly in CXCR3⁻ cells which are considered as Th2. We conclude that the peritoneal cavity does not change the Th-lineage of the cells, but that domination of this anatomic niche by Th1 cells rather results from preferential migration to this compartment.

Universal expression and dual function of the atypical chemokine receptor D6 on innate-like B cells in mice

Mouse innate-like B cells are a heterogeneous collection of multifunctional cells that control infection, play housekeeping roles, contribute to adaptive immunity, and suppress inflammation. We show that, among leukocytes, chemokine internalization by the D6 receptor is a unique and universal feature of all known innate-like B-cell populations and, to our knowledge, the most effective unifying marker of these cells. Moreover, we identify novel D6(active) B1-cell subsets, including those we term B1d, which lack CD5 and CD11b but exhibit typical B1-cell properties, including spontaneous ex vivo production of IgM, IL-10, and anti-phosphorylcholine antibody. The unprecedented opportunity to examine D6 on primary cells has allowed us to clarify its ligand specificity and show that, consistent with a scavenging role, D6 internalizes chemokines but cannot induce Ca(2+) fluxes or chemotaxis. Unexpectedly, however, D6 can also suppress the function of CXCR5, a critical chemokine receptor in innate-like B-cell biology. This is associated with a reduction in B1 cells and circulating class-switched anti-phosphorylcholine antibody in D6-deficient mice. Therefore, in the present study, we identify a unifying marker of innate-like B cells, describe novel B1-cell subsets, reveal a dual role for D6, and provide the first evidence of defects in resting D6-deficient mice.

CCR7 essentially contributes to the homing of plasmacytoid dendritic cells to lymph nodes under steady-state as well as inflammatory conditions

The chemokine receptor CCR7 represents an important determinant for circulating lymphocytes to enter lymph nodes (LN) via high endothelial venules. High endothelial venules also represent the major site of entry for plasmacytoid dendritic cells (pDC). In the steady-state, murine pDC have been suggested to home to LN engaging the chemokine receptors CXCR3, CXCR4, and CCR5, whereas responsiveness to CCR7 ligands is thought to be acquired only upon activation. In this study, we show that already resting pDC express minute amounts of CCR7 that suffice to trigger migration to CCL19/CCL21 in vitro. Upon activation with TLR ligands, CCR7 levels on pDC are strongly increased. Notably, CCR7-deficient mice display substantially reduced pDC counts in LN but not in bone marrow and spleen. Adoptive cell transfer experiments revealed that under both steady-state as well as inflammatory conditions, the homing of CCR7-deficient pDC is severely impaired, indicating that the reduced cell counts of naive pDC observed in CCR7(-/-) mice reflect an intrinsic homing defect of pDC. Together, these observations provide strong evidence that similar to naive lymphocytes, nonstimulated pDC exploit CCR7 to gain entry into LN. This adds to the repertoire of chemokine receptors permitting them to enter diverse tissues.

Roles of B-1 and B-2 cells in innate and acquired IgA-mediated immunity

The gut harbors an extremely dense and complex community of microorganisms that are in constant dialog with our immune cells. The gut bacteria provide strong selective pressure to the host to evolve innate and adaptive immune responses required for the maintenance of local and systemic homeostasis. One of the most conspicuous responses of the gut immune system following microbial colonization is the production of immunoglobulin A (IgA). In this review, we discuss the roles of B-1 and B-2 cells in IgA-mediated immunity and present an updated view for the sites and mechanisms of IgA synthesis in the gut. We summarize the role of secretory IgAs for regulation of microbial communities and provide clues as to how the gut microbiota contributes to the development of the gut-associated lymphoid tissues.

Chemokines and chemokine receptors in chronic lymphocytic leukemia (CLL): from understanding the basics towards therapeutic targeting

Chemokines and their receptors organize the recruitment and positioning of cells at each stage of the immune response, a system critically dependent upon coordination to get the right cells to the right place at the right time. Chemokine receptors expressed on CLL B cells are thought to function in a similar fashion, regulating the trafficking of the leukemia cells between blood, lymphoid organs, and the bone marrow, and within sub compartments within these tissues, in concert with adhesion molecules and other guidance cues. CLL cells not only respond to chemokines secreted in the microenvironment, the leukemia cells also secrete chemokines in response to external signals, such as B cell receptor engagement. These CLL cell-derived chemokines facilitate interactions between CLL cells, T cells, and other immune cells that shape the CLL microenvironment. CXCR4, the most prominent chemokine receptor in CLL, is now targeted in a first clinical trial, emphasizing that chemokines and their receptors have become a highly dynamic translational research field.

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.

CCR7 regulates lymphocyte egress and recirculation through body cavities

T and B lymphocytes recirculate among blood, lymph, and extralymphoid tissues to ensure immune surveillance and the establishment of self-tolerance. The underlying mechanisms regulating homeostatic lymphocyte recirculation through body cavities are not fully understood. Here, we demonstrate that the homeostatic chemokine receptor CCR7 regulates homeostatic recirculation of lymphocytes through body cavities. CCR7 deficiency results in massive accumulation of CD4(+) and CD8(+) T cells and B-2 B cells in the peritoneal and pleural cavities. The increase in B-2 B and T lymphocytes is not associated with an altered maturation and/or activation status of these cells. Mechanistically, an increase in peritoneal lymphocyte numbers is caused by impaired egress of CCR7-deficient lymphocytes from body cavities. These results establish that CCR7 plays a crucial role in lymphocyte exit from the PerC.

Antigen-dependent rescue of nose-associated lymphoid tissue (NALT) development independent of LTbetaR and CXCR5 signaling

Nose-associated lymphoid tissue (NALT) in the rodent upper respiratory tract develops postnatally and is considered to be independent of several factors known to be involved in the organogenesis of LN and Peyer's patches (PP). In this study we demonstrate that at least two different pathways result in NALT development. Following NALT anlage formation the intrinsic pathway relies on a signaling cascade including those mediated through the chemokine receptor CXCR5 and the lymphotoxin beta receptor (LTbetaR). This allows for the formation of high endothelial venules and thereby the recruitment of lymphocytes into NALT. Alternatively, high endothelial venule formation and lymphocyte recruitment can be induced in the NALT anlage by environmental signals, which are independent of LT-betaR and chemokine receptor CXCR5 signaling but in part rely on CD40 ligand. Thus, our study identifies a novel mechanism that facilitates the rescue of NALT development at late stages in adult life independent of the canonical LTbetaR-CXCR5 signaling axis.

Identification of a chemokine receptor profile characteristic for mediastinal large B-cell lymphoma

Mediastinal large B-cell lymphomas (MLBCLs) are considered as a subtype of diffuse large B-cell lymphoma; however, they exhibit completely different patterns of dissemination. Since they share a number of surface markers with thymic B cells, a close relationship was assumed. MLBCLs arise extranodally within the anterior mediastinum and have a low propensity to metastasize. To address the preferential positioning of MLBCL, we focused on homeostatic chemokines involved in B-cell compartmental homing in secondary lymphoid organs, which are also capable of shaping lymphoid niches in ectopic sites. Here, we applied immunohistochemistry to assess chemokine receptor and ligand expression in situ. Flow cytometry was used to identify the chemokine receptor profile on an MLBCL-derived cell line, Karpas1106 and on thymic B cells. Migration assays were performed to examine functionality of chemokine receptors. Electrophoretic mobility shift assay was applied to score for NF-kappaB activity. Using immunohistochemistry, we obtained an unexpectedly low-expression frequency for the chemokine receptors CXCR5 and CCR7 in primary lesions. Although the mature B-cell marker CCR6 was absent in most cases, the lineage aberrant marker CCR9 emerged in the majority of MLBCL cases. Given the role of NF-kappaB in the transcriptional activation of CCR7, we identified the involvement of the noncanonical activation pathway in MLBCLs. MLBCLs exhibit a diagnostic chemokine receptor profile that is instrumental in the discrimination from diffuse large B-cell lymphoma not otherwise specified and classical Hodgkin lymphoma. Furthermore, we suggest that low-abundance expression of CCR7 and CXCR5 may hinder lymphoma cells from nodal dissemination.

Suppression of CXCR4 expression in mast cells upon IgE-mediated antigen stimulation

OBJECTIVE

Recent studies have demonstrated that a variety of chemokine receptors are expressed in mast cells. We investigated the changes in mRNA expression of CXCRs in murine IL-3-dependent bone marrow-derived mast cells (BMMCs) to clarify how the CXCR expression is regulated in mast cells.

METHODS

Expression of CXCR mRNA was measured by RNase protection assay. Functional expression of CXCRs was confirmed by monitoring intracellular Ca(2+) mobilization.

RESULTS

CXCR4 mRNA expression was transiently induced in BMMCs in serum-dependent fashion and was completely suppressed upon IgE-mediated antigen stimulation. In contrast, CXCR5 mRNA expression was induced upon IgE-mediated antigen stimulation. Changes in the intracellular Ca(2+) mobilization induced by CXCL12 strongly indicated the functional expression of CXCR4. The decrease in CXCR4 and the increase in CXCR5 mRNA expression was also observed in BMMCs stimulated with thapsigargin, a phorbol ester, and stem cell factor.

CONCLUSION

The mRNA expression of CXCR4 is differentially regulated in BMMCs upon various stimuli including IgE-mediated antigen stimulation.

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.

Chemokine receptor CXCR5 supports solitary intestinal lymphoid tissue formation, B cell homing, and induction of intestinal IgA responses

Solitary intestinal lymphoid tissue (SILT) comprises a spectrum of phenotypically diverse lymphoid aggregates interspersed throughout the small intestinal mucosa. Manifestations of SILT range from tiny lymphoid aggregates almost void of mature lymphocytes to large structures dominated by B cells. Large SILT phenotypically resemble a single Peyer's patch follicle, suggesting that SILT might contribute to intestinal humoral immune responses. In this study, we track the fate of individual SILT in vivo over time and analyze SILT formation and function in chemokine receptor CXCR5-deficient mice. We show that, in analogy to Peyer's patches, formation of SILT is invariantly determined during ontogeny and depends on CXCR5. Young CXCR5-deficient mice completely lack SILT, suggesting that CXCR5 is essential for SILT formation during regular postnatal development. However, microbiota and other external stimuli can induce the formation of aberrant SILT distinguished by impaired development of B cell follicles in CXCR5-deficient mice. Small intestinal transplantation and bone marrow transplantation reveal that defect follicle formation is due to impaired B cell homing. Moreover, oral immunization with cholera toxin or infection with noninvasive Salmonella fail to induce efficient humoral immune responses in CXCR5-deficient mice. Bone marrow transplantation of CXCR5-deficient recipients with wild-type bone marrow rescued B cell follicle formation in SILT but failed to restore full humoral immune responses. These results reveal an essential role of CXCR5 in Peyer's patch and SILT development and function and indicate that SILT do not fully compensate for the lack of Peyer's patches in T cell-dependent humoral immune responses.