Mechanism of TLR4-Mediated Anti-Inflammatory Response Induced by Exopolysaccharide from the Probiotic Bacillus subtilis.

Intestinal inflammatory diseases affect millions of people worldwide, and one class of drugs showing promise toward treatment of several inflammatory diseases is probiotics. Numerous studies have been performed using probiotics to prevent and treat intestinal inflammatory diseases. Most of these studies used intact bacteria, and neither the active molecule nor the molecular mechanisms by which they affect immune responses are known. We have shown that the probiotic Bacillus subtilis is anti-inflammatory and can protect mice from acute colitis induced by the enteric pathogen Citrobacter rodentium. We identified and purified the active molecule, exopolysaccharide (EPS), and showed that it protects mice from C. rodentium-induced colitis by inducing anti-inflammatory M2 macrophages or inhibitory dendritic cells (DCs), both of which inhibit excessive T cell responses. We showed previously that EPS affects macrophages and DCs in a TLR4-dependent manner, and in the current study we asked how EPS induces these anti-inflammatory cells and how they function to inhibit T cells. By investigating the signaling downstream of TLR4 that leads to acquisition of inhibitory properties of macrophages and DCs, we found that EPS induces expression of the inhibitory molecule IDO in bone marrow-derived DCs, and that inhibition of T cell proliferation by IDO-expressing bone marrow-derived DCs utilizes the kynurenine/aryl hydrocarbon receptor circuit. Furthermore, unlike LPS, EPS does not induce inflammatory cytokines upon injection in vivo, directly demonstrating different outcomes induced by two different TLR4 agonists.

Post COVID-19 conditions in an Australian pediatric cohort, 3 months following a Delta outbreak.

BACKGROUND: Pediatric long COVID remains incompletely understood with scant Australian data available. We aimed to assess the impacts of the 2021 Delta variant of SARS-CoV-2 outbreak on symptoms and functioning 12 weeks post-acute infection in a cohort of children and adolescents. METHODS: The parents/carers of 11,864 patients with PCR-confirmed SARS-CoV-2 were invited, via email or text message, to complete an online survey assessing symptoms and functional impairment. FINDINGS: 1731 (17.6%) responded to the survey. 203 (11.7%) reported continued symptoms and/or functional impairment which were flagged for clinical review, all others reported recovery. Of the 169 subsequently clinically reviewed, 63 had already recovered (37.3%) and 17 had exacerbation of pre-existing condition(s) (10.1%); 63 (37.3%) were diagnosed with a Post COVID Condition (PCC). Of these, 21 (12.4%) were considered to have features compatible with the United Kingdom consensus cases definition for Long COVID. INTERPRETATION: During an outbreak of SARS-CoV-2 an online questionnaire with subsequent clinical review revealed self-reported non-recovery at 12 weeks in a minority of cases, with a spectrum of features. Long COVID comprised only a subset of cases with self-reported non-recovery, and is infrequent in children and adolescents, but still comprises a likely significant burden that warrants attention. IMPACT: Our study provides the only comprehensive estimate of the frequency and spectrum of post-COVID conditions in children from Australia. The high frequency of self-reported recovery, and low frequency of Long COVID compatible illness adds to the literature from other settings. Risk factors for post-COVID conditions in children are identified and include: age >11 year, and previous medical co-morbidity.

Youth suicidality risk relative to ambient temperature and heatwaves across climate zones: A time series analysis of emergency department presentations in New South Wales, Australia.

OBJECTIVE: Youth suicidality prevalence continues to rise alongside hot weather severity. Links between these two variables are underexplored. We examined associations between daily temperature and emergency department suicidality presentations by young people. We assessed these associations for five regions covering New South Wales as determined by 'climate zone' and analysed for heatwave effects as well as based on demographic subgroups. METHODS: Daily emergency department presentations for suicidality by people aged 12-24 years across New South Wales, Australia, during warmer months (November to March) from 2012 to 2019 were examined in relation to daily mean temperature and heatwaves (⩾3 consecutive days ⩾ 95th percentile of long-term daily mean temperature) and by climate zone, using a generalised additive model with negative binomial distribution. Risks for age- and sex-based subgroups were also calculated. RESULTS: New South Wales youth suicidality presentation rates were significantly higher on hotter days. For every 1°C rise above average daily mean temperature, youth suicidality presentations to New South Wales emergency departments increased by 1.3%. Heatwaves did not increase presentation rates beyond single-day daily mean temperature effects. These findings were predominantly replicated across climate zones and demographic subgroups, though the association between suicidality and ambient temperature was weaker in coastal regions including Eastern Sydney. CONCLUSION: There is a positive linear association between ambient temperature and youth suicidality presentations to emergency departments. Risks are increased on single hot days, not only during heatwaves. Public health, broader societal approaches to heat and health system planning should consider impacts on youth suicidality of predicted increases in hot weather severity and frequency.

Amelioration of Graft-versus-Host Disease by Exopolysaccharide from a Commensal Bacterium.

Acute graft-versus-host disease (aGvHD) is a severe, often lethal, complication of hematopoietic stem cell transplantation, and although prophylactic regimens are given as standard pretransplantation therapy, up to 60% of these patients develop aGvHD, and require additional immunosuppressive intervention. We treated mice with a purified probiotic molecule, exopolysaccharide (EPS) from Bacillus subtilis, shortly before and after induction of aGvHD and found that, whereas only 10% of control mice survived to day 80, 70% of EPS-treated mice survived to 80 d. EPS treatment of donor-only mice resulted in ∼60% survival. Using a biosensor mouse model to assess inflammation in live mice during aGvHD, we found that EPS prevented the activation of alloreactive donor T cells. In vitro, EPS did not affect T cells directly but, instead, induced bone marrow-derived dendritic cells (BMDCs) that displayed characteristics of inhibitory dendritic cells (DCs). Development of these BMDCs required TLR4 signaling through both MyD88 and TRIF pathways. Using BMDCs derived from IDO knockout mice, we showed that T cell inhibition by EPS-treated BMDCs was mediated through the suppressive effects of IDO. These studies describe a bacterial molecule that modulates immune responses by inducing inhibitory DCs in a TLR4-dependent manner, and these cells have the capacity to inhibit T cell activation through IDO. We suggest that EPS or EPS-treated DCs can serve as novel agents for preventing aGvHD.

Growth in emergency department self-harm or suicidal ideation presentations in young people: Comparing trends before and since the COVID-19 first wave in New South Wales, Australia.

INTRODUCTION: Self-harm presentations in children and young people have increased internationally over the last decade. The COVID-19 pandemic has the potential to worsen these trends. OBJECTIVE: To describe trends in emergency department self-harm or suicidal ideation presentations for children and young people in New South Wales before and since the COVID-19 pandemic. METHODS: We studied presentations for self-harm or suicidal ideation by 10- to 24-year-olds to New South Wales emergency departments, using interrupted time series analysis to compare annualised growth before COVID (2015 to February 2020) and since COVID (March 2020 to June 2021). Subgroup analyses compared age group, gender, triage category, rurality and disadvantage. Time series decomposition via generalised additive models identified long-term, seasonal and short-term trends. RESULTS: Self-harm or suicidal ideation presentations by young people in New South Wales increased by 8.4% per annum pre-COVID. Growth accelerated since COVID, to 19.2% per annum, primarily due to increased presentations by females aged 13-17 years (47.1% per annum since COVID, from 290 per 10,000 in 2019 to 466 per 10,000 in 2021). Presentations in males aged 10-24 years did not increase since COVID (105.4 per 10,000 in 2019, 109.8 per 10,000 in 2021) despite growing 9.9% per annum before COVID. Presentation rates accelerated significantly in socio-economically advantaged areas. Presentations in children and adolescents were strongly linked to school semesters. CONCLUSION: Emergency department self-harm or suicidal ideation presentations by New South Wales young people grew steadily before COVID. Understanding the sustained increase remains a priority. Growth has increased since COVID particularly for adolescent females, but not among adolescent males. Surprisingly, the largest post-COVID increases in annual growth occurred in socio-economically advantaged and urban regions. The COVID-19 pandemic appears to have added new challenges, particularly in females in the developmentally critical early adolescent and teenage years.

Human CD36hi monocytes induce Foxp3+ CD25+ T cells with regulatory functions from CD4 and CD8 subsets.

The fetal and neonatal immune systems are uniquely poised to generate tolerance to self, maternal and environmental antigens encountered in the womb and shortly after birth. However, the tolerogenic nature of fetal and neonatal immunity can be detrimental in the context of pathogens, leading to overwhelming bacterial infections or chronic viral infections. A variety of mechanisms contribute to fetal and neonatal tolerance, including a propensity to generate Foxp3+ regulatory T cells (Treg cells). However, the mechanism(s) of fetal Foxp3+ T-cell differentiation, the specific antigen-presenting cells required and factors that inhibit Treg generation after the neonatal period are poorly understood. Here, we demonstrate that a subset of CD14+ monocytes expressing the scavenger molecule, CD36, can generate CD4+ and CD8+ T cells that coexpress Foxp3 and T-bet from both umbilical cord blood. These Foxp3+ T-bet+ T cells potently suppress T-cell proliferation and ameliorate xenogeneic graft-versus-host disease. CD14+  CD36+ monocytes provide known Treg-inducing signals: membrane-bound transforming growth factor-beta and retinoic acid. Unexpectedly, adult peripheral blood monocytes are also capable of inducing Foxp3+ T cells from both cord blood and adult peripheral naïve T cells. The induction of Foxp3+ T cells in umbilical cord blood by monocytes was inhibited by the lymphoid fraction of adult peripheral blood cells. These studies highlight a novel immunoregulatory role of monocytes and suggest that antigen presentation by CD36hi monocytes may contribute to the peripheral development of Foxp3+ T-bet+ T cells with regulatory functions in both neonates and adults.

A Caspase-1 Biosensor to Monitor the Progression of Inflammation In Vivo.

Inflammasomes are multiprotein complexes that coordinate cellular inflammatory responses and mediate host defense. Following recognition of pathogens and danger signals, inflammasomes assemble and recruit and activate caspase-1, the cysteine protease that cleaves numerous downstream targets, including pro-IL-1ß and pro-IL-18 into their biologically active form. In this study, we sought to develop a biosensor that would allow us to monitor the initiation, progression, and resolution of inflammation in living animals. To this end, we inserted a known caspase-1 target sequence into a circularly permuted luciferase construct that becomes bioluminescent upon protease cleavage. This biosensor was activated in response to various inflammatory stimuli in human monocytic cell lines and murine bone marrow-derived macrophages. Next, we generated C57BL/6 transgenic mice constitutively expressing the caspase-1 biosensor. We were able to monitor the spatiotemporal dynamics of caspase-1 activation and onset of inflammation in individual animals in the context of a systemic bacterial infection, colitis, and acute graft-versus-host disease. These data established a model whereby the development and progression of inflammatory responses can be monitored in the context of these and other mouse models of disease.

Suppression of Staphylococcus aureus Superantigen-Independent Interferon Gamma Response by a Probiotic Polysaccharide.

Staphylococcus aureus is a Gram-positive opportunistic pathogen that causes a variety of diseases. Bloodstream infection is the most severe, with mortality rates reaching 20 to 50%. Exopolysaccharide (EPS) from the probiotic Bacillus subtilis reduces bacterial burden and inflammation during S. aureus bloodstream infection in mice. Protection is due, in part, to hybrid macrophages that restrict S. aureus growth through reactive oxygen species and to limiting superantigen-induced T cell activation and interferon gamma (IFN-γ) production during infection. A decrease in IFN-γ production was observed within 24 h after infection, and here, we investigated how EPS abrogates its production. We discovered that S. aureus uses a rapid, superantigen-independent mechanism to induce host IFN-γ and that this is mediated by interleukin-12 (IL-12) activation of NK cells. Furthermore, we found that EPS limits IFN-γ production by modulating host immunity in a Toll-like receptor 4 (TLR4)-dependent manner, a signaling pathway that is required for EPS-mediated protection from S. aureus infection in vivo We conclude that EPS protects hosts from acute bloodstream S. aureus infection not only by inducing macrophages that restrict S. aureus growth and inhibit superantigen-activated T cells but also by limiting NK cell production of IFN-γ after S. aureus infection in a TLR4-dependent manner.

Probiotic Exopolysaccharide Protects against Systemic Staphylococcus aureus Infection, Inducing Dual-Functioning Macrophages That Restrict Bacterial Growth and Limit Inflammation.

Staphylococcus aureus causes severe systemic infection with high mortality rates. We previously identified exopolysaccharide (EPS) from a probiotic, Bacillus subtilis, that induces anti-inflammatory macrophages with an M2 phenotype and protects mice from Citrobacter rodentium-induced colitis. We tested if EPS could protect from systemic infection induced by S. aureus and found that EPS-treated mice had enhanced survival as well as reduced weight loss, systemic inflammation, and bacterial burden. While macrophages from EPS-treated mice display an M2 phenotype, they also restrict growth of internalized S. aureus through reactive oxygen species (ROS), reminiscent of proinflammatory phagocytes. These EPS-induced macrophages also limit T cell activation by S. aureus superantigens, and EPS abrogates systemic induction of gamma interferon after infection. We conclude that B. subtilis EPS is an immunomodulatory agent that induces hybrid macrophages that bolster antibacterial immunity and simultaneously limit inflammation, reducing disease burden and promoting host survival.

Inflammatory Changes in Bone Marrow Microenvironment Associated with Declining B Lymphopoiesis.

B lymphopoiesis arrests precipitously in rabbits such that by 2-4 mo of age, before sexual maturity, little to no B lymphopoiesis occurs in the bone marrow (BM). Previously, we showed that in mice, adipocytes inhibit B lymphopoiesis in vitro by inducing inflammatory myeloid cells, which produce IL-1ß. In this study, we characterized rabbit BM after the arrest of B lymphopoiesis and found a dramatic increase in fat, increased CD11b+ myeloid cells, and upregulated expression of the inflammatory molecules, IL-1ß and S100A9, by the myeloid cells. We added BM fat, CD11b+ myeloid cells, and recombinant S100A9 to B lymphopoiesis cultures and found that they inhibited B lymphopoiesis and enhanced myelopoiesis. Unlike IL-1ß, which inhibits B lymphopoiesis by acting on early lymphoid progenitors, S100A9 inhibits B lymphopoiesis by acting on myeloid cells and promoting the release of inflammatory molecules, including IL-1ß. Many molecules produced by adipocytes activate the NLRP3 inflammasome, and the NLRP3 inhibitor, glibenclamide, restored B lymphopoiesis and minimized induction of myeloid cells induced by adipocyte-conditioned medium in vitro. We suggest that fat provides an inflammatory microenvironment in the BM and promotes/activates myeloid cells to produce inflammatory molecules such as IL-1ß and S100A9, which negatively regulate B lymphopoiesis.

Exopolysaccharide from Bacillus subtilis Induces Anti-Inflammatory M2 Macrophages That Prevent T Cell-Mediated Disease.

Commensal bacteria contribute to immune homeostasis in the gastrointestinal tract; however, the underlying mechanisms for this are not well understood. A single dose of exopolysaccharide (EPS) from the probiotic spore-forming bacterium Bacillus subtilis protects mice from acute colitis induced by the enteric pathogen Citrobacter rodentium Adoptive transfer of macrophage-rich peritoneal cells from EPS-treated mice confers protection from disease to recipient mice. In vivo, EPS induces development of anti-inflammatory M2 macrophages in a TLR4-dependent manner, and these cells inhibit T cell activation in vitro and in C. rodentium-infected mice. In vitro, M2 macrophages inhibit CD4+ and CD8+ T cells. The inhibition of CD4+ T cells is dependent on TGF-ß, whereas inhibition of CD8+ T cells is dependent on TGF-ß and PD-L1. We suggest that administration of B. subtilis EPS can be used to broadly inhibit T cell activation and, thus, control T cell-mediated immune responses in numerous inflammatory diseases.

Inhibition of B Lymphopoiesis by Adipocytes and IL-1-Producing Myeloid-Derived Suppressor Cells.

B lymphopoiesis declines with age, and this decline correlates with increased adipose tissue in the bone marrow (BM). Also, adipocyte-derived factors are known to inhibit B lymphopoiesis. Using cocultures of mouse BM cells with OP9 stromal cells, we found that adipocyte-conditioned medium induces the generation of CD11b(+)Gr1(+) myeloid cells, which inhibit B cell development in vitro. Adipocyte-conditioned medium-induced CD11b(+)Gr1(+) cells express Arg1 (arginase) and Nos2 (inducible NO synthase) and suppress CD4(+) T cell proliferation, indicating that these cells are myeloid-derived suppressor cells (MDSCs). Blocking arginase and inducible NO synthase did not restore B lymphopoiesis, indicating that inhibition is not mediated by these molecules. Transwell and conditioned-medium experiments showed that MDSCs inhibit B lymphopoiesis via soluble factors, and by cytokine array we identified IL-1 as an important factor. Addition of anti-IL-1 Abs restored B lymphopoiesis in BM cultures containing MDSCs, showing that MDSC inhibition of B lymphopoiesis is mediated by IL-1. By treating hematopoietic precursors with IL-1, we found that multipotent progenitors are targets of IL-1. This study uncovers a novel function for MDSCs to inhibit B lymphopoiesis through IL-1. We suggest that inflammaging contributes to a decline of B lymphopoiesis in aged individuals, and furthermore, that MDSCs and IL-1 provide therapeutic targets for restoration of B lymphopoiesis in aged and obese individuals.

Protection from intestinal inflammation by bacterial exopolysaccharides.

Host inflammatory responses against pathogenic organisms can be abrogated by commensals; however, the molecular mechanisms by which pathogenesis is prevented are still poorly understood. Previous studies demonstrated that administration of a single dose of Bacillus subtilis prevented disease and inflammation by the enteric mouse pathogen Citrobacter rodentium, which causes disease similar to the human pathogen enteropathogenic Escherichia coli. No protection was observed when an exopolysaccharide (EPS)-deficient mutant of B. subtilis was used, suggesting that EPS are the protective factor. In this study, we isolated and characterized EPS and showed that they also prevent C. rodentium-associated intestinal disease after a single injection. Protection is TLR4 dependent because EPS-treated TLR4 knockout mice developed disease. Furthermore, protection could be conveyed to wild-type mice by adoptive transfer of macrophage-rich peritoneal cells from EPS-treated mice. We found that EPS specifically bind peritoneal macrophages, and because mice lacking MyD88 signaling in myeloid cells were not protected by EPS, we conclude that bacterial EPS prevent colitis in a TLR4-dependent manner that requires myeloid cells. These studies provide a simple means of preventing intestinal inflammation caused by enteric pathogens.

Chemokine-mediated B cell trafficking during early rabbit GALT development.

Microbial and host cell interactions stimulate rabbit B cells to diversify the primary Ab repertoire in GALT. B cells at the base of appendix follicles begin proliferating and diversifying their V-(D)-J genes around 1 wk of age, ∼5 d after B cells first begin entering appendix follicles. To gain insight into the microbial and host cell interactions that stimulate B cells to diversify the primary Ab repertoire, we analyzed B cell trafficking within follicles during the first week of life. We visualized B cells, as well as chemokines that mediate B cell homing in lymphoid tissues, by in situ hybridization, and we examined B cell chemokine receptor expression by flow cytometry. We found that B cells were activated and began downregulating their BCRs well before a detectable B cell proliferative region appeared at the follicle base. The proliferative region was similar to germinal center dark zones, in that it exhibited elevated CXCL12 mRNA expression, and B cells that upregulated CXCR4 mRNA in response to signals acquired from selected intestinal commensals localized in this region. Our results suggest that after entering appendix follicles, B cells home sequentially to the follicle-associated epithelium, the follicular dendritic cell network, the B cell/T cell boundary, and, ultimately, the base of the follicle, where they enter a proliferative program and diversify the primary Ab repertoire.

Induction of antitoxin responses in Clostridium-difficile-infected patients compared to healthy blood donors.

According to the literature Clostridium difficile antitoxins are present in up to 66% of humans. In a survey of ∼400 plasma samples from healthy blood donors we found that less than 6% were positive for anti-TcdA or anti-TcdB antitoxins. Using the same standard immunoassay protocol, we looked for IgG and IgA antitoxins in the blood and stool samples from 25 patients with C. difficile infection (CDI). Some patients with CDI had no antitoxin detected at all, while others had high levels of specific IgG- and IgA-antitoxins against both TcdA and TcdB in blood and IgA-anti-TcdA and -anti-TcdB antibodies in stool. Systemic responses to TcdB and mucosal responses to TcdA predominated. Among patients infected with the NAP1/027/BI strain, systemic IgG-anti-TcdB responses were particularly elevated. In contrast, patients infected with non-027 strains had more elevated mucosal IgA-anti-TcdA responses. Furthermore, high titer sera did not correlate with high neutralizing potential. We hypothesize that paradoxical killing of primed B-cells by antibody-mediated endosomal uptake of the Large Clostridial Toxins, TcdA and TcdB leads to clonal elimination of the fittest B-cells. If this hypothesis is confirmed, immune suppression rather than protective humoral immunity might be the consequence in some patients infected with toxigenic C. difficile.

Analysis of Bacterial Communities during Clostridium difficile Infection in the Mouse.

Clostridium difficile infection (CDI) is a major cause of health care-associated disease. CDI initiates with ingestion of C. difficile spores, germination in the gastrointestinal (GI) tract, and then colonization of the large intestine. The interactions between C. difficile cells and other bacteria and with host mucosa during CDI remain poorly understood. Here, we addressed the hypothesis that, in a mouse model of CDI, C. difficile resides in multicellular communities (biofilms) in association with host mucosa. To do this, we paraffin embedded and then sectioned the GI tracts of infected mice at various days postinfection (p.i.). We then used fluorescent in situ hybridization (FISH) with 16S rRNA probes targeting most bacteria as well as C. difficile specifically. The results revealed that C. difficile is present as a minority member of communities in the outer (loose) mucus layer, in the cecum and colon, starting at day 1 p.i. To generate FISH probes that identify bacteria within mucus-associated communities harboring C. difficile, we characterized bacterial populations in the infected mouse GI tract using 16S rRNA gene sequence analysis of bacterial DNA prepared from intestinal content. This analysis revealed the presence of genera of several families belonging to Bacteroidetes and Firmicutes. These data suggest that formation of multispecies communities associated with the mucus of the cecum and colon is an important early step in GI tract colonization. They raise the possibility that other bacterial species in these communities modulate the ability of C. difficile to successfully colonize and, thereby, cause disease.

Applying refinement to the use of mice and rats in rheumatoid arthritis research.

Rheumatoid arthritis (RA) is a painful, chronic disorder and there is currently an unmet need for effective therapies that will benefit a wide range of patients. The research and development process for therapies and treatments currently involves in vivo studies, which have the potential to cause discomfort, pain or distress. This Working Group report focuses on identifying causes of suffering within commonly used mouse and rat 'models' of RA, describing practical refinements to help reduce suffering and improve welfare without compromising the scientific objectives. The report also discusses other, relevant topics including identifying and minimising sources of variation within in vivo RA studies, the potential to provide pain relief including analgesia, welfare assessment, humane endpoints, reporting standards and the potential to replace animals in RA research.

Protection from Clostridium difficile infection in CD4 T Cell- and polymeric immunoglobulin receptor-deficient mice.

Clostridium difficile rivals methicillin-resistant Staphylococcus aureus as the primary hospital-acquired infection. C. difficile infection (CDI) caused by toxins A and/or B can manifest as mild diarrhea to life-threatening pseudomembranous colitis. Although most patients recover fully from CDI, ~20% undergo recurrent disease. Several studies have demonstrated a correlation between anti-toxin antibody (Ab) and decreased recurrence; however, the contributions of the systemic and mucosal Ab responses remain unclear. Our goal was to use the CDI mouse model to characterize the protective immune response to C. difficile. C57BL/6 mice infected with epidemic C. difficile strain BI17 developed protective immunity against CDI and did not develop CDI upon rechallenge; they generated systemic IgG and IgA as well as mucosal IgA Ab to toxin. To determine if protective immunity to C. difficile could be generated in immunodeficient individuals, we infected CD4(-/-) mice and found that they generated both mucosal and serum IgA anti-toxin Abs and were protected from CDI upon rechallenge, with protection dependent on major histocompatibility complex class II (MHCII) expression; no IgG anti-toxin Ab was found. We found that protection was likely due to neutralizing mucosal IgA Ab. In contrast, pIgR(-/-) mice, which lack the receptor to transcytose polymeric Ab across the epithelium, were also protected from CDI, suggesting that although mucosal anti-toxin Ab may contribute to protection, it is not required. We conclude that protection from CDI can occur by several mechanisms and that the mechanism of protection is determined by the state of immunocompetence of the host.

Development of CD27+ marginal zone B cells requires GALT.

In species other than mouse, little is known about the origin and development of marginal zone (MZ) B cells. Using cross-reactive antibodies, we identified and characterized splenic MZ B cells in rabbits as CD27(+) CD23(-). In rabbits in which organized gut-associated lymphoid tissue (GALT) was surgically removed at birth, we found only CD23(+) follicular (FO) B cells and almost no CD27(+) MZ B cells in the spleen, indicating that GALT is required for the development of splenic MZ B cells. These findings lead us to suggest that commensal microbiota contribute to the development of MZ B cells.