Early development of Ig-secreting cells in young of germ-free BALB/c mice fed a chemically defined ultrafiltered diet

Neonatal Exposure to Commensal-Bacteria-Derived Antigens Directs Polysaccharide-Specific B-1 B Cell Repertoire Development

B-1 B cells derive from a developmental program distinct from that of conventional B cells, through B cell receptor (BCR)-dependent positive selection of fetally derived precursors. Here, we used direct labeling of B cells reactive with the N-acetyl-D-glucosamine (GlcNAc)-containing Lancefield group A carbohydrate of Streptococcus pyogenes to study the effects of bacterial antigens on the emergent B-1 B cell clonal repertoire. The number, phenotype, and BCR clonotypes of GlcNAc-reactive B-1 B cells were modulated by neonatal exposure to heat-killed S. pyogenes bacteria. GlcNAc-reactive B-1 clonotypes and serum antibodies were reduced in germ-free mice compared with conventionally raised mice. Colonization of germ-free mice with a conventional microbiota promoted GlcNAc-reactive B-1 B cell development and concomitantly elicited clonally related IgA⁺ plasma cells in the small intestine. Thus, exposure to microbial antigens in early life determines the clonality of the mature B-1 B cell repertoire and ensuing antibody responses, with implications for vaccination approaches and schedules.

Spleen development is modulated by neonatal gut microbiota

The full development of the mammalian immune system occurs after birth upon exposure to non self-antigens. The gut is the first site of bacterial colonization where it is crucial to create the appropriate microenvironment able to balance effector or tolerogenic responses to external stimuli. It is a well-established fact that at mucosal sites bacteria play a key role in developing the immune system but we ignore how colonising bacteria impact the maturation of the spleen. Here we addressed this issue. Taking advantage of the fact that milk SIgA regulates bacterial colonization of the newborn intestine, we generated immunocompetent mice born either from IgA pro-efficient or IgA deficient females. Having demonstrated that SIgA in maternal milk modulates neonatal gut microbiota by promoting an increased diversity of the colonizing species we also found that immunocompetent pups, not exposed to milk SIgA, fail to properly develop the FDC network and primary follicles in the spleen compromising the response to T-dependent antigens. The presence of a less diverse microbiota with a higher representation of pathogenic species leads to a fast replenishment of the marginal zone and the IgM plasma cell compartment of the spleen as well as IgA plasma cells in the gut.

Microbiota-Induced Antibodies Are Essential for Host Inflammatory Responsiveness to Sterile and Infectious Stimuli

The indigenous intestinal microbiota is frequently considered an additional major organ of the human body and exerts profound immunomodulating activities. Germ-free (GF) mice display a significantly different inflammatory responsiveness pattern compared with conventional (CV) mice, and this was dubbed a "hyporesponsive phenotype." Taking into account that the deposition of immune complexes is a major event in acute inflammation and that GF mice have a distinct Ig repertoire and B cell activity, we aimed to evaluate whether this altered Ig repertoire interferes with the inflammatory responsiveness of GF mice. We found that serum transfer from CV naive mice was capable of reversing the inflammatory hyporesponsiveness of GF mice in sterile inflammatory injury induced by intestinal ischemia and reperfusion, as well as in a model of lung infection by Klebsiella pneumoniae Transferring serum from Ig-deficient mice to GF animals did not alter their response to inflammatory insult; however, injecting purified Abs from CV animals restored inflammatory responsiveness in GF mice, suggesting that natural Abs present in serum were responsible for altering GF responsiveness. Mechanistically, injection of serum and Ig from CV mice into GF animals restored IgG deposition, leukocyte influx, NF-κB activation, and proinflammatory gene expression in inflamed tissues and concomitantly downregulated annexin-1 and IL-10 production. Thus, our data show that microbiota-induced natural Abs are pivotal for host inflammatory responsiveness to sterile and infectious insults.

Manipulation of the glycan-specific natural antibody repertoire for immunotherapy

Natural immunoglobulin derived from innate-like B lymphocytes plays important roles in the suppression of inflammatory responses and represents a promising therapeutic target in a growing number of allergic and autoimmune diseases. These antibodies are commonly autoreactive and incorporate evolutionarily conserved specificities, including certain glycan-specific antibodies. Despite this conservation, exposure to bacterial polysaccharides during innate-like B lymphocyte development, through either natural exposure or immunization, induces significant changes in clonal representation within the glycan-reactive B cell pool. Glycan-reactive natural antibodies (NAbs) have been reported to play protective and pathogenic roles in autoimmune and inflammatory diseases. An understanding of the composition and functions of a healthy glycan-reactive NAb repertoire is therefore paramount. A more thorough understanding of NAb repertoire development holds promise for the design of both biological diagnostics and therapies. In this article, we review the development and functions of NAbs and examine three glycan specificities, represented in the innate-like B cell pool, to illustrate the complex roles environmental antigens play in NAb repertoire development. We also discuss the implications of increased clonal plasticity of the innate-like B cell repertoire during neonatal and perinatal periods, and the prospect of targeting B cell development with interventional therapies and correct defects in this important arm of the adaptive immune system.

Long-term effects of early life microbiota disturbance on adaptive immunity in laying hens

Due to an interplay between intestinal microbiota and immune system, disruption of intestinal microbiota composition during immune development may have consequences for immune responses later in life. The present study investigated the effects of antibiotic treatment in the first weeks of life on the specific antibody response later in life in chickens. Layer chicks received an antibiotic cocktail consisting of vancomycin, neomycin, metronidazole, and amphotericin-B by oral gavage every 12 h, and ampicillin and colistin in drinking water for the first week of life. After the first week of life, chicks received ampicillin and colistin in drinking water for two more weeks. Control birds received no antibiotic cocktail and plain drinking water. Fecal microbiota composition was determined during antibiotic treatment (d 8 and 22), two weeks after cessation of antibiotic treatment (d 36), and at the end of the experimental period at d 175 using a 16S ribosomal RNA gene targeted microarray, the Chicken Intestinal Tract Chip (ChickChip). During antibiotic treatment fecal microbiota composition differed strongly between treatment groups. Fecal microbiota of antibiotic treated birds consisted mainly of Proteobacteria, and in particular E.coli, whereas fecal microbiota of control birds consisted mainly of Firmicutes, such as lactobacilli and clostridia. Two weeks after cessation of antibiotic treatment fecal microbiota composition of antibiotic treated birds had recovered and was similar to that of control birds. On d 105, 12 weeks after cessation of antibiotic treatment, chicks of both treatment groups received an intra-tracheal lipopolysaccharide (LPS)/human serum albumin (HuSA) challenge. Antibody titers against LPS and HuSA were measured 10 days after administration of the challenge. While T cell independent antibody titers (LPS) were not affected by antibiotic treatment, antibiotic treated birds showed lower T cell dependent antibody titers (HuSA) compared with control birds. In conclusion, intestinal microbial dysbiosis early in life may still have effects on the specific antibody response months after cessation of antibiotic treatment and despite an apparent recovery in microbiota composition.

The germfree murine animal: an important animal model for research on the relationship between gut microbiota and the host

Scientific findings in recent decades have demonstrated that the commensal intestinal microbiota has profound effects on the physiology and diseases of the host. It is estimated that the human microbiota is composed of 10(14) bacterial cells, a number 10 times greater than the total number of human cells. The variety and the complex interactions of the intestinal microbiota are associated with physiological details that remain largely unknown. Germfree hosts, especially murine (rat or mouse) animals that have been maintained free from demonstrable microbial associates such as bacteria, viruses, fungi, and parasites throughout life, have become a powerful tool for exploring the interplay between the host and microorganisms inhabiting the human intestine. This review and survey of recent findings will argue that the germfree mouse model can produce its greatest potential benefits in the study of the metabolism and immunity of the host.

Oxidation-specific epitopes are dominant targets of innate natural antibodies in mice and humans

Atherosclerosis is a chronic inflammatory disease characterized by the accumulation of oxidized lipoproteins and apoptotic cells. Adaptive immune responses to various oxidation-specific epitopes play an important role in atherogenesis. However, accumulating evidence suggests that these epitopes are also recognized by innate receptors, such as scavenger receptors on macrophages, and plasma proteins, such as C-reactive protein (CRP). Here, we provide multiple lines of evidence that oxidation-specific epitopes constitute a dominant, previously unrecognized target of natural Abs (NAbs) in both mice and humans. Using reconstituted mice expressing solely IgM NAbs, we have shown that approximately 30% of all NAbs bound to model oxidation-specific epitopes, as well as to atherosclerotic lesions and apoptotic cells. Because oxidative processes are ubiquitous, we hypothesized that these epitopes exert selective pressure to expand NAbs, which in turn play an important role in mediating homeostatic functions consequent to inflammation and cell death, as demonstrated by their ability to facilitate apoptotic cell clearance. These findings provide novel insights into the functions of NAbs in mediating host homeostasis and into their roles in health and diseases, such as chronic inflammatory diseases and atherosclerosis.

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

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

Irf4 is a positional and functional candidate gene for the control of serum IgM levels in the mouse

Natural IgM are involved in numerous immunological functions but the genetic factors that control the homeostasis of its secretion and upholding remain unknown. Prompted by the finding that C57BL/6 mice had significantly lower serum levels of IgM when compared with BALB/c mice, we performed a genome-wide screen and found that the level of serum IgM was controlled by a QTL on chromosome 13 reaching the highest level of association at marker D13Mit266 (LOD score=3.54). This locus was named IgMSC1 and covered a region encompassing the interferon-regulatory factor 4 gene (Irf4). The number of splenic mature B cells in C57BL/6 did not differ from BALB/c mice but we found that low serum levels of IgM in C57BL/6 mice correlated with lower frequency of IgM-secreting cells in the spleen and in the peritoneal cavity. These results suggested that C57BL/6 mice have lower efficiency in late B-cell maturation, a process that is highly impaired in Irf4 knockout mice. In fact, we also found reduced Irf4 gene expression in B cells of C57BL/6 mice. Thus, we propose Irf4 as a candidate for the IgMSC1 locus, which controls IgM homeostatic levels at the level of B-cell terminal differentiation.

Oxidation-specific epitopes are important targets of innate immunity

During the oxidation of LDL, a central pathophysiological component of atherogenesis, a wide variety of chemical and physical changes occur leading to the generation of oxidation-specific neoepitopes. These epitopes are not only immunogenic, leading to adaptive humoral responses, but are also a prominent target of multiple arcs of innate immunity. The pattern recognition receptors (PRRs) of innate immunity are germ line encoded, conserved by natural selection, and bind to pathogen-associated molecular patterns (PAMPs) common on multiple structures. However, it is not intuitive as to why they should recognize oxidation-specific neoepitopes. Yet it is clear that multiple macrophage scavenger receptors, which are classic PRRs, recognize oxidation-specific epitopes, such as those found on oxidized LDL (OxLDL). Other innate proteins, such as C-reactive protein, also bind to OxLDL. Natural antibodies (NAbs), the humoral arc of innate immunity, provide a nonredundant role in the first line of defence against pathogens, but are also believed to provide important homeostatic house-keeping functions against self-antigens. Our work demonstrates that oxidation-specific epitopes, as found on OxLDL, are a major target of NAbs. In this review, we will discuss the specific example of the prototypic NAb T15/E06, which is increased in atherosclerotic mice and mediates atheroprotection, and discuss the potential role of NAbs in atherogenesis, and in inflammation in general. We also review data that oxidation-specific epitopes are generated whenever cells undergo programmed cell death, forming a common set of PAMPs recognized by oxidation-specific PRRs on macrophages, NAbs and innate proteins. We present the hypothesis that oxidation-specific epitopes on apoptotic cells exerted evolutionary pressure for the conservation of these PRRs and also serve to maintain the expansion of a substantial proportion of NAbs directed to these stress-induced self-antigens.

Use of axenic animals in studying the adaptation of mammals to their commensal intestinal microbiota

Vertebrates are essentially born germ-free but normally acquire a complex intestinal microbiota soon after birth. Most of these organisms are non-pathogenic to immunocompetent hosts; in fact, many are beneficial, supplying vitamins for host nutrition and filling the available microbiological niche to limit access and consequent pathology when pathogens are encountered. Thus, mammalian health depends on mutualism between host and flora. This is evident in inflammatory conditions such as inflammatory bowel disease, where aberrant responses to microbiota can result in host pathology. Studies with axenic (germ-free) or deliberately colonised animals have revealed that commensal organisms are required for the development of a fully functional immune system and affect many physiological processes within the host. Here, we describe the technical requirements for raising and maintaining axenic and gnotobiotic animals, and highlight the extreme diversity of changes within and beyond the immune system that occur when a germ-free animal is colonized with commensal bacteria.

Immunological activities are modulated by enteral administration of an elemental diet in mice

BACKGROUND & AIMS

Elemental diets (EDs) have been used successfully in treatment of some intestinal inflammatory diseases; however, the mechanism that mediates their effects is still unclear. In this study we evaluated the immunological effect of enteral administration of an ED in mice.

METHODS

C57BL/6 mice were fed an ED (El-Diet) from weaning up to adulthood and immunological parameters were analyzed.

RESULTS

El-Diet-fed mice presented an underdeveloped gut-associated-lymphoid tissue with lower numbers of TCRalphabeta+IELs and lamina propria cells and low levels of secretory IgA when compared to chow-fed mice. They showed a systemic decrease in the production of IgG and IgA as well as a skewing towards a Th2 profile of cytokine production upon in vitro stimulation with an increase in IL-4 and a reduction in IFN-gamma and IL-6 secretion.

CONCLUSION

Our study demonstrated the role of EDs in modulating immunological activities in mice and proposes a rational for their successful use in treatment of some intestinal inflammatory diseases.

The role of natural antibodies in atherogenesis

Atherosclerosis is now widely recognized as a chronic inflammatory disease that involves innate and adaptive immune responses. Both cellular and humoral components of the immune system have been implicated in atherogenesis. Natural antibodies can be considered humoral factors of innate immunity, and their functional role in health and disease has been reexamined in recent years. Natural antibodies exhibit a remarkably conserved repertoire that includes a broad specificity for self-antigens. For this reason, they are believed to be a product of natural selection and have been suggested to play an important role in "housekeeping" functions. Recent evidence has revealed that oxidation-specific epitopes are important and maybe immunodominant targets of natural antibodies, suggesting an important function for these antibodies in the host response to consequences of oxidative stress, for example, to the oxidative events that occur when cells undergo apoptosis. This review will focus on these recent findings and discuss the emerging evidence for an important role of natural antibodies in atherogenesis.

Natural serum IgM maintains immunological homeostasis and prevents autoimmunity

Natural (i.e. low-affinity, preimmune) IgM has a wide range of actions in the immune system. That IgM is important in defence against infection has been recognised for many years but recently, due to the generation of mouse models specifically deficient in serum IgM, other functions of serum IgM have been revealed. The participation of natural IgM in autoimmunity has been a subject of speculation for some time, but there is recent definitive evidence that demonstrates its protective role. This review focuses on these new areas of research, which also show that many pathologies not normally recognised as having an immunological basis are modulated by serum IgM.

MHC-restricted Ig V region-driven T-B lymphocyte collaboration: B cell receptor ligation facilitates switch to IgG production

B cells spontaneously process their endogenous Ig and present V region peptides on their MHC class II molecules. We have here investigated whether B cells collaborate with V region-specific CD4+ T cells in vivo. By use of paired Ig L chain-transgenic and TCR-transgenic mice and cell transfer into normal hosts, we demonstrate that B cell presentation of a V(L) region peptide to CD4+ T cells results in germinal centers, plasma cells, and Ab secretion. Because the transgenic B cells have a fixed L chain but polyclonal H chains, their B cell receptor (BCR) repertoire is diverse and may bind a multitude of ligands. In a hapten-based system, BCR ligation concomitant with V region-driven T-B collaboration induced germinal center formation and an IgM --> IgG isotype switch. In the absence of BCR ligation, mainly IgM was produced. Consistent with this, prolonged V region-driven T-B collaboration resulted in high titers of IgG autoantibodies against ubiquitous self-Ags, while natural-type Abs against exotic bacteria remained IgM. Taken together, V region-driven T-B collaboration may explain induction of natural IgM Abs (absence of BCR ligation) and IgG autoantibodies (BCR ligation by autoantigen) and may be involved in the development of autoimmunity.

Sequence-specific binding of normal serum immunoglobulin M to exposed protein C-termini

Both the timely clearance of degraded endogenous structures and the presence of secreted natural immunoglobulin M (IgM) are needed to avoid autoimmunity. These requirements may be causally related provided that natural IgM preferentially reacts with degraded antigens and, by activating complement, mediates their non-inflammatory clearance through complement receptors. We have previously shown that normal serum IgM reacts in vivo and in vitro with virtually all randomly generated C-terminal peptides displayed on T7 phage. The resultant multivalent IgM-peptide complexes activate complement and are detected by a loss of phage infectivity. A striking feature of these reactions is that different C-terminal peptides ( approximately 3-4 amino acids) specifically react with different 'C-terminal' IgM (C-IgM) antibodies. This suggests that degraded supramolecular structures, expressing elevated levels of identical C-termini as a result of proteolysis, denaturation and abnormal exposure of repetitive protein constituents, may be preferential targets of C-IgM-mediated complement activation in the physiological environment. The specificity of C-IgM-peptide reactions is much higher than one would expect, assuming that normal serum IgM mostly comprises polyspecific natural antibodies. However, it is possible that polyspecific IgM is not adequately registered by our 'functional' phage-inactivation assays. In this study, we resolve the issue of C-IgM specificity by directly characterizing the binding reactivity of normal serum IgM with phage-displayed C-terminal peptides.

The Dual Phases of the Response to Neonatal Exposure to a VH Family-Restricted Staphylococcal B Cell Superantigen

In vitro studies of several naturally occurring proteins have characterized VH family-specific B lymphocyte binding and stimulatory properties that appear analogous to those of T cell superantigens. To examine the in vivo consequences of exposure to a putative B cell superantigen, we treated neonatal BALB/c mice with a form of staphylococcal protein A (MS) devoid of Fcγ binding activity, which retains the clan VHIII Fab binding specificity. In naive adults, about 5% of peripheral B cells and &gt;13% of splenic IgM-secreting cells display MS binding activity, in association with high IgM and low IgG circulating anti-MS Ab titers. Neonatal exposure to MS elicited two distinct temporal phases of immune responsiveness. The early phase, representing the first approximately 5 wk of life, was associated with MS-specific B cell and T cell tolerance. Microfluorometric assays revealed that exposure caused a dramatic MS-specific B cell clonal loss in bone marrow and spleen, but levels normalized by about 3 wk of life. The late phase (&gt;6 wk of age) was associated with spontaneous priming for MS-specific T cell responses and production of MS-specific IgG1 Abs despite long term persistently depressed in vivo and in vitro MS-specific IgM responses. In vivo challenge during the late phase induced high frequencies of MS-specific IgG-secreting cells, indicating recruitment of highly focused Ab responses that were predominantly encoded by rearrangements of the S107 family, a member of the VHIII clan. These studies document the immunodominance of the VH-restricted Fab binding site on staphylococcal protein A and demonstrate the diverse effects of a B cell superantigen on the emerging peripheral B cell compartment.

The repertoire of serum IgM in normal mice is largely independent of external antigenic contact

Antigen-free (AGF) and germ-free (GF) mice, although essentially free of serum IgG, maintain normal levels of circulating IgM. Using a quantitative immunoblot assay, we have now analyzed the repertoire of serum IgM from AGF, GF, and specific pathogen-free (SPF) BALB/c mice, on large panels of natural antigens from homologous tissues and bacteria. The reactivity profiles were very similar in the three groups of mice. Multiparametric statistic evaluation of the data showed that BALB/c animals, SPF, GF, and AGF mice constitute an homogeneous group with similar immunoreactivity profiles when compared to C57BL/6. Differences between immunoreactivity profiles of GF and AGF mice were observed, but were not statistically significant. These results suggest that the serum IgM repertoire of normal mice is strictly regulated and selected by endogenous ligands.

The role of natural antibodies and ABO (H) blood groups in transplantation of human lymphoid cells into mice

Recently, evidence was presented that natural antibodies (NAb) are a crucial barrier to human cellular engraftment in severely immunosuppressed normal mice (Eur. J. Immunol. 1992. 22: 197.). In this report we show that normal mouse serum contains low titers of NAb against human cells of blood groups type O (H) and B and high titers against human cells of blood group A. Accordingly, human peripheral blood leukocytes (PBL) of group O (H) and B donors could be grafted successfully into normal BCBA mice (H-2b/k) following irradiation with high dose total body irradiation (TBI). PBL of blood group type A donors did not engraft in normal mice but could be transplanted without difficulty in B cell-deficient CBA/N mice which lack NAb, after conditioning with high dose TBI. Treatment of lethally irradiated normal BCBA mice with cobra venom factor (COF), which eliminates the third factor of complement, and liposomes containing dichloromethylene diphosphate (Cl2DMP), which eliminates macrophages, resulted in engraftment of human blood group type A PBL. This implies that the NAb barrier for discordant xenogeneic cell transplantation can be abrogated. A method utilizing directly labeled probes and flow cytometry is described for the quantitation in mouse serum of NAb, reacting with human cells. Using sera of H-2b/k mice we show that murine NAb react with human stem cells, granulocytes, lymphocytes and monocytes of blood group A and only weakly with similar cells from blood group O (H) and B donors. Sera of H-2b, H-2d and H-2k mice of different ages and microflora possess NAb against human erythrocytes of blood group type A and occasionally demonstrate weak titers against erythrocytes of blood groups B and O (H) and the Rhesus factor.

Autoantibody activity and V gene usage by B-cell malignancies

Autoreactive B cells account for a substantial part of the B-cell repertoire. They frequently secrete polyspecific natural autoantibodies, which probably bind with low affinity to the different antigens they recognize and which express germinal genes. The role of this pre-immune repertoire still remains to be defined but it has been suggested that it participates in the elimination of cell breakdown products, serves as a first barrier of defense or acts as a template upon which antigen driven selection and somatic recombinations could induce the emergence of high affinity induced antibodies. The present study, reviews the evidence favouring the idea that this autoreactive B-cell repertoire, which expresses a restricted set of V genes, frequently undergoes malignant transformation. This evidence arises from the study of the autoantibody activity and V gene usage in three different models of B-cell malignancies namely monoclonal immunoglobulins; chronic lymphocytic leukemia; and follicular non-Hodgkin's lymphomas.

Complement-mediated follicular localization of T-independent type-2 antigens: the role of marginal zone macrophages revisited

In this study we demonstrate a hitherto undescribed phenomenon, namely that thymus-independent type-2 antigens (TI-2 Ag) localize in splenic follicles within 1 h after administration. The follicular localization of 2,4,6-trinitrophenyl (TNP)-Ficoll was not antibody mediated. In addition in case of high-dose administration we observed a relatively large amount of TI-2 Ag in marginal zone macrophages. However, after low-dose administration we observed a preferential localization of TNP-Ficoll in the splenic follicles. Detection of TNP-haptenated Ag in cryostat sections of murine spleens was performed with a high-affinity TNP-specific monoclonal antibody conjugated to beta-galactosidase. Within minutes after injection the TI-2 Ag localized in the marginal zone, attached to marginal zone macrophages and B cells. Twenty minutes after injection the Ag was also detected in the follicles and gradually accumulated there until 7 h after injection. Thereafter, the amount of follicular Ag gradually decreased but was still detectable up to 14 days after immunization. The follicular localization of TNP-Ficoll was complement dependent in contrast to the binding to and uptake by marginal zone macrophages. Double staining revealed that Ag was bound by macrophages, B cells and follicular dendritic cells. Haptenated thymus-dependent (TD) Ag localized exclusively in the red pulp macrophages. In vivo macrophage elimination drastically increased the amount of TNP-Ficoll in the follicles, and enhanced the humoral immune response at low doses of Ag. Moreover, complement deprivation of mice abrogated the localization of TI-2 Ag in the follicles, and led to a decreased humoral TI-2 immune response. In conclusion, we demonstrate for the first time that TI-2 Ag localize in follicles. Moreover, the presented results provide further evidence that B cells and follicular localized Ag play an important role in the induction of humoral TI-2 immune responses.

Background (spontaneous) immunoglobulin production in the murine small intestine before and after weaning

The ontogeny of the murine intestinal B-cell compartment before and after weaning was studied by quantitative analysis of immunoglobulin-secreting cells (Ig-SC) in the small intestine (SI). Before weaning, few Ig-SC were detected in the SI, whereas spleen and bone marrow already contained many Ig-SC. The number of Ig-SC in the SI started to increase immediately after weaning. Comparing early-weaned mice with non-weaned mice of the same age clearly demonstrated that weaning brought on the development of Ig-SC in the SI. The influence of a gut flora on the number of Ig-SC in the SI was examined by comparing the number of Ig-SC in the SI of conventionally housed, specific pathogen free (SPF) and germ-free mice. A bacterial flora was apparently needed for the normal development of Ig-SC in the SI. Comparing mice containing an aerobic Gram-negative bacterial flora with mice containing only an anaerobic Gram-positive bacterial flora demonstrated that the type of bacterial flora is relatively unimportant. No evidence was found that circulating maternal antibodies suppressed the development of the "spontaneous" intestinal and systemic B cell response. The results show that bacterial colonization of the intestine plays a pivotal role in the development of the Ig-SC compartment in the SI.

Effect of the gastrointestinal microflora on induction and maintenance of oral tolerance to ovalbumin in C3H/HeJ mice

The effect of the digestive microflora on oral tolerance to ovalbumin was studied by using axenic (germfree) and conventional C3H/HeJ mice. In contrast to reported results of studies with sheep erythrocytes, oral administration of ovalbumin induced tolerance in axenic mice, but the maintenance of tolerance was found to be of shorter duration than was with conventional mice. These data indicate that the contribution of the microflora to oral tolerance depends on the antigen used.

The influence of exogenous antigenic stimulation on the specificity repertoire of background immunoglobulin-secreting cells of different isotypes

The total number of spontaneously occurring ("background") IgM-, IgG-, and IgA-secreting cells and the frequency of antigen-specific IgM-, IgG-, and IgA-secreting cells were determined in germ-free BALB/c mice fed a chemically defined ultrafiltered diet (GF-CD), in specific pathogen-free BALB/c mice fed an autoclaved natural ingredient diet (SPF-NI), and in conventional BALB/c mice fed nonautoclaved natural ingredients (CV-NI). This was done by means of the ELISA-plaque assay. The results did not show differences among the various groups of mice with regard to the total numbers of IgM-secreting cells in the various lymphoid organs. Also the frequencies of IgM-secreting cells specific for DNP27-BSA and the anti-idiotypic monoclonal antibodies Ac38 and Ac146 did not differ significantly among GF-CD, SPF-NI, and CV-NI mice. GF-CD mice, however, did show substantially decreased numbers of IgG- and IgA-secreting cells in their lymphoid organs. Furthermore, there were striking differences in the frequencies of antigen-specific IgG- and IgA-secreting cells between GF-CD mice and the two other groups of mice. These results indicate that exogenous antigenic stimulation has a great effect on both the total numbers and the specificity repertoires of background IgG- and IgA-secreting cells. Such an influence could not be detected with regard to the background IgM-secreting cells. This suggests two distinct compartments of background Ig-secreting cells: a very stable, endogenously regulated compartment consisting mainly of IgM-secreting cells, and another compartment, consisting mainly of IgG- and IgA-secreting cells, whose numbers and specificity repertoire appeared to be influenced by exogenous antigenic stimulation.