Commensal enteric bacteria engender a self-limiting humoral mucosal immune response while permanently colonizing the gut

Roundtrip ticket for secretory IgA: role in mucosal homeostasis?

An important activity of mucosal surfaces is the production of Ab referred to as secretory IgA (SIgA). SIgA serves as the first line of defense against microorganisms through a mechanism called immune exclusion. In addition, SIgA adheres selectively to M cells in intestinal Peyer's patches, thus mediating the transepithelial transport of the Ab molecule from the intestinal lumen to underlying gut-associated organized lymphoid tissue. In Peyer's patches, SIgA binds and is internalized by dendritic cells in the subepithelial dome region. When used as carrier for Ags in oral immunization, SIgA induces mucosal and systemic responses associated with production of anti-inflammatory cytokines and limits activation of dendritic cells. In terms of humoral immunity at mucosal surfaces, SIgA appears thus to combine properties of a neutralizing agent (immune exclusion) and of a mucosal immunopotentiator inducing effector immune responses in a noninflammatory context favorable to preserve local homeostasis of the gastrointestinal tract.

Cross-talk between probiotic bacteria and the host immune system

Among the numerous purported health benefits attributed to probiotic bacteria, their capacity to interact with the immune system of the host is now supported by an increasing number of in vitro and in vivo experiments. In addition to these, a few well-controlled human intervention trials aimed at preventing chronic immune dysregulation have been reported. Even though the precise molecular mechanisms governing the cross-talk between these beneficial bacteria and the intestinal ecosystem remain to be discovered, a new and fascinating phase of research has been initiated in this area as demonstrated by a series of recent articles. This article summarizes the status and latest progress of the field in selected areas and aims at identifying key questions that remain to be addressed, especially concerning the translocation of ingested bacteria, the identification of major immunomodulatory compounds of probiotics, and specific aspects of the host-microbe cross-talk. The interaction with immunocompetent cells and the role of secretory IgA in gut homeostasis are also evoked. Finally, a brief overview is provided on the potential use of recombinant DNA technology to enhance the health benefits of probiotic strains and to unravel specific mechanisms of the host-microbe interaction.

Mucosal vaccine development for botulinum intoxication

Botulism has classically been considered to be a food- and water-borne disease. However, it was recently classified by the US National Institute of Allergy and Infectious Diseases (National Institute of Health) and the US Centers for Disease Control and Prevention as a Category A agent. Thus, the botulinum exotoxin, a neurotoxin, could be easily disseminated by bioterrorists through the air-borne route with a high morbidity and mortality rate. In this regard, a high priority should be given to the development of a safe and effective mucosal vaccine to protect against botulinum neurotoxins (BoNTs) since it is well known that the mucosal immune system is the first line of defense against major pathogens. Further, mucosal immunization has been shown to induce both mucosal and systemic immunity to pathogens. By contrast, the current injection-type vaccine only provides protective immunity in the systemic compartment. Clearly, the development of a safe and effective mucosal vaccine against this toxin should be a high priority. In this regard, it has been shown that both nasal and oral immunization approaches have been taken in order to protect from BoNT intoxication. In this article, we will discuss the importance of the development of a mucosal vaccine against botulinum and introduce current aspects of BoNT mucosal vaccines, which show that they effectively prevent mucosal BoNT intoxication.

Biological Functions of IgA

Immunoglobulin A (IgA) is the most enigmatic of immunoglobulins. It is by far the most abundant of human Igs, being present in the blood plasma at concentrations approximating 2–3mg/mL, as well as the dominant isotype in most secretions where its output amounts to some 5–8g/day in adults. Furthermore, its evolutionary origins appear to precede the synapsid– diapsid divergence in tetrapod phylogeny (>300 million years ago) because it is present in both mammals and birds and therefore possibly also in reptiles (reviewed in Peppard et al., 2005); an IgA-like molecule has now been identified in a lizard (Deza et al., 2007).

Immune privilege in the gut: the establishment and maintenance of non-responsiveness to dietary antigens and commensal flora

Immune privilege in the gut is the result of a complex interplay between the gut microbiome, gut luminal antigens, and the intestinal epithelial barrier. Composed of both physical and immunochemical components, the intestinal barrier secretes immunoregulatory mediators that promote the generation of tolerogenic antigen-presenting cells, phagocytic innate immune cells characterized by 'inflammatory anergy', and regulatory cells of the adaptive immune system. Innate immune cells mediate controlled transepithelial transport of luminal antigens as far as the mesenteric lymph nodes, where the intestinal and peripheral immune systems intersect. This promotes the generation of adaptive regulatory lymphocytes that actively suppress effector cell responses against gut luminal antigens and flora. The net result is the generation of tolerance to dietary antigens and the maintenance of gut homeostasis. Dysregulation of this complex immunoregulatory network leads to diseases such as food allergy and inflammatory bowel disease. Future therapies for these diseases will likely involve the functional restoration of the barrier and regulatory cell functions at the epithelial/luminal interface.

Comparison of fecal biota from specific pathogen free and feral mice

Specific pathogen free (SPF) rodents are derived from germfree animals that are colonized with Schaedler's flora, a cocktail of eight bacterial strains isolated from the natural biota of mice. During successive generations SPF animals acquire a complex biota, but it is not known how similar it is to natural mouse biota. Therefore, fecal pellets of two feral mice and three SPF mice were studied by small subunit ribosomal DNA sequence analysis. After amplification of 16S rDNA by Bacterial Kingdom-specific primers, 132 rDNA clones from feral mice and 219 clones from SPF mice were placed phylogenetically. Forty-four percent of recovered rDNAs from feral mice were from organisms belonging to the Ribosomal Database Project's Bacteroides Group with significant proportions also coming from lactobacilli, the Clostridium coccoides Group and the Clostridium leptum Group. Although the SPF biota appeared equally complex at lower phylogenetic levels, the major phylogenetic groups represented were less diverse in that 92% of rDNA's from SPF mice mapped to groups of clostridia with 79% to the C. coccoides Group alone. Given the number of physiological parameters influenced by the gut biota and the importance of mice in biomedical research, further investigations are warranted.

B cell recruitment and selection in mouse GALT germinal centers

In conventionally reared mice germinal centers (GCs) are chronically induced in Peyer's patches (PP), mesenteric lymph node (MLN), and isolated lymphoid follicles (ILF) of gut-associated lymphoid tissues (GALT), as a result of continuous B cell stimulation by commensal bacteria. It is generally thought that BCR-mediated antigen recognition controls the recruitment and thus selection of B cells within GALT GCs. However, recent results challenge this view and suggest that engagement of innate immune receptors by microbial antigens promotes B cell recruitment to, and maintenance within, the GC, irrespective of BCR specificity. We propose a scenario in which microbial determinants presented by follicular dendritic cells (FDCs) to innate receptors on B cells within the GC support the survival and concomitant expansion of somatically mutated, IgA-class-switched B cell clones expressing a variety of BCR specificities. From this pool, B cell mutants recognizing gut-derived antigens through their BCR are either, in GCs, drawn into the process of affinity maturation, or, in the lamina propria (LP) of the gut, locally selected to differentiate into plasmablasts, thus contributing to the continuous production of IgA antibodies required for an efficient protection against commensal and pathogenic microorganisms.

IgA adaptation to the presence of commensal bacteria in the intestine

The lower intestine of mammals is colonised by a dense flora composed mainly of non-pathogenic commensal bacteria. These intestinal bacteria have a wide-ranging impact on host immunity and physiology. One adaptation following intestinal colonisation is increased production and secretion of polyspecific intestinal IgA. In contrast to the strong mucosal immune response to bacterial colonisation, the systemic immune system remains ignorant of these organisms in pathogen-free mice. Small numbers of bacteria can penetrate the epithelial surface overlying Peyer's patches and survive in dendritic cells to induce IgA by T-dependent and T-independent mechanisms. These dendritic cells loaded with live commensal organisms can home to the mesenteric lymph nodes but do not reach systemic secondary lymphoid structures, so induction of mucosal responses is focused in mucosal lymphoid tissues. The secretion of antibodies across the intestinal epithelial surface in turn limits the penetration of commensal organisms, but this is one of many mechanisms which adapt the intestinal mucosa to co-existence with commensal bacteria.

Host gene expression in the colon of gnotobiotic interleukin-2-deficient mice colonized with commensal colitogenic or noncolitogenic bacterial strains: common patterns and bacteria strain specific signatures

BACKGROUND

Specific pathogen-free (SPF), but not germfree (GF), interleukin (IL)-2-deficient (IL-2-/-) mice develop inflammatory bowel disease (IBD) at 10 to 15 weeks of age. Gnotobiotic IL-2-/- mice monocolonized with E. coli mpk develop IBD at 25 to 33 weeks of age but not B. vulgatus mpk, E. coli Nissle 1917, or mice cocolonized with both E. coli mpk and B. vulgatus.

METHODS

To determine genes regulated by these commensal bacteria, host gene expression in the colon of 8-week-old IL-2-/- mice was compared by using microarrays and semiquantitative reverse-transcription polymerase chain reaction. Colonization with E. coli mpk/B. vulgatus or SPF microbiota altered the gene expression profile more profoundly than monocolonization with either B. vulgatus, E. coli mpk or E. coli Nissle indicating that the complexity of the gene expression pattern is influenced by the diversity of the microbiota.

RESULTS

A small but distinct group of genes could be defined which might be associated with colitis development. Thus, 8 week old E. coli mpk IL-2-/- mice rone to colitis compared to E. coli Nissle, B. vulgatus and E. coli mpk/B. vulgatus IL-2-/- mice displayed a lower expression of the anti-inflammatory RegIII family genes such as RegIII[gamma] and pancreatitis associated protein (PAP) and peroxisome proliferator-activated receptor-[gamma] regulated genes such as adipsin and adiponectin.

CONCLUSION

The increased expression of these genes in B. vulgatus colonized mice might be associated with prevention of E. coli mpk triggered colitis in E. coli mpkM/B. vulgatus IL-2-/- mice.

Tight mucosal compartmentation of the murine immune response to antigens of the enteric microbiota

BACKGROUND & AIMS

The normal host immune response to antigens of the enteric microbiota is poorly defined. In this study, we isolated recombinant microbial antigens from commensal bacteria and used them to probe the normal murine immune response.

METHODS

A plasmid DNA expression library was generated from cecal bacteria of C3H/HeJ mice and used to express 20 recombinant intestinal bacterial proteins (rIBs). Antibody responses in serum and secretions were measured by an enzyme-linked immunosorbent assay, and CD4+ T-cell responses were measured by [3H]-thymidine incorporation. Two immunodominant commensal flagellins were also tested.

RESULTS

No baseline serum immunoglobulin (Ig)G antibody or splenic CD4+ T-cell systemic response to any rIB or to either flagellin was detected in normal C3H/HeJ mice. However, there were strong systemic responses to all 20 rIBs after parenteral immunization, which were equivalent to the responses to ovalbumin. Substantial levels of intestinal IgA were detected to half the rIBs and to both commensal flagellins. Mucosal immunization with flagellin plus ovalbumin stimulated an intestinal IgA but not a serum IgG response. Antigen-pulsed dendritic cells (DCs) stimulated production of specific IgA in the absence of T-cell help via costimulation by BAFF and/or APRIL, members of the TNF family.

CONCLUSIONS

The host immune response to enteric bacteria is tightly compartmentalized to the mucosa in normal mice, with systemic B cells and CD4+ T cells remaining naive rather than tolerant. We postulate that mucosal DCs play a crucial role in this compartmentation.

Innate secretory antibodies protect against natural Salmonella typhimurium infection

The production of IgA is induced in an antigen-unspecific manner by commensal flora. These secretory antibodies (SAbs) may bind multiple antigens and are thought to eliminate commensal bacteria and self-antigens to avoid systemic recognition. In this study, we addressed the role of "innate" SAbs, i.e., those that are continuously produced in normal individuals, in protection against infection of the gastrointestinal tract. We used polymeric immunoglobulin receptor (pIgR-/-) knock-out mice, which are unable to bind and actively transport dimeric IgA and pentameric IgM to the mucosae, and examined the role of innate SAbs in protection against the invasive pathogen Salmonella typhimurium. In vitro experiments suggested that innate IgA in pIgR-/- serum bound S. typhimurium in a cross-reactive manner which inhibited epithelial cell invasion. Using a "natural" infection model, we demonstrated that pIgR-/- mice are profoundly sensitive to infection with S. typhimurium via the fecal-oral route and, moreover, shed more bacteria that readily infected other animals. These results imply an important evolutionary role for innate SAbs in protecting both the individual and the herd against infections, and suggest that the major role of SAbs may be to prevent the spread of microbial pathogens throughout the population, rather than protection of local mucosal surfaces.

The B-cell system in inflammatory bowel disease

Secretory immunity is the best-defined part ot the mucosal immune system. This adaptive humoral defense mechanism depends on a fine-tuned cooperation between secretory epithelia and local plasma cells. Such mucosal immunocytes produce preferentially dimers and larger polymers of immunoglobulin A (collectively called pIgA), which contain J chain and therefore can bind to the epithelial secretory component (SC). This transmembrane glycoprotein functions as pIg receptor (pIgR) that also translocates pentameric IgM to the epithelial surface. B cells with a high level of J-chain expression and pIg-pIgR interactions at mucosal effector sites are thus necessary for the generation of secretory antibodies (SIgA and SIgM). Secretory antibodies perform immune exclusion in a first-line defense, thereby counteracting microbial colonization and mucosal penetration of soluble antigens. However, local production of pIgA is significantly down-regulated in inflammatory bowel disease (IBD), as revealed by strikingly decreased J-chain expression. Although the total increase of the immunocyte population in IBD lesions probably compensates for the relatively reduced pIgA production, decreased pIgR/SC expression in regenerating and dysplastic epithelium signifies that the SIgA system is topically deficient. There is, moreover, a significant shift from IgA2 to IgA1 production, the latter subclass being less resistant to proteolytic degradation. These changes--together with activation of mucosal macrophages and a dramatic increase of IgG-producing cells--may reflect local establishment of a second defense line which, however, is unsuccessful in its attempt to eliminate antigens derived from the indigenous microbial flora. Such a 'frustrated' local humoral immune system results in altered immunological homeostasis and jeopardized mucosal integrity. Complement activation observed in relation to epithelium-bound IgG1 in ulcerative colitis indicates, moreover, that the surface epithelium is subjected to immunological attack by an autoimmune reaction. These luminal deposits regularly contain terminal cytotoxic complement, and often also C3b as a sign of persistent activation. Comparison of identical twins, discordant with regard to ulcerative colitis, suggests that the markedly skewed local IgG1 response seen in this IBD entity may be genetically determined. The initial event(s) eliciting B-cell driven immunopathology in IBD remains unknown. Abrogation of oral tolerance to certain antigens from commensal bacteria has been suggested as a putative early mechanism, and lymphoid neogenesis and hyperplasia in the lesions most likely signify massive microbial overstimulation of the local B-cell system. Such ectopic lymphoid microcompartments may contribute substantially to the proinflammatory systemic-type of B-cell responses occurring in established IBD lesions.

The development of mucosal immunity

The development of the intestinal immune system is a complex sequence of events that begins in utero under various genetic influences, but continues after birth, being modified by factors such as bacteria, hormones and feeds. This review discusses what is known about the ontogeny of each aspect of the mucosal immune system so as to provide a better understanding of how aberrations in the system might lead to systemic disease.

Management of food allergy: vitamins, fatty acids or probiotics?

The dietary approach to allergic disease in infancy is evolving from passive allergen avoidance to active stimulation of the immature immune system, the aim of which is to support the establishment of tolerance. This may include probiotics providing maturational signals for the gut-associated lymphoid tissue and by balancing the generation of pro and anti-inflammatory cytokines in addition to their capacity to reduce the dietary antigen load by degrading and modifying macromolecules. Probiotics have also been shown to reverse the increased intestinal permeability characteristic of children with food allergy and to enhance specific IgA responses frequently defective in children with food allergy. The promotion of gut barrier functions by probiotics also includes the normalization of the gut microecology, alterations in which have been demonstrated in allergic individuals. Dietary lipids, especially long-chain polyunsaturated fatty acids, regulate immune function and may modify the adherence of microbes in the mucosa thereby contributing to host-microbe interactions. The properties of specific dietary compounds in optimal combinations and the joint effects of nutrients can be exploited in the development of specific prophylactic and therapeutic interventions. To meet these targets, rigorous scientific effort is required to elucidate how the food matrix and the dietary content impacts on the complex cascade of interrelated immunological mechanisms in food allergy.

Mucosal B cells: phenotypic characteristics, transcriptional regulation, and homing properties

Mucosal antibody defense depends on a complex cooperation between local B cells and secretory epithelia. Mucosa-associated lymphoid tissue gives rise to B cells with striking J-chain expression that are seeded to secretory effector sites. Such preferential homing constitutes the biological basis for local production of polymeric immunoglobulin A (pIgA) and pentameric IgM with high affinity to the epithelial pIg receptor that readily can export these antibodies to the mucosal surface. This ultimate functional goal of mucosal B-cell differentiation appears to explain why the J chain is also expressed by IgG- and IgD-producing plasma cells (PCs) occurring at secretory tissue sites; these immunocytes may be considered as 'spin-offs' from early effector clones that through class switch are on their way to pIgA production. Abundant evidence supports the notion that intestinal PCs are largely derived from B cells initially activated in gut-associated lymphoid tissue (GALT). Nevertheless, insufficient knowledge exists concerning the relative importance of M cells, major histocompatibility complex class II-expressing epithelial cells, and professional antigen-presenting cells for the uptake, processing, and presentation of luminal antigens in GALT to accomplish the extensive and sustained priming and expansion of mucosal B cells. Likewise, it is unclear how the germinal center reaction in GALT so strikingly can promote class switch to IgA and expression of J chain. Although B-cell migration from GALT to the intestinal lamina propria is guided by rather well-defined adhesion molecules and chemokines/chemokine receptors, the cues directing preferential homing to different segments of the gut require better definition. This is even more so for the molecules involved in homing of mucosal B cells to secretory effector sites beyond the gut, and in this respect, the role of Waldever's ring (including the palatine tonsils and adenoids) as a regional inductive tissue needs further characterization. Data suggest a remarkable compartmentalization of the mucosal immune system that must be taken into account in the development of effective local vaccines to protect specifically the airways, eyes, oral cavity, small and large intestines, and urogenital tract.

Innate mechanisms for Bifidobacterium lactis to activate transient pro-inflammatory host responses in intestinal epithelial cells after the colonization of germ-free rats

Bifidobacteria comprise a dominant microbial population group in the human intestinal tract with purported beneficial health effects on the host. In this study, we characterized the molecular mechanisms for the initial interaction of probiotic Bifidobacterium lactis strain BB12 with native and intestinal epithelial cell (IEC) lines. We showed that B. lactis-monoassociated Fisher F344 rats transiently induce phosphorylation/activation of the NF-kappaB transcriptionally active subunit RelA and the mitogen-activated protein kinase (MAPK) p38 in native IEC at day 5 after initial bacterial colonization. In addition, Interleukin 6 (IL-6) gene expression was significantly increased at day 5, demonstrating the physiological relevance of transient transcription factor activation in IEC. In contrast, Bacteroides vulgatus-monoassociated Fisher rats revealed RelA but not p38 MAPK phosphorylation and failed to trigger significant IL-6 gene expression in native IEC. Moreover, we demonstrated that B. lactis triggers NF-kappaB RelA and p38 MAPK phosphorylation in IEC lines. Adenoviral delivery of mutant IKK-beta (Ad5dnIKKbeta) and inhibition of the p38 MAPK pathway through the pharmacological inhibitor SB203580 significantly blocked B. lactis-induced IL-6 gene expression in IEC, suggesting that B. lactis triggers NF-kappaB and MAPK signaling to induce gene expression in the intestinal epithelium. Regarding the mechanisms of bacteria epithelial cell cross-talk, B. lactis-induced IL-6 gene expression was completely inhibited in TLR2 deficient mouse embryogenic fibroblasts (MEF TLR2-/-) as well as TLR2DeltaTIR transfected Mode-K cells. In conclusion, we demonstrated that probiotic bacteria transiently trigger innate signal transduction and pro-inflammatory gene expression in the intestinal epithelium at early stages of bacterial colonization.

Enterococcus faecium SF68 enhances the immune response to Giardia intestinalis in mice

We studied the ability of the probiotic organism Enterococcus faecium SF68 to antagonize Giardia intestinalis infection in mice. Oral feeding of E. faecium strain SF68 starting 7 d before inoculation with Giardia trophozoites significantly increased the production of specific anti-Giardia intestinal IgA and blood IgG. This humoral response was mirrored at the cellular level by an increased percentage of CD4(+) T cells in the Peyer's patches and in the spleens of SF68-fed mice. The improvement of specific immune responses in probiotic-fed mice was associated with a diminution in the number of active trophozoites in the small intestine as well as decreased shedding of fecal Giardia antigens (GSA65 protein). The ability of SF68 to stimulate the immune system at both mucosal and systemic levels highlights mechanisms by which this probiotic might antagonize pathogens in vivo. Taken together, the data demonstrate the strong potential of strain SF68 to prevent protozoa from causing intestinal infections.

Comparison of Salmonella enterica serovar Typhimurium colitis in germfree mice and mice pretreated with streptomycin

Salmonella enterica subspecies 1 serovar Typhimurium is a common cause of bacterial enterocolitis. Mice are generally protected from Salmonella serovar Typhimurium colonization and enterocolitis by their resident intestinal microflora. This phenomenon is called "colonization resistance" (CR). Two murine Salmonella serovar Typhimurium infection models are based on the neutralization of CR: (i) in specific-pathogen-free mice pretreated with streptomycin (StrSPF mice) antibiotics disrupt the intestinal microflora; and (ii) germfree (GF) mice are raised without any intestinal microflora, but their intestines show distinct physiologic and immunologic characteristics. It has been unclear whether the same pathogenetic mechanisms trigger Salmonella serovar Typhimurium colitis in GF and StrSPF mice. In this study, we compared the two colitis models. In both of the models Salmonella serovar Typhimurium efficiently colonized the large intestine and triggered cecum and colon inflammation starting 8 h postinfection. The type III secretion system encoded in Salmonella pathogenicity island 1 was essential in both disease models. Thus, Salmonella serovar Typhimurium colitis is triggered by similar pathogenetic mechanisms in StrSPF and GF mice. This is remarkable considering the distinct physiological properties of the GF mouse gut. One obvious difference was more pronounced damage and reduced regenerative response of the cecal epithelium in GF mice. Overall, StrSPF mice and GF mice provide similar but not identical models for Salmonella serovar Typhimurium colitis.

Immune responses that adapt the intestinal mucosa to commensal intestinal bacteria

Animals contain an enormous load of non-pathogenic bacteria in the lower intestine, which exploit an environment with a stable temperature and abundant carbon sources. Our load of bacteria outnumbers our own cells. In order to survive with such a high number of organisms in very close proximity to host tissues the intestinal mucosa and its immune system is highly adapted. Mucosal immune responses are induced by small numbers of live commensal organisms penetrating the Peyer's patches and persisting in dendritic cells (DC). These DC can induce immunoglobulin A+ (IgA+) B cells, which recirculate through the lymph and bloodstream to populate the lamina propria and secrete protective IgA. Because DC loaded with commensal bacteria do not penetrate further than the mesenteric lymph nodes, immune induction to commensals is confined to the mucosa, allowing strong mucosal immune responses to be induced whilst the systemic immune system remains relatively ignorant of these organisms.

Compartmentalization of the mucosal immune responses to commensal intestinal bacteria

Mammals coexist with a luxuriant load of bacteria in the lower intestine (up to 10(12) organisms/g of intestinal contents). Although these bacteria do not cause disease if they remain within the intestinal lumen, they contain abundant immunostimulatory molecules that trigger immunopathology if the bacteria penetrate the body in large numbers. The physical barrier consists only of a single epithelial cell layer with overlying mucus, but comparisons between animals kept in germ-free conditions and those colonized with bacteria show that bacteria induce both mucosal B cells and some T cell subsets; these adaptations are assumed to function as an immune barrier against bacterial penetration, but the mechanisms are poorly understood. In mice with normal intestinal flora, but no pathogens, there is a secretory IgA response against bacterial membrane proteins and other cell wall components. Whereas induction of IgA against cholera toxin is highly T help dependent, secretory IgA against commensal bacteria is induced by both T independent and T dependent pathways. When animals are kept in clean conditions and free of pathogens, there is still a profound intestinal secretory IgA response against the commensal intestinal flora. However, T dependent serum IgG responses against commensal bacteria do not occur in immunocompetent animals unless they are deliberately injected intravenously with 10(4) to 10(6) organisms. In other words, unmanipulated pathogen-free mice are systemically ignorant but not tolerant of their commensal flora despite the mucosal immune response to these organisms. In mice that are challenged with intestinal doses of commensal bacteria, small numbers of commensals penetrate the epithelial cell layer and survive within dendritic cells (DC). These commensal-loaded DC induce IgA, but because they are confined within the mucosal immune system by the mesenteric lymph nodes, they do not induce systemic immune responses. In this way the mucosal immune responses to commensals are geographically and functionally separated from systemic immunity.

Accumulation of intestinal intraepithelial lymphocytes in association with lack of polymeric immunoglobulin receptor

Immunoglobulin A (IgA) is transported by the polymeric immunoglobulin receptor (pIgR) through epithelial cells of the gut, the airways, the tear and salivary glands, and the lactating mammary gland, and IgA accumulates in serum and the intestinal lamina propria of pIgR-deficient (pIgR(-/-)) mice. Intraepithelial lymphocytes (IEL) increased in number and Thy-1(+)CD8alphabeta(+)TCRalphabeta(+) IEL preferentially expanded in the small intestine (SI) of pIgR(-/-) mice. Cytotoxic activity of SI-IEL was comparable in pIgR(+/+) and pIgR(-/-) mice. Accumulation and cytotoxic activity of SI-IEL was attenuated in germ-free pIgR(-/-) mice. Furthermore, Thy-1(+)CD8alphabeta(+) IEL did not expand in pIgR(-/-)TCRbetadelta(-/-) mice compared with TCRbetadelta(-/-) mice, and SI-IEL from pIgR(-/-)TCRbetadelta(-/-) mice as well as TCRbetadelta(-/-) mice expressed perforin and granzyme B mRNA and serine esterase. The proliferative status of SI-IEL from pIgR(+/+) and pIgR(-/-) mice was similar, but adoptive transfer experiment showed that SI-IEL from pIgR(-/-) mice might have a stronger tendency to migrate into the intestinal epithelia than those from pIgR(+/+) mice. These results demonstrate that the accumulation of Thy-1(+)CD8alphabeta(+)TCRalphabeta(+) IEL in pIgR(-/-) mice triggered by intestinal microorganisms needed the expression of functional TCR and might be caused by lymphocyte migration into the intestinal epithelia.

Restricted IgA repertoire in both B-1 and B-2 cell-derived gut plasmablasts

Mucosal IgA is the most abundantly produced Ig upon colonization of the intestinal tract with commensal organisms in the majority of mammals. The repertoire of these IgA molecules is still largely unknown; a large amount of the mucosal IgA cannot be shown to react with the inducing microorganisms. Analysis of the repertoire of used H chain Ig (V(H)) genes by H-CDR3 spectrotyping, cloning, and sequencing of V(H) genes from murine intestinal IgA-producing plasma cells reveals a very restricted usage of V(H) genes and multiple clonally related sequences. The restricted usage of V(H) genes is a very consistent observation, and is observed for IgA plasma cells derived from B-1 or conventional B-2 cells from different mouse strains. Clonal patterns from all analyzed V(H) gene sequences show mainly independently acquired somatic mutations in contrast to the clonal evolution patterns often observed as a consequence of affinity maturation in germinal center reactions in peripheral lymphoid organs and Peyer's patches. Our data suggest a model of clonal expansion in which many mucosal IgA-producing B cells develop in the absence of affinity maturation. The affinity of most produced IgA might not be the most critical factor for its possible function to control the commensal organisms, but simply the abundance of large amounts of IgA that can bind with relatively unselected affinity to redundant epitopes on such organisms.

Study of humoral immunity to commensal oral bacteria in human infants demonstrates the presence of secretory immunoglobulin A antibodies reactive with Actinomyces naeslundii genospecies 1 and 2 ribotypes

The mouths of three human infants were examined from birth to age 2 years to detect colonization of Actinomyces naeslundii genospecies 1 and 2. These bacteria did not colonize until after tooth eruption. The diversity of posteruption isolates was determined by ribotyping. Using immunoblotting and enzyme-linked immunosorbent assay, we determined the reactivity of secretory immunoglobulin A (SIgA) antibodies in saliva samples collected from each infant before and after colonization against cell wall proteins from their own A. naeslundii strains and carbohydrates from standard A. naeslundii genospecies 1 and 2 strains. A. naeslundii genospecies 1 and 2 carbohydrate-reactive SIgA antibodies were not detected in any saliva sample. However, SIgA antibodies reactive with cell wall proteins were present in saliva before these bacteria colonized the mouth. These antibodies could be almost completely removed by absorption with A. odontolyticus, a species known to colonize the human mouth shortly after birth. However, after colonization by A. naeslundii genospecies 1 and 2, specific antibodies were induced that could not be removed by absorption with A. odontolyticus. Cluster analysis of the patterns of reactivity of postcolonization salivary antibodies from each infant with antigens from their own strains showed that not only could these antibodies discriminate among strains but antibodies in saliva samples collected at different times showed different reactivity patterns. Overall, these data suggest that, although much of the salivary SIgA antibodies reactive with A. naeslundii genospecies 1 and 2 are directed against genus-specific or more broadly cross-reactive antigens, species, genospecies, and possibly strain-specific antibodies are induced in response to colonization.

Lactobacillus paracasei normalizes muscle hypercontractility in a murine model of postinfective gut dysfunction

BACKGROUND & AIMS

The effects of probiotics on gut dysfunction in postinfective irritable bowel syndrome are unknown. We tested whether probiotics influence persistent muscle hypercontractility in mice after recovery from infection with Trichinella spiralis and analyzed the underlying mechanisms.

METHODS

Mice were gavaged with Lactobacillus paracasei, Lactobacillus johnsonii, Bifidobacterium longum, or Bifidobacterium lactis in spent culture medium from days 10 to 21 after infection. Additional mice received heat-inactivated Lactobacillus paracasei, Lactobacillus paracasei -free spent culture medium, or heat-inactivated Lactobacillus paracasei -free spent culture medium. Lactobacilli enumeration, immunohistochemistry, and cytokine detection (enzyme-linked immunosorbent assay) were performed. Mice were also treated with Lactobacillus paracasei or Lactobacillus paracasei -free spent culture medium from days 18 to 28 after infection. Contractility was measured on days 21 and 28 after infection.

RESULTS

Lactobacillus paracasei, but not Lactobacillus johnsonii, Bifidobacterium lactis, or Bifidobacterium longum, attenuated muscle hypercontractility. This was associated with a reduction in the Trichinella spiralis -associated T-helper 2 response and a reduction in transforming growth factor-beta1, cyclooxygenase-2, and prostaglandin E 2 levels in muscle. Attenuation of muscle hypercontractility by Lactobacillus paracasei -free spent culture medium was abolished after heat treatment. Improvement of muscle hypercontractility at day 28 after infection was also observed after the administration of Lactobacillus paracasei or Lactobacillus paracasei -free spent culture medium from day 18 after infection.

CONCLUSIONS

Probiotics show strain-dependent attenuation of muscle hypercontractility in an animal model of postinfective irritable bowel syndrome. This likely occurs via both a modulation of the immunologic response to infection and a direct effect of Lactobacillus paracasei or a heat-labile metabolite on postinfective muscle hypercontractility. Lactobacillus paracasei may be useful in the treatment of postinfective irritable bowel syndrome.

Oral tolerance in T cells is accompanied by induction of effector function in lymphoid organs after systemic immunization

The physiological ramifications of oral tolerance remain poorly understood. We report here that mice fed ovalbumin (OVA) exhibit oral tolerance to subsequent systemic immunization with OVA in adjuvant, and yet they clear systemic infection with a recombinant OVA-expressing strain of Salmonella enterica serovar Typhimurium better than unfed mice do. Mice fed a sonicated extract of S. enterica serovar Typhimurium are also protected against systemic bacterial challenge, and the protection is Th1 mediated, as feeding enhances clearance in interleukin-4-null (IL-4(-/-)) and IL-10(-/-) mice but not in gamma interferon-null (IFN-gamma(-/-)) mice. When T-cell priming in vivo is tracked temporally in T-cell receptor-transgenic mice fed a single low dose of OVA, CD4 T-cell activation and expansion are restricted largely to mucosal lymphoid organs. However, T cells from spleens and peripheral lymph nodes of fed mice proliferate and secrete IFN-gamma when restimulated with OVA in vitro, indicating the presence of primed T cells in systemic tissues following oral exposure to antigen. Nonetheless, oral tolerance can be observed in the fed mice as reduced recall responses following subsequent systemic immunization with OVA in adjuvant. Soluble OVA administered systemically has similar effects in vivo, and the "tolerance" seen in both cases can be partially reversed if the initial priming is made more immunogenic. Together, the results indicate that antigen exposure under poor adjuvantic conditions, whether oral or systemic, may lead to T-cell commitment to effector rather than proliferative capabilities, necessitating a reassessment of therapeutic modalities for induction of oral tolerance in allergic or autoimmune states.

Mechanisms of natural tolerance in the intestine: implications for inflammatory bowel disease

Tolerance, the regulated inability to respond to a specific immunologic stimulant, is a physiological event important to normal immune function. Just as loss of tolerance to self-proteins results in autoimmune diseases, we assert that loss of tolerance to commensal flora in the intestinal lumen leads to inflammatory bowel disease (IBD). Mechanisms through which the mucosal immune system establishes and remains hyporesponsive toward the presence of food proteins and commensal flora, which we define as natural tolerance, are discussed. In addition to the contributions by commensal flora, the innate host defense and the adaptive immune systems promote natural tolerance to sustain normal mucosal homeostasis. Understanding the molecular and cellular events that mediate natural tolerance will lead to more advanced insights into IBD pathogenesis and improved therapeutic options.

Getting a grip on things: how do communities of bacterial symbionts become established in our intestine?

The gut contains our largest collection of resident microorganisms. One obvious question is how microbial communities establish and maintain themselves within a perfused intestine. The answers, which may come in part from observations made by environmental engineers and glycobiologists, have important implications for immunologists who wish to understand how indigenous microbial communities are accommodated. Here we propose that the mucus gel layer overlying the intestinal epithelium is a key contributor to the structural and functional stability of this microbiota and its tolerance by the host.

Interactions of commensal gut microbes with subsets of B- and T-cells in the murine host

Although mechanisms operative in the induction and maintenance of specific, adaptive immunity, including 'cognate' B/T interactions, have been extensively studied and defined, we still know little about the mechanisms operative in developing and maintaining B- and T-cell dependent 'natural' immunity. Particularly, we are still rather ignorant concerning gut microbial/gut or systemic APC, T cell and B cell interactions that lead to lymphoid cell mediated 'natural' immunity: specific or broadly reactive, activation via TCR and BCR and/or via other receptors such as the TLR series, and whether T/B interactions are operative at this level? Here we will address: (1) the general role of gut microbes in the development and maintenance of the intestinal, humoral immune system; (2) the general role of gut microbes in the development of B1 cell mediated, 'natural' gut IgA and the dependence of these B1 cells on bystander T cell help; (3) the relative contributions of B1 versus B2 cells to gut 'natural' and specific IgA responses; (4) the role for particular 'normal' gut microbes in the initiation of inflammatory bowel diseases (IBD) in mice with a dysregulated immune system; and (5) the possible roles of gut microbes in facilitating oral tolerance, a mechanism likely operative in forestalling or ameliorating IBD. A central theme of this paper is to attempt to define the specificities of activated, functional CD4+ T cells in the gut for Ags of particular, usually benign gut microbes. We will also consider the still-unresolved issue of whether the contributions of B1-derived IgA in the gut to the 'natural' Ab pool are Ag-selected and driven to proliferation/differentiation or whether the main stimuli are not via BCRs but rather other receptors (TLRs, etc.). The main experimental approach has been to use antigen-free, germ-free, or gnotobiotic (mono- or oligo-associated with precisely known bacterial species) mice.

Role of commensal bacteria in development of gut-associated lymphoid tissues and preimmune antibody repertoire

Intestinal bacteria are required for development of gut-associated lymphoid tissues (GALT), which mediate a variety of host immune functions, such as mucosal immunity and oral tolerance. In rabbits, the intestinal microflora are also required for developing the preimmune Ab repertoire by promoting somatic diversification of Ig genes in B cells that have migrated to GALT. We studied the mechanism of bacteria-induced GALT development. Bacteria were introduced into rabbits in which the appendix had been rendered germfree by microsurgery (we refer to these rabbits as germfree-appendix rabbits). We then identified specific members of the intestinal flora that promote GALT development. The combination of Bacteroides fragilis and Bacillus subtilis consistently promoted GALT development and led to development of the preimmune Ab repertoire, as shown by an increase in somatic diversification of VDJ-C micro genes in appendix B cells. Neither species alone consistently induced GALT development, nor did Clostridium subterminale, Escherichia coli, or Staphylococcus epidermidis. B. fragilis, which by itself is immunogenic, did not promote GALT development; hence, GALT development in rabbits does not appear to be the result of an Ag-specific immune response. To identify bacterial pathways required for GALT development, we introduced B. fragilis along with stress-response mutants of B. subtilis into germfree-appendix rabbits. We identified two Spo0A-controlled stress responses, sporulation and secretion of the protein YqxM, which are required for GALT development. We conclude that specific members of the commensal, intestinal flora drive GALT development through a specific subset of stress responses.

Microbial-gut interactions in health and disease. Immune responses

The indigenous bacterial microflora colonize the gut at birth and remain there throughout life. Approximately 10(14) bacteria are present in the ileum and colon and they are clearly immunogenic. The evidence is strong that the vast majority of IgA plasma cells in normal human gut are responding to the antigens of the flora, and although the flora is also responsible for producing the large numbers of T cells which are present in the gut of healthy individuals, the types of T cell response which the flora elicits are less well understood. A major challenge for the immune system is to distinguish between the antigens of the flora and the antigens of pathogens. There is also a growing realization that the normal flora can also influence gene expression in antigen-presenting cells in the gut and so set the context in which T cells respond to food antigen and vaccines.

Effect of probiotic bacteria on induction and maintenance of oral tolerance to beta-lactoglobulin in gnotobiotic mice

In this study, the effect of Lactobacillus paracasei (NCC 2461), Lactobacillus johnsonii (NCC 533) and Bifidobacterium lactis Bb12 (NCC 362) on the induction and maintenance of oral tolerance to bovine beta-lactoglobulin (BLG) was investigated in mice. Germfree mice were monocolonized with one of the three strains before oral administration of whey protein to induce tolerance. Mice were then injected with BLG and sacrificed 28 or 50 days after whey protein feeding for humoral and cellular response measurement. Conventional and germfree mice were used as controls. Both humoral and cellular responses were better suppressed in conventional mice than in germfree and monoassociated mice throughout the experiment and better suppressed in L. paracasei-associated mice than in mice colonized with B. lactis or L. johnsonii. The latter two mono-associations suppressed humoral responses only partially and cellular responses not at all. This study provides evidence that probiotics modulate the oral tolerance response to BLG in mice. The mono-colonization effect is strain-dependant, the best result having been obtained with L. paracasei.

Intestinal microflora in early infancy: composition and development

The neonatal intestinal microbiota is a complex ecosystem composed of numerous genera, species and strains of bacteria. This enormous cell mass performs a variety of unique activities that affect both the colonic and systemic physiology. Its primary activities include nutritive, metabolic, immunological and protective functions. Most studies of infants have been based on faecal samples using the classical plating techniques with culturing on specific media. The limitations of these methods must be taken into account when evaluating the varying results of the different studies. The establishment of the gut microbial population is not strictly a succession in the ecological sense; it is rather a complex process influenced by microbial and host interactions and by external and internal factors. The climax intestinal flora is attained in successive stages. The foetal intestine is sterile and bathed in swallowed amniotic fluid. Following delivery, multiple different antigens challenge the intestine of the newborn. The maternal intestinal flora is a source of bacteria for the neonatal gut. The bacterial flora is usually heterogeneous during the first few days of life, independently of feeding habits. After the first week of life, a stable bacterial flora is usually established. In full-term infants a diet of breast milk induces the development of a flora rich in Bifidobacterium spp. Other obligate anaerobes, such as Clostridium spp. and Bacteroides spp., are more rarely isolated and also enterobacteria and enterococci are relatively few. During the corresponding period, formula-fed babies are often colonized by other anaerobes in addition to bifidobacteria and by facultatively anaerobic bacteria; the development of a "bifidus flora" is unusual. In other studies the presence of a consistent number of bifidobacteria in infants delivered in large urban hospitals has not been demonstrated, whether the babies were bottle fed or exclusively breastfed. The predominant faecal bacteria were coliforms and bacteroides. According to these studies, environmental factors may be more important than breastfeeding in gut colonization after delivery. Environmental factors are indeed extremely important for the intestinal colonization of infants born by caesarean section. In these infants, the establishment of a stable flora characterized by a low incidence of Bacteroides spp. and by the isolation of few other bacteria is consistently delayed. In extremely low-birthweight infants, hospitalization in neonatal intensive care units, characterized by prolonged antibiotic therapy, parenteral nutrition, delayed oral feedings and intubation seems to affect the composition of the intestinal microbiota. The gut is colonized by a small number of bacterial species; Lactobacillus and Bifidobacteria spp. are seldom, if ever, identified. According to the few studies so far performed, the predominant species are Enterococcus faecalis, E. coli, Enterobacter cloacae, Klebsiella pneumoniae, Staphylococcus epidermidis and Staphylococcus haemolyticus. Hygienic conditions and antimicrobial procedures strongly influence the intestinal colonization pattern.

Human secretory immunoglobulin A may contribute to biofilm formation in the gut

It is critical, both for the host and for the long-term benefit of the bacteria that colonize the gut, that bacterial overgrowth with subsequent bacterial translocation, which may lead to sepsis and death of the host, be avoided. Secretory IgA (sIgA) is known to be a key factor in this process, agglutinating bacteria and preventing their translocation in a process termed 'immune exclusion'. To determine whether human sIgA might facilitate the growth of normal enteric bacteria under some conditions, the growth of human enteric bacteria on cultured, fixed human epithelial cells was evaluated in the presence of sIgA or various other proteins. Human sIgA was found to facilitate biofilm formation by normal human gut flora and by Escherichia coli on cultured human epithelial cell surfaces under conditions in which non-adherent bacteria were repeatedly washed away. In addition, the presence of sIgA resulted in a 64% increase in adherence of E. coli to live cultured epithelial cells over a 45-min period. Mucin, another defence factor thought to play a key role in immune exclusion, was found to facilitate biofilm formation by E. coli. Our findings suggest that sIgA may contribute to biofilm formation in the gut.

Immune responses against Salmonella enterica serovar enteritidis infection in virally immunosuppressed chickens

To understand the role of immune mechanisms in protecting chickens from Salmonella infections, we examined the immune responses of Salmonella enterica serovar Enteritidis-infected chickens and the effect of chicken anemia virus (CAV), a T-cell-targeted virus, on S. enterica serovar Enteritidis-induced immune responses. One-day-old chicks were orally inoculated with S. enterica serovar Enteritidis with or without intramuscular injection of CAV. The bacterial infection, pathology, and immune responses of chickens were evaluated at 14, 28, and 56 days postinoculation. The infection increased the levels of S. enterica serovar Enteritidis-specific mucosal immunoglobulin A (IgA), the number of gut-associated T cells, and the titer of serum IgG specific for S. enterica serovar Enteritidis surface antigens. CAV infection depressed these immune responses, especially the mucosal immune responses, but did not increase the number of S. enterica serovar Enteritidis-infected cells in the intestine. The severity of pathological lesions appeared to be reciprocal to the level of immune responses, but the S. enterica serovar Enteritidis infection persisted. These results suggest that oral infection of S. enterica serovar Enteritidis in chickens induces both mucosal and systemic immune responses, which have a limited effect on the S. enterica serovar Enteritidis infection under conditions designed to mimic the field situation.

Escherichia coli in infants' intestinal microflora: colonization rate, strain turnover, and virulence gene carriage

Colonization by Escherichia. coli in infants might have decreased in the last decades, owing to changes in hospital routines and family lifestyle. In this study, the E. coli flora was characterized in 70 healthy Swedish infants followed for the first year of life. E. coli was isolated from rectal swabs obtained at 3 d of age and quantified in fecal samples collected at 1, 2, 4, and 8 wk of age and at 6 and 12 mo of age. Strains were typed using random amplified polymorphic DNA, and their virulence factor genes were identified by multiplex PCR. Colonization by E. coli occurred late; only 61% of the infants were positive by 2 mo of age. The turnover of individual strains in the microflora was slow (1.5 strains per infant during 6 mo, 2.1 during 1 y). Environmental factors, such as siblings, pets, or feeding mode, did not influence colonization kinetics or strain turnover rate. Genes encoding type 1 fimbriae, P fimbriae, and hemolysin were significantly more common in E. coli strains persisting for at least 3 wk in the microflora than in transient strains. The P-fimbrial class III adhesin gene was more common in E. coli from children who had a cat in their homes than in E. coli from children without pets (p = 0.01); this adhesin type is common in E. coli from cats. The late colonization and low E. coli strain turnover rate suggest limited exposure of Swedish infants to E. coli. Our results confirm that P fimbriae and other virulence factors facilitate persistence of E. coli in the human colonic microflora.

Bacteroides vulgatus protects against Escherichia coli-induced colitis in gnotobiotic interleukin-2-deficient mice

BACKGROUND & AIMS

The microflora plays a crucial role in inflammatory bowel diseases (IBDs). Specific pathogen-free (SPF), but not germ-free, interleukin (IL)-2-deficient (IL-2-/-) mice develop colitis. The colitogenicity of commensal bacteria was determined.

METHODS

Gnotobiotic IL-2-/- and IL-2+/+ mice were colonized with Escherichia coli mpk, Bacteroides vulgatus mpk, or both bacterial strains, or with E. coli strain Nissle 1917. DNA arrays were used to characterize E. coli mpk. Colitis was analyzed by histology and real-time reverse-transcription polymerase chain reaction (RT-PCR) for interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, IL-10, and CD14 messenger RNA (mRNA) expression. Bacterial numbers in feces and bacterial localization in the colon was determined by culture and fluorescence in situ hybridization (FISH).

RESULTS

IL-2-/- but not IL-2+/+ mice monocolonized with E. coli mpk developed colitis, whereas mono-association with B. vulgatus mpk, or E. coli Nissle, or co-colonization with E. coli mpk and B. vulgatus mpk, did not induce colitis. DNA array experiments and cellular studies revealed that E. coli mpk is a nonpathogenic strain. FISH and culture methods revealed that the anticolitogenic effect of B. vulgatus mpk on E. coli mpk cannot be explained by a significant reduction in numbers of E. coli in the colon. E. coli mpk-induced colitis was associated with increased IFN-gamma, TNF-alpha, CD14, and IL-10 mRNA expression in the colon.

CONCLUSIONS

In IL-2-/- mice, B. vulgatus mpk protects against E. coli mpk-triggered colitis by an unknown mechanism. E. coli Nissle does not induce colitis. Various bacterial species common to the microflora differ in their ability to trigger IBD.

Inflammatory bowel disease: dysfunction of GALT and gut bacterial flora (II)

The precise cause(s) of Crohn's disease and ulcerative colitis are unknown. From animal models and human studies it is well established that gut bacterial flora are essential for inducing the bowel inflammation. Animal models, when kept in a germ-free environment, do not develop colitis until the gut flora is reconstituted. It is not clear whether the bacterial antigens (Ags) from the normal flora or some other pathogenic bacterial Ags induce/propagate the inflammatory process in inflammatory bowel disease (IBD). Despite extensive research it has not been possible to identify any specific bacteria or virus as a credible cause of IBD. Recent understanding of quorum sensing molecules (QSMs) secreted by bacteria helps to explain the community behaviour in bacterial species. When QSMs reach a defined concentration, they activate bacterial proliferation and a number of virulence genes. Also, these molecules have been found to modulate the immune system to the advantage of the gut bacteria. They have not been well studied, however, in the gut. Inappropriate secretion of QSMs may alter the gut-associated lymphoid tissue (GALT) and, thereby, deregulate the immune tolerance normally present. Usefulness of probiotics and their immune modulating effects are being increasingly reported. Probiotics are also being used in the treatment of IBD. The interaction between the epithelial cells and the gut flora is very important as this is the first line of contact; this interaction may determine the induction of tolerance and mucosal integrity or immune activity, tissue inflammation and abnormal permeability. The latter is documented in patients with IBD and their healthy relatives. This may be an important factor in disruption of mucosal integrity and GALT dysfunction.

B1 cells contribute to serum IgM, but not to intestinal IgA, production in gnotobiotic Ig allotype chimeric mice

B1 cells are a significant source of natural serum IgM, thereby serving as a first line of defense against systemic bacterial and viral infections. They can migrate to the intestinal lamina propria and differentiate into IgA-producing plasma cells and thus might play a similar role in mucosal immunity. To investigate the contribution of B1 cells to the intestinal IgA response induced by the commensal flora in immunocompetent animals, we generated gnotobiotic and conventionally reared Ig allotype chimeric mice. In this system B1- and B2-derived Abs can be distinguished based on different allotypes. FACS analysis of peritoneal cavity cells and analysis of B1- and B2-derived serum IgM indicated stable B1/B2 chimerism and the establishment of a functional B1 population. Monoassociation with either Morganella morganii, Bacteroides distasonis, or segmented filamentous bacteria induced germinal center reactions in Peyer's patches and led to the production of intestinal IgA, partially reactive with bacterial Ag. A considerable amount of serum IgM was B1 cell derived in both monoassociated and conventionally reared mice. However, most of the total as well as bacteria-specific intestinal IgA was produced by B2 cells. These data suggest that intestinal IgA production induced by commensal bacteria is mainly performed by B2, not B1, cells.

Supplementation of food with Enterococcus faecium (SF68) stimulates immune functions in young dogs

The gut microflora play a crucial role in several physiologic functions of the host, including maturation of the gut-associated lymphoid tissues during the first months of life. Oral administration of probiotic lactic acid bacteria (LAB) modulates the immune system of humans and some laboratory animals. This effect has never been examined in dogs; therefore, our aim was to study the capacity of a probiotic LAB to stimulate immune functions in young dogs. Puppies were allotted to two groups receiving either a control diet or a diet supplemented with 5 x 10(8) colony forming units (cfu)/d of probiotic Enterococcus faecium (SF68) from weaning to 1 y of age. Fecal and blood samples were collected from the dogs at different time points for the measurement of fecal immunoglobulin (Ig)A, circulating IgG and IgA, and the proportions of lymphoid cell subsets. Fecal IgA and canine distemper virus (CDV) vaccine-specific circulating IgG and IgA were higher in the group receiving the probiotic than in controls. There were no differences in the percentages of CD4(+) and CD8(+) T cells between the groups, but the proportion of mature B cells [CD21(+)/major histocompatibility complex (MHC) class II(+)] was greater in those fed the probiotic. These data show for the first time that a dietary probiotic LAB enhance specific immune functions in young dogs, thus offering new opportunities for the utilization of probiotics in canine nutrition.