The secretory immunoglobulin system: regulation and biological significance. Focusing on human mammary glands

Commensal microbiota modulates phenotypic characteristics and gene expression in piglet Peyer's patches

The gastrointestinal tract contains a complex microbial community. Peyer's patches (PPs) play an important role in inducing mucosal immune responses in the gastrointestinal tract. However, little is known about the effect of commensal microbiota on the host's PPs. Here, we analyzed the phenotypic-to-transcriptome changes in the intestine PPs of specific pathogen-free (SPF) and germ-free (GF) piglets (fed in an environment with and without commensal microbiota, respectively) to elucidate the role of commensal microbiota in host intestine mucosal immunity. Analyses of anatomical and histological characteristics showed that commensal microbiota deficiency led to PP hypoplasia, especially regarding B and T cells. A total of 12,444 mRNAs were expressed in 12 libraries; 2,156 and 425 differentially expressed (DE) mRNAs were detected in the jejunal PP (JPP) and ileal PP (IPP), respectively (SPF vs. GF). The shared DE mRNAs of the JPP and IPP were mainly involved in basic physiological and metabolic processes, while the specific DE mRNAs were enriched in regulating immune cells in the JPP and microbial responses and cellular immunity in the IPP. Commensal microbiota significantly modulated the expression of genes related to B-cell functions, including activation, proliferation, differentiation, apoptosis, receptor signaling, germinal center formation, and IgA isotype class switching, particularly in the JPP. TLR4 pathway-related genes were induced in response to microbial colonization and in LPS/SCFA-treated B cells. We also detected 69 and 21 DE lncRNAs in the JPP and IPP, respectively, and four one-to-one lncRNA-mRNA pairs were identified. These findings might represent key regulatory axes for host intestine mucosal immunity development during microbial colonization. Overall, the findings of this study revealed that commensal microbiota modulated phenotypic characteristics and gene expression in the piglet intestine PPs and underscored the importance of early microbial colonization for host mucosal immunity development.

Oral Application of Mother's Own Milk for Prevention of Late Onset Sepsis in Preterm Very Low Birth Weight Neonates: A Randomized Controlled Trial

Background: Oral application of mother's own milk (MOM) in very low birth weight (VLBW) neonates may provide an effective method of cleaning the oral cavity and influencing the immunocompetent cells there. Methods: A double-blind randomized controlled trial was conducted in a tertiary level neonatal unit on VLBW neonates; that is, neonates with birth weight <1,500 g, hospitalized within 72 hours of birth. The intervention was oral care with MOM. The primary objective was to observe the effect of oral care with MOM on late onset sepsis (LOS) and also on mortality, days to attain full enteral feed, necrotizing enterocolitis (NEC), exclusive breastfeeding rates at discharge, and total hospital stay (THS). Results: One hundred ten neonates were randomized to intervention and no-intervention group; 55 in each group. Neonates in the intervention group developed LOS in 9.1% versus 27.3% in no-intervention group (p = 0.013). Mortality was 9.1% versus 16.4% (p = 0.252); days to attain full enteral feeds were 6.94 ± 2.34 versus7.4 ± 2.31 days (p = 0.306); exclusive BF rates at discharge were 83% versus 75% (p = 0.449); THS was 30.72 ± 8.72 versus 33.5 ± 8.7 days (p = 0.267) in the two groups, respectively. There was no effect on NEC. Conclusion: Oral care of VLBW neonates with MOM reduces LOS and mortality.

Effect of Oropharyngeal Administration of Colostrum in Premature Newborns ≤32 Weeks of Gestation on the Immune Response and Neonatal Morbidity: A Double-Blind Randomized Clinical Trial

INTRODUCTION

The mother's colostrum carries immunological components, such as cytokines and immunoglobulins (Igs), derived from the maternal circulation with bacteriostatic properties.

OBJECTIVE

The objective of this study was to evaluate the effect of oropharyngeal administration of colostrum (OPAC) vs. placebo in the first 4 days of life in premature newborns ≤32 weeks of gestation on serum Ig concentration, neonatal morbidity, and total days of hospitalization.

HYPOTHESIS

The OPAC increases serum Igs and decreases morbidity and total days of hospitalization.

MATERIALS AND METHODS

A double-blind randomized controlled trial was carried out. Participants were randomly assigned to one of the two groups, namely, group 1: placebo (P) (n = 50) and group 2: colostrum (C) (n = 46). A blood sample was obtained at baseline and 7 and 28 days of life to quantify immunoglobulin G (IgG), immunoglobulin A (IgA), and IgM. Results: The C group showed an increase in serum IgA on day 28 expressed as median and [interquartile range]; C: 25 [12-35] vs. P: 11 [8-18], p < 0.001. There were no significant differences in neonatal morbidity. Newborns in the colostrum group showed the completed enteral feeding earlier (days), C: 13.9 ± 7 vs. P: 17.4 ± 8.4, p < 0.04; they reached the birth weight earlier, C: 10.9 ± 2.8 vs. P: 12.9 ± 4, p < 0.01, and had less days of hospitalization, C: 60.2 ± 33.8 vs. P: 77.2 ± 47.3, p < 0.04. Neonatal mortality was lower in the colostrum group than the placebo group 0% vs. 12%, respectively, without a statistical difference (p = 0.06).

CONCLUSION

In premature newborns ≤32 weeks of gestation, the OPAC within 4 days after birth increases serum IgA concentration at day 28 compared to placebo. Similarly, OPAC decreased the days to complete enteral feeding and reach the birth weight and total days of hospitalization.

CLINICAL TRIAL REGISTRATION

[https://clinicaltrials.gov/ct2/show/NCT03578341], identifier: [NCT03578341].

New insight into function and dysfunction of gut microfold cells

Microfold cells (M cells), derived from intestinal crypt Lgr5⁺ stem cells, are distributed in gut-associated lymphoid tissue (GALT), nasopharyngeal-associated lymphoid tissue (NALT), and bronchial-associated lymphoid tissue (BALT). The basement membrane of mature M cells protrudes upward, showing a "pocket-like" shape. M cell differentiation is mainly regulated by two pathways, one is the non-canonical NF-κB pathway, and the other is the canonical NF-κB pathway. The differentiation and maturation of M cells are closely related to RANKL and S100A4. M cells can not only transport antigens and trigger an immune response, but also are the gateway for various pathogens to invade the body. The occurrence and development of tuberculosis, prion disease, and Crohn's disease are closely related to M cells.

Oropharyngeal Colostrum Administration in Very Low Birth Weight Infants: A Randomized Controlled Trial

OBJECTIVES

Studies have confirmed the safety of oropharyngeal administration of colostrum in very low birth weight infants. However, the effect of oropharyngeal administration of colostrum on immune system is inconclusive. This study aims to evaluate the effect of oropharyngeal administration of colostrum on secretory immunoglobulin A and lactoferrin in very low birth weight infants.

DESIGN

Randomized controlled trial.

SETTING

Forty-bedded neonatal ICU in a university children's hospital in the People's Republic of China.

PATIENTS

Very low birth weight infants were allocated to the study group (n = 32) and control group (n = 32).

INTERVENTION

The intervention was oropharyngeal administration of 0.2 mL of their mother's colostrum every 4 hours for 7 days. The control group received saline solution.

MEASUREMENTS AND MAIN RESULTS

Secretory immunoglobulin A and lactoferrin in urine and saliva were measured within 24 hours of life (baseline) and at 7 and 21 days. Primary outcomes were changes of secretory immunoglobulin A and lactoferrin in urine and saliva between baseline and at 7 and 21 days. Infant's clinical data were also collected during hospitalization. Change from baseline in lactoferrin in saliva at 7 days (5.18 ± 7.07 vs -1.74 ± 4.67 µg/mL; p < 0.001) and 21 days (5.31 ± 9.74 vs -1.17 ± 10.38 µg/mL; p = 0.02) shows statistic difference. No differences were found of lactoferrin in urine and also no differences of secretory immunoglobulin A in urine and saliva. There were also no differences between days to full enteral feeding, occurrence rate of clinical sepsis, proven sepsis, and necrotizing enterocolitis.

CONCLUSIONS

Oropharyngeal administration of colostrum can increases the level of lactoferrin in saliva in very low birth weight infants. No effect could be documented of secretory immunoglobulin A and lactoferrin in urine. Larger trials are needed to better describe the benefit of oropharyngeal administration of colostrum, if any, in very low birth weight infants.

Effects of Mediterranean diet in patients with recurring colds and frequent complications

INTRODUCTION

In recent years, traditional diets enriched with fresh plant-based foods have been gradually abandoned, increasing the consumption of animal foods and highly processed food. The aim of this study was to assess the effects of a nutritional intervention with a Traditional Mediterranean Diet in patients with recurring colds (RC) and frequent inflammatory complications (IC).

METHODS

Prospective before-after comparison study of 63 girls and 65 boys aged 1-5 years were included over a year in the nutritional programme "Learning to eat from the Mediterranean". We studied clinical and therapeutic variables and various anthropometric parameters.

RESULTS

All the studied indicators (number of catarrhal episodes CB, degree of intensity, emergency and hospital admissions) showed a positive and statistically significant evolution, evidenced from the first weeks of starting treatment, until the end of the year, after which 53.9% of patients had no CB, 25% had only one, and 16.4% had two episodes, compared to the 4.64 episodes on average in the previous year. Antibiotic use decreased by 87.4%, from 3.85±1.27 times/patient/year to 0.49±0.79 (p<0.001). Symptomatic treatment decreased by 56.7%, from 7.03±2.76 to 3.05±1.69 (p<0.001). The satisfaction of the families was very high. The Kidmed index, which assesses the quality of the Mediterranean Diet, increased from 7.8 to 10.9 points.

CONCLUSION

The adoption of a Traditional Mediterranean Diet could be a major contribution to the improvement of patients with recurring colds and frequent inflammatory complications.

Amniotic fluid: Source of trophic factors for the developing intestine

The gastrointestinal tract (GIT) is a complex system, which changes in response to requirements of the body. GIT represents a barrier to the external environment. To achieve this, epithelial cells must renew rapidly. This renewal of epithelial cells starts in the fetal life under the influence of many GIT peptides by swallowing amniotic fluid (AF). Development and maturation of GIT is a very complex cascade that begins long before birth and continues during infancy and childhood by breast-feeding. Many factors like genetic preprogramming, local and systemic endocrine secretions and many trophic factors (TF) from swallowed AF contribute and modulate the development and growth of the GIT. GIT morphogenesis, differentiation and functional development depend on the activity of various TF in the AF. This manuscript will review the role of AF borne TF in the development of GIT.

Bayesian modeling suggests that IL-12 (p40), IL-13 and MCP-1 drive murine cytokine networks in vivo

Background

Cytokine-hormone network deregulations underpin pathologies ranging from autoimmune disorders to cancer, but our understanding of these networks in physiological/pathophysiological states remains patchy. We employed Bayesian networks to analyze cytokine-hormone interactions in vivo using murine lactation as a dynamic, physiological model system.

Results

Circulatory levels of estrogen, progesterone, prolactin and twenty-three cytokines were profiled in post partum mice with/without pups. The resultant networks were very robust and assembled about structural hubs, with evidence that interleukin (IL)-12 (p40), IL-13 and monocyte chemoattractant protein (MCP)-1 were the primary drivers of network behavior. Network structural conservation across physiological scenarios coupled with the successful empirical validation of our approach suggested that in silico network perturbations can predict in vivo qualitative responses. In silico perturbation of network components also captured biological features of cytokine interactions (antagonism, synergy, redundancy).

Conclusion

These findings highlight the potential of network-based approaches in identifying novel cytokine pharmacological targets and in predicting the effects of their exogenous manipulation in inflammatory/immune disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0226-3) contains supplementary material, which is available to authorized users.

Establishment of a functional secretory IgA transcytosis model system in vitro for functional food screening

A characteristic mucosal immune response involves the production of antigen-specific secretory immunoglobulin A (SIgA) antibodies. In order to study transcytosis by mimicking the SIgA secretion and to screen for SIgA secretion-promoting substances, we developed a model system of a transfected Madin-Darby canine kidney (MDCK) cell line that expresses the human polymeric immunoglobulin receptor (pIgR). We thus isolated the human dIgA (dimeric IgA)/pIgA (polymeric IgA) complex as the binding ligands. In the present study, a recombinant vector encoding the human pIgR gene was constructed and infected into MDCK cells. Following reverse transcription polymerase chain reaction (RT-PCR), immunoblotting, and immunofluorescence staining, we confirmed that pIgR was expressed in the transfectant MDCK-pIgR cells and was located at the basolateral side of the cell surface. We also confirmed the coexistence of the dIgA/pIgA complex in the IgA myeloma serum. The covalent dIgA/pIgA complex was then isolated from the serum of an IgA multiple myeloma patient using an ÄKTA purifier operation system with a HiPrep 16/60 Sephacryl S-300 HR column, in order to utilize the complex as transcytosis ligands for human pIgR. Finally, we confirmed the uptake of the isolated human dIgA/pIgA complex into MDCK-pIgR cells. We demonstrated that the human dIgA/pIgA complex was transcytosed into the apical side of the monolayer cells. Therefore, our MDCK-human pIgR cell transcytosis model is an operational system and can be used for screening functional food components that promote dIgA/pIgR transcytosis as well as SIgA secretion.

Diseases of the nose and paranasal sinuses in child

Diseases of the pediatric nose and nasal sinuses as well as neighboring anatomical structures encompass a variety of pathologies, especially of inflammatory nature. Congenital disease, such as malformations and structural deviations of the nasal septum, as well as systemic metabolic pathologies affecting the nose and sinuses, rarely require medical therapy from an Otolaryngologist. The immunological function of the mucosa and genetic factors play a role in the development of disease in the pediatric upper airway tract, especially due to the constantly changing anatomy in this growth phase. Disease description of the nose and nasal sinuses due to mid-facial growth must also take developmental age differences (infant, toddler, preschool, and school age) into account. Epidemiological examinations and evidence based studies are often lacking in the pediatric population. The wide range of inflammatory diseases of the nose and paranasal sinuses, such as the acute and chronic rhinosinusitis, the allergic rhinitis, and adenoid disease, play a role in the susceptibility of a child to infection. The susceptibility to infection depends on the pediatric age structure (infant, young child) and has yet to be well defined. The acute rhinosinusitis in children develops after a viral infection of the upper airways, also referred to as the "common cold" in the literature. It usually spontaneously heals within ten days without any medical therapy. Antibiotic therapy is prudent in complicated episodes of ARS. The antibiotic therapy is reserved for children with complications or associated disease, such as bronchial asthma and/or chronic bronchitis. A chronic rhinosinusitis is defined as the inflammatory change in the nasal mucosa and nasal sinus mucosa, in which the corresponding symptoms persist for over 12 weeks. The indication for CT-imaging of the nasal sinuses is reserved for cases of chronic rhinosinusitis that have been successfully treated with medication. A staged therapeutic concept is followed in CRS based on conservative and surgical methods. Nasal sinus surgery is considered nowadays as effective and safe in children. Based on the assumption that adenoids are a reservoir for bacteria, from which recurrent infections of the nose and nasal sinus originate, the adenoidectomy is still defined as a cleansing procedure in rhinosinusitis. 69.3% of the children had benefit from adenoidectomy. Comorbidities, such as pediatric bronchial asthma, presently play an even more important role in the therapy of rhinosinusitis; therefore, it is often wise to have the support of pediatricians. In western European countries 40% of children presently suffer from allergic rhinitis, in which pronounced nasal obstruction can cause disturbed growth in facial bones. An early therapy with SIT may prevent the development of bronchial asthma and secondary sensitization to other allergens. Therefore, SIT is recommended in treatment of allergic rhinitis whenever, if possible. The assessment of diagnostic tools is for the examiner not often possible due to the lack of evidence. Rhinosurgical approaches are often described in study reports; however, they lack the standard prospective randomized long-term study design required nowadays and can only be evaluated with caution in the literature.

High intestinal IgA associates with reduced risk of IgE-associated allergic diseases

Development of oral tolerance and its stimulation by probiotics are still incomprehensible. Microbial stimulation of the gut may induce a subtle inflammation and induce secretion of mucosal IgA, which participates in antigen elimination. In a cohort of allergy-prone infants receiving probiotics and prebiotics or placebo we studied intestinal IgA and inflammation in the development of eczema, food allergy, asthma, and rhinitis (allergic diseases). We performed a nested unmatched case-control study of 237 infants participating in a randomized double-blind placebo-controlled allergy-prevention trial using a combination of four probiotic strains pre-natally and during 6 months form birth. We measured faecal IgA, alpha1-antitrypsin (alpha1-AT), tumour necrosis factor-alpha (TNF-alpha), and calprotectin at the age of 3 and 6 months. By age 2 yr, 124 infants had developed allergic disease or IgE-sensitization (cases) and 113 had not (controls). In infants with high faecal IgA concentration at the age of 6 months, the risk of having any allergic disease before the age of 2 yr tended to reduce [odds ratio (OR: 0.52)] and the risk for any IgE-associated (atopic) disease reduced significantly (OR: 0.49). High faecal calprotectin at the age of 6 months associated also with lower risk for IgE-associated diseases up to age 2 yr (OR: 0.49). All faecal inflammation markers (alpha1-AT, TNF-alpha, and calprotectin) correlated positively with faecal IgA (p < 0.001). Probiotics tended to augment faecal IgA (p = 0.085) and significantly increased faecal alpha1-AT (p = 0.001). High intestinal IgA in early life associates with minimal intestinal inflammation and indicates reduced risk for IgE-associated allergic diseases.

Does hapten exposure predispose to atopic disease? The hapten-atopy hypothesis

Contact allergy data indicates that atopics have heightened oral tolerance to haptens (chemical allergens). We speculate here, that artificially increased oral exposure to chemicals compete with dietary proteins for the development of oral tolerance, predisposing to the acquisition of food protein allergy and representing one driver for the increasing prevalence of protein allergy and/or atopy. Hapten exposure via other surfaces such as the skin and airways might also be important in promoting atopic disease. Consistent with this hypothesis it is notable that over 40 years, with the huge increase in atopic disease, there has also been an increase in dietary hapten exposure through processed food, formula milk and oral antibiotic and drug use.

Host factors in amniotic fluid and breast milk that contribute to gut maturation

The gut represents a complex organ system with regional differences, which reflect selective digestive and absorptive functions that change constantly in response to bodily requirements and the outside milieu. As a barrier to the external environment, gut epithelium must be renewed rapidly and repeatedly. Growth and renewal of gut epithelial cells is dependent on controlled cell stimulation and proliferation by a number of signaling processes and agents, including gut peptides-both endogenous and exogenous sources. This cascade of events begins during fetal development; with the ingestion of amniotic fluid, this process is enhanced and continued during infancy and early childhood through the ingestion of human milk. Events influenced by amniotic fluid during fetal development and those influenced by human milk that unfold after birth and early childhood to render the gut mature are presented.

Induction of secretory immunity and memory at mucosal surfaces

Mucosal epithelia comprise an extensive vulnerable barrier which is reinforced by numerous innate defence mechanisms cooperating intimately with adaptive immunity. Local generation of secretory IgA (SIgA) constitutes the largest humoral immune system of the body. Secretory antibodies function both by performing antigen exclusion at mucosal surfaces and by virus and endotoxin neutralization within epithelial cells without causing tissue damage. SIgA is thus persistently containing commensal bacteria outside the epithelial barrier but can also target invasion of pathogens and penetration of harmful antigens. Resistance to toxin-producing bacteria such as Vibrio cholerae and enterotoxigenic Escherichia coli appears to depend largely on SIgA, and so does herd protection against horizontal faecal-oral spread of enteric pathogens under naïve or immunized conditions--with a substantial innate impact both on cross-reactivity and memory. Like natural infections, live mucosal vaccines or adequate combinations of non-replicating vaccines and mucosal adjuvants, give rise not only to SIgA antibodies but also to longstanding serum IgG and IgA responses. However, there is considerably disparity with regard to migration of memory/effector cells from mucosal inductive sites to secretory effector sites and systemic immune organs. Also, although immunological memory is generated after mucosal priming, this may be masked by a self-limiting response protecting the inductive lymphoid tissue in the gut. The intranasal route of vaccine application targeting nasopharynx-associated lymphoid tissue may be more advantageous for certain infections, but only if successful stimulation is achieved without the use of toxic adjuvants that might reach the central nervous system. The degree of protection obtained after mucosal vaccination ranges from reduction of symptoms to complete inhibition of re-infection. In this scenario, it is often difficult to determine the relative importance of SIgA versus serum antibodies, but infection models in knockout mice strongly support the notion that SIgA exerts a decisive role in protection and cross-protection against a variety of infectious agents. Nevertheless, relatively few mucosal vaccines have been approved for human use, and more basic work is needed in vaccine and adjuvant design, including particulate or live-vectored combinations.

The postnatal maturation of the immunoglobulin heavy chain IgG repertoire in human preterm neonates is slower than in term neonates

During the perinatal period the development of the IgH chain CDR3 (CDR-H3) repertoire of IgM transcripts is maturity-dependent and not influenced by premature exposure to Ag. To study whether maturity-dependent restrictions also predominate in the perinatal IgG repertoire we compared 1000 IgG transcripts from cord blood and venous blood of extremely preterm neonates (24-28 wk of gestation) and of term neonates from birth until early infancy with those of adults. We found the following. First, premature contact with the extrauterine environment induced the premature development of an IgG repertoire. However after preterm birth the diversification of the IgG repertoire was slower than that after term birth. Second, the IgG repertoire of preterm neonates retained immature characteristics such as short CDR-H3 regions and overrepresentation of D(H)7-27. Third, despite premature exposure to the extrauterine environment, somatic mutation frequency in IgG transcripts of preterm infants remained low until they reached a postconceptional age corresponding to the end of term gestation. Thereafter, somatic mutations accumulated with age at similar rates in preterm and term neonates and reached 30% of the adult level after 6 mo. In conclusion, class switch was inducible already at the beginning of the third trimester of gestation, but the developing IgG repertoire was characterized by similar restrictions as those of the developing IgM repertoire. Those B cells expressing more "mature" H chain sequences were not preferentially selected into the IgG repertoire. Therefore, the postnatal IgG repertoire of preterm infants until the expected date of delivery differs from the postnatal repertoire of term neonates.

IgA antibodies, TGF-beta1 and -beta2, and soluble CD14 in the colostrum and development of atopy by age 4

Specific defense factors in breast milk together with length of breast-feeding and genetic predisposition may modulate the development of allergy. We studied whether IgA, soluble CD14 (sCD14), or transforming growth factor (TGF)-beta in colostrum could affect the development of atopy in children up to age 4. From a cohort of 4676, we selected four groups of children with either long or short exclusive breast-feeding (>3.5 or <0.5 mo); these groups further differed in the presence or absence of atopic heredity. In colostrum from mothers, we measured total IgA, IgA antibodies to cow's milk (CM) and casein, sCD14, and TGF-beta1 and -beta2. The children were divided into three groups: those with no atopic symptoms or IgE, those with allergic symptoms, and those with both outcomes. Mothers of infants later showing atopic symptoms or, in addition, having IgE sensitization (verified atopy) had a lower concentration of IgA casein antibodies in their colostrum than did mothers of infants with no indication of atopy at age 4. Low concentration of IgA casein antibodies was a significant risk for verified atopy. sCD14 levels were lower in colostrum of mothers with infants developing atopic symptoms and IgE sensitization than of those of infants with no atopy. Specific IgA antibodies to CM antigens and sCD14 in colostrum significantly associated with the appearance of both symptomatic and verified atopy by age 4.

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.