Ontogeny of the secretory immune system: maturation of a functional polymeric immunoglobulin receptor regulated by gene expression

Expression characteristics of polymeric immunoglobulin receptor in Bactrian camel (Camelus bactrianus) lungs

Polymeric immunoglobulin receptor (pIgR), the transmembrane transporter of polymeric immunoglobulin A and M, has multiple immune functions. To explore the characteristics of pIgR expression in Bactrian camel lungs, twelve healthy adult (2-7 years old) Bactrian camels were systematically studied. The results showed that pIgR was mainly expressed in the cytoplasm and membrane of ciliated cells, as well as in the cytoplasm and membrane of basal cells, serous cells of bronchial glands, club cells and alveolar type 2 cells in Bactrian camel lungs. Specially, as the bronchial branches extended, the pIgR expression level in ciliated cells significantly declined (p<0.05), and the corresponding bronchial luminal areas obviously decreased (p<0.05). However, pIgR was not expressed in goblet cells, endocrine cells, alveolar type 1 cells and mucous cells of bronchial glands. The results demonstrated that ciliated cells continuously distributed throughout the whole bronchial tree mucosa were the major expression sites of pIgR, and pIgR was also expressed in basal cells, serous cells of bronchial glands, club cells and alveolar type 2 cells, which would facilitate secretory immunoglobulin A (SIgA) transmembrane transport by pIgR and form an intact protective barrier. Moreover, the pIgR expression level in ciliated cells was positively correlated with the bronchial luminal areas; but negatively correlated with the cleanliness of airflow through the bronchial cross-sections, showing that the pIgR expression level in the bronchial epithelium was inhomogeneous. Our study provided a foundation for further exploring the regulatory functions of immunoglobulins (i.e., SIgA) after transport across the membrane by pIgR in Bactrian camel lungs.

Age-related expression of the polymeric immunoglobulin receptor (pIgR) in the gastric mucosa of young pigs

To date few studies have addressed the development and function of the porcine gastric mucosal immune system and this is a major limitation to understanding the immunopathogenesis of infections occurring in young pigs. The polymeric immunoglobulin receptor (pIgR) mediates the transport of secretory immunoglobulins until luminal surface of the gut mucosa and the aim of this study was to investigate the time course of pIgR expression and to determine its localization in three functionally different porcine gastric sites during the suckling period and after weaning. An additional goal was to investigate the time course expression of toll-like receptors (TLRs) in relation to pIgR expression. Gastric samples were collected from the cardiac-to-oxyntic transition (Cd), the oxyntic (Ox), and the pyloric (Py) regions in 84 pigs, slaughtered before weaning (14, 21 and 28 days of age; 23, 23 and 19 pigs, respectively) and 14 days post-weaning (42 days of age, 23 pigs). PIgR was expressed in the mucosa of all the three gastric sites, and its transcript levels were modulated during suckling and after weaning, with regional differences. PIgR expression increased linearly during suckling (P=0.019) and also increased post-weaning (P=0.001) in Cd, it increased post-weaning in Py (P=0.049) and increased linearly during suckling in Ox (P=0.036). TLRs expression was also modulated during development: in Cd, TLR2 increased linearly during suckling (P=0.003); in Ox, TLR2 decreased after weaning (P=0.038) while TLR4 increased linearly during suckling(P=0.008). The expression of TLR2, 3 and 4 in Ox was positively correlated with pIgR expression (P<0.001).

Importantly, both pIgR protein and mRNA were localized, by immunohistochemistry and in situ hybridization, respectively, in the gastric glands of the lamina propria. These results indicate that pIgR is actively synthesized in the gastric mucosa and suggest that pIgR could play a crucial role in gastric mucosal immune defense of growing pigs.

Quercetin alleviates predator stress-induced anxiety-like and brain oxidative signs in pregnant rats and immune count disturbance in their offspring

This study was performed in rats to investigate the effect of a psychogenic stress during late gestation on the immediate behavior and brain oxidative status in dams as well as on the immune cell counts in their offspring up to weaning. Besides, the ability of quercetin (a natural flavonoid) to prevent stress effects was evaluated. Quercetin was orally administered for 6 consecutive days before the pregnant rats were acutely exposed to predator stress on gestational day 19. Post-stress corticosterone level, brain oxidative stress parameters and anxiety-like behavior were assessed in dams, whereas immune cell counts were postnatally determined in both male and female pups. Predator stress caused an oxidative stress in the brain and elicited an elevation in plasma corticosterone with concomitant behavioral impairment in dams. Prenatally-stressed pups mainly showed a decrease in total leukocytes and lymphocytes along with monocytosis and granulocytosis, but these changes were sex-dependent throughout the postnatal period studied. Quercetin pretreatment blocked the stress-induced corticosterone release and alleviated the brain oxidative stress with the maternal anxiety measures being slightly attenuated, whereas its effects on the offspring immune cell counts were mostly revealed at birth. Our findings suggest that late gestational exposure to traumatic events may cause anxiety symptoms in dams, in which corticosterone and brain oxidative stress play a certain role, and trigger negative immune changes in the early postnatal life of progeny. Notably, quercetin intake before such adverse events seems to be beneficial against negative outcomes in both dams and offspring.

The suckling rat as a model for immunonutrition studies in early life

Diet plays a crucial role in maintaining optimal immune function. Research demonstrates the immunomodulatory properties and mechanisms of particular nutrients; however, these aspects are studied less in early life, when diet may exert an important role in the immune development of the neonate. Besides the limited data from epidemiological and human interventional trials in early life, animal models hold the key to increase the current knowledge about this interaction in this particular period. This paper reports the potential of the suckling rat as a model for immunonutrition studies in early life. In particular, it describes the main changes in the systemic and mucosal immune system development during rat suckling and allows some of these elements to be established as target biomarkers for studying the influence of particular nutrients. Different approaches to evaluate these immune effects, including the manipulation of the maternal diet during gestation and/or lactation or feeding the nutrient directly to the pups, are also described in detail. In summary, this paper provides investigators with useful tools for better designing experimental approaches focused on nutrition in early life for programming and immune development by using the suckling rat as a model.

Successive immunoglobulin and cytokine expression in the small intestine of juvenile chicken

The intestinal mucosa is of major importance for immune development. To further study the ontogeny of avian mucosal immunity, mRNA levels of IgM, IgY and IgA, the polymeric immunoglobulin receptor (pIgR) and a number of cytokines were determined at different ages in jejunum and ileum of non-immunized healthy juvenile layer chickens. Immunoglobulin genes were successively expressed in jejunum and ileum. IgM expression was maximal in week 1, IgY expression peaked in week 5, and IgA expression was most dominant after week 7 post hatch. PIgR gene expression was relatively low in the first 2 weeks post hatch, but increased thereafter. Generally, increased expression levels of IL-1, IL-10, IL-12p40, iNOS and interferon-γ mRNA levels were found between days 14-42 as compared to days 3 and 49-70 post hatch (p<0.05). Correlation was found between IgA and IL-10, TGF-β and IFN-γ expression levels on days 21, 28 and 35. Cytokine mRNA expression levels decreased to basal levels between 49 and 70 days post hatch, whereas IgA reached its maximum levels in this period. Based on the current results, we hypothesize that chicken sIgA, as mammalian sIgA, may contribute to the maintenance of intestinal homeostasis.

Role of passive and adaptive immunity in influencing enterocyte-specific gene expression

Numerous genes expressed by intestinal epithelial cells are developmentally regulated, and the influence that adaptive (AI) and passive (PI) immunity have in controlling their expression has not been evaluated. In this study, we tested the hypothesis that both PI and AI influenced enterocyte gene expression by developing a breeding scheme that used T and B cell-deficient recombination-activating gene (RAG) mice. RNA was isolated from the liver and proximal/distal small intestine at various ages, and the steady-state levels of six different transcripts were evaluated by RNase protection assay. In wild-type (WT) pups, all transcripts [Fc receptor of the neonate (FcRn), polymeric IgA receptor (pIgR), GLUT5, lactase-phlorizin hydrolase (lactase), apical sodium-dependent bile acid transporter (ASBT), and Na+/glucose cotransporter (SGLT1)] studied were developmentally regulated at the time of weaning, and all transcripts except ASBT had the highest levels of expression in the proximal small intestine. In WT suckling pups reared in the absence of PI, pIgR mRNA levels were increased 100% during the early phase of development. In mice lacking AI, the expression of pIgR and lactase were significantly attenuated, whereas FcRn and GLUT5 levels were higher compared with WT mice. Finally, in the absence of both passive and active immunity, expression levels of pIgR and lactase were significantly lower than similarly aged WT mice. In summary, we report that the adaptive and passive immune status of mice influences steady-state mRNA levels of several important, developmentally regulated enterocyte genes during the suckling and weaning periods of life.

Functional pleiotropy of an intramolecular triplex-forming fragment from the 3'-UTR of the rat Pigr gene

A microsatellite-containing 359-bp restriction fragment, isolated from the rat Pigr gene (murine polymeric immunoglobulin receptor gene) 3'-untranslated region (3'-UTR) and inserted into 3'-UTR or 3' flanking positions in transcription units of supercoiled plasmids, attenuates luciferase reporter gene expression in orientation- and position-dependent ways following transient transfection of human 293 cells. The same fragment stimulates orientation-dependent gene expression in a 5' flanking position. Plasmid linearization abrogates both orientation- and position-dependent responses. Cell-free translation reveals that 5' and 3' flanking expression responses are proportional to increased and decreased luciferase mRNA levels, whereas 3'-UTR expression is associated with control mRNA levels. Hypersensitivity to nucleases S1 and P1, gel mobility differences between supercoiled plasmids carrying opposing microsatellite orientations, and anomalous melting profiles of this fragment are also observed. These results suggest that functional pleiotropy of this fragment depends on the DNA context of its purine-rich microsatellite strand and on DNA supercoiling. Intramolecular triplexes stabilized by supercoiling and secondary structures of purine repeat-rich mRNAs may also confer regulatory properties to similar genomic elements.

Characterization of the 5'-flanking region of the murine polymeric IgA receptor gene

The regulatory elements that control basal and activated transcriptional expression of the polymeric IgA receptor gene (pIgR) have not been defined. In this study, we performed functional analysis of the murine pIgR 5'-upstream region. Transient transfection studies identified the gene's minimal promoter to reside within 110 nucleotides upstream from the start of transcription. Substitution mutations of this region identified both a putative activator (-78 to -70) and a repressor (-66 to -52) element. DNase I footprint analysis confirmed an area of protection that spans from nucleotides -85 to -62. Mobility shift assays of the putative region confirmed binding of upstream stimulatory factor 1 (USF1) to an E box element at positions -75 and -70, representing the putative enhancer. Overexpression studies using various forms of USF suggest that both USF1 and USF2 enhance activity of the pIgR minimal promoter. We report the identification and characterization of the murine pIgR minimal promoter, as well as the critical role of USF in enhancing its basal level of transcription in Caco-2 cells.

Isolation and genomic analysis of the rat polymeric immunoglobulin receptor gene terminal domain and transcriptional control region

The polymeric immunoglobulin receptor (pIgR) transports IgA and IgM across secretory epithelial cells and is essential in external immunity maintenance. We report here the structural characterization of the single-copy rat gene distributed over 30 kb of chromosomal DNA and analysis of its transcriptional control region. RNA sequencing and genomic analysis show a 5' terminal region originates at a major (+1) and a minor site producing an unusual 124-bp nontranslated exon I separated from a small 96-bp initiator ATG coding exon II by a 7.5-kb intron. The pIgR 5' region comprises a structured promoter with abundant helix-loop-helix (bHLH) cis elements positioned within an equivalent internal -70, -290, -528, and three centered at -745. The three latter bHLH elements each occur within 30-bp repeats at -690 to -780. Transient expression assays show a 1.3-kb 5' region is sufficient to drive expression in rat primary hepatocyte monolayer cultures, transformed human hepatic (HepG2) cells, and a mammary epithelial tumor cell line MCF-7, but is inactive in the rodent fibroblast 3T3 cell line. A minimal transcriptional promoter domain was deduced from sequentially deleted vectors revealing a +40 to -922 sequence to be sufficient for full activity. Further deletions within this region yield incremental losses in cis activity, indicating that multiple subregions comprise an extended transcriptional control region.

Discordant expression and variable numbers of neighboring GGA- and GAA-rich triplet repeats in the 3' untranslated regions of two groups of messenger RNAs encoded by the rat polymeric immunoglobulin receptor gene

An unusual S1-nuclease sensitive microsatellite (STMS) has been found in the single copy, rat polymeric immunoglobulin receptor gene (PIGR) terminal exon. In Fisher rats, elements within or beyond the STMS are expressed variably in the 3' untranslated regions (3'UTRs) of two 'Groups' of PIGR-encoded hepatic mRNAs (pIg-R) during liver regeneration. STMS elements include neighboring constant regions (a 60-bp d[GA]-rich tract with a chi-like octamer, followed by 15 tandem d[GGA] repeats) that merge directly with 36 or 39 tandem d[GAA] repeats (Fisher or Wistar strains, respectively) interrupted by d[AA] between their 5th-6th repeat units. The Wistar STMS is flanked upstream by two regions of nearly contiguous d[CA] or d[CT] repeats in the 3' end of intron 8; and downstream, by a 283 bp 'unit' containing several inversions at its 5' end, and two polyadenylation signals at its 3' end. The 283 nt unit is expressed in Group 1 pIg-R mRNAs; but it is absent in the Group 2 family so that their GAA repeats merge with their poly A tails. In contrast to genomic sequence, GGA triplet repeats are amplified (n > or = 24-26), whereas GAA triplet repeats are truncated variably (n < or = 9-37) and expressed uninterruptedly in both mRNA Groups. These results suggest that 3' end processing of the rat PIGR gene may involve misalignment, slippage and premature termination of RNA polymerase II. The function of this unusual processing and possible roles of chi-like octamers in quiescent or extrahepatic tissues are discussed.

Gene transfer into the airway epithelium of animals by targeting the polymeric immunoglobulin receptor

Genes of interest can be targeted specifically to respiratory epithelial cells in intact animals with high efficiency by exploiting the receptor-mediated endocytosis of the polymeric immunoglobulin receptor. A DNA carrier, consisting of the Fab portion of polyclonal antibodies raised against rat secretory component covalently linked to poly-L-lysine, was used to introduce plasmids containing different reporter genes into airway epithelial cells in vivo. We observed significant levels of luciferase enzyme activity in protein extracts from the liver and lung, achieving maximum values of 13,795 +/- 4,431 and 346,954 +/- 199,120 integrated light units (ILU) per milligram of protein extract, respectively. No luciferase activity was detected in spleen or heart, which do not express the receptor. Transfections using complexes consisting of an irrelevant plasmid (pCMV lacZ) bound to the bona fide carrier or the expression plasmid (pGEMluc) bound to a carrier based on an irrelevant Fab fragment resulted in background levels of luciferase activity in all tissues examined. Thus, only tissues that contain cells bearing the polymeric immunoglobulin receptor are transfected, and transfection cannot be attributed to the nonspecific uptake of an irrelevant carrier-DNA complex. Specific mRNA from the luciferase gene was also detected in the lungs of transfected animals. To determine which cells in the lungs are transfected by this method, DNA complexes were prepared containing expression plasmids with genes encoding the bacterial beta-galactosidase or the human interleukin 2 receptor. Expression of these genes was localized to the surface epithelium of the airways and the submucosal glands, and not the bronchioles and alveoli. Receptor-mediated endocytosis can be used to introduce functional genes into the respiratory epithelium of rats, and may be a useful technique for gene therapy targeting the lung.