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The polymeric immunoglobulin receptor (pIgR) plays an important role in mediating the transcytosis of polymeric immunoglobulins (pIgs) to protect organisms against pathogen invasion. Here, a polyclonal antibody against grass carp (Ctenopharyngodon idellus) recombinant pIgR was developed by immunizing New Zealand white rabbit, and the responses of pIgR, IgM and IgZ were analyzed after bath immunization and intraperitoneal administration with Flavobacterium columnare. The results showed that pIgR transcription level was similar to IgM and IgZ, but pIgR rose much faster and peaked earlier than IgM and IgZ; the pIgR mRNA levels were higher in the skin and spleen for both immunized groups, while IgM and IgZ mRNA expression were higher in skin, gills, and intestines in bath immersion group, or spleen and head kidney in intraperitoneal immunization group. ELISA revealed that the IgM, IgZ and pIgR protein levels were up-regulated in skin mucus, gill mucus, gut mucus and bile, reaching a higher peak level earlier in skin mucus and gill mucus in bath immersion group, but a higher peak level in bile in injection group. Moreover, secretory component molecules were detected in grass carp's skin, gill and intestine mucus and bile, but not in serum, which molecular mass was near the theoretical mass obtained from the sequence of grass carp pIgR. These results demonstrated that bath and intraperitoneal immunization up-regulated pIgR and secretory Ig expression in secretions, which provided more insights into the role of pIgR in immunity and offer insight into ways of protecting teleost against pathogen invasion.

The receptor responsible for maternofetal transmission of immunoglobulin (Igs) in the teleosts is not clear. Polymeric immunoglobulin receptor (pIgR) specifically binds with IgA and IgM and mediates the transcytosis of intracellular polymeric immunoglobulins (pIgs) at the mucosal surface to protect against pathogens. Hence there is a possibility that it may be involved in the transmission of maternal Igs. The aim of the present study was to detect the expression and localization of pIgR during embryonal development in turbot (Scophthalmus maximus). pIgR gene was first cloned from eggs and embryos of turbot with or without parent immunization. The expression and distribution of pIgR in unfertilized egg and in embryos ranging from day 1 to day 5 after fertilization were analyzed using reverse transcriptase quantitative polymerase chain reaction and in situ hybridization. pIgR gene was detected in all eggs and embryos at different stages of development, with the highest level detected on the 5th day. pIgR mRNA was observed to be first located in the whole blastoderm and enveloped the yolk sac. Later, it was located around entoderm including primary digestive tract and pronephric tubule tract, and finally it was located at the joint of abdomen and vitelline membrane. Then, Eukaryotic expression plasmid carrying pIgR gene was constructed and transfected into HEK293T cells. Results showed mature pIgR protein located on the cellular membrane, and could bound IgM in vitro. Our findings provide information for studying the involvement of pIgR in maternal Igs transportation in turbot.

The polymeric immunoglobulin receptor (pIgR) plays a pivotal role in mucosal immune response by transporting polymeric immunoglobulins onto the surface of mucosal epithelia to protect animals from invading pathogens. In this study, the full-length cDNA of pIgR was firstly cloned in Qihe crucian carp ( Carassius auratus ), hereafter designated as CapIgR , by using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends. The molecular characterization and expression of CapIgR were investigated. The full-length cDNA sequence of CapIgR was composed of 1409 bp, which included a 112 bp 5ʹ-untranslated region (UTR), a 984 bp ORF, and a 313 bp 3ʹ-UTR, with a putative polyadenylation signal sequence AATAAA located upstream of the poly(A) tail. The deduced amino acid sequence indicated that CapIgR was a single-spanning transmembrane protein with 327 amino acids and possessed a signal peptide, an extracellular region containing two immunoglobulin-like domains, a transmembrane region, and an intracellular region. The mRNA expression levels of CapIgR were detected in different tissues of healthy C. auratus by quantitative real-time PCR, and the highest expression level was found in the liver. After Aeromonas hydrophila challenge, CapIgR expression was upregulated in different tissues at certain time points, and temporal expression changes of CapIgR fluctuated in a time-dependent manner. CapIgR exhibited rapid immune response to A. hydrophila challenge and played an important role in the immune defense of fish. These findings provided insights into the structure, function, and immune defense mechanism of CapIgR in C. auratus . This study can serve as a basis for developing disease control strategies in aquaculture.

The polymeric immunoglobulin receptor (pIgR) is one of the most important mucosal effectors mediating the transcytosis of polymeric immunoglobulins (pIgs) to protect the organisms. In this paper, a full-length cDNA of pIgR was firstly cloned from flounder (Paralichthys olivaceus) using rapid amplification of cDNA ends approaches, and it was of 1384 bp, containing an open reading frame (ORF) of 1005 bp encoding a polypeptide of 335 amino acids with the predicted molecular mass of 37.6 kDa. The flounder pIgR exhibited a unique structure containing only two immunoglobulin-like domains (ILD) corresponding to mammalian pIgR ILD1 and ILD5. The mRNA transcripts of pIgR were detected in all the tested tissues of flounder by semi-quantitative RT-PCR, and the pIgR was expressed at the highest level in liver and higher levels in intestine, gill, skin, spleen and head kidney than in stomach and muscle. The ORF was successfully expressed in Escherichia coli BL21 (DE3) and the recombinant protein displayed binding capability to the purified mucus IgM and serum IgM of flounder by ELISA. The polyclonal antibody against flounder recombinant pIgR was developed by immunization of Balb/C mice, which specifically reacted to the recombinant pIgR in Western blot. Moreover, a secretory component-like molecule was detected in the skin mucus but not in the serum of flounder, which molecular mass (about 37 kDa) was near the theoretical mass obtained from the sequence of flounder pIgR. All these results indicated that flounder pIgR probably involved in the pIgs transport and provided insights into the roles of fish pIgR in the mucosal immunity.

The mucosal immune system seems to be an important defence mechanism for fish but the binding of IgM in mucosal organs is poorly described in fish. In this study the gene encoding the polymeric Immunoglobulin Receptor (pIgR) in carp has been isolated and sequenced from a liver cDNA-library and aligned with other species. The pIgR of carp consists of 2 Ig domains, a transmembrane and an intracellular region, together 327 amino acids. In situ hybridisations with sense and anti-sense DIG-labelled pIgR RNA probes were performed on liver, gut and skin of common carp (Cyprinus carpio L.) and in these organs only anti-sense probes were found to hybridise. In liver the majority of hepatocytes was stained around the nucleus. In gut and skin, staining could be detected around the nucleus of the epithelial cells, but in gut also a subpopulation of lymphoid cells was stained in epithelium and lamina propria. The specific in situ hybridisation of the epithelia and hepatocytes coincides with the in situ binding of FITC-labelled carp IgM to the same cells. RT-PCR results indicate the expression of the pIgR gene in all lymphoid organs of carp, but not in muscle. Macrophages/neutrophils enriched by adherence or sorted B cells (MACS) did not show expression of the pIgR gene and are excluded as the pIgR expressing lymphoid cells in the intestine. The relevance of pIgR staining and gene expression in mucosal organs is discussed.