Biophysical and Biochemical Characterization of Avian Secretory Component Provides Structural Insights into the Evolution of the Polymeric Ig Receptor

Structural and Biochemical Requirements for Secretory Component Interactions with Dimeric IgA

Secretory (S) IgA is the predominant mucosal Ab that protects host epithelial barriers and promotes microbial homeostasis. SIgA production occurs when plasma cells assemble two copies of monomeric IgA and one joining chain (JC) to form dimeric (d) IgA, which is bound by the polymeric Ig receptor (pIgR) on the basolateral surface of epithelial cells and transcytosed to the apical surface. There, pIgR is proteolytically cleaved, releasing SIgA, a complex of the dIgA and the pIgR ectodomain, called the secretory component (SC). The pIgR's five Ig-like domains (D1-D5) undergo a conformational change upon binding dIgA, ultimately contacting four IgA H chains and the JC in SIgA. In this study, we report structure-based mutational analysis combined with surface plasmon resonance binding assays that identify key residues in mouse SC D1 and D3 that mediate SC binding to dIgA. Residues in D1 CDR3 are likely to initiate binding, whereas residues that stabilize the D1-D3 interface are likely to promote the conformational change and stabilize the final SIgA structure. Additionally, we find that the JC's three C-terminal residues play a limited role in dIgA assembly but a significant role in pIgR/SC binding to dIgA. Together, these results inform models for the intricate mechanisms underlying IgA transport across epithelia and functions in the mucosa.

Structural and biochemical requirements for secretory component interactions with dimeric Immunoglobulin A

Secretory (S) Immunoglobulin (Ig) A is the predominant mucosal antibody that protects host epithelial barriers and promotes microbial homeostasis. SIgA production occurs when plasma cells assemble two copies of monomeric IgA and one joining-chain (JC) to form dimeric (d) IgA, which is bound by the polymeric Ig receptor (pIgR) on the basolateral surface of epithelial cells and transcytosed to the apical surface. There, pIgR is proteolytically cleaved, releasing SIgA, a complex of the dIgA and the pIgR ectodomain, called secretory component (SC). The pIgR's five Ig-like domains (D1-D5) undergo a conformational change upon binding dIgA, ultimately contacting four IgA heavy chains and the JC in SIgA. Here we report structure-based mutational analysis combined with surface plasmon resonance binding assays that identify key residues in mouse SC D1 and D3 that mediate SC binding to dIgA. Residues in D1 CDR3 are likely to initiate binding whereas residues that stabilize the D1-D3 interface are likely to promote the conformation change and stabilize the final SIgA structure. Additionally, we find that the JC's three C-terminal residues play a limited role in dIgA assembly but a significant role in pIgR/SC binding to dIgA. Together results inform new models for the intricate mechanisms underlying IgA transport across epithelia and functions in the mucosa.

License to Clump: Secretory IgA Structure-Function Relationships Across Scales

Secretory antibodies are the only component of our adaptive immune system capable of attacking mucosal pathogens topologically outside of our bodies. All secretory antibody classes are (a) relatively resistant to harsh proteolytic environments and (b) polymeric. Recent elucidation of the structure of secretory IgA (SIgA) has begun to shed light on SIgA functions at the nanoscale. We can now begin to unravel the structure-function relationships of these molecules, for example, by understanding how the bent conformation of SIgA enables robust cross-linking between adjacent growing bacteria. Many mysteries remain, such as the structural basis of protease resistance and the role of noncanonical bacteria-IgA interactions. In this review, we explore the structure-function relationships of IgA from the nano- to the metascale, with a strong focus on how the seemingly banal "license to clump" can have potent effects on bacterial physiology and colonization.

Molecular cloning and functional analysis of polymeric immunoglobulin receptor, pIgR, gene in mandarin fish Siniperca chuatsi

Polymeric immunoglobulin receptor (pIgR) can bind and transport immunoglobulins (Igs), thus playing a role in mucosal immunity. In this study, pIgR gene was cloned in mandarin fish, Siniperca chuatsi, with the open reading frame (ORF) of 1011 bp, encoding 336 amino acids. The pIgR protein consists of a signal peptide, an extracellular domain, a transmembrane domain and an intracellular region, with the presence of two Ig-like domains (ILDs) in the extracellular domain, as reported in other species of fish. The pIgR gene was expressed in all organs/tissues of healthy mandarin fish, with higher level observed in liver and spleen. Following the immersion infection of Flavobacterium columnare, pIgR transcripts were detected in immune related, especially mucosal tissues, with significantly increased transcription during the first two days of infection. Through transfection of plasmids expressing pIgR, IgT and IgM, pIgR was found to be interacted with IgT and IgM as revealed by co-immunoprecipitation and immunofluorescence.

Immunity in Sea Turtles: Review of a Host-Pathogen Arms Race Millions of Years in the Running

The immune system of sea turtles is not completely understood. Sea turtles (as reptiles) bridge a unique evolutionary gap, being ectothermic vertebrates like fish and amphibians and amniotes like birds and mammals. Turtles are ectotherms; thus, their immune system is influenced by environmental conditions like temperature and season. We aim to review the turtle immune system and note what studies have investigated sea turtles and the effect of the environment on the immune response. Turtles rely heavily on the nonspecific innate response rather than the specific adaptive response. Turtles' innate immune effectors include antimicrobial peptides, complement, and nonspecific leukocytes. The antiviral defense is understudied in terms of the diversity of pathogen receptors and interferon function. Turtles also mount adaptive responses to pathogens. Lymphoid structures responsible for lymphocyte activation and maturation are either missing in reptiles or function is affected by season. Turtles are a marker of health for their marine environment, and their immune system is commonly dysregulated because of disease or contaminants. Fibropapillomatosis (FP) is a tumorous disease that afflicts sea turtles and is thought to be caused by a virus and an environmental factor. We aim, by exploring the current understanding of the immune system in turtles, to aid the investigation of environmental factors that contribute to the pathogenesis of this disease and provide options for immunotherapy.

Quantification of secretory IgA and mucin excretion and their contributions to total endogenous amino acid losses in roosters that were fasted or precision-fed a nitrogen-free diet or various highly digestible protein sources

The objective of this study was to quantify total secretory IgA (sIgA) and mucin excretion via excreta in roosters fed diets containing highly digestible protein sources and to determine their proportional contributions to total endogenous amino acid (AA) losses. Precision-fed rooster assays with 24 h excreta collections were conducted using conventional White Leghorn roosters (4-8 roosters per treatment). In Experiment 1, roosters were fasted or precision-fed 30 g (crop intubation) of a nitrogen-free (NF) or semi-purified diet containing 10% casein. Roosters in Experiment 2 received a NF or semi-purified diet containing either 10% casein, 17% whole egg, 10% egg white, 9.8% soy protein isolate, 10.2% chicken breast meat, 11.2% spray-dried animal plasma (SDAP), or an AA mixture containing the same AA as casein. A Latin square design was used in Experiment 3, where roosters received NF or semi-purified diets containing either 10% casein, 17% whole egg, or 9.6% of a crystalline AA mixture to evaluate both diet and individual bird effects. In Experiment 1, mucin excretion did not differ (P > 0.05) among treatments; however, total sIgA excretion was lower for fasted birds, intermediate for the NF diet, and highest for casein (P < 0.05). Total endogenous AA losses (proportion of the total) from sIgA were higher for roosters fed casein, whereas mucin contributions were higher for fasted roosters (P < 0.05). In Experiment 2, sIgA excretion did not differ (P > 0.05) among treatments; however, mucin excretion was reduced for NF, whole egg, egg white, and chicken breast compared with casein and SDAP. In Experiment 3, sIgA and mucin excretion did not differ (P > 0.05) among treatments; however, sIgA excretion differed among individual roosters (7-27 mg/24 h; P < 0.05). Overall, fasting reduced sIgA excretion and sIgA and mucin excretion were affected by dietary protein source. Further, roosters excreted a substantial amount of sIgA, and sIgA and mucin were considerable contributors to total endogenous AA losses.

Molecular cloning and characteristic analysis of polymeric immunoglobulin receptor-like (plgRL) in large yellow croaker (Larimichthys crocea)

In the present study, the polyimmunoglobulin receptor-like (pIgRL) of large yellow croaker (Larimichthys crocea) was first cloned and characterized. LcpIgRL's full-length cDNA was 1610 bp, encoding 377 amino acids, and the protein's predicted molecular weight was 41.9 kDa, containing two immunoglobulin-like structural domains. The transcript levels of LcpIgRL in different tissues of healthy large yellow croaker were examined by real-time fluorescence quantitative PCR, and the results showed that the gills and head kidney had the highest levels. Within 36 h of the large yellow croaker being infected with Vibrio harveyi, pIgRL mRNA first increased and then decreased in all determined tissues, with the highest expression in the skin and hindgut. Furthermore, a recombinant protein of the extracellular region of LcpIgRL was expressed in E. coli BL21, and a murine rLcpIgRL polyclonal antibody was prepared, which could react specifically with the natural LcpIgRL in skin mucus, but no natural LcpIgRL was detected in serum. Meanwhile, it was found that the rLcpIgRL could bind to the recombinant IgM and the natural IgM, indicating that LcpIgRL could mediate the transport of IgM in mucus. In addition, rLcpIgRL binds to Aeromonas hydrophila and V. harveyi, as well as lipopolysaccharide (LPS) and various saccharides, and reduced binding to bacteria was observed under LPS treatment, suggesting that LcpIgRL can bind to bacteria to prevent infection and that saccharide binding is an important mechanism of interaction between pIgRL and bacteria.

Characterization of the Pelodiscus sinensis polymeric immunoglobulin receptor (P. sinensis pIgR) and its response to LPS and Aeromonas sobria

The polymeric immunoglobulin receptor (pIgR) is one of the most vital components of mucosal immunity that plays a pivotal role in mediating transcytosis of polymeric immunoglobulin (pIg) on epithelial surfaces for protection against invading pathogens. Herein, we cloned the full-length cDNA of Pelodiscus sinensis pIgR, designated as P. sinensis pIgR, made of an open reading frame (ORF) of 1848 bp, molecular weight of 68.2 kDa and estimated isoelectric point of 7.00. The deduced P. sinensis pIgR sequence had a leader peptide, extracellular region containing four immunoglobulin-like domains (Ig like domains), transmembrane and intracellular regions comparable with other vertebrates. P. sinensis pIgR contained four Ig like domains that corresponded with mammalian D1, D3, D4 and D5 similar with reptile and avian Ig like domains. It had 40 potential phosphorylation sites, four putative N-glycosylation sites and several motifs resembling mammalian pIgR motifs. Phylogenetic analysis showed a close relationship between P. sinensis pIgR with avian and reptile pIgRs. P. sinensis pIgR basal levels were higher in the esophagus, small intestine and intestinnum crissum than in other organs of health turtles. Intragastric delivery of LPS and Aeromonassobria led to significant upregulation of P. sinensis pIgR in tissues of the gastrointestinal tract. A polyclonal anti- P. sinensis pIgR antibody produced in rabbit reacted with the recombinant P. sinensis pIgR protein expressed in Escherichia coli in Western blot. These studies demonstrate the existence and immune response of P. sinensis pIgR to stimulation in mucosal organs in Chinese soft-shelled turtles.

Role of Polymeric Immunoglobulin Receptor in IgA and IgM Transcytosis

Transcytosis of polymeric IgA and IgM from the basolateral surface to the apical side of the epithelium and subsequent secretion into mucosal fluids are mediated by the polymeric immunoglobulin receptor (pIgR). Secreted IgA and IgM have vital roles in mucosal immunity in response to pathogenic infections. Binding and recognition of polymeric IgA and IgM by pIgR require the joining chain (J chain), a small protein essential in the formation and stabilization of polymeric Ig structures. Recent studies have identified marginal zone B and B1 cell-specific protein (MZB1) as a novel regulator of polymeric IgA and IgM formation. MZB1 might facilitate IgA and IgM transcytosis by promoting the binding of J chain to Ig. In this review, we discuss the roles of pIgR in transcytosis of IgA and IgM, the roles of J chain in the formation of polymeric IgA and IgM and recognition by pIgR, and focus particularly on recent progress in understanding the roles of MZB1, a molecular chaperone protein.

Molecular cloning and binding analysis of polymeric immunoglobulin receptor in largemouth bass (Micropterus salmoides)

The polymeric immunoglobulin receptor (pIgR) is an important molecule in the mucosal immunity of teleosts. Previous studies have shown that pIgR can bind and transport polymeric immunoglobulins (pIgs), but few studies have focused on the binding of teleost pIgR to bacteria. In this study, we identified a gene encoding pIgR in largemouth bass (Micropterus salmoides). The pIgR gene contained two Ig-like domains (ILDs), which were homologous to ILD1 and ILD5 of mammalian pIgR. Our results showed that largemouth bass pIgR-ILD could combine with IgM. Moreover, we also found that largemouth bass pIgR-ILD could bind to Aeromonas hydrophila and Micrococcus luteus. Further analysis showed that largemouth bass pIgR-ILD could also combine with lipopolysaccharide (LPS), peptidoglycan (PGN) and various saccharides, and reduced binding to bacteria was observed with LPS and PGN treatment, indicating that largemouth bass pIgR could bind to bacteria to prevent infection and that saccharide binding is an important interaction mechanism between pIgR and bacteria. These results collectively demonstrated that largemouth bass pIgR not only combines with IgM but also binds to bacteria by various saccharides.

The structures of secretory and dimeric immunoglobulin A

Secretory (S) Immunoglobulin (Ig) A is the predominant mucosal antibody, which binds pathogens and commensal microbes. SIgA is a polymeric antibody, typically containing two copies of IgA that assemble with one joining-chain (JC) to form dimeric (d) IgA that is bound by the polymeric Ig-receptor ectodomain, called secretory component (SC). Here, we report the cryo-electron microscopy structures of murine SIgA and dIgA. Structures reveal two IgAs conjoined through four heavy-chain tailpieces and the JC that together form a β-sandwich-like fold. The two IgAs are bent and tilted with respect to each other, forming distinct concave and convex surfaces. In SIgA, SC is bound to one face, asymmetrically contacting both IgAs and JC. The bent and tilted arrangement of complex components limits the possible positions of both sets of antigen-binding fragments (Fabs) and preserves steric accessibility to receptor-binding sites, likely influencing antigen binding and effector functions.

Molecular cloning of polymeric immunoglobulin receptor-like (pIgRL) in flounder (Paralichthys olivaceus) and its expression in response to immunization with inactivated Vibrio anguillarum

In the present work, the polymeric immunoglobulin receptor-like (pIgRL) from flounder (Paralichthys olivaceus) was firstly cloned and identified. The full length cDNA of flounder pIgRL was of 1393 bp including an open reading frame of 1053 bp, and the deduced pIgRL sequence encoded 350 amino acids, with a predicted molecular mass of 39 kDa. There were two immunoglobulin-like domains in flounder pIgRL. In healthy flounder, the transcriptional level of pIgRL was detected in different tissues by real-time PCR, showing the highest level in the skin and gills, and higher levels in the spleen and hindgut. After flounders were vaccinated with inactivated Vibrio anguillarum via intraperitoneal injection and immersion, the pIgRL mRNA level increased firstly and then declined in all tested tissues during 48 h, and the maximum expression levels in the gills, skin, spleen and hindgut in immersion group, or in the spleen, head kidney, skin and gills in injection group, were higher than in other tested tissues. In addition, recombinant protein of the extracellular region of flounder pIgRL was expressed in Escherichia coli BL21 (DE3), and rabbit anti-pIgRL polyclonal antibodies were prepared, which specifically reacted with the recombinant pIgRL, and a 39 kDa protein confirmed as natural pIgRL by liquid chromatography-mass spectrometry in skin mucus of flounder. Co-immunoprecipitation assay and western-blotting demonstrated that the pIgRL, together with IgM, could be immunoprecipitated by anti-pIgRL antibody in gut, skin and gill mucus of flounder, suggesting the existence of pIgRL-IgM complexes. These results indicated that the flounder pIgRL was probably involved in the mucosal IgM transportation and played important roles in mucosal immunity.

Innate immune functions of avian intestinal epithelial cells: Response to bacterial stimuli and localization of responding cells in the developing avian digestive tract

Intestinal epithelial cells are multi-tasked cells that participate in digestion and absorption as well as in protection of the digestive tract. While information on the physiology and immune functions of intestinal epithelial cells in mammals is abundant, little is known of their immune function in birds and other species. Our main objectives were to study the development of anti-bacterial innate immune functions in the rapidly developing gut of the pre- and post-hatch chick and to determine the functional diversity of epithelial cells. After establishing primary intestinal epithelial cell cultures, we demonstrated their capacity to uptake and process bacteria. The response to bacterial products, LPS and LTA, induced expression of pro-inflammatory cytokine genes (IL-6, IL-18) as well as the expression of the acute phase proteins avidin, lysozyme and the secretory component derived from the polymeric immunoglobulin receptor. These proteins were then localized in gut sections, and the goblet cell was shown to store avidin, lysozyme as well as secretory component. Lysozyme staining was also located in a novel rod-shaped intestinal cell, situated at different loci along the villus, thus deviating from the classical Paneth cell in the mammal, that is restricted to crypts. Thus, in the chicken, the intestinal epithelium, and particularly goblet cells, are committed to innate immune protection. The unique role of the goblet cell in chicken intestinal immunity, as well as the unique distribution of lysozyme-positive cells highlight alternative solutions of gut protection in the bird.

The contribution of modern EPR to structural biology

Electron paramagnetic resonance (EPR) spectroscopy combined with site-directed spin labelling is applicable to biomolecules and their complexes irrespective of system size and in a broad range of environments. Neither short-range nor long-range order is required to obtain structural restraints on accessibility of sites to water or oxygen, on secondary structure, and on distances between sites. Many of the experiments characterize a static ensemble obtained by shock-freezing. Compared with characterizing the dynamic ensemble at ambient temperature, analysis is simplified and information loss due to overlapping timescales of measurement and system dynamics is avoided. The necessity for labelling leads to sparse restraint sets that require integration with data from other methodologies for building models. The double electron–electron resonance experiment provides distance distributions in the nanometre range that carry information not only on the mean conformation but also on the width of the native ensemble. The distribution widths are often inconsistent with Anfinsen's concept that a sequence encodes a single native conformation defined at atomic resolution under physiological conditions.

Polymeric immunoglobulin receptor in dojo loach (Misgurnus anguillicaudatus): Molecular characterization and expression analysis in response to bacterial and parasitic challenge

The polymeric immunoglobulin receptor (pIgR) is an essential component of the mucosal immune system in jawed vertebrates including teleost fish, which mediate transepithelial transport of secretory immunoglobulins (sIgs) to protect organisms against environmental pathogens. In this study, we firstly cloned and identified the pIgR from dojo loach (Misgurnus anguillicaudatus). The full-length cDNA of Ma-pIgR was of 1145 bp, containing an open reading frame (ORF) of 1101 bp encoded a predicted protein of 336 amino acids. The structure of Ma-pIgR is comprised of a signal peptide, a transmembrane region, an intracellular region and an extracellular region with two Ig-like domains (ILDs), which are similar to their counterparts described in other teleosts. Multiple sequence alignment and phylogenetic analysis showed the dojo loach is closely related to the fish family Cyprinidae. The transcriptional level of Ma-pIgR was detected by quantitative real-time PCR (qRT-PCR) in different tissues and high expression was found in liver, skin, kidney, eye, fin and gills. Two infection models of the loach with bacteria (Aeromonas hydrophila) and parasite (Ichthyophthirius multifiliis) were constructed for the first time. Histological studies showed the goblet cells in skin significantly increased and the ratio of gill length to width also significantly changed after challenged with A.hydrophila. Both challenge experiments resulted in the significant up-regulated expression of Ma-pIgR not only in kidney and spleen, but also in skin and gills. Our results suggest that pIgR may play an important role in skin and gill mucosal immunity to protect the loach against bacteria and parasite.