Structure of the secretory immunoglobulin A core

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.

Structure characterization and intestinal immune promotion effect of polysaccharide purified from Alhagi camelorum Fisch

This study investigated the structure of acid Alhagi camelorum Fischa polysaccharide (aAP) and its impact on intestinal activity in mice. The results showed that aAP comprised of the fucose, arabinose, rhamnose, galactose, glucose, xylose, mannose, galacturonic acid, glucuronic acid with the molar ratio of 0.81:14.97:10.84:11.14:3.26:0.80:0.80:54.92:2.47 with the molecular weight (Mw) of 22.734 kDa. Additionally, the composition of aAP was assessed via FT-IR, methylation, and NMR analyses, indicating that the backbone of the aAP was consisted of →4)-α-D-GalpA-6-OMe-(1 → 4)-α-GalpA-(1 → and →4)-α-D-GalpA-6-OMe-(1 → 2)-α-L-Rhap-(1→, as well as →4)-β-D-Galp- and →5)-α-L-Araf- for the branched chain. Furthermore, ICR mice underwent intragastric administration of different concentrations of aAP for 7 consecutive days. The results showed that aAP enhanced the murine spleen and thymus indices, promoted the secretion of serum lgG antibody, intestinal lgA antibody and intestinal cytokines, improved the morphology of intestinal villi and crypts, enhanced quantity of intestinal IELs and IgA+ cells, and activated T lymphocytes and DC cells in MLNs. In summary, these findings suggest that the utilization of aAP could enhance the immune response of the murine intestinal 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.

Expanding Differential Ion Mobility Separations into the MegaDalton Range

Along with mass spectrometry (MS), ion mobility separations (IMS) are advancing to ever larger biomolecules. The emergence of electrospray ionization (ESI) and native MS enabled the IMS/MS analyses of proteins up to ∼100 kDa in the 1990s and whole protein complexes and viruses up to ∼10 MDa since the 2000s. Differential IMS (FAIMS) is substantially orthogonal to linear IMS based on absolute mobility K and offers exceptional resolution, unique selectivity, and steady filtering readily compatible with slower analytical methods such as electron capture or transfer dissociation (ECD/ETD). However, the associated MS stages had limited FAIMS to ions with m/z < 8000 and masses under ∼300 kDa. Here, we integrate high-definition FAIMS with the Q-Exactive Orbitrap UHMR mass spectrometer that can handle m/z up to 80,000 and MDa-size ions in the native ESI regime. In the initial evaluation, the oligomers of monoclonal antibody adalimumab (148 kDa) are size-selected up to at least the nonamers (1.34 MDa) with m/z values up to ∼17,000. This demonstrates the survival and efficient separation of noncovalent MDa assemblies in the FAIMS process, opening the door to novel analyses of the heaviest macromolecules.

Functional studies with IgM and IgA immunoglobulins: binding to pIgR, FcαμR, FcμR, and CDC activities

IgMs are the first antibodies produced by the immune system upon encounter of a possible pathogen and are one of five antibody subclasses in humans. For IgG, the most intensively studied antibody class, the N-linked glycosylation site located in the Fc-domain is directly involved in high affinity binding to the respective receptors and initiation of corresponding immune response. IgM molecules have five N-glycosylation sites and one N-glycosylation site in the J-chain, which can be incorporated in IgM or IgA molecules. There is only limited knowledge available concerning the function of these N-glycosylations in IgMs. To address this question, we produced IgM molecules lacking a particular N-glycosylation site and tested these variants as well as IgA molecules for binding to the known receptors: the polymeric immunoglobulin receptor (pIgR), the dual receptor for IgA and IgM, FcαμR, and the specific receptor for IgM, FcμR. The single glycosylation sites did not show an impact on expression and multimerization, except for variant N402Q, which could not be expressed. In SPR measurements, no major impact on the binding to the receptors by particular glycosylation sites could be detected. In cellular assays, deglycosylated variants showed some alterations in induction of CDC activity. Most strikingly, we observed also binding of IgA to the FcμR in the same affinity range as IgM, suggesting that this might have a physiological role. To further substantiate the binding of IgA to FcμR we used IgA from different origins and were able to confirm binding of IgA preparations to the FcμR.

Recent advances in the molecular understanding of immunoglobulin A

Immunoglobulin A (IgA) plays a crucial role in the human immune system, particularly in mucosal immunity. IgA antibodies that target the mucosal surface are made up of two to five IgA monomers linked together by the joining chain, forming polymeric molecules. These IgA polymers are transported across mucosal epithelial cells by the polymeric immunoglobulin receptor pIgR, resulting in the formation of secretory IgA (SIgA). This review aims to explore recent advancements in our molecular understanding of IgA, with a specific focus on SIgA, and the interaction between IgA and pathogen molecules.

Each N-glycan on human IgA and J-chain uniquely affects oligomericity and stability

BACKGROUND

Immunoglobulin A (IgA) plays a pivotal role in various immune responses, especially that of mucosal immunity. IgA is usually assembled into dimers with the contribution of J-chains. There are two N-glycosylation sites in human IgA1-Fc and one in the J-chain. There is no consensus as yet on the functional role of the N-glycosylation.

METHODS

To gain a better understanding of their role, we designed a series of IgA1-Fc mutants, which were expressed in the absence or presence of the J-chain.

RESULTS

IgA1-Fc without the J-chain, was predominantly expressed as a monomer, and in its presence dimers and some polymers appeared. N263 (Fc Cα2), N459 (Fc tailpiece) and N49 (J-chain) were shown to be site-specifically modified with N-glycans by mass spectrometry analysis. Mutant IgA1-Fc N459Q failed to form a proper dimer in the presence of the J-chain, instead higher-order aggregates appeared. Fluorescence experiments suggest that the N459-glycans cover a hydrophobic surface at the Fc tailpiece that prevents other Fc molecules from approaching the dimeric IgA. A thermofluor assay revealed that the N-glycans at N263 (Fc) and N49 (J-chain) both contribute in different ways to the thermal stability of the Fc-J-chain complex. NMR analysis of 13C-labeled Fc suggests that the N459-glycan is relatively flexible while the N263-glycan is more rigid.

CONCLUSIONS

We conclude that the N459-glycan of IgA1-Fc is essential for dimer formation and prevention of higher-order aggregates while those at N263 (Fc) and N49 (J-chain) stabilize the Fc-J-chain complex.

GENERAL SIGNIFICANCE

Site-specific role for N-glycan in molecular assembly is addressed.

The immunoglobulin J chain is an evolutionarily co-opted chemokine

The joining (J) chain regulates polymerization of multimeric Immunoglobulin(Ig)M and IgA, forming a disulfide bond to the C termini of their Ig heavy chains, and it controls IgM/IgA transport across mucosal epithelia. Like Ig itself and human-like adaptive immunity, J chain emerged in jawed vertebrates (gnathostomes), but its origin has remained mysterious since its discovery over 50 y ago. Here, we show unexpectedly that J chain is a member of the CXCL chemokine family. The J chain gene (JCHAIN) is linked to clustered CXCL chemokine loci in all gnathostomes except actinopterygians that lost JCHAIN. JCHAIN and most CXCL genes have four exons with the same intron phases, including the same cleavage site for the signal peptide/mature protein. The second exon of both genes encodes a CXC motif at the same position, and the lengths of exons 1 to 3 are similar. No other gene in the human secretome shares all of these characteristics. In contrast, intrachain disulfide bonds of the two proteins are completely different, likely due to modifications in J chain to direct Ig polymerization and mucosal transport. Crystal structures of CXCL8 and J chain share a conserved beta-strand core but diverge otherwise due to different intrachain disulfide bonds and extension of the J chain C terminus. Identification of this ancestral affiliation between J chain and CXCL chemokines addresses an age-old problem in immunology.

Immunoglobulin A response to SARS-CoV-2 infection and immunity

The novel coronavirus disease (COVID-19) and its infamous "Variants" of the etiological agent termed Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2) has proven to be a global health concern. The three antibodies, IgA, IgM, and IgG, perform their dedicated role as main workhorses of the host adaptive immune system in virus neutralization. Immunoglobulin-A (IgA), also known as "Mucosal Immunoglobulin", has been under keen interest throughout the viral infection cycle. Its importance lies because IgA is predominant mucosal antibody and SARS family viruses primarily infect the mucosal surfaces of human respiratory tract. Therefore, IgA can be considered a diagnostic and prognostic marker and an active infection biomarker for SARS CoV-2 infection. Along with molecular analyses, serological tests, including IgA detection tests, are gaining ground in application as an early detectable marker and as a minimally invasive detection strategy. In the current review, it was emphasized the role of IgA response in diagnosis, host defense strategies, treatment, and prevention of SARS-CoV-2 infection. The data analysis was performed through almost 100 published peer-reviewed research reports and comprehended the importance of IgA in antiviral immunity against SARS-CoV-2 and other related respiratory viruses. Taken together, it is concluded that secretory IgA- Abs can serve as a promising detection tool for respiratory viral diagnosis and treatment parallel to IgG-based therapeutics and diagnostics. Vaccine candidates that target and trigger mucosal immune response may also be employed in future dimensions of research against other respiratory viruses.

An engineered bispecific nanobody in tetrameric secretory IgA format confers broad neutralization against SARS-CoV-1&2 and most variants

SARS-CoV-2, a type of respiratory virus, has exerted a great impact on global health and economy over the past three years. Antibody-based therapy was initially successful but later failed due to the accumulation of mutations in the spike protein of the virus. Strategies that enable antibodies to resist virus escape are therefore of great significance. Here, we engineer a bispecific SARS-CoV-2 neutralizing nanobody in secretory Immunoglobulin A (SIgA) format, named S2-3-IgA2m2, which shows broad and potent neutralization against SARS-CoV-1, SARS-CoV-2 and its variants of concern (VOCs) including XBB and BQ.1.1. S2-3-IgA2m2 is ∼1800-fold more potent than its parental IgG counterpart in neutralizing XBB. S2-3-IgA2m2 is stable in mouse lungs at least for three days when administrated by nasal delivery. In hamsters infected with BA.5, three intranasal doses of S2-3-IgA2m2 at 1 mg/kg significantly reduce viral RNA loads and completely eliminate infectious particles in the trachea and lungs. Notably, even at single dose of 1 mg/kg, S2-3-IgA2m2 prophylactically administered through the intranasal route drastically reduces airway viral RNA loads and infectious particles. This study provides an effective weapon combating SARS-CoV-2, proposes a new strategy overcoming the virus escape, and lays strategic reserves for rapid response to potential future outbreaks of "SARS-CoV-3".

The immunoglobulin A isotype of the Arabian camel (Camelus dromedarius) preserves the dualistic structure of unconventional single-domain and canonical heavy chains

Introduction

The evolution of adaptive immunity in Camelidae resulted in the concurrent expression of classic heterotetrameric and unconventional homodimeric heavy chain-only IgG antibodies. Heavy chain-only IgG bears a single variable domain and lacks the constant heavy (CH) γ1 domain required for pairing with the light chain. It has not been reported whether this distinctive feature of IgG is also observed in the IgA isotype.

Methods

Gene-specific primers were used to generate an IgA heavy chain cDNA library derived from RNA extracted from the dromedary's third eyelid where isolated lymphoid follicles and plasma cells abound at inductive and effector sites, respectively.

Results

Majority of the cDNA clones revealed hallmarks of heavy chain-only antibodies, i.e. camelid-specific amino acid substitutions in framework region 1 and 2, broad length distribution of complementarity determining region 3, and the absence of the CHα1 domain. In a few clones, however, the cDNA of the canonical IgA heavy chain was amplified which included the CHα1 domain, analogous to CHγ1 domain in IgG1 subclass. Moreover, we noticed a short, proline-rich hinge, and, at the N-terminal end of the CHα3 domain, a unique, camelid-specific pentapeptide of undetermined function, designated as the inter-α region. Immunoblots using rabbit anti-camel IgA antibodies raised against CHα2 and CHα3 domains as well as the inter-α region revealed the expression of a ~52 kDa and a ~60 kDa IgA species, corresponding to unconventional and canonical IgA heavy chain, respectively, in the third eyelid, trachea, small and large intestine. In contrast, the leporine anti-CHα1 antibody detected canonical, but not unconventional IgA heavy chain, in all the examined tissues, milk, and serum, in addition to another hitherto unexplored species of ~45 kDa in milk and serum. Immunohistology using anti-CHα domain antibodies confirmed the expression of both variants of IgA heavy chains in plasma cells in the third eyelid's lacrimal gland, conjunctiva, tracheal and intestinal mucosa.

Conclusion

We found that in the dromedary, the IgA isotype has expanded the immunoglobulin repertoire by co-expressing unconventional and canonical IgA heavy chains, comparable to the IgG class, thus underscoring the crucial role of heavy chain-only antibodies not only in circulation but also at the mucosal frontiers.

The structure of the teleost Immunoglobulin M core provides insights on polymeric antibody evolution, assembly, and function

Polymeric (p) immunoglobulins (Igs) serve broad functions during vertebrate immune responses. Typically, pIgs contain between two and six Ig monomers, each with two antigen binding fragments and one fragment crystallization (Fc). In addition, many pIgs assemble with a joining-chain (JC); however, the number of monomers and potential to include JC vary with species and heavy chain class. Here, we report the cryo-electron microscopy structure of IgM from a teleost (t) species, which does not encode JC. The structure reveals four tIgM Fcs linked through eight C-terminal tailpieces (Tps), which adopt a single β-sandwich-like domain (Tp assembly) located between two Fcs. Specifically, two of eight heavy chains fold uniquely, resulting in a structure distinct from mammalian IgM, which typically contains five IgM monomers, one JC and a centrally-located Tp assembly. Together with mutational analysis, structural data indicate that pIgs have evolved a range of assembly mechanisms and structures, each likely to support unique antibody effector functions.

Engineered Secretory Immunoglobulin A provides insights on antibody-based effector mechanisms targeting Clostridiodes difficile

Secretory (S) Immunoglobin (Ig) A is the predominant mucosal antibody, which mediates host interactions with commensal and pathogenic microbes, including Clostridioides difficile. SIgA adopts a polymeric IgA structure that is bound by secretory component (SC). Despite significance, how SIgA supports diverse effector mechanisms is poorly characterized and SIgA-based therapies nonexistent. We engineered chimeric (c) SIgAs, in which we replaced SC domain D2 with a single domain antibody or a monomeric fluorescent protein, allowing us to investigate and enhance SIgA effector mechanisms. cSIgAs exhibited increased neutralization potency against C. difficile toxins, promoted bacterial clumping and cell rupture, and decreased cytotoxicity. cSIgA also allowed us to visualize and/or quantify C. difficile morphological changes and clumping events. Results reveal mechanisms by which SIgA combats C. difficile infection, demonstrate that cSIgA design can modulate these mechanisms, and demonstrate cSIgA's adaptability to modifications that might target a broad range of antigens and effector mechanisms.

Development of AAV-delivered broadly neutralizing anti-human ACE2 antibodies against SARS-CoV-2 variants

The ongoing evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), resulting in the emergence of new variants that are resistant to existing vaccines and therapeutic antibodies, has raised the need for novel strategies to combat the persistent global COVID-19 epidemic. In this study, a monoclonal anti-human angiotensin-converting enzyme 2 (hACE2) antibody, ch2H2, was isolated and humanized to block the viral receptor-binding domain (RBD) binding to hACE2, the major entry receptor of SARS-CoV-2. This antibody targets the RBD-binding site on the N terminus of hACE2 and has a high binding affinity to outcompete the RBD. In vitro, ch2H2 antibody showed potent inhibitory activity against multiple SARS-CoV-2 variants, including the most antigenically drifted and immune-evading variant Omicron. In vivo, adeno-associated virus (AAV)-mediated delivery enabled a sustained expression of monoclonal antibody (mAb) ch2H2, generating a high concentration of antibodies in mice. A single administration of AAV-delivered mAb ch2H2 significantly reduced viral RNA load and infectious virions and mitigated pulmonary pathological changes in mice challenged with SARS-CoV-2 Omicron BA.5 subvariant. Collectively, the results suggest that AAV-delivered hACE2-blocking antibody provides a promising approach for developing broad-spectrum antivirals against SARS-CoV-2 and potentially other hACE2-dependent pathogens that may emerge in the future.

SIgA structures bound to Streptococcus pyogenes M4 and human CD89 provide insights into host-pathogen interactions

Immunoglobulin (Ig) A functions as monomeric IgA in the serum and Secretory (S) IgA in mucosal secretions. Host IgA Fc receptors (FcαRs), including human FcαR1/CD89, mediate IgA effector functions; however, human pathogen Streptococcus pyogenes has evolved surface-protein virulence factors, including M4, that also engage the CD89-binding site on IgA. Despite human mucosa serving as a reservoir for pathogens, SIgA interactions with CD89 and M4 remain poorly understood. Here we report cryo-EM structures of M4-SIgA and CD89-SIgA complexes, which unexpectedly reveal different SIgA-binding stoichiometry for M4 and CD89. Structural data, supporting experiments, and modeling indicate that copies of SIgA bound to S. pyogenes M4 will adopt similar orientations on the bacterium surface and leave one host FcαR binding site open. Results suggest unappreciated functional consequences associated with SIgA binding to host and bacterial FcαRs relevant to understanding host-microbe co-evolution, IgA effector functions and improving the outcomes of group A Streptococcus infection.

Gut bacteria induce IgA expression in pituitary hormone-secreting cells during aging

Pituitary hormone decline is a hallmark of aging. However, the precise gene regulation mechanism during pituitary aging is unclear. Here, we characterized the cell population alteration and global transcriptional change during pituitary aging through single-cell RNA sequencing (scRNA-seq). We found that mRNA-encoding components of protein translational machinery declined the most in the pituitary during aging. Remarkably, Immunoglobulin A (IgA) was found to be expressed in hormone-secreting cells, and the IgA expression level increased dramatically in aged pituitary. Moreover, the pituitary IgA expression was regulated by gut microbiota. The non-hematopoietic origin of the IgA+ cells in the pituitary was further confirmed through bone marrow transplantation. Somatotropes were identified as the most prominent IgA-producing cells through lineage tracing. Thus, pituitary hormone-secreting cells can generate IgA in an age-dependent manner, and such a process is influenced by gut bacteria.

Examining the impact of nanoplastics and PFAS exposure on immune functions through inhibition of secretory immunoglobin A in human breast milk

Emerging contaminants such as nanoplastics (NPs) and per- and polyfluoroalkyl substances (PFAS), have been detected in the environment and breast milk, thus exposing infants to potentially harmful chemicals during breastfeeding. Breast milk contains secretory immunoglobulin A (SIgA), an antibody that plays a vital role in disease protection and the development of the infant's immune system. This study employed molecular simulation and fractional factorial designs to assess the toxicity of NPs and PFAS on breast milk and their influence on infant immunity by inhibiting SIgA. The research found that NPs and PFAS have higher binding affinities to SIgA compared to the control compound. Polycarbonate (-10.7 kcal/mol) had the highest binding affinity among plastics, while Perfluorodecanoic acid (PFDA, - 8.0 kcal/mol) had the highest binding affinity among PFAS. The relative toxic index was higher for PFAS (2.4) than for plastics (1.9), suggesting that PFAS may pose a higher overall toxicity burden on the protein. The presence of specific combinations of NPs and PFAS in breast milk may potentially harm breastfeeding infants, although additional experimental studies are required to validate these findings. These results underscore the potential risks associated with these emerging contaminants in breast milk and their impact on infant immunity.

Impacts of Solid-State Fermented Barley with Fibrolytic Exogenous Enzymes on Feed Utilization, and Antioxidant Status of Broiler Chickens

The present and future high demand of common cereals as corn and wheat encourage the development of feed processing technology that allows for the dietary inclusion of other cereals of low nutritional value in poultry feeding. Barley grains contain anti-nutritional factors that limit their dietary inclusion in the poultry industry. The treatment of barley with solid-state fermentation and exogenous enzymes (FBEs) provides a good alternative to common cereals. In this study, barley grains were subjected to solid-state microbial fermentation using Lactobacillus plantarum, Bacillus subtilis and exogenous fibrolytic enzymes. This study aimed to assess the impact of FBEs on growth, feed utilization efficiency, immune modulation, antioxidant status and the expression of intestinal barrier and nutrient transporter-related genes. One-day-old broiler chicks (Ross 308, n = 400) comprised four representative groups with ten replicates (10 chicks/replicate) and were fed corn-soybean meal basal diets with inclusions of FBEs at 0, 5, 10 and 15% for 38 days. Solid-state fermentation of barley grains with fibrolytic enzymes increased protein content, lowered crude fiber and reduced sugars compared to non-fermented barley gains. In consequence, the group fed FBEs10% had the superior feed utilization efficiency and body weight gain (increased by 4.7%) with higher levels of nutrient metabolizability, pancreatic digestive enzyme activities and low digesta viscosity. Notably, the group fed FBEs10% showed an increased villi height and a decreased crypt depth with a remarkable hyperactivity of duodenal glands. In addition, higher inclusion levels of FBEs boosted serum immune-related parameters and intestinal and breast muscle antioxidants status. Intestinal nutrient transporters encoding genes (GLUT-1, CAAT-1, LAT1 and PepT-1) and intestinal barriers encoding genes (MUC-2, JAM-2, occludin, claudins-1 and β-defensin 1) were upregulated with higher dietary FBEs levels. In conclusion, feeding on FBEs10% positively enhanced broiler chickens' performance, feed efficiency and antioxidant status, and boosted intestinal barrier nutrient transporters encoding genes.

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.

The structure of the teleost Immunoglobulin M core provides insights on polymeric antibody evolution, assembly, and function

Polymeric (p) immunoglobulins (Igs) serve broad functions during vertebrate immune responses. Typically, pIgs contain between two and six Ig monomers, each with two antigen binding fragments and one fragment crystallization (Fc). In addition, many pIgs assemble with a joining-chain (JC); however, the number of monomers and potential to include JC varies with species and heavy chain class. Here, we report the cryo-electron microscopy structure of IgM from a teleost (t) species, which does not encode JC. The structure revealed four tIgM Fcs linked through eight C-terminal tailpieces (Tps), which adopt a single β-sandwich-like domain (Tp assembly) located between two Fcs. Remarkably, two of eight heavy chains fold uniquely, resulting in a structure distinct from mammalian IgM, which typically contains five IgM monomers, one JC and a centrally-located Tp assembly. Together with mutational analysis, structural data indicate that pIgs have evolved a range of assembly mechanisms and structures, each likely to support unique antibody effector functions.

AT2 cell-derived IgA trapped by the extracellular matrix in silica-induced pulmonary fibrosis

Pulmonary fibrosis is an interstitial lung disease caused by various factors such as exposure to workplace environmental contaminants, drugs, or X-rays. Epithelial cells are among the driving factors of pulmonary fibrosis. Immunoglobulin A (IgA), traditionally thought to be secreted by B cells, is an important immune factor involved in respiratory mucosal immunity. In the current study, we found that lung epithelial cells are involved in IgA secretion, which, in turn, promotes pulmonary fibrosis. Spatial transcriptomics and single-cell sequencing suggest that Igha transcripts were highly expressed in the fibrotic lesion areas of lungs from silica-treated mice. Reconstruction of B-cell receptor (BCR) sequences revealed a new cluster of AT2-like epithelial cells with a shared BCR and high expression of genes related to IgA production. Furthermore, the secretion of IgA by AT2-like cells was trapped by the extracellular matrix and aggravated pulmonary fibrosis by activating fibroblasts. Targeted blockade of IgA secretion by pulmonary epithelial cells may be a potential strategy for treating pulmonary fibrosis.

Recovery from olfactory and gustatory dysfunction following COVID-19 acquired during Omicron BA.1 wave in Italy

BACKGROUND

Despite alterations in the sense of smell and taste have dominated the symptoms of SARS-CoV-2 infection, the prevalence and the severity of self-reporting COVID-19 associated olfactory and gustatory dysfunction has dropped significantly with the advent of the Omicron BA.1 subvariant. However, data on the evolution of Omicron-related chemosensory impairment are still lacking.

OBJECTIVE

The aim of the present study was to estimate the prevalence and the recovery rate of self-reported chemosensory dysfunction 6-month after SARS-CoV-2 infection acquired during the predominance of the Omicron BA.1 subvariant in Italy.

METHODS

Prospective observational study based on the sino-nasal outcome tool 22 (SNOT-22), item "sense of smell or taste" and additional outcomes conducted in University hospitals and tertiary referral centers in Italy.

RESULTS

Of 338 patients with mild-to-moderate COVID-19 completing the baseline survey, 294 (87.0 %) responded to the 6-month follow-up interview. Among them, 101 (34.4 %) and 4 (1.4 %) reported an altered sense of smell or taste at baseline and at 6 months, respectively. Among the 101 patients with COVID-19-associated smell or taste dysfunction during the acute phase of the disease, 97 (96.0 %) reported complete resolution at 6 months. The duration of smell or taste impairment was significantly shorter in vaccinated patients (p = 0.007).

CONCLUSIONS

Compared with that observed in subjects infected during the first wave of the pandemic, the recovery rate from chemosensory dysfunctions reported in the present series of patients infected during the predominance of the Omicron BA.1 subvariant was more favorable with a shorter duration being positively influenced by vaccination.

The Structures of Secretory IgA in complex with Streptococcus pyogenes M4 and human CD89 provide insights on mucosal host-pathogen interactions

Immunoglobulin (Ig) A functions as monomeric IgA in the serum and Secretory (S) IgA in mucosal secretions. Host IgA Fc receptors (FcαRs), including human FcαR1/CD89, mediate IgA effector functions; however human pathogen Streptococcus pyogenes has evolved surface-protein virulence factors, including M4, that also engage the CD89 binding site on IgA. Despite human mucosa serving as a reservoir for pathogens, SIgA interactions with CD89 and M4 remain poorly understood. Here we report cryo-EM structures of M4-SIgA and CD89-SIgA complexes, which unexpectedly reveal different SIgA-binding stoichiometry for M4 and CD89. Structural data, supporting experiments, and modeling indicate that copies of SIgA bound to S. pyogenes M4 will adopt similar orientations on the bacterium surface and leave one host FcαR binding site open. Results suggest unappreciated functional consequences associated with SIgA binding to host and bacterial FcαRs relevant to understanding host-microbe co-evolution, IgA effector functions and to improving the outcomes of group A Streptococcus infection.

Respiratory delivery of passive immunotherapies for SARS-CoV-2 prophylaxis and therapy

Convalescent plasma has been extensively tested during the COVID-19 pandemic as a transfusion product. Similarly, monoclonal antibodies have been largely administered either intravenously or intramuscularly. Nevertheless, when used against a respiratory pathogen, respiratory delivery is preferable to maximize the amount of antibody that reaches the entry door in order to prevent sustained viral multiplication. In this narrative review, we review the different types of inhalation device and summarize evidence from animal models and early clinical trials supporting the respiratory delivery (for either prophylactic or therapeutic purposes) of convalescent plasma or monoclonal antibodies (either full antibodies, single-chain variable fragments, or camelid-derived monoclonal heavy-chain only antibodies). Preliminary evidences from animal models suggest similar safety and noninferior efficacy, but efficacy evaluation from clinical trials is still limited.

Evolution of the Immunoglobulin Isotypes-Variations of Biophysical Properties among Animal Classes

The adaptive immune system arose around 500 million years ago in jawed fish, and, since then, it has mediated the immune defense against pathogens in all vertebrates. Antibodies play a central role in the immune reaction, recognizing and attacking external invaders. During the evolutionary process, several immunoglobulin isotypes emerged, each having a characteristic structural organization and dedicated function. In this work, we investigate the evolution of the immunoglobulin isotypes, in order to highlight the relevant features that were preserved over time and the parts that, instead, mutated. The residues that are coupled in the evolution process are often involved in intra- or interdomain interactions, meaning that they are fundamental to maintaining the immunoglobulin fold and to ensuring interactions with other domains. The explosive growth of available sequences allows us to point out the evolutionary conserved residues and compare the biophysical properties among different animal classes and isotypes. Our study offers a general overview of the evolution of immunoglobulin isotypes and advances the knowledge of their characteristic biophysical properties, as a first step in guiding protein design from evolution.

An overview of host-derived molecules that interact with gut microbiota

The gut microbiota comprises bacteria, archaea, fungi, protists, and viruses that live together and interact with each other and with host cells. A stable gut microbiota is vital for regulating host metabolism and maintaining body health, while a disturbed microbiota may induce different kinds of disease. In addition, diet is also considered to be the main factor that influences the gut microbiota. The host could shape the gut microbiota through other factors. Here, we reviewed the mechanisms that mediate host regulation on gut microbiota, involved in gut-derived molecules, including gut-derived immune system molecules (secretory immunoglobulin A, antimicrobial peptides, cytokines, cluster of differentiation 4+ effector T cell, and innate lymphoid cells), sources related to gut-derived mucosal molecules (carbon sources, nitrogen sources, oxygen sources, and electron respiratory acceptors), gut-derived exosomal noncoding RNA (ncRNAs) (microRNAs, circular RNA, and long ncRNA), and molecules derived from organs other than the gut (estrogen, androgen, neurohormones, bile acid, and lactic acid). This study provides a systemic overview for understanding the interplay between gut microbiota and host, a comprehensive source for potential ways to manipulate gut microbiota, and a solid foundation for future personalized treatment that utilizes gut microbiota.

Structural basis for Fc receptor recognition of immunoglobulin M

Immunoglobulin Fc receptors are cell surface transmembrane proteins that bind to the Fc constant region of antibodies and play critical roles in regulating immune responses by activation of immune cells, clearance of immune complexes and regulation of antibody production. FcμR is the immunoglobulin M (IgM) antibody isotype-specific Fc receptor involved in the survival and activation of B cells. Here we reveal eight binding sites for the human FcμR immunoglobulin domain on the IgM pentamer by cryogenic electron microscopy. One of the sites overlaps with the binding site for the polymeric immunoglobulin receptor (pIgR), but a different mode of FcμR binding explains its antibody isotype specificity. Variation in FcμR binding sites and their occupancy reflects the asymmetry of the IgM pentameric core and the versatility of FcμR binding. The complex explains engagement with polymeric serum IgM and the monomeric IgM B-cell receptor (BCR).

Immunoglobulin M perception by FcμR

Immunoglobulin M (IgM) is the first antibody to emerge during embryonic development and the humoral immune response¹. IgM can exist in several distinct forms, including monomeric, membrane-bound IgM within the B cell receptor (BCR) complex, pentameric and hexameric IgM in serum and secretory IgM on the mucosal surface. FcμR, the only IgM-specific receptor in mammals, recognizes different forms of IgM to regulate diverse immune responses^2-5. However, the underlying molecular mechanisms remain unknown. Here we delineate the structural basis of the FcμR-IgM interaction by crystallography and cryo-electron microscopy. We show that two FcμR molecules interact with a Fcμ-Cμ4 dimer, suggesting that FcμR can bind to membrane-bound IgM with a 2:1 stoichiometry. Further analyses reveal that FcμR-binding sites are accessible in the context of IgM BCR. By contrast, pentameric IgM can recruit four FcμR molecules to bind on the same side and thereby facilitate the formation of an FcμR oligomer. One of these FcμR molecules occupies the binding site of the secretory component. Nevertheless, four FcμR molecules bind to the other side of secretory component-containing secretory IgM, consistent with the function of FcμR in the retrotransport of secretory IgM. These results reveal intricate mechanisms of IgM perception by FcμR.

Identification of common and distinct origins of human serum and breastmilk IgA1 by mass spectrometry-based clonal profiling

The most abundant immunoglobulin present in the human body is IgA. It has the highest concentrations at the mucosal lining and in biofluids such as milk and is the second most abundant class of antibodies in serum. We assessed the structural diversity and clonal repertoire of IgA1-containing molecular assemblies longitudinally in human serum and milk from three donors using a mass spectrometry-based approach. IgA-containing molecules purified from serum or milk were assessed by the release and subsequent analysis of their Fab fragments. Our data revealed that serum IgA1 consists of two distinct structural populations, namely monomeric IgA1 (∼80%) and dimeric joining (J-) chain coupled IgA1 (∼20%). Also, we confirmed that IgA1 in milk is present solely as secretory (S)IgA, consisting of two (∼50%), three (∼33%) or four (∼17%) IgA1 molecules assembled with a J-chain and secretory component (SC). Interestingly, the serum and milk IgA1-Fab repertoires were distinct between monomeric, and J-chain coupled dimeric IgA1. The serum dimeric J-chain coupled IgA1 repertoire contained several abundant clones also observed in the milk IgA1 repertoire. The latter repertoire had little to no overlap with the serum monomeric IgA1 repertoire. This suggests that human IgA1s have (at least) two distinct origins; one of these produces dimeric J-chain coupled IgA1 molecules, shared in human serum and milk, and another produces monomeric IgA1 ending up exclusively in serum.

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.

Complex probiotics alleviate ampicillin-induced antibiotic-associated diarrhea in mice

Aim

Antibiotic-associated diarrhea (AAD) is a common side effect during antibiotic treatment, which can cause dysbacteriosis of the gut microbiota. Previous studies have shown beneficial effects in AAD treatment with Bifidobacterium lactis XLTG11, Lactobacillus casei Zhang, Lactobacillus plantarum CCFM8661, and Lactobacillus rhamnosus Probio-M9. However, no studies have been conducted on the immunomodulatory effects and protective intestinal barrier function of four complex probiotics. The aim of our study is to investigate the alleviation effects of complex probiotics on ampicillin-induced AAD.

Methods

Thirty-six BALB/c mice were randomly divided into six groups: normal control group (NC), model control group (MC), low-, medium-, and high-dose probiotics groups (LD, MD, and HD), and positive drug (Bifico, 1 × 10⁷ cfu) control group (PDC; Bifico, also known as Bifidobacterium Triple Live Capsule, is composed of Bifidobacterium longum, Lactobacillus acidophilus, and Enterococcus faecalis). An AAD model was established by intragastric administration of ampicillin, by gavage of different doses of complex probiotics and Bifico. The weight gain, fecal water content, loose stool grade, intestinal permeability, total protein and albumin levels, intestinal barrier, cytokine levels, and gut microbiota were determined.

Results

The results showed that complex probiotics significantly decreased the fecal water content, loose stool grade, intestinal permeability, and ileum tissue damage. Their application increased the weight gain, SIgA, TP, and ALB levels. Additionally, complex probiotics significantly decreased the levels of pro-inflammatory cytokines and increased those of anti-inflammatory cytokines. Meanwhile, the mRNA expression levels of ZO-1, occludin, claudin-1, and MUC2 were significantly upregulated in the probiotic-treated group. Furthermore, the complex probiotics increased the gut microbiota diversity and modulated the changes in the gut microbiota composition caused by ampicillin. At the phylum level, the abundance of Proteobacteria in the HD group was lower than that in the MC group, whereas that of Bacteroidetes was higher. At the genus level, the abundances of Klebsiella and Parabacteroides in the HD group were lower, whereas those of Bacteroides, Muribaculaceae, and Lactobacillus were higher than those in the MC group. Moreover, Spearman's correlation analysis also found that several specific gut microbiota were significantly correlated with AAD-related indicators.

Conclusion

We found that complex probiotics improved the diarrhea-related indexes, regulated gut microbiota composition and diversity, increased the expression levels of intestinal protective barrier-related genes, preserved the intestinal barrier function, and relieved inflammation and intestinal injury, thereby effectively improving AAD-associated symptoms. Graphical Abstract.

Kidney microbiota dysbiosis contributes to the development of hypertension

Gut microbiota dysbiosis promotes metabolic syndromes (e.g., hypertension); however, the patterns that drive hypertensive pathology and could be targeted for therapeutic intervention are unclear. We hypothesized that gut microbes might translocate to the kidney to trigger hypertension. We aimed to uncover their method of colonization, and thereby how to maintain blood pressure homeostasis. Using combined approaches based on fluorescence in situ hybridization (FISH) and immunofluorescence staining, electron microscopy analysis, bacterial cultures, species identification, and RNA-sequencing-based meta-transcriptomics, we first demonstrated the presence of bacteria within the kidney of spontaneously hypertensive rats (SHRs) and its normotensive counterpart, Wistar-Kyoto rats (WKYs), and patients with hypertension. Translocated renal bacteria were coated with secretory IgA (sIgA) or remained dormant in the L-form. Klebsiella pneumoniae (K.pn) was identified in the kidneys of germ-free (GF) mice following intestinal transplantation, which suggested an influx of gut bacteria into the kidneys. Renal bacterial taxa and their function are associated with hypertension. Hypertensive hosts showed increased richness in the pathobionts of their kidneys, which were partly derived from the gastrointestinal tract. We also demonstrated the indispensable role of bacterial IgA proteases in the translocation of live microbes. Furthermore, Tartary buckwheat dietary intervention reduced blood pressure and modulated the core renal flora-host ecosystem to near-normal states. Taken together, the unique patterns of viable and dormant bacteria in the kidney provide insight into the pathogenesis of non-communicable chronic diseases and cardiometabolic diseases (e.g., hypertension), and may lead to potential novel microbiota-targeted dietary therapies.

Cryo-electron Microscopic Analysis of Single-Pass Transmembrane Receptors

Single-pass transmembrane receptors (SPTMRs) represent a diverse group of integral membrane proteins that are involved in many essential cellular processes, including signal transduction, cell adhesion, and transmembrane transport of materials. Dysregulation of the SPTMRs is linked with many human diseases. Despite extensive efforts in past decades, the mechanisms of action of the SPTMRs remain incompletely understood. One major hurdle is the lack of structures of the full-length SPTMRs in different functional states. Such structural information is difficult to obtain by traditional structural biology methods such as X-ray crystallography and nuclear magnetic resonance (NMR). The recent rapid development of single-particle cryo-electron microscopy (cryo-EM) has led to an exponential surge in the number of high-resolution structures of integral membrane proteins, including SPTMRs. Cryo-EM structures of SPTMRs solved in the past few years have tremendously improved our understanding of how SPTMRs function. In this review, we will highlight these progresses in the structural studies of SPTMRs by single-particle cryo-EM, analyze important structural details of each protein involved, and discuss their implications on the underlying mechanisms. Finally, we also briefly discuss remaining challenges and exciting opportunities in the field.

Gut microbiome and breast-feeding: Implications for early immune development

Establishment of the gut microbiome during early life is a complex process with lasting implications for an individual's health. Several factors influence microbial assembly; however, breast-feeding is recognized as one of the most influential drivers of gut microbiome composition during infancy, with potential implications for function. Differences in gut microbial communities between breast-fed and formula-fed infants have been consistently observed and are hypothesized to partially mediate the relationships between breast-feeding and decreased risk for numerous communicable and noncommunicable diseases in early life. Despite decades of research on the gut microbiome of breast-fed infants, there are large scientific gaps in understanding how human milk has evolved to support microbial and immune development. This review will summarize the evidence on how breast-feeding broadly affects the composition and function of the early-life gut microbiome and discuss mechanisms by which specific human milk components shape intestinal bacterial colonization, succession, and function.

Cryo-EM structures of two human B cell receptor isotypes

The B cell receptor (BCR) complex plays a critical role in B cell development and immune responses. The assembly mechanisms underlying the BCR complex remain unknown. We determined the cryo-electron microscopy (cryo-EM) structures of human IgG-BCR and IgM-BCR, which consist of membrane-bound immunoglobulin molecules (mIg) and Igα/β subunits at a 1:1 stoichiometry. Assembly of both BCR complexes involves their extracellular domains, membrane-proximal connection peptides, and transmembrane (TM) helices. The TM helices of mIgG and mIgM share a conserved set of hydrophobic and polar interactions with Igα/β TM helices. By contrast, the IgG-Cγ3 and IgM-Cμ4 domains interact with extracellular Ig-like domains of Igα/β through head-to-tail and side-by-side modes, respectively. This work reveals the structural basis for BCR assembly and provides insights into BCR triggering.

Cryo-EM structure of the human IgM B cell receptor

The B cell receptor (BCR) initiates immune responses through antigen recognition. We report a 3.3-angstrom cryo-electron microscopy structure of human immunoglobulin M (IgM)-BCR in the resting state. IgM-BCR comprises two heavy chains, two light chains, and the Igα/Igβ heterodimer. The ectodomains of the heavy chains closely stack against those of Igα/Igβ, with one heavy chain locked between Igα and Igβ in the juxtamembrane region. Extracellular interactions may determine isotype specificity of the BCR. The transmembrane helices of IgM-BCR form a four-helix bundle that appears to be conserved among all BCR isotypes. This structure contains 14 glycosylation sites on the IgM-BCR ectodomains and reveals three potential surface binding sites. Our work reveals the organizational principles of the BCR and may facilitate the design of antibody-based therapeutics.

Antibody Interfaces Revealed Through Structural Mining

Antibodies are fundamental effectors of humoral immunity, and have become a highly successful class of therapeutics. There is increasing evidence that antibodies utilize transient homotypic interactions to enhance function, and elucidation of such interactions can provide insights into their biology and new opportunities for their optimization as drugs. Yet the transitory nature of weak interactions makes them difficult to investigate. Capitalizing on their rich structural data and high conservation, we have characterized all the ways that antibody fragment antigen-binding (Fab) regions interact crystallographically. This approach led to the discovery of previously unrealized interfaces between antibodies. While diverse interactions exist, β-sheet dimers and variable-constant elbow dimers are recurrent motifs. Disulfide engineering enabled interactions to be trapped and investigated structurally and functionally, providing experimental validation of the interfaces and illustrating their potential for optimization. This work provides first insight into previously undiscovered oligomeric interactions between antibodies, and enables new opportunities for their biotherapeutic optimization.

Early life administration of Bifidobacterium bifidum BD-1 alleviates long-term colitis by remodeling the gut microbiota and promoting intestinal barrier development

Inflammatory bowel disease (IBD) is a chronic intestinal disease characterized by microbiota disturbance and intestinal mucosal damage. The current study aimed to investigate the preventive effects of Bifidobacterium bifidum BD-1 (BD-1) against long-term IBD and possible mechanism by which it alters the gut microbiota, immune response, and mucosal barrier. Our study found that early treatment of BD-1 + Ceftri (ceftriaxone followed by BD-1) and BD-1 confers a certain protective effect against the occurrence of long-term Dextran sulfate sodium-induced colitis, which manifests as a decrease in inflammation scores and MPO activity levels, as well as a relatively intact intestinal epithelial structure. Moreover, compared to BD-1, Ceftri, and NS, early treatment with BD-1 + Ceftri promoted greater expression levels of mucosal barrier-related proteins [KI67, MUC2, ZO-1, secretory immunoglobulin A (slgA), Clauding-1, and Occludin], better local immune responses activation, and moderately better modulation of systemic immune responses during long-term colitis. This may be due to the fact that BD-1 + Ceftri can deliberately prolong the colonization time of some beneficial microbiota (e.g., Bifidobacterium) and reduce the relative abundance of inflammation-related microbiota (e.g., Escherichia/Shigella and Ruminococcus). Interestingly, we found that the changes in the gut barrier and immunity were already present immediately after early intervention with BD-1 + Ceftri, implying that early effects can persist with appropriate intervention. Furthermore, intervention with BD-1 alone in early life confers an anti-inflammatory effect to a certain degree in the long-term, which may be due to the interaction between BD-1 and the host’s native gut microbiota affecting intestinal metabolites. In conclusion, BD-1 was not as effective as BD-1 + Ceftri in early life, perhaps due to its failure to fully play the role of the strain itself under the influence of the host’s complex microbiota. Therefore, further research is needed to explore specific mechanisms for single strain and native microbiota or the combination between probiotics and antibiotics.

Cytokines and Production of Aberrantly O-Glycosylated IgA1, the Main Autoantigen in IgA Nephropathy

Immunoglobulin A (IgA) nephropathy is the most common primary glomerulonephritis worldwide, with no disease-specific treatment and up to 40% of patients progressing to kidney failure. IgA nephropathy (IgAN), characterized by IgA1-containing immunodeposits in the glomeruli, is considered to be an autoimmune disease in which the kidneys are injured as innocent bystanders. Glomerular immunodeposits are thought to originate from the circulating immune complexes that contain aberrantly O-glycosylated IgA1, the main autoantigen in IgAN, bound by IgG autoantibodies. A common clinical manifestation associated with IgAN includes synpharyngitic hematuria at disease onset or during disease activity. This observation suggests a connection of disease pathogenesis with an activated mucosal immune system of the upper-respiratory and/or gastrointestinal tract and IgA1 glycosylation. In fact, some cytokines can enhance production of aberrantly O-glycosylated IgA1. This process involves abnormal cytokine signaling in IgA1-producing cells from patients with IgAN. In this article, we present our view of pathogenesis of IgAN and review how some cytokines can contribute to the disease process by enhancing production of aberrantly glycosylated IgA1. We also review current clinical trials of IgAN based on cytokine-targeting therapeutic approaches.

Review of glucose oxidase as a feed additive: production, engineering, applications, growth-promoting mechanisms, and outlook

The regulation and prohibition of antibiotics used as growth promoters (AGP) in the feed field are increasing because they cause antimicrobial resistance and drug residue issues and threaten community health. Recently, glucose oxidase (GOx) has attracted increasing interest in the feed industry as an alternative to antibiotics. GOx specifically catalyzes the production of gluconic acid (GA) and hydrogen peroxide (H₂O₂) by consuming molecular oxygen, and plays an important role in relieving oxidative stress, preserving health, and promoting animal growth. To expand the application of GOx in the feed field, considerable efforts have been made to mine new genetic resources. Efforts have also been made to heterologously overexpress relevant genes to reduce production costs and to engineer proteins by modifying enzyme properties, both of which are bottleneck problems that limit industrial feed applications. Herein, the: different sources, diverse biochemical properties, distinct structural features, and various strategies of GOx engineering and heterologous overexpression are summarized. The mechanism through which GOx promotes growth in animal production, including the improvement of antioxidant capacity, maintenance of intestinal microbiota homeostasis, and enhancement of gut function, are also systematically addressed. Finally, a new perspective is provided for the future development of GOx applications in the feed field.

Distinct Immunological Profiles Help in the Maintenance of Salivary Secretory IgA Production in Mild Symptoms COVID-19 Patients

Background

Relevant aspects regarding the SARS-CoV-2 pathogenesis and the systemic immune response to this infection have been reported. However, the mucosal immune response of the upper airways two months after SARS-CoV-2 infection in patients with mild/moderate symptoms is still not completely described. Therefore, we investigated the immune/inflammatory responses of the mucosa of the upper airways of mild/moderate symptom COVID-19 patients two months after the SARS-CoV-2 infection in comparison to a control group composed of non-COVID-19 healthy individuals.

Methods

A cohort of 80 volunteers (age 37.2 ± 8.2), including non-COVID-19 healthy individuals (n=24) and COVID-19 patients (n=56) who presented mild/moderate symptoms during a COVID-19 outbreak in Brazil in November and December of 2020. Saliva samples were obtained two months after the COVID-19 diagnosis to assess the levels of SIgA by ELISA and the cytokines by multiplex analysis.

Results

Salivary levels of SIgA were detected in 39 volunteers into the COVID-19 group and, unexpectedly, in 14 volunteers in the control group. Based on this observation, we distributed the volunteers of the control group into without SIgA or with SIgA sub-groups, and COVID-19 group into without SIgA or with SIgA sub-groups. Individuals with SIgA showed higher levels of IL-10, IL-17A, IFN-γ, IL-12p70, IL-13, and IFN-α than those without SIgA. In intergroup analysis, the COVID-19 groups showed higher salivary levels of IL-10, IL-13, IL-17A, and IFN-α than the control group. No statistical differences were verified in the salivary levels of IL-6 and IFN-β. Lower IL-12p70/IL-10 and IFN-γ/IL-10 ratios were found in the control group without SIgA than the control group with SIgA and the COVID-19 group with SIgA.

Conclusion

We were able to present, for the first time, that associations between distinct immunological profiles can help the mucosal immunity to maintain the salivary levels of SIgA in COVID-19 patients two months after the SARS-CoV-2 infection.

Transcytosis of IgA Attenuates Salmonella Invasion in Human Enteroids and Intestinal Organoids

Secretory IgA (SIgA) is the most abundant antibody type in intestinal secretions where it contributes to safeguarding the epithelium from invasive pathogens like the Gram-negative bacterium, Salmonella enterica serovar Typhimurium (STm). For example, we recently reported that passive oral administration of the recombinant monoclonal SIgA antibody, Sal4, to mice promotes STm agglutination in the intestinal lumen and restricts bacterial invasion of Peyer's patch tissues. In this report, we sought to recapitulate Sal4-mediated protection against STm in human Enteroids and human intestinal organoids (HIOs) as models to decipher the molecular mechanisms by which antibodies function in mucosal immunity in the human gastrointestinal tract. We confirm that Enteroids and HIO-derived monolayers are permissive to STm infection, dependent on HilD, the master transcriptional regulator of the SPI-I type three secretion system (T3SS). Stimulation of M-like cells in both Enteroids and HIOs by the addition of RANKL further enhanced STm invasion. The apical addition of Sal4 mouse IgA, as well as recombinant human Sal4 dimeric IgA (dIgA) and SIgA resulted a dose-dependent reduction in bacterial invasion. Moreover, basolateral application of Sal4 dIgA to Enteroid and HIO monolayers gave rise to SIgA in the apical compartment via a pathway dependent on expression of the polymeric immunoglobulin receptor (pIgR). The resulting Sal4 SIgA was sufficient to reduce STm invasion of Enteroid and HIO epithelial cell monolayers by ~20-fold. Recombinant Sal4 IgG was also transported in the Enteroid and HIOs, but to a lesser degree and via a pathway dependent on the neonatal Fc receptor (FCGRT). The models described lay the foundation for future studies into detailed mechanisms of IgA and IgG protection against STm and other pathogens.

Chaotropic Perturbation of Noncovalent Interactions of the Hemagglutinin Tag Monoclonal Antibody Fragment Enables Superresolution Molecular Census

Antibody-antigen interactions represent one of the most exploited biomolecular interactions in experimental biology. While numerous techniques harnessed immobilized antibodies for nanoscale fluorescence imaging, few utilized their reversible binding kinetics. Here, we investigated noncovalent interactions of the monoclonal hemagglutinin (HA) epitope tag antibody, 12CA5, in the fixed cellular environment. We observed that the use of a chaotropic agent, potassium thiocyanate (KSCN), promoted the dissociation of the 12CA5 antibody fragment (Fab), which already displayed faster dissociation compared to its immunoglobulin G (IgG) counterpart. Molecular dynamic simulations revealed notable root-mean-square deviations and destabilizations in the presence of KSCN, while the hydrogen-bonding network remained primarily unaffected at the antigen-binding site. The reversible interactions enabled us to achieve a superresolution molecular census of local populations of 3xHA tagged microtubule fibers with improved molecular quantification consistency compared to single-molecule localization microscopy (SMLM) techniques utilizing standard immunofluorescence staining for sample labeling. Our technique, termed superresolution census of molecular epitope tags (SR-COMET), highlights the utilization of reversible antibody-antigen interactions for SMLM-based quantitative superresolution imaging.

Molecular Mechanisms of Multimeric Assembly of IgM and IgA

As central effectors of the adaptive immune response, immunoglobulins, or antibodies, provide essential protection from pathogens through their ability to recognize foreign antigens, aid in neutralization, and facilitate elimination from the host. Mammalian immunoglobulins can be classified into five isotypes—IgA, IgD, IgE, IgG, and IgM—each with distinct roles in mediating various aspects of the immune response. Of these isotypes, IgA and IgM are the only ones capable of multimerization, arming them with unique biological functions. Increased valency of polymeric IgA and IgM provides high avidity for binding low-affinity antigens, and their ability to be transported across the mucosal epithelium into secretions by the polymeric immunoglobulin receptor allows them to play critical roles in mucosal immunity. Here we discuss the molecular assembly, structure, and function of these multimeric antibodies.

Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

[Mucosal immunity and vaccines against viral infections]

Mucosal immunity is realized through a structural and functional system called mucose-associated lymphoid tissue (MALT). MALT is subdivided into parts (clusters) depending on their anatomical location, but they all have a similar structure: mucus layer, epithelial tissue, lamina propria and lymphoid follicles. Plasma cells of MALT produce a unique type of immunoglobulins, IgA, which have the ability to polymerize. In mucosal immunization, the predominant form of IgA is a secretory dimer, sIgA, which is concentrated in large quantities in the mucosa. Mucosal IgA acts as a first line of defense and neutralizes viruses efficiently at the portal of entry, preventing infection of epithelial cells and generalization of infection. To date, several mucosal antiviral vaccines have been licensed, which include attenuated strains of the corresponding viruses: poliomyelitis, influenza, and rotavirus. Despite the tremendous success of these vaccines, in particular, in the eradication of poliomyelitis, significant disadvantages of using attenuated viral strains in their composition are the risk of reactogenicity and the possibility of reversion to a virulent strain during vaccination. Nevertheless, it is mucosal vaccination, which mimics a natural infection, is able to induce a fast and effective immune response and thus help prevent and possibly stop outbreaks of many viral infections. Currently, a number of intranasal vaccines based on a new vector approach are successfully undergoing clinical trials. In these vaccines, the safe viral vectors are used to deliver protectively significant immunogens of pathogenic viruses. The most tested vector for intranasal vaccines is adenovirus, and the most significant immunogen is SARSCoV-2 S protein. Mucosal vector vaccines against human respiratory syncytial virus and human immunodeficiency virus type 1 based on Sendai virus, which is able to replicate asymptomatically in cells of bronchial epithelium, are also being investigated.

Engineering a pure and stable heterodimeric IgA for the development of multispecific therapeutics

ABBREVIATIONS

CE-SDS: capillary electrophoresis sodium dodecyl sulfate; DSC: differential scanning calorimetry; FACS: fluorescence-activated cell sorting; FSA: full-sized antibody; Her2: human epidermal growth factor receptor 2; MFI: mean fluorescent intensity; OAA: one-armed antibody; PBS: phosphate-buffered saline; PDB: Protein Data Bank; SEC: size-exclusion chromatography; prepSEC (preparative SEC); RMSD: root-mean-square deviation; RU: resonance units; SPR: surface plasmon resonance; TAA: tumor-associated antigen; WT: wild-type.

A Direct MS-Based Approach to Profile Human Milk Secretory Immunoglobulin A (IgA1) Reveals Donor-Specific Clonal Repertoires With High Longitudinal Stability

Recently, a mass spectrometry-based approach was introduced to directly assess the IgG1 immunoglobulin clonal repertoires in plasma. Here we expanded upon this approach by describing a mass spectrometry-based technique to assess specifically the clonal repertoire of another important class of immunoglobulin molecules, IgA1, and show it is efficiently and robustly applicable to either milk or plasma samples. Focusing on two individual healthy donors, whose milk was sampled longitudinally during the first 16 weeks of lactation, we demonstrate that the total repertoire of milk sIgA1 is dominated by only 50-500 clones, even though the human body theoretically can generate several orders of magnitude more clones. We show that in each donor the sIgA1 repertoire only changes marginally and quite gradually over the monitored 16-week period of lactation. Furthermore, the observed overlap in clonal repertoires between the two individual donors is close to non-existent. Mothers provide protection to their newborn infants directly by the transfer of antibodies via breastfeeding. The approach introduced here, can be used to visualize the clonal repertoire transferred from mother to infant and to detect changes in-time in that repertoire adapting to changes in maternal physiology.

The rapidly evolving role of cryo-EM in drug design

Since the early 2010s, cryo-electron microscopy (cryo-EM) has evolved to a mainstream structural biology method in what has been dubbed the "resolution revolution". Pharma companies also began to use cryo-EM in drug discovery, evidenced by a growing number of industry publications. Hitherto limited in resolution, throughput and attainable molecular weight, cryo-EM is rapidly overcoming its main limitations for more widespread use through a new wave of technological advances. This review discusses how cryo-EM has already impacted drug discovery, and how the state-of-the-art is poised to further revolutionize its application to previously intractable proteins as well as new use cases.

3D Structures of IgA, IgM, and Components

Immunoglobulin G (IgG) is currently the most studied immunoglobin class and is frequently used in antibody therapeutics in which its beneficial effector functions are exploited. IgG is composed of two heavy chains and two light chains, forming the basic antibody monomeric unit. In contrast, immunoglobulin A (IgA) and immunoglobulin M (IgM) are usually assembled into dimers or pentamers with the contribution of joining (J)-chains, which bind to the secretory component (SC) of the polymeric Ig receptor (pIgR) and are transported to the mucosal surface. IgA and IgM play a pivotal role in various immune responses, especially in mucosal immunity. Due to their structural complexity, 3D structural study of these molecules at atomic scale has been slow. With the emergence of cryo-EM and X-ray crystallographic techniques and the growing interest in the structure-function relationships of IgA and IgM, atomic-scale structural information on IgA-Fc and IgM-Fc has been accumulating. Here, we examine the 3D structures of IgA and IgM, including the J-chain and SC. Disulfide bridging and N-glycosylation on these molecules are also summarized. With the increasing information of structure–function relationships, IgA- and IgM-based monoclonal antibodies will be an effective option in the therapeutic field.