IgE, IgE Receptors and Anti-IgE Biologics: Protein Structures and Mechanisms of Action.

The evolution of IgE in mammals added an extra layer of immune protection at body surfaces to provide a rapid and local response against antigens from the environment. The IgE immune response employs potent expulsive and inflammatory forces against local antigen provocation, at the risk of damaging host tissues and causing allergic disease. Two well-known IgE receptors, the high-affinity FcεRI and low-affinity CD23, mediate the activities of IgE. Unlike other known antibody receptors, CD23 also regulates IgE expression, maintaining IgE homeostasis. This mechanism evolved by adapting the function of the complement receptor CD21. Recent insights into the dynamic character of IgE structure, its resultant capacity for allosteric modulation, and the potential for ligand-induced dissociation have revealed previously unappreciated mechanisms for regulation of IgE and IgE complexes. We describe recent research, highlighting structural studies of the IgE network of proteins to analyze the uniquely versatile activities of IgE and anti-IgE biologics.

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.

Production and Function of Immunoglobulin A.

Among antibodies, IgA is unique because it has evolved to be secreted onto mucosal surfaces. The structure of IgA and the associated secretory component allow IgA to survive the highly proteolytic environment of mucosal surfaces but also substantially limit IgA's ability to activate effector functions on immune cells. Despite these characteristics, IgA is critical for both preventing enteric infections and shaping the local microbiome. IgA's function is determined by a distinct antigen-binding repertoire, composed of antibodies with a variety of specificities, from permissive polyspecificity to cross-reactivity to exquisite specificity to a single epitope, which act together to regulate intestinal bacteria. Development of the unique function and specificities of IgA is shaped by local cues provided by the gut-associated lymphoid tissue, driven by the constantly changing environment of the intestine and microbiota.

The Way We Walked with Immunology.

It has been a little more than 50 years since we discovered IgE, a key molecule for the allergic response and a target for treating allergies and severe asthma. Here, I trace my career, from the kindling of my interest in immunochemistry to groundbreaking discoveries in the biology and chemistry of immunoglobulins. I describe my service to the broader community of immunologists and my role in shaping departments and research institutes. My course starts in Japan and includes Southern California, Baltimore, and Denver.

IgA Function in Relation to the Intestinal Microbiota.

IgA is the dominant immunoglobulin isotype produced in mammals, largely secreted across the intestinal mucosal surface. Although induction of IgA has been a hallmark feature of microbiota colonization following colonization in germ-free animals, until recently appreciation of the function of IgA in host-microbial mutualism has depended mainly on indirect evidence of alterations in microbiota composition or penetration of microbes in the absence of somatic mutations in IgA (or compensatory IgM). Highly parallel sequencing techniques that enable high-resolution analysis of either microbial consortia or IgA sequence diversity are now giving us new perspectives on selective targeting of microbial taxa and the trajectory of IgA diversification according to induction mechanisms, between different individuals and over time. The prospects are to link the range of diversified IgA clonotypes to specific antigenic functions in modulating the microbiota composition, position and metabolism to ensure host mutualism.

Host and pathogen response to bacteriophage engineered against Mycobacterium abscessus lung infection.

Two mycobacteriophages were administered intravenously to a male with treatment-refractory Mycobacterium abscessus pulmonary infection and severe cystic fibrosis lung disease. The phages were engineered to enhance their capacity to lyse M. abscessus and were selected specifically as the most effective against the subject's bacterial isolate. In the setting of compassionate use, the evidence of phage-induced lysis was observed using molecular and metabolic assays combined with clinical assessments. M. abscessus isolates pre and post-phage treatment demonstrated genetic stability, with a general decline in diversity and no increased resistance to phage or antibiotics. The anti-phage neutralizing antibody titers to one phage increased with time but did not prevent clinical improvement throughout the course of treatment. The subject received lung transplantation on day 379, and systematic culturing of the explanted lung did not detect M. abscessus. This study describes the course and associated markers of a successful phage treatment of M. abscessus in advanced lung disease.

TCR+/BCR+ dual-expressing cells and their associated public BCR clonotype are not enriched in type 1 diabetes.

Ahmed and colleagues recently described a novel hybrid lymphocyte expressing both a B and T cell receptor, termed double expresser (DE) cells. DE cells in blood of type 1 diabetes (T1D) subjects were present at increased numbers and enriched for a public B cell clonotype. Here, we attempted to reproduce these findings. While we could identify DE cells by flow cytometry, we found no association between DE cell frequency and T1D status. We were unable to identify the reported public B cell clone, or any similar clone, in bulk B cells or sorted DE cells from T1D subjects or controls. We also did not observe increased usage of the public clone VH or DH genes in B cells or in sorted DE cells. Taken together, our findings suggest that DE cells and their alleged public clonotype are not enriched in T1D. This Matters Arising paper is in response to Ahmed et al. (2019), published in Cell. See also the response by Ahmed et al. (2021), published in this issue.

The Immunoglobulin Superfamily Receptome Defines Cancer-Relevant Networks Associated with Clinical Outcome.

Cell surface receptors and their interactions play a central role in physiological and pathological signaling. Despite its clinical relevance, the immunoglobulin superfamily (IgSF) remains uncharacterized and underrepresented in databases. Here, we present a systematic extracellular protein map, the IgSF interactome. Using a high-throughput technology to interrogate most single transmembrane receptors for binding to 445 IgSF proteins, we identify over 500 interactions, 82% previously undocumented, and confirm more than 60 receptor-ligand pairs using orthogonal assays. Our study reveals a map of cell-type-specific interactions and the landscape of dysregulated receptor-ligand crosstalk in cancer, including selective loss of function for tumor-associated mutations. Furthermore, investigation of the IgSF interactome in a large cohort of cancer patients identifies interacting protein signatures associated with clinical outcome. The IgSF interactome represents an important resource to fuel biological discoveries and a framework for understanding the functional organization of the surfaceome during homeostasis and disease, ultimately informing therapeutic development.

Immunoglobulin germline gene polymorphisms influence the function of SARS-CoV-2 neutralizing antibodies.

The human immunoglobulin heavy-chain (IGH) locus is exceptionally polymorphic, with high levels of allelic and structural variation. Thus, germline IGH genotypes are personal, which may influence responses to infection and vaccination. For an improved understanding of inter-individual differences in antibody responses, we isolated SARS-CoV-2 spike-specific monoclonal antibodies from convalescent health care workers, focusing on the IGHV1-69 gene, which has the highest level of allelic variation of all IGHV genes. The IGHV1-69∗20-using CAB-I47 antibody and two similar antibodies isolated from an independent donor were critically dependent on allele usage. Neutralization was retained when reverting the V region to the germline IGHV1-69∗20 allele but lost when reverting to other IGHV1-69 alleles. Structural data confirmed that two germline-encoded polymorphisms, R50 and F55, in the IGHV1-69 gene were required for high-affinity receptor-binding domain interaction. These results demonstrate that polymorphisms in IGH genes can influence the function of SARS-CoV-2 neutralizing antibodies.

Rhesus and cynomolgus macaque immunoglobulin heavy-chain genotyping yields comprehensive databases of germline VDJ alleles.

Definition of the specific germline immunoglobulin (Ig) alleles present in an individual is a critical first step to delineate the ontogeny and evolution of antigen-specific antibody responses. Rhesus and cynomolgus macaques are important animal models for pre-clinical studies, with four main sub-groups being used: Indian- and Chinese-origin rhesus macaques and Mauritian and Indonesian cynomolgus macaques. We applied the (Ig) gene inference tool IgDiscover and performed extensive Sanger sequencing-based genomic validation to define germline VDJ alleles in these 4 sub-groups, comprising 45 macaques in total. There was allelic overlap between Chinese- and Indian-origin rhesus macaques and also between the two macaque species, which is consistent with substantial admixture. The island-restricted Mauritian cynomolgus population displayed the lowest number of alleles of the sub-groups, yet maintained high individual allelic diversity. These comprehensive databases of germline IGH alleles for rhesus and cynomolgus macaques provide a resource toward the study of B cell responses in these important pre-clinical models.

The cholesterol metabolite 25-hydroxycholesterol restrains the transcriptional regulator SREBP2 and limits intestinal IgA plasma cell differentiation.

Diets high in cholesterol alter intestinal immunity. Here, we examined how the cholesterol metabolite 25-hydroxycholesterol (25-HC) impacts the intestinal B cell response. Mice lacking cholesterol 25-hydroxylase (CH25H), the enzyme generating 25-HC, had higher frequencies of immunoglobulin A (IgA)-secreting antigen-specific B cells upon immunization or infection. 25-HC did not affect class-switch recombination but rather restrained plasma cell (PC) differentiation. 25-HC was produced by follicular dendritic cells and increased in response to dietary cholesterol. Mechanistically, 25-HC restricted activation of the sterol-sensing transcription factor SREBP2, thereby regulating B cell cholesterol biosynthesis. Ectopic expression of SREBP2 in germinal center B cells induced rapid PC differentiation, whereas SREBP2 deficiency reduced PC output in vitro and in vivo. High-cholesterol diet impaired, whereas Ch25h deficiency enhanced, the IgA response against Salmonella and the resulting protection from systemic bacterial dissemination. Thus, a 25-HC-SREBP2 axis shapes the humoral response at the intestinal barrier, providing insight into the effect of high dietary cholesterol in intestinal immunity.

Control of Synaptic Connectivity by a Network of Drosophila IgSF Cell Surface Proteins.

We have defined a network of interacting Drosophila cell surface proteins in which a 21-member IgSF subfamily, the Dprs, binds to a nine-member subfamily, the DIPs. The structural basis of the Dpr-DIP interaction code appears to be dictated by shape complementarity within the Dpr-DIP binding interface. Each of the six dpr and DIP genes examined here is expressed by a unique subset of larval and pupal neurons. In the neuromuscular system, interactions between Dpr11 and DIP-γ affect presynaptic terminal development, trophic factor responses, and neurotransmission. In the visual system, dpr11 is selectively expressed by R7 photoreceptors that use Rh4 opsin (yR7s). Their primary synaptic targets, Dm8 amacrine neurons, express DIP-γ. In dpr11 or DIP-γ mutants, yR7 terminals extend beyond their normal termination zones in layer M6 of the medulla. DIP-γ is also required for Dm8 survival or differentiation. Our findings suggest that Dpr-DIP interactions are important determinants of synaptic connectivity.

Next-Generation Sequencing of T and B Cell Receptor Repertoires from COVID-19 Patients Showed Signatures Associated with Severity of Disease.

We profiled adaptive immunity in COVID-19 patients with active infection or after recovery and created a repository of currently >14 million B and T cell receptor (BCR and TCR) sequences from the blood of these patients. The B cell response showed converging IGHV3-driven BCR clusters closely associated with SARS-CoV-2 antibodies. Clonality and skewing of TCR repertoires were associated with interferon type I and III responses, early CD4+ and CD8+ T cell activation, and counterregulation by the co-receptors BTLA, Tim-3, PD-1, TIGIT, and CD73. Tfh, Th17-like, and nonconventional (but not classical antiviral) Th1 cell polarizations were induced. SARS-CoV-2-specific T cell responses were driven by TCR clusters shared between patients with a characteristic trajectory of clonotypes and traceability over the disease course. Our data provide fundamental insight into adaptive immunity to SARS-CoV-2 with the actively updated repository providing a resource for the scientific community urgently needed to inform therapeutic concepts and vaccine development.

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.

Immunomodulatory and anti-inflammatory properties of immunoglobulin G antibodies.

Antibodies provide an essential layer of protection from infection and reinfection with microbial pathogens. An impaired ability to produce antibodies results in immunodeficiency and necessitates the constant substitution with pooled serum antibodies from healthy donors. Among the five antibody isotypes in humans and mice, immunoglobulin G (IgG) antibodies are the most potent anti-microbial antibody isotype due to their long half-life, their ability to penetrate almost all tissues and due to their ability to trigger a wide variety of effector functions. Of note, individuals suffering from IgG deficiency frequently produce self-reactive antibodies, suggesting that a normal serum IgG level also may contribute to maintaining self-tolerance. Indeed, the substitution of immunodeficient patients with pooled serum IgG fractions from healthy donors, also referred to as intravenous immunoglobulin G (IVIg) therapy, not only protects the patient from infection but also diminishes autoantibody induced pathology, providing more direct evidence that IgG antibodies play an active role in maintaining tolerance during the steady state and during resolution of inflammation. The aim of this review is to discuss different conceptual models that may explain how serum IgG or IVIg can contribute to maintaining a balanced immune response. We will focus on pathways depending on the IgG fragment crystallizable (Fc) as pre-clinical data in various mouse model systems as well as human clinical data have demonstrated that the IgG Fc-domain recapitulates the ability of intact IVIg with respect to its ability to trigger resolution of inflammation. We will further discuss how the findings already have or are in the process of being translated to novel therapeutic approaches to substitute IVIg in treating autoimmune inflammation.

Nanobody-based heavy chain antibodies and chimeric antibodies.

Nanobodies are the products of an intriguing invention in the evolution of immunoglobulins. This invention can be traced back approximately 45 million years to the common ancestor of extant dromedaries, camels, llamas, and alpacas. Next to conventional heterotetrameric H2L2 antibodies, these camelids produce homodimeric nanobody-based heavy chain antibodies, composed of shortened heavy chains that a lack the CH1 domain. Nanobodies against human target antigens are derived from immunized animals and/or synthetic nanobody libraries. As a robust, highly soluble, single immunoglobulin domain, a nanobody can easily be fused to another protein, for example to another nanobody and/or the hinge and constant domains of other immunoglobulins. Nanobody-derived heavy chain antibodies hold promise as a new form of immunotherapeutics.

The Janus (dual) model of immunoglobulin isotype evolution: Conservation and plasticity are the defining paradigms.

The study of antibodies in jawed vertebrates (gnathostomes) provides every immunologist with a bird's eye view of how human immunoglobulins (Igs) came into existence and subsequently evolved into their present forms. It is a fascinating Darwinian history of conservation on the one hand and flexibility on the other, exemplified by the Ig heavy chain (H) isotypes IgM and IgD/W, respectively. The cartilaginous fish (e.g., sharks) Igs provide a glimpse of "how everything got off the ground," while the amphibians (e.g., the model Xenopus) reveal how the adaptive immune system made an about face with the emergence of Ig isotype switching and IgG-like structure/function. The evolution of mucosal Igs is a captivating account of malleability, convergence, and conservation, and a call to arms for future study! In between there are spellbinding chronicles of antibody evolution in each class of vertebrates and rather incredible stories of how antibodies can adapt to occupy niches, for example, single-domain variable regions, cold-adapted Igs, convergent mechanisms to dampen antibody function, provision of mucosal defense, and many more. The purpose here is not to provide an encyclopedic examination of antibody evolution, but rather to hit the high points and entice readers to appreciate how things "came to be."

Airway Epithelial Cell-Derived Colony Stimulating Factor-1 Promotes Allergen Sensitization.

Airway epithelial cells (AECs) secrete innate immune cytokines that regulate adaptive immune effector cells. In allergen-sensitized humans and mice, the airway and alveolar microenvironment is enriched with colony stimulating factor-1 (CSF1) in response to allergen exposure. In this study we found that AEC-derived CSF1 had a critical role in the production of allergen reactive-IgE production. Furthermore, spatiotemporally secreted CSF1 regulated the recruitment of alveolar dendritic cells (DCs) and enhanced the migration of conventional DC2s (cDC2s) to the draining lymph node in an interferon regulatory factor 4 (IRF4)-dependent manner. CSF1 selectively upregulated the expression of the chemokine receptor CCR7 on the CSF1R+ cDC2, but not the cDC1, population in response to allergen stimuli. Our data describe the functional specification of CSF1-dependent DC subsets that link the innate and adaptive immune responses in T helper 2 (Th2) cell-mediated allergic lung inflammation.

B cells and the intestinal microbiome in time, space and place.

The gut immune system is shaped by the continuous interaction with the microbiota. Here we dissect temporal, spatial and contextual layers of gut B cell responses. The microbiota impacts on the selection of the developing pool of pre-immune B cells that serves as substrate for B cell activation, expansion and differentiation. However, various aspects of the gut B cell response display unique features. In particular, occurrence of somatically mutated B cells, chronic gut germinal centers in T cell-deficient settings and polyreactive binding of gut IgA to the microbiota questioned the nature and microbiota-specificity of gut germinal centers. We propose a model to reconcile these observations incorporating recent work demonstrating microbiota-specificity of gut germinal centers. We speculate that adjuvant effects of the microbiota might modify permissiveness for B cell to enter and exit gut germinal centers. We propose that separating aspects of time, space and place facilitate the occasionally puzzling discussion of gut B cell responses to the microbiota.

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.