Solution structure of human and mouse immunoglobulin M by synchrotron X-ray scattering and molecular graphics modelling. A possible mechanism for complement activation

Defining the Features of Complement-Active IgM

Immunoglobulin M (IgM) is a class of mammalian antibody that is critical for the early stages of adaptive immunity, and is the most potent Ig-activator of the classical complement cascade. While the relationship between IgM and complement has been appreciated for decades, the structural transitions within IgM upon antigen binding that promote the activation of complement component C1 remain unresolved. Here we examine in vitro complement activation, C1 binding kinetics, and conformational changes within IgM in different antigen-bound states. Binding studies using biolayer interferometry revealed that only in a multivalent complex with a surface-displayed antigen was IgM fully capable of initiating complement activation. Hydrogen/Deuterium exchange with mass spectrometry revealed the predominant structural changes within the Fc domains during transition to the active conformation. Collectively, this work establishes key structural and functional qualities that define the complement-active form of IgM.

Antibody density on bacteria regulates C1q recruitment by monoclonal IgG but not IgM

Antibodies that trigger the complement system play a pivotal role in the immune defense against pathogenic bacteria and offer potential therapeutic avenues for combating antibiotic-resistant bacterial infections, a rising global concern. To gain a deeper understanding of the key parameters regulating complement activation by monoclonal antibodies, we developed a novel bioassay for quantifying classical complement activation at the monoclonal antibody level, and employed this assay to characterize rare complement-activating antibacterial antibodies on the single-antibody level in postimmunization murine antibody repertoires. We characterized monoclonal antibodies from various antibody isotypes against specific pathogenic bacteria (Bordetella pertussis and Neisseria meningitidis) to broaden the scope of our findings. We demonstrated activation of the classical pathway by individual IgM- and IgG-secreting cells, that is, monoclonal IgM and IgG2a/2b/3 subclasses. Additionally, we could observe different epitope density requirements for efficient C1q binding depending on antibody isotype, which is in agreement with previously proposed molecular mechanisms. In short, we found that antibody density most crucially regulated C1q recruitment by monoclonal IgG isotypes, but not IgM isotypes. This study provides additional insights into important parameters for classical complement initiation by monoclonal antibodies, a knowledge that might inform antibody screening and vaccination efforts.

The evolution of flexibility and function in the Fc domains of IgM, IgY, and IgE

Introduction

Antibody Fc regions harbour the binding sites for receptors that mediate effector functions following antigen engagement by the Fab regions. An extended "hinge" region in IgG allows flexibility between Fab and Fc, but in both the most primitive antibody, IgM, and in the evolutionarily more recent IgE, the hinge is replaced by an additional domain pair in the homodimeric six-domain Fc region. This permits additional flexibility within the Fc region, which has been exploited by nature to modulate antibody effector functions. Thus, in pentameric or hexameric IgM, the Fc regions appear to adopt a planar conformation in solution until antigen binding causes a conformational change and exposes the complement binding sites. In contrast, IgE-Fc principally adopts an acutely bent conformation in solution, but the binding of different receptors is controlled by the degree of bending, and there is allosteric communication between receptor binding sites.

Methods

We sought to trace the evolution of Fc conformational diversity from IgM to IgE via the intermediate avian IgY by studying the solution conformations of their Fc regions by small-angle X-ray scattering. We compared four extant proteins: human IgM-Fc homodimer, chicken IgY-Fc, platypus IgE-Fc, and human IgE-Fc. These are examples of proteins that first appeared in the jawed fish [425 million years ago (mya)], tetrapod (310 mya), monotreme (166 mya), and hominid (2.5 mya) clades, respectively.

Results and discussion

We analysed the scattering curves in terms of contributions from a pool of variously bent models chosen by a non-negative linear least-squares algorithm and found that the four proteins form a series in which the proportion of acutely bent material increases: IgM-Fc < IgY-Fc < plIgE-Fc < huIgE-Fc. This follows their order of appearance in evolution. For the huIgM-Fc homodimer, although none are acutely bent, and a significant fraction of the protein is sufficiently bent to expose the C1q-binding site, it predominantly adopts a fully extended conformation. In contrast, huIgE-Fc is found principally to be acutely bent, as expected from earlier studies. IgY-Fc, in this first structural analysis of the complete Fc region, exhibits an ensemble of conformations from acutely bent to fully extended, reflecting IgY's position as an evolutionary intermediate between IgM and IgE.

Engineered IgM and IgG cleaving enzymes for mitigating antibody neutralization and complement activation in AAV gene transfer

Systemic dosing of adeno-associated viral (AAV) vectors poses potential risk of adverse side effects including complement activation triggered by anti-capsid immunity. Due to the multifactorial nature of toxicities observed in this setting, a wide spectrum of immune modulatory regimens are being investigated in the clinic. Here, we discover an IgM cleaving enzyme (IceM) that degrades human IgM, a key trigger in the anti-AAV immune cascade. We then engineer a fusion enzyme (IceMG) with dual proteolytic activity against human IgM and IgG. IceMG cleaves B cell surface antigen receptors and inactivates phospholipase gamma signaling in vitro. Importantly, IceMG is more effective at inhibiting complement activation compared with an IgG cleaving enzyme alone. Upon IV dosing, IceMG rapidly and reversibly clears circulating IgM and IgG in macaques. Antisera from these animals treated with IceMG shows decreased ability to neutralize AAV and activate complement. Consistently, pre-conditioning with IceMG restores AAV transduction in mice passively immunized with human antisera. Thus, IgM cleaving enzymes show promise in simultaneously addressing multiple aspects of anti-AAV immunity mediated by B cells, circulating antibodies and complement. These studies have implications for improving safety of AAV gene therapies and possibly broader applications including organ transplantation and autoimmune diseases.

Interplay between Liposomes and IgM: Principles, Challenges, and Opportunities

Liposomes have received tremendous attention as a class of versatile pharmaceutical vehicles of great potential over the past several decades. However, the application of liposomes encounters major challenges due to the knowledge gaps in their in vivo delivery process. Immunoglobulin M (IgM) displays both pervasiveness and complexity in regulating the biological functions as well as eliciting adverse effects of liposomes. Understanding, mitigating, and exploiting the duality of IgM are prerequisites for achieving various biomedical applications of liposomes. In this review, the intricate relationship between liposomes and their biological environments has been summarized, with an emphasis on the regulatory effects of IgM on in vivo performance of liposomes. Corresponding solutions have also been discussed to evade IgM-mediated opsonization for safe and efficient drug delivery.

The primitive complement system in molluscs

The complement system is an important immune defense mechanism that plays essential roles in both innate and adaptive immunity of vertebrates. Since complement components are identified in deuterostome and even primitive protostome species, the origin and evolution of complement system in invertebrates have been of great interest. Recently, research on the complement system in mollusc immunity has been increasing due to their importance in worldwide aquaculture, and their phylogenetic position. Complement components including C3, C1q domain containing protein (C1qDCP), C-type lectin (CTL), ficolin-like, mannose-binding lectin (MBL)-associated serine proteases like (MASPL), and factor B have been identified, suggesting the existence of complement system in molluscs. The lectin pathway has been outlined in molluscs, which is initiated by CTL with CCP domain and MASPL protein to generate C3 cleavage fragments. The molluscan C1qDCP exhibits the capability to bind human IgG, indicating the existence of possible C1qDCP-mediated activation pathway in molluscs. The activation of C3 regulates the expressions of immune effectors (cytokines and antibacterial peptides), mediates the haemocyte phagocytosis, and inhibits the bacterial growth. Some MACPF domain containing proteins may replace the missing terminal pathway in molluscs. This article provides a review of complement system in molluscs, including its components, activation mechanisms and functions in the immune response of molluscs.

A Zika virus-specific IgM elicited in pregnancy exhibits ultrapotent neutralization

Congenital Zika virus (ZIKV) infection results in neurodevelopmental deficits in up to 14% of infants born to ZIKV-infected mothers. Neutralizing antibodies are a critical component of protective immunity. Here, we demonstrate that plasma IgM contributes to ZIKV immunity in pregnancy, mediating neutralization up to 3 months post-symptoms. From a ZIKV-infected pregnant woman, we isolated a pentameric ZIKV-specific IgM (DH1017.IgM) that exhibited ultrapotent ZIKV neutralization dependent on the IgM isotype. DH1017.IgM targets an envelope dimer epitope within domain II. The epitope arrangement on the virion is compatible with concurrent engagement of all ten antigen-binding sites of DH1017.IgM, a solution not available to IgG. DH1017.IgM protected mice against viremia upon lethal ZIKV challenge more efficiently than when expressed as an IgG. Our findings identify a role for antibodies of the IgM isotype in protection against ZIKV and posit DH1017.IgM as a safe and effective candidate immunotherapeutic, particularly during pregnancy.

Modular Nucleic Acid Scaffolds for Synthesizing Monodisperse and Sequence-Encoded Antibody Oligomers

Synthesizing protein oligomers that contain exact numbers of multiple different proteins in defined architectures is challenging. DNA-DNA interactions can be used to program protein assembly into oligomers; however, existing methods require changes to DNA design to achieve different numbers and oligomeric sequences of proteins. Herein, we develop a modular DNA scaffold that uses only six synthetic oligonucleotides to organize proteins into defined oligomers. As a proof-of-concept, model proteins (antibodies) are oligomerized into dimers and trimers, where antibody function is retained. Illustrating the modularity of this technique, dimer and trimer building blocks are then assembled into pentamers containing three different antibodies in an exact stoichiometry and oligomeric sequence. In sum, this report describes a generalizable method for organizing proteins into monodisperse, sequence-encoded oligomers using DNA. This advance will enable studies into how oligomeric protein sequences affect material properties in areas spanning pharmaceutical development, cascade catalysis, synthetic photosynthesis, and membrane transport.

Reconstruction of 3D density from solution scattering

Ab initio modeling methods have proven to be powerful means of interpreting solution scattering data. In the absence of atomic models, or complementary to them, ab initio modeling approaches can be used for generating low-resolution particle envelopes using only solution scattering profiles. Recently, a new ab initio reconstruction algorithm has been introduced to the scientific community, called DENSS. DENSS is unique among ab initio modeling algorithms in that it solves the inverse scattering problem, i.e., the 1D scattering intensities are directly used to determine the 3D particle density. The reconstruction of particle density has several advantages over conventional uniform density modeling approaches, including the ability to reconstruct a much wider range of particle types and the ability to visualize low-resolution density fluctuations inside the particle envelope. In this chapter we will discuss the theory behind this new approach, how to use DENSS, and how to interpret the results. Several examples with experimental and simulated data will be provided.

Nanometer- and angstrom-scale characteristics that modulate complement responses to nanoparticles

The contribution of the complement system to non-specific host defence and maintenance of homeostasis is well appreciated. Many particulate systems trigger complement activation but the underlying mechanisms are still poorly understood. Activation of the complement cascade could lead to particle opsonisation by the cleavage products of the third complement protein and might promote inflammatory reactions. Antibody binding in a controlled manner and/or sensing of particles by the complement pattern-recognition molecules such as C1q and mannose-binding lectin can trigger complement activation. Particle curvature and spacing arrangement/periodicity of surface functional groups/ligands are two important parameters that modulate complement responses through multivalent engagement with and conformational regulation of surface-bound antibodies and complement pattern-recognition molecules. Thus, a better fundamental understanding of nanometer- and angstrom-scale parameters that modulate particle interaction with antibodies and complement proteins could portend new possibilities for engineering of particulate drug carriers and biomedical platforms with tuneable complement responses and is discussed here.

Conjugated Detergent Micelles as a Platform for IgM Purification

Immunoglobulin M (IgM) antibodies hold promise as anticancer drugs and as agents for promoting immune homeostasis. This promise has not been realized due to low expression levels in mammalian cells producing IgM class antibodies, and the failure of protein A chromatography for IgM purification. Here, we describe a nonchromatographic platform for quantitatively capturing IgMs at neutral pH, which is then recovered with 86%–94% yield and >95% purity at pH 3. The platform contains micelles conjugated with the [(bathophenanthroline)₃:Fe²⁺] amphiphilic complex. Inclusion of amino acid monomers, for example, phenylalanine or tyrosine, during conjugation of detergent micelles, allows subsequent extraction of IgMs at close to neutral pH. With the successful implementation of this purification platform for both polyclonal humans and bovine IgMs, we anticipate similar results for monoclonal IgMs, most relevant for the pharmaceutical industry.

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.

Therapeutic antibodies for COVID-19: is a new age of IgM, IgA and bispecific antibodies coming?

Early humoral immune responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are dominated by IgM and IgA antibodies, which greatly contribute to virus neutralization at mucosal sites. Given the essential roles of IgM and IgA in the control and elimination of SARS-CoV-2 infection, the mucosal immunity could be exploited for therapeutic and prophylactic purposes. However, almost all neutralizing antibodies that are authorized for emergency use and under clinical development are IgG antibodies, and no vaccine has been developed to boost mucosal immunity for SARS-CoV-2 infection. In addition to IgM and IgA, bispecific antibodies (bsAbs) combine specificities of two antibodies in one molecule, representing an important alternative to monoclonal antibody cocktails. Here, we summarize the latest advances in studies on IgM, IgA and bsAbs against SARS-CoV-2. The current challenges and future directions in vaccine design and antibody-based therapeutics are also discussed.

Extending Native Top-Down Electron Capture Dissociation to MDa Immunoglobulin Complexes Provides Useful Sequence Tags Covering Their Critical Variable Complementarity-Determining Regions

Native top-down mass spectrometry (MS) is gaining traction for the analysis and sequencing of intact proteins and protein assemblies, giving access to their mass and composition, as well as sequence information useful for identification. Herein, we extend and apply native top-down MS, using electron capture dissociation, to two submillion Da IgM- and IgG-based oligomeric immunoglobulins. Despite structural similarities, these two systems are quite different. The ∼895 kDa noncovalent IgG hexamer consists of six IgG subunits hexamerizing in solution due to three specifically engineered mutations in the Fc region, whereas the ∼935 kDa IgM oligomer results from the covalent assembly of one joining (J) chain and 5 IgM subunits into an asymmetric “pentamer” stabilized by interchain disulfide bridges. Notwithstanding their size, structural differences, and complexity, we observe that their top-down electron capture dissociation spectra are quite similar and straightforward to interpret, specifically providing informative sequence tags covering the highly variable CDR3s and FR4s of the Ig subunits they contain. Moreover, we show that the electron capture dissociation fragmentation spectra of immunoglobulin oligomers are essentially identical to those obtained for their respective monomers. Demonstrated for recombinantly produced systems, the approach described here opens up new prospects for the characterization and identification of IgMs circulating in plasma, which is important since IgMs play a critical role in the early immune response to pathogens such as viruses and bacteria.

Structure, Function, and Therapeutic Use of IgM Antibodies

Natural immunoglobulin M (IgM) antibodies are pentameric or hexameric macro-immunoglobulins and have been highly conserved during evolution. IgMs are initially expressed during B cell ontogeny and are the first antibodies secreted following exposure to foreign antigens. The IgM multimer has either 10 (pentamer) or 12 (hexamer) antigen binding domains consisting of paired µ heavy chains with four constant domains, each with a single variable domain, paired with a corresponding light chain. Although the antigen binding affinities of natural IgM antibodies are typically lower than IgG, their polyvalency allows for high avidity binding and efficient engagement of complement to induce complement-dependent cell lysis. The high avidity of IgM antibodies renders them particularly efficient at binding antigens present at low levels, and non-protein antigens, for example, carbohydrates or lipids present on microbial surfaces. Pentameric IgM antibodies also contain a joining (J) chain that stabilizes the pentameric structure and enables binding to several receptors. One such receptor, the polymeric immunoglobulin receptor (pIgR), is responsible for transcytosis from the vasculature to the mucosal surfaces of the lung and gastrointestinal tract. Several naturally occurring IgM antibodies have been explored as therapeutics in clinical trials, and a new class of molecules, engineered IgM antibodies with enhanced binding and/or additional functional properties are being evaluated in humans. Here, we review the considerable progress that has been made regarding the understanding of biology, structure, function, manufacturing, and therapeutic potential of IgM antibodies since their discovery more than 80 years ago.

Immunoglobulins or Antibodies: IMGT® Bridging Genes, Structures and Functions

IMGT®, the international ImMunoGeneTics® information system founded in 1989 by Marie-Paule Lefranc (Université de Montpellier and CNRS), marked the advent of immunoinformatics, a new science at the interface between immunogenetics and bioinformatics. For the first time, the immunoglobulin (IG) or antibody and T cell receptor (TR) genes were officially recognized as 'genes' as well as were conventional genes. This major breakthrough has allowed the entry, in genomic databases, of the IG and TR variable (V), diversity (D) and joining (J) genes and alleles of Homo sapiens and of other jawed vertebrate species, based on the CLASSIFICATION axiom. The second major breakthrough has been the IMGT unique numbering and the IMGT Collier de Perles for the V and constant (C) domains of the IG and TR and other proteins of the IG superfamily (IgSF), based on the NUMEROTATION axiom. IMGT-ONTOLOGY axioms and concepts bridge genes, sequences, structures and functions, between biological and computational spheres in the IMGT® system (Web resources, databases and tools). They provide the IMGT Scientific chart rules to identify, to describe and to analyse the IG complex molecular data, the huge diversity of repertoires, the genetic (alleles, allotypes, CNV) polymorphisms, the IG dual function (paratope/epitope, effector properties), the antibody humanization and engineering.

Not all therapeutic antibody isotypes are equal: the case of IgM versus IgG in Pertuzumab and Trastuzumab

The therapeutic potential of immunoglobulin M (IgM) is of considerable interest in immunotherapy due to its complement-activating and cell-agglutinating abilities. Pertuzumab and Trastuzumab are monoclonal antibodies used to treat human epidermal growth factor receptor 2 (HER2)-positive breast cancer but exhibit significantly different binding affinities as IgM when compared to its IgG isotype. Using integrative multiscale modelling and simulations of complete antibody assemblies, we show that Pertuzumab IgM is able to utilize all of its V-regions to bind multiple HER2 receptors simultaneously, while similar binding in Trastuzumab IgM is prohibited by steric clashes caused by the large globular domain of HER2. This is subsequently validated by confirming that Pertuzumab IgM inhibits proliferation in HER2 over-expressing live cells more effectively than its IgG counterpart and Trastuzumab IgM. Our study highlights the importance of understanding the molecular details of antibody-antigen interactions for the design and isotype selection of therapeutic antibodies.

Antibody Complexes

Monoclonal based therapeutics have always been looked at as a futuristic natural way we could take care of pathogens and many diseases. However, in order to develop, establish and realize monoclonal based therapy we need to understand how the immune system contains or kill pathogens. Antibody complexes serve the means to decode this black box. We have discussed examples of antibody complexes both at biochemical and structural levels to understand and appreciate how discoveries in the field of antibody complexes have started to decoded mechanism of viral invasion and create potential vaccine targets against many pathogens. Antibody complexes have made advancement in our knowledge about the molecular interaction between antibody and antigen. It has also led to identification of potent protective monoclonal antibodies. Further use of selective combination of monoclonal antibodies have provided improved protection against deadly diseases. The administration of newly designed and improved immunogen has been used as potential vaccine. Therefore, antibody complexes are important tools to develop new vaccine targets and design an improved combination of monoclonal antibodies for passive immunization or protection with very little or no side effects.

Peptides in proteins

During evolution C-terminal peptide extensions were added to proteins on the gene level. These convey additional functions such as interaction with partner proteins or oligomerisation. IgM antibodies and molecular chaperones are two prominent examples discussed.

Mapping of the binding site for FcμR in human IgM-Fc

FcμR is a high-affinity receptor for the Fc portion of human IgM. It participates in B cell activation, cell survival and proliferation, but the full range of its functions remains to be elucidated. The receptor has an extracellular immunoglobulin (Ig)-like domain homologous to those in Fcα/μR and pIgR, but unlike these two other IgM receptors which also bind IgA, FcμR exhibits a binding specificity for only IgM-Fc. Previous studies have suggested that the IgM/FcμR interaction mainly involves the Cμ4 domains with possible contributions from either Cμ3 or Cμ2. To define the binding site more precisely, we generated three recombinant IgM-Fc proteins with specific mutations in the Cμ3 and Cμ4 domains, as well as a construct lacking the Cμ2 domains, and analyzed their interaction with the extracellular Ig-like domain of FcμR using surface plasmon resonance analysis. There is a binding site for FcμR in each IgM heavy chain. Neither the absence of the Cμ2 domains nor the quadruple mutant D340S/Q341G/D342S/T343S (in Cμ3 adjacent to Cμ2) affected FcμR binding, whereas double mutant K361D/D416R (in Cμ3 at the Cμ4 interface) substantially decreased binding, and a single mutation Q510R (in Cμ4) completely abolished FcμR binding. We conclude that glutamine at position 510 in Cμ4 is critical for IgM binding to FcμR. This will facilitate discrimination between the distinct effects of FcμR interactions with soluble IgM and with the IgM BCR.

Immunoglobulin G structure and rheumatoid factor epitopes

Antibodies are important for immunity and exist in several classes (IgM, IgD, IgA, IgG, IgE). They are composed of symmetric dimeric molecules with two antigen binding regions (Fab) and a constant part (Fc), usually depicted as Y-shaped molecules. Rheumatoid factors found in patients with rheumatoid arthritis are autoantibodies binding to IgG and paradoxically appear to circulate in blood alongside with their antigen (IgG) without reacting with it. Here, it is shown that rheumatoid factors do not react with native IgG in solution, and that their epitopes only become accessible upon certain physico-chemical treatments (e.g. heat treatment at 57 °C), by physical adsorption on a hydrophobic surface or by antigen binding. Moreover, chemical cross-linking in combination with mass spectrometry showed that the native state of IgG is a compact (closed) form and that the Fab parts of IgG shield the Fc region and thereby control access of rheumatoid factors and presumably also some effector functions. It can be inferred that antibody binding to pathogen surfaces induces a conformational change, which exposes the Fc part with its effector sites and rheumatoid factor epitopes. This has strong implications for understanding antibody structure and physiology and necessitates a conceptual reformulation of IgG models.

Insights into IgM-mediated complement activation based on in situ structures of IgM-C1-C4b

IgM antibodies protect mammals against humoral microbial infection and mediate clearance of cellular debris. IgM activates the immune complement system only after binding to cell-surface antigens. Here we report the in situ 3D structures of surface-antigen–bound IgM antibodies in complex with both C1 and C4b complement components. The data indicate the structural arrangement of pentameric and hexameric IgM upon antigen binding, exposing the C1q-binding sites with both adopting hexagonal symmetry. The structures reveal the entire C1qr₂s₂ complex and elucidate several protein–protein interactions with C4b and IgM. Based on the structural data, we hypothesize a C1q-transmitted surface trigger that activates C1, leading to C4 cleavage and C4b deposition on membranes.

Antigen binding by serum Ig-M (IgM) protects against microbial infections and helps to prevent autoimmunity, but causes life-threatening diseases when mistargeted. How antigen-bound IgM activates complement-immune responses remains unclear. We present cryoelectron tomography structures of IgM, C1, and C4b complexes formed on antigen-bearing lipid membranes by normal human serum at 4 °C. The IgM-C1-C4b complexes revealed C4b product release as the temperature-limiting step in complement activation. Both IgM hexamers and pentamers adopted hexagonal, dome-shaped structures with Fab pairs, dimerized by hinge domains, bound to surface antigens that support a platform of Fc regions. C1 binds IgM through widely spread C1q-collagen helices, with C1r proteases pointing outward and C1s bending downward and interacting with surface-attached C4b, which further interacts with the adjacent IgM-Fab₂ and globular C1q-recognition unit. Based on these data, we present mechanistic models for antibody-mediated, C1q-transmitted activation of C1 and for C4b deposition, while further conformational rearrangements are required to form C3 convertases.

Kingella kingae Surface Polysaccharides Promote Resistance to Human Serum and Virulence in a Juvenile Rat Model

Kingella kingae is a Gram-negative coccobacillus that is increasingly being recognized as an important cause of invasive disease in young children. The pathogenesis of K. kingae disease begins with colonization of the oropharynx, followed by invasion of the bloodstream, survival in the intravascular space, and dissemination to distant sites. Recent studies have revealed that K. kingae produces a number of surface factors that may contribute to the pathogenic process, including a polysaccharide capsule and an exopolysaccharide. In this study, we observed that K. kingae was highly resistant to the bactericidal effects of human serum complement. Using mutant strains deficient in expression of capsule, exopolysaccharide, or both in assays with human serum, we found that elimination of both capsule and exopolysaccharide was required for efficient binding of IgG, IgM, C4b, and C3b to the bacterial surface and for complement-mediated killing. Abrogation of the classical complement pathway using EGTA-treated human serum restored survival to wild-type levels by the mutant lacking both capsule and exopolysaccharide, demonstrating that capsule and exopolysaccharide promote resistance to the classical complement pathway. Consistent with these results, loss of both capsule and exopolysaccharide eliminated invasive disease in juvenile rats with an intact complement system but not in rats lacking complement. Based on these observations, we conclude that the capsule and the exopolysaccharide have important redundant roles in promoting survival of K. kingae in human serum. Each of these surface factors is sufficient by itself to fully prevent serum opsonin deposition and complement-mediated killing of K. kingae, ultimately facilitating intravascular survival and promoting K. kingae invasive disease.

Structures of C1-IgG1 provide insights into how danger pattern recognition activates complement

Danger patterns on microbes or damaged host cells bind and activate C1, inducing innate immune responses and clearance through the complement cascade. How these patterns trigger complement initiation remains elusive. Here, we present cryo-electron microscopy analyses of C1 bound to monoclonal antibodies in which we observed heterogeneous structures of single and clustered C1-immunoglobulin G1 (IgG1) hexamer complexes. Distinct C1q binding sites are observed on the two Fc-CH2 domains of each IgG molecule. These are consistent with known interactions and also reveal additional interactions, which are supported by functional IgG1-mutant analysis. Upon antibody binding, the C1q arms condense, inducing rearrangements of the C1r₂s₂ proteases and tilting C1q's cone-shaped stalk. The data suggest that C1r may activate C1s within single, strained C1 complexes or between neighboring C1 complexes on surfaces.

Mouse Antibody of IgM Class is Prone to Non-Enzymatic Cleavage between CH1 and CH2 Domains

IgM is a multivalent antibody which evolved as a first line defense of adaptive immunity. It consists of heavy and light chains assembled into a complex oligomer. In mouse serum there are two forms of IgM, a full-length and a truncated one. The latter contains μ’ chain, which lacks a variable region. Although μ’ chain was discovered many years ago, its origin has not yet been elucidated. Our results indicate that μ’ chain is generated from a full-length heavy chain by non-enzymatic cleavage of the protein backbone. The cleavage occurred specifically after Asn209 and is prevented by mutating this residue into any other amino acid. The process requires the presence of other proteins, preferentially with an acidic isoelectric point, and is facilitated by neutral or alkaline pH. This unique characteristic of the investigated phenomenon distinguishes it from other, already described, Asn-dependent protein reactions. A single IgM molecule is able to bind up to 12 epitopes via its antigen binding fragments (Fabs). The cleavage at Asn209 generates truncated IgM molecules and free Fabs, resulting in a reduced IgM valence and probably affecting IgM functionality in vivo.

Atomistic modelling of scattering data in the Collaborative Computational Project for Small Angle Scattering (CCP-SAS)

The capabilities of current computer simulations provide a unique opportunity to model small-angle scattering (SAS) data at the atomistic level, and to include other structural constraints ranging from molecular and atomistic energetics to crystallography, electron microscopy and NMR. This extends the capabilities of solution scattering and provides deeper insights into the physics and chemistry of the systems studied. Realizing this potential, however, requires integrating the experimental data with a new generation of modelling software. To achieve this, the CCP-SAS collaboration (http://www.ccpsas.org/) is developing open-source, high-throughput and user-friendly software for the atomistic and coarse-grained molecular modelling of scattering data. Robust state-of-the-art molecular simulation engines and molecular dynamics and Monte Carlo force fields provide constraints to the solution structure inferred from the small-angle scattering data, which incorporates the known physical chemistry of the system. The implementation of this software suite involves a tiered approach in which GenApp provides the deployment infrastructure for running applications on both standard and high-performance computing hardware, and SASSIE provides a workflow framework into which modules can be plugged to prepare structures, carry out simulations, calculate theoretical scattering data and compare results with experimental data. GenApp produces the accessible web-based front end termed SASSIE-web, and GenApp and SASSIE also make community SAS codes available. Applications are illustrated by case studies: (i) inter-domain flexibility in two- to six-domain proteins as exemplified by HIV-1 Gag, MASP and ubiquitin; (ii) the hinge conformation in human IgG2 and IgA1 antibodies; (iii) the complex formed between a hexameric protein Hfq and mRNA; and (iv) synthetic 'bottlebrush' polymers.

Dependence of the volume of an antibody on the force applied in a force microscopy experiment in liquid

The volume of a protein can be estimated from its molecular weight. This approach has also been applied in force microscopy experiments. Two factors contribute to the determination of the volume from a force microscope image, the applied force and the tip radius. Those factors act in opposite directions. Here, we demonstrate that in the optimum conditions to image a protein, the apparent volume deduced from an AFM image overestimates the real protein volume. The lateral broadening due to the tip finite size, makes the simulated volume to exceed the real protein volume value, while the force applied by the tip tends to decrease the measured volume. The measured volume could coincide with the real volume for either a point-size tip at zero force or when the compression exerted by the tip compensates its dilation effects. The interplay between the above factors make unsuitable to apply the molecular weight method to determine the volume of a protein from AFM data.

Complement inhibitors to treat IgM-mediated autoimmune hemolysis

Complement activation in autoimmune hemolytic anemia may exacerbate extravascular hemolysis and may occasionally result in intravascular hemolysis. IgM autoantibodies as characteristically found in cold autoantibody autoimmune hemolytic anemia, in cold agglutinin disease but also in a considerable percentage of patients with warm autoantibodies are very likely to activate complement in vivo. Therapy of IgM-mediated autoimmune hemolytic anemia mainly aims to decrease autoantibody production. However, most of these treatments require time to become effective and will not stop immediate ongoing complement-mediated hemolysis nor prevent hemolysis of transfused red blood cells. Therefore pharmacological inhibition of the complement system might be a suitable approach to halt or at least attenuate ongoing hemolysis and improve the recovery of red blood cell transfusion in autoimmune hemolytic anemia. In recent years, several complement inhibitors have become available in the clinic, some of them with proven efficacy in autoimmune hemolytic anemia. In the present review, we give a short introduction on the pathogenesis of autoimmune hemolytic anemia, followed by an overview on the complement system with a special focus on its regulation. Finally, we will discuss complement inhibitors with regard to their potential efficacy to halt or attenuate hemolysis in complement-mediated autoimmune hemolytic anemia.

Architecture of Human IgM in Complex with P. falciparum Erythrocyte Membrane Protein 1

Plasmodium falciparum virulence is associated with sequestration of infected erythrocytes. Microvascular binding mediated by PfEMP1 in complex with non-immune immunoglobulin M (IgM) is common among parasites that cause both severe childhood malaria and pregnancy-associated malaria. Here, we present cryo-molecular electron tomography structures of human IgM, PfEMP1 and their complex. Three-dimensional reconstructions of IgM reveal that it has a dome-like core, randomly oriented Fab2s units, and the overall shape of a turtle. PfEMP1 is a C- shaped molecule with a flexible N terminus followed by an arc-shaped backbone and a bulky C terminus that interacts with IgM. Our data demonstrate that the PfEMP1 binding pockets on IgM overlap with those of C1q, and the bulkiness of PfEMP1 limits the capacity of IgM to interact with PfEMP1. We suggest that P. falciparum exploits IgM to cluster PfEMP1 into an organized matrix to augment its affinity to host cell receptors.

Transcriptomic analysis and biomarkers (Rag1 and Igμ) for probing the immune system development in Pacific cod, Gadus macrocephalus

Mortality (>90%) is a big concern in larval rearing facilities of Pacific cod, Gadus macrocephalus, limiting its culture presently still in the experimental stages. Understanding the immune system development of G. macrocephalus is crucial to optimize the aquaculture of this species, to improve the use of economic resources and to avoid abuse of antibiotics. For the transcriptome analysis, using an Illumina sequencing platform, 61,775,698 raw reads were acquired. After a de novo assembly, 77,561 unigenes were obtained. We have classified functionally these transcripts by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). 27 genes mainly related to hematopoietic or lymphoid organ development and somatic diversification of immune receptors have been reported for the first time in Pacific cod, and 14 Ig heavy chain (μ chain) locuses were assembled using Trinity. Based on our previous achievement, we have chosen Rag1 and Igμ as immune system development biomarkers. Full length cDNA of Rag1 and Igμ as biomarkers were obtained respectively using RACE PCR. Concerning Rag1, the deduced amino acid of Rag1 and protein immunodetection revealed a Rag1 isoform of 69 kDa, significantly different from other fish orthologs, such as Oncorhynchus mykiss (121 kDa). Phylogenetic analysis reveals a unique immune system for the Gadus genre, not exclusive for Atlantic cod, among vertebrates. Meanwhile, full length cDNA of Igμ included an ORF of 1710 bp and the deduced amino acid was composed of a leader peptide, a variable domain, CH1, CH2, Hinge, CH3, CH4 and C-terminus, which was in accordance with most teleost. Absolute quantification PCR revealed that significant expression of Rag1 appeared earlier than Igμ, 61 and 95 dph compared to 95 dph, respectively. Here we report the first transcriptomic analysis of G. macrocephalus as the starting point for genetic research on immune system development towards improving the Pacific cod aquaculture.

SCT: a suite of programs for comparing atomistic models with small-angle scattering data

Small-angle X-ray and neutron scattering techniques characterize proteins in solution and complement high-resolution structural studies. They are of particular utility when large proteins cannot be crystallized or when the structure is altered by solution conditions. Atomistic models of the averaged structure can be generated through constrained modelling, a technique in which known domain or subunit structures are combined with linker models to produce candidate global conformations. By randomizing the configuration adopted by the different elements of the model, thousands of candidate structures are produced. Next, theoretical scattering curves are generated for each model for trial-and-error fits to the experimental data. From these, a small family of best-fit models is identified. In order to facilitate both the computation of theoretical scattering curves from atomistic models and their comparison with experiment, the SCT suite of tools was developed. SCT also includes programs that provide sequence-based estimates of protein volume (either incorporating hydration or not) and add a hydration layer to models for X-ray scattering modelling. The original SCT software, written in Fortran, resulted in the first atomistic scattering structures to be deposited in the Protein Data Bank, and 77 structures for antibodies, complement proteins and anionic oligosaccharides were determined between 1998 and 2014. For the first time, this software is publicly available, alongside an easier-to-use reimplementation of the same algorithms in Python. Both versions of SCT have been released as open-source software under the Apache 2 license and are available for download from https://github.com/dww100/sct.

C1q, antibodies and anti-C1q autoantibodies

The complement system has long been known for its role in combating infections. More recently the complement system is becoming increasingly appreciated for its role in processes that range from waste transport, immune tolerance and shaping of the adaptive immune response. Antibodies represent the humoral part of the adaptive immune response and the complement system interacts with antibodies in several ways. Activated complement fragments impact on the production of antibodies, the complement system gets activated by antibodies and complement proteins can be the target of (auto)antibodies. In this review, written to celebrate the contributions of Prof. Dr. M.R. Daha to the field of immunology and especially complement, we will focus on C1q and its various interactions with antibodies. We will specifically focus on the mechanisms by which C1q will interact with monomeric IgG versus polymerized IgG and fluid-phase IgM versus solid-phase IgM. In addition in this review we will discuss in detail how C1q itself is targeted by autoantibodies and how these autoantibodies are currently considered to play a role in human disease.

Ambiguity assessment of small-angle scattering curves from monodisperse systems

A novel approach is presented for an a priori assessment of the ambiguity associated with spherically averaged single-particle scattering. The approach is of broad interest to the structural biology community, allowing the rapid and model-independent assessment of the inherent non-uniqueness of three-dimensional shape reconstruction from scattering experiments on solutions of biological macromolecules. One-dimensional scattering curves recorded from monodisperse systems are nowadays routinely utilized to generate low-resolution particle shapes, but the potential ambiguity of such reconstructions remains a major issue. At present, the (non)uniqueness can only be assessed by a posteriori comparison and averaging of repetitive Monte Carlo-based shape-determination runs. The new a priori ambiguity measure is based on the number of distinct shape categories compatible with a given data set. For this purpose, a comprehensive library of over 14,000 shape topologies has been generated containing up to seven beads closely packed on a hexagonal grid. The computed scattering curves rescaled to keep only the shape topology rather than the overall size information provide a `scattering map' of this set of shapes. For a given scattering data set, one rapidly obtains the number of neighbours in the map and the associated shape topologies such that in addition to providing a quantitative ambiguity measure the algorithm may also serve as an alternative shape-analysis tool. The approach has been validated in model calculations on geometrical bodies and its usefulness is further demonstrated on a number of experimental X-ray scattering data sets from proteins in solution. A quantitative ambiguity score (a-score) is introduced to provide immediate and convenient guidance to the user on the uniqueness of the ab initio shape reconstruction from the given data set.

The solution structures of two human IgG1 antibodies show conformational stability and accommodate their C1q and FcγR ligands

The human IgG1 antibody subclass shows distinct properties compared with the IgG2, IgG3, and IgG4 subclasses and is the most exploited subclass in therapeutic antibodies. It is the most abundant subclass, has a half-life as long as that of IgG2 and IgG4, binds the FcγR receptor, and activates complement. There is limited structural information on full-length human IgG1 because of the challenges of crystallization. To rectify this, we have studied the solution structures of two human IgG1 6a and 19a monoclonal antibodies in different buffers at different temperatures. Analytical ultracentrifugation showed that both antibodies were predominantly monomeric, with sedimentation coefficients s20,w (0) of 6.3-6.4 S. Only a minor dimer peak was observed, and the amount was not dependent on buffer conditions. Solution scattering showed that the x-ray radius of gyration Rg increased with salt concentration, whereas the neutron Rg values remained unchanged with temperature. The x-ray and neutron distance distribution curves P(r) revealed two peaks, M1 and M2, whose positions were unchanged in different buffers to indicate conformational stability. Constrained atomistic scattering modeling revealed predominantly asymmetric solution structures for both antibodies with extended hinge structures. Both structures were similar to the only known crystal structure of full-length human IgG1. The Fab conformations in both structures were suitably positioned to permit the Fc region to bind readily to its FcγR and C1q ligands without steric clashes, unlike human IgG4. Our molecular models for human IgG1 explain its immune activities, and we discuss its stability and function for therapeutic applications.

Introduction of germline residues improves the stability of anti-HIV mAb 2G12-IgM

Immunoglobulins M (IgMs) are gaining increasing attention as biopharmaceuticals since their multivalent mode of binding can give rise to high avidity. Furthermore, IgMs are potent activators of the complement system. However, they are frequently difficult to express recombinantly and can suffer from low conformational stability. Here, the broadly neutralizing anti-HIV-1 antibody 2G12 was class-switched to IgM and then further engineered by introduction of 17 germline residues. The impact of these changes on the structure and conformational stability of the antibody was then assessed using a range of biophysical techniques. We also investigated the effects of the class switch and germline substitutions on the ligand-binding properties of 2G12 and its capacity for HIV-1 neutralization. Our results demonstrate that the introduced germline residues improve the conformational and thermal stability of 2G12-IgM without altering its overall shape and ligand-binding properties. Interestingly, the engineered protein was found to exhibit much lower neutralization potency than its wild-type counterpart, indicating that potent antigen recognition is not solely responsible for IgM-mediated HIV-1 inactivation.

Human immunoglobulin E flexes between acutely bent and extended conformations

Crystallographic and solution studies have shown that IgE molecules are acutely bent in their Fc region. Crystal structures reveal the Cɛ2 domain pair folded back onto the Cɛ3-Cɛ4 domains, but is the molecule exclusively bent or can the Cɛ2 domains adopt extended conformations and even 'flip' from one side of the molecule to the other? We report the crystal structure of IgE-Fc captured in a fully extended, symmetrical conformation and show by molecular dynamics, calorimetry, stopped-flow kinetic, surface plasmon resonance (SPR) and Förster resonance energy transfer (FRET) analyses that the antibody can indeed adopt such extended conformations in solution. This diversity of conformational states available to IgE-Fc offers a new perspective on IgE function in allergen recognition, as part of the B-cell receptor and as a therapeutic target in allergic disease.

Teaching the structure of immunoglobulins by molecular visualization and SDS-PAGE analysis

This laboratory class combines molecular visualization and laboratory experimentation to teach the structure of the immunoglobulins (Ig). In the first part of the class, the three-dimensional structures of the human IgG and IgM molecules available through the RCSB PDB database are visualized using freely available software. In the second part, IgG and IgM are studied using electrophoretic methods. Through SDS-PAGE analysis under reducing conditions, the students determine the number and molecular masses of the polypeptide chains, while through SDS-PAGE under nonreducing conditions, the students assess the oligomerization of these Ig molecules. The aims of this class are to expand upon the knowledge and understanding of the Ig structure that the students have gained from classroom lectures. The combination of this molecular visualization of the Ig molecules and the SDS-PAGE experimentation ensures variety in the teaching techniques, while the implication of the Ig molecules in human disease promotes interest for biomedical students.

Low-affinity IgM antibodies lacking somatic hypermutations are produced in the secondary response of C57BL/6 mice to (4-hydroxy-3-nitrophenyl)acetyl hapten

Class-switched memory B cells, which are generated through the processes of somatic hypermutation (SHM) and affinity-based selection in germinal centers, contribute to the production of affinity-matured IgG antibodies in the secondary immune response. However, changes in the affinity of IgM antibodies during the immune response have not yet been studied, although IgM(+) memory B cells have been shown to be generated. In order to understand the relationship between IgM affinity and the recall immune response, we prepared hybridomas producing anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) IgM antibodies from C57BL/6 mice and from activation-induced cytidine deaminase (AID)-deficient mice. Binding analysis by ELISA showed that mAbs obtained from the secondary immune response contained IgM mAbs with affinity lower than the affinity of mAbs obtained from the primary response. By analyzing sequences of the IgM genes of hybridomas and plasma cells, we found many unmutated VH genes. VH genes that had neither tyrosine nor glycine at position 95 were frequent. The repertoire change may correlate with the lower affinity of IgM antibodies in the secondary response. The sequence and affinity changes in IgM antibodies were shown to be independent of SHM by analyzing hybridomas from AID-deficient mice. A functional assay revealed a reciprocal relationship between affinity and complement-dependent hemolytic activity toward NP-conjugated sheep RBCs; IgM antibodies with lower affinities had higher hemolytic activity. These findings indicate that lower affinity IgM antibodies with enhanced complement activation function are produced in the secondary immune response.

High-resolution structures of the IgM Fc domains reveal principles of its hexamer formation

IgM is the first antibody produced during the humoral immune response. Despite its fundamental role in the immune system, IgM is structurally only poorly described. In this work we used X-ray crystallography and NMR spectroscopy to determine the atomic structures of the constant IgM Fc domains (Cµ2, Cµ3, and Cµ4) and to address their roles in IgM oligomerization. Although the isolated domains share the typical Ig fold, they differ substantially in dimerization properties and quaternary contacts. Unexpectedly, the Cµ4 domain and its C-terminal tail piece are responsible and sufficient for the specific polymerization of Cµ4 dimers into covalently linked hexamers of dimers. Based on small angle X-ray scattering data, we present a model of the ring-shaped Cµ4 structure, which reveals the principles of IgM oligomerization.

The immunoglobulin, IgG Fc receptor and complement triangle in autoimmune diseases

Immunoglobulin G (IgG)-mediated activation of complement and IgG Fc receptors (FcγRs) are important defense mechanisms of the innate immune system to ward off infections. However, the same mechanisms can drive severe and harmful inflammation, when IgG antibodies react with self-antigens in solution or tissues, as described for several autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, and immune vasculitis. More specifically, IgG immune complexes (ICs) can activate all three pathways of the complement system resulting in the generation of C3 and C5 cleavage products that can activate a panel of different complement receptors on innate and adaptive immune cells. Importantly, complement and FcγRs are often co-expressed on inflammatory immune cells such as neutrophils, monocytes, macrophages or dendritic cells and act in concert to mediate the inflammatory response in autoimmune diseases. In this context, the cross-talk between the receptor for the anaphylatoxin C5a, i.e. C5ar1 (CD88) and FcγRs is of major importance. Recent data suggest a model of bidirectional regulation, in which CD88 acts upstream of FcγRs and sets the threshold for FcγR-dependent effector responses by regulating the ratio between activating and inhibitory FcγRs. Vice versa, FcγR ligation can either amplify or block C5aR-mediated effector functions, depending on whether IgG IC aggregate activating or inhibitory FcγRs. Further, complement and FcγRs cooperate on B cells and on follicular dendritic cells to regulate the development of autoreactive B cells, their differentiation into plasma cells and, eventually, the production of autoantibodies. Here, we will give an update on recent findings regarding this complex regulatory network between complement and FcγRs, which may also regulate the inflammatory response in allergy, cancer and infection.

On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines

Immunology often conveys the image of large molecules, either in the soluble state or in the membrane of leukocytes, forming multiple contacts with a target for actions of the immune system. Avidity names the ability of a polyvalent molecule to form multiple connections of the same kind with ligands tethered to the same surface. Polyvalent interactions are vastly stronger than their monovalent equivalent. In the present review, the functional consequences of polyvalent interactions are explored in a perspective of recent theoretical advances in understanding the thermodynamics of such binding. From insights on the structural biology of soluble pattern recognition molecules as well as adhesion molecules in the cell membranes or in their proteolytically shed form, this review documents the prominent role of polyvalent interactions in making the immune system a formidable barrier to microbial infection as well as constituting a significant challenge to the application of nanomedicines.

The anti-cancer IgM monoclonal antibody PAT-SM6 binds with high avidity to the unfolded protein response regulator GRP78

The monoclonal IgM antibody PAT-SM6 derived from human tumours induces apoptosis in tumour cells and is considered a potential anti-cancer agent. A primary target for PAT-SM6 is the unfolded protein response regulator GRP78, over-expressed externally on the cell surface of tumour cells. Small angle X-ray scattering (SAXS) studies of human GRP78 showed a two-domain dumbbell-shaped monomer, while SAXS analysis of PAT-SM6 revealed a saucer-shaped structure accommodating five-fold symmetry, consistent with previous studies of related proteins. Sedimentation velocity analysis of GRP78 and PAT-SM6 mixtures indicated weak complex formation characterized by dissociation constants in the high micromolar concentration range. In contrast, enzyme-linked immunosorbant assays (ELISAs) showed strong and specific interactions between PAT-SM6 and immobilized GRP78. The apparent binding constant estimated from a PAT-SM6 saturation curve correlated strongly with the concentration of GRP78 used to coat the microtiter tray. Experiments using polyclonal antiGRP78 IgG antibodies or a monoclonal IgG derivative of PAT-SM6 did not show a similar dependence. Competition experiments with soluble GRP78 indicated more effective inhibition of PAT-SM6 binding at low GRP78 coating concentrations. These observations suggest an avidity-based binding mechanism that depends on the multi-point attachment of PAT-SM6 to GRP78 clustered on the surface of the tray. Analysis of ELISA data at high GRP78 coating concentrations yielded an apparent dissociation constant of approximately 4 nM. We propose that the biological action of PAT-SM6 in tumour cell apoptosis may depend on the multivalent nature of PAT-SM6 and the high avidity of its interaction with multiple GRP78 molecules clustered on the tumour cell surface.

The IgM CH2 domain as covalently linked homodimerization module for the generation of fusion proteins with dual specificity

Dimeric assembly of antibody fragments and other therapeutic molecules can result in increased binding and improved bioactivity. Here, we investigated the use of the IgM heavy chain domain 2 (MHD2) as covalently linked homodimerization module. Fusion of single-chain fragment variable (scFv) molecules directed against epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 to the N- and/or C-terminus of the MHD2, respectively, resulted in molecules with single or dual specificity for tumor cells. Bispecific tetravalent molecules were further generated by fusing a bispecific single-chain diabody directed against EGFR and epithelial cell adhesion molecule to the N-terminus of the MHD2. By combining an anti-EGFR scFv with a single-chain derivative of tumor necrosis factor, a tetravalent bifunctional fusion protein was produced. This fusion protein exhibited improved TNF activity, also mimicking the membrane-bound form of TNF, as shown by the activation of TNFR2-mediated cell killing. Furthermore, the scFv moiety allowed for an antigen-dependent delivery of TNF to EGFR-positive cells and an improved stimulatory TNF action on these cells. Thus, we established the MHD2 as a versatile module for the generation of bispecific and bifunctional fusion proteins.