Immunoglobulin M perception by FcμR

Functions of IgM fc receptor (FcµR) related to autoimmunity

Unlike Fc receptors for switched immunoglobulin (Ig) isotypes, Fc receptor for IgM (FcµR) is selectively expressed by lymphocytes. The ablation of the FcµR gene in mice impairs B cell tolerance as evidenced by concomitant production of autoantibodies of IgM and IgG isotypes. In this essay, we reiterate the autoimmune phenotypes observed in mutant mice, ie IgM homeostasis, dysregulated humoral immune responses including autoantibodies, and Mott cell formation. We also propose the potential phenotypes in individuals with FCMR deficiency and the model for FcµR-mediated regulation of self-reactive B cells.

Stellabody: A novel hexamer-promoting mutation for improved IgG potency

Advances in antibody engineering are being directed at the development of next generation immunotherapeutics with improved potency. Hexamerisation of IgG is a normal physiological aspect of IgG biology and recently described mutations that facilitate this process have a substantial impact upon monoclonal antibody behavior resulting in the elicitation of dramatically enhanced complement-dependent cytotoxicity, Fc receptor function, and enhanced antigen binding effects, such as targeted receptor agonism or microbe neutralization. Whereas the discovery of IgG hexamerisation enhancing mutations has largely focused on residues with exposure at the surface of the Fc-Fc and CH2-CH3 interfaces, our unique approach is the engineering of the mostly buried residue H429 in the CH3 domain. Selective substitution at position 429 forms the basis of Stellabody technology, where the choice of amino acid results in distinct hexamerisation outcomes. H429F results in monomeric IgG that hexamerises after target binding, so called "on-target" hexamerisation, while the H429Y mutant forms pH-sensitive hexamers in-solution prior to antigen binding. Moreover, Stellabody technologies are broadly applicable across the family of antibody-based biologic therapeutics, including conventional mAbs, bispecific mAbs, and Ig-like biologics such as Fc-fusions, with applications in diverse diseases.

Challenges in the Development of NK-92 Cells as an Effective Universal Off-the-Shelf Cellular Therapeutic

The human-derived NK-92 cell-based CAR-NK therapy exhibits inconsistency with overall suboptimal efficacy and rapid in vivo clearance of CAR-NK92 cells in cancer patients. Analysis indicates that although pre-existing IgM in healthy individuals (n = 10) strongly recognizes both NK-92 and CAR-NK92 cells, IgG and IgE do not. However, only a subset of cancer patients (3/8) exhibit strong IgM recognition of these cells, with some (2/8) showing pre-existing IgG recognition. These results suggest a natural immunoreactivity between NK-92 and CAR-NK92 cells and pre-existing human Abs. Furthermore, the therapy's immunogenicity is evidenced by enhanced IgG and IgM recognition postinfusion of CAR-NK92 cells. We also confirmed that healthy plasma's cytotoxicity toward these cells is reduced by complement inhibitors, suggesting that Abs may facilitate the rapid clearance of CAR-NK92 cells through complement-dependent cytotoxicity. Given that NK-92 cells lack known receptors for IgG and IgM, identifying and modifying the recognition targets for these Abs on NK-92 and CAR-NK92 cells may improve clinical outcomes. Moreover, we discovered that the 72nd amino acid of the NKG2D receptor on NK-92 cells is alanine. Previous studies have demonstrated polymorphism at the 72nd amino acid of the NKG2D on human NK cells, with NKG2D72Thr exhibiting a superior activation effect on NK cells compared with NKG2D72Ala. We confirmed this conclusion also applies to NK-92 cells by in vitro cytotoxicity experiments. Therefore, reducing the immunoreactivity and immunogenicity of CAR-NK92 and directly switching NK-92 bearing NKG2D72Ala to NKG2D72Thr represent pressing challenges in realizing NK-92 cells as qualified universal off-the-shelf cellular therapeutics.

Cryo-EM reveals structural basis for human AIM/CD5L recognition of polymeric immunoglobulin M

Cell surface scavenger receptors contribute to homoeostasis and the response to pathogens and products associated with damage by binding to common molecular features on a wide range of targets. Apoptosis inhibitor of macrophage (AIM/CD5L) is a soluble protein belonging to the scavenger receptor cysteine-rich (SRCR) superfamily that contributes to prevention of a wide range of diseases associated with infection, inflammation, and cancer. AIM forms complexes with IgM pentamers which helps maintain high-levels of circulating AIM in serum for subsequent activation on release from the complex. The structural basis for AIM recognition of IgM as well as other binding targets is unknown. Here we apply cryogenic electron microscopy imaging (cryo-EM) to show how interfaces on both of AIM's C-terminal SRCR domains interact with the Fcμ constant region and J chain components of the IgM core. Both SRCR interfaces are also shown to contribute interactions important for AIM binding to damage-associated molecular patterns (DAMPs).

The Emerging Role of Immunoglobulins and Complement in the Stimulation of Neuronal Activity and Repair: Not as Simple as We Thought

Neurologic disorders such as traumatic brain injury, multiple sclerosis, Alzheimer's disease, and drug-resistant epilepsy have a high socioeconomic impact around the world. Current therapies for these disorders are often not effective. This creates a demand for the development of new therapeutic approaches to treat these disorders. Recent data suggest that autoreactive naturally occurring immunoglobulins produced by subsets of B cells, called B1 B cells, combined with complement, are actively involved in the processes of restoration of neuronal functions during pathological conditions and remyelination. The focus of this review is to discuss the possibility of creating specific therapeutic antibodies that can activate and fix complement to enhance neuronal survival and promote central nervous system repair after injuries associated with many types of neurodegenerative diseases.

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.

CD5L associates with IgM via the J chain

CD5 antigen-like (CD5L), also known as Spα or AIM (Apoptosis inhibitor of macrophage), emerges as an integral component of serum immunoglobulin M (IgM). However, the molecular mechanism underlying the interaction between IgM and CD5L has remained elusive. In this study, we present a cryo-electron microscopy structure of the human IgM pentamer core in complex with CD5L. Our findings reveal that CD5L binds to the joining chain (J chain) in a Ca2+-dependent manner and further links to IgM via a disulfide bond. We further corroborate recently published data that CD5L reduces IgM binding to the mucosal transport receptor pIgR, but does not impact the binding of the IgM-specific receptor FcμR. Additionally, CD5L does not interfere with IgM-mediated complement activation. These results offer a more comprehensive understanding of IgM and shed light on the function of the J chain in the immune system.

Multiple trait comparison and global intestine transcriptional provide new insights into bases of heterosis in hybrid tilapia (Oreochromis niloticus × Oreochromis aureus)

Heterosis has been utilized in aquaculture for many years, yet its molecular basis remains elusive. Therefore, a comprehensive analysis of heterosis was conducted by comparing growth, digestion and biochemistry indices, as well as the intestinal gene expression profiles of Nile tilapia, blue tilapia and their hybrids. The results revealed that hybrid tilapia demonstrated an enhanced growth traits and elevated digestive enzyme activity compared to Nile and blue tilapia. Additionally, the hybrid tilapia displayed superior antioxidants and non-specific immune levels, with increased levels of catalase (CAT), alkaline phosphatase (AKP), acid phosphatase (ACP), glutathione (GSH), superoxide dismutase (SOD), total antioxidant capacity (TAOC), lysozyme, and immunoglobulin M (IgM) relative to Nile and blue tilapia. Moreover, 3392, 2470 and 1261 differentially expressed genes (DEGs) were identified in the intestinal tissues when comparing Nile tilapia to blue tilapia, hybrid tilapia to blue tilapia, and hybrid tilapia to Nile tilapia. Upon classifying the differentially expressed genes (DEGs), non-additively expressed DEGs accounted for 68.1 % of the total DEGs, with dominant and over-dominant expressed DEGs comprising 63.7 % and 4.4 % in the intestines, respectively. These non-additively expressed DEGs were primarily associated with metabolic, digestive, growth, and developmental pathways. This enrichment enhances our comprehension of the molecular underpinnings of growth heterosis in aquatic species.

A natural IgM hitchhiking strategy for delivery of cancer nanovaccines to splenic marginal zone B cells

B cell-targeted cancer vaccines are receiving increasing attention in immunotherapy due to the combined antibody-secreting and antigen-presenting functions. In this study, we propose a natural IgM-hitchhiking delivery strategy to co-deliver tumor antigens and adjuvants to splenic marginal zone B (MZB) cells. We constructed nanovaccines (FA-sLip/OVA/MPLA) consisting of classical folic acid (FA)-conjugated liposomes co-loaded with ovalbumin (OVA) and toll-like receptor 4 agonists, MPLA. We found that natural IgM absorption could be manipulated at the bio-nano interface on FA-sLip/OVA/MPLA, enabling targeted delivery to splenic MZB cells. Systemic administration of FA-sLip/OVA/MPLA effectively activated splenic MZB cells via IgM-mediated multiplex pathways, eliciting antigen-specific humoral and cytotoxic T lymphocyte responses, and ultimately retarding E.G7-OVA tumor growth. In addition, combining FA-sLip/OVA/MPLA immunization with anti-PD-1 treatments showed improved antitumor efficiency. Overall, this natural IgM-hitchhiking delivery strategy holds great promise for efficient, splenic MZB cell-targeted delivery of cancer vaccines in future applications.

Secreted IgM modulates IL-10 expression in B cells

IL-10+ B cells are critical for immune homeostasis and restraining immune responses in infection, cancer, and inflammation; however, the signals that govern IL-10+ B cell differentiation are ill-defined. Here we find that IL-10+ B cells expand in mice lacking secreted IgM ((s)IgM-/-) up to 10-fold relative to wildtype (WT) among all major B cell and regulatory B cell subsets. The IL-10+ B cell increase is polyclonal and presents within 24 hours of birth. In WT mice, sIgM is produced prenatally and limits the expansion of IL-10+ B cells. Lack of the high affinity receptor for sIgM, FcμR, in B cells translates into an intermediate IL-10+ B cell phenotype relative to WT or sIgM-/- mice. Our study thus shows that sIgM regulates IL-10 programming in B cells in part via B cell-expressed FcμR, thereby revealing a function of sIgM in regulating immune homeostasis.

Exploring Immunome and Microbiome Interplay in Reproductive Health: Current Knowledge, Challenges, and Novel Diagnostic Tools

The dynamic interplay between the immunome and microbiome in reproductive health is a complex and rapidly advancing research field, holding tremendously vast possibilities for the development of reproductive medicine. This immunome-microbiome relationship influences the innate and adaptive immune responses, thereby affecting the onset and progression of reproductive disorders. However, the mechanisms governing these interactions remain elusive and require innovative approaches to gather more understanding. This comprehensive review examines the current knowledge on reproductive microbiomes across various parts of female reproductive tract, with special consideration of bidirectional interactions between microbiomes and the immune system. Additionally, it explores innate and adaptive immunity, focusing on immunoglobulin (Ig) A and IgM antibodies, their regulation, self-antigen tolerance mechanisms, and their roles in immune homeostasis. This review also highlights ongoing technological innovations in microbiota research, emphasizing the need for standardized detection and analysis methods. For instance, we evaluate the clinical utility of innovative technologies such as Phage ImmunoPrecipitation Sequencing (PhIP-Seq) and Microbial Flow Cytometry coupled to Next-Generation Sequencing (mFLOW-Seq). Despite ongoing advancements, we emphasize the need for further exploration in this field, as a deeper understanding of immunome-microbiome interactions holds promise for innovative diagnostic and therapeutic strategies for reproductive health, like infertility treatment and management of pregnancy.

The clarifying lens of cryo-electron microscopy in immunoglobulin M biology

In this article, we discuss recent advances into the structural analyses of immunoglobulin M complexes, which are enabling comprehensive characterization of these enigmatic antibodies, to reveal central tenets of immunoglobulin M immunobiology and inform their immunotherapeutic use.

Plasmodium falciparum has evolved multiple mechanisms to hijack human immunoglobulin M

Plasmodium falciparum causes the most severe malaria in humans. Immunoglobulin M (IgM) serves as the first line of humoral defense against infection and potently activates the complement pathway to facilitate P. falciparum clearance. A number of P. falciparum proteins bind IgM, leading to immune evasion and severe disease. However, the underlying molecular mechanisms remain unknown. Here, using high-resolution cryo-electron microscopy, we delineate how P. falciparum proteins VAR2CSA, TM284VAR1, DBLMSP, and DBLMSP2 target IgM. Each protein binds IgM in a different manner, and together they present a variety of Duffy-binding-like domain-IgM interaction modes. We further show that these proteins interfere directly with IgM-mediated complement activation in vitro, with VAR2CSA exhibiting the most potent inhibitory effect. These results underscore the importance of IgM for human adaptation of P. falciparum and provide critical insights into its immune evasion mechanism.