Structural and functional anatomy of the globular domain of complement protein C1q

Complement-Coagulation Cross-talk: Factor H-mediated regulation of the Complement Classical Pathway activation by fibrin clots

The classical pathway of the complement system is activated by the binding of C1q in the C1 complex to the target activator, including immune complexes. Factor H is regarded as the key downregulatory protein of the complement alternative pathway. However, both C1q and factor H bind to target surfaces via charge distribution patterns. For a few targets, C1q and factor H compete for binding to common or overlapping sites. Factor H, therefore, can effectively regulate the classical pathway activation through such targets, in addition to its previously characterized role in the alternative pathway. Both C1q and factor H are known to recognize foreign or altered-self materials, e.g., bacteria, viruses, and apoptotic/necrotic cells. Clots, formed by the coagulation system, are an example of altered self. Factor H is present abundantly in platelets and is a well-known substrate for FXIIIa. Here, we investigated whether clots activate the complement classical pathway and whether this is regulated by factor H. We show here that both C1q and factor H bind to the fibrin formed in microtiter plates and the fibrin clots formed under in vitro physiological conditions. Both C1q and factor H become covalently bound to fibrin clots, and this is mediated via FXIIIa. We also show that fibrin clots activate the classical pathway of complement, as demonstrated by C4 consumption and membrane attack complex detection assays. Thus, factor H downregulates the activation of the classical pathway induced by fibrin clots. These results elucidate the intricate molecular mechanisms through which the complement and coagulation pathways intersect and have regulatory consequences.

LYVE-1-expressing Macrophages Modulate the Hyaluronan-containing Extracellular Matrix in the Mammary Stroma and Contribute to Mammary Tumor Growth

Macrophages represent a heterogeneous myeloid population with diverse functions in normal tissues and tumors. While macrophages expressing the cell surface marker lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1) have been identified in stromal regions of the normal mammary gland and in the peritumoral stroma, their functions within these regions are not well understood. Using a genetic mouse model of LYVE-1+ macrophage depletion, we demonstrate that loss of LYVE-1+ macrophages is associated with altered extracellular matrix remodeling in the normal mammary gland and reduced mammary tumor growth in vivo. In further studies focused on investigating the functions of LYVE-1+ macrophages in the tumor microenvironment, we demonstrate that LYVE-1 expression correlates with an increased ability of macrophages to bind, internalize, and degrade hyaluronan. Consistent with this, we show that depletion of LYVE-1+ macrophages correlates with increased hyaluronan accumulation in both the normal mammary gland and in mammary tumors. Analysis of single-cell RNA sequencing of macrophages isolated from these tumors reveals that depletion of LYVE-1+ macrophages in tumors drives a shift in the majority of the remaining macrophages toward a proinflammatory phenotype, as well as an increase in CD8+ T-cell infiltration. Together, these findings indicate that LYVE-1+ macrophages represent a tumor-promoting anti-inflammatory subset of macrophages that contributes to hyaluronan remodeling in the tumor microenvironment.

SIGNIFICANCE

We have identified a macrophage subset in mouse mammary tumors associated with tumor structural components. When this macrophage subset is absent in tumors, we report a delay in tumor growth and an increase in antitumor immune cells. Understanding the functions of distinct macrophage subsets may allow for improved therapeutic strategies for patients with breast cancer.

Identification of the C1qDC gene family in grass carp (Ctenopharyngodon idellus) and the response of C1qA, C1qB, and C1qC to GCRV infection in vivo and in vitro

Proteins from the C1q domain-containing (C1qDC) family recognize self-, non-self-, and altered-self ligands and serves as an initiator molecule for the classical complement pathway as well as recognizing immune complexes. In this study, C1qDC gene family members were identified and analyzed in grass carp (Ctenopharyngodon idellus). Members of the C1q subfamily were cloned, and their response to infection with the grass carp virus was investigated. In the grass carp genome, 54 C1qDC genes and 67 isoforms have been identified. Most were located on chromosome 3, with 52 shared zebrafish homologies. Seven substantially differentially expressed C1qDC family genes were identified in the transcriptomes of cytokine-induced killer (CIK) cells infected with grass carp reovirus (GCRV), all of which exhibited sustained upregulation. The opening reading frames of grass carp C1qA, C1qB, and C1qC, belonging to the C1q subfamily, were determined to be 738, 732, and 735 base pairs, encoding 245, 243, and 244 amino acids with molecular weights of 25.81 kDa, 25.63 kDa and 26.16 kDa, respectively. Three genes were detected in the nine collected tissues, and their expression patterns were similar, with the highest expression levels observed in the spleen. In vivo after GCRV infection showed expression trends of C1qA, C1qB, and C1qC in the liver, spleen, and kidney. An N-type pattern in the liver and kidney was characterized by an initial increase followed by a decrease, with the highest expression occurring during the recovering period, and a V-type pattern in the spleen with the lowest expression levels during the death period. In vitro, after GCRV infection showed expression trends of C1qA, C1qB, and C1qC, and this gradually increased within the first 24 h, with a notable increase observed at the 24 h time point. After CIK cells incubation with purified recombinant proteins, rC1qA, rC1qB, and rC1qC for 3 h, followed by GCRV inoculation, the GCRV replication indicated that rC1qC exerted a substantial inhibitory effect on viral replication in CIK cells after 24 h of GCRV inoculation. These findings offer valuable insights into the structure, evolution, and function of the C1qDC family genes and provide a foundational understanding of the immune function of C1q in grass carp.

C1QBP is a critical component in the immune response of large yellow croaker (Larimichthys crocea) against visceral white spot disease caused by Pseudomonas plecoglossicida

The large yellow croaker (Larimichthys crocea) stands as a cornerstone of mariculture in China due to its significant value. However, the threat of Pseudomonas plecoglossicida infection looms large, capable of triggering "visceral white spot disease" and subsequently inflicting severe economic ramifications. Through a prior genome-wide association analysis (GWAS) aimed at understanding the resistance of the large yellow croaker to this ailment, a pivotal player emerged: the complement component 1q binding protein, aptly named LcC1qbp. This protein assumes a crucial role in the activation of the complement system. This study delves deeper into the immune response by examining the expression patterns of LcC1QBP when confronted with P. plecoglossicida. The investigation into gene expression patterns reveals LcC1qbp's widespread presence, with its highest transcriptional abundance identified in the kidney tissues. Upon infection by P. plecoglossicida, the up-regulation of LcC1qbp in major immune organs manifests at both the transcriptional and translational levels. In the context of RNA interference, transcriptome analysis of C1qbp in HEK 293T cells uncovers 1327 differentially expressed genes (DEGs), featuring 41 significant immune genes. This includes pivotal components such as C1S and C3 of the complement system, along with IL11, IL12RB2, and Myd88, among others. The outcomes of enrichment analysis spotlight the prevalence of DEGs within key pathways like immune system development, myeloid leukocyte-mediated immunity, MAPK signaling, and other immune-related routes. By unveiling the immune response mechanisms of the large yellow croaker to P. plecoglossicida infection, this study bolsters our understanding. Furthermore, it lays the groundwork for pursuing effective strategies in both preventing and treating "visceral white spot disease" in the large yellow croaker.

Next generation of multispecific antibody engineering

Multispecific antibodies recognize two or more epitopes located on the same or distinct targets. This added capability through protein design allows these man-made molecules to address unmet medical needs that are no longer possible with single targeting such as with monoclonal antibodies or cytokines alone. However, the approach to the development of these multispecific molecules has been met with numerous road bumps, which suggests that a new workflow for multispecific molecules is required. The investigation of the molecular basis that mediates the successful assembly of the building blocks into non-native quaternary structures will lead to the writing of a playbook for multispecifics. This is a must do if we are to design workflows that we can control and in turn predict success. Here, we reflect on the current state-of-the-art of therapeutic biologics and look at the building blocks, in terms of proteins, and tools that can be used to build the foundations of such a next-generation workflow.

Complement in Human Brain Health: Potential of Dietary Food in Relation to Neurodegenerative Diseases

The complement pathway is a major component of the innate immune system, which is critical for recognizing and clearing pathogens that rapidly react to defend the body against external pathogens. Many components of this pathway are expressed throughout the brain and play a beneficial role in synaptic pruning in the developing central nervous system (CNS). However, excessive complement-mediated synaptic pruning in the aging or injured brain may play a contributing role in a wide range of neurodegenerative diseases. Complement Component 1q (C1q), an initiating recognition molecule of the classical complement pathway, can interact with a variety of ligands and perform a range of functions in physiological and pathophysiological conditions of the CNS. This review considers the function and immunomodulatory mechanisms of C1q; the emerging role of C1q on synaptic pruning in developing, aging, or pathological CNS; the relevance of C1q; the complement pathway to neurodegenerative diseases; and, finally, it summarizes the foods with beneficial effects in neurodegenerative diseases via C1q and complement pathway and highlights the need for further research to clarify these roles. This paper aims to provide references for the subsequent study of food functions related to C1q, complement, neurodegenerative diseases, and human health.

Multi-omics investigation of the resistance mechanisms of pomalidomide in multiple myeloma

Background

Despite significant therapeutic advances over the last decade, multiple myeloma remains an incurable disease. Pomalidomide is the third Immunomodulatory drug that is commonly used to treat patients with relapsed/refractory multiple myeloma. However, approximately half of the patients exhibit resistance to pomalidomide treatment. While previous studies have identified Cereblon as a primary target of Immunomodulatory drugs' anti-myeloma activity, it is crucial to explore additional mechanisms that are currently less understood.

Methods

To comprehensively investigate the mechanisms of drug resistance, we conducted integrated proteomic and metabonomic analyses of 12 plasma samples from multiple myeloma patients who had varying responses to pomalidomide. Differentially expressed proteins and metabolites were screened, and were further analyzed using pathway analysis and functional correlation analysis. Also, we estimated the cellular proportions based on ssGSEA algorithm. To investigate the potential role of glycine in modulating the response of MM cells to pomalidomide, cell viability and apoptosis were analyzed.

Results

Our findings revealed a consistent decrease in the levels of complement components in the pomalidomide-resistant group. Additionally, there were significant differences in the proportion of T follicular helper cell and B cells in the resistant group. Furthermore, glycine levels were significantly decreased in pomalidomide-resistant patients, and exogenous glycine administration increased the sensitivity of MM cell lines to pomalidomide.

Conclusion

These results demonstrate distinct molecular changes in the plasma of resistant patients that could be used as potential biomarkers for identifying resistance mechanisms for pomalidomide in multiple myeloma and developing immune-related therapeutic strategies.

Could a Non-Cellular Molecular Interactome in the Blood Circulation Influence Pathogens' Infectivity?

We advance the notion that much like artificial nanoparticles, relatively more complex biological entities with nanometric dimensions such as pathogens (viruses, bacteria, and other microorganisms) may also acquire a biomolecular corona upon entering the blood circulation of an organism. We view this biomolecular corona as a component of a much broader non-cellular blood interactome that can be highly specific to the organism, akin to components of the innate immune response to an invading pathogen. We review published supporting data and generalize these notions from artificial nanoparticles to viruses and bacteria. Characterization of the non-cellular blood interactome of an organism may help explain apparent differences in the susceptibility to pathogens among individuals. The non-cellular blood interactome is a candidate therapeutic target to treat infectious and non-infectious conditions.

Complement Inhibition in Kidney Transplantation: Where Are We Now?

Kidney transplantation is a life-saving strategy for patients with end-stage renal disease. Although progress has been made in the field of transplantation medicine in recent decades in terms of surgical techniques and immunosuppression, long-term organ survival remains a challenge. Also, for reasons of organ shortage, there is an unmet need for new therapeutic approaches to improve the long-term survival of transplants. There is increasing evidence that the complement system plays a crucial role in various pathological events after transplantation, including ischemia/reperfusion injury as well as rejection episodes. The complement system is part of the innate immune system and plays a crucial role in the defense against pathogens but is also involved in tissue homeostasis. However, the tightly regulated complement system can become dysregulated or activated by non-infectious stimuli, then targeting the organism's own cells and leading to inflammatory tissue damage that exacerbates injury. In this review, we will highlight the role of the complement system after transplantation and discuss ongoing and potential therapeutic approaches.

Nanolattice-Forming Hybrid Collagens in Protective Shark Egg Cases

Nanoscopic structural control with long-range ordering remains a profound challenge in nanomaterial fabrication. The nanoarchitectured egg cases of elasmobranchs rely on a hierarchically ordered latticework for their protective function─serving as an exemplary system for nanoscale self-assembly. Although the proteinaceous precursors are known to undergo intermediate liquid crystalline phase transitions before being structurally arrested in the final nanolattice architecture, their sequences have so far remained unknown. By leveraging RNA-seq and proteomic techniques, we identified a cohort of nanolattice-forming proteins comprising a collagenous midblock flanked by domains typically associated with innate immunity and network-forming collagens. Structurally homologous proteins were found in the genomes of other egg-case-producing cartilaginous fishes, suggesting a conserved molecular self-assembly strategy. The identity and stabilizing role of cross-links were subsequently elucidated using mass spectrometry and in situ small-angle X-ray scattering. Our findings provide a new design approach for protein-based liquid crystalline elastomers and the self-assembly of nanolattices.

How to get away with murder: The multiple strategies employed by pathogenic protozoa to avoid complement killing

Parasitic protozoa are eukaryotic unicellular organisms that depend on a variety of living organisms and can develop intra- and extracellularly inside their hosts. In humans, these parasites cause diseases with a significant impact on public health, such as malaria, toxoplasmosis, Chagas disease, leishmaniasis and amebiasis. The ability of a parasite in establishing a successful infection depends on a series of intricate evolutionarily selected adaptations, which include the development of molecular and cellular strategies to evade the host immune system effector mechanisms. The complement system is one of the main effector mechanisms and the first humoral shield of hosts innate immunity against pathogens. For unicellular pathogens, such as protozoa, bacteria and fungi, the activation of the complement system may culminate in the elimination of the invader mainly via 1- the formation of a pore that depolarizes the plasma membrane of the parasite, causing cell lysis; 2- opsonization and killing by phagocytes; 3- increasing vascular permeability while also recruiting neutrophils to the site of activation. Numerous strategies to avoid complement activation have been reported for parasitic protozoa, such as 1- sequestration of complement system regulatory proteins produced by the host, 2- expression of complement system regulatory proteins, 3- proteolytic cleavage of different complement effector molecules, 4- formation of a physical glycolipid barrier that prevents deposition of complement molecules on the plasma membrane, and 5- removal, by endocytosis, of complement molecules bound to plasma membrane. In this review, we revisit the different strategies of blocking various stages of complement activation described for the main species of parasitic protozoa, present the most recent discoveries in the field and discuss new perspectives on yet neglected strategies and possible new evasion mechanisms.

The yin and the yang of early classical pathway complement disorders

The classical pathway of the complement cascade has been recognized as a key activation arm, partnering with the lectin activation arm and the alternative pathway to cleave C3 and initiate the assembly of the terminal components. While deficiencies of classical pathway components have been recognized since 1966, only recently have gain-of-function variants been described for some of these proteins. Loss-of-function variants in C1, C4, and C2 are most often associated with lupus and systemic infections with encapsulated bacteria. C3 deficiency varies slightly from this phenotypic class with membranoproliferative glomerulonephritis and infection as the dominant phenotypes. The gain-of- function variants recently described for C1r and C1s lead to periodontal Ehlers Danlos syndrome, a surprisingly structural phenotype. Gain-of-function in C3 and C2 are associated with endothelial manifestations including hemolytic uremic syndrome and vasculitis with C2 gain-of-function variants thus far having been reported in patients with a C3 glomerulopathy. This review will discuss the loss-of-function and gain-of-function phenotypes and place them within the larger context of complement deficiencies.

Understanding the contextual functions of C1q and LAIR-1 and their applications

The importance of the complement component C1q has been highlighted by its involvement in autoimmunity, infection, inflammatory diseases, and tumors. The unique tulip-like structure of C1q has both a collagen-like stalk (C1q tail) and heterotrimeric globular head (gC1q), each with different binding specificities, and the binding of these components to their respective receptors leads to functional complexities in the body and bridges innate and adaptive immunity. This review describes the fundamental roles of C1q in various microenvironments and focuses on the importance of the interactions of C1q and its receptors with the inhibitory receptor LAIR-1 in maintaining homeostasis. Current therapeutic opportunities modulating LAIR-1 are also discussed.

Research into the activities of the protein C1q, involved in a cascade of molecular interactions of the immune response called complement activation, is revealing new details of the protein’s role and opening up possible new therapeutic opportunities. Myoungsun Son at Feinstein Institutes for Medical Research in Manhasset, USA, reviews the involvement of C1q in infection, autoimmunity, inflammatory diseases and tumors. The interaction of C1q with a receptor protein called LAIR-1 seems to be particularly significant. LAIR-1 is present in the membrane of most blood-forming cells and is involved in maintaining the healthy balance of cellular activities referred to as homeostasis. Emerging research suggests that targeting the interactions between C1q and LAIR-1 could enable the development of new treatments for many diseases, including inflammatory diseases, the autoimmune condition lupus, a variety of cancers, and possibly Covid-19.

N'-terminal- and Ca2+-induced stabilization of high-order oligomers of full-length Danio rerio and Homo sapiens otolin-1

Otolin-1 is a C1q family member and a major component of the organic matrix of fish otoliths and human otoconia. To date, the protein molecular properties have not been characterized. In this work, we describe biochemical characterization and comparative studies on saccular-specific otolin-1 derived from Danio rerio and Homo sapiens. Due to the low abundance of proteins in the otoconial matrix, we developed a production and purification method for both recombinant homologues of otolin-1. Danio rerio and Homo sapiens otolin-1 forms higher-order oligomers that can be partially disrupted under reducing conditions. The presence of Ca²⁺ stabilizes the oligomers and significantly increases the thermal stability of the proteins. Despite the high sequence coverage, the oligomerization of Danio rerio otolin-1 is more affected by the reducing conditions and presence of Ca²⁺ than the human homologue. The results show differences in molecular behaviour, which may be reflected in Danio rerio and Homo sapiens otolin-1 role in otolith and otoconia formation.

Globular C1q domain-containing protein from Pinctada fucata martensii participates in the immune defense process

The globular C1q domain-containing (C1qDC) protein can recognize a variety of ligands, such as pathogen-associated molecular patterns, and plays an important role in the innate immune response. Our previous studies showed that a novel globular C1q domain-containing protein (PmC1qDC-1) is involved in the damage repair process of pearl oyster shells. However, the function of PmC1qDC-1 in pearl oyster innate immunity remains unknown. In the present study, the high-level structural analysis showed that PmC1qDC-1 was a spherical structure composed of 10 strands and was similar to the AiC1qDC-2 of bay scallop (Argopecten irradians). In situ hybridization indicated that PmC1qDC-1 had strong fluorescence signal in gills. Furthermore, the mRNA expression of PmC1qDC-1 was highly induced at 6-48 h in gill after lipopolysaccharide, peptidoglycan and polyinosinic-polycytidylic acid stimulation. Additionally, we obtained the recombinant protein of PmC1qDC-1 (rPmC1qDC-1) and found that rPmC1qDC-1 had antibacterial activity against Gram-negative (i.e., Pseudomonas aeruginosa, Vibrio parahaemolyticus, Escherichia coli, and Aeromonas hydrophila) and Gram-positive (i.e., Staphylococcus aureus and Bacillus subtilis) bacteria. These results indicated that PmC1qDC-1 might play an important role in the immune response against bacteria and viruses. This study provides clues for further studying the immune defense of Pinctada fucata martensii against pathogens and exploring the evolution of the classic pathway of complement system.

Cryo-Electron Microscopy and Biochemical Analysis Offer Insights Into the Effects of Acidic pH, Such as Occur During Acidosis, on the Complement Binding Properties of C-Reactive Protein

The pentraxin family of proteins includes C-reactive protein (CRP), a canonical marker for the acute phase inflammatory response. As compared to normal physiological conditions in human serum, under conditions associated with damage and inflammation, such as acidosis and the oxidative burst, CRP exhibits modulated biochemical properties that may have a structural basis. Here, we explore how pH and ligand binding affect the structure and biochemical properties of CRP. Cryo-electron microscopy was used to solve structures of CRP at pH 7.5 or pH 5 and in the presence or absence of the ligand phosphocholine (PCh), which yielded 7 new high-resolution structures of CRP, including pentameric and decameric complexes. Structures previously derived from crystallography were imperfect pentagons, as shown by the variable angles between each subunit, whereas pentameric CRP derived from cryoEM was found to have C5 symmetry, with subunits forming a regular pentagon with equal angles. This discrepancy indicates flexibility at the interfaces of monomers that may relate to activation of the complement system by the C1 complex. CRP also appears to readily decamerise in solution into dimers of pentamers, which obscures the postulated binding sites for C1. Subtle structural rearrangements were observed between the conditions tested, including a putative change in histidine protonation that may prime the disulphide bridges for reduction and enhanced ability to activate the immune system. Enzyme-linked immunosorbent assays showed that CRP had markedly increased association to the C1 complex and immunoglobulins under conditions associated with acidosis, whilst a reduction in the Ca²⁺ concentration lowered this pH-sensitivity for C1q, but not immunoglobulins, suggesting different modes of binding. These data suggest a model whereby a change in the ionic nature of CRP and immunological proteins can make it more adhesive to potential ligands without large structural rearrangements.

Multimerin-1 and cancer: a review

Multimerin-1 (MMRN1) is a platelet protein with a role in haemostasis and coagulation. It is also present in endothelial cells (ECs) and the extracellular matrix (ECM), where it may be involved in cell adhesion, but its molecular functions and protein–protein interactions in these cellular locations have not been studied in detail yet. In recent years, MMRN1 has been identified as a differentially expressed gene (DEG) in various cancers and it has been proposed as a possible cancer biomarker. Some evidence suggest that MMRN1 expression is regulated by methylation, protein interactions, and non-coding RNAs (ncRNAs) in different cancers. This raises the questions if a functional role of MMRN1 is being targeted during cancer development, and if MMRN1’s differential expression pattern correlates with cancer progression. As a result, it is timely to review the current state of what is known about MMRN1 to help inform future research into MMRN1’s molecular mechanisms in cancer.

The role of complement in the tumor microenvironment

Tumorigenesis has long been linked to the evasion of the immune system and the uncontrolled proliferation of transformed cells. The complement system, a major arm of innate immunity, is a key factor in the progression of cancer because many of its components have critical regulatory roles in the tumor microenvironment. For example, complement anaphylatoxins directly and indirectly inhibit antitumor T-cell responses in primary and metastatic sites, enhance proliferation of tumor cells, and promote metastasis and tumor angiogenesis. Many recent studies have provided evidence that cancer is able to hijack the immunoregulatory components of the complement system which fundamentally are tasked with protecting the body against abnormal cells and pathogens. Indeed, recent evidence shows that many types of cancer use C1q receptors (C1qRs) to promote tumor growth and progression. More importantly, most cancer cells express both C1q and its major receptors (gC1qR and cC1qR) on their surface which are essential for cell proliferation and survival. In this review, we discuss the ability of cancer to control and manipulate the complement system in the tumor microenvironment and identify possible therapeutic targets, including C1q and gC1qR.

C1q tumor necrosis factor-related protein 1: a promising therapeutic target for atherosclerosis

ABSTRACT

Atherosclerosis serves as the pathological basis of most cardiovascular and cerebrovascular diseases. C1q tumor necrosis factor-related protein (CTRP1) is a 35-kDa glycoprotein synthesized by various tissues and cells, such as adipose tissue and macrophages. As an adiponectin paralog, CTRP1 signals through adiponectin receptor 1 (AdipoR1) and participates in a variety of pathophysiological processes. Circulating CTRP1 levels are significantly increased in patients with coronary artery disease. Importantly, CTRP1 was shown to accelerate the development of atherosclerosis by promoting vascular inflammation, macrophage foam cell formation and endothelial barrier dysfunction. This review focused on recent advances regarding the role of CTRP1 in atherogenesis with an emphasis on its potential as a novel biomarker and a promising therapeutic target for atherosclerosis-related diseases.

The collagen structure of C1q induces wound healing by engaging discoidin domain receptor 2

Background

C1q has been reported to reveal complement-independent roles in immune and non-immune cells. C1q binds to its specific receptors to regulate distinct functions that rely on the environment and cell types. Discoidin domain receptor 2 (DDR2) is activated by collagen and functions in wound healing by controlling matrix metalloproteinase (MMP) expression. Since C1q exhibits a collagen-like structure, we hypothesized that C1q might engage DDR2 to regulate wound healing and extracellular matrix (ECM) remodeling.

Methods

Cell-based assay, proximity ligation assay, ELISA, and surface plasmon analysis were utilized to investigate DDR2 and C1q binding. We also investigate the C1q-mediated in vitro wound healing ability using the human fibrosarcoma cell line, HT1080.

Results

C1q induced the phosphorylation of DDR2, p38 kinase, and ERK1/2. C1q and DDR2 binding improved cell migration and induced MMP2 and MMP9 expression. DDR2-specific shRNA reduced C1q-mediated cell migration for wound healing.

Conclusions

C1q is a new DDR2 ligand that promotes wound healing. These findings have therapeutic implications in wound healing-related diseases.

On-target IgG hexamerisation driven by a C-terminal IgM tail-piece fusion variant confers augmented complement activation

The majority of depleting monoclonal antibody (mAb) drugs elicit responses via Fc-FcγR and Fc-C1q interactions. Optimal C1q interaction is achieved through hexameric Fc:Fc interactions at the target cell surface. Herein is described an approach to exploit the tailpiece of the naturally multimeric IgM to augment hexamerisation of IgG. Fusion of the C-terminal tailpiece of IgM promoted spontaneous hIgG hexamer formation, resulting in enhanced C1q recruitment and complement-dependent cytotoxicity (CDC) but with off-target complement activation and reduced in-vivo efficacy. Mutation of the penultimate tailpiece cysteine to serine (C575S) ablated spontaneous hexamer formation, but facilitated reversible hexamer formation after concentration in solution. C575S mutant tailpiece antibodies displayed increased complement activity only after target binding, in-line with the concept of 'on-target hexamerisation', whilst retaining efficient in-vivo efficacy and augmented target cell killing in the lymph node. Hence, C575S-tailpiece technology represents an alternative format for promoting on-target hexamerisation and enhanced CDC.

Dendrimer end-terminal motif-dependent evasion of human complement and complement activation through IgM hitchhiking

Complement is an enzymatic humoral pattern-recognition defence system of the body. Non-specific deposition of blood biomolecules on nanomedicines triggers complement activation through the alternative pathway, but complement-triggering mechanisms of nanomaterials with dimensions comparable to or smaller than many globular blood proteins are unknown. Here we study this using a library of <6 nm poly(amido amine) dendrimers bearing different end-terminal functional groups. Dendrimers are not sensed by C1q and mannan-binding lectin, and hence do not trigger complement activation through these pattern-recognition molecules. While, pyrrolidone- and carboxylic acid-terminated dendrimers fully evade complement response, and independent of factor H modulation, binding of amine-terminated dendrimers to a subset of natural IgM glycoforms triggers complement activation through lectin pathway-IgM axis. These findings contribute to mechanistic understanding of complement surveillance of dendrimeric materials, and provide opportunities for dendrimer-driven engineering of complement-safe nanomedicines and medical devices.

Understanding nanomaterials interactions with complement is important for a number of applications. Here, the authors study the interaction of sub 6 nm dendrimers with complement and show the small dendrimers escape complement activation but do interact with IgM to trigger lectin-pathway complement activation.

Anti-Idiotype scFv Localizes an Autoepitope in the Globular Domain of C1q

We addressed the issue of C1q autoantigenicity by studying the structural features of the autoepitopes recognized by the polyclonal anti-C1q antibodies present in Lupus Nephritis (LN) sera. We used six fractions of anti-C1q as antigens and selected anti-idiotypic scFv antibodies from the phage library "Griffin.1". The monoclonal scFv A1 was the most potent inhibitor of the recognition of C1q and its fragments ghA, ghB and ghC, comprising the globular domain gC1q, by the lupus autoantibodies. It was sequenced and in silico folded by molecular dynamics into a 3D structure. The generated 3D model of A1 elucidated CDR similarity to the apical region of gC1q, thus mapping indirectly for the first time a globular autoepitope of C1q. The V_H CDR2 of A1 mimicked the ghA sequence GSEAD suggested as a cross-epitope between anti-DNA and anti-C1q antibodies. Other potential inhibitors of the recognition of C1q by the LN autoantibodies among the selected recombinant antibodies were the monoclonal scFv F6, F9 and A12.

The Enhanced Immune Protection in Small Abalone Haliotis diversicolor Against a Secondary Infection With Vibrio harveyi

In recent years, more and more studies have shown that early pathogenic bacterial infection in invertebrates can enhance immunity and significantly reduce mortality when reinfected with the same pathogen. There are mechanisms to explain this phenomenon, but they are relatively few. In addition, dose-dependent primary infection is also associated with increased immunity. In the present study, the initial infection dose and mortality of abalone Haliotis diversicolor after reinfection with Vibrio harveyi were recorded, and the mechanism of immune enhancement was investigated by the transcriptomic response of abalone after two successive stimuli with V. harveyi. Priming with different concentrations of pathogen can enhance immunity; however, higher concentration is not always better. Compared with the first exposure, more genes were up-regulated after the second exposure. Among the commonly expressed genes, the immune related genes were significantly or persistently highly expressed after two infections and included pattern recognition receptors as well as immune effectors, such as toll-like receptors, perlucin 4, scavenger receptor class B-like protein, cytochrome P450 1B1-like, glutathione S-transferase 6, lysozyme and so on; in addition, these immune-related genes were mainly distributed in the pathways related to phagocytosis and calcium signaling. Among the specifically expressed genes, compared with the first infection, more genes were involved in the immune, metabolic and digestive pathways after the second infection, which would be more conducive to preventing the invasion of pathogens. This study outlined the mechanism of immune enhancement in abalone after secondary infection at the global molecular level, which is helpful for a comprehensive understanding of the mechanism of immune priming in invertebrates.

C1QL3 promotes cell-cell adhesion by mediating complex formation between ADGRB3/BAI3 and neuronal pentraxins

Synapses are the fundamental structural unit by which neurons communicate. An orchestra of proteins regulates diverse synaptic functions, including synapse formation, maintenance, and elimination-synapse homeostasis. Some proteins of the larger C1q super-family are synaptic organizers involved in crucial neuronal processes in various brain regions. C1Q-like (C1QL) proteins bind to the adhesion G protein-coupled receptor B3 (ADGRB3) and act at synapses in a subset of circuits. To investigate the hypothesis that the secreted C1QL proteins mediate tripartite trans-synaptic adhesion complexes, we conducted an in vivo interactome study and identified new binding candidates. We demonstrate that C1QL3 mediates a novel cell-cell adhesion complex involving ADGRB3 and two neuronal pentraxins, NPTX1 and NPTXR. Analysis of single-cell RNA-Seq data from the cerebral cortex shows that C1ql3, Nptx1, and Nptxr are highly co-expressed in the same excitatory neurons. Thus, our results suggest the possibility that in vivo the three co-expressed proteins are presynaptically secreted and form a complex capable of binding to postsynaptically localized ADGRB3, thereby creating a novel trans-synaptic adhesion complex. Identifying new binding partners for C1QL proteins and deciphering their underlying molecular principles will accelerate our understanding of their role in synapse organization.

Strong, non-specific adhesion using C-lectin heterotrimers in a molluscan defensive secretion

The defensive mucus produced by the slug Arion subfuscus is tough and remarkably sticky. It spreads rapidly and adheres strongly to a wide range of surfaces. The adhesion is equally strong on wettable (glass) and non-wettable (plastic) surfaces. The adhesion appears to depend on a group of proteins that adsorb equally well to a wide range of different natural and artificial surfaces. Prominent among these proteins were those that distinguish the adhesive secretion from the non-adhesive mucus. The adhesive proteins were not washed off by non-ionic detergent, nor was the adhesion of the glue as a whole affected by this treatment. In contrast, high salt concentrations washed the most abundant adhesive proteins off the surfaces, and correspondingly weakened the glue's attachment. The most abundant of the adhesive proteins were C-lectins, which appear to form heterotrimers. These and other lectin-like proteins in slug glue have a high proportion of aromatic amino acids at conserved locations, and are relatively small and often basic. The aromatic and cationic side chains may provide a powerful combination promoting and maintaining surface adhesion.

Systems Biology Modeling of the Complement System Under Immune Susceptible Pathogens

The complement system is assembled from a network of proteins that function to bring about the first line of defense of the body against invading pathogens. However, complement deficiencies or invasive pathogens can hijack complement to subsequently increase susceptibility of the body to infections. Moreover, invasive pathogens are increasingly becoming resistant to the currently available therapies. Hence, it is important to gain insights into the highly dynamic interaction between complement and invading microbes in the frontlines of immunity. Here, we developed a mathematical model of the complement system composed of 670 ordinary differential equations with 328 kinetic parameters, which describes all three complement pathways (alternative, classical, and lectin) and includes description of mannose-binding lectin, collectins, ficolins, factor H-related proteins, immunoglobulin M, and pentraxins. Additionally, we incorporate two pathogens: (type 1) complement susceptible pathogen and (type 2) Neisseria meningitidis located in either nasopharynx or bloodstream. In both cases, we generate time profiles of the pathogen surface occupied by complement components and the membrane attack complex (MAC). Our model shows both pathogen types in bloodstream are saturated by complement proteins, whereas MACs occupy <<1.0% of the pathogen surface. Conversely, the MAC production in nasopharynx occupies about 1.5-10% of the total N. meningitidis surface, thus making nasal MAC levels at least about eight orders of magnitude higher. Altogether, we predict complement-imbalance, favoring overactivation, is associated with nasopharynx homeostasis. Conversely, orientating toward complement-balance may cause disruption to the nasopharynx homeostasis. Thus, for sporadic meningococcal disease, our model predicts rising nasal levels of complement regulators as early infection biomarkers.

Complement in Tumourigenesis and the Response to Cancer Therapy

In recent years, our knowledge of the complement system beyond innate immunity has progressed significantly. A modern understanding is that the complement system has a multifaceted role in malignancy, impacting carcinogenesis, the acquisition of a metastatic phenotype and response to therapies. The ability of local immune cells to produce and respond to complement components has provided valuable insights into their regulation, and the subsequent remodeling of the tumour microenvironment. These novel discoveries have advanced our understanding of the immunosuppressive mechanisms supporting tumour growth and uncovered potential therapeutic targets. This review discusses the current understanding of complement in cancer, outlining both direct and immune cell-mediated roles. The role of complement in response to therapies such as chemotherapy, radiation and immunotherapy is also presented. While complement activities are largely context and cancer type-dependent, it is evident that promising therapeutic avenues have been identified, in particular in combination therapies.

On Origin and Evolution of the Antibody Molecule

Simple Summary

Like many other molecules playing vital functions in animals, the antibody molecule possesses a complex structure with distinctive features. The structure of the basic unit, i.e., the immunoglobulin domain of very ancient origin is substantially simple. However, high complexity resides in the types and numbers of the domains composing the whole molecule. The emergence of the antibody molecule during evolution overturned the effectiveness of the organisms’ defense system. The particular organization of the coding genes, the mechanisms generating antibody diversity, and the plasticity of the overall protein structure, attest to an extraordinary successful evolutionary history. Here, we attempt to trace, across the evolutionary scale, the very early origins of the most significant features characterizing the structure of the antibody molecule and of the molecular mechanisms underlying its major role in recognizing an almost unlimited number of pathogens.

Abstract

The vertebrate immune system provides a powerful defense because of the ability to potentially recognize an unlimited number of pathogens. The antibody molecule, also termed immunoglobulin (Ig) is one of the major mediators of the immune response. It is built up from two types of Ig domains: the variable domain, which provides the capability to recognize and bind a potentially infinite range of foreign substances, and the constant domains, which exert the effector functions. In the last 20 years, advances in our understanding of the molecular mechanisms and structural features of antibody in mammals and in a variety of other organisms have uncovered the underlying principles and complexity of this fundamental molecule. One notable evolutionary topic is the origin and evolution of antibody. Many aspects have been clearly stated, but some others remain limited or obscure. By considering a wide range of prokaryotic and eukaryotic organisms through a literature survey about the topic, we have provided an integrated view of the emergence of antibodies in evolution and underlined the very ancient origins.

Molecular and functional analysis of PmC1qDC in nacre formation of Pinctada fucata martensii

The C1q-domain-containing (C1qDC) proteins are a family of proteins characterized by a globular C1q (gC1q) domain in their C-terminus which hold the potential function in the shell formation as shell matrix proteins. In this study, a C1qDC protein was identified and characterized in pearl oyster (Pinctada fucata martensii) (PmC1qDC) to explore its function in nacre formation. The PmC1qDC-deduced protein sequence carried a typical globular C1q (gC1q) domain that possessed the typical 10-stranded β-sandwich fold with a jelly-roll topology common to all C1qDC family members and shared high homology with other gC1q domains. Homologous analysis of PmC1qDC presented it contained conserved secondary structure and Phe135, Phe155, Tyr166, Phe173, Tyr181, Phe183, and Phe256 amino acid residues. Expression pattern analysis showed that PmC1qDC expressed in all the detected tissues and exhibited a significantly higher expression level in nacre formation-associated tissues. After the shell notching, the expression level of PmC1qDC showed significantly up-regulation after 12 h in the central zone of mantle (MC). PmC1qDC expression significantly decreased in the MC after RNA interference (RNAi). Furthermore, disordered crystals with evident rough surface and irregular crystal tablets were observed in the nacre after RNAi. Results suggested that PmC1qDC affects the shell nacre formation, which is significant to improve the pearl production of pearl oyster.

Microglia-Astrocyte Communication via C1q Contributes to Orofacial Neuropathic Pain Associated with Infraorbital Nerve Injury

Trigeminal nerve injury causes a distinct time window of glial activation in the trigeminal spinal subnucleus caudalis (Vc), which are involved in the initiation and maintenance phases of orofacial neuropathic pain. Microglia-derived factors enable the activation of astrocytes. The complement component C1q, which promotes the activation of astrocytes, is known to be synthesized in microglia. However, it is unclear whether microglia–astrocyte communication via C1q is involved in orofacial neuropathic pain. Here, we analyzed microglia-astrocyte communication in a rat model with infraorbital nerve injury (IONI). The orofacial mechanical hypersensitivity induced by IONI was significantly attenuated by preemptive treatment with minocycline. Immunohistochemical analyses revealed that minocycline inhibited the increase in c-Fos immune-reactive (IR) cells and the fluorescence intensity of both Iba1 and glial fibrillary acidic protein (GFAP) in the Vc following IONI. Intracisternal administration of C1q caused orofacial mechanical hypersensitivity and an increase in the number of c-Fos-IR cells and fluorescence intensity of GFAP. C1q-induced orofacial mechanical hypersensitivity was completely abrogated by intracisternal administration of fluorocitrate. The present findings suggest that the enhancement in the excitability of Vc nociceptive neurons is produced by astrocytic activation via the signaling of C1q released from activated microglia in the Vc following IONI, resulting in persistent orofacial neuropathic pain.

Immune Evasion Strategies of Relapsing Fever Spirochetes

Relapsing fever (RF) is claimed a neglected arthropod-borne disease caused by a number of diverse human pathogenic Borrelia (B.) species. These RF borreliae are separated into the groups of tick-transmitted species including B. duttonii, B. hermsii, B. parkeri, B. turicatae, B. hispanica, B. persica, B. caucasica, and B. myiamotoi, and the louse-borne Borrelia species B. recurrentis. As typical blood-borne pathogens achieving high cell concentrations in human blood, RF borreliae (RFB) must outwit innate immunity, in particular complement as the first line of defense. One prominent strategy developed by RFB to evade innate immunity involves inactivation of complement by recruiting distinct complement regulatory proteins, e.g., C1 esterase inhibitor (C1-INH), C4b-binding protein (C4BP), factor H (FH), FH-like protein-1 (FHL-1), and factor H-related proteins FHR-1 and FHR-2, or binding of individual complement components and plasminogen, respectively. A number of multi-functional, complement and plasminogen-binding molecules from distinct Borrelia species have previously been identified and characterized, exhibiting considerable heterogeneity in their sequences, structures, gene localization, and their capacity to bind host-derived proteins. In addition, RFB possess a unique system of antigenic variation, allowing them to change the composition of surface-exposed variable major proteins, thus evading the acquired immune response of the human host. This review focuses on the current knowledge of the immune evasion strategies by RFB and highlights the role of complement-interfering and infection-associated molecules for the pathogenesis of RFB.

Functional and Structural Characterization of a Potent C1q Inhibitor Targeting the Classical Pathway of the Complement System

The classical pathway of complement is important for protection against pathogens and in maintaining tissue homeostasis, but excessive or aberrant activation is directly linked to numerous pathologies. We describe the development and in vitro characterization of C1qNb75, a single domain antibody (nanobody) specific for C1q, the pattern recognition molecule of the classical pathway. C1qNb75 binds to the globular head modules of human C1q with sub-nanomolar affinity and impedes classical pathway mediated hemolysis by IgG and IgM. Crystal structure analysis revealed that C1qNb75 recognizes an epitope primarily located in the C1q B-chain that overlaps with the binding sites of IgG and IgM. Thus, C1qNb75 competitively prevents C1q from binding to IgG and IgM causing blockade of complement activation by the classical pathway. Overall, C1qNb75 represents a high-affinity nanobody-based inhibitor of IgG- and IgM-mediated activation of the classical pathway and may serve as a valuable reagent in mechanistic and functional studies of complement, and as an efficient inhibitor of complement under conditions of excessive CP activation.

Identification of immune-related proteins of Dreissena polymorpha hemocytes and plasma involved in host-microbe interactions by differential proteomics

Biological responses of zebra mussel Dreissena polymorpha are investigated to assess the impact of contaminants on aquatic organisms and ecosystems. In addition to concentrate chemical contaminants in their tissues, zebra mussels accumulate several microorganisms such as viruses, protozoa and bacteria. In order to understand the molecular mechanisms involved in the defence against microorganisms this study aims at identifying immune proteins from D. polymorpha hemolymph involved in defence against protozoa and viruses. For this purpose, hemolymph were exposed ex vivo to Cryptosporidium parvum and RNA poly I:C. Differential proteomics on both hemocytes and plasma revealed immune proteins modulated under exposures. Different patterns of response were observed after C. parvum and RNA poly I:C exposures. The number of modulated proteins per hemolymphatic compartments suggest that C. parvum is managed in cells while RNA poly I:C is managed in plasma after 4 h exposure. BLAST annotation and GO terms enrichment analysis revealed further characteristics of immune mechanisms. Results showed that many proteins involved in the recognition and destruction of microorganisms were modulated in both exposure conditions, while proteins related to phagocytosis and apoptosis were exclusively modulated by C. parvum. This differential proteomic analysis highlights in zebra mussels modulated proteins involved in the response to microorganisms, which reflect a broad range of immune mechanisms such as recognition, internalization and destruction of microorganisms. This study paves the way for the identification of new markers of immune processes that can be used to assess the impact of both chemical and biological contaminations on the health status of aquatic organisms.

Transient increase of microglial C1q expression in the circumventricular organs of adult mouse during LPS-induced inflammation

The circumventricular organs (CVOs) are the brain regions that lack the blood-brain barrier and allow free entry of blood-derived molecules, offering specialized niche to initiate rapid and early neuroinflammatory responses in the brain. Complement component 1q (C1q) is shown to be the first recognition component of the complement pathway and has a crucial function in the brain under pathological conditions. In the present study, we found that C1q expression in CX3CR1-positive microglia was increased in the CVOs and their neighbouring brain regions of adult mice at 1 day after a single administration of 1 mg/kg lipopolysaccharide (LPS), whereas it returned to control levels at 3 days after LPS stimulation. C1q expression was also seen to localize at synapsin-positive presynaptic axonal terminals in various brain regions. Thus, the present study demonstrates a transient upregulation of microglial C1q expression in the CVOs and their adjacent brain regions, indicating that a transient upregulation of C1q is possibly concerned with physiological responses at early phase of brain inflammation. SIGNIFICANCE OF THE STUDY: The circumventricular organs (CVOs) are specialized brain regions that lack the blood-brain barrier (BBB) and initiate neuroinflammatory responses in the brains. The present study showed that the expression of complement protein C1q was highly increased in microglia of the CVOs and their adjacent brain regions. Moreover, C1q expression was observed to localize specifically at presynaptic axonal terminals in the CVOs and their neighbouring brain regions. Thus, the present study indicates that C1q is possibly correlated with physiological responses at early phase of brain inflammation.

Effects of the surface polarity of nanomaterials on their interaction with complement protein gC1q

The dependence of the recognition mode of gC1q on the surface polarity of nanomaterials is mainly attributed to its distinct surface residue composition.

Pattern recognition receptors in annelids

The existence of pattern recognition receptors (PRRs) on immune cells was discussed in 1989 by Charles Janeway, Jr., who proposed a general concept of the ability of PRRs to recognize and bind conserved molecular structures of microorganisms known as pathogen-associated molecular patterns (PAMPs). Upon PAMP engagement, PRRs trigger intracellular signaling cascades resulting in the expression of various proinflammatory molecules. These recognition molecules represent an important and efficient innate immunity tool of all organisms. As invertebrates lack the instruments of the adaptive immune system, based on "true" lymphocytes and functional antibodies, the importance of PRRs are even more fundamental. In the present review, the structure, specificity, and expression profiles of PRRs characterized in annelids are discussed, and their role in innate defense is suggested.

Complement activation by IgG containing immune complexes regulates the interaction of C1q with its ligands

Classical pathway activation of the compl ement system is initiated by the binding of the globular head domains of glycoprotein C1q to its corresponding ligand leading to both C1 activation and C3 convertase formation. However, the whereabouts and function of C1q after complement activation have only been marginally investigated. This report presents two mechanisms of action that remove bound C1q from a complement activating IgG immune complex in concentrated serum. The first mechanism details that sequential activation of the classical and alternative pathways releases bound C1q from an immune complex and that the dissociated C1q is subsequently found in complex with complement fragment C3c. The second mechanism is the displacement of C1q from an immune complex by the addition of near physiologic concentrations of purified or serum C1q. This activity can also be demonstrated using serum depleted of C3, normal serum chelated in EDTA, or purified C1. Fresh C1q in C3-depleted serum was found to replace dissociated C1q on the immune complex. C1q dissociated from immune complexes by the mechanism of C1q displacement is able to bind B and T lymphoblastoid cells that express receptors and ligands for both the collagen like region and the globular head domains of C1q. C1q dissociated from immune complexes by the mechanism of C3 activation do not bind these cells. This result suggests that C3 bound to C1q during complement activation and dissociation interferes with the ability of released C1q to access C1q receptors and ligands, particularly receptors for the globular head domains. These underlying mechanisms that regulate the interaction of C1q with its ligands reveal a novel function for complement activation during the immune response.

Conceptual Approaches to Modulating Antibody Effector Functions and Circulation Half-Life

Antibodies and Fc-fusion antibody-like proteins have become successful biologics developed for cancer treatment, passive immunity against infection, addiction, and autoimmune diseases. In general these biopharmaceuticals can be used for blocking protein:protein interactions, crosslinking host receptors to induce signaling, recruiting effector cells to targets, and fixing complement. With the vast capability of antibodies to affect infectious and genetic diseases much effort has been placed on improving and tailoring antibodies for specific functions. While antibody:antigen engagement is critical for an efficacious antibody biologic, equally as important are the hinge and constant domains of the heavy chain. It is the hinge and constant domains of the antibody that engage host receptors or complement protein to mediate a myriad of effector functions and regulate antibody circulation. Molecular and structural studies have provided insight into how the hinge and constant domains from antibodies across different species, isotypes, subclasses, and alleles are recognized by host cell receptors and complement protein C1q. The molecular details of these interactions have led to manipulation of the sequences and glycosylation of hinge and constant domains to enhance or reduce antibody effector functions and circulating half-life. This review will describe the concepts being applied to optimize the hinge and crystallizable fragment of antibodies, and it will detail how these interactions can be tuned up or down to mediate a biological function that confers a desired disease outcome.

Emerging Roles of Complement Protein C1q in Neurodegeneration

The innate immune system is an ancient and primary component system that rapidly reacts to defend the body against external pathogens. C1 is the initial responder of classical pathway of the innate immune system. C1 is comprised of C1q, C1r, and C1s. Among them, C1q is known to interact with diverse ligands, which can perform various functions in physiological and pathophysiological conditions. Because C1q participates in the clearance of pathogens, its interaction with novel receptors is expected to facilitate apoptosis induction, which could prevent the onset or progression of neurodegenerative diseases and could delay the aging process. Because senescence-associated secreting phenotype determinants are generally inflammatory cytokines or immune factors to activate immune cells. In the central nervous system, C1q has diverse neuroprotective roles against pathogens and inflammation. Most of neurodegenerative diseases show region specific pathology feature in the brain. It has been suggested the evidences that the active site and amount of C1q may be disease specific. This review considers currently the emerging and under-recognized roles of C1q in neurodegeneration and highlights the need for further research to clarify these roles. Future studies on the roles of C1q in regulating disease progression should consider these aspects, including the age-dependent onset time of each neurodegenerative disease progression.

Is the Complement Protein C1q a Pro- or Anti-tumorigenic Factor? Bioinformatics Analysis Involving Human Carcinomas

C1q is the first subcomponent of the classical pathway of the complement system and belongs to the C1q/Tumor Necrosis Factor superfamily. C1q can perform a diverse range of immune and non-immune functions in a complement-dependent as well as -independent manner. Being a pattern recognition molecule of the innate immunity, C1q can recognize a number of self, non-self and altered-self ligands and bring about effector mechanisms designed to clear pathogens via opsonisation and inflammatory response. C1q is locally synthesized by macrophages and dendritic cells, and thus, can get involved in a range of biological processes, such as angiogenesis and tissue remodeling, immune modulation, and immunologic tolerance. The notion of C1q involvement in the pathogenesis of cancer is still evolving. C1q appears to have a dual role in cancer: tumor promoting as well as tumor-protective, depending on the context of the disease. In the current study, we performed a bioinformatics analysis to investigate whether C1q can serve as a potential prognostic marker for human carcinoma. We used the Oncomine database and the survival analysis platforms Kaplan-Meier plotter. Our results showed that high levels of C1q have a favorable prognostic index in basal-like breast cancer for disease-free survival, and in HER2-positive breast cancer for overall survival, while it showed a pro-tumorigenic role of C1q in lung adenocarcinoma, and in clear cell renal cell carcinoma. This in silico study, if validated via a retrospective study, can be a step forward in establishing C1q as a new tool as a prognostic biomarker for various carcinoma.

A novel C1q domain containing protein in black rockfish (Sebastes schlegelii) serves as a pattern recognition receptor with immunoregulatory properties and possesses binding activity to heat-aggregated IgG

C1q-domain-containing (C1qDC) proteins, which are involved in a series of immune responses, are important pattern recognition receptors in innate immunity in vertebrates and invertebrates. Functional studies of C1qDC proteins in vertebrates are scarce. In the present study, a C1qDC protein (SsC1qDC) from the teleost black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. The open reading frame of SsC1qDC is 636 bp, and the predicted amino acid sequence of SsC1qDC shares 62%-69% overall identity with the C1qDC proteins of several fish species. SsC1qDC possesses conserved C1qDC features, including a signal sequence and a C1q domain. SsC1qDC was expressed in different tissues and its expression was up-regulated by bacterial and viral infection. Recombinant SsC1qDC (rSsC1qDC) exhibited apparent binding activities against PAMPs including LPS and PGN. rSsC1qDC had antibacterial activity against Vibrio parahaemolyticus, and was able to enhance the phagocytic activity of macrophages towards Vibrio anguillarum. rSsC1qDC interacted with human heat-aggregated IgG. Furthermore, in the presence of rSsC1qDC, fish exhibited enhanced resistance against bacterial infection. Collectively, these results indicated that SsC1qDC serves as a pattern recognition receptor and plays a vital role in the defense system of black rockfish.

A novel globular C1q domain containing protein (C1qDC-7) from Crassostrea gigas acts as pattern recognition receptor with broad recognition spectrum

The globular C1q domain containing (C1qDC) proteins are a family of versatile pattern recognition receptors (PRRs) to bind various ligands by their globular C1q (gC1q) domain. In the present study, a novel globular C1qDC (CgC1qDC-7) was characterized from Pacific oyster Crassostrea gigas. The open reading frame of CgC1qDC-7 was of 555 bp, encoding a polypeptide of 185 amino acids. Phylogenetic analysis indicated that CgC1qDC-7 shared high homology with C1qDCs from Crassostrea virginica, Mytilus galloprovincialis, and Mizuhopecten yessoensis. The mRNA transcripts of CgC1qDC-7 were widely expressed in all the tested tissues including mantle, gonad, gills, adductor muscle, hemocytes, hepatopancreas and labial palps, with the highest expression level in hemocytes and gills. The recombinant protein of CgC1qDC-7 (rCgC1qDC-7) exhibited binding activity towards Gram-negative bacteria (Vibrio splendidus, V. anguillarum, Escherichia coli, V. alginolyticus, and Aeromonas hydrophila), Gram-positive bacteria (Micrococcus luteus and Staphylococcus aureus) and fungi (Pichia pastoris and Yarrowia lipolytica), and displayed strongest binding affinity towards Gram-negative bacteria V. splendidus and V. anguillarum. It also exhibited affinity to vital pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), peptidoglycan (PGN), mannan (MAN) and Poly (I:C) with high affinity towards LPS and PGN, and low affinity to MAN and Poly (I:C). These results collectively indicated that CgC1qDC-7 was a novel PRR in C. gigas with high binding affinity towards LPS and PGN as well as Gram-negative bacteria.

Critical Role for a Subset of Intestinal Macrophages in Shaping Gut Microbiota in Adult Zebrafish

The gut microbiota is strongly influenced by environmental factors, although host contribution is far less understood. We leveraged macrophage-deficient interferon regulatory factor irf8 zebrafish mutants to investigate the role of macrophages in this process. In conventionally raised adult irf8-deficient mutants, we found a significant loss of intestinal macrophages associated with a strikingly altered gut microbiota when compared to co-housed siblings. The destabilization of the gut commensal microbiota was associated with a severe reduction in complement C1q genes and outgrowth of a rare bacterial species. Consistent with a critical function of irf8 in adult intestinal macrophages, irf8 is abundantly expressed in these cells normally, and restoring macrophage irf8 expression in irf8 mutants was sufficient to recover commensal microbes and C1q genes expression. This study reports an important subpopulation of intestinal macrophages that requires irf8 to establish in the gut, ensure normal colonization of gut microbes, and prevent immune dysregulation.

Chimeric Proteins Containing MAP-1 and Functional Domains of C4b-Binding Protein Reveal Strong Complement Inhibitory Capacities

The complement system is a tightly regulated network of proteins involved in defense against pathogens, inflammatory processes, and coordination of the innate and adaptive immune responses. Dysregulation of the complement cascade is associated with many inflammatory disorders. Thus, inhibition of the complement system has emerged as an option for treatment of a range of different inflammatory diseases. MAP-1 is a pattern recognition molecule (PRM)-associated inhibitor of the lectin pathway of the complement system, whereas C4b-binding protein (C4BP) regulates both the classical and lectin pathways. In this study we generated chimeric proteins consisting of MAP-1 and the first five domains of human C4BP (C4BP¹⁻⁵) in order to develop a targeted inhibitor acting at different levels of the complement cascade. Two different constructs were designed and expressed in CHO cells where MAP-1 was fused with C4BP¹⁻⁵ in either the C- or N-terminus. The functionality of the chimeric proteins was assessed using different in vitro complement activation assays. Both chimeric proteins displayed the characteristic Ca²⁺-dependent dimerization and binding to PRMs of native MAP-1, as well as the co-factor activity of native C4BP. In ELISA-based complement activation assays they could effectively inhibit the lectin and classical pathways. Notably, MAP-1:C4BP¹⁻⁵ was five times more effective than rMAP-1 and rC4BP¹⁻⁵ applied at the same time, emphasizing the advantage of a single inhibitor containing both functional domains. The MAP-1/C4BP chimeras exert unique complement inhibitory properties and represent a novel therapeutic approach targeting both upstream and central complement activation.

Comparative genomic analysis of eutherian adiponectin genes

The present study proposed updated and standardized classification and nomenclature of eutherian adiponectin genes implicated in regulation of systemic metabolism and inflammation and activation of classical complement pathway. The revisions of comprehensive adiponectin gene data sets used eutherian comparative genomic analysis protocol and public reference genomic sequence assemblies. Among 438 potential coding sequences, the tests of reliability of eutherian public genomic sequences annotated most comprehensive curated third-party data gene data set of eutherian adiponectin genes that included 211 complete coding sequences. There were 18 major gene clusters of eutherian adiponectin genes described, one of which included evidence of differential gene expansions. For example, the present analysis initially described human ADIF2 and ADIR genes. Finally, the tests of protein molecular evolution using relative synonymous codon usage statistics confirmed protein primary structure similarities between eutherian adiponectins and tumor necrosis factor ligands.

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.

Pathogen-Derived Carbohydrate Recognition in Molluscs Immune Defense

Self-nonself discrimination is a common theme for all of the organisms in different evolutionary branches, which is also the most fundamental step for host immune protection. Plenty of pattern recognition receptors (PRRs) with great diversity have been identified from different organisms to recognize various pathogen-associated molecular patterns (PAMPs) in the last two decades, depicting a complicated scene of host-pathogen interaction. However, the detailed mechanism of the complicate PAMPs–PRRs interactions at the contacting interface between pathogens and hosts is still not well understood. All of the cells are coated by glycosylation complex and thick carbohydrates layer. The different polysaccharides in extracellular matrix of pathogen-host are important for nonself recognition of most organisms. Coincidentally, massive expansion of PRRs, majority of which contain recognition domains of Ig, leucine-rich repeat (LRR), C-type lectin (CTL), C1q and scavenger receptor (SR), have been annotated and identified in invertebrates by screening the available genomic sequence. The phylum Mollusca is one of the largest groups in the animal kingdom with abundant biodiversity providing plenty of solutions about pathogen recognition and immune protection, which might offer a suitable model to figure out the common rules of immune recognition mechanism. The present review summarizes the diverse PRRs and common elements of various PAMPs, especially focusing on the structural and functional characteristics of canonical carbohydrate recognition proteins and some novel proteins functioning in molluscan immune defense system, with the objective to provide new ideas about the immune recognition mechanisms.

Is the A-Chain the Engine That Drives the Diversity of C1q Functions? Revisiting Its Unique Structure

The immunopathological functions associated with human C1q are still growing in terms of novelty, diversity, and pathologic relevance. It is, therefore, not surprising that C1q is being recognized as an important molecular bridge between innate and adaptive immunity. The secret of this functional diversity, in turn, resides in the elegant but complex structure of the C1q molecule, which is assembled from three distinct gene products: A, B, and C, each of which has evolved from a separate and unique ancestral gene template. The C1q molecule is made up of 6A, 6B, and 6C polypeptide chains, which are held together through strong covalent and non-covalent bonds to form the 18-chain, bouquet-of-flower-like protein that we know today. The assembled C1q protein displays at least two distinct structural and functional regions: the collagen-like region (cC1q) and the globular head region (gC1q), each being capable of driving a diverse range of ligand- or receptor-mediated biological functions. What is most intriguing, however, is the observation that most of the functions appear to be predominantly driven by the A-chain of the molecule, which begs the question: what are the evolutionary modifications or rearrangements that singularly shaped the primordial A-chain gene to become a pluripotent and versatile component of the intact C1q molecule? Here, we revisit and discuss some of the known unique structural and functional features of the A-chain, which may have contributed to its versatility.