Structural Immunology of Complement Receptors 3 and 4

αMI-domain of integrin Mac-1 binds the cytokine pleiotrophin using multiple mechanisms

The integrin Mac-1 (αMβ2, CD11b/CD18, CR3) is an adhesion receptor expressed on macrophages and neutrophils. Mac-1 is also a promiscuous integrin that binds a diverse set of ligands through its αMI-domain. However, the binding mechanism of most ligands remains unclear. We have characterized the interaction of αMI-domain with the cytokine pleiotrophin (PTN), a protein known to bind αMI-domain and induce Mac-1-mediated cell adhesion and migration. Our data show that PTN's N-terminal domain binds a unique site near the N- and C-termini of the αMI-domain using a metal-independent mechanism. However, a stronger interaction is achieved when an acidic amino acid in a zwitterionic motif in PTN's C-terminal domain chelates the divalent cation in the metal ion-dependent adhesion site of active αMI-domain. These results indicate that αMI-domain can bind ligands using multiple mechanisms and that the active αMI-domain has a preference for motifs containing both positively and negatively charged amino acids.

Complement receptor 4 mediates the clearance of extracellular tau fibrils by microglia

Tauopathies exhibit a characteristic accumulation of misfolded tau aggregates in the brain. Tau pathology shows disease-specific spatiotemporal propagation through intercellular transmission, which is closely correlated with the progression of clinical manifestations. Therefore, identifying molecular mechanisms that prevent tau propagation is critical for developing therapeutic strategies for tauopathies. The various innate immune receptors, such as complement receptor 3 (CR3) and complement receptor 4 (CR4), have been reported to play a critical role in the clearance of various extracellular toxic molecules by microglia. However, their role in tau clearance has not been studied yet. In the present study, we investigated the role of CR3 and CR4 in regulating extracellular tau clearance. We found that CR4 selectively binds to tau fibrils but not to tau monomers, whereas CR3 does not bind to either of them. Inhibiting CR4, but not CR3, significantly reduces the uptake of tau fibrils by BV2 cells and primary microglia. By contrast, inhibiting CR4 has no effect on the uptake of tau monomers by BV2 cells. Furthermore, inhibiting CR4 suppresses the clearance of extracellular tau fibrils, leading to more seed-competent tau fibrils remaining in the extracellular space relative to control samples. We also provide evidence that the expression of CR4 is upregulated in the brains of human Alzheimer's disease patients and the PS19 mouse model of tauopathy. Taken together, our data strongly support that CR4 is a previously undescribed receptor for the clearance of tau fibrils in microglia and may represent a novel therapeutic target for tauopathy.

Complement receptor 3-dependent engagement by Candida glabrata β-glucan modulates dendritic cells to induce regulatory T-cell expansion

Candida glabrata is an important pathogen causing invasive infection associated with a high mortality rate. One mechanism that causes the failure of Candida eradication is an increase in regulatory T cells (Treg), which play a major role in immune suppression and promoting Candida pathogenicity. To date, how C. glabrata induces a Treg response remains unclear. Dendritic cells (DCs) recognition of fungi provides the fundamental signal determining the fate of the T-cell response. This study investigated the interplay between C. glabrata and DCs and its effect on Treg induction. We found that C. glabrata β-glucan was a major component that interacted with DCs and consequently mediated the Treg response. Blocking the binding of C. glabrata β-glucan to dectin-1 and complement receptor 3 (CR3) showed that CR3 activation in DCs was crucial for the induction of Treg. Furthermore, a ligand-receptor binding assay showed the preferential binding of C. glabrata β-glucan to CR3. Our data suggest that C. glabrata β-glucan potentially mediates the Treg response, probably through CR3-dependent activation in DCs. This study contributes new insights into immune modulation by C. glabrata that may lead to a better design of novel immunotherapeutic strategies for invasive C. glabrata infection.

Low-frequency inherited complement receptor variants are associated with purpura fulminans

ABSTRACT

Extreme disease phenotypes can provide key insights into the pathophysiology of common conditions, but studying such cases is challenging due to their rarity and the limited statistical power of existing methods. Herein, we used a novel approach to pathway-based mutational burden testing, the rare variant trend test (RVTT), to investigate genetic risk factors for an extreme form of sepsis-induced coagulopathy, infectious purpura fulminans (PF). In addition to prospective patient sample collection, we electronically screened over 10.4 million medical records from 4 large hospital systems and identified historical cases of PF for which archived specimens were available to perform germline whole-exome sequencing. We found a significantly increased burden of low-frequency, putatively function-altering variants in the complement system in patients with PF compared with unselected patients with sepsis (P = .01). A multivariable logistic regression analysis found that the number of complement system variants per patient was independently associated with PF after controlling for age, sex, and disease acuity (P = .01). Functional characterization of PF-associated variants in the immunomodulatory complement receptors CR3 and CR4 revealed that they result in partial or complete loss of anti-inflammatory CR3 function and/or gain of proinflammatory CR4 function. Taken together, these findings suggest that inherited defects in CR3 and CR4 predispose to the maladaptive hyperinflammation that characterizes severe sepsis with coagulopathy.

Monocyte NLRP3 inflammasome and interleukin-1β activation modulated by alpha-1 antitrypsin therapy in deficient individuals

INTRODUCTION

Altered complement component 3 (C3) activation in patients with alpha-1 antitrypsin (AAT) deficiency (AATD) has been reported. To understand the potential impact on course of inflammation, the aim of this study was to investigate whether C3d, a cleavage-product of C3, triggers interleukin (IL)-1β secretion via activation of NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome. The objective was to explore the effect of AAT augmentation therapy in patients with AATD on the C3d/complement receptor 3 (CR3) signalling axis of monocytes and on circulating pro-inflammatory markers.

METHODS

Inflammatory mediators were detected in blood from patients with AATD (n=28) and patients with AATD receiving augmentation therapy (n=19). Inflammasome activation and IL-1β secretion were measured in monocytes of patients with AATD, and following C3d stimulation in the presence or absence of CR3 or NLRP3 inhibitors.

RESULTS

C3d acting via CR3 induces NLRP3 and pro-IL-1β production, and through induction of endoplasmic reticulum (ER) stress and calcium flux, triggers caspase-1 activation and IL-1β secretion. Treatment of individuals with AATD with AAT therapy results in decreased plasma levels of C3d (3.0±1.2 µg/mL vs 1.3±0.5 µg/mL respectively, p<0.0001) and IL-1β (115.4±30 pg/mL vs 73.3±20 pg/mL, respectively, p<0.0001), with a 2.0-fold decrease in monocyte NLRP3 protein expression (p=0.0303), despite continued ER stress activation.

DISCUSSION

These results provide strong insight into the mechanism of complement-driven inflammation associated with AATD. Although the described variance in C3d and NLRP3 activation decreased post AAT augmentation therapy, results demonstrate persistent C3d and monocyte ER stress, with implications for new therapeutics and clinical practice.

The Promise of Complement Therapeutics in Solid Organ Transplantation

Transplantation is the ideal therapy for end-stage organ failure, but outcomes for all transplant organs are suboptimal, underscoring the need to develop novel approaches to improve graft survival and function. The complement system, traditionally considered a component of innate immunity, is now known to broadly control inflammation and crucially contribute to induction and function of adaptive T-cell and B-cell immune responses, including those induced by alloantigens. Interest of pharmaceutical industries in complement therapeutics for nontransplant indications and the understanding that the complement system contributes to solid organ transplantation injury through multiple mechanisms raise the possibility that targeting specific complement components could improve transplant outcomes and patient health. Here, we provide an overview of complement biology and review the roles and mechanisms through which the complement system is pathogenically linked to solid organ transplant injury. We then discuss how this knowledge has been translated into novel therapeutic strategies to improve organ transplant outcomes and identify areas for future investigation. Although the clinical application of complement-targeted therapies in transplantation remains in its infancy, the increasing availability of new agents in this arena provides a rich environment for potentially transformative translational transplant research.

The human factor H protein family - an update

Complement is an ancient and complex network of the immune system and, as such, it plays vital physiological roles, but it is also involved in numerous pathological processes. The proper regulation of the complement system is important to allow its sufficient and targeted activity without deleterious side-effects. Factor H is a major complement regulator, and together with its splice variant factor H-like protein 1 and the five human factor H-related (FHR) proteins, they have been linked to various diseases. The role of factor H in inhibiting complement activation is well studied, but the function of the FHRs is less characterized. Current evidence supports the main role of the FHRs as enhancers of complement activation and opsonization, i.e., counter-balancing the inhibitory effect of factor H. FHRs emerge as soluble pattern recognition molecules and positive regulators of the complement system. In addition, factor H and some of the FHR proteins were shown to modulate the activity of immune cells, a non-canonical function outside the complement cascade. Recent efforts have intensified to study factor H and the FHRs and develop new tools for the distinction, quantification and functional characterization of members of this protein family. Here, we provide an update and overview on the versatile roles of factor H family proteins, what we know about their biological functions in healthy conditions and in diseases.

Amyloid-β aggregates activate peripheral monocytes in mild cognitive impairment

The peripheral immune system is important in neurodegenerative diseases, both in protecting and inflaming the brain, but the underlying mechanisms remain elusive. Alzheimer's Disease is commonly preceded by a prodromal period. Here, we report the presence of large Aβ aggregates in plasma from patients with mild cognitive impairment (n = 38). The aggregates are associated with low level Alzheimer's Disease-like brain pathology as observed by 11C-PiB PET and 18F-FTP PET and lowered CD18-rich monocytes. We characterize complement receptor 4 as a strong binder of amyloids and show Aβ aggregates are preferentially phagocytosed and stimulate lysosomal activity through this receptor in stem cell-derived microglia. KIM127 integrin activation in monocytes promotes size selective phagocytosis of Aβ. Hydrodynamic calculations suggest Aβ aggregates associate with vessel walls of the cortical capillaries. In turn, we hypothesize aggregates may provide an adhesion substrate for recruiting CD18-rich monocytes into the cortex. Our results support a role for complement receptor 4 in regulating amyloid homeostasis.

αMI-domain of Integrin Mac-1 Binds the Cytokine Pleiotrophin Using Multiple Mechanisms

The integrin Mac-1 (αMβ2, CD11b/CD18, CR3) is an important adhesion receptor expressed on macrophages and neutrophils. Mac-1 is also the most promiscuous member of the integrin family that binds a diverse set of ligands through its αMI-domain. However, the binding mechanism of most ligands is not clear. We have determined the interaction of αMI-domain with the cytokine pleiotrophin (PTN), a cationic protein known to bind αMI-domain and induce Mac-1-mediated cell adhesion and migration. Our data show that PTN's N-terminal domain binds a unique site near the N- and C-termini of the αMI-domain using a metal-independent mechanism. However, stronger interaction is achieved when an acidic amino acid in a zwitterionic motif in PTN's C-terminal domain chelates the divalent cation in the metal ion-dependent adhesion site of the active αMI-domain. These results indicate that αMI-domain can bind ligands using multiple mechanisms, and suggest that active αMI-domain prefers acidic amino acids in zwitterionic motifs.

Antibody-mediated phagocytosis in cancer immunotherapy

Unconjugated monoclonal antibodies (mAb) have revolutionized the treatment of many types of cancer. Some of these mAbs promote the clearance of malignant cells via direct cytotoxic effects. More recently, antibody-dependent cellular phagocytosis (ADCP) has been appreciated as a major mechanism of action for a number of widely-used mAbs, including anti-CD20 (rituximab, obinutuzumab), anti-HER2 (trazituzumab), and anti-CD38 (daratumumab). However, as a monotherapy these ADCP-inducing mAbs produce insufficient levels of cytotoxicity in vivo and are not curative. As a result, these mAbs are most effectively used in combination therapies. The efficacy of these mAbs is further hampered by the apparent development of drug resistance by many patients. Here we will explore the role of ADCP in cancer immunotherapy and discuss the key factors that could limit the efficacy of ADCP-inducing mAbs in vivo. Finally, we will discuss current insights and approaches being applied to overcome these limitations.

Structural biology of complement receptors

The complement system plays crucial roles in a wide breadth of immune and inflammatory processes and is frequently cited as an etiological or aggravating factor in many human diseases, from asthma to cancer. Complement receptors encompass at least eight proteins from four structural classes, orchestrating complement-mediated humoral and cellular effector responses and coordinating the complex cross-talk between innate and adaptive immunity. The progressive increase in understanding of the structural features of the main complement factors, activated proteolytic fragments, and their assemblies have spurred a renewed interest in deciphering their receptor complexes. In this review, we describe what is currently known about the structural biology of the complement receptors and their complexes with natural agonists and pharmacological antagonists. We highlight the fundamental concepts and the gray areas where issues and problems have been identified, including current research gaps. We seek to offer guidance into the structural biology of the complement system as structural information underlies fundamental and therapeutic research endeavors. Finally, we also indicate what we believe are potential developments in the field.

Trypanosoma brucei Invariant Surface gp65 Inhibits the Alternative Pathway of Complement by Accelerating C3b Degradation

Trypanosomes are known to activate the complement system on their surface, but they control the cascade in a manner such that the cascade does not progress into the terminal pathway. It was recently reported that the invariant surface glycoprotein ISG65 from Trypanosoma brucei interacts reversibly with complement C3 and its degradation products, but the molecular mechanism by which ISG65 interferes with complement activation remains unknown. In this study, we show that ISG65 does not interfere directly with the assembly or activity of the two C3 convertases. However, ISG65 acts as a potent inhibitor of C3 deposition through the alternative pathway in human and murine serum. Degradation assays demonstrate that ISG65 stimulates the C3b to iC3b converting activity of complement factor I in the presence of the cofactors factor H or complement receptor 1. A structure-based model suggests that ISG65 promotes a C3b conformation susceptible to degradation or directly bridges factor I and C3b without contact with the cofactor. In addition, ISG65 is observed to form a stable ternary complex with the ligand binding domain of complement receptor 3 and iC3b. Our data suggest that ISG65 supports trypanosome complement evasion by accelerating the conversion of C3b to iC3b through a unique mechanism.

Counteracting Immunosenescence-Which Therapeutic Strategies Are Promising?

Aging attenuates the overall responsiveness of the immune system to eradicate pathogens. The increased production of pro-inflammatory cytokines by innate immune cells under basal conditions, termed inflammaging, contributes to impaired innate immune responsiveness towards pathogen-mediated stimulation and limits antigen-presenting activity. Adaptive immune responses are attenuated as well due to lowered numbers of naïve lymphocytes and their impaired responsiveness towards antigen-specific stimulation. Additionally, the numbers of immunoregulatory cell types, comprising regulatory T cells and myeloid-derived suppressor cells, that inhibit the activity of innate and adaptive immune cells are elevated. This review aims to summarize our knowledge on the cellular and molecular causes of immunosenescence while also taking into account senescence effects that constitute immune evasion mechanisms in the case of chronic viral infections and cancer. For tumor therapy numerous nanoformulated drugs have been developed to overcome poor solubility of compounds and to enable cell-directed delivery in order to restore immune functions, e.g., by addressing dysregulated signaling pathways. Further, nanovaccines which efficiently address antigen-presenting cells to mount sustained anti-tumor immune responses have been clinically evaluated. Further, senolytics that selectively deplete senescent cells are being tested in a number of clinical trials. Here we discuss the potential use of such drugs to improve anti-aging therapy.

You complete me: tumor cell-myeloid cell nuclear fusion as a facilitator of organ-specific metastasis

Every cancer genome is unique, resulting in potentially near infinite cancer cell phenotypes and an inability to predict clinical outcomes in most cases. Despite this profound genomic heterogeneity, many cancer types and subtypes display a non-random distribution of metastasis to distant organs, a phenomenon known as organotropism. Proposed factors in metastatic organotropism include hematogenous versus lymphatic dissemination, the circulation pattern of the tissue of origin, tumor-intrinsic factors, compatibility with established organ-specific niches, long-range induction of premetastatic niche formation, and so-called "prometastatic niches" that facilitate successful colonization of the secondary site following extravasation. To successfully complete the steps required for distant metastasis, cancer cells must evade immunosurveillance and survive in multiple new and hostile environments. Despite substantial advances in our understanding of the biology underlying malignancy, many of the mechanisms used by cancer cells to survive the metastatic journey remain a mystery. This review synthesizes the rapidly growing body of literature demonstrating the relevance of an unusual cell type known as "fusion hybrid" cells to many of the hallmarks of cancer, including tumor heterogeneity, metastatic conversion, survival in circulation, and metastatic organotropism. Whereas the concept of fusion between tumor cells and blood cells was initially proposed over a century ago, only recently have technological advancements allowed for detection of cells containing components of both immune and neoplastic cells within primary and metastatic lesions as well as among circulating malignant cells. Specifically, heterotypic fusion of cancer cells with monocytes and macrophages results in a highly heterogeneous population of hybrid daughter cells with enhanced malignant potential. Proposed mechanisms behind these findings include rapid, massive genome rearrangement during nuclear fusion and/or acquisition of monocyte/macrophage features such as migratory and invasive capability, immune privilege, immune cell trafficking and homing, and others. Rapid acquisition of these cellular traits may increase the likelihood of both escape from the primary tumor site and extravasation of hybrid cells at a secondary location that is amenable to colonization by that particular hybrid phenotype, providing a partial explanation for the patterns observed in some cancers with regard to sites of distant metastases.

Mass cytometric analysis of the immune cell landscape after traumatic brain injury elucidates the role of complement and complement receptors in neurologic outcomes

Following traumatic brain injury (TBI), a neuroinflammatory response can persist for years and contribute to the development of chronic neurological manifestations. Complement plays a central role in post-TBI neuroinflammation, and C3 opsonins and the anaphylatoxins (C3a and C5a) have been implicated in promoting secondary injury. We used single cell mass cytometry to characterize the immune cell landscape of the brain at different time points after TBI. To specifically investigate how complement shapes the post-TBI immune cell landscape, we analyzed TBI brains in the context of CR2-Crry treatment, an inhibitor of C3 activation. We analyzed 13 immune cell types, including peripheral and brain resident cells, and assessed expression of various receptors. TBI modulated the expression of phagocytic and complement receptors on both brain resident and infiltrating peripheral immune cells, and distinct functional clusters were identified within same cell populations that emerge at different phases after TBI. In particular, a CD11c+ (CR4) microglia subpopulation continued to expand over 28 days after injury, and was the only receptor to show continuous increase over time. Complement inhibition affected the abundance of brain resident immune cells in the injured hemisphere and impacted the expression of functional receptors on infiltrating cells. A role for C5a has also been indicated in models of brain injury, and we found significant upregulation of C5aR1 on many immune cell types after TBI. However, we demonstrated experimentally that while C5aR1 is involved in the infiltration of peripheral immune cells into the brain after injury, it does not alone affect histological or behavioral outcomes. However, CR2-Crry improved post-TBI outcomes and reduced resident immune cell populations, as well as complement and phagocytic receptor expression, indicating that its neuroprotective effects are mediated upstream of C5a generation, likely via modulating C3 opsonization and complement receptor expression.

Undernutrition Disrupts Cecal Microbiota and Epithelium Interactions, Epithelial Metabolism, and Immune Responses in a Pregnant Sheep Model

Undernutrition may change cecal microbiota-epithelium interactions to influence cecal feed fermentation, nutrient absorption and metabolism, and immune function. Sixteen late-gestation Hu-sheep were randomly divided into control (normal feeding) and treatment (feed restriction) groups to establish an undernourished sheep model. Cecal digesta and epithelium were collected to analyze microbiota-host interactions based on 16S rRNA gene and transcriptome sequencing. Results showed that cecal weight and pH were decreased, volatile fatty acids and microbial proteins concentrations were increased, and epithelial morphology was changed upon undernutrition. Undernutrition reduced the diversity, richness, and evenness of cecal microbiota. The relative abundances of cecal genera involved in acetate production (Rikenellaceae dgA-11 gut group, Rikenellaceae RC9 gut group, and Ruminococcus) and negatively correlated with butyrate proportion (Clostridia vadinBB60 group_norank) were decreased, while genera related to butyrate (Oscillospiraceae_uncultured and Peptococcaceae_uncultured) and valerate (Peptococcaceae_uncultured) production were increased in undernourished ewes. These findings were consistent with the decreased molar proportion of acetate and the increased molar proportions of butyrate and valerate. Undernutrition changed the overall transcriptional profile and substance transport and metabolism in cecal epithelium. Undernutrition suppressed extracellular matrix-receptor interaction and intracellular phosphatidyl inositol 3-kinase (PI3K) signaling pathway then disrupted biological processes in cecal epithelium. Moreover, undernutrition repressed phagosome antigen processing and presentation, cytokine-cytokine receptor interaction, and intestinal immune network. In conclusion, undernutrition affected cecal microbial diversity and composition and fermentation parameters, inhibited extracellular matrix-receptor interaction and the PI3K signaling pathway, and then disrupted epithelial proliferation and renewal and intestinal immune functions. Our findings exposed cecal microbiota-host interactions upon undernutrition and contribute to their further exploration. IMPORTANCE Undernutrition is commonly encountered in ruminant production, especially during pregnancy and lactation in females. Undernutrition not only induces metabolic diseases and threatens pregnant mothers' health, but also inhibits fetal growth and development, leading to weakness or even death of fetuses. Cecum works importantly in hindgut fermentation, providing volatile fatty acids and microbial proteins to the organism. Intestinal epithelial tissue plays a role in nutrient absorption and transport, barrier function, and immune function. However, little is known about cecal microbiota and epithelium interactions upon undernutrition. Our findings showed that undernutrition affected bacterial structures and functions, which changed fermentation parameters and energy regimens, and therefore affected the substance transport and metabolism in cecal epithelium. Extracellular matrix-receptor interactions were inhibited, which repressed cecal epithelial morphology and cecal weight via the PI3K signaling pathway and lowered immune response function upon undernutrition. These findings will help in further exploring microbe-host interactions.

The Complement System in Kidney Transplantation

Kidney transplantation is the therapy of choice for patients who suffer from end-stage renal diseases. Despite improvements in surgical techniques and immunosuppressive treatments, long-term graft survival remains a challenge. A large body of evidence documented that the complement cascade, a part of the innate immune system, plays a crucial role in the deleterious inflammatory reactions that occur during the transplantation process, such as brain or cardiac death of the donor and ischaemia/reperfusion injury. In addition, the complement system also modulates the responses of T cells and B cells to alloantigens, thus playing a crucial role in cellular as well as humoral responses to the allograft, which lead to damage to the transplanted kidney. Since several drugs that are capable of inhibiting complement activation at various stages of the complement cascade are emerging and being developed, we will discuss how these novel therapies could have potential applications in ameliorating outcomes in kidney transplantations by preventing the deleterious effects of ischaemia/reperfusion injury, modulating the adaptive immune response, and treating antibody-mediated rejection.

The echinoid complement system inferred from genome sequence searches

The vertebrate complement cascade is an essential host protection system that functions at the intersection of adaptive and innate immunity. However, it was originally assumed that complement was present only in vertebrates because it was activated by antibodies and functioned with adaptive immunity. Subsequently, the identification of the key component, SpC3, in sea urchins plus a wide range of other invertebrates significantly expanded the concepts of how complement functions. Because there are few reports on the echinoid complement system, an alternative approach to identify complement components in echinoderms is to search the deduced proteins encoded in the genomes. This approach identified known and putative members of the lectin and alternative activation pathways, but members of the terminal pathway are absent. Several types of complement receptors are encoded in the genomes. Complement regulatory proteins composed of complement control protein (CCP) modules are identified that may control the activation pathways and the convertases. Other regulatory proteins without CCP modules are also identified, however regulators of the terminal pathway are absent. The expansion of genes encoding proteins with Macpf domains is noteworthy because this domain is a signature of perforin and proteins in the terminal pathway. The results suggest that the major functions of the echinoid complement system are detection of foreign targets by the proteins that initiate the activation pathways resulting in opsonization by SpC3b fragments to augment phagocytosis and destruction of the foreign targets by the immune cells.

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.

The localization of Toll and Imd pathway and complement system components and their response to Vibrio infection in the nemertean Lineus ruber

BACKGROUND

Innate immunity is the first line of defense against pathogens. In animals, the Toll pathway, the Imd pathway, the complement system, and lectins are well-known mechanisms involved in innate immunity. Although these pathways and systems are well understood in vertebrates and arthropods, they are understudied in other invertebrates.

RESULTS

To shed light on immunity in the nemertean Lineus ruber, we performed a transcriptomic survey and identified the main components of the Toll pathway (e.g., myD88, dorsal/dif/NFκB-p65), the Imd pathway (e.g., imd, relish/NFκB-p105/100), the complement system (e.g., C3, cfb), and some lectins (FreD-Cs and C-lectins). In situ hybridization showed that TLRβ1, TLRβ2, and imd are expressed in the nervous system; the complement gene C3-1 is expressed in the gut; and the lectins are expressed in the nervous system, the blood, and the gut. To reveal their potential role in defense mechanisms, we performed immune challenge experiments, in which Lineus ruber specimens were exposed to the gram-negative bacteria Vibrio diazotrophicus. Our results show the upregulation of specific components of the Toll pathway (TLRα3, TLRβ1, and TLRβ2), the complement system (C3-1), and lectins (c-lectin2 and fred-c5).

CONCLUSIONS

Therefore, similarly to what occurs in other invertebrates, our study shows that components of the Toll pathway, the complement system, and lectins are involved in the immune response in the nemertean Lineus ruber. The presence of these pathways and systems in Lineus ruber, but also in other spiralians; in ecdysozoans; and in deuterostomes suggests that these pathways and systems were involved in the immune response in the stem species of Bilateria.

Microglial heterogeneity and complement component 3 elimination within emerging multisensory midbrain compartments during an early critical period

The lateral cortex of the inferior colliculus (LCIC) is a midbrain shell region that receives multimodal inputs that target discrete zones of its compartmental (modular-matrix) framework. This arrangement emerges perinatally in mice (postnatal day, P0-P12) as somatosensory and auditory inputs segregate into their respective modular and matrix terminal patterns. Microglial cells (MGCs) perform a variety of critical functions in the developing brain, among them identifying areas of active circuit assembly and selectively pruning exuberant or underutilized connections. Recent evidence in other brain structures suggest considerable MGC heterogeneity across the lifespan, particularly during established developmental critical periods. The present study examines the potential involvement of classical complement cascade signaling (C3-CR3/CD11b) in refining early multisensory networks, and identifies several microglial subsets exhibiting distinct molecular signatures within the nascent LCIC. Immunostaining was performed in GAD67-green fluorescent protein (GFP) and CX3CR1-GFP mice throughout and after the defined LCIC critical period. GAD labeling highlights the emerging LCIC modularity, while CX3CR1 labeling depicts MGCs expressing the fractalkine receptor. C3 expression is widespread throughout the LCIC neuropil early on, prior to its conspicuous absence from modular zones at P8, and more global disappearance following critical period closure. CD11b-expressing microglia while homogeneously distributed at birth, are biased to modular fields at P8 and then the surrounding matrix by P12. Temporal and spatial matching of the disappearance of C3 by LCIC compartment (i.e., modules then matrix) with CD11b-positive MGC occupancy implicates complement signaling in the selective refinement of early LCIC connectivity. Multiple-labeling studies for a variety of established MGC markers (CD11b, CX3CR1, Iba1, TMEM119) indicate significant MGC heterogeneity in the LCIC as its compartments and segregated multisensory maps emerge. Marker colocalization was the exception rather than the rule, suggesting that unique MGC subpopulations exist in the LCIC and perhaps serve distinct developmental roles. Potential mechanisms whereby microglia sculpt early multisensory LCIC maps and how such activity/inactivity may underlie certain neurodevelopmental conditions, including autism spectrum disorder and schizophrenia, are discussed.

Age-related macular degeneration: A disease of extracellular complement amplification

Age-related macular degeneration (AMD) is a major cause of vision impairment in the Western World, and with the aging world population, its incidence is increasing. As of today, for the majority of patients, no treatment exists. Multiple genetic and biochemical studies have shown a strong association with components in the complement system and AMD, and evidence suggests a major role of remodeling of the extracellular matrix underlying the outer blood/retinal barrier. As part of the innate immune system, the complement cascade acts as a first-line defense against pathogens, and upon activation, its amplification loop ensures a strong, rapid, and sustained response. Excessive activation, however, can lead to host tissue damage and cause complement-associated diseases like AMD. AMD patients present with aberrant activation of the alternative pathway, especially in ocular tissues but also on a systemic level. Here, we review the latest findings of complement activation in AMD, and we will discuss in vivo observations made in human tissue, cellular models, the potential synergy of different AMD-associated pathways, and conclude on current clinical trials and the future outlook.

The Q163C/Q309C mutant of αMI-domain is an active variant suitable for NMR characterization

Integrin αMβ2 (Mac-1, CD11b/CD18, CR3) is an important adhesion receptor expressed on monocytes. Mac-1 is responsible for mediating cell migration, phagocytosis, degranulation as well as cell-cell fusion. It is also the most promiscuous integrin in terms of ligand specificity with over 100 ligands, most of which use the αMI-domain as their binding site. Despite the importance of αMI-domain in defining ligand interactions of Mac-1, structural studies of αMI-domain's interactions with ligands are lacking. In particular, solution NMR studies of αMI-domain's interaction with ligands have not been possible because the most commonly used active αMI-domain mutants (I316G and ΔK315) are not sufficiently stable and soluble to be used in solution NMR. The goal of this study is to identify an αMI-domain active mutant that's amenable to NMR characterization. By screening known activating mutations of αMI-domain, we determined that the Q163C/Q309C mutant, which converts the αMI-domain into its active form through the formation of an intramolecular disulfide bond, can be produced with a high yield and is more stable than other active mutants. In addition, the Q163C/Q309C mutant has better NMR spectral quality than other active mutants and its affinity for ligands is comparable to other active mutants. Analysis of the Co2+-induced pseudocontact shifts in the Q163C/Q309C mutant showed the structure of the mutant is consistent with the active conformation. Finally, we show that the minor fraction of the Q163C/Q309C mutant without the disulfide bond can be removed through the use of carboxymethyl sepharose chromatography. We think the availability of this mutant for NMR study will significantly enhance structural characterizations of αMI-domain-ligand interactions.

C3-dependent effector functions of complement

C3 is the central effector molecule of the complement system, mediating its multiple functions through different binding sites and their corresponding receptors. We will introduce the C3 forms (native C3, C3 [H₂ O], and intracellular C3), the C3 fragments C3a, C3b, iC3b, and C3dg/C3d, and the C3 expression sites. To highlight the important role that C3 plays in human biological processes, we will give an overview of the diseases linked to C3 deficiency and to uncontrolled C3 activation. Next, we will present a structural description of C3 activation and of the C3 fragments generated by complement regulation. We will proceed by describing the C3a interaction with the anaphylatoxin receptor, followed by the interactions of opsonins (C3b, iC3b, and C3dg/C3d) with complement receptors, divided into two groups: receptors bearing complement regulatory functions and the effector receptors without complement regulatory activity. We outline the molecular architecture of the receptors, their binding sites on the C3 activation fragments, the cells expressing them, the diversity of their functions, and recent advances. With this review, we aim to give an up-to-date analysis of the processes triggered by C3 activation fragments on different cell types in health and disease contexts.

Heterotropic roles of divalent cations in the establishment of allostery and affinity maturation of integrin αXβ2

Allosteric activation and silencing of leukocyte β2-integrins transpire through cation-dependent structural changes, which mediate integrin biosynthesis and recycling, and are essential to designing leukocyte-specific drugs. Stepwise addition of Mg²⁺ reveals two mutually coupled events for the αXβ2 ligand-binding domain-the αX I-domain-corresponding to allostery establishment and affinity maturation. Electrostatic alterations in the Mg²⁺-binding site establish long-range couplings, leading to both pH- and Mg²⁺-occupancy-dependent biphasic stability change in the αX I-domain fold. The ligand-binding sensorgrams show composite affinity events for the αX I-domain accounting for the multiplicity of the αX I-domain conformational states existing in the solution. On cell surfaces, increasing Mg²⁺ concentration enhanced adhesiveness of αXβ2. This work highlights how intrinsically flexible pH- and cation-sensitive architecture endows a unique dynamic continuum to the αI-domain structure on the intact integrin, thereby revealing the importance of allostery establishment and affinity maturation in both extracellular and intracellular integrin events.

β2 Integrins on Dendritic Cells Modulate Cytokine Signaling and Inflammation-Associated Gene Expression, and Are Required for Induction of Autoimmune Encephalomyelitis

Heterodimeric β2 integrin surface receptors (CD11a-d/CD18) are specifically expressed by leukocytes that contribute to pathogen uptake, cell migration, immunological synapse formation and cell signaling. In humans, the loss of CD18 expression results in leukocyte adhesion deficiency syndrome (LAD-)1, largely characterized by recurrent severe infections. All available mouse models display the constitutive and ubiquitous knockout of either α or the common β2 (CD18) subunit, which hampers the analysis of the cell type-specific role of β2 integrins in vivo. To overcome this limitation, we generated a CD18 gene floxed mouse strain. Offspring generated from crossing with CD11c-Cre mice displayed the efficient knockdown of β2 integrins, specifically in dendritic cells (DCs). Stimulated β2-integrin-deficient splenic DCs showed enhanced cytokine production and the concomitantly elevated activity of signal transducers and activators of transcription (STAT) 1, 3 and 5, as well as the impaired expression of suppressor of cytokine signaling (SOCS) 2–6 as assessed in bone marrow-derived (BM) DCs. Paradoxically, these BMDCs also showed the attenuated expression of genes involved in inflammatory signaling. In line, in experimental autoimmune encephalomyelitis mice with a conditional DC-specific β2 integrin knockdown presented with a delayed onset and milder course of disease, associated with lower frequencies of T helper cell populations (Th)1/Th17 in the inflamed spinal cord. Altogether, our mouse model may prove to be a valuable tool to study the leukocyte-specific functions of β2 integrins in vivo.

Role of Complement System in Kidney Transplantation: Stepping From Animal Models to Clinical Application

Kidney transplantation is a life-saving strategy for patients with end-stage renal diseases. Despite the advances in surgical techniques and immunosuppressive agents, the long-term graft survival remains a challenge. Growing evidence has shown that the complement system, part of the innate immune response, is involved in kidney transplantation. Novel insights highlighted the role of the locally produced and intracellular complement components in the development of inflammation and the alloreactive response in the kidney allograft. In the current review, we provide the updated understanding of the complement system in kidney transplantation. We will discuss the involvement of the different complement components in kidney ischemia-reperfusion injury, delayed graft function, allograft rejection, and chronic allograft injury. We will also introduce the existing and upcoming attempts to improve allograft outcomes in animal models and in the clinical setting by targeting the complement system.

Chemo- and mechanosensing by dendritic cells facilitate antigen surveillance in the spleen

Spleen dendritic cells (DC) are critical for initiation of adaptive immune responses against blood-borne invaders. Key to DC function is their positioning at sites of pathogen entry, and their abilities to selectively capture foreign antigens and promptly engage T cells. Focusing on conventional DC2 (cDC2), we discuss the contribution of chemoattractant receptors (EBI2 or GPR183, S1PR1, and CCR7) and integrins to cDC2 positioning and function. We give particular attention to a newly identified role in cDC2 for adhesion G-protein coupled receptor E5 (Adgre5 or CD97) and its ligand CD55, detailing how this mechanosensing system contributes to splenic cDC2 positioning and homeostasis. Additional roles of CD97 in the immune system are reviewed. The ability of cDC2 to be activated by circulating missing self-CD47 cells and to integrate multiple red blood cell (RBC)-derived inputs is discussed. Finally, we describe the process of activated cDC2 migration to engage and prime helper T cells. Throughout the review, we consider the insights into cDC function in the spleen that have emerged from imaging studies.

The Effects of Porphyromonas gingivalis on Atherosclerosis-Related Cells

Atherosclerosis (AS), one of the most common types of cardiovascular disease, has initially been attributed to the accumulation of fats and fibrous materials. However, more and more researchers regarded it as a chronic inflammatory disease nowadays. Infective disease, such as periodontitis, is related to the risk of atherosclerosis. Porphyromonas gingivalis (P. gingivalis), one of the most common bacteria in stomatology, is usually discovered in atherosclerotic plaque in patients. Furthermore, it was reported that P. gingivalis can promote the progression of atherosclerosis. Elucidating the underlying mechanisms of P. gingivalis in atherosclerosis attracted attention, which is thought to be crucial to the therapy of atherosclerosis. Nevertheless, the pathogenesis of atherosclerosis is much complicated, and many kinds of cells participate in it. By summarizing existing studies, we find that P. gingivalis can influence the function of many cells in atherosclerosis. It can induce the dysfunction of endothelium, promote the formation of foam cells as well as the proliferation and calcification of vascular smooth muscle cells, and lead to the imbalance of regulatory T cells (Tregs) and T helper (Th) cells, ultimately promoting the occurrence and development of atherosclerosis. This article summarizes the specific mechanism of atherosclerosis caused by P. gingivalis. It sorts out the interaction between P. gingivalis and AS-related cells, which provides a new perspective for us to prevent or slow down the occurrence and development of AS by inhibiting periodontal pathogens.

C3d Elicits Neutrophil Degranulation and Decreases Endothelial Cell Migration, with Implications for Patients with Alpha-1 Antitrypsin Deficiency

Alpha-1 antitrypsin (AAT) deficiency (AATD) is characterized by increased risk for emphysema, chronic obstructive pulmonary disease (COPD), vasculitis, and wound-healing impairment. Neutrophils play a central role in the pathogenesis of AATD. Dysregulated complement activation in AATD results in increased plasma levels of C3d. The current study investigated the impact of C3d on circulating neutrophils. Blood was collected from AATD (n = 88) or non-AATD COPD patients (n = 10) and healthy controls (HC) (n = 40). Neutrophils were challenged with C3d, and degranulation was assessed by Western blotting, ELISA, or fluorescence resonance energy transfer (FRET) substrate assays. Ex vivo, C3d levels were increased in plasma (p < 0.0001) and on neutrophil plasma membranes (p = 0.038) in AATD compared to HC. C3d binding to CR3 receptors triggered primary (p = 0.01), secondary (p = 0.004), and tertiary (p = 0.018) granule release and increased CXCL8 secretion (p = 0.02). Ex vivo plasma levels of bactericidal-permeability-increasing-protein (p = 0.02), myeloperoxidase (p < 0.0001), and lactoferrin (p < 0.0001) were significantly increased in AATD patients. In endothelial cell scratch wound assays, C3d significantly decreased cell migration (p < 0.0001), an effect potentiated by neutrophil degranulated proteins (p < 0.0001). In summary, AATD patients had increased C3d in plasma and on neutrophil membranes and, together with neutrophil-released granule enzymes, reduced endothelial cell migration and wound healing, with potential implications for AATD-related vasculitis.

Crosstalk between macrophages and natural killer cells in the tumor microenvironment

The tumor microenvironment (TME) is jointly constructed by a variety of cell types, including tumor cells, immune cells, fibroblasts, and epithelial cells, among others. The cells within the TME interact with each other and with tumor cells to influence tumor development and progression. As the most abundant immune cells in the TME, macrophages regulate the immune network by not only secreting a large amount of versatile cytokines but also expressing a series of ligands or receptors on the surface to interact with other cells directly. Due to their strong plasticity, they exert both immunostimulatory and immunosuppressive effects in the complex TME. The major effector cells of the immune system that directly target cancer cells include but are not limited to natural killer cells (NKs), dendritic cells (DCs), macrophages, polymorphonuclear leukocytes, mast cells, and cytotoxic T lymphocytes (CTLs). Among them, NK cells are the predominant innate lymphocyte subsets that mediate antitumor and antiviral responses. The activation and inhibition of NK cells are regulated by cytokines and the balance between activating and inhibitory receptors. There is an inextricable regulatory relationship between macrophages and NK cells. Herein, we systematically elaborate on the regulatory network between macrophages and NK cells through soluble mediator crosstalk and cell-to-cell interactions. We believe that a better understanding of the crosstalk between macrophages and NKs in the TME will benefit the development of novel macrophage- or NK cell-focused therapeutic strategies with superior efficacies in cancer therapy.

Complement Factor H Family Proteins Modulate Monocyte and Neutrophil Granulocyte Functions

Besides being a key effector arm of innate immunity, a plethora of non-canonical functions of complement has recently been emerging. Factor H (FH), the main regulator of the alternative pathway of complement activation, has been reported to bind to various immune cells and regulate their functions, beyond its role in modulating complement activation. In this study we investigated the effect of FH, its alternative splice product FH-like protein 1 (FHL-1), the FH-related (FHR) proteins FHR-1 and FHR-5, and the recently developed artificial complement inhibitor mini-FH, on two key innate immune cells, monocytes and neutrophilic granulocytes. We found that, similar to FH, the other factor H family proteins FHL-1, FHR-1 and FHR-5, as well as the recombinant mini-FH, are able to bind to both monocytes and neutrophils. As a functional outcome, immobilized FH and FHR-1 inhibited PMA-induced NET formation, but increased the adherence and IL-8 production of neutrophils. FHL-1 increased only the adherence of the cells, while FHR-5 was ineffective in altering these functions. The adherence of monocytes was increased on FH, recombinant mini-FH and FHL-1 covered surfaces and, except for FHL-1, the same molecules also enhanced secretion of the inflammatory cytokines IL-1β and TNFα. When monocytes were stimulated with LPS in the presence of immobilized FH family proteins, FH, FHL-1 and mini-FH enhanced whereas FHR-1 and FHR-5 decreased the secretion of TNFα; FHL-1 and mini-FH also enhanced IL-10 release compared to the effect of LPS alone. Our results reveal heterogeneous effects of FH and FH family members on monocytes and neutrophils, altering key features involved in pathogen killing, and also demonstrate that FH-based complement inhibitors, such as mini-FH, may have effects beyond their function of inhibiting complement activation. Thus, our data provide new insight into the non-canonical functions of FH, FHL-1, FHR-1 and FHR-5 that might be exploited during protection against infections and in vaccine development.

The Role of B Cells in Adult and Paediatric Liver Injury

B lymphocytes are multitasking cells that direct the immune response by producing pro- or anti-inflammatory cytokines, by presenting processed antigen for T cell activation and co-stimulation, and by turning into antibody-secreting cells. These functions are important to control infection in the liver but can also exacerbate tissue damage and fibrosis as part of persistent inflammation that can lead to end stage disease requiring a transplant. In transplantation, immunosuppression increases the incidence of lymphoma and often this is of B cell origin. In this review we bring together information on liver B cell biology from different liver diseases, including alcohol-related and metabolic fatty liver disease, autoimmune hepatitis, primary biliary and primary sclerosing cholangitis, viral hepatitis and, in infants, biliary atresia. We also discuss the impact of B cell depletion therapy in the liver setting. Taken together, our analysis shows that B cells are important in the pathogenesis of liver diseases and that further research is necessary to fully characterise the human liver B cell compartment.

Leveling Up the Controversial Role of Neutrophils in Cancer: When the Complexity Becomes Entangled

Neutrophils are the most abundant immune cell in the circulation of human and act as gatekeepers to discard foreign elements that have entered the body. They are essential in initiating immune responses for eliminating invaders, such as microorganisms and alien particles, as well as to act as immune surveyors of cancer cells, especially during the initial stages of carcinogenesis and for eliminating single metastatic cells in the circulation and in the premetastatic organs. Since neutrophils can secrete a whole range of factors stored in their many granules as well as produce reactive oxygen and nitrogen species upon stimulation, neutrophils may directly or indirectly affect carcinogenesis in both the positive and negative directions. An intricate crosstalk between tumor cells, neutrophils, other immune cells and stromal cells in the microenvironment modulates neutrophil function resulting in both anti- and pro-tumor activities. Both the anti-tumor and pro-tumor activities require chemoattraction towards the tumor cells, neutrophil activation and ROS production. Divergence is seen in other neutrophil properties, including differential secretory repertoire and membrane receptor display. Many of the direct effects of neutrophils on tumor growth and metastases are dependent on tight neutrophil–tumor cell interactions. Among them, the neutrophil Mac-1 interaction with tumor ICAM-1 and the neutrophil L-selectin interaction with tumor-cell sialomucins were found to be involved in the neutrophil-mediated capturing of circulating tumor cells resulting in increased metastatic seeding. On the other hand, the anti-tumor function of neutrophils was found to rely on the interaction between tumor-surface-expressed receptor for advanced glycation end products (RAGE) and Cathepsin G expressed on the neutrophil surface. Intriguingly, these two molecules are also involved in the promotion of tumor growth and metastases. RAGE is upregulated during early inflammation-induced carcinogenesis and was found to be important for sustaining tumor growth and homing at metastatic sites. Cathepsin G was found to be essential for neutrophil-supported lung colonization of cancer cells. These data level up the complexity of the dual role of neutrophils in cancer.

Insights Into the Structure-Function Relationships of Dimeric C3d Fragments

Cleavage of C3 to C3a and C3b plays a central role in the generation of complement-mediated defences. Although the thioester-mediated surface deposition of C3b has been well-studied, fluid phase dimers of C3 fragments remain largely unexplored. Here we show C3 cleavage results in the spontaneous formation of C3b dimers and present the first X-ray crystal structure of a disulphide-linked human C3d dimer. Binding studies reveal these dimers are capable of crosslinking complement receptor 2 and preliminary cell-based analyses suggest they could modulate B cell activation to influence tolerogenic pathways. Altogether, insights into the physiologically-relevant functions of C3d(g) dimers gained from our findings will pave the way to enhancing our understanding surrounding the importance of complement in the fluid phase and could inform the design of novel therapies for immune system disorders in the future.

Growth, Chemical Composition, Histology and Antioxidant Genes of Atlantic Salmon (Salmo salar) Fed Whole or Pre-Processed Nannochloropsis oceanica and Tetraselmis sp

New sustainable feed ingredients are a necessity for the salmon aquaculture industry. In this study, we examined the effect of pre-extrusion processing of two microalgae, Nannochloropsis oceanica and Tetraselmis sp., on the growth, fatty acid content in the flesh and health of Atlantic salmon. The fish were fed one of the following five diets for nine weeks: (1) CO: a fish meal-based control (basal) diet, (2) NU: a Nannochloropsis diet, (3) NE: a pre-extruded Nannochloropsis diet, (4) TU: a Tetraselmis diet, and (5) TE: a pre-extruded Tetraselmis diet. The algae-incorporated diets contained 30% of the respective microalgae. Our results showed that the best growth performance was achieved by the CO diet, followed by the NE diets. Feeding of unprocessed Nannochloropsis and Tetraselmis resulted in a significant reduction in enterocyte vacuolization compared to the CO feeding. A significant effect of processing was noted in the fillet fatty acid content, the intestine and liver structure and the expression of selected genes in the liver. The expression of antioxidant genes in both the liver and intestine, and the accumulation of different fatty acids in the fillet and liver of the extruded algae-fed groups, warrants further investigation. In conclusion, based on the short-term study, 30% inclusion of the microalgae Nannochloropsis oceanica and Tetraselmis sp. can be considered in Atlantic salmon feeds.

Characterization of DNA-protein complexes by nanoparticle tracking analysis and their association with systemic lupus erythematosus

Nanotechnology enables investigations of single biomacromolecules, but technical challenges have limited the application in liquid biopsies, for example, blood plasma. Nonetheless, tools to characterize single molecular species in such samples represent a significant unmet need with the increasing appreciation of the physiological importance of protein structural changes at nanometer scale. Mannose-binding lectin (MBL) is an oligomeric plasma protein and part of the innate immune system through its ability to activate complement. MBL also serves a role as a scavenger for cellular debris, especially DNA. This may link functions of MBL with several inflammatory diseases in which cell-free DNA now appears to play a role, but mechanistic insight has been lacking. By making nanoparticle tracking analysis possible in human plasma, we now show that superoligomeric structures of MBL form nanoparticles with DNA. These oligomers correlate with disease activity in systemic lupus erythematosus patients. With the direct quantification of the hydrodynamic radius, calculations following the principles of Taylor dispersion in the blood stream connect the size of these complexes to endothelial inflammation, which is among the most important morbidities in lupus. Mechanistic insight from an animal model of lupus supported that DNA-stabilized superoligomers stimulate the formation of germinal center B cells and drive loss of immunological tolerance. The formation involves an inverse relationship between the concentration of MBL superoligomers and antibodies to double-stranded DNA. Our approach implicates the structure of DNA-protein nanoparticulates in the pathobiology of autoimmune diseases.

Targeting complement components C3 and C5 for the retina: Key concepts and lingering questions

Age-related macular degeneration (AMD) remains a major cause of legal blindness, and treatment for the geographic atrophy form of AMD is a significant unmet need. Dysregulation of the complement cascade is thought to be instrumental for AMD pathophysiology. In particular, C3 and C5 are pivotal components of the complement cascade and have become leading therapeutic targets for AMD. In this article, we discuss C3 and C5 in detail, including their roles in AMD, biochemical and structural aspects, locations of expression, and the functions of C3 and C5 fragments. Further, the article critically reviews developing therapeutics aimed at C3 and C5, underscoring the potential effects of broad inhibition of complement at the level of C3 versus more specific inhibition at C5. The relationships of complement biology to the inflammasome and microglia/macrophage activity are highlighted. Concepts of C3 and C5 biology will be emphasized, while we point out questions that need to be settled and directions for future investigations.

Halting targeted and collateral damage to red blood cells by the complement system

The complement system is an important defense mechanism against pathogens; however, in certain pathologies, the system also attacks human cells, such as red blood cells (RBCs). In paroxysmal nocturnal hemoglobinuria (PNH), RBCs lack certain complement regulators which sensitize them to complement-mediated lysis, while in autoimmune hemolytic anemia (AIHA), antibodies against RBCs may initiate complement-mediated hemolysis. In recent years, complement inhibition has improved treatment prospects for these patients, with eculizumab now the standard of care for PNH patients. Current complement inhibitors are however not sufficient for all patients, and they come with high costs, patient burden, and increased infection risk. This review gives an overview of the underlying pathophysiology of complement-mediated hemolysis in PNH and AIHA, the role of therapeutic complement inhibition nowadays, and the high number of complement inhibitors currently under investigation, as for almost every complement protein, an inhibitor is being developed. The focus lies with novel therapeutics that inhibit complement activity specifically in the pathway that causes pathology or those that reduce costs or patient burden through novel administration routes.

New aspects in the regulation of human B cell functions by complement receptors CR1, CR2, CR3 and CR4

The involvement of complement in the regulation of antibody responses has been known for long. By now several additional B cell functions - including cytokine production and antigen presentation - have also been shown to be regulated by complement proteins. Most of these important activities are mediated by receptors interacting with activation fragments of the central component of the complement system C3, such as C3b, iC3b and C3d, which are covalently attached to antigens and immune complexes. This review summarizes the role of complement receptors interacting with these ligands, namely CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) expressed by B cells in health and disease. Although we focus on human B lymphocytes, we also aim to call the attention to important differences between human and mouse systems.

Immune Dysregulation and the Increased Risk of Complications and Mortality Following Respiratory Tract Infections in Adults With Down Syndrome

The risk of severe outcomes following respiratory tract infections is significantly increased in individuals over 60 years, especially in those with chronic medical conditions, i.e., hypertension, diabetes, cardiovascular disease, dementia, chronic respiratory disease, and cancer. Down Syndrome (DS), the most prevalent intellectual disability, is caused by trisomy-21 in ~1:750 live births worldwide. Over the past few decades, a substantial body of evidence has accumulated, pointing at the occurrence of alterations, impairments, and subsequently dysfunction of the various components of the immune system in individuals with DS. This associates with increased vulnerability to respiratory tract infections in this population, such as the influenza virus, respiratory syncytial virus, SARS-CoV-2 (COVID-19), and bacterial pneumonias. To emphasize this link, here we comprehensively review the immunobiology of DS and its contribution to higher susceptibility to severe illness and mortality from respiratory tract infections.

Complement Receptor 3 Forms a Compact High-Affinity Complex with iC3b

Complement receptor 3 (CR3, also known as Mac-1, integrin α_Mβ₂, or CD11b/CD18) is expressed on a subset of myeloid and certain activated lymphoid cells. CR3 is essential for the phagocytosis of complement-opsonized particles such as pathogens and apoptotic or necrotic cells opsonized with the complement fragment iC3b and, to a lesser extent, C3dg. Although the interaction between the iC3b thioester domain and the ligand binding CR3 α_M I-domain is structurally and functionally well characterized, the nature of additional CR3-iC3b interactions required for phagocytosis of complement-opsonized objects remains obscure. In this study, we analyzed the interaction between iC3b and the 150-kDa headpiece fragment of the CR3 ectodomain. Surface plasmon resonance experiments demonstrated a 30 nM affinity of the CR3 headpiece for iC3b compared with 515 nM for the iC3b thioester domain, whereas experiments monitoring binding of iC3b to CR3-expressing cells suggested an affinity of 50 nM for the CR3-iC3b interaction. Small angle x-ray scattering analysis revealed that iC3b adopts an extended but preferred conformation in solution. Upon interaction with CR3, iC3b rearranges to form a compact receptor-ligand complex. Overall, the data suggest that the iC3b-CR3 interaction is of high affinity and relies on minor contacts formed between CR3 and regions outside the iC3b thioester domain. Our results rationalize the more efficient phagocytosis elicited by iC3b than by C3dg and pave the way for the development of specific therapeutics for the treatment of inflammatory and neurodegenerative diseases that do not interfere with the recognition of noncomplement CR3 ligands.

Analysis of the role of Purα in the pathogenesis of Alzheimer's disease based on RNA-seq and ChIP-seq

Purine rich element binding protein A (Purα), encoded by the Purα gene, is an important transcriptional regulator that binds to DNA and RNA and is involved in processes such as DNA replication and RNA translation. Purα also plays an important role in the nervous system. To identify the function of Pura, we performed RNA sequence (RNA-seq) analysis of Purɑ-KO mouse hippocampal neuron cell line (HT22) to analyze the effect of Purα deletion on neuronal expression profiles. And combined with ChIP-seq analysis to explore the mechanism of Purα on gene regulation. In the end, totaly 656 differentially expressed genes between HT22 and Purα-KO HT22 cells have been found, which include 7 Alzheimer’s disease (AD)-related genes and 5 Aβ clearance related genes. 47 genes were regulated by Purα directly, the evidence based on CHIP-seq, which include Insr, Mapt, Vldlr, Jag1, etc. Our study provides the important informations of Purα in neuro-development. The possible regulative effects of Purα on AD-related genes consist inthe direct and indirect pathways of Purα in the pathogenesis of AD.

Cell Adhesion Molecules are Upregulated and May Drive Inflammation in Chronic Rhinosinusitis with Nasal Polyposis

Purpose

Chronic rhinosinusitis with nasal polyps (CRSwNP) is predominantly characterized by eosinophil- and T helper 2 cell (Th2)-biased inflammation. Integrins and intercellular adhesion molecules (ICAMs) are superfamilies of cell adhesion molecules (CAMs) that facilitate the recruitment and trafficking of immune cells and have been implicated in coordinating eosinophil and Th2 cell adhesion and signaling in asthma. The roles of CAMs in CRSwNP, however, remain poorly understood. The purpose of this study was to characterize the systemic and local expression of CAMs and identify which CAMs are potentially involved in CRSwNP pathology.

Materials and Methods

A prospective observational study was conducted using peripheral blood and anterior ethmoid tissues of patients with CRSwNP (n=32) and controls (n=15). Multiplex gene analysis and Pearson correlations were performed to identify associations between systemically and locally expressed CAMs. Based on the gene expression results, immunohistochemical evaluation and quantification of cells expressing integrins ITGAM, ITGAX, and ITGB2, as well as ICAM-3 in sinonasal tissues were conducted to compare local protein expression patterns.

Results

Integrin and ICAM genes were significantly elevated in the blood (p<0.001 to p<0.05) and sinuses (p<0.0003 to p<0.05) of patients with CRSwNP compared to controls. Strong positive correlations of genes expressed in the blood (p<0.01 to p<0.05) and sinuses (p<0.01) were observed between ITGAM, ITGAX, ITGB2, and ICAM3. ITGAM-, ITGB2-, ICAM-3-, and ICAM-3/ITGB2-positive cell counts were significantly increased in CRSwNP compared to controls (p<0.0001 to p<0.04), and a positive correlation between ICAM-3/ITGB2- and ITGAM-positive cell counts was observed (p<0.02).

Conclusion

The systemic and local expression of ITGAM, ITGB2, and ICAM-3 is significantly upregulated in CRSwNP, suggesting that integrin complex ITGAM/ITGB2 and ICAM-3 serve a potential role in inflammation-mediated signaling in CRSwNP.

The complement system in age-related macular degeneration

Age-related macular degeneration (AMD) is a chronic and progressive degenerative disease of the retina, which culminates in blindness and affects mainly the elderly population. AMD pathogenesis and pathophysiology are incredibly complex due to the structural and cellular complexity of the retina, and the variety of risk factors and molecular mechanisms that contribute to disease onset and progression. AMD is driven by a combination of genetic predisposition, natural ageing changes and lifestyle factors, such as smoking or nutritional intake. The mechanism by which these risk factors interact and converge towards AMD are not fully understood and therefore drug discovery is challenging, where no therapeutic attempt has been fully effective thus far. Genetic and molecular studies have identified the complement system as an important player in AMD. Indeed, many of the genetic risk variants cluster in genes of the alternative pathway of the complement system and complement activation products are elevated in AMD patients. Nevertheless, attempts in treating AMD via complement regulators have not yet been successful, suggesting a level of complexity that could not be predicted only from a genetic point of view. In this review, we will explore the role of complement system in AMD development and in the main molecular and cellular features of AMD, including complement activation itself, inflammation, ECM stability, energy metabolism and oxidative stress.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

Complement-Opsonized Nano-Carriers Are Bound by Dendritic Cells (DC) via Complement Receptor (CR)3, and by B Cell Subpopulations via CR-1/2, and Affect the Activation of DC and B-1 Cells

The development of nanocarriers (NC) for biomedical applications has gained large interest due to their potential to co-deliver drugs in a cell-type-targeting manner. However, depending on their surface characteristics, NC accumulate serum factors, termed protein corona, which may affect their cellular binding. We have previously shown that NC coated with carbohydrates to enable biocompatibility triggered the lectin-dependent complement pathway, resulting in enhanced binding to B cells via complement receptor (CR)1/2. Here we show that such NC also engaged all types of splenic leukocytes known to express CR3 at a high rate when NC were pre-incubated with native mouse serum resulting in complement opsonization. By focusing on dendritic cells (DC) as an important antigen-presenting cell type, we show that CR3 was essential for binding/uptake of complement-opsonized NC, whereas CR4, which in mouse is specifically expressed by DC, played no role. Further, a minor B cell subpopulation (B-1), which is important for first-line pathogen responses, and co-expressed CR1/2 and CR3, in general, engaged NC to a much higher extent than normal B cells. Here, we identified CR-1/2 as necessary for binding of complement-opsonized NC, whereas CR3 was dispensable. Interestingly, the binding of complement-opsonized NC to both DC and B-1 cells affected the expression of activation markers. Our findings may have important implications for the design of nano-vaccines against infectious diseases, which codeliver pathogen-specific protein antigen and adjuvant, aimed to induce a broad adaptive cellular and humoral immune response by inducing cytotoxic T lymphocytes that kill infected cells and pathogen-neutralizing antibodies, respectively. Decoration of nano-vaccines either with carbohydrates to trigger complement activation in vivo or with active complement may result in concomitant targeting of DC and B cells and thereby may strongly enhance the extent of dual cellular/humoral immune responses.

Nanobodies Provide Insight into the Molecular Mechanisms of the Complement Cascade and Offer New Therapeutic Strategies

The complement system is part of the innate immune response, where it provides immediate protection from infectious agents and plays a fundamental role in homeostasis. Complement dysregulation occurs in several diseases, where the tightly regulated proteolytic cascade turns offensive. Prominent examples are atypical hemolytic uremic syndrome, paroxysmal nocturnal hemoglobinuria and Alzheimer’s disease. Therapeutic intervention targeting complement activation may allow treatment of such debilitating diseases. In this review, we describe a panel of complement targeting nanobodies that allow modulation at different steps of the proteolytic cascade, from the activation of the C1 complex in the classical pathway to formation of the C5 convertase in the terminal pathway. Thorough structural and functional characterization has provided a deep mechanistic understanding of the mode of inhibition for each of the nanobodies. These complement specific nanobodies are novel powerful probes for basic research and offer new opportunities for in vivo complement modulation.

Complement Receptors and Their Role in Leukocyte Recruitment and Phagocytosis

The complement system is deeply embedded in our physiology and immunity. Complement activation generates a multitude of molecules that converge simultaneously on the opsonization of a target for phagocytosis and activation of the immune system via soluble anaphylatoxins. This response is used to control microorganisms and to remove dead cells, but also plays a major role in stimulating the adaptive immune response and the regeneration of injured tissues. Many of these effects inherently depend on complement receptors expressed on leukocytes and parenchymal cells, which, by recognizing complement-derived molecules, promote leukocyte recruitment, phagocytosis of microorganisms and clearance of immune complexes. Here, the plethora of information on the role of complement receptors will be reviewed, including an analysis of how this functionally and structurally diverse group of molecules acts jointly to exert the full extent of complement regulation of homeostasis.