The versatile functions of complement C3-derived ligands

Increased expression of complement C3c, iC3b, and cells containing CD11b or CD14 in experimentally induced psoriatic lesion

Psoriasis is a chronic inflammatory skin disease with a characteristic isomorphic reaction, i.e. the Köbner reaction, induced by slight epidermal trauma. In this study, the tape-stripping technique was used to induce the development of Köbner reaction in 18 subjects with psoriasis. Eight subjects developed a positive reaction. To study the early cellular changes, skin biopsies were taken at the baseline and subsequent time points of 2 h, 1 d, 3 d, and 7 d for the immunostaining of complement C3c, iC3b, and cells expressing complement receptor 3 (CD11b/CD18; a receptor of iC3b) or CD14. The results show that the positive Köbner reaction is associated with rapid (2 h-1 d) and sustained (3-7 d) increase in the expression of epidermal C3c and iC3b and dermal C3c. In addition, there was a positive correlation between CD11b+ and CD14+ cells in baseline and 2 h-1 d biopsies with a subsequent increase in CD11b+ and CD14+ cells in 3-7 d biopsies in the Köbner-positive group. In the Köbner-negative group, only a transient increase in epidermal iC3b at 2 h-1 d, as well as rapid (2 h-1 d) and sustained increase (3-7 d) in dermal iC3b and CD14+ cells, was observed. In experiments with cultured monolayer keratinocytes, a slight cell damage already at 30 mJ/cm2 ultraviolet B irradiation led to increased expression of C3c, but not iC3b. Therefore, there are marked differences between Köbner groups in respect to the expression of C3c, iC3b, and cells expressing CD11b or CD14. Of note is the rapid and sustained increase in epidermal C3c and iC3b in the positive Köbner reaction.

Sequential Increase in Complement Factor I, iC3b, and Cells Expressing CD11b or CD14 in Cutaneous Vasculitis

Mast cells contribute to the pathogenesis of cutaneous vasculitis through complement C3 that is cleaved to C3b and then to iC3b by complement factor I. The receptor of iC3b, CD11b, is expressed on neutrophils and monocytes and CD14 on monocytes. Their role in vasculitis is obscure. In this study, frozen skin biopsies from the nonlesional skin, initial petechial lesion, and palpable purpura lesion from 10 patients with immunocomplex-mediated small vessel vasculitis were studied immunohistochemically for complement factor I, iC3b, CD11b, and CD14. Peripheral blood mononuclear cells from 5 healthy subjects were used to study cell migration and cytokine secretion. Already, the nonlesional skin revealed marked immunostaining of complement factor I, iC3b, CD11b, and CD14, and their expression increased sequentially in initial petechial and palpable purpura lesions. Mast cell C3c correlated to iC3b, and both of them correlated to CD11b⁺ and CD14⁺ cells, in the nonlesional skin. The stimulation of mononuclear cells with 0.01-0.1 μg/ml iC3b induced cell migration in the transwell assay. C3a stimulated slightly interleukin-8 secretion, whereas 1 μg/ml iC3b inhibited it slightly, in 4/5 subjects. In conclusion, the C3-C3b-iC3b axis is activated already in the early vasculitis lesion leading to progressive accumulation of CD11b⁺ and CD14⁺ cells.

Bioinformatics analysis of C3 in brain low-grade gliomas as potential therapeutic target and promoting immune cell infiltration

Low-grade gliomas is the malignant nervous tumor with distinct biological and clinical characteristics. Despite advances in diagnostic and therapeutic methods, how to significantly elongate the survival of low-grade gliomas is still challengeable. Complement 3, as the critical component in the innate immune system, plays an essential role in local immune response and participating into regulation of the epithelial-mesenchymal transition and tumor microenvironment. In this study, we systematically determined the expression levels and immunological roles of C3 in low-grade gliomas using various public databases. Then, we further identified the impact of C3 expression on immune cell infiltration compared to normal tissue, indicating the effect of cellular microenvironment on overall survival of LGG patients. We obtained clinical characteristics, transcriptome, and survival of C3 in LGG from the TCGA, GEPIA2.0, and cBioportal databases. Two differentially expressed genes (DEGs) were obtained, DEGs compared to normal tissue (DEG_G1) and DEGs between C3 high expression and C3 low expression in LGG patients (DEG_G2). By performing the GO analysis and protein-protein interaction (PPI) network of DEG_G1, we have identified the top-ranked 10 hub genes, which are highly associated with regulation of cell cycle. The gene set enrichment analysis demonstrated that overexpression of C3 in LGG patient is positively correlated with regulation of cell cycle. The relative PPI analysis and GSEA of DEG_G2 were performed and analysis results indicated that higher expression of C3 in the LGG can activate immune-related pathways. Finally, immune cell infiltration analysis of C3 in the LGG patients was employed and clearly indicated that higher neutrophil infiltration can worsen the survival of the LGG patients with higher expression of C3. These results were confirmed by the Human Protein Atlas database, in which expression level of C3 protein in gliomas patients always higher. This investigation implied that C3 can be as diagnostic biomarker and potential targets of precise therapy for the LGG patients.

Natural Compounds as Target Biomolecules in Cellular Adhesion and Migration: From Biomolecular Stimulation to Label-Free Discovery and Bioactivity-Based Isolation

Plants and fungi can be used for medical applications because of their accumulation of special bioactive metabolites. These substances might be beneficial to human health, exerting also anti-inflammatory and anticancer (antiproliferative) effects. We propose that they are mediated by influencing cellular adhesion and migration via various signaling pathways and by directly inactivating key cell adhesion surface receptor sites. The evidence for this proposition is reviewed (by summarizing the natural metabolites and their effects influencing cellular adhesion and migration), along with the classical measuring techniques used to gain such evidence. We systematize existing knowledge concerning the mechanisms of how natural metabolites affect adhesion and movement, and their role in gene expression as well. We conclude by highlighting the possibilities to screen natural compounds faster and more easily by applying new label-free methods, which also enable a far greater degree of quantification than the conventional methods used hitherto. We have systematically classified recent studies regarding the effects of natural compounds on cellular adhesion and movement, characterizing the active substances according to their organismal origin (plants, animals or fungi). Finally, we also summarize the results of recent studies and experiments on SARS-CoV-2 treatments by natural extracts affecting mainly the adhesion and entry of the virus.

Why current quantitative serology is not quantitative and how systems immunology could provide solutions

Determination of the presence of antibodies against infectious agents, self-antigens, allogeneic antigens and environmental antigens is the goal of medical serology. Along with the standardization of these tests the community also started to use the expression “quantitative serology,” referring to the fact that arbitrary units are used for the expression of results. In this review I will argue against the use of the term quantitative serology for current tests. Because each test and each antibody isotype determination uses its own references, the term semiquantitative better describes these methods. The introduction of really quantitative serology could both benefit from and drive forward systems immunological approach to immunity.

Biologia Futura: stories about the functions of β2-integrins in human phagocytes

Integrins are essential membrane proteins that provide a tightly regulated link between the extracellular matrix and the intracellular cytoskeletal network. These cell surface proteins are composed of a non-covalently bound α chain and β chain. The leukocyte-specific complement receptor 3 (CR3, α_Mβ₂, CD11b/CD18) and complement receptor 4 (CR4, α_Xβ₂, CD11c/CD18) belong to the family of β₂-integrins. These receptors bind multiple ligands like iC3b, ICAMs, fibrinogen or LPS, thus allowing them to partake in phagocytosis, cellular adhesion, extracellular matrix rearrangement and migration. CR3 and CR4 were generally expected to mediate identical functions due to their structural homology, overlapping ligand specificity and parallel expression on human phagocytes. Despite their similarities, the expression level and function of these receptors differ in a cell-type-specific manner, both under physiological and inflammatory conditions.We investigated comprehensively the individual role of CR3 and CR4 in various functions of human phagocytes, and we proved that there is a "division of labour" between these two receptors. In this review, I will summarize our current knowledge about this area.

Circulating Complement C3-Alpha Chain Levels Predict Survival of Septic Shock Patients

BACKGROUND

Circulating complement C3 fragments released during septic shock might contribute to the development of complications such as profound hypotension and disseminated intravascular coagulation. The role of C3 in the course of septic shock varies in the literature, possibly because circulating C3 exists in different forms indistinguishable via traditional ELISA-based methods. We sought to test the relationship between C3 forms, measured by Western blotting with its associated protein size differentiation feature, and clinical outcomes.

METHODS

Secondary analysis of two prospective cohorts of patients with septic shock: a discovery cohort of 24 patents and a validation cohort of 181 patients. C3 levels were measured by Western blotting in both cohorts using blood obtained at enrollment. Differences between survivors and non-survivors were compared, and the independent prognostic values of C3 forms were assessed.

RESULTS

In both cohorts there were significantly lower levels of the C3-alpha chain in non-survivors than in survivors, and persisted after controlling for sequential organ failure assessment score. Area under the receiver operating characteristics to predict survival was 0.65 (95% confidence interval: 0.56-0.75). At a best cutoff value (Youden) of 970.6 μg/mL, the test demonstrated a sensitivity of 68.5% and specificity of 61.5%. At this cutoff point, Kaplan-Meier survival analysis showed that patients with lower levels of C3-alpha chain had significantly lower survival than those with higher levels (P < 0.001).

CONCLUSION

Circulating C3-alpha chain levels is a significant independent predictor of survival in septic shock patients.

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.

Revisiting the Coreceptor Function of Complement Receptor Type 2 (CR2, CD21); Coengagement With the B-Cell Receptor Inhibits the Activation, Proliferation, and Antibody Production of Human B Cells

The positive coreceptor function of complement receptor type 2 [CR2 (CD21)] on B cells is generally accepted, although its role in the enhancement of antibody production had only been proven in mice. The importance of this phenomenon prompted reinvestigation of the functional consequences of coclustering CD21 and the B cell receptor (BCR) on primary human cells. We found that, at non-stimulatory concentrations of anti-IgG/A/M, coclustering the BCR and CR2 enhanced the Ca²⁺ response, while activation marker expression, cytokine production, proliferation, and antibody production were all inhibited upon the coengagement of CR2 and BCR on human B cells. Thus, the “textbook dogma” claiming that C3d acts as an adjuvant to enhance humoral immunity is relevant only to mice and not to humans.

Soluble Membrane Attack Complex: Biochemistry and Immunobiology

The soluble membrane attack complex (sMAC, a.k.a., sC5b-9 or TCC) is generated on activation of complement and contains the complement proteins C5b, C6, C7, C8, C9 together with the regulatory proteins clusterin and/or vitronectin. sMAC is a member of the MACPF/cholesterol-dependent-cytolysin superfamily of pore-forming molecules that insert into lipid bilayers and disrupt cellular integrity and function. sMAC is a unique complement activation macromolecule as it is comprised of several different subunits. To date no complement-mediated function has been identified for sMAC. sMAC is present in blood and other body fluids under homeostatic conditions and there is abundant evidence documenting changes in sMAC levels during infection, autoimmune disease and trauma. Despite decades of scientific interest in sMAC, the mechanisms regulating its formation in healthy individuals and its biological functions in both health and disease remain poorly understood. Here, we review the structural differences between sMAC and its membrane counterpart, MAC, and examine sMAC immunobiology with respect to its presence in body fluids in health and disease. Finally, we discuss the diagnostic potential of sMAC for diagnostic and prognostic applications and potential utility as a companion diagnostic.

Clonally expanding smooth muscle cells promote atherosclerosis by escaping efferocytosis and activating the complement cascade

Atherosclerosis is the process underlying heart attack and stroke. Despite decades of research, its pathogenesis remains unclear. Dogma suggests that atherosclerotic plaques expand primarily via the accumulation of cholesterol and inflammatory cells. However, recent evidence suggests that a substantial portion of the plaque may arise from a subset of "dedifferentiated" vascular smooth muscle cells (SMCs) which proliferate in a clonal fashion. Herein we use multicolor lineage-tracing models to confirm that the mature SMC can give rise to a hyperproliferative cell which appears to promote inflammation via elaboration of complement-dependent anaphylatoxins. Despite being extensively opsonized with prophagocytic complement fragments, we find that this cell also escapes immune surveillance by neighboring macrophages, thereby exacerbating its relative survival advantage. Mechanistic studies indicate this phenomenon results from a generalized opsonin-sensing defect acquired by macrophages during polarization. This defect coincides with the noncanonical up-regulation of so-called don't eat me molecules on inflamed phagocytes, which reduces their capacity for programmed cell removal (PrCR). Knockdown or knockout of the key antiphagocytic molecule CD47 restores the ability of macrophages to sense and clear opsonized targets in vitro, allowing for potent and targeted suppression of clonal SMC expansion in the plaque in vivo. Because integrated clinical and genomic analyses indicate that similar pathways are active in humans with cardiovascular disease, these studies suggest that the clonally expanding SMC may represent a translational target for treating atherosclerosis.

Complement Receptor 2 Based Immunoassay Measuring Activation of the Complement System at C3-Level in Plasma Samples From Mice and Humans

We aimed at establishing a sensitive and robust assay for estimation of systemic complement activation at complement component C3 level in mouse and human plasma samples. In order to capture the activation products iC3b and C3dg in a specific and physiological relevant manner we utilized a construct consisting of the iC3b/C3dg-binding site of human complement receptor 2 (CR2) attached to an Fc-part of mouse IgG. This construct binds C3dg and iC3b from both mice and humans. We purified the CR2-IgG construct from mouse B myeloma cell line supernatants, J558L-CR2-IgG, by protein G affinity chromatography. The CR2-IgG construct was used for capturing C3 fragments in microtiter wells and an anti-mouse or an anti-human-C3 antibody was used for detection of bound C3 fragments. Initially we tested the specificity of the assays with the use of purified C3 fragments. Further, with the use of the CR2-based assay, we measured an up to three-fold higher signal in activated mouse serum as compared to non-activated mouse serum, whereas activated serum from a C3 knock-out mouse gave no signal. We tested in vivo generated samples from a mouse experiment; complement activation was induced by injecting cobra venom factor or heat aggregated IgG into C57bl6 mice, followed by withdrawal of EDTA blood samples at different time points and measurement of iC3b/C3dg. We observed a clear time-dependent distinction in signals between samples with expected high and low complement activation. Furthermore, with the use of the assay for human C3 fragments, we observed that patients with systemic lupus erythematosus (SLE) (n = 144) had significantly higher iC3b/C3dg levels as compared to healthy individuals (n = 144) (p < 0.0001). We present two functional immunoassays, that are able to measure systemic levels of the C3-activation products iC3b and C3dg in mice and humans. To our knowledge, these are the first assays for complement activation that use a physiological relevant capture construct such as CR2. These assays will be a relevant tool when investigating mouse models and human diseases involving the complement system.

European Society for Immunodeficiencies (ESID) and European Reference Network on Rare Primary Immunodeficiency, Autoinflammatory and Autoimmune Diseases (ERN RITA) Complement Guideline: Deficiencies, Diagnosis, and Management

This guideline aims to describe the complement system and the functions of the constituent pathways, with particular focus on primary immunodeficiencies (PIDs) and their diagnosis and management. The complement system is a crucial part of the innate immune system, with multiple membrane-bound and soluble components. There are three distinct enzymatic cascade pathways within the complement system, the classical, alternative and lectin pathways, which converge with the cleavage of central C3. Complement deficiencies account for ~5% of PIDs. The clinical consequences of inherited defects in the complement system are protean and include increased susceptibility to infection, autoimmune diseases (e.g., systemic lupus erythematosus), age-related macular degeneration, renal disorders (e.g., atypical hemolytic uremic syndrome) and angioedema. Modern complement analysis allows an in-depth insight into the functional and molecular basis of nearly all complement deficiencies. However, therapeutic options remain relatively limited for the majority of complement deficiencies with the exception of hereditary angioedema and inhibition of an overactivated complement system in regulation defects. Current management strategies for complement disorders associated with infection include education, family testing, vaccinations, antibiotics and emergency planning.

Regulation of anti-microbial autophagy by factors of the complement system

The complement system is a major component of innate immunity that participates in the defense of the host against a myriad of pathogenic microorganisms. Activation of complement allows for both local inflammatory response and physical elimination of microbes through phagocytosis or lysis. The system is highly efficient and is therefore finely regulated. In addition to these well-established properties, recent works have revealed that components of the complement system can be involved in a variety of other functions including in autophagy, the conserved mechanism that allows for the targeting and degradation of cytosolic materials by the lysosomal pathway after confining them into specialized organelles called autophagosomes. Besides impacting cell death, development or metabolism, the complement factors-autophagy connection can greatly modulate the cell autonomous, anti-microbial activity of autophagy: xenophagy. Both surface receptor-ligand interactions and intracellular interactions are involved in the modulation of the autophagic response to intracellular microbes by complement factors. Here, we review works that relate to the recently discovered connections between factors of the complement system and the functioning of autophagy in the context of host-pathogen relationship.

Complement Signals Determine Opposite Effects of B Cells in Chemotherapy-Induced Immunity

Understanding molecular mechanisms that dictate B cell diversity is important for targeting B cells as anti-cancer treatment. Through the single-cell dissection of B cell heterogeneity in longitudinal samples of patients with breast cancer before and after neoadjuvant chemotherapy, we revealed that an ICOSL⁺ B cell subset emerges after chemotherapy. Using three immunocompetent mouse models, we recapitulated the subset switch of human tumor-infiltrating B cells during chemotherapy. By employing B-cell-specific deletion mice, we showed that ICOSL in B cells boosts anti-tumor immunity by enhancing the effector to regulatory T cell ratio. The signature of ICOSL⁺ B cells is imprinted by complement-CR2 signaling, which is triggered by immunogenic cell death. Moreover, we identified that CD55, a complement inhibitory protein, determines the opposite roles of B cells in chemotherapy. Collectively, we demonstrated a critical role of the B cell subset switch in chemotherapy response, which has implications in designing novel anti-cancer therapies. VIDEO ABSTRACT.

New insights into the immune functions of complement

The recognition of microbial or danger-associated molecular patterns by complement proteins initiates a cascade of events that culminates in the activation of surface complement receptors on immune cells. Such signalling pathways converge with those activated downstream of pattern recognition receptors to determine the type and magnitude of the immune response. Intensive investigation in the field has uncovered novel pathways that link complement-mediated signalling with homeostatic and pathological T cell responses. More recently, the observation that complement proteins also act in the intracellular space to shape T cell fates has added a new layer of complexity. Here, we consider fundamental mechanisms and novel concepts at the interface of complement biology and immunity and discuss how these affect the maintenance of homeostasis and the development of human pathology.

Compensatory role of C3 convertase on the strain difference for C3 protein expression in FVB/N, C3H/HeN and C57BL/6N mice

To investigate the role of complement C3 (C3) convertase on the strain difference for C3 protein expression in three inbred mice strains, we compared the levels of C2, C3 and C4 mRNA, as well as C3 protein and C3 convertase activity in the serum and liver tissue of FVB/N, C3H/HeN and C57BL/6N mice. The level of mRNA, inactive form (InACF) and active form (ACF) for C3 showed a regular pattern, which they were higher in the FVB/N and C57BL/6N mice than C3H/HeN mice. However, the level of C3b fragments (C3bα and β) derived from C3 protein were constantly maintained in the liver of FVB/N, C3H/HeN and C57BL/6N mice in spite of the strain difference on the transcriptional and translation level of C3. Especially, a reverse pattern of the level of mRNA, InACF and ACF for C3 was observed on the activity level of C3 convertase activity. The highest level of C3 convertase activity was measured in C3H/HeN mice, followed by C57BL/6N and FVB/N mice. In case of C3 convertase components, the level of C2 mRNA was higher in C3H/HeN mice than FVB/N and C57BL/6 N mice, while levels of C4 mRNA were higher in FVB/N and C57BL/6N mice than C3H/HeN mice. The current study results provide the first scientific evidence that C3 convertase may play complementary role to overcome the strain difference on the C3 protein expression in FVB/N, C3H/HeN and C57BL/6N mice.

Complement Receptor Type 1 (CR1, CD35), the Inhibitor of BCR-Mediated Human B Cell Activation, Differentially Regulates TLR7, and TLR9 Induced Responses

The complement system and Toll-like receptors (TLRs) are essential contributors of innate immunity. Separate activation of these systems has been shown to play a role in initiating and shaping the adaptive immune response, however the modulation of various B cell functions by the simultaneous involvement of these two systems has not yet been uncovered. We demonstrate here that occupancy of complement receptor type 1 (CR1, CD35) by its natural, complement component C3-derived ligand significantly and dose dependently reduces the TLR9-induced expression of activation markers, cytokine production, proliferation, and antibody production by human B cells, but has no effect on the TLR7-induced functions. The synergistic response to the simultaneous engagement of either TLR9 or TLR7 along with the BCR however, is significantly inhibited by CR1 occupancy. Our findings imply that both under physiological and pathological conditions, when complement- and TLR-activating microbial and damage products are present in the B cell environment, the cooperation between CR1 and TLR7 or TLR9 provides additional levels of the regulation of human B cell functions.

Cutaneous Vasculitis and Digital Ischaemia Caused by Heterozygous Gain-of-Function Mutation in C3

It is now increasingly recognized that some monogenic autoinflammatory diseases and immunodeficiencies cause vasculitis, although genetic causes of vasculitis are extremely rare. We describe a child of non-consanguineous parents who presented with cutaneous vasculitis, digital ischaemia and hypocomplementaemia. A heterozygous p.R1042G gain-of-function mutation (GOF) in the complement component C3 gene was identified as the cause, resulting in secondary C3 consumption and complete absence of alternative complement pathway activity, decreased classical complement activity, and low levels of serum C3 with normal C4 levels. The same heterozygous mutation and immunological defects were also identified in another symptomatic sibling and his father. C3 deficiency due GOF C3 mutations is thus now added to the growing list of monogenic causes of vasculitis and should always be considered in vasculitis patients found to have persistently low levels of C3 with normal C4.

Carboxypeptidase B2 and carboxypeptidase N in the crosstalk between coagulation, thrombosis, inflammation, and innate immunity

Two basic carboxypeptidases, carboxypeptidase B2 (CPB2) and carboxypeptidase N (CPN) are present in plasma. CPN is constitutively active, whereas CPB2 circulates as a precursor, procarboxypeptidase B2 (proCPB2), that needs to be activated by the thrombin-thrombomodulin complex or plasmin bound to glycosaminoglycans. The substrate specificities of CPB2 and CPN are similar; they both remove C-terminal basic amino acids from bioactive peptides and proteins, thereby inactivating them. The complement cascade is a cascade of proteases and cofactors activated by pathogens or dead cells, divided into two phases, with the second phase only being triggered if sufficient C3b is present. Complement activation generates anaphylatoxins: C3a, which stimulates macrophages; and C5a, which is an activator and attractant for neutrophils. Pharmacological intervention with inhibitors has shown that CPB2 delays fibrinolysis, whereas CPN is responsible for systemic inactivation of C3a and C5a. Among mice genetically deficient in either CPB2 or CPN, in a model of hemolytic-uremic syndrome, Cpb2-/- mice had the worst disease, followed by Cpn-/- mice, with wild-type (WT) mice being the most protected. This model is driven by C5a, and shows that CPB2 is important in inactivating C5a. In contrast, when mice were challenged acutely with cobra venom factor, the reverse phenotype was observed; Cpn-/- mice had markedly worse disease than Cpb2-/- mice, and WT mice were resistant. These observations need to be confirmed in humans. Therefore, CPB2 and CPN have different roles. CPN inactivates C3a and C5a generated spontaneously, whereas proCPB2 is activated at specific sites, where it inactivates bioactive peptides that would overwhelm CPN.

Phagocytic and Bactericidal Properties of Channel Catfish Peritoneal Macrophages Exposed to Edwardsiella ictaluri Live Attenuated Vaccine and Wild-Type Strains

Edwardsiella ictaluri (E. ictaluri), a Gram-negative, intracellular, facultative bacterium, is the causative agent of enteric septicemia of catfish (ESC), which is one of the most significant diseases of farmed channel catfish. Macrophages have a critical role in major defense mechanisms against bacterial infections by migrating to the site of infection, engulfing and killing pathogens, and priming adaptive immune responses. Vaccination of catfish with E. ictaluri live attenuated vaccine (LAV) strains increased the efficiency of phagocytosis and bacterial killing in catfish peritoneal macrophages compared in vitro with macrophages from non-vaccinated fish. Recently, our group developed several protective LAV strains from E. ictaluri. However, their effects on the antigen uptake and bacterial killing in catfish macrophages have not been evaluated. In this study, we assessed the phagocytic and bactericidal activity of peritoneal macrophages in the uptake of E. ictaluri wild-type (WT) and two LAV strains. We found that phagocytosis of LAV strains was significantly higher compared to their WT counterpart in peritoneal macrophages. Moreover, the uptake of E. ictaluri opsonized with sera from vaccinated catfish was more efficient than when opsonized with sera from sham-vaccinated fish. Notably, catfish macrophages did not lose their phagocytic properties at 4°C, as described previously in mammalian and zebrafish models. Also, opsonization of E. ictaluri with inactivated sera from vaccinated and sham-vaccinated catfish decreased significantly phagocytic uptake of bacteria at 32°C, and virtually suppressed endocytosis at 4°C, suggesting the important role of complement-dependent mechanisms in catfish macrophage phagocytosis. In conclusion, our data on enhanced phagocytic capacity and effective killing ability in macrophages of vaccine strains suggested the LAVs’ advantage if processed and presented in the form of peptides to specific lymphocytes of an adaptive immune system and emphasize the importance of macrophage-mediated immunity against ESC. Furthermore, we showed the role of complement-dependent mechanisms in the phagocytic uptakes of E. ictaluri in catfish peritoneal macrophages at 4 and 32°C. Finally, LAV vaccine-induced bacterial phagocytosis and killing properties of peritoneal macrophages emphasized the importance of the innate immune responses in ESC.

Complement component C3 - The "Swiss Army Knife" of innate immunity and host defense

As a preformed defense system, complement faces a delicate challenge in providing an immediate, forceful response to pathogens even at first encounter, while sparing host cells in the process. For this purpose, it engages a tightly regulated network of plasma proteins, cell surface receptors, and regulators. Complement component C3 plays a particularly versatile role in this process by keeping the cascade alert, acting as a point of convergence of activation pathways, fueling the amplification of the complement response, exerting direct effector functions, and helping to coordinate downstream immune responses. In recent years, it has become evident that nature engages the power of C3 not only to clear pathogens but also for a variety of homeostatic processes ranging from tissue regeneration and synapse pruning to clearing debris and controlling tumor cell progression. At the same time, its central position in immune surveillance makes C3 a target for microbial immune evasion and, if improperly engaged, a trigger point for various clinical conditions. In our review, we look at the versatile roles and evolutionary journey of C3, discuss new insights into the molecular basis for C3 function, provide examples of disease involvement, and summarize the emerging potential of C3 as a therapeutic target.