The Nano-War Against Complement Proteins

Targeted drug delivery and nanomedicine hold the potential promise of delivering drugs solely to target organs or cell types, thus decreasing off-target side effects and improving efficacy. However, nano-scale drug carriers face several barriers to this goal, with one of the most formidable being the complement cascade. Complement proteins, especially C3, opsonize not just the microbes they evolved to contain, but also nanocarriers. This results in multiple problems, including marking the nanocarriers for clearance by leukocytes, likely fouling of the targeting moieties on nanocarriers, and release of toxins which produce deleterious local and systemic effects. Here, we review how complement achieves its blockade of nanomedicine, which nanocarrier materials properties best avoid complement, and current and future strategies to control complement to unleash nanomedicine’s potential.

Novel methods to determine complement activation in human serum induced by the complex of Dezamizumab and serum amyloid P

Lack of simple and robust methods to determine complement activation in human serum induced by antigen–antibody complexes is a major hurdle for monitoring therapeutic antibody drug quality and stability. Dezamizumab is a humanized IgG1 monoclonal antibody that binds to serum amyloid P component (SAP) for potential treatment of systemic amyloidosis. The mechanism of action of Dezamizumab includes the binding of SAP, complement activation through classical pathway, and phagocytosis; however, the steps in this process cannot be easily monitored. We developed two novel methods to determine Dezamizumab-SAP complex-induced complement activation. Complement component 3 (C3) depletion was detected by homogeneous time-resolved fluorescence (HTRF), and C3a desArg fragment, formed after the cleavage of C3 to yield C3a followed by removal of its C-terminal arginine residue, was determined using Meso Scale Discovery (MSD) technology. We found that the presence of both Dezamizumab and SAP was required for complement activation via both methods. The optimal molar ratio of Dezamizumab:SAP was 6:1 in order to obtain maximal complement activation. The relative potency from both methods showed a good correlation to Dezamizumab-SAP-dependent complement component 1q (C1q) binding activity in Dezamizumab thermal-stressed samples. Both SAP and C1q binding, as determined by surface plasmon resonance and the two complement activation potency methods described here, reflect the mechanism of action of Dezamizumab. We conclude that these methods can be used to monitor Dezamizumab quality for drug release and stability testing, and the novel potency methods reported here can be potentially used to evaluate complement activity induced by other antigen–antibody complexes.

Complement Component C3 Loss leads to Locomotor Deficits and Altered Cerebellar Internal Granule Cell In Vitro Synaptic Protein Expression in C57BL/6 Mice

Complement component 3 (C3) expression is increased in the cerebellum of aging mice that demonstrate locomotor impairments and increased excitatory synapse density. However, C3 regulation of locomotion, as well as C3 roles in excitatory synapse function, remains poorly understood. Here, we demonstrate that constitutive loss of C3 function in mice evokes a locomotor phenotype characterized by decreased speed, increased active state locomotor probability, and gait ataxia. C3 loss does not alter metabolism or body mass composition. No evidence of significant muscle weakness or degenerative arthritis was found in C3 knockout mice to explain decreased gait speeds. In an enriched primary cerebellar granule cell culture model, loss of C3 protein results in increased excitatory synaptic density and increased response to KCl depolarization. Our analysis of excitatory synaptic density in the cerebellar internal granule cell and molecular layers did not demonstrate increased synaptic density in vivo, suggesting the presence of compensatory mechanisms regulating synaptic development. Functional deficits in C3 knockout mice are therefore more likely to result from altered synaptic function and/or connectivity than gross synaptic deficits. Our data demonstrate a novel role for complement proteins in cerebellar regulation of locomotor output and control.

The Inflammatory Feed-Forward Loop Triggered by the Complement Component C3 as a Potential Target in Endometriosis

The complement system is a major component of humoral innate immunity, acting as a first line of defense against microbes via opsonization and lysis of pathogens. However, novel roles of the complement system in inflammatory and immunological processes, including in cancer, are emerging. Endometriosis (EM), a benign disease characterized by ectopic endometrial implants, shows certain unique features of cancer, such as the capacity to invade surrounding tissues, and in severe cases, metastatic properties. A defective immune surveillance against autologous tissue deposited in the peritoneal cavity allows immune escape for endometriotic lesions. There is evidence that the glandular epithelial cells found in endometriotic implants produce and secrete the complement component C3. Here, we show, using immunofluorescence and RT-qPCR, the presence of locally synthesized C3 in the ectopic endometriotic tissue, but not in the eutopic tissue. We generated a murine model of EM via injection of minced uterine tissue from a donor mouse into the peritoneum of recipient mice. The wild type mice showed greater amount of cyst formation in the peritoneum compared to C3 knock-out mice. Peritoneal washings from the wild type mice with EM showed more degranulated mast cells compared to C3 knock-out mice, consistent with higher C3a levels in the peritoneal fluid of EM patients. We provide evidence that C3a participates in an auto-amplifying loop leading to mast cell infiltration and activation, which is pathogenic in EM. Thus, C3 can be considered a marker of EM and its local synthesis can promote the engraftment of the endometriotic cysts.

The Contribution of Serum Complement Component 3 Levels to 90-Day Mortality in Living Donor Liver Transplantation

The contributions of the complement system have been elucidated in the process of solid organ transplantation, including kidney transplantation. However, the role of complement in liver transplantation is unknown. We sought to elucidate the time-dependent changes of peritransplantational serum complement levels and the relationships with posttransplant outcomes and other immunological biomarkers. We enrolled 82 patients who underwent living-related donor liver transplantation (LDLT). Nine patients (11%) died within 90 days after LDLT (non-survivors). The following immunomarkers were collected preoperatively and at 1, 2, and 4 week(s) after LDLT: serum C3, C4, immunoglobulin G (IgG), and peripheral blood leukocyte populations characterized by CD3, CD4, CD8, CD16, CD19, CD20, CD22, and CD56. Consequently, C3 and C4 increased time-dependently after LDLT. Preoperatively, C3 was negatively correlated with the MELD score, Child–Pugh score, CD16-positive leukocyte percentage, and the CD56-positive leukocyte percentage. Non-survivors had lower levels of C3 at 2 weeks in comparison to survivors (median [interquartile range]: 56 [49-70] mg/dL vs. 88 [71-116] mg/dL, p=0.0059). When the cutoff value of C3 at 2 weeks to distinguish non-survivors was set to 71 mg/dL, the sensitivity, specificity, and area under the ROC curve were 87.5%, 75.0%, and 0.80, respectively. A principal component analysis showed an inverse relationship between the C3 and C4 levels and the percentage of CD8-, CD16-, and CD56-positive leukocytes at 1 and 2 week(s). All non-survivors were included in the cluster that showed higher percentages of CD8-, CD16-, and CD56-positive leukocytes at 2 weeks. In conclusion, we demonstrated the relationship between complement, outcomes, and other immunomarkers in LDLT and suggested the usefulness of C3 at 2 weeks after LDLT in distinguishing the mortality.

The Complement System: A Powerful Modulator and Effector of Astrocyte Function in the Healthy and Diseased Central Nervous System

The complement system, an effector arm of the innate immune system that plays a critical role in tissue inflammation, the elimination of pathogens and the clearance of dead cells and cell debris, has emerged as a regulator of many processes in the central nervous system, including neural cell genesis and migration, control of synapse number and function, and modulation of glial cell responses. Complement dysfunction has also been put forward as a major contributor to neurological disease. Astrocytes are neuroectoderm-derived glial cells that maintain water and ionic homeostasis, and control cerebral blood flow and multiple aspects of neuronal functioning. By virtue of their expression of soluble as well as membrane-bound complement proteins and receptors, astrocytes are able to both send and receive complement-related signals. Here we review the current understanding of the multiple functions of the complement system in the central nervous system as they pertain to the modulation of astrocyte activity, and how astrocytes use the complement system to affect their environment in the healthy brain and in the context of neurological disease.

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.

The role of complement in arterial hypertension and hypertensive end organ damage

Increasing evidence indicates that hypertension and hypertensive end organ damage are not only mediated by haemodynamic injury but that inflammation also plays an important role. The complement system protects the host from a hostile microbial environment and maintains tissue and cell integrity through the elimination of altered or dead cells. As an important effector arm of innate immunity, it plays also central roles in the regulation of adaptive immunity. Thus, complement activation may drive the pathology of hypertension through its effects on innate and adaptive immune responses, aside from direct effects on the vasculature. Recent experimental data strongly support a role for complement in all stages of arterial hypertension. The remarkably similar clinical and histopathological features of malignant nephrosclerosis and atypical haemolytic uraemic syndrome suggest also a role for complement in the development of malignant nephrosclerosis. Here, we review the role of complement in hypertension and hypertensive end organ damage. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Astrocytic Kir6.1 deletion aggravates neurodegeneration in the lipopolysaccharide-induced mouse model of Parkinson's disease via astrocyte-neuron cross talk through complement C3-C3R signaling

Complement pathway over-activation has been implicated in a variety of neurological diseases. However, the signaling pathways governing astrocytic complement activation in Parkinson's disease (PD) are poorly understood. Kir6.1, a pore-forming subunit of ATP-sensitive potassium (K-ATP) channel, is prominently expressed in astrocytes and exhibits anti-inflammatory effects. Therefore, we hypothesize that Kir6.1/K-ATP channel may regulate astrocytic complement activation in the pathogenesis of PD. In this study, astrocytic Kir6.1 knockout (KO) mice were used to examine the effect of astrocytic Kir6.1/K-ATP channel on astrocytic complement activation triggered by the lipopolysaccharide (LPS). Here, we found that astrocytic Kir6.1 KO mice showed more dopaminergic neuron loss and more astrocyte reactivity in substantia nigra compacta than controls. We also found that astrocytic Kir6.1 KO increased the expression of complement C3 in astrocytes in LPS-induced mouse model of PD. Mechanistically, astrocytic Kir6.1 KO promoted astroglial NF-κB activation to elicit extracellular release of C3, which in turn interacted with neuronal C3aR to induce neuron death. Blocking complement function by NF-κB inhibitor or C3aR antagonist rescued the aggravated neuron death induced by Kir6.1 KO. Collectively, our findings reveal that astrocytic Kir6.1/K-ATP channel prevents neurodegeneration in PD via astrocyte-neuron cross talk through NF-κB/C3/C3aR signaling and suggest that targeting astroglial Kir6.1/K-ATP channel-NF-κB-C3-neuronal C3aR signaling represents a novel therapeutic strategy for PD.

Role of intracellular complement activation in kidney fibrosis

Increased expression of complement C1r, C1s and C3 in kidney cells plays an important role in the pathogenesis of kidney fibrosis. Our studies suggest that activation of complement in kidney cells with increased generation of C3 and its fragments occurs by activation of classical and alternative pathways. Single nuclei RNA sequencing studies in kidney tissue from unilateral ureteral obstruction mice show that increased synthesis of complement C3 and C5 occurs primarily in renal tubular epithelial cells (proximal and distal), while increased expression of complement receptors C3ar1 and C5ar1 occurs in interstitial cells including immune cells like monocytes/macrophages suggesting compartmentalization of complement components during kidney injury. Although global deletion of C3 and macrophage ablation prevent inflammation and reduced kidney tissue scarring, the development of mice with cell-specific deletion of complement components and their regulators could bring further insights into the mechanisms by which intracellular complement activation leads to fibrosis and progressive kidney disease. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Interactive Impacts from Hepatitis C Virus Infection and Mixed Cryoglobulinemia on Complement Levels

BACKGROUND/AIM

How hepatitis C virus (HCV) infection and mixed cryoglobulinemia interactively affect complement levels remains elusive, and we aimed to elucidate it.

METHODS

A prospective cohort study of 678 consecutive chronic HCV-infected (CHC) patients was conducted. Of 678, 438 had completed a course of anti-HCV therapy and 362 had achieved a sustained virological response (SVR). The baseline and 24-week post-therapy variables including complement levels and mixed cryoglobulinemia status were surveyed.

RESULTS

At baseline, lower complement component 3 (C3) and component 4 (C4) levels were noted in patients with than those without mixed cryoglobulinemia. The differences between pre-therapy (in 678 CHC patients) and 24-week post-therapy (in 362 SVR patients) factors associated with C3 levels were interferon λ3 (IFNL3) genotype, triglycerides, cirrhosis, and estimated glomerular filtration rate; the different associations with C4 levels were cirrhosis, sex and high sensitivity C-reactive protein. Compared with baseline, SVR patients without pre- and post-therapy mixed cryoglobulinemia had increased C3 levels, and SVR patients with pre-therapy mixed cryoglobulinemia had increased C4 levels. Lower C3 levels were noted in SVR patients with than those without post-therapy mixed cryoglobulinemia.

CONCLUSIONS

HCV might affect C3 levels through IFNL3 genotype, triglycerides, cirrhosis, and renal function; and affect C4 with a link to sex, inflammation, and cirrhosis. That C3 levels decreased in CHC patients without mixed cryoglobulinemia or in SVR patients with post-therapy mixed cryoglobulinemia, and C4 levels decreased in CHC patients with mixed cryoglobulinemia, suggested that mixed cryoglobulinemia and HCV infection antagonistically and synergistically decrease C3 and C4 levels, respectively.

Complement in metabolic disease: metaflammation and a two-edged sword

We are currently experiencing an enduring global epidemic of obesity and diabetes. It is now understood that chronic low-grade tissue inflammation plays an important role in metabolic disease, brought upon by increased uptake of a so-called Western diet, and a more sedentary lifestyle. Many evolutionarily conserved links exist between metabolism and the immune system, and an imbalance in this system induced by chronic over-nutrition has been termed 'metaflammation'. The complement system is an important and evolutionarily ancient part of innate immunity, but recent work has revealed that complement not only is involved in the recognition of pathogens and induction of inflammation, but also plays important roles in cellular and tissue homeostasis. Complement can therefore contribute both positively and negatively to metabolic control, depending on the nature and anatomical site of its activity. This review will therefore focus on the interactions of complement with mechanisms and tissues relevant for metabolic control, obesity and diabetes.

Human CD3+CD56+NKT-like cells express a range of complement receptors and C3 activation has negative effects on these cell activity and effector function

CD3⁺CD56⁺NKT-like cells are a rare population of lymphocytes that serve important roles in various types of immune-related diseases, and particularly in cancer. The complement system regulates inflammatory and immune responses by interacting with complement receptors expressed on a range of immune cells. However, whether CD3⁺CD56⁺NKT-like cells are regulated by the complement system has still not been definitively determined. In the present study, the expression of complement receptors and regulators in gated CD3⁺CD56⁺NKT-like cells isolated from human peripheral blood was assessed using PCR and flow cytometry. The results showed that human CD3⁺CD56⁺NKT-like cells expressed a range of complement receptors and regulators, such as CR3, C3aR, C5aR, C5L2, CD46 and CD55. Furthermore, the presence of complement component 3 (C3), a key component in complement activation in culture supernatant, mitigated the activity, IFN-γ production and killing function of CD3⁺CD56⁺NKT-like cells. The present study provides evidences supporting the relationship between complement activation and functional modulation of CD3⁺CD56⁺NKT-like cells, expanding our knowledge of the complement regulatory network, and also highlighting a potential target for treatment of numerous immune-related diseases, particularly NKT cell-based tumor adoptive immunotherapy.

A Review of Completed and Ongoing Complement Inhibitor Trials for Geographic Atrophy Secondary to Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness among older adults in the Western world. While therapies exist for patients with exudative AMD, there are currently no approved therapies for non-exudative AMD and its advanced form of geographic atrophy (GA). The discovery of genetic variants in complement protein loci with increased susceptibility to AMD has led to the investigation of the role of complement inhibition in AMD with a focus on GA. Here, we review completed and ongoing clinical trials evaluating the safety and efficacy of these studies. Overall, complement inhibition in GA has yielded mixed results. The inhibition of complement factor D has failed pivotal phase 3 trials. Studies of C3 and C5 inhibition meeting their primary endpoint are limited by high rates of discontinuation and withdrawal in the treatment arm and higher risks of conversion to exudative AMD. Studies evaluating other complement members (CFB, CFH, CFI and inhibitors of membrane attack complex—CD59) are ongoing and could offer other viable strategies.

Leaf gas exchange and δ13C in cowpea and triticale under water stress and well-watered conditions

Leaf gas exchanges play a critical role in determining crop productivity as they control both CO₂ gain and water loss. CO₂ gain and water loss influence water use efficiency (WUE) and carbon isotope composition (δ¹³C). Responses in leaf gas exchanges to water stress are species-specific. However, the extent of this variation in C3 crops is less studied. A field study was carried out to investigate the influence of water stress on leaf gas exchanges of triticale and cowpea. Crops were grown under water stress and well-watered conditions and leaf gas exchanges were determined at flowering. The results showed that triticale maintained a higher stomatal conductance (gs), transpiration rate(E) and intercellular CO₂ concentration (ci) compared to cowpea but did not differ in photosynthetic rate(A). As a result, triticale discriminated against ¹³C more than cowpea. These results suggest a higher influence of ci on δ¹³C than A. Despite triticale maintaining higher rates of ci, A and gs, it had lower WUE compared to cowpea. Consequently, triticale grain yield was more sensitive to water stress than cowpea. The findings of this study showed significant variation in leaf gas exchanges and δ¹³C between two drought-tolerant C3 crops suggesting differences in their response mechanism to water stress.

Serum Complement C3 and C4 and COVID-19 Severity and Mortality: A Systematic Review and Meta-Analysis With Meta-Regression

Activation of the complement system has been observed in coronavirus disease 19 (COVID-19). We conducted a systematic review and meta-analysis with meta-regression to investigate possible differences in the serum concentrations of two routinely measured complement components, C3 and C4, in COVID-19 patients with different severity and survival status. We searched PubMed, Web of Science and Scopus, between January 2020 and February 2021, for studies reporting serum complement C3 and C4, measures of COVID-19 severity, and survival. Eligibility criteria were a) reporting continuous data on serum C3 and C4 concentrations in COVID-19 patients, -b) investigating COVID-19 patients with different disease severity and/or survival status, c) adult patients, d) English language, e) ≥10 patients, and f) full-text available. Using a random-effects model, standardized mean differences (SMD) with 95% confidence intervals (CIs) were calculated to evaluate differences in serum C3 and C4 concentrations between COVID-19 patients with low vs. high severity or survivor vs. non-survivor status. Risk of bias was assessed using the Newcastle-Ottawa scale whereas publication bias was assessed with the Begg’s and Egger’s tests. Certainty of evidence was assessed using GRADE. Nineteen studies in 3,764 COVID-19 patients were included in the meta-analysis. Both C3 and C4 concentrations were significantly lower in patients with high disease severity or non-survivor status than patients with low severity or survivor status (C3 SMD=-0.40, 95% CI -0.60 to -0.21, p<0.001; C4 SMD=-0.29, 95% CI -0.49 to -0.09, p=0.005; moderate certainty of evidence). Extreme between-study heterogeneity was observed (C3, I² = 82.1%; C4, I² = 84.4%). Sensitivity analysis, performed by sequentially removing each study and re-assessing the pooled estimates, showed that the magnitude and direction of the effect size was not modified. There was no publication bias. In meta-regression, the SMD of C3 was significantly associated with white blood cell count, C-reactive protein (CRP), and pro-thrombin time, whereas the SMD of C4 was significantly associated with CRP, pro-thrombin time, D-dimer, and albumin. In conclusion, lower concentrations of C3 and C4, indicating complement activation, were significantly associated with higher COVID-19 severity and mortality. C3 and C4 might be useful to predict adverse clinical consequences in these patients.

Systematic Review Registration: PROSPERO, Registration number: CRD42021239634.

Platelet-Derived Growth Factor-D Activates Complement System to Propagate Macrophage Polarization and Neovascularization

Platelet-derived growth factor-D (PDGF-D) is highly expressed in immune cells. However, the potential role of PDGF-D in immune system remains thus far unclear. Here, we reveal a novel function of PDGF-D in activating both classical and alternative complement pathways that markedly increase chemokine and cytokine responses to promote macrophage polarization. Pharmacological targeting of the complement C3a receptor using SB290157 alleviated PDGF-D-induced neuroinflammation by blocking macrophage polarization and inhibited pathological choroidal neovascularization. Our study thus suggests that therapeutic strategies targeting both PDGF-D and the complement system may open up new possibilities for the treatment of neovascular diseases.

Methylprednisolone Induces Neuro-Protective Effects via the Inhibition of A1 Astrocyte Activation in Traumatic Spinal Cord Injury Mouse Models

Traumatic spinal cord injury (TSCI) leads to pathological changes such as inflammation, edema, and neuronal apoptosis. Methylprednisolone (MP) is a glucocorticoid that has a variety of beneficial effects, including decreasing inflammation and ischemic reaction, as well as inhibiting lipid peroxidation. However, the efficacy and mechanism of MP in TSCI therapy is yet to be deciphered. In the present study, MP significantly attenuated the apoptotic effects of H₂O₂ in neuronal cells. Western blot analysis demonstrated that the levels of apoptotic related proteins, Bax and cleaved caspase-3, were reduced while levels of anti-apoptotic Bcl-2 were increased. In vivo TUNEL assays further demonstrated that MP effectively protected neuronal cells from apoptosis after TSCI, and was consistent with in vitro studies. Furthermore, we demonstrated that MP could decrease expression levels of IBA1, Il-1α, TNFα, and C3 and suppress A1 neurotoxic reactive astrocyte activation in TSCI mouse models. Neurological function was evaluated using the Basso Mouse Scale (BMS) and Footprint Test. Results demonstrated that the neurological function of MP-treated injured mice was significantly increased. In conclusion, our study demonstrated that MP could attenuate astrocyte cell death, decrease microglia activation, suppress A1 astrocytes activation, and promote functional recovery after acute TSCI in mouse models.

Clinico-Laboratory Features and Associated Factors of Lupus Mesenteric Vasculitis

Introduction

Lupus mesenteric vasculitis (LMV) is a rare but potentially life-threatening clinical entity in systemic lupus erythematosus (SLE) patients.

Objective

The present study was initiated to explore the clinical features and associated factors of LMV in SLE patients.

Methods

We conducted a retrospective study on 50 cases of SLE patients with lupus mesenteric vasculitis (LMV) from January 2010 to December 2019 and 89 cases of non-LMV-SLE patients with similar demographic and comorbidities were included as control. All the data regarding clinical features, laboratory findings, and treatment were reviewed independently by two experts in the field. Both univariate and multivariate logistic regression analyses were employed to identify the associated factors of LMV.

Results

The incidence of LMV was 2.9% among hospitalized SLE patients in the current study. The most frequent symptom and physical sign of LMV were respectively abdominal pain (48, 96%) and abdominal tenderness (45, 90%). Through univariate and subsequent multivariate analysis, oral ulcer (OR, 4.25; P = 0.024), urinary tract involvement (OR, 5.23; P = 0.021), and elevated D-dimer (OR, 1.121; P = 0.008) were demonstrated to be positively associated with LMV, while percentage of lymphocytes (OR, 0.928; P = 0.004) and complement 3 (OR, 0.048; P = 0.008) were negatively correlated with LMV.

Conclusions

Oral ulcer, urinary tract involvement, reduced percentage of lymphocytes and complement 3, elevated D-dimer could be associated factors for LMV in SLE patients.

C3 Deficiency Leads to Increased Angiogenesis and Elevated Pro-Angiogenic Leukocyte Recruitment in Ischemic Muscle Tissue

The complement system is a potent inflammatory trigger, activator, and chemoattractant for leukocytes, which play a crucial role in promoting angiogenesis. However, little information is available about the influence of the complement system on angiogenesis in ischemic muscle tissue. To address this topic and analyze the impact of the complement system on angiogenesis, we induced muscle ischemia in complement factor C3 deficient (C3-/-) and wildtype control mice by femoral artery ligation (FAL). At 24 h and 7 days after FAL, we isolated the ischemic gastrocnemius muscles and investigated them by means of (immuno-)histological analyses. C3-/- mice showed elevated ischemic damage 7 days after FAL, as evidenced by H&E staining. In addition, angiogenesis was increased in C3-/- mice, as demonstrated by increased capillary/muscle fiber ratio and increased proliferating endothelial cells (CD31+/BrdU+). Moreover, our results showed that the total number of leukocytes (CD45+) was increased in C3-/- mice, which was based on an increased number of neutrophils (MPO+), neutrophil extracellular trap formation (MPO+/CitH3+), and macrophages (CD68+) displaying a shift toward an anti-inflammatory and pro-angiogenic M2-like polarized phenotype (CD68+/MRC1+). In summary, we show that the deficiency of complement factor C3 increased neutrophil and M2-like polarized macrophage accumulation in ischemic muscle tissue, contributing to angiogenesis.

Slab fractures of the third carpal bone involving both radial and intermediate facets and outcomes after surgical repair in racing Quarter Horses

BACKGROUND

Characteristics and outcomes after surgical repair of third carpal bone (C3) slab fractures involving both radial and intermediate facets in racing Quarter Horses are unknown.

OBJECTIVES

To describe the pre- and intraoperative characteristics of C3 slab fractures of both radial and intermediate facets in Quarter Horses and to report on the long-term outcomes after internal fixation.

STUDY DESIGN

Retrospective case series.

METHODS

Case records were collected from racehorses with C3 slab fractures between 2008 and 2020. Inclusion criteria required arthroscopic-guided repair of C3 slab fractures involving both radial and intermediate facets in Quarter Horses. Routine C3 slab fractures (single facet), fractures in other breeds or those repaired with other techniques were excluded. Outcomes were obtained by standardised questionnaire. Data were presented as mean ± SD or as proportions with 95% confidence interval (CI).

RESULTS

Of 22 Quarter Horses with C3 slab fractures involving both radial and intermediate facets, 91% (CI 79%-100%; n = 20) were collapsing and 91% (CI 79%-100%; n = 20) had avulsion of the medial palmar intercarpal ligament. Articular cartilage erosion and osteochondral fracture of the radial carpal bone was observed in 91% (CI 79%-100%; n = 20) and 41% (CI 20%-62%; n = 9) cases respectively. At 5.5 ± 3.9 years after surgery, 86% (CI 72%-100%; n = 19) were alive and used for breeding or retirement. Of 18 horses with follow-up >1 year, 39% (CI 16%-61%; n = 7) resumed some athletic activity.

MAIN LIMITATIONS

Cases were referred specifically for surgical repair and horses with fractures considered too severe for surgical intervention or euthanasia at owner request were not included. Questionnaire responses are susceptible to recall bias.

CONCLUSIONS

Horses with C3 slab fractures of both radial and intermediate facets that are repaired have a favourable prognosis for retirement, breeding and potentially low-level athletic activity.

Lipoteichoic Acid from Staphylococcus aureus Activates the Complement System via C3 Induction and CD55 Inhibition

Staphylococcus aureus inhibits complement activity by secreting a variety of toxins. However, the underlying mechanism of complement component regulation by lipoteichoic acid (LTA), a cell wall component of S. aureus, has not been elucidated. In this study, we observed that aLTA (LTA of S. aureus) increased C3 expression in THP-1 cells. The mechanism of aLTA-mediated C3 induction includes an aLTA-toll-like receptor (TLR) 2 interaction, interleukin 1 receptor associated kinase (IRAK) 2 recruitment, and nuclear factor kappa B (NF-kB) activation. In HepG2 cells, C3 protein production begins to increase from 3 h and increases steadily until 48 h. On the other hand, CD55 levels increased up to 6 h after aLTA treatment and started to decrease after 24 h and levels were decreased at 48 h by more than 50% compared to untreated cells. The expression of CD55 in HepG2 cells was shown to be regulated by IRAK-M induced by aLTA. Serum C3 levels increased in mice injected with aLTA, which resulted in an increase in the amount and activity of the membrane attack complex (MAC). We also observed that CD55 mRNA was increased in the liver 24 h after aLTA injection, but was decreased 48 h after injection. These results suggest that aLTA increases complement levels via induction of C3 and inhibition of CD55, which may cause associated MAC-mediated liver damage.

Acquired decrease of the C3b/C4b receptor (CR1, CD35) and increased C4d deposits on erythrocytes from ICU COVID-19 patients

In order to study the mechanisms of COVID-19 damage following the complement activation phase occurring during the innate immune response to SARS-CoV-2, CR1 (the regulating complement activation factor, CD35, the C3b/C4b receptor), C4d deposits on Erythrocytes (E), and the products of complement activation C3b/C3bi, were assessed in 52 COVID-19 patients undergoing O₂ therapy or assisted ventilation in ICU units in Rheims France. An acquired decrease of CR1 density on E from COVID-19 patients was observed (Mean = 418, SD = 162, N = 52) versus healthy individuals (Mean = 592, SD = 287, N = 400), Student’s t-test p < 10⁻⁶, particularly among fatal cases, and in parallel with several parameters of clinical severity. Large deposits of C4d on E in patients were well above values observed in normal individuals, mostly without concomitant C3 deposits, in more than 80% of the patients. This finding is reminiscent of the increased C4d deposits on E previously observed to correlate with sub endothelial pericapillary deposits in organ transplant rejection, and with clinical SLE flares. Conversely, significant C3 deposits on E were only observed among ¼ of the patients. The decrease of CR1/E density, deposits of C4 fragments on E and previously reported detection of virus spikes or C3 on E among COVID-19 patients, suggest that the handling and clearance of immune complex or complement fragment coated cell debris may play an important role in the pathophysiology of SARS-CoV-2. Measurement of C4d deposits on E might represent a surrogate marker for assessing inflammation and complement activation occurring in organ capillaries and CR1/E decrease might represent a cumulative index of complement activation in COVID-19 patients.

Taken together, these original findings highlight the participation of complement regulatory proteins and indicate that E are important in immune pathophysiology of COVID-19 patients. Besides a potential role for monitoring the course of disease, these observations suggest that novel therapies such as the use of CR1, or CR1-like molecules, in order to down regulate complement activation and inflammation, should be considered.

Molecular Characterization of Complement Component 3 (C3) in the Pearl Oyster Pinctada fucata Improves Our Understanding of the Primitive Complement System in Bivalve

As the central component in the complement system, complement component 3 (C3) plays essential roles in both the innate and adaptive immune responses. Here, a C3 gene (designated as pf-C3) was obtained from the pearl oyster Pinctada fucata by RT-PCR and rapid amplification of cDNA ends (RACE). The pf-C3 cDNA consists of 5,634 bp with an open reading frame (ORF) of 5,193 bp encoding a protein of 1,730 amino acids with a 19 residue signal peptide. The deduced pf-C3 protein possessed the characteristic structural features present in its homologs and contained the A2M_N_2, ANATO, A2M, A2M_comp, A2M_recep, and C345C domains, as well as the C3 convertase cleavage site, thioester motif, and conserved Cys, His, and Glu residues. Phylogenetic analysis revealed that pf-C3 is closely related to the C3s from other mollusks. Pf-C3 mRNA was expressed in all examined tissues including gill, digestive gland, adductor muscle, mantle and foot, while the highest expression was found in the digestive gland. Following the challenge with Vibrio alginolyticus, pf-C3 expression was significantly induced in hemocytes. Luciferase reporter assays indicated that pf-C3a could activate the NF-κB signal pathway in HEK293T cells. Further knockdown of pf-C3 by specific siRNA could significantly reduce the phagocytosis of V. alginolyticus by hemocytes in vitro. These results would help increase understanding of the function of C3 in the invertebrate immune system and therefore provide new insights into the roles of the primitive complement system in invertebrates.

LncRNA MIAT enhances systemic lupus erythematosus by upregulating CFHR5 expression via miR-222 degradation

Systemic lupus erythematosus (SLE), a complex polygenic autoimmune disease, is associated with increased complement activation. Complement factor H related protein 5 (CFHR5) may contribute to dysfunctional complement activation, thus predisposing to SLE. The expression levels of anti-dsDNA, C3 and CFHR5 in blood samples from 50 SLE patients and 50 healthy individuals were evaluated, and also their expression levels were measured in an MRL/lpr mouse model and control MRL/MPJ mice. The results showed that CFHR5 expression increased in SLE patients together with the increase of anti-dsDNA in comparison with the healthy control. Furthermore, CFHR5 expression was inversely correlated with C3, down-regulation of which was associated with worse SLE. Previous studies indicated that long noncoding RNA (lncRNA) regulates mRNA synthesis via microRNA (miRNA) inhibition. The present bioinformatics analysis revealed that the target miRNA (miR-222) was combined with both lncRNA MIAT and mRNA CFHR5. H&E staining of the kidney tissues of the MRL/lpr mice revealed that lncRNA MIAT, as a competitive inhibitor of miR-222, enhanced SLE by upregulating CFHR5 expression through the degradation of miR-222 in vivo. Thus, our study revealed for the first time the role of lncRNA MIAT in regulating CFHR5 expression in SLE in vivo and provided new insights into the role of lncRNA in regulation and complement function of SLE pathogenesis.