The C3a/C3aR axis mediates anti-inflammatory activity and protects against uropathogenic E coli-induced kidney injury in mice

Inhibition of colon C5a/C5a receptor signalling pathway confers protection against LPS-induced acute kidney injury via gut microbiota-kidney axis

Acute kidney injury (AKI) is a critical condition often associated with systemic inflammation and dysregulated gut microbiota. This study aimed to investigate the effects of the C5a receptor antagonist W54011 on lipopolysaccharide (LPS)-induced AKI, focusing on the colon's C5a/C5a receptor pathway, intestinal barrier integrity, and gut microbiota. Our findings demonstrate that W54011 effectively ameliorated kidney injury in the LPS-induced AKI model by selectively inhibiting the colon's C5a/C5a receptor signalling pathway. Additionally, C5a receptor blockade resulted in the inhibition of colonic inflammation and the reconstruction of the intestinal mucosal barrier. Furthermore, W54011 administration significantly impacted the composition and stability of the gut microbiota, restoring the abundance of dominant bacteria to levels observed in the normal state of the intestinal flora and reducing the abundance of potentially harmful bacterial groups. In conclusion, W54011 alleviates LPS-induced AKI by modulating the interplay between the colon, gut microbiota, and kidneys. It preserves the integrity of the intestinal barrier and reinstates gut microbiota, thereby mitigating AKI symptoms. These findings suggest that targeting the colon and gut microbiota could be a promising therapeutic strategy for AKI treatment.

From complement to complosome in non-alcoholic fatty liver disease: When location matters

Non-alcoholic fatty liver disease (NAFLD) is a growing public health threat and becoming the leading cause of liver transplantation. Nevertheless, no approved specific treatment is currently available for NAFLD. The pathogenesis of NAFLD is multifaceted and not yet fully understood. Accumulating evidence suggests a significant role of the complement system in the development and progression of NAFLD. Here, we provide an overview of the complement system, incorporating the novel concept of complosome, and summarise the up-to-date evidence elucidating the association between complement dysregulation and the pathogenesis of NAFLD. In this process, the extracellular complement system is activated through various pathways, thereby directly contributing to, or working together with other immune cells in the disease development and progression. We also introduce the complosome and assess the evidence that implicates its potential influence in NAFLD through its direct impact on hepatocytes or non-parenchymal liver cells. Additionally, we expound upon how complement system and the complosome may exert their effects in relation with hepatic zonation in NAFLD. Furthermore, we discuss the potential therapeutic implications of targeting the complement system, extracellularly and intracellularly, for NAFLD treatment. Finally, we present future perspectives towards a better understanding of the complement system's contribution to NAFLD.

Revisiting the role of the complement system in intracerebral hemorrhage and therapeutic prospects

Intracerebral hemorrhage (ICH) is a stroke subtype characterized by non-traumatic rupture of blood vessels in the brain, resulting in blood pooling in the brain parenchyma. Despite its lower incidence than ischemic stroke, ICH remains a significant contributor to stroke-related mortality, and most survivors experience poor outcomes that significantly impact their quality of life. ICH has been accompanied by various complex pathological damage, including mechanical damage of brain tissue, hematoma mass effect, and then leads to inflammatory response, thrombin activation, erythrocyte lysis, excitatory amino acid toxicity, complement activation, and other pathological changes. Accumulating evidence has demonstrated that activation of complement cascade occurs in the early stage of brain injury, and the excessive complement activation after ICH will affect the occurrence of secondary brain injury (SBI) through multiple complex pathological processes, aggravating brain edema, and pathological brain injury. Therefore, the review summarized the pathological mechanisms of brain injury after ICH, specifically the complement role in ICH, and its related pathological mechanisms, to comprehensively understand the specific mechanism of different complements at different stages after ICH. Furthermore, we systematically reviewed the current state of complement-targeted therapies for ICH, providing a reference and basis for future clinical transformation of complement-targeted therapy for ICH.

Protective role for C3aR in experimental chronic pyelonephritis

Emerging evidence suggest that C3aR plays important roles in homeostasis, host defense and disease. Although it is known that C3aR is protective in several models of acute bacterial infections, the role for C3aR in chronic infection is largely unknown. Here we show that C3aR is protective in experimental chronic pyelonephritis. Global C3aR deficient (C3ar^-/- ) mice had higher renal bacterial load, more pronounced renal histological lesions, increased renal apoptotic cell accumulation, tissue inflammation and extracellular matrix deposition following renal infection with uropathogenic E. coli (UPEC) strain IH11128, compared to WT control mice. Myeloid C3aR deficient (Lyz2-C3ar^-/- ) mice exhibited a similar disease phenotype to global C3ar^-/- mice. Pharmacological treatment with a C3aR agonist reduced disease severity in experimental chronic pyelonephritis. Furthermore, macrophages of C3ar^-/- mice exhibited impaired ability to phagocytose UPEC. Our data clearly demonstrate a protective role for C3aR against experimental chronic pyelonephritis, macrophage C3aR plays a major role in the protection, and C3aR is necessary for phagocytosis of UPEC by macrophages. Our observation that C3aR agonist curtailed the pathology suggests a therapeutic potential for activation of C3aR in chronic infection.

The role of complement in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has become a leading cause of chronic liver diseases globally. NAFLD includes a range of hepatic manifestations, starting with liver steatosis and potentially evolving towards nonalcoholic steatohepatitis, cirrhosis or even hepatocellular carcinoma. Although the pathogenesis of NAFLD is incompletely understood, insulin resistance and lipid metabolism disorder are implicated. The complement system is an essential part of the immune system, but it is also involved in lipid metabolism. In particular, activation of the alternative complement pathway and the production of complement activation products such as C3a, C3adesArg (acylation stimulating protein or ASP) and C5a, are strongly associated with insulin resistance, lipid metabolism disorder, and hepatic inflammation. In this review, we briefly summarize research on the role of the complement system in NAFLD, aiming to provide a basis for the development of novel therapeutic strategies for NAFLD.

Genome-scale CRISPR screening reveals that C3aR signaling is critical for rapid capture of fungi by macrophages

The fungal pathogen Histoplasma capsulatum (Hc) invades, replicates within, and destroys macrophages. To interrogate the molecular mechanisms underlying this interaction, we conducted a host-directed CRISPR-Cas9 screen and identified 361 genes that modify macrophage susceptibility to Hc infection, greatly expanding our understanding of host gene networks targeted by Hc. We identified pathways that have not been previously implicated in Hc interaction with macrophages, including the ragulator complex (involved in nutrient stress sensing), glycosylation enzymes, protein degradation machinery, mitochondrial respiration genes, solute transporters, and the ER membrane complex (EMC). The highest scoring protective hits included the complement C3a receptor (C3aR), a G-protein coupled receptor (GPCR) that recognizes the complement fragment C3a. Although it is known that complement components react with the fungal surface, leading to opsonization and release of small peptide fragments such as C3a, a role for C3aR in macrophage interactions with fungi has not been elucidated. We demonstrated that whereas C3aR is dispensable for macrophage phagocytosis of bacteria and latex beads, it is critical for optimal macrophage capture of pathogenic fungi, including Hc, the ubiquitous fungal pathogen Candida albicans, and the causative agent of Valley Fever Coccidioides posadasii. We showed that C3aR localizes to the early phagosome during Hc infection where it coordinates the formation of actin-rich membrane protrusions that promote Hc capture. We also showed that the EMC promotes surface expression of C3aR, likely explaining its identification in our screen. Taken together, our results provide new insight into host processes that affect Hc-macrophage interactions and uncover a novel and specific role for C3aR in macrophage recognition of fungi.

C3aR plays both sides in regulating resistance to bacterial infections

Activation of the complement pathway results in the production of bioactive C3a, a product of C3 cleavage, which interacts with membrane-bound receptor C3aR to regulate innate immune cell function and outcome of bacterial infection. Specifically, previous research has identified mechanistically distinct and cell type–specific roles for C3aR in regulating innate immune cell inflammatory state, antimicrobial killing capacity, and metabolism. Historically, the production of C3a has been relegated to the serum; however, recent studies have provided evidence that various cell types can produce intracellular C3a that stimulates intracellular C3aR. In light of these new results, it is imperative that we revisit previous studies regarding the role of C3aR in controlling bacterial infections and analyze these results in the context of both extracellular and intracellular C3a production and C3aR activation. Thus, this review will cover specific roles of C3aR in driving cell type–specific and tissue specific responses during bacterial infections and emphasize the contribution of the C3a–C3aR axis in regulating host resistance to bacterial infection.

The complement C3a‑C3aR and C5a‑C5aR pathways promote viability and inflammation of human retinal pigment epithelium cells by targeting NF‑κB signaling

Retinal detachment (RD) and its special form of rhegmatogenous RD associated with choroidal detachment (RRDCD) feature similar pathological alterations, including enhanced retinal cell inflammation. Although the importance of the complement components C3a and C5a and their corresponding receptors in retinal maintenance has been demonstrated, the relevance of these molecules to the pathogenesis of RD or RRDCD remains to be investigated. The contents of C3a, C5a and inflammatory factors, such as TNF-α, IL-1β, IL-6 and prostaglandin (PG)E2, in related clinical samples were examined by ELISA. Subsequently, human retinal pigment epithelial (HRPE) cells were subjected to challenge with the C3a and C5a recombinant proteins with or without C3a and C5a antagonists and NF-κB inhibitor, and the cell viability and inflammatory cytokines were then determined by a Cell Counting Kit-8 assay and ELISA, respectively. In addition, reverse transcription-quantitative PCR and western blot analyses were utilized to examine the mRNA or/and protein levels of C3a and its receptor C3aR, as well as C5a and its receptor C5aR, and NF-κB. In addition, the correlation of C3a and C5a with the aforementioned inflammatory factors was analyzed. The inflammatory factor levels of C3a and C5a were considerably elevated in patients with RRDCD compared to those in the controls. Consistently, C3a and C5a treatment led to increased cell viability and aggravated inflammation in HRPE cells. Accordingly, C3a and C5a induced upregulation of their corresponding receptors C3aR and C5aR, which was in turn observed to be linked to the activation of the NF-κB signaling pathway. Furthermore, there was a positive correlation of the complements C3a and C5a with individual TNF-α, IL-1β, IL-6 and PGE2. Taken together, the C3a-C3aR and C5a-C5aR pathways were indicated to promote cell viability and inflammation of HRPE cells by targeting the NF-κB signaling pathway.

Inhibition of the C3a receptor attenuates sepsis-induced acute lung injury by suppressing pyroptosis of the pulmonary vascular endothelial cells

Acute lung injury (ALI) is the leading cause of bacterial sepsis-related death because of disrupted pulmonary endothelial barrier, resulting in protein-rich pulmonary oedema, an influx of pro-inflammatory cells and refractory hypoxaemia. Several studies have reported that C3a levels are significantly higher in organs with sepsis and their peripheral organs and are closely associated with organ dysfunction and poor prognosis in sepsis. However, the role of the C3a complement in sepsis ALI remains unclear. Therefore, this study aimed to investigate the important role and mechanism of C3a in preventing the occurrence of pyroptosis (a pro-inflammatory form of cell death) to protect the lung endothelial cells (ECs) in sepsis-induced ALI. A septic mouse model was established with cecal ligation and puncture (CLP), which demonstrated that C3a mediated EC pyroptosis through its C3aR receptor. Furthermore, inhibition of the C3a-C3aR axis could block both NLRP3/caspase-1 and caspase-11 pathways, thus preventing pulmonary EC from pyroptosis. These results indicate that inhibition of the C3A-C3AR complement axis can inhibit pulmonary vascular EC pyroptosis, a potential target for the treatment of ALI.

Interaction Between the Complement System and Infectious Agents - A Potential Mechanistic Link to Neurodegeneration and Dementia

As part of the innate immune system, complement plays a critical role in the elimination of pathogens and mobilization of cellular immune responses. In the central nervous system (CNS), many complement proteins are locally produced and regulate nervous system development and physiological processes such as neural plasticity. However, aberrant complement activation has been implicated in neurodegeneration, including Alzheimer’s disease. There is a growing list of pathogens that have been shown to interact with the complement system in the brain but the short- and long-term consequences of infection-induced complement activation for neuronal functioning are largely elusive. Available evidence suggests that the infection-induced complement activation could be protective or harmful, depending on the context. Here we summarize how various infectious agents, including bacteria (e.g., Streptococcus spp.), viruses (e.g., HIV and measles virus), fungi (e.g., Candida spp.), parasites (e.g., Toxoplasma gondii and Plasmodium spp.), and prion proteins activate and manipulate the complement system in the CNS. We also discuss the potential mechanisms by which the interaction between the infectious agents and the complement system can play a role in neurodegeneration and dementia.

The Complement C3a and C3a Receptor Pathway in Kidney Diseases

The pathogenesis of some kidney diseases is closely associated with complement activation, where the C3a/C3a receptor (C3aR) might play a crucial role. C3a/C3aR has dual roles and may exert anti-inflammatory or pro-inflammatory effects depending on different cell types and diseases. In the kidneys, C3aR is primarily expressed on the tubular epithelium and less in glomerular podocytes. C3aR expression is enhanced and the levels of C3a in the plasma and urine are increased in kidney diseases of several types, and are associated with disease progression and severity. The C3a/C3aR pathway facilitates the progression of glomerular and tubulointerstitial diseases, while it has opposite effects on urinary tract infections. Clinical trials targeting C3a/C3aR in kidney diseases are lacking. Here, we reviewed the studies on the C3a/C3aR pathway in kidney disease, with the aim of understanding in-depth its controversial roles and its potential therapeutic value.

The C5a/C5aR2 axis promotes renal inflammation and tissue damage

C5a is a potent inflammatory mediator that binds C5aR1 and C5aR2. Although pathogenic roles of the C5a/C5aR1 axis in inflammatory disorders are well documented, the roles for the C5a/C5aR2 axis in inflammatory disorders and underlying mechanisms remain unclear. Here, we show that the C5a/C5aR2 axis contributes to renal inflammation and tissue damage in a mouse model of acute pyelonephritis. Compared with WT littermates, C5ar2-/- mice had significantly reduced renal inflammation, tubular damage, and renal bacterial load following bladder inoculation with uropathogenic E. coli. The decrease in inflammatory responses in the kidney of C5ar2-/- mice was correlated with reduced intrarenal levels of high mobility group box-1 protein (HMGB1), NLRP3 inflammasome components, cleaved caspase-1, and IL-1β. In vitro, C5a stimulation of macrophages from C5ar1-/- mice (lacking C5aR1 but expressing C5aR2) led to significant upregulation of HMGB1 release, NLRP3/cleaved caspase-1 inflammasome activation, and IL-1β secretion. Furthermore, blockade of HMGB1 significantly reduced C5a-mediated upregulation of NLRP3/cleaved caspase-1 inflammasome activation and IL-1β secretion in the macrophages, implying a HMGB1-dependent upregulation of NLRP3/cleaved caspase-1 inflammasome activation in macrophages. Our findings demonstrate a pathogenic role for the C5a/C5aR2 axis in renal injury following renal infection and suggest that the C5a/C5aR2 axis contributes to renal inflammation and tissue damage through upregulation of HMGB1 and NLRP3/cleaved caspase-1 inflammasome.