TNF-alpha regulation of C3 gene expression and protein biosynthesis in rat glomerular endothelial cells

Changes in cytokine and cytokine receptor levels during postnatal development of the human dorsolateral prefrontal cortex

In addition to their traditional roles in immune cell communication, cytokines regulate brain development. Cytokines are known to influence neural cell generation, differentiation, maturation, and survival. However, most work on the role of cytokines in brain development investigates rodents or focuses on prenatal events. Here, we investigate how mRNA and protein levels of key cytokines and cytokine receptors change during postnatal development of the human prefrontal cortex. We find that most cytokine transcripts investigated (IL1B, IL18, IL6, TNF, IL13) are lowest at birth and increase between 1.5 and 5 years old. After 5 years old, transcriptional patterns proceeded in one of two directions: decreased expression in teens and young adults (IL1B, p = 0.002; and IL18, p = 0.004) or increased mean expression with maturation, particularly in teenagers (IL6, p = 0.004; TNF, p = 0.002; IL13, p < 0.001). In contrast, cytokine proteins tended to remain elevated after peaking significantly around 3 years of age (IL1B, p = 0.012; IL18, p = 0.026; IL6, p = 0.039; TNF, p < 0.001), with TNF protein being highest in teenagers. An mRNA-only analysis of cytokine receptor transcripts found that early developmental increases in cytokines were paralleled by increases in their ligand-binding receptor subunits, such as IL1R1 (p = 0.033) and IL6R (p < 0.001) transcripts. In contrast, cytokine receptor-associated signaling subunits, IL1RAP and IL6ST, did not change significantly between age groups. Of the two TNF receptors, the 'pro-death' TNFRSF1A and 'pro-survival' TNFRSF1B, only TNFRSF1B was significantly changed (p = 0.028), increasing first in toddlers and again in young adults. Finally, the cytokine inhibitor, IL13, was elevated first in toddlers (p = 0.006) and again in young adults (p = 0.053). While the mean expression of interleukin-1 receptor antagonist (IL1RN) was highest in toddlers, this increase was not statistically significant. The fluctuations in cytokine expression reported here support a role for increases in specific cytokines at two different stages of human cortical development. The first is during the toddler/preschool period (IL1B, IL18, and IL13), and the other occurs at adolescence/young adult maturation (IL6, TNF and IL13).

Stem Cell-Derived Extracellular Vesicles as Potential Therapeutic Approach for Acute Kidney Injury

Acute kidney injury is a frequent complication of hospitalized patients and significantly increases morbidity and mortality, worsening costs and length of hospital stay. Despite this impact on healthcare system, treatment still remains only supportive (dialysis). Stem cell-derived extracellular vesicles are a promising option as they recapitulate stem cells properties, overcoming safety issues related to risks or rejection or aberrant differentiation. A growing body of evidence based on pre-clinical studies suggests that extracellular vesicles may be effective to treat acute kidney injury and to limit fibrosis through direct interference with pathogenic mechanisms of vascular and tubular epithelial cell damage. We herein analyze the state-of-the-art knowledge of therapeutic approaches with stem cell-derived extracellular vesicles for different forms of acute kidney injury (toxic, ischemic or septic) dissecting their cytoprotective, regenerative and immunomodulatory properties. We also analyze the potential impact of extracellular vesicles on the mechanisms of transition from acute kidney injury to chronic kidney disease, with a focus on the pivotal role of the inhibition of complement cascade in this setting. Despite some technical limits, nowadays the development of therapies based on stem cell-derived extracellular vesicles holds promise as a new frontier to limit acute kidney injury onset and progression.

Complement Components C3 and C4 Indicate Vasculitis Manifestations to Distinct Renal Compartments in ANCA-Associated Glomerulonephritis

Acute kidney injury (AKI) is a common and severe complication of antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) causing progressive chronic kidney disease (CKD), end-stage renal disease (ESRD) or death. Pathogenic ANCAs, in particular proteinase 3 (PR3) and myeloperoxidase (MPO), trigger a deleterious immune response resulting in pauci-immune necrotizing and crescentic glomerulonephritis (GN), a common manifestation of glomerular injury in AAV. However, there is growing evidence that activation of the complement pathway contributes to the pathogenesis and progression of AAV. We here aimed to compare glomerular and tubulointerstitial lesions in ANCA GN and extrarenal manifestation of AAV in association with levels of circulating complement components C3c and C4.

METHODS

Plasma levels of C3c and C4 in a total number of 53 kidney biopsies with ANCA GN were retrospectively included between 2015 and 2020. Glomerular and tubulointerstitial lesions were evaluated according to established scoring systems for ANCA GN and analogous to the Banff classification.

RESULTS

We here show that circulating levels of C3c and C4 in ANCA GN were comparable to the majority of other renal pathologies. Furthermore, hypocomplementemia was only detectable in a minor subset of ANCA GN and not correlated with renal or extrarenal AAV manifestations. However, low levels of circulating C3c correlated with AKI severity in ANCA GN independent of systemic disease activity or extrarenal AAV manifestation. By systematic scoring of glomerular and tubulointerstitial lesions, we provide evidence that low levels of circulating C3c and C4 correlated with vasculitis manifestations to distinct renal compartments in ANCA GN.

CONCLUSIONS

We here expand our current knowledge about distinct complement components in association with vasculitis manifestations to different renal compartments in ANCA GN. While low levels of C4 correlated with glomerulitis, our observation that low levels of circulating complement component C3c is associated with interstitial vasculitis manifestation reflected by intimal arteritis implicates that C3c contributes to tubulointerstitial injury in ANCA GN.

PDE inhibition in distinct cell types to reclaim the balance of synaptic plasticity

Synapses are the functional units of the brain. They form specific contact points that drive neuronal communication and are highly plastic in their strength, density, and shape. A carefully orchestrated balance between synaptogenesis and synaptic pruning, i.e., the elimination of weak or redundant synapses, ensures adequate synaptic density. An imbalance between these two processes lies at the basis of multiple neuropathologies. Recent evidence has highlighted the importance of glia-neuron interactions in the synaptic unit, emphasized by glial phagocytosis of synapses and local excretion of inflammatory mediators. These findings warrant a closer look into the molecular basis of cell-signaling pathways in the different brain cells that are related to synaptic plasticity. In neurons, intracellular second messengers, such as cyclic guanosine or adenosine monophosphate (cGMP and cAMP, respectively), are known mediators of synaptic homeostasis and plasticity. Increased levels of these second messengers in glial cells slow down inflammation and neurodegenerative processes. These multi-faceted effects provide the opportunity to counteract excessive synapse loss by targeting cGMP and cAMP pathways in multiple cell types. Phosphodiesterases (PDEs) are specialized degraders of these second messengers, rendering them attractive targets to combat the detrimental effects of neurological disorders. Cellular and subcellular compartmentalization of the specific isoforms of PDEs leads to divergent downstream effects for these enzymes in the various central nervous system resident cell types. This review provides a detailed overview on the role of PDEs and their inhibition in the context of glia-neuron interactions in different neuropathologies characterized by synapse loss. In doing so, it provides a framework to support future research towards finding combinational therapy for specific neuropathologies.

Treatment of Rare Inflammatory Kidney Diseases: Drugs Targeting the Terminal Complement Pathway

The complement system comprises the frontline of the innate immune system. Triggered by pathogenic surface patterns in different pathways, the cascade concludes with the formation of a membrane attack complex (MAC; complement components C5b to C9) and C5a, a potent anaphylatoxin that elicits various inflammatory signals through binding to C5a receptor 1 (C5aR1). Despite its important role in pathogen elimination, priming and recruitment of myeloid cells from the immune system, as well as crosstalk with other physiological systems, inadvertent activation of the complement system can result in self-attack and overreaction in autoinflammatory diseases. Consequently, it constitutes an interesting target for specialized therapies. The paradigm of safe and efficacious terminal complement pathway inhibition has been demonstrated by the approval of eculizumab in paroxysmal nocturnal hematuria. In addition, complement contribution in rare kidney diseases, such as lupus nephritis, IgA nephropathy, atypical hemolytic uremic syndrome, C3 glomerulopathy, or antineutrophil cytoplasmic antibody-associated vasculitis has been demonstrated. This review summarizes the involvement of the terminal effector agents of the complement system in these diseases and provides an overview of inhibitors for complement components C5, C5a, C5aR1, and MAC that are currently in clinical development. Furthermore, a link between increased complement activity and lung damage in severe COVID-19 patients is discussed and the potential for use of complement inhibitors in COVID-19 is presented.

DDX5 promotes oncogene C3 and FABP1 expressions and drives intestinal inflammation and tumorigenesis

Tumorigenesis in different segments of the intestinal tract involves tissue-specific oncogenic drivers. In the colon, complement component 3 (C3) activation is a major contributor to inflammation and malignancies. By contrast, tumorigenesis in the small intestine involves fatty acid-binding protein 1 (FABP1). However, little is known of the upstream mechanisms driving their expressions in different segments of the intestinal tract. Here, we report that the RNA-binding protein DDX5 binds to the mRNA transcripts of C3 and Fabp1 to augment their expressions posttranscriptionally. Knocking out DDX5 in epithelial cells protected mice from intestinal tumorigenesis and dextran sodium sulfate (DSS)-induced colitis. Identification of DDX5 as a common upstream regulator of tissue-specific oncogenic molecules provides an excellent therapeutic target for intestinal diseases.

Podocytes Produce and Secrete Functional Complement C3 and Complement Factor H

Podocytes are an important part of the glomerular filtration barrier and the key player in the development of proteinuria, which is an early feature of complement mediated renal diseases. Complement factors are mainly liver-born and present in circulation. Nevertheless, there is a growing body of evidence for additional sites of complement protein synthesis, including various cell types in the kidney. We hypothesized that podocytes are able to produce complement components and contribute to the local balance of complement activation and regulation. To investigate the relevant balance between inhibiting and activating sides, our studies focused on complement factor H (CFH), an important complement regulator, and on C3, the early key component for complement activation. We characterized human cultured podocytes for the expression and secretion of activating and regulating complement factors, and analyzed the secretion pathway and functional activity. We studied glomerular CFH and C3 expression in puromycin aminonucleoside (PAN) -treated rats, a model for proteinuria, and the physiological mRNA-expression of both factors in murine kidneys. We found, that C3 and CFH were expressed in cultured podocytes and expression levels differed from those in cultivated glomerular endothelial cells. The process of secretion in podocytes was stimulated with interferon gamma and located in the Golgi apparatus. Cultured podocytes could initiate the complement cascade by the splitting of C3, which can be shown by the generation of C3a, a functional C3 split product. C3 contributed to external complement activation. Podocyte-secreted CFH, in conjunction with factor I, was able to split C3b. Podocytes derived from a patient with a CFH mutation displayed impaired cell surface complement regulation. CFH and C3 were synthesized in podocytes of healthy C57Bl/6-mice and were upregulated in podocytes of PAN treated rats. These data show that podocytes produce functionally active complement components, and could therefore influence the local glomerular complement activation and regulation. This modulating effect should therefore be considered in all diseases where glomerular complement activation occurs. Furthermore, our data indicate a potential novel role of podocytes in the innate immune system.

Germ-free and microbiota-associated mice yield small intestinal epithelial organoids with equivalent and robust transcriptome/proteome expression phenotypes

Intestinal epithelial organoids established from gut tissue have become a widely used research tool. However, it remains unclear how environmental cues, divergent microbiota composition and other sources of variation before, during and after establishment confound organoid properties, and how these properties relate to the original tissue. While environmental influences cannot be easily addressed in human organoids, mice offer a controlled assay-system. Here, we probed the effect of donor microbiota differences, previously identified as a confounding factor in murine in vivo studies, on organoids. We analysed the proteomes and transcriptomes of primary organoid cultures established from two colonised and one germ-free mouse colony of C57BL/6J genetic background, and compared them to their tissue of origin and commonly used cell lines. While an imprint of microbiota-exposure was observed on the proteome of epithelial samples, the long-term global impact of donor microbiota on organoid expression patterns was negligible. Instead, stochastic culture-to-culture differences accounted for a moderate variability between independently established organoids. Integration of transcriptome and proteome datasets revealed an organoid-typic expression signature comprising 14 transcripts and 10 proteins that distinguished organoids across all donors from murine epithelial cell lines and fibroblasts and closely mimicked expression patterns in the gut epithelium. This included the inflammasome components ASC, Naip1-6, Nlrc4 and Caspase-1, which were highly expressed in all organoids compared to the reference cell line m-ICc12 or mouse embryonic fibroblasts. Taken together, these results reveal that the donor microbiota has little effect on the organoid phenotype and suggest that organoids represent a more suitable culture model than immortalised cell lines, in particular for studies of intestinal epithelial inflammasomes.

Inflammaging and Complement System: A Link Between Acute Kidney Injury and Chronic Graft Damage

The aberrant activation of complement system in several kidney diseases suggests that this pillar of innate immunity has a critical role in the pathophysiology of renal damage of different etiologies. A growing body of experimental evidence indicates that complement activation contributes to the pathogenesis of acute kidney injury (AKI) such as delayed graft function (DGF) in transplant patients. AKI is characterized by the rapid loss of the kidney's excretory function and is a complex syndrome currently lacking a specific medical treatment to arrest or attenuate progression in chronic kidney disease (CKD). Recent evidence suggests that independently from the initial trigger (i.e., sepsis or ischemia/reperfusions injury), an episode of AKI is strongly associated with an increased risk of subsequent CKD. The AKI-to-CKD transition may involve a wide range of mechanisms including scar-forming myofibroblasts generated from different sources, microvascular rarefaction, mitochondrial dysfunction, or cell cycle arrest by the involvement of epigenetic, gene, and protein alterations leading to common final signaling pathways [i.e., transforming growth factor beta (TGF-β), p16 ink4a , Wnt/β-catenin pathway] involved in renal aging. Research in recent years has revealed that several stressors or complications such as rejection after renal transplantation can lead to accelerated renal aging with detrimental effects with the establishment of chronic proinflammatory cellular phenotypes within the kidney. Despite a greater understanding of these mechanisms, the role of complement system in the context of the AKI-to-CKD transition and renal inflammaging is still poorly explored. The purpose of this review is to summarize recent findings describing the role of complement in AKI-to-CKD transition. We will also address how and when complement inhibitors might be used to prevent AKI and CKD progression, therefore improving graft function.

Vaccination with Alpha-Gal Protects Against Mycobacterial Infection in the Zebrafish Model of Tuberculosis

The alpha-Gal syndrome (AGS) is associated with tick bites that can induce in humans high levels of IgE antibodies against the carbohydrate Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) present in glycoproteins and glycolipids from tick saliva that mediate primarily delayed anaphylaxis to mammalian meat consumption. It has been proposed that humans evolved by losing the capacity to synthesize α-Gal to increase the protective immune response against pathogens with this modification on their surface. This evolutionary adaptation suggested the possibility of developing vaccines and other interventions to induce the anti-α-Gal IgM/IgG protective response against pathogen infection and multiplication. However, the protective effect of the anti-α-Gal immune response for the control of tuberculosis caused by Mycobacterium spp. has not been explored. To address the possibility of using vaccination with α-Gal for the control of tuberculosis, in this study, we used the zebrafish-Mycobacterium marinum model. The results showed that vaccination with α-Gal protected against mycobacteriosis in the zebrafish model of tuberculosis and provided evidence on the protective mechanisms in response to vaccination with α-Gal. These mechanisms included B-cell maturation, antibody-mediated opsonization of mycobacteria, Fc-receptor (FcR)-mediated phagocytosis, macrophage response, interference with the α-Gal antagonistic effect of the toll-like receptor 2 (TLR2)/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB)-mediated immune response, and upregulation of pro-inflammatory cytokines. These results provided additional evidence supporting the role of the α-Gal-induced immune response in the control of infections caused by pathogens with this modification on their surface and the possibility of using this approach for the control of multiple infectious diseases.

Molecular cloning of complement component C3 gene from pearl mussel, Hyriopsis cumingii and analysis of the gene expression in response to tissue transplantation

Complement component C3 is well recognized as the central mediator of complement system, whose activation is responsible for the immune surveillance and elimination of non-self-antigens. In this study, C3 gene (HcC3) from a pearl making mussel, Hyriopsis cumingii, was successfully identified. The putative HcC3 possessed the canonical domains and highly conserved functional residues of C3 family members. In phylogenetic analysis, HcC3 was also clustered into C3 subfamily and separated from α2 macroglobulin clade. HcC3 gene was constitutively expressed in a wide range of tissues of pearl mussels, among which the immune-related tissues like hemocytes got highest expression. After allograft surgery of mantle tissues for aquaculture pearl production, the gene expression of HcC3 exhibited a rapid upregulation on day 1, dropped back on day 3, peaked the value on day 7, and restored to the level similar to control samples on day 14 after mantle allograft. The biphasic expression within the two weeks post the surgery suggests the important roles for HcC3 in alloimmune responses and an intricate complement activation mechanism in mollusks during tissue allograft.

Endothelial cells: source, barrier, and target of defensive mediators

Endothelium is strategically located at the interface between blood and interstitial tissues, placing thus endothelial cell as a key player in vascular homeostasis. Endothelial cells are in a dynamic equilibrium with their environment and constitute concomitantly a source, a barrier, and a target of defensive mediators. This review will discuss the recent advances in our understanding of the complex crosstalk between the endothelium, the complement system and the hemostasis in health and in disease. The first part will provide a general introduction on endothelial cells heterogeneity and on the physiologic role of the complement and hemostatic systems. The second part will analyze the interplay between complement, hemostasis and endothelial cells in physiological conditions and their alterations in diseases. Particular focus will be made on the prototypes of thrombotic microangiopathic disorders, resulting from complement or hemostasis dysregulation-mediated endothelial damage: atypical hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. Novel aspects of the pathophysiology of the thrombotic microangiopathies will be discussed.

Generation of complement protein C3 deficient pigs by CRISPR/Cas9-mediated gene targeting

Complement protein C3 is the pivotal component of the complement system. Previous studies have demonstrated that C3 has implications in various human diseases and exerts profound functions under certain conditions. However, the delineation of pathological and physiological roles of C3 has been hampered by the insufficiency of suitable animal models. In the present study, we applied the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system to target the C3 gene in porcine fetal fibroblasts. Our results indicated that CRISPR/Cas9 targeting efficiency was as high as 84.7%, and the biallelic mutation efficiency reached at 45.7%. The biallelic modified colonies were used as donor for somatic cell nuclear transfer (SCNT) technology to generate C3 targeted piglets. A total of 19 C3 knockout (KO) piglets were produced and their plasma C3 protein was undetectable by western blot analysis and ELISA. The hemolytic complement activity and complement-dependent cytotoxicity assay further confirmed that C3 was disrupted in these piglets. These C3 KO pigs could be utilized as a valuable large animal model for the elucidation of the roles of C3.

Rhodopsin T17M Mutant Inhibits Complement C3 Secretion in Retinal Pigment Epithelium via ROS Induced Downregulation of TWIST1

Rhodopsin mutations cause autosomal dominant form of retinitis pigmentosa (RP). T17M rhodopsin predisposes cells to endoplasmic reticulum stress induced apoptosis. However, the pathogenic role of T17M rhodopsin in RP is not completely understood. Complement C3 has a protective role in RP pathogenesis. This study aimed to investigate whether T17M rhodopsin regulates C3 secretion in retinal pigment epithelium. The human retinal pigment epithelial cell line (ARPE-19) was engineered to overexpress wide-type (WT) and T17M rhodopsin. Gene expression was detected by RT-PCR and Western blot analysis. C3 secretion was detected by ELISA. The overexpression of T17M rhodopsin significantly induced ROS and reduced C3 secretion and transcription in ARPE-19 cells, but ROS scavengers could partially rescue reduced C3 secretion and transcription. Mechanistically, we found that ROS suppressed transcription factor TWIST1 which is responsible for activated transcription of C3. In conclusion, our data provide the first evidence that T17M rhodopsin mutant disrupts C3 secretion via the induction of ROS and the suppression of TWIST1. These findings reveal novel insight into the pathogenic role of mutant rhodopsin in RP. J. Cell. Biochem. 118: 4914-4920, 2017. © 2017 Wiley Periodicals, Inc.

Depression of Complement Regulatory Factors in Rat and Human Renal Grafts Is Associated with the Progress of Acute T-Cell Mediated Rejection

Background

The association of complement with the progression of acute T cell mediated rejection (ATCMR) is not well understood. We investigated the production of complement components and the expression of complement regulatory proteins (Cregs) in acute T-cell mediated rejection using rat and human renal allografts.

Methods

We prepared rat allograft and syngeneic graft models of renal transplantation. The expression of Complement components and Cregs was assessed in the rat grafts using quantitative real-time PCR (qRT-PCR) and immunofluorescent staining. We also administered anti-Crry and anti-CD59 antibodies to the rat allograft model. Further, we assessed the relationship between the expression of membrane cofactor protein (MCP) by immunohistochemical staining in human renal grafts and their clinical course.

Results

qRT-PCR results showed that the expression of Cregs, CD59 and rodent-specific complement regulator complement receptor 1-related gene/protein-y (Crry), was diminished in the rat allograft model especially on day 5 after transplantation in comparison with the syngeneic model. In contrast, the expression of complement components and receptors: C3, C3a receptor, C5a receptor, Factor B, C9, C1q, was increased, but not the expression of C4 and C5, indicating a possible activation of the alternative pathway. When anti-Crry and anti-CD59 mAbs were administered to the allograft, the survival period for each group was shortened. In the human ATCMR cases, the group with higher MCP expression in the grafts showed improved serum creatinine levels after the ATCMR treatment as well as a better 5-year graft survival rate.

Conclusions

We conclude that the expression of Cregs in allografts is connected with ATCMR. Our results suggest that controlling complement activation in renal grafts can be a new strategy for the treatment of ATCMR.

Complement activation in progressive renal disease

Chronic kidney disease (CKD) is common and the cause of significant morbidity and mortality. The replacement of functioning nephrons by fibrosis is characteristic of progressive disease. The pathways that lead to fibrosis are not fully understood, although chronic non-resolving inflammation in the kidney is likely to drive the fibrotic response that occurs. In patients with progressive CKD there is histological evidence of inflammation in the interstitium and strategies that reduce inflammation reduce renal injury in pre-clinical models of CKD. The complement system is an integral part of the innate immune system but also augments adaptive immune responses. Complement activation is known to occur in many diverse renal diseases, including glomerulonephritis, thrombotic microangiopathies and transplant rejection. In this review we discuss current evidence that complement activation contributes to progression of CKD, how complement could cause renal inflammation and whether complement inhibition would slow progression of renal disease.

Overview of complement activation and regulation

Complement is an important component of the innate immune system that is crucial for defense from microbial infections and for clearance of immune complexes and injured cells. In normal conditions complement is tightly controlled by a number of fluid-phase and cell surface proteins to avoid injury to autologous tissues. When complement is hyperactivated, as occurs in autoimmune diseases or in subjects with dysfunctional regulatory proteins, it drives a severe inflammatory response in numerous organs. The kidney appears to be particularly vulnerable to complement-mediated inflammatory injury. Injury may derive from deposition of circulating active complement fragments in glomeruli, but complement locally produced and activated in the kidney also may have a role. Many kidney disorders have been linked to abnormal complement activation, including immune-complex–mediated glomerulonephritis and rare genetic kidney diseases, but also tubulointerstitial injury associated with progressive proteinuric diseases or ischemia-reperfusion.

Human skin mast cells express complement factors C3 and C5

We examine whether complement factor C3 or C5 is synthesized by human skin-derived mast cells and whether their synthesis is regulated by cytokines. C3 and C5 mRNAs were assessed by RT-PCR, and proteins by flow cytometry, confocal microscopy, Western blotting, and ELISA. C3 and C5 mRNAs were each expressed, and baseline protein levels/10(6) cultured mast cells were 0.9 and 0.8 ng, respectively, and located in the cytoplasm outside of secretory granules. C3 accumulated in mast cell culture medium over time and by 3 d reached a concentration of 9.4 ± 8.0 ng/ml, whereas C5 levels were not detectable (<0.15 ng/ml). Three-day incubations of mast cells with IL-1α, IL-1β, IL-17, IFN-γ, IL-6, or anti-FcεRI did not affect C3 protein levels in culture medium, whereas incubations with PMA, TNF-α, IL-13, or IL-4 enhanced levels of C3 1.7- to 3.3-fold. In contrast with C3, levels of C5 remained undetectable. Importantly, treatment with TNF-α together with either IL-4 or IL-13 synergistically enhanced C3 (but not C5) production in culture medium by 9.8- or 7.1-fold, respectively. This synergy was blocked by attenuating the TNF-α pathway with neutralizing anti-TNF-α Ab, soluble TNFR, or an inhibitor of NF-κB, or by attenuating the IL-4/13 pathway with Jak family or Erk antagonists. Inhibitors of PI3K, Jnk, and p38 MAPK did not affect this synergy. Thus, human mast cells can produce and secrete C3, whereas β-tryptase can act on C3 to generate C3a and C3b, raising the likelihood that mast cells engage complement to modulate immunity and inflammation in vivo.

Which pathways trigger the role of complement in ischaemia/reperfusion injury?

Investigations into the role of complement in ischemia/reperfusion (I/R) injury have identified common effector mechanisms that depend on the production of C5a and C5b-9 through the cleavage of C3. These studies have also defined an important role for C3 synthesized within ischemic kidney. Less clear however is the mechanism of complement activation that leads to the cleavage of C3 in ischemic tissues and to what extent the potential trigger mechanisms are organ dependent - an important question which informs the development of therapies that are more selective in their ability to limit the injury, yet preserve the other functions of complement where possible. Here we consider recent evidence for each of the three major pathways of complement activation (classical, lectin, and alternative) as mediators of I/R injury, and in particular highlight the role of lectin molecules that increasingly seem to underpin the injury in different organ models and in addition reveal unusual routes of complement activation that contribute to organ damage.

Mammalian DNA is an endogenous danger signal that stimulates local synthesis and release of complement factor B

Complement factor B plays a critical role in ischemic tissue injury and autoimmunity. Factor B is dynamically synthesized and released by cells outside of the liver, but the molecules that trigger local factor B synthesis and release during endogenous tissue injury have not been identified. We determined that factor B is upregulated early after cold ischemia-reperfusion in mice, using a heterotopic heart transplant model. These data suggested upregulation of factor B by damage-associated molecular patterns (DAMPs), but multiple common DAMPs did not induce factor B in RAW264.7 mouse macrophages. However, exogenous DNA induced factor B mRNA and protein expression in RAW cells in vitro, as well as in peritoneal and alveolar macrophages in vivo. To determine the cellular mechanisms involved in DNA-induced factor B upregulation we then investigated the role of multiple known DNA receptors or binding partners. We stimulated peritoneal macrophages from wild-type (WT), toll-like receptor 9 (TLR9)-deficient, receptor for advanced glycation end products (RAGE)⁻/⁻ and myeloid differentiation factor 88 (MyD88)⁻/⁻ mice, or mouse macrophages deficient in high-mobility group box proteins (HMGBs), DNA-dependent activator of interferon-regulatory factors (DAI) or absent in melanoma 2 (AIM2), with DNA in the presence or absence of lipofection reagent. Reverse transcription-polymerase chain reaction, Western blotting and immunocytochemical analysis were employed for analysis. Synthesis of factor B was independent of TLR9, RAGE, DAI and AIM2, but was dependent on HMGBs, MyD88, p38 and NF-κB. Our data therefore show that mammalian DNA is an endogenous molecule that stimulates factor B synthesis and release from macrophages via HMGBs, MyD88, p38 and NF-κB signaling. This activation of the immune system likely contributes to damage following sterile injury such as hemorrhagic shock and ischemia-reperfusion.

Complement-mediated injury and protection of endothelium: lessons from atypical haemolytic uraemic syndrome

The complement system provides a vital defence against invading pathogens. As an intrinsic system it is always 'on', in a state of constant, low level activation. This activation is principally mediated through the deposition of C3b on to pathogenic surfaces and host tissues. C3b is generated by spontaneous 'tick over' and formal activation of the alternative pathway, and by activation of the classical and lectin pathways. If the deposited C3b is not appropriately regulated, there is progression to terminal pathway complement activation via the C5 convertases, generating the potent anaphylotoxin C5a and the membrane attack complex C5b-9. Unsurprisingly, these highly active components have the potential to cause injury to bystander host tissue, including the vascular endothelium. As such, complement activation on endothelium is normally tightly controlled by a large number of fluid-phase and membrane bound inhibitors, in an attempt to ensure that propagation of complement activation is appropriately restricted to invading pathogens and altered 'self', e.g. apoptotic and necrotic cells. The kidney is increasingly recognised as a site at particular risk from complement-mediated endothelial injury. Both genetic and acquired defects which impact on complement regulation predispose to this susceptibility. The thrombotic microangiopathy, haemolytic uraemic syndrome (HUS), will be used to illustrate the mechanisms by which the endothelial cell injury occurs. Finally, the underlying rationale for current and future potential therapeutic interventions in HUS and also the opportunities for enhancing endothelial defence to prevent relapsing disease through increased complement cytoprotective strategies will be summarised.

Crosstalk between complement and Toll-like receptor activation in relation to donor brain death and renal ischemia-reperfusion injury

Two central pathways of innate immunity, complement and Toll-like receptors (TLRs), play an important role in the pathogenesis of renal injury inherent to kidney transplantation. Recent findings indicate close crosstalk between complement and TLR signaling pathways. It is suggested that mitogen activated protein kinases (MAPKs) might be the key molecules linking both the complement and TLR pathways together. Complement and TLRs are important mediators of renal ischemia-reperfusion injury (IRI). Besides IRI, complement C3 can also be upregulated and activated in the kidney before transplantation as a direct result of brain death (BD) in the donor. This local upregulation and activation of complement in the donor kidney has been proven to be detrimental for renal allograft outcome. Also TLR4 and several of its major ligands are upregulated by donor BD compared to living donors. Important and in line with the observations above, kidney transplant recipients have a benefit when receiving a kidney from a TLR4 Asp299Gly/Thr399Ile genotypic donor. The role of complement and TLRs and crosstalk between these two innate immune systems in relation to renal injury during donor BD and ischemia-reperfusion are focus of this review. Future strategies to target complement and TLR activation in kidney transplantation are considered.

Primed CD8(+) T-cell responses to allogeneic endothelial cells are controlled by local complement activation

CD8 T cells primed by transplantation recognize allogeneic class I MHC molecules expressed on graft vascular endothelium and contribute to allograft injury. We previously showed that immune cell-derived complement activation fragments are integral to T cell activation/expansion. Herein we tested the impact of local complement production/activation on T cell/endothelial cell (EC) interactions. We found that proinflammatory cytokines upregulated alternative pathway complement production by ECs, yielding C5a. We further found that ECs deficient in the cell surface C3/C5 convertase regulator decay accelerating factor (DAF, CD55) induced greater CD8 T-cell proliferation and more IFNgamma(+) and perforin(+) effector cells than wild-type (WT) ECs. Allogeneic C3(-/-) EC induced little or no CD8 responses. Abrogation of responses following C5a receptor (C5aR) blockade, or augmentation following addition of recombinant C5a demonstrated that the effects were mediated through T-cell-expressed-C5aR interactions. Analyses of in vivo CD8 cell responses to transplanted heart grafts deficient in EC DAF showed similar augmentation. The findings reveal that EC-derived complement triggers secondary CD8 T-cell differentiation and expansion and argue that targeting complement and/or C5aR could limit T-cell-mediated graft injury.

Immunoregulatory role of TNFalpha in inflammatory kidney diseases

Tumor necrosis factor alpha (TNFalpha), a pleiotropic cytokine, plays important inflammatory roles in renal diseases such as lupus nephritis, anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis and renal allograft rejection. However, TNFalpha also plays critical immunoregulatory roles that are required to maintain immune homeostasis. These complex biological functions of TNFalpha are orchestrated by its two receptors, TNFR1 and TNFR2. For example, TNFR2 promotes leukocyte infiltration and tissue injury in an animal model of immune complex-mediated glomerulonephritis. On the other hand, TNFR1 plays an immunoregulatory function in a murine lupus model with a deficiency in this receptor that leads to more severe autoimmune symptoms. In humans, proinflammatory and immunoregulatory roles for TNFalpha are strikingly illustrated in patients on anti-TNFalpha medications: These treatments are greatly beneficial in certain inflammatory diseases such as rheumatoid arthritis but, on the other hand, are also associated with the induction of autoimmune lupus-like syndromes and enhanced autoimmunity in multiple sclerosis patients. The indication for anti-TNFalpha treatments in renal inflammatory diseases is still under discussion. Ongoing clinical trials may help to clarify the potential benefit of such treatments in lupus nephritis and ANCA-associated glomerulonephritis. Overall, the complex biology of TNFalpha is not fully understood. A greater understanding of the function of its receptors may provide a framework to understand its contrasting proinflammatory and immunoregulatory functions. This may lead the development of new, more specific anti-inflammatory drugs.

Anti-inflammatory and renal protective actions of stanniocalcin-1 in a model of anti-glomerular basement membrane glomerulonephritis

We have previously shown that stanniocalcin-1 (STC1) inhibits the transendothelial migration of macrophages and T cells, suppresses superoxide generation in macrophages, and attenuates macrophage responses to chemoattractants. To study the effects of STC1 on inflammation, in this study we induced a macrophage- and T-cell-mediated model of anti-glomerular basement membrane disease in STC1 transgenic mice, which display elevated serum STC1 levels and preferentially express STC1 in both endothelial cells and macrophages. We examined the following parameters both at baseline and after anti-glomerular basement membrane antibody treatment: blood pressure; C(3a) levels; urine output; proteinuria; blood urea nitrogen; and kidney C(3) deposition, fibrosis, histological changes, cytokine expression, and number of T cells and macrophages. Compared with wild-type mice, after anti-glomerular basement membrane treatment STC1 transgenic mice exhibited: i) diminished infiltration of inflammatory macrophages in the glomeruli; ii) marked reduction in crescent formation and sclerotic glomeruli; iii) decreased interstitial fibrosis; iv) preservation of kidney function and lower blood pressure; v) diminished C(3) deposition in the glomeruli; and vi) reduced expression of macrophage inhibitory protein-2 and transforming growth factor-beta2 in the kidney. Compared with baseline, wild-type mice, but not STC1 transgenic mice, had higher proteinuria and a marked reduction in urine output. STC1 had minimal effects, however, on both T-cell number in the glomeruli and interstitium and on cytokine expression characteristic of either TH1 or TH2 activation. These data suggest that STC1 is a potent anti-inflammatory and renal protective protein.

Complement gene expression in human cardiac allograft biopsies as a correlate of histologic grade of injury

Complement activation contributes to antibody-mediated allograft rejection, but increasing evidence also implicates complement proteins produced locally within the graft, in part by infiltrating mononuclear cells, as important mediators of tissue injury. To test this concept in transplant recipients, we evaluated complement, complement regulator, and T cell/proinflammatory marker gene expression by quantitative real-time polymerase chain reaction in 71 archived heart transplant biopsies and correlated the results with the histologic grade of rejection. Significantly more transcripts encoding alternative pathway components factor B, C3 and properdin, and C3a receptor and C5a receptor were detected in grade 3 versus grade 0 or 1 biopsies. The grade 3 rejections also contained significantly higher amounts of CD3, interferon gamma, perforin, and granzyme B genes. In addition to providing supportive evidence for a pathogenic role of graft-derived complement in human heart transplant injury, these correlations suggest that molecular profiling of complement gene expression could be useful in the diagnosis of human allograft rejection.

Complement and renal transplantation: from donor to recipient

Long-term kidney graft survival is affected by different variables including donor condition, ischemia-reperfusion injury, and graft rejection during the transplantation process. The complement system is an important mediator of renal ischemia-reperfusion injury and in rejecting allografts. However, donor complement C3 seems to be crucial in renal transplantation-related injury as renal injury is attenuated in C3 deficient kidney grafts. Interestingly, before ischemia-reperfusion induced C3 expression, C3 is already induced in donors suffering from brain death. Therefore, strategies targeting complement activation in the brain-dead donor may increase graft viability and transplant outcome.

The relative importance of local and systemic complement production in ischaemia, transplantation and other pathologies

Besides a critical role in innate host defence, complement activation contributes to inflammatory and immunological responses in a number of pathological conditions. Many tissues outside the liver (the primary source of complement) synthesise a variety of complement proteins, either constitutively or response to noxious stimuli. The significance of this local synthesis of complement has become clearer as a result of functional studies. It revealed that local production not only contributes to the systemic pool of complement but also influences local tissue injury and provides a link with the antigen-specific immune response. Extravascular production of complement seems particularly important at locations with poor access to circulating components and at sites of tissue stress responses, notably portals of entry of invasive microbes, such as interstitial spaces and renal tubular epithelial surfaces. Understanding the relative importance of local and systemic complement production at such locations could help to explain the differential involvement of complement in organ-specific pathology and inform the design of complement-based therapy. Here, we will describe the lessons we have learned over the last decade about the local synthesis of complement and its association with inflammatory and immunological diseases, placing emphasis on the role of local synthesis of complement in organ transplantation.

The role of complement and Toll-like receptors in organ transplantation

The innate immune system not only participates in host defence but also contributes to the control of adaptive immune responses. Complement and Toll-like receptors (TLR) are key components of innate immunity. Emerging evidence suggests their activation is involved in all major aspects of transplantation. This paper reviews the current understanding of how the complement and TLR on impact transplant injury.

Effect of chronic ethanol consumption on the expression of complement components and acute-phase proteins in liver

The complement system can provoke but also participate in the repair of liver injury. Here we investigated by microarray analysis the effect of chronic ethanol consumption on hepatic mRNA expression of complement components and acute-phase proteins in complement C3-deficient (C3(-/-)) and wild-type (C3(+/+)) mice. Up-regulation by ethanol of factor B, C1qA-chain and clusterin but down-regulation of factor H, Masp-2, factor D and the terminal components C6, C8alpha and C9 was seen in both strains. Ethanol up-regulated C2 and down-regulated C4bp only in C3(+/+) mice, while in C3(-/-) mice up-regulation of C1qB-chain and vitronectin was observed. The expression of factor B, C6, C1qB and factor I was lower but that of factor D higher in C3(-/-) than in C3(+/+) mice. Ethanol induced mRNA synthesis of many acute-phase proteins including SPARC and lipocalin-2, but reduced the expression of SAP. The induction of early classical and alternative pathway components and suppression of terminal pathway components and soluble regulators may thus contribute to alcohol-induced liver injury. Lipocalin-2 and SPARC emerge as new candidate markers for early detection of liver damage.

Cross-talk between the complement system and endothelial cells in physiologic conditions and in vascular diseases

The endothelial layer represents a continuous physical barrier that controls coagulation and allows selective passage of soluble molecules and circulating cells across the vessel wall into the tissue. The functional activity of the endothelial cells may be influenced by their interaction with components of the complement system. In this review we shall discuss the complex interplay that can be established between the endothelium and complement proteins or activation products. Endothelial cells may also secrete several complement components which contribute to the circulating pool. This process can be regulated by cytokines and other pro-inflammatory stimuli. In addition, complement activation products stimulate endothelial cells to acquire a pro-inflammatory and pro-coagulant status. Expression of regulatory molecules on the cell surface provides protection against an undesired attack by complement activation products. Unrestricted complement activation under pathological conditions may lead to structural and functional changes of the endothelium resulting in vascular disease.

Role of dendritic cell synthesis of complement in the allospecific T cell response

Although extrahepatic synthesis of complement and particularly C3 has been widely studied in most cells and tissues, new information is emerging on dendritic cells (DCs). This research has shown that mouse bone marrow (BM) derived DCs are able to synthesise C3 and this synthesis has a substantial impact on DC activation, affecting the diverse range of DC functions relevant to the allospecific T cell response. Thus, local production of C3 appears to regulate the capacity of DCs to trigger the primary T cell response against donor alloantigen. Understanding of the key mechanisms by which complement activation modulates DC maturation could lead to the development of therapeutic strategies to down regulate DC activation thus reduce allograft rejection.

Macrophages from C3-deficient mice have impaired potency to stimulate alloreactive T cells

Impaired T-cell reactivity is a feature of C3-deficient mice in several disease models. The mechanism behind the reduced T-cell response is, however, poorly understood. We explored the hypothesis that antigen-presenting cells (APCs) from C3-/- mice have impaired potency to stimulate antigen-specific T cells, in an alloantigen-dependent model. Our results show that C3-/- macrophages have reduced ability to elicit alloreactive T-cell responses in vitro and in vivo, affecting both the primary and secondary responses. The C3 status of donor macrophages had a major impact on the CD4 T-cell response. The impaired CD4 T-cell response was associated with reduced expression of MHC class II on the surface of C3-/- macrophages, without loss of class II gene expression. Furthermore, inhibition of C3 gene expression in C3+/+ macrophages reduced their ability to stimulate alloreactive T cells, suggesting that endogenous production of C3 could in part contribute to the potency of APCs. Our data provide compelling evidence that C3 deficiency modulates the potency of APCs to stimulate the T-cell response, suggesting a critical role for complement in the maintenance of APC function. This could offer a partial explanation as to why the T-cell response is impaired in C3-/- mice. (Blood. 2006;107:2461-2469)

Local extravascular pool of C3 is a determinant of postischemic acute renal failure

The third complement component (C3) is an acute phase protein that plays a central role in reperfusion injury in several organ models. To investigate the contribution of local synthesis of C3 and distinguish it from that of circulating complement mainly produced by hepatic synthesis, we employed a mouse renal isograft model. Our model demonstrated a close relationship between the extent of intrarenal expression of C3 and cold-ischemia induced injury. Ischemic C3-positive donor kidneys transplanted into C3-positive or C3-negative recipients developed widespread tissue damage and severe acute renal failure. In contrast, ischemic C3-negative isografts exhibited only mild degrees of functional and structural disturbance, even when transplanted into normal C3-positive recipients. Thus local synthesis of C3, mostly identified in the tubular epithelium, was essential for complement-mediated reperfusion damage, whereas circulating C3 had a negligible effect. Our results suggest a two-compartment model for the pathogenic function of C3, in which the extravascular compartment is the domain of local synthesis of C3, and where the role of circulating C3 is redundant. Our data cast new light on the mechanism of complement-mediated tissue injury in nonimmunological disorders, and challenges the longstanding dogma that circulating components are the main complement effectors of extravascular tissue damage.

Accumulation of immune complexes in glomerular disease is independent of locally synthesized c3

Although complement activation can make immune complex glomerulonephritis worse, the third complement component also can solubilize immune complexes and thus reduce the severity of disease. How C3 that is produced within the kidney contributes to this balance is unknown. This study therefore investigated the relative roles of systemic and local C3 production in a model of glomerular immune complex disease. Injection of sheep anti-glomerular basement membrane antibody into preimmunized mice resulted in accumulation of immune complexes and progressive loss of function over 14 d that was much more marked in C3-deficient (C3-/-) mice. In C3-sufficient mice that received a transplant of a C3-/- mouse kidney and in C3-/- mice with C3-sufficient mouse kidney transplants, the severity and the pattern of injury went with the complement status of the recipient. That is, mice with deficient circulating C3 developed severe glomerular immune complex disease, whereas those with a high level of circulating C3 had well-preserved glomerular structure and function. It is concluded that circulating C3 is a critical factor in reducing the glomerular accumulation of immune complexes. Local synthesis of C3 did not have a major influence on this aspect of glomerular disease.

Allograft rejection: effect of local synthesis of complement

The complement system is known for its ability to participate in non-specific inflammation and membrane injury as well as contributing to antigen-specific immune stimulation. In renal transplantation, the complement cascade behaves true to form in that both non-immune- and immune-mediated destruction of the renal tubules are complement dependent. What is remarkable, however, is the extent of involvement of local synthesis of complement in both of these injuries, suggesting that the extravascular tissue compartment is the domain of local synthesis, whereas the effect of circulating complement is much less. This creates a new paradigm for studying the influence of local synthesis of complement in other organ-based diseases and underlines the need for tissue-targeting strategies in successful therapeutic development.

Minireview: functions of the renal tract epithelium in coordinating the innate immune response to infection

Infection of the urinary tract remains one of the most common infections affecting mankind. Renal epithelial cells, being one of the first cells to come into contact with invading organisms, are in a key position to coordinate host defense. The epithelium not only provides a physical barrier to infection, but can also augment the immune response via the production of a number of inflammatory mediators and antimicrobial proteins. Recent work has demonstrated that cells of the innate immune system, including epithelial cells, express toll-like receptors (TLRs), with the capacity to recognize bacterial components. Although the exact mechanisms remain unclear, engagement of TLRs can lead to epithelial cell activation and the production of inflammatory mediators. These include complement proteins, other bactericidal peptides, and chemotactic cytokines. The resulting inflammatory infiltrate serves to aid bacterial clearance, but can also lead to renal damage. In this review, we describe how renal epithelial cells contribute to the innate immune response to ascending urinary tract infection. We specifically relate previous work to more recent developments in this field. An improved understanding of the mechanisms involved may highlight potential therapeutic avenues to aid bacterial clearance and prevent the renal scarring associated with infection.

Complement synthesis and activation in the brain of SIV-infected monkeys

Complement is one of the most critical defence tools against cerebral infections, but uncontrolled complement biosynthesis and activation can induce profound brain tissue damage. To clarify the role of complement in the pathogenesis of AIDS-associated neurological disorders, we analysed the synthesis of complement in the brains of SIV-infected rhesus macaques. Using immunohistochemical staining we could show that the cerebral synthesis of complement factors C1q and C3 was strongly upregulated in SIV-infected monkeys compared to the spontaneous synthesis in uninfected control monkeys. Astrocytes, neurons, microglia, infiltrating macrophages and multinuclear giant cells all contribute to the high amounts of C1q and C3 in the brain. Secreted C1q and C3 are also deposited on the membrane of neurons, a prerequisite for formation of the membrane-driven lytic membrane attack complex. The membrane deposition thus might suggest complement-induced lysis of bystander neurons as a potential mechanism for cell damage during viral infection of the brain.

Nontransgenic hyperexpression of a complement regulator in donor kidney modulates transplant ischemia/reperfusion damage, acute rejection, and chronic nephropathy

Complement activation during ischemia and reperfusion contributes to the development of tissue injury with severe negative impact on outcomes in transplantation. To counter the effect of complement, we present a strategy to deliver a novel complement regulator stabilized on cell surfaces within donor organs. The membrane-bound complement regulator is able to inhibit complement activation when the donor organ is revascularized and exposed to host-circulating complement. Application of this construct to donor kidneys protected transplanted tissues from ischemia/reperfusion injury and reduced the deposition of activated complement and histological signs of damage under conditions in which a nontargeted control construct was ineffective. Treatment of donor organs in this way improved graft performance in the short and long term. An analysis of the immune response in allograft recipients showed that reducing graft damage at the time of transplantation through complement regulation also modulated the alloresponse. Additionally, the results of perfusion studies with human kidneys demonstrated the feasibility of targeting endothelial and epithelial surfaces with this construct, to allow investigation in clinical transplantation.

Locally produced complement and its role in renal allograft rejection

The role of innate immunity in allograft injury is just beginning to become clear, and complement is probably one of a number of factors that are activated very early in the course of transplantation. Kidney transplantation into complement-inhibited rats reduces subsequent inflammation of the graft, probably as a result of reduction of ischemia reperfusion damage as well as diminution of immune mediated damage. Closer analysis of the role of locally synthesised components in mice has suggested that regional synthesis of complement proteins, in particular by the renal tubule, may play a more important role than circulating components. A marked effect on the antidonor T cell response may be explained by the triggering of complement receptors present on antigen presenting cells or T cells infiltrating the graft, or by a more direct effect of complement on the liaison between proximal tubule cells and T cells. Therapeutic control is likely to require a shift to a more targeted approach, directed at complement components produced in the extravascular tissue compartment.

Mechanism of human immunodeficiency virus-induced complement expression in astrocytes and neurons

The cerebral complement system is hypothesized to contribute to neurodegeneration in the pathogenesis of AIDS-associated neurological disorders. Our former results have shown that the human immunodeficiency virus (HIV) strongly induces the synthesis of complement factor C3 in astrocytes. This upregulation explains in vivo data showing elevated complement levels in the cerebrospinal fluid of patients with AIDS-associated neurological symptoms. Since inhibition of complement synthesis and activation in the brain may represent a putative therapeutic goal to prevent virus-induced damage, we analyzed in detail the mechanisms of HIV-induced modulation of C3 expression. HIV-1 increased the C3 levels in astrocyte culture supernatants from 30 to up to 400 ng/ml; signal transduction studies revealed that adenylate cyclase activation with upregulation of cyclic AMP is the central signaling pathway to mediate that increase. Furthermore, activity of protein kinase C is necessary for HIV induction of C3, since inhibition of protein kinase C by prolonged exposure to the phorbol ester tetradecanoyl phorbol acetate partly abolished the HIV effect. The cytokines tumor necrosis factor alpha and gamma interferon were not involved in mediating the HIV-induced C3 upregulation, since neutralizing antibodies had no effect. Besides whole HIV virions, the purified viral proteins Nef and gp41 are biologically active in upregulating C3, whereas Tat, gp120, and gp160 were not able to modulate C3 synthesis. Further experiments revealed that neurons were also able to respond on incubation with HIV with increased C3 synthesis, although the precise pattern was slightly different from that in astrocytes. This strengthens the hypothesis that HIV-induced complement synthesis represents an important mechanism for the pathogenesis of AIDS in the brain.

Intrarenal synthesis of complement

During the past decade, research has shown that the kidney has the capacity to synthesize most of the activation pathway components of the complement cascade. As well as implying physiological roles in local clearance of immune complexes and defense against invasive organisms, an increasing amount of evidence indicates that the intrarenal synthesis of complement makes an important contribution in the pathogenesis of renal injury. Here we review this evidence and present a case for more definitive investigation of these functions.

Human immunodeficiency virus type 1 induces expression of complement factors in human astrocytes

Since the brain is separated from the blood immune system by a tight barrier, the brain-resident complement system may represent a central player in the immune defense of this compartment against human immunodeficiency virus (HIV). Chronic complement activation, however, may participate in HIV-associated neurodegeneration. Since the level of complement factors in the cerebrospinal fluid is known to be elevated in AIDS-associated neurological disorders, we evaluated the effect of HIV type 1 (HIV-1) on the complement synthesis of brain astrocytes. Incubation of different astrocytic cell lines and primary astrocytes with HIV-1 induced a marked upregulation of the expression of the complement factors C2 and C3. The synthesis of other secreted or membrane-bound complement proteins was not found to be altered. The enhancement of C3 production was measured both on the mRNA level and as secreted protein in the culture supernatants. HIV-1 laboratory strains as well as primary isolates were capable of inducing C3 production with varied effectiveness. The usage of viral coreceptors by HIV-1 was proved to be a prerequisite for the upregulation of C3 synthesis, which was modulated by the simultaneous addition of cytokines. The C3 protein which is secreted after incubation of the cells with HIV was shown to be biologically active as it can participate in the complement cascade.

In situ localization of C3 synthesis in experimental acute renal allograft rejection

Recent evidence has implicated complement in renal transplant injury and identified the kidney as a source of complement components. We therefore investigated the local gene expression of complement component C3, pivotal to complement activation pathways and a mediator of inflammatory injury, in a rat renal transplant model. By reverse transcriptase-polymerase chain reaction, the expression of C3 mRNA increased in two phases. The first phase coincided with post-ischemic injury over 2 days post-transplantation and was localized by in situ hybridization to vessels and glomerular mesangial cells in allogeneic and syngeneic (control) kidney transplants. In allografts only, a second phase was found in tubular epithelial cells, glomerular parietal cells, vessel walls and some infiltrating cells, which peaked on day 4 together with rapid influx of leukocytes, tubule cell damage, the induction of interleukin-2 and interferon-gamma mRNA, and the up-regulation of tumor necrosis factor-alpha and interleukin-1beta mRNA in the graft. In vitro studies showed that interleukin-2 and interferon-gamma up-regulate C3 production in renal tubule cells. We conclude that post-ischemic injury led to transient up-regulation of glomerular expression of C3 mRNA. Subsequent cellular rejection was associated with tubulointerstitial/glomerular parietal cell expression of C3 mRNA. This differential expression of local C3, immediately post-transplant or associated with acute rejection, may have implications for putative therapeutic complement inhibition in clinical transplantation.

Expression of complement messenger RNAs by human endothelial cells

This study evaluated complement mRNA expression in human brain microvessel endothelial cells (HBMEC), human umbilical vein endothelial cells (HUVEC), and cells of the human derived ECV304 line. Cerebral endothelial cells and HUVEC expressed detectable levels of complement gene mRNAs for the C1q B-chain, C1r, C1s, C2, C3, C4, C5, C7, C8 gamma-subunit and C9. In addition to C6 mRNA, C1q and C9 were not detected in ECV304 cells. These results indicate that endothelial cells may be a source of complement proteins in brain and other organs of the body.

Synthesis of complement components C3 and factor B in human keratinocytes is differentially regulated by cytokines

The complement system plays an important part in host defense and inflammation. Locally synthesized complement may perform these functions at tissue and organ level. In skin the keratinocyte is the major cell type, it is known to produce two soluble complement components, C3 and factor B. In this study we investigated the regulation of synthesis of these components in foreskin keratinocytes by cytokines. Human keratinocytes were cultured in the presence of supernatant of activated peripheral blood mononuclear cells, interleukin-1alpha, interleukin-2, interleukin-6, transforming growth factor-beta1, tumor necrosis factor-alpha, or interferon-gamma. C3 and factor B proteins were measured in culture supernatant by enzyme-linked immunosorbent assay and C3 and factor B transcripts in harvested cells by reverse transcriptase-polymerase chain reaction. Cultured keratinocytes constitutively produced C3 and factor B. Supernatant of activated mononuclear cells upregulated C3 and factor B production by 27- and 15-fold, respectively. interleukin-1alpha, interferon-gamma, and tumor necrosis factor-alpha upregulated C3 synthesis by 7-, 8-, and 22-fold, and interleukin-1alpha, interleukin-6, and interferon-gamma upregulated factor B synthesis by 3-, 3-, and 34-fold, respectively. Tumor necrosis factor-alpha induced production of C3 and interferon-gamma induced production of factor B were inhibited by cycloheximide. Cytokine induced upregulation of C3 and factor B proteins was always associated with the upregulation of levels of C3 and factor B mRNA. This indicated that, as expected, cytokine-induced enhancement in C3 and factor B levels was due to an increase in synthesis rather than their possible release from intracellular stores. In conclusion, synthesis of C3 and factor B in keratinocytes is regulated by some cytokines, known to be produced by inflammatory cells and keratinocytes.

Renal C3 synthesis in idiopathic membranous nephropathy: correlation to urinary C5b-9 excretion

Renal C3 synthesis in idiopathic membranous nephropathy: Correlation to urinary C5b-9 excretion.

BACKGROUND

Complement activation plays a central pathogenetic role in idiopathic membranous nephropathy (IMN). Urinary excretion of C5b-9 correlates to the immunologic activity of this disease. Recently, renal cortical C3 gene expression has been described in several nephropathies.

METHODS

The aim of this study was to investigate the renal C3 gene expression by in situ hybridization in IMN and to correlate it with histopathologic, pathophysiologic, and immunologic (urinary C5b-9) indices of disease activity.

RESULTS

C3 was expressed in 77% of 22 renal biopsies of IMN patients, mainly at the cortical tubular and glomerular parietal epithelial cell levels. C3 protein synthesis by tubular cells was demonstrated by immunofluorescence. The intensity of C3 gene expression by both glomerular and tubulointerstitial compartments correlated with the glomerular stage of disease (P = 0. 0023 and P = 0.0214, respectively). Although no correlation was found with proteinuria, serum creatinine at renal biopsy time was strongly associated with renal C3 expression. IMN patients showed a trend of increased urinary C5b-9 levels, which correlated to C3 at the tubulointerstitial level (P = 0.0143).

CONCLUSION

Renal C3 production, mainly at the tubular level, may be induced by urinary excretion of C5b-9 in IMN and may have a pathogenetic role in the tubulointerstitial damage that can be associated with this disease.

mRNA expression of complement components and regulators in rat arterial smooth muscle cells

The presence of C5b-9 complexes, some complement regulators, and abundant cytokines in atherosclerotic lesions has been reported. However, it is unclear whether these complement-associated proteins are produced by vascular smooth muscle cells (SMCs) and how they are influenced by the cytokines. In the present study, we demonstrated, by the reverse transcription-polymerase chain reaction method, the mRNA expression of complement components (C3, C4, and C5) and membrane regulators (decay-accelerating factor, membrane cofactor protein, Crry, and CD59) in cultured SMCs derived from the rat carotid artery. The expression of C9 mRNA was also induced upon stimulation by interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and/or lipopolysaccharide (LPS). Northern blot analysis showed that the mRNA expression of C3, C4, DAF and Crry was up-regulated, but that of CD59 was down-regulated by IFN-gamma, TNF-alpha and/or LPS alone or by synergy. The increase of C3 mRNA by TNF-alpha or LPS and that of C4 mRNA by IFN-gamma was induced in a dose-dependent manner. The results indicate that the arterial SMCs of rat have the ability to produce complement components and regulators, which is affected by cytokines and/or LPS. Since atherosclerosis is characterized by the intimal proliferation of SMCs, the complement system including its regulators may be involved in the pathogenesis of the disease.

Cutting Edge: C1q Protects Against the Development of Glomerulonephritis Independently of C3 Activation

C1q-deficient (C1qa−/−) mice develop antinuclear Abs and glomerulonephritis (GN) characterized by multiple apoptotic bodies. To explore the contribution of C3 activation to the induction of spontaneous GN, C1qa−/− mice were crossed with factor B- and C2-deficient (H2-Bf/C2−/−) mice. GN was present in 64% of the 45 C1qa/H2-Bf/C2−/− mice compared with 8% of the 65 H2-Bf/C2−/− mice and none of the 24 wild-type controls. IgG was detected in the glomeruli of diseased C1qa/H2-Bf/C2−/− kidneys. However, glomerular staining for C3 was absent. Increased numbers of glomerular apoptotic bodies were detected in undiseased C1qa/H2-Bf/C2−/− kidneys. These findings support the hypothesis that C1q may play a role in the clearance of apoptotic cells without the necessity for C3 activation and demonstrate that the activation of C3 is not essential for the development of GN in this spontaneous model of lupus-like disease.