CTRP9 prevents atherosclerosis progression through changing autophagic status of macrophages by activating USP22 mediated-de-ubiquitination on Sirt1 in vitro

BACKGROUND

Atherosclerosis (AS) is commonly regarded as a key driver accounted for the leading causes of morbidity and mortality among cardiovascular and cerebrovascular diseases. A growing body of evidence indicates that autophagy in macrophages involved in AS might be a potential therapeutic target. C1q/TNF-related protein 9 (CTRP9) has been proven to delay the progression of cardiovascular diseases. However, the relations between CTRP9 and Sirt1, as well as their effects on macrophages autophagy have not been fully explored.

METHODS

Macrophages were differentiated from mononuclear cells collected from peripheral blood samples of healthy donors. The in vitro AS models were constructed by ox-LDL treatment. Cell viability was determined by CCK-8 assay. Immunofluorescence assay of LC3 was implemented for evaluating autophagy activity. Oil Red O staining was performed for lipid accumulation detection. ELISA, cholesterol concentration assay and cholesterol efflux analysis were conducted using commercial kits. Cycloheximide assay was implemented for revealing protein stability. RT-qPCR was used for mRNA expression detection, and western blotting was performed for protein level monitoring.

RESULTS

CTRP9 attenuated impaired cell viability, autophagy inhibition and increased lipid accumulation induced by ox-LDL. Moreover, CTRP9 maintained Sirt1 protein level through enhancing its stability through de-ubiquitination, which was mediated by upregulated USP22 level. CRTP9 exerted its protective role in promoting autophagy and reducing lipid accumulation through the USP22/Sirt1 axis.

CONCLUSION

Collectively, CTRP9 alleviates lipid accumulation and facilitated the macrophages autophagy by upregulating USP22 level and maintaining Sirt1 protein expression, thereby exerting a protective role in AS progression in vitro.

Identification of C1q/TNF-related protein 4 as a novel appetite-regulating peptide that reduces food intake in Siberian sturgeon (Acipenser baerii)

Emerging findings point to a role for C1q/TNF-related protein 4 (CTRP4) in feeding in mammals. However, it remains unknown whether CTRP4 regulates feeding in fish. This study aimed to determine the feeding regulation function of CTRP4 in Siberian sturgeon (Acipenser baerii). In this study, the Siberian sturgeon ctrp4 (Abctrp4) gene was cloned, and Abctrp4 mRNA was shown to be highly expressed in the hypothalamus. In the hypothalamus, Abctrp4 mRNA decreased during fasting and reversed after refeeding. Subsequently, we obtained the AbCTRP4 recombinant protein by prokaryotic expression and optimized the expression and purification conditions. Siberian sturgeon (81.28 ± 14.75 g) were injected intraperitoneally using 30, 100, and 300 ng/g Body weight (BW) AbCTRP4 to investigate its effect on feeding. The results showed that 30, 100, and 300 ng/g BW of the AbCTRP4 significantly reduced the cumulative food intake of Siberian sturgeon at 1, 3, and 6 h. Finally, to investigate the potential mechanism of CTRP4 feeding inhibition, 300 ng/g BW AbCTRP4 was injected intraperitoneally. The findings demonstrated that AbCTRP4 treatment for 1 h significantly promoted the mRNA levels of anorexigenic peptides (pomc, cart, and leptin) while suppressing the mRNA abundances of orexigenic peptides (npy and agrp).In addition, the jak2/stat3 pathway in the hypothalamus was significantly activated after 1 h of AbCTRP4 treatment. In conclusion., this study confirms the anorexigenic effect of CTRP4 in Siberian sturgeon.

C1q/Tumor Necrosis Factor-Related Protein-9 Is a Novel Vasculoprotective Cytokine That Restores High Glucose-Suppressed Endothelial Progenitor Cell Functions by Activating the Endothelial Nitric Oxide Synthase

BACKGROUND

This study investigated whether gCTRP9 (globular C1q/tumor necrosis factor-related protein-9) could restore high-glucose (HG)-suppressed endothelial progenitor cell (EPC) functions by activating the endothelial nitric oxide synthase (eNOS).

METHODS AND RESULTS

EPCs were treated with HG (25 mmol/L) and gCTRP9. Migration, adhesion, and tube formation assays were performed. Adiponectin receptor 1, adiponectin receptor 2, and N-cadherin expression and AMP-activated protein kinase, protein kinase B, and eNOS phosphorylation were measured by Western blotting. eNOS activity was determined using nitrite production measurement. In vivo reendothelialization and EPC homing assays were performed using Evans blue and immunofluorescence in mice. Treatment with gCTRP9 at physiological levels enhanced migration, adhesion, and tube formation of EPCs. gCTRP9 upregulated the phosphorylation of AMP-activated protein kinase, protein kinase B, and eNOS and increased nitrite production in a concentration-dependent manner. Exposure of EPCs to HG-attenuated EPC functions induced cellular senescence and decreased eNOS activity and nitric oxide synthesis; the effects of HG were reversed by gCTRP9. Protein kinase B knockdown inhibited eNOS phosphorylation but did not affect gCTRP9-induced AMP-activated protein kinase phosphorylation. HG impaired N-cadherin expression, but treatment with gCTRP9 restored N-cadherin expression after HG stimulation. gCTRP9 restored HG-impaired EPC functions through both adiponectin receptor 1 and N-cadherin-mediated AMP-activated protein kinase /protein kinase B/eNOS signaling. Nude mice that received EPCs treated with gCTRP9 under HG medium showed a significant enhancement of the reendothelialization capacity compared with those with EPCs incubated under HG conditions.

CONCLUSIONS

CTRP9 promotes EPC migration, adhesion, and tube formation and restores these functions under HG conditions through eNOS-mediated signaling mechanisms. Therefore, CTRP9 modulation could eventually be used for vascular healing after injury.

Responses of multifunctional immune complement component 1q (C1q) and apoptosis-related genes in Macrophthalmus japonicus tissues and human cells following exposure to environmental pollutants

Apoptosis is a key defense process for multiple immune system functions, playing a central role in maintaining homeostasis and cell development. The purpose of this study was to evaluate the effects of environmental pollutant exposure on immune-related apoptotic pathways in crab tissues and human cells. To do this, we characterized the multifunctional immune complement component 1q (C1q) gene and analyzed C1q expression in Macrophthalmus japonicus crabs after exposure to di(2-ethylhexyl) phthalate (DEHP) or hexabromocyclododecanes (HBCDs). Moreover, the responses of apoptotic signal-related genes were observed in M. japonicus tissues and human cell lines (HEK293T and HCT116). C1q gene expression was downregulated in the gills and hepatopancreas of M. japonicus after exposure to DEHP or HBCD. Pollutant exposure also increased antioxidant enzyme activities and altered transcription of 15 apoptotic signaling genes in M. japonicus. However, patterns in apoptotic signaling in response to these pollutants differed in human cells. HBCD exposure generated an apoptotic signal (cleaved caspase-3) and inhibited cell growth in both cell lines, whereas DEHP exposure did not produce such a response. These results suggest that exposure to environmental pollutants induced different levels of immune-related apoptosis depending on the cell or tissue type and that this induction of apoptotic signaling may trigger an initiation of carcinogenesis in M. japonicus and in humans as consumers.

C1q/TNF-related protein 3 alleviates heart failure via attenuation of oxidative stress in myocardial infarction rats

C1q-tumor necrosis factor-related protein 3 (CTRP3), an adipokine, has been reported to be closely related to cardiovascular diseases (CVDs). However, the effect of CTRP3 on heart failure (HF) remains dimness. This study was to explore the role of CTRP3 in HF and its potential interaction mechanism. Heart failure model was established by inducing ischemia myocardial infarction (MI) through ligation of the left anterior descending artery in Sprague-Dawley rats. Four weeks later, the rats were detected by transthoracic echocardiography and masson staining. Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), cardiac troponin I (cTnI) levels, creatine kinase-MB (CK-MB) and oxidative stress levels were recorded. The level of CTRP3 was reduced in the cardiomyocytes (CMs) treated with oxygen-glucose deprivation (OGD) and in the heart of MI rats. CTRP3 overexpression alleviated cardiac dysfunction, attenuated the cardiac fibrosis, and inhibited the increases of ANP, BNP, cTnI and CK-MB in the serum of MI rats. The increases of ANP and BNP in the CMs, and the collagen I and collagen III in the cardiac fibroblasts (CFs) induced by OGD were inhibited by CTRP3 overexpression. The enhancement of oxidative stress in the heart of MI rats was inhibited by CTRP3 overexpression. These results indicated that overexpression of CTRP3 could improve cardiac function and the related cardiac fibrosis in MI-induced HF rats via inhibition of oxidative stress. Upregulation of CTRP3 may be a strategy for HF therapy in the future.

Deficiency of C1QL1 Reduced Murine Ovarian Follicle Reserve Through Intraovarian and Endocrine Control

Ovarian aging is associated with depletion of the ovarian follicle reserve, which is the key determinant of fertility potential in females. In this study, we found that the small, secreted protein complement 1Q-like (C1QL1) is involved in the regulation of follicle depletion through intraovarian and endocrine control in a multidimensional collaborative manner. C1ql1 was detected to be conserved in the ovary and showed high transcript levels during folliculogenesis. Blockade of C1QL1 by IP and ovarian intrabursal injection of C1QL1 antiserum into prepubertal mice impaired folliculogenesis accompanied by reductions in body weight, fat mass, and intraovarian lipid accumulation. An elevation of circulating estradiol levels, reduction of hypothalamic KISS1 and GnRH expression, and a decrease in serum FSH levels were found in C1QL1-deficient mice. In C1QL1-deficient ovaries, many primordial follicles were recruited and developed into medium follicles but underwent atresia at the large follicle stages, which resulted in depletion of follicle reserve. Depletion of C1QL1 alleviated the inhibitory effect of C1QL1 on granulosa cell apoptosis and the stimulatory effect of C1QL1 on granulosa cell autophagy, which resulted in accumulation in the preantral and early antral follicles and an increase in the atretic follicles. The abnormal profile of endocrine hormones accelerated the intraovarian effect of C1QL1 deficiency and further led to depletion of ovarian reserve. Altogether, this study revealed the expression patterns and the mechanism of action of C1QL1 during folliculogenesis and demonstrated that deficiency of C1QL1 caused ovarian follicular depletion.

Amyloid beta acts synergistically as a pro-inflammatory cytokine

The amyloid beta (Aβ) peptide is believed to play a central role in Alzheimer's disease (AD), the most common age-related neurodegenerative disorder. However, the natural, evolutionarily selected functions of Aβ are incompletely understood. Here, we report that nanomolar concentrations of Aβ act synergistically with known cytokines to promote pro-inflammatory activation in primary human astrocytes (a cell type increasingly implicated in brain aging and AD). Using transcriptomics (RNA-seq), we show that Aβ can directly substitute for the complement component C1q in a cytokine cocktail previously shown to induce astrocyte immune activation. Furthermore, we show that astrocytes synergistically activated by Aβ have a transcriptional signature similar to neurotoxic "A1" astrocytes known to accumulate with age and in AD. Interestingly, we find that this biological action of Aβ at low concentrations is distinct from the transcriptome changes induced by the high/supraphysiological doses of Aβ often used in in vitro studies. Collectively, our results suggest an important, cytokine-like function for Aβ and a novel mechanism by which it may directly contribute to the neuroinflammation associated with brain aging and AD.

The regulatory role of C1q on Helicobacter pylori-induced inflammatory cytokines secretion in THP-1 cells

C1q, as a LAIR-1 ligand, maintains monocytes quiescence and possess immunosuppressive properties. To understand the roles and molecular mechanisms, C1q mediated inflammation cytokines and several pivotal proteins in THP-1 cells after H. pylori infection were detected. The results showed that the expression of IL-8, IL-10, LAIR-1, phosphorylated/total JNK, phosphorylated/total p38-MAPK, phosphorylated/total AKT and phosphorylated/total NF-κB were up-regulated significantly in THP-1 cells after H. pylori infection. There was significant upregulation in IL-10 concentration, phosphorylated/total p38-MAPK and phosphorylated/total AKT, and downregulation in phosphorylated/total JNK in non-H. pylori infected THP-1 cells pretreated with C1q. C1q was also able to increase IL-8 and IL-10 production, and reduce LAIR-1 and phosphorylated/total p38-MAPK expression in pretreatment-C1q THP-1 cells after H. pylori infection. These results together indicated that H. pylori might induce IL-8 and IL-10 production through JNK, p38-MAPK, PI3K/AKT and NF-κB signaling pathway. C1q manipulate LAIR-1 to regulation IL-8 and IL-10 secretion in THP-1 cells after H. pylori infection through the p38-MAPK signaling pathway. This information is helpful to further understand the role and mechanisms of C1q on inflammation cytokines secretion in monocytes after H. pylori infection.

Tissue factor pathway inhibitor-2 (TFPI-2) recognizes the complement and kininogen binding protein gC1qR/p33 (gC1qR): implications for vascular inflammation

Evidence is accumulating to suggest that TFPI-2 is involved in regulating pericellular proteases implicated in a variety of physiologic and pathologic processes including cancer cell invasion, vascular inflammation, and atherosclerosis. Recent immunohistochemical studies of advanced atherosclerotic lesions, demonstrated a similar tissue distribution for TFPI-2, High Molecular Weight Kininogen (HK), and gC1qR/p33 (gC1qR), a ubiquitously expressed, multicompartmental cellular protein involved in modulating complement, coagulation, and kinin cascades. Further studies to evaluate TFPI-2 interactions with gC1qR demonstrated direct interactions between gC1qR and TFPI-2 using immunoprecipitation and solid phase binding studies. Specific and saturable binding between TFPI-2 and gC1qR (estimated Kd: ∼ 70 nM) was observed by ELISA and surface plasmon resonance (Biacore) binding assays. Binding was inhibited by antibodies to gC1qR, and was strongly dependent on the Kunitz-2 domain of TFPI-2, as deletion of this domain reduced gC1qR-TFPI-2 interactions by approximately 75%. Deletion of gC1qR amino acids 74-95, involved in C1q binding, had no effect on gC1qR binding to TFPI-2, although antibodies to this region and purified C1q both inhibited binding, most likely via allosteric effects. In contrast, HK did not affect TFPI-2 binding to gC1qR. Binding of TFPI-2 to gC1qR produced statistically significant but modest reductions in TFPI-2 inhibition of plasmin, but had no effect on kallikrein inhibition in fluid phase chromogenic assays. Taken together, these data suggest that gC1qR may participate in tissue remodeling and inflammation by localizing TFPI-2 to the pericellular environment to modulate local protease activity and regulate HK activation.

C1q, the recognition subcomponent of the classical pathway of complement, drives microglial activation

Microglia, central nervous system (CNS) resident phagocytic cells, persistently police the integrity of CNS tissue and respond to any kind of damage or pathophysiological changes. These cells sense and rapidly respond to danger and inflammatory signals by changing their cell morphology; by release of cytokines, chemokines, or nitric oxide; and by changing their MHC expression profile. We have shown previously that microglial biosynthesis of the complement subcomponent C1q may serve as a reliable marker of microglial activation ranging from undetectable levels of C1q biosynthesis in resting microglia to abundant C1q expression in activated, nonramified microglia. In this study, we demonstrate that cultured microglial cells respond to extrinsic C1q with a marked intracellular Ca(2+) increase. A shift toward proinflammatory microglial activation is indicated by the release of interleukin-6, tumor necrosis factor-alpha, and nitric oxide and the oxidative burst in rat primary microglial cells, an activation and differentiation process similar to the proinflammatory response of microglia to exposure to lipopolysaccharide. Our findings indicate 1) that extrinsic plasma C1q is involved in the initiation of microglial activation in the course of CNS diseases with blood-brain barrier impairment and 2) that C1q synthesized and released by activated microglia is likely to contribute in an autocrine/paracrine way to maintain and balance microglial activation in the diseased CNS tissue.

Differential chemokine response of fibroblast subtypes to complement C1q

BACKGROUND AND OBJECTIVE

The pathogenesis of periodontitis includes an inappropriate activation of the classical complement cascade (C') with accumulation of inflammatory C' products in fluids and tissues. Our hypothesis is that in vivo the C' product, C1q, may act as a regulatory component of the innate immune response of distinct matrix fibroblasts to the inflammatory environment. This study analyzed the C1q induction of pro-inflammatory cytokine secretion in fibroblast subtypes derived from distinct periodontal tissues, and identified a mechanism of the cell response.

MATERIAL AND METHODS

Primary human gingival fibroblast, periodontal ligament fibroblast, and granulation tissue fibroblast cultures were treated for 24 h with C1q. Protein arrays assessed the secretory profile of constitutive and C1q-inducible pro-inflammatory cytokines, and enzyme-linked immunosorbent assays were used to quantify the kinetics of each inducible cytokine.

RESULTS

Granulation tissue fibroblast cultures were unresponsive to C1q challenge. In contrast, periodontal ligament fibroblasts responded with a release of monocyte chemoattractant protein (MCP)-1, interleukin-6, interleukin-8, and macrophage inflammatory protein (MIP)-1beta higher than the basal level by 8.2-, 7.0-, 3.8-, and 7.2-fold, respectively. Human gingival fibroblast cultures increased secretion of these chemokines by 5.2-, 4.5-, 3.0-, and 9.8-fold, respectively. Inhibitor studies revealed that C1q-inducible release of chemokines by the human gingival fibroblast and periodontal ligament cultures was contingent upon p38 mitogen-activated protein kinase activity.

CONCLUSION

The ability of C1q to stimulate secretion of pro-inflammatory chemokines depends upon which specific fibroblast subtype is involved. Targeting C1q-activated intracellular signaling pathways may be an effective means to inhibit the production of chemokines that promote inflammatory cell infiltration into gingival and periodontal ligament tissues.

Minocycline does not affect amyloid beta phagocytosis by human microglial cells

Activated microglia accumulate in amyloid beta (Abeta) plaques containing amyloid associated factors SAP and C1q in Alzheimer's disease (AD) brain. Microglia are involved in AD pathogenesis by promoting Abeta plaque formation and production of pro-inflammatory cytokines. On the other hand, phagocytosis of Abeta by activated microglia may prevent Abeta-mediated neurotoxicity and Abeta plaque formation. Minocycline, a tetracycline derivative, is neuroprotective in various neurodegenerative models as well as human chronic neurological disorders. Minocycline attenuates the release of TNF-alpha by human microglia upon exposure to a mixture of Abeta, SAP and C1q. Here, we demonstrate that minocycline down-regulates the production of pro-inflammatory cytokines by human microglia without affecting their beneficial activity, phagocytosis of amyloid beta fibrils.

Generation of inhibitory NFkappaB complexes and phosphorylated cAMP response element-binding protein correlates with the anti-inflammatory activity of complement protein C1q in human monocytes

The interaction of C1q with specific cells of the immune system induces activities, such as enhancement of phagocytosis in monocytes and stimulation of superoxide production in neutrophils. In contrast to some other monocyte activators, C1q itself does not induce pro-inflammatory cytokine production, but rather inhibits the lipopolysaccharide (LPS)-stimulated induction of certain pro-inflammatory cytokines and induces expression of interleukin-10. To investigate the molecular mechanism by which C1q exerts this effect on gene expression, the influence of C1q on the activation of transcription factors of the NFkappaB family and cAMP response element-binding protein (CREB) was assessed. C1q treatment increased kappaB binding activity in freshly isolated human monocytes in a time-dependent fashion as assessed by electrophoretic mobility shift assays. In antibody supershift experiments, anti-p50 antibody supershifted the C1q-induced NFkappaB complex, whereas anti-p65 antibody had little effect, suggesting that C1q induced the translocation of NFkappaB p50p50 homodimers. This is in contrast to the dominant induction of p65 containing complexes in parallel monocyte cultures stimulated with LPS. C1q treatment also induced cAMP response element (CRE)-binding activity as demonstrated by electrophoretic mobility shift assay, increased phosphorylation of CREB, and induction of CRE driven gene expression. In contrast, CREB activation was not detected in LPS-treated monocytes. These results suggest that C1q may modulate the cytokine profile expressed in response to inflammatory stimuli (e.g. LPS), by triggering inhibitory and/or competing signals. Because C1q and other defense collagens have been shown to enhance clearance of apoptotic cells, this regulatory pathway may be beneficial in avoiding autoimmunity and/or resolving inflammation.

Chemotaxis of human monocyte-derived dendritic cells to complement component C1q is mediated by the receptors gC1qR and cC1qR

Dendritic cells (DCs) are recruited to inflammatory sites where they phagocytose and process antigens for subsequent presentation to the T lymphocytes in the lymphoid tissue. Several leukocyte chemoattractants and their specific receptors have been shown to induce the migration of DC. The complement protein C1q has multiple immune functions including acting as a chemoattractant for neutrophils, eosinophils and mast cells. Therefore, the objective of this study was to determine if soluble C1q can induce chemotaxis of DC. Culturing cells in GM-CSF and IL-4 for 5 to 7 days generated human monocyte-derived DCs. In addition, LPS was added from day 5 to 7 to induce DC maturation. Cells were classified as either immature or mature DC by assessing the cell surface markers by flow cytometry, phagocytosis of dextran-FITC and T cell proliferation in an allogenic MLR. Immature DCs express the C1q receptors (C1qR), gC1qR and cC1qR/CR and, accordingly, display a vigorous migratory response to soluble C1q with maximal cell movement observed at 10-50nM. In contrast, mature DCs neither express C1qR nor do move to a gradient of soluble C1q. Varying the concentration gradient of C1q (checkerboard assay) showed that the protein largely induces a chemotactic response. Finally, blocking gC1qR and cC1qR/CR by using specific antibodies abolished the chemotactic response to C1q but had no effect on a different chemoattractant C5a. These results clearly demonstrate that C1q functions as a chemotactic factor for immature DC, and migration is mediated through ligation of both gC1qR and cC1qR/CR.

Activation of human microglia by fibrillar prion protein-related peptides is enhanced by amyloid-associated factors SAP and C1q

Complement activation products C1q and C3d, serum amyloid P component (SAP) and activated glial cells accumulate in amyloid deposits of conformationally changed prion protein (PrP(Sc)) in Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and scrapie-infected mouse brain. Biological properties, including the potential to activate microglia, relate to prion (PrP) peptide fibrillogenic abilities. We investigated if SAP and C1q influence the fibrillogenic properties of human and mouse PrP peptide and concomitantly their stimulatory effects on human microglia in vitro. PrP-peptide induced microglial IL-6 and TNF-alpha release significantly increased in the presence of SAP and C1q. Also, SAP and C1q enhanced PrP-peptide fibril formation as revealed by electron microscopy and thioflavin S-based quantitative assays. This suggests that SAP and C1q contribute to fibrillar state-dependent cellular effects of PrP. Combined, ultrastructural and thioflavin assays, together with microglial cytokine release measurements, provide a test system to screen potential, fibrillarity impeding therapeutics for prion disease.

C1-esterase inhibitor and a novel peptide inhibitor improve contractile function in reperfused skeletal muscle

To determine the role of inhibition of complement activation in the contractile function of skeletal muscle ischemia-reperfusion (I/R) injury, the rat extensor digitorum longus (EDL) muscles underwent 3 h ischemia and received human C1-esterase inhibitor (C1-INH, 100 IU/kg), a synthetic C1q A chain peptide with a similar inhibitory effect on activated C1 (peptide, 5 mg/kg), or human serum albumin control. Results showed a significant overall increase in tetanic contractile forces of the reperfused EDL in both C1-INH and peptide groups compared to controls. Maximum improvement occurred with peptide treatment at 120-Hz stimulation, with an increase in force from 38 +/- 4% of normal in controls to 52 +/- 4% in peptide-treated rats. There were no significant differences between C1-INH and peptide groups. Plasma C3 and C4 activities were significantly increased in both treated groups, suggesting inhibition of complement activation. Our results suggest that complement activation is involved in I/R injury, and inhibition of complement activation may therefore represent a potential therapeutic approach to reducing or preventing I/R injury.

Complement component C1q induces endothelial cell adhesion and spreading through a docking/signaling partnership of C1q receptors and integrins

The interaction of C1q with endothelial cells elicits a multiplicity of biologic responses. Although these specific responses are thought to be mediated by the interaction of C1q with proteins of the endothelial cell surface, the molecular identity of the participant(s) has not been clearly defined. In this study, we examined the role of two C1q-binding proteins, cC1q-R/CR and gC1q-R/p33, on C1q-mediated adhesion and spreading of human dermal microvascular endothelial cells (HDMVECs). A specific and dose-dependent adhesion and spreading was observed when HDMVECs were cultured in microtiter plate wells coated with concentrations of C1q ranging from 0 to 50 microg/ml. The extent of adhesion and spreading was similar to the adhesion seen on collagen-coated wells. Furthermore, the effect of C1q was mimicked by either polyclonal anti-cC1q-R or mAb 60.11, but not with isotype- and species-matched control IgG. More importantly, however, a 100% inhibition of spreading but not adhesion to C1q-coated wells was observed when HDMVECs were cultured in the presence of 30 mM of the peptide GRRGDSP but not GRRGESP. Furthermore, while anti-beta1 integrin antibody blocked adhesion and spreading, antialpha5 integrin only blocked spreading. Since earlier studies have shown that zinc induces the exposure of hydrophobic sites in the C-terminus of gC1q-R including the putative high-molecular weight kininogen (HK)-binding site corresponding to residues 204-218, we also examined the effect of zinc on antibody binding to cell surface gC1q-R. Flow cytometric data show that the binding of mAb 74.5.2, which recognizes residues 204-218, is greatly enhanced when endothelial cells were incubated in the presence of 50 microM zinc. In summary, our data show that: (a) C1q-mediated endothelial cell adhesion and spreading requires the cooperation of both C1q receptors and 1 integrins, and possibly other membrane-spanning molecules, and (b) zinc can induce the exposure of hydrophobic sites in the C-terminal domain of gC1q-R allowing a more efficient binding of mAb 74.5.2 and HK.

Surfactant protein A modulates the differentiation of murine bone marrow-derived dendritic cells

Surfactant protein A (SP-A) is an innate immune molecule that regulates pathogen clearance and lung inflammation. SP-A modulates innate immune functions such as phagocytosis, cytokine production, and chemotaxis; however, little is known about regulation of adaptive immunity by SP-A. Dendritic cells (DCs) are the most potent antigen-presenting cell with the unique capacity to activate naive T cells and initiate adaptive immunity. The goal of this study was to test the hypothesis that SP-A regulates the differentiation of immature DCs into potent T cell stimulators. The data show that incubation of immature DCs for 24 h with SP-A inhibits basal- and LPS-mediated expression of major histocompatibility complex class II and CD86. Stimulation of immature DCs by SP-A also inhibits the allostimulation of T cells, enhances dextran endocytosis, and alters DC chemotaxis toward RANTES and secondary lymphoid tissue chemokine. The effects on DC phenotype and function are similar for the structurally homologous C1q, but not for SP-D. These studies demonstrate that SP-A participates in the adaptive immune response by modulating important immune functions of DCs.

Surfactant protein A enhances the phagocytosis of C1q-coated particles by alveolar macrophages

Surfactant protein-A (SP-A) plays multiple roles in pulmonary host defense, including stimulating bacterial phagocytosis by innate immune cells. Previously, SP-A was shown to interact with complement protein C1q. Our goal was to further characterize this interaction and elucidate its functional consequences. Radiolabeled SP-A bound solid-phase C1q but not other complement proteins tested. The lectin activity of SP-A was not required for binding to C1q. Because C1q is involved in bacterial clearance but alone does not efficiently enhance the phagocytosis of most bacteria, we hypothesize that SP-A enhances phagocytosis of C1q-coated antigens. SP-A enhanced by sixfold the percentage of rat alveolar macrophages in suspension that phagocytosed C1q-coated fluorescent beads. Furthermore, uptake of C1q-coated beads was enhanced when either beads or alveolar macrophages were preincubated with SP-A. In contrast, SP-A had no significant effect on the uptake of C1q-coated beads by alveolar macrophages adhered to plastic slides. We conclude that SP-A may serve a protective role in the lung by interacting with C1q to enhance the clearance of foreign particles.

Production of thromboxane A2 and prostaglandin i2 affected by interaction of heat aggregated IgG, endothelial cells, and platelets in lupus nephritis

OBJECTIVE

To examine the role of immune complexes in the prostanoid metabolism of glomerular capillary endothelial cells (EC) and platelets in lupus nephritis. Heat aggregated IgG (HA-IgG), instead of immune complexes, was incubated using an in vitro coculture system with human umbilical vein EC, instead of glomerular capillary EC, and platelets. The effect of complement component C1q and a novel imidazole-type thromboxane A2 (TXA2) synthetase inhibitor, DP-1904, on this prostanoid metabolism change was also investigated.

METHODS

EC monolayers (1.5x10(5) cells/well) were incubated with various concentrations of HA-IgG, monomeric IgG, or medium alone for 1 h at 37 degrees C, and then incubated with platelet suspensions (1x10(8) cells/ml) for various times. Concentrations of TXB2 and 6-keto-prostaglandin F(1alpha) (6-keto-PGF(1alpha)), the stable hydrolysis products of TXA2 and prostaglandin I2 (PGI2), respectively, released in the supernatants were measured by ELISA.

RESULTS

HA-IgG bound to EC monolayers produced TXB2 and 6-keto-PGF(1alpha) in a concentration dependent manner and much more than monomeric IgG or medium alone did. However, the production of 6-keto-PGF(1alpha) stimulated with HA-IgG was much lower than that of TXB2, indicating a large imbalance between TXA2 and PGI2. Preincubation of HA-IgG with purified C1q partially suppressed the production of TXB2, but not that of 6-keto-PGF(1alpha). DP-1904 suppressed the production of TXB2 completely, but by sharp contrast, it dramatically increased the production of 6-keto-PGF(1alpha) from EC and platelets by HA-IgG.

CONCLUSION

The large imbalance of TXA2 and PGI2 produced by the interaction of EC, immune complexes, and platelets may be associated with alterations in glomerular pathological findings and hemodynamics mediated by immune complexes in lupus nephritis. C1q and a TXA2 synthetase inhibitor may improve the abnormal prostanoid metabolism change of lupus nephritis.

Evidence for a sustained increase in clonal beta-cell basal intracellular Ca2+ levels after incubation in the presence of newly diagnosed Type-1 diabetic patient sera. Possible role in serum-induced inhibition of insulin secretion

We have previously reported that newly diagnosed Type-1 diabetic patient sera potently suppressed insulin secretion from a clonal rat pancreatic beta-cell line (BRIN BD11) but did not alter cell viability. Here, we report that apoptosis in BRIN BD11 cells incubated in various sera types (fetal calf serum (FCS), normal human serum and Type-1 diabetic patient) was virtually undetectable. Although low levels of necrosis were detected, these were not significantly different between cells incubated in sera from different sources. ATP levels were reduced by approximately 30% while nitrite production increased twofold from BRIN BD11 cells incubated for 24 h in the presence of Type-1 diabetic patient sera compared with normal human sera. Additionally, ATP levels were reduced by approximately 40% and DNA fragmentation increased by more than 20-fold in BRIN BD11 cells incubated in FCS in the presence of a pro-inflammatory cytokine cocktail (interleukin-1beta, tumour necrosis factor-alpha and interferon-gamma), compared with cells incubated in the absence of cytokines. Nitric oxide production from BRIN BD11 cells was markedly increased (up to 10-fold) irrespective of sera type when the cytokine cocktail was included in the incubation medium. Type-1 diabetic patient sera significantly (P<0.001) raised basal levels of intracellular free Ca(2+ )concentration ([Ca(2+)](i)) in BRIN BD11 cells after a 24-h incubation. The alteration in [Ca(2+)](i) concentration was complement dependent, as removal of the early complement components C1q and C3 resulted in a significant reduction (P<0.01) of sera-induced [Ca(2+)](i )changes. We propose that the mechanism of Type-1 diabetic patient sera-induced inhibition of insulin secretion from clonal beta-cells may involve complement-stimulated elevation of [Ca(2+)](i) which attenuates the nutrient-induced insulin secretory process possibly by desensitizing the cell to further changes in Ca(2+).

Cooperation of C1q receptors and integrins in C1q-mediated endothelial cell adhesion and spreading

The interaction of C1q with endothelial cells elicits a multiplicity of biologic responses. Although these responses are presumed to be mediated by the interaction of C1q with endothelial cell surface proteins, the identity of the participants is not known. In this study we examined the roles of two C1q binding proteins, cC1q-R/calreticulin and gC1q-R/p33, in C1q-mediated adhesion and spreading of human dermal microvascular endothelial cells (HDMVEC). When HDMVEC were cultured in microtiter plate wells coated with concentrations of C1q ranging from 0 to 50 microg/ml, a specific and dose-dependent adhesion and spreading was observed. The extent of adhesion and spreading was similar to the adhesion seen on collagen-coated wells. Spreading (68 +/- 12%) and to a moderate extent adhesion (47 +/- 9%) were inhibited by anti-gC1q-R mAb 60.11. Similar effects were noted with polyclonal anti-cC1q-R but not with control nonimmune IgG. The two Abs had a slight additive effect (75 +/- 13% inhibition) when mixed together in the proportion of 100 microg/ml anti-gC1q-R and 30 microg/ml anti-cC1q-R. More importantly, a 100% inhibition of spreading, but not adhesion, to C1q-coated wells was observed when HDMVEC were cultured in the presence of 30 microM of the peptide GRRGDSP but not GRRGESP. Furthermore, while anti-beta(1) integrin Ab blocked both adhesion and spreading, anti-alpha(5) integrin blocked only spreading and not adhesion. Ag capture ELISA of endothelial cell membrane proteins using polyclonal anti-gC1q-R showed the presence of not only beta(1) and alpha(5) integrins but also CD44. Taken together these results suggest that endothelial cell adhesion and spreading require the cooperation of both C1qRs and beta(1) integrins and possibly other membrane-spanning molecules.

Surfactant protein A differentially regulates IFN-gamma- and LPS-induced nitrite production by rat alveolar macrophages

Although several studies have demonstrated that the pulmonary collectins surfactant protein (SP)-A and SP-D contribute to innate immunity by enhancing pathogen phagocytosis, the role of SP-A and SP-D in regulating production of free radicals and cytokines is controversial. We hypothesized that the state and mechanism of activation of the immune cell influence its response to SP-A. The effects of SP-A and SP-D on production of nitric oxide (NO) and inducible nitric oxide synthase (iNOS) were assessed in isolated rat alveolar macrophages activated with lipopolysaccharide (LPS), interferon gamma (IFN-gamma), or both agonists. SP-A inhibited production of NO and iNOS in macrophages stimulated with smooth LPS, which did not significantly bind SP-A, or rough LPS, which avidly bound SP-A. In contrast, SP-A enhanced production of NO and iNOS in cells stimulated with IFN-gamma or INF-gamma plus LPS. Neither SP-A nor SP-D affected baseline NO production, and SP-D did not significantly affect production of NO in cells stimulated with either LPS or IFN-gamma. These results suggest that SP-A contributes to the lung inflammatory response by exerting differential effects on the responses of immune cells, depending on their state and mechanism of activation.

Intercellular adhesion molecule 1 and beta2 integrins in C1q-stimulated superoxide production by human neutrophils: an example of a general regulatory mechanism governing acute inflammation

OBJECTIVE

To investigate the role of intercellular adhesion molecule 1 (ICAM-1) and beta2 integrins in the production of superoxide (O2-) by C1q-stimulated human polymorphonuclear leukocytes (PMN).

METHODS

PMN were pretreated with F(ab')2 fragments of monoclonal antibodies (mAb) that blocked or did not block beta2 integrin-mediated adhesion. The cells were added to wells coated with C1q, and the production of O2- was monitored kinetically as a color change due to reduction of cytochrome c. In some experiments, C1q was co-immobilized with purified ICAM-1.

RESULTS

Blocking mAb to the shared beta2 integrin subunit, CD18, completely inhibited the O2- response triggered by immobilized C1q, while blocking mAb to the alpha subunits of the beta2 integrins each partially blocked the O2- response. PMN treated with C1q were found to activate the beta2 integrins lymphocyte function-associated antigen 1 and CR3 for binding to ICAM-1. Co-immobilization of ICAM-1 with C1q cooperatively triggered O2- production by PMN.

CONCLUSION

beta2 integrin binding to an ICAM provided an essential costimulatory signal for O2-production triggered by C1q in PMN. Our findings suggest a model for PMN activation in which 2 stimuli are required for O2- production: a first signal that also activates PMN beta2 integrins, followed by a second, beta2 integrin-mediated signal, which occurs physiologically upon PMN binding to ICAM-1. The requirement for this dual signal for PMN generation of O2- would serve as a regulatory mechanism to limit the production of O2- to a tissue environment where C1q, or some other stimulus, is colocalized with stromal cells bearing up-regulated ICAM-1. This mechanism may explain why all tissues can express ICAM-1 and may explain in part why inhibitors of tumor necrosis factor alpha, a major physiologic stimulus of ICAM-1 up-regulation, are potent antiinflammatory agents.

Synthetic peptides which inhibit the interaction between C1q and immunoglobulin and prolong xenograft survival

BACKGROUND

Acute vascular xenograft rejection (AVXR), also termed delayed xenograft rejection (DXR), occurs when hyperacute rejection (HAR) is prevented by strategies directed at xenoreactive natural antibodies and/or complement activation. We have hypothesized that AVXR/DXR is initiated in part by early components of the complement cascade, notably C1q. We have developed synthetic peptides (termed CBP2 and WY) that interfere with the interaction between C1q and antibody.

METHODS

CBP2 and the WY-conjugates were used as inhibitors of immunoglobulin aggregate binding to solid phase C1q. Inhibition of complement activation by the peptides of the classical system was determined using lysis assays with sensitized sheep red blood cells or porcine aortic endothelial cells as targets and of the alternate complement pathway using guinea pig red blood cells as targets. Two transplant models were used to study the effects of administering peptides to recipients: rat heart transplant to presensitized mouse, and guinea heart transplant to PVG C6-deficient rats.

RESULTS

CBP2 and WY-conjugates inhibited immunoglobulin aggregate binding to C1q. The peptides also inhibited human complement-mediated lysis of sensitized sheep red blood cells and porcine aortic endothelial cells in a dose-dependent manner and the WY-conjugates prevented activation of the alternate complement pathway as shown by inhibition of guinea pig red blood cells lysis with human serum. In addition, the use of the peptides and conjugates resulted in significant prolongation of xenograft survival.

CONCLUSIONS

The CBP2 and WY peptides exhibit the functional activity of inhibition of complement activation. These peptides also prolong xenograft survival and thus provide reagents for the study of the importance of C1q and other complement components in transplant rejection mechanisms.

Complement component C1q modulates the phagocytosis of Abeta by microglia

Recent studies showing that microglia internalize the amyloid beta-peptide (Abeta) suggest that these cells have the potential for clearing Abeta deposits in Alzheimer's disease, and mechanisms that regulate the removal of Abeta may therefore be of clinical interest. Previous studies from this laboratory showing that C1q enhances phagocytosis of cellular targets by rat microglia prompted the current investigations characterizing the effects of C1q on microglial phagocytosis of Abeta. Microglia were shown to phagocytose Abeta1-42, in agreement with observations of other investigators. Uptake of Abeta1-42 was observed for concentrations of 5-50 microM, and phagocytosis of peptides containing (14)C or fluorescein (FM) labels was not affected by the interaction of microglia with C1q-coated surfaces. However, inclusion of C1q (125 nM-1.4 microM) in solutions of 50 microM Abeta1-42 inhibited the uptake of (14)C-Abeta1-42 and FM-Abeta1-42, suggesting that C1q blocks the interaction of Abeta with microglia. Uptake of Abeta was partially blocked by the scavenger receptor ligands polyinosinic acid and maleylated BSA. Inhibition of Abeta uptake by C1q may contribute to the accumulation of fibrillar, C1q-containing plaques that occurs in parallel with disease progression. These data suggest that mechanisms which interfere with the binding of C1q to Abeta may be of therapeutic value both through inhibition of the inflammatory events resulting from complement activation and via altered access of Abeta sites necessary for ingestion by microglia.

Digestion of C1q collagen-like domain with MMPs-1,-2,-3, and -9 further defines the sequence involved in the stimulation of neutrophil superoxide production

C1q, a subunit of the first component (C1) of the classical complement pathway, binds to neutrophils via its collagen-like region (C1q-CLR) stimulating superoxide production. We previously identified a region of C1q-CLR, defined by fragments generated by trypsin and endoLys-C digestion, that was required for triggering this respiratory burst. To further localize that critical site, purified human C1q was digested with pepsin to generate C1q-CLR, and subsequently cleaved with the matrix metalloproteinases, MMP-1, MMP-2, MMP-3, and MMP-9. Digestion of C1q-CLR with any of these MMPs did not alter the circular dichroism spectra, demonstrating that the fragments generated had maintained the secondary structure observed in the native molecule. All fragments retained the ability to trigger superoxide production by neutrophils. Analysis of the amino acid sequences of the purified cleavage products (none of which are identical to the published cleavage site specificities for these enzymes) demonstrated that it is the C-chain, but not the A-chain of C1q, that is critical for stimulating this activity, and thus may be a target for future therapeutic intervention.

The mouse C1q A-chain sequence alters beta-amyloid-induced complement activation

In transgenic models of Alzheimer's disease (AD) neuronal loss has not been widely observed. The loss of neurons in AD may be due to chronic activation of complement (C') by beta-amyloid (A beta). A beta has been shown to activate C' by binding to a site on the C1q A-chain. The mouse A-chain sequence differs significantly from human, and a peptide based on the mouse A-chain sequence was ineffective at blocking activation of C' by A beta in contrast to the inhibition seen with the human peptide. Comparison of mouse and human serum showed that human C' was activated more effectively by A beta than was mouse C'. Therefore, additional genetic manipulations may be necessary to replicate in the murine model the inflammation and neurodegeneration that occur in AD.

Comparison of C1q-receptors on rat microglia and peritoneal macrophages

A comparison of the expression and ligand specificity of the C1q (first complement component) receptor on rat microglia and peritoneal macrophages was made. This revealed that radiolabelled C1q was competed from the peritoneal macrophages with intact C1q, and additively displaced by calf-skin collagen and purified C1q globular heads, suggesting the presence of at least two receptors. This was in contrast to microglia, where radiolabelled C1q was displaced with intact C1q and to a modest degree with collagen, but not with globular heads. Taken together, this implies that under these conditions, peritoneal macrophages and microglia both express a C1q receptor which binds to the collagen-like region, and that peritoneal macrophages additionally express a molecule which binds to the globular head of C1q. Analysis of the ligand bound by these cells reflected the differences observed in the competitive binding experiments, with the novel identification of naturally-occurring peptides from the globular head of C1q bound to the peritoneal macrophages, but not the microglia.

Surfactant proteins A and D specifically stimulate directed actin-based responses in alveolar macrophages

Surfactant protein (SP) A and SP-D are the pulmonary members of the collectin family, structurally related proteins involved in innate immune responses. Here, we have examined the abilities of SP-A, SP-D, mannose-binding protein (MBP), and the complement component C1q to stimulate actin-based cellular functions in rat alveolar macrophages and peripheral blood monocytes. Our goal in this study was to examine the cell specificity of the effects of the collectins to understand further the mechanisms by which SP-A and SP-D stimulate alveolar macrophages. We found that SP-A and SP-D have lung cell-specific effects at physiologically relevant concentrations; they stimulate directional actin polymerization and chemotaxis in alveolar macrophages but not in monocytes. Although C1q and MBP weakly stimulate the rearrangement of actin in both cell types, C1q is chemotactic only for peripheral blood monocytes and MBP does not stimulate chemotaxis of either cell type. Neither C1q nor MBP stimulates actin polymerization in alveolar macrophages. These results support the hypothesis that alveolar macrophages express receptors specific for the pulmonary collectins SP-A and SP-D and provide insight into the potential roles of collectins in the recruitment and maturation of mononuclear phagocytes in the lung.

The first subcomponent of complement, C1q, triggers the production of IL-8, IL-6, and monocyte chemoattractant peptide-1 by human umbilical vein endothelial cells

We and others have demonstrated previously the occurrence of cC1qR/CaR, a receptor for the collagen-like stalks of complement component C1q, on endothelial cells. In the present study we investigated whether binding of C1q to endothelial cells resulted in enhancement of cytokine or chemokine production. HUVEC produced 82 +/- 91 pg/ml of IL-8, 79 +/- 113 pg/ml of IL-6, and 503 +/- 221 pg/ml of monocyte chemoattractant peptide-1 (MCP-1) under basal conditions. Incubation with C1q resulted in a time- and dose-dependent up-regulation of IL-8 (1012 +/- 43 pg/ml), IL-6 (392 +/- 20 pg/ml), and MCP-1 (2450 +/- 101 pg/ml). This production is dependent on de novo protein synthesis, as demonstrated by the detection of specific mRNA after C1q stimulation, and inhibition of peptide production in the presence of cycloheximide. The production of all factors was inhibited (69 +/- 7%) by the collagenous fragments of C1q, while the C1q globular heads only induced 13 +/- 11% inhibition. When HUVEC were incubated with C1q in the presence of aggregated IgM, enhanced production of IL-8 (2500 +/- 422 pg/ml), IL-6 (997 +/- 21 pg/ml), and MCP-1 (5343 +/- 302 pg/ml) was found. Furthermore, F(ab')2 anti-calreticulin partially inhibited the production of IL-8, confirming at least the involvement of cC1qR/CaR. These experiments suggest that in an inflammatory response C1q not only is able to activate the complement pathway, but when presented in a proper fashion also might induce the production of factors that contribute to acute phase responses and recruitment of inflammatory cells.

Complement C1q inhibits cellular spreading and stimulates adenylyl cyclase activity of fibroblasts

C1q selectively localizes at injured tissues, where it may function as a regulator of cell-matrix interactions. We show here that purified C1q, added to the culture medium of human gingival fibroblasts (HF) spread onto fibronectin substrates, elicited a round morphology that was accompanied by altered F-actin and correlated with inhibition of cellular spreading. Shape modification required integrity of the molecule and was specific, dose dependent, nontoxic, and reversible. Antispreading activity was mediated, at least in part, by specific cell-surface C1q receptors. We hypothesized that ligand occupancy of C1q receptors could influence shape by affecting intracellular levels of cyclic AMP (cAMP). Within 20 min of exposure of adhering HF to C1q, we detected an increase in adenylyl cyclase activity (six- to ninefold) in cAMP accumulation (by 20%) and in cAMP-dependent protein kinase activity (by 20%). These changes suggested that the rounding effect of C1q may be associated with activation of the adenylyl cyclase pathway.

C1q augments platelet activation in response to aggregated Ig

Immune complexes and aggregated IgG (agg-IgG) induce platelet aggregation and the release reaction. Immune complexes also activate the complement system and interact with the complement component C1q. Since platelets possess both Fc and C1q receptors capable of signal transduction, the present study focused on the interaction between these binding sites and platelet activation. Subaggregating doses of agg-IgG (20-400 microg/ml) were identified for washed platelets from each of 11 healthy donors, and platelet aggregation was monitored in the presence or the absence of increasing concentrations of C1q (5-100 microg/ml). C1q produced a dose-dependent potentiation of platelet alphaIIb/beta3 integrin activation, platelet aggregation, and granule secretion when combined with low doses of agg-IgG. C1q alone was without effect. Maximal enhancement of agg-IgG-induced platelet activation was noted at C1q concentrations ranging from 50 to 100 microg/ml. The observed C1q-induced potentiation of platelet aggregation in response to agg-IgG was blocked by polyclonal antibody F(ab')2 directed against platelet binding sites recognizing the collagen-like domain of C1q (cC1qR) or by mAb Fab (IV.3) directed against platelet FcgammaRII receptors. These data suggest a cooperative interaction between platelet FcgammaRII and cC1q receptors and support a potential role for platelet cC1q receptors in pathologic platelet activation by circulating immune complexes often associated with in vivo thrombosis and thrombocytopenia.

Two parallel routes of the complement-mediated antibody-dependent enhancement of HIV-1 infection

OBJECTIVE

To study the mechanism of the complement-mediated antibody-dependent enhancement (C'-ADE) of HIV infection which may play a significant role in the progression of HIV-disease.

METHODS

In vitro complement activating and complement-mediated HIV-infection enhancing abilities of three human anti-gp41 monoclonal antibodies (MAb) were tested. C'-ADE was estimated using HIV-1IIIB and CR2 (CD21)-carrying MT-4 target cells. Normal human serum (NHS), purified C1q, C1q-deficient (C1qD) and C2-deficient (C2D) human sera were applied as complement sources.

RESULTS

All MAb mediated increased C1q binding to solid-phase gp41. All MAb had a marked dose-dependent and strictly complement-mediated HIV-infection enhancing effect. Mixtures of the MAb with purified C1q also significantly increased HIV-1 infection. C1qD serum had a markedly lower enhancing effect than NHS, which could be raised to normal level by addition of purified C1q. Pretreatment of the target cells with anti-CR2 antibodies only partially inhibited the enhancing effect of the MAb plus normal human serum.

CONCLUSION

These novel findings indicate that besides the well-known facilitation of entry of HIV-1 by the interaction between virus-bound C3 fragments and CR2 present on the target cells, fixation of C1q to intact virions also results in an enhanced productive HIV-1 infection in the MT-4 cell cultures.

Release of calreticulin from neutrophils may alter C1q-mediated immune functions

Calreticulin is an abundant intracellular protein which is involved in a number of cellular functions. During cytomegalovirus infection, as well as inflammatory episodes in autoimmune disease, calreticulin can be released from cells and detected in the circulation, where it may act as an immunodominant autoantigen in diseases such as systemic lupus erythematosus. Calreticulin is known to bind to the molecules of innate immunity, such as C1q, the first subcomponent of complement. However, the functional implications of C1q-calreticulin interactions are unknown. In the present study we sought to investigate, in greater detail, the interaction between these two proteins following the release of calreticulin from neutrophils upon stimulation. In order to pinpoint the regions of interaction, recombinant calreticulin and its discrete domains (N-, P- and C-domains) were produced in Escherichia coli. Both the N- and P-domains of calreticulin were shown to bind to the globular head regions of C1q. Calreticulin also appeared to alter C1q-mediated immune functions. Binding of calreticulin to C1q inhibited haemolysis of IgM-sensitized erythrocytes. Both the N- and P-domains of calreticulin were found to contain sites involved in the inhibition of C1q-induced haemolysis. Full-length calreticulin, and its N- and P-domains, were also able to reduce the C1q-dependent binding of immune complexes to neutrophils. We conclude that calreticulin, once released from neutrophils during inflammation, may not only induce an antigenic reaction, but, under defined conditions, may also interfere with C1q-mediated inflammatory processes.

Phagocytosis of viable Candida albicans by alveolar macrophages: lack of opsonin function of surfactant protein A

Surfactant protein A (SP-A) contributes to host defense by opsonizing microbial organisms for phagocytosis by alveolar macrophages (AM). The role of SP-A as opsonin for phagocytosis of Candida albicans was analyzed. AM in suspension exhibited no phagocytosis of nonopsonized yeast. This was not increased by SP-A, whether provided for preincubation of AM or yeast or present during coincubation. However, the engulfment of serum-opsonized yeast by AM in suspension was inhibited by SP-A. This inhibitory effect was mimicked by complement subcomponent C1q and concanavalin A but not by type IV collagen. SP-A did not interfere with phagocytosis of serum-opsonized yeast by adherent AM, monocytes, neutrophils, or peritoneal macrophages. SP-A lacks function as an opsonin for the phagocytosis of C. albicans by AM but interferes with binding of yeast to AM, inhibiting subsequent ingestion. The role of SP-A as an alveolar space opsonin may thus critically depend on the microbial species involved.

Human C1q induces eosinophil migration

Eosinophils (Eo) play a significant role in allergic inflammation and the host's immunity to parasitic infections. Although the presence of C1q-binding cell surface molecule(s) (C1q-R) on Eo had been previously implicated by the ability of C1q to augment IgG-dependent, Eo-mediated killing of schistosomula, little is known about the structure or the function of this receptor. The present studies were therefore undertaken to immunochemically demonstrate and to examine the biology of Eo C1q-R. Eo were purified to homogeneity (>90%) and viability (>98%) from hypereosinophilic donors by Percoll density gradient. Western blot analysis using antibodies to cC1q-R and gC1q-R showed distinct bands corresponding to cC1q-R (60 kDa) and gC1q-R (33 kDa) when immunoblotted with their respective antibodies. The Eo C1q-R was tested for its ability to induce chemokinesis and/or chemotaxis as assessed by the modified Boyden microchamber assay utilizing 5-micrometer-pore polycarbonate membranes and using C1q, cC1q, or gC1q (10 micrograms/ml) as agonists. The known chemotactic factors C5a and RANTES (10(-8)M) were used as positive controls. The results showed that at this concentration, cC1q was most efficient in its ability to induce Eo migration (20 +/- SEM 12, n = 4) followed by C1q (107 +/- SEM 7, n=7) and gC1q (77 +/- SEM 10, n = 10). When checkerboard analysis was performed, the data indicated that the observed phenomenon was likely to be due largely to chemokinesis. As expected, C5a (145 +/- SEM 15, n = 7) and RANTES (145 +/- SEM 43, n = 7) were both chemotactic. Furthermore, incubation of Eo with 50 micrograms of either C1q, gC1q, or cC1q (1 hr, 37 degrees C) did not cause release of eosinophil cationic protein as measured by RIA, nor did it enhance the expression of CD11b or CD29 as assessed by FACS analysis. The data presented in this paper show that Eo express both cC1q-R and gC1q-R and may participate in Eo function by providing a primary signal for locomotion.

Characterization of the binding of serum amyloid P to type IV collagen

Serum amyloid P (SAP), a member of the evolutionarily conserved pentraxin family, is a normal component of a number of basement membranes, including glomerular and alveolar. In vitro SAP binds to a variety of proteins including fibronectin, proteoglycans, and the collagen-like region of the complement component C1q. In these studies, binding of SAP to type IV collagen, a major component of basement membrane, was examined. Purified SAP binds to human and mouse type IV collagen but not type I, II, or III collagens. Binding of SAP to type IV collagen is dependent on the presence of Ca2+. This binding is saturable with a Kd approximately 1.2 x 10(-7) M based on solid phase binding and 4 x 10(-8) M based on the IC50 value from fluid phase binding data. Binding of SAP to type IV collagen was inhibited by both SAP and C-reactive protein (CRP). However, a 5-fold molar excess of CRP as compared with SAP was required to inhibit the SAP binding by 50%. Binding of SAP to type IV collagen was inhibited by both collagen IV and C1q but not by phosphatidylethanolamine or bovine serum albumin. The inhibition data indicate that SAP may bind to the triple helical region of type IV collagen via a site distinct from its galactan binding site. The most likely site of SAP involved in its interaction with type IV collagen may be the region spanning amino acid residues 108-120, which shows a great deal of sequence homology (60% strict identity) with the CRP region implicated in its binding to the collagen-like region of the C1q molecule.

Surfactant protein A stimulates phagocytosis of specific pulmonary pathogens by alveolar macrophages

Surfactant protein A (SP-A) regulates alveolar macrophage function and has been implicated in the mediation of pulmonary host defense. Our goals were to characterize the interaction of SP-A with various pulmonary pathogens, to investigate the mechanism of SP-A-mediated phagocytosis using an assay that distinguishes bound from internalized bacteria by quenching the fluorescence of extracellular bacteria, and to examine further the interactions of SP-A and the structurally homologous protein complement component 1q (C1q) with alveolar macrophages and peripheral blood monocytes. We found that SP-A binds to and increases the phagocytosis of Haemophilus influenzae, Streptococcus pneumoniae, and Group A Streptococcus; SP-A aggregates only H. influenzae. SP-A neither binds to, aggregates, nor stimulates the phagocytosis of Pseudomonas aeruginosa. We have also found that bronchoalveolar lavage stimulates phagocytosis and that this stimulation is reduced by an anti-SP-A antibody. While the enhancement of phagocytosis by SP-A is inhibited in blood monocytes adhered to C1q-coated surfaces, which presumably clusters the C1q receptor on the basal surface of the cell, alveolar macrophages on C1q-coated slides show no significant change in their response to SP-A. In summary, SP-A stimulates the phagocytosis by alveolar macrophages of specific pulmonary pathogens to which it binds, but aggregation is not required for the effect. Additionally, the role of the C1q receptor in the response to SP-A may differ between monocytes and alveolar macrophages.

Murine mast cells express two types of C1q receptors that are involved in the induction of chemotaxis and chemokinesis

Although previous studies have shown that different cells and cell lines of murine origin bind human C1q, suggesting that they display cell surface receptors for C1q, no information is available to indicate whether mouse or human mast cells express C1q receptors. This paper presents the first evidence to show that murine mast cells express specific receptors for C1q. Western blot analysis of cell membrane proteins prepared from a bone marrow-derived mouse cell line using two monospecific polyclonal Abs, one directed against the 60-kDa C1q receptor (C1q-R) that binds to the collagen-like stalk of C1q (cC1q-R) and the other directed against the 33-kDa molecule that binds to the globular "heads" of C1q (gC1q-R), show that both of these receptors are present on these cells. In addition, C1q can induce mast cell migration in a specific and dose-dependent manner. Interestingly, the C1q-induced migratory response was found to be biphasic; the first response peaked at a C1q concentration of 0.1 nM, whereas the second phase peaked at approximately 40 nM. Checkerboard analysis of the mast cell migratory response to C1q showed that the first phase was primarily due to chemotaxis and the second phase was attributable to chemokinesis. Preincubation of C1q with Abs specific for the collagen-like tail of the molecule abolished both its chemotactic and chemokinetic response, whereas heat inactivation of C1q (56 degrees C, 1 h) resulted in 85% abrogation of the chemotactic phase and 42% reduction in the chemokinetic phase. The observed mast cell migratory responses were mediated by cell surface C1q-R(s), as inclusion of a mixture of anti-C1q-R and anti-gC1q-R Abs with the cells inhibited their migratory response toward C1q. However, incubation of cells with various doses of C1q did not result in histamine release. Furthermore, engagement of mast cell C1q-Rs by the ligand C1q induced an antiproliferative response, as coculturing of mast cells with C1q resulted in a specific and dose-dependent decrease in DNA synthesis. These data suggest that C1q-Rs may play a significant role in mast cell function and regulation by providing an important signal through which mast cells can be recruited to inflammatory sites of increased C1q concentration.

C1q triggers neutrophil superoxide production by a unique CD18-dependent mechanism

Complement protein C1q induces the production of superoxide (O2-) by neutrophils via an as yet unidentified receptor or receptor complex. Several strategies were therefore used to identify cell surface molecules involved in the response of neutrophils to C1q and its collagen-like domain (C1q-CLR). Treatment of neutrophils with phosphatidylinositol-specific phospholipase C effectively removed the phosphatidylinositol-linked surface molecules CD14 and CD16, yet did not reduce O2- production in response to C1q. Next, 17 monoclonal antibodies (mAbs) recognizing various neutrophil surface antigens were tested for their ability to inhibit C1q-CLR-mediated O2- production. Only two of the mAbs, 44a and IB4, which recognize CD11b/CD18 (complement receptor 3 or Mac-1), were inhibitory. In addition, neutrophils from a patient with leukocyte adhesion deficiency, which are CD18 deficient, did not produce O2- in response to C1q or C1q-CLR. Because CD11b/CD18 is recognized to play a role in cell adhesion, the role of adherence in C1q-mediated O2- production was explored. Adherence of neutrophils to C1q-CLR-coated surfaces occurred with kinetics, which usually paralleled those of O2- production, and was invariably abolished by the anti-CD11b mAb 44a. However, this mAb often only partially inhibited O2- production, indicating that an avid attachment of neutrophils to the C1q-CLR-coated surface is not required for O2- production.

Characterization of the human neutrophil C1q receptor and functional effects of free ligand on activated neutrophils

The partial characterization and expression of the C1q receptor (C1q-R) in relation to other complement receptors present on the surface of neutrophils has been examined, as well as the effects of free C1q on cell function. A polyclonal anti-C1q-R antibody recognizes a 68-kD neutrophil surface protein. C1q-R expression was not upregulated upon warming, priming, or exposure to FMLP, but decreased after exposure to phorbol myristate acetate (PMA), because of shedding of the receptor into the extracellular medium, as detected by enzyme-linked immunosorbent assay. CR3 and CR1 expression was upregulated from intracellular pools after cell stimulation by PMA. No evidence of intracellular pools of C1q-R was found, as assessed by immunoblotting of subcellular fractions. But C1q-R appeared to be expressed early in cell differentiation, was detected on undifferentiated HL-60 cells, and like CR3 expression, increased upon 5 days differentiation towards a neutrophil lineage. However, C1q-R expression decreased upon additional culture, whereas CR3 expression continued to increase. A large variation in the percentage of peripheral cells expressing C1q receptors in donors was observed, ranging from 13% to 100%, contrasting with CR3 receptors that exhibited less variability. Interactions between free monomeric C1q and neutrophils were also studied. Incubation of stimulated neutrophils with 10 to 100 micrograms/mL C1q resulted in a further increase in CR3 expression and adherence to albumin-coated surfaces. Staphylococci opsonized with low quantities of C1q (0.1 to 1 microgram/mL) mediated a moderate and sustained respiratory burst in neutrophils, whereas a burst of similar magnitude was generated only with free C1q at concentrations 10- to 100-fold higher. Stimulation was only partially inhibited if cells were first treated with anti-C1q-R antibody, suggesting other C1q binding proteins may be present on the cell surface. In summary, neutrophil C1q receptor is approximately 68-kD, exhibits varying expression on different subjects, and is not upregulated from intracellular stores on exposure to soluble stimuli. Stimulated, but not resting, neutrophils selectively respond to raised levels of free C1q, resulting in altered cell function and enhanced CR3 receptor expression. These studies thus suggest complex roles for C1q in neutrophil function.

The effects of histidine residue modification on the immune precipitating ability of rabbit IgG

Treatment of anti-ovalbumin rabbit IgG with diethylpyrocarbonate (DEPC) at concentrations up to 100 microM led to a progressive decrease in the rates of formation of insoluble immune complexes, without affecting the final extent of immune complex formation. DEPC concentrations approximately 10-fold higher were needed to give comparable decreases in the rates of immune complex formation by F(ab')2. Treatment of DEPC-treated IgG with hydroxylamine led to substantial restoration of the rates of formation of insoluble immune complexes. Carbethoxylation of two histidine residues per IgG molecule had little effect on rates of formation of insoluble immune complexes, but these rates were markedly decreased in samples of IgG with four to five histidines per molecule modified. There were parallel decreases in the protein A-binding activity and in the rates of formation of insoluble immune complexes in IgG treated with increasing concentrations of DEPC. The presence of complement protein C1q restored the rates of formation of insoluble immune complexes of DEPC-treated IgG.

Up-regulation of the MHC class II molecules on B cells by peptide ligands

MHC class I and class II molecules present peptide Ag to T lymphocytes. Peptides are critical in class I heavy chain folding and/or stable association with beta 2m. A recent study suggests the role of peptide Ag binding for MHC class II alpha- and beta-chain heterodimers to enter into a compact state and allow their transport to the cell surface. We have investigated the effect of peptide ligands on the expression of MHC class II I-A(d) molecules on the B cell hybridoma, TA3. These cells, when cultured in vitro, gradually lost the surface expression of I-A(d) molecules. Incubation with peptides, having high affinity for binding to intact I-A(d) molecules, significantly increased the surface expression of I-A(d) in less than 24 h. The ability of peptides to induce increased expression of I-A(d) correlated with the affinity of peptide to intact I-A(d), and an I-Ak-restricted peptide did not have an effect on I-A(d) expression. The effect could be reversed after the removal of the peptide Ag. Based on our studies with inhibitors of protein synthesis and intracellular transport, the mechanism for up-regulation of I-A(d) expression by peptides seems to involve intracellular pathway but appears not to require new protein synthesis or transport from ER. Our results suggest that the decrease in surface expression of I-A(d) on TA3 cells may result from their failure to generate or to be saturated with naturally processed peptide ligands. Thus peptide ligands are evidently important in regulating surface expression of MHC class II molecules and their recognition by T lymphocytes.

Signal transduction mechanisms of C1q-mediated superoxide production. Evidence for the involvement of temporally distinct staurosporine-insensitive and sensitive pathways

C1q, a plasma glycoprotein and the recognition component of the classical complement pathway, interacts with specific cells of the immune system resulting in the enhancement of cell function. For example, interaction of C1q with its cell-surface receptor on neutrophils induces the activation of the respiratory burst, a finding previously documented using a chemiluminescent assay to detect oxygen radical formation. In an alternative approach we have now used a modified cytochrome c reduction assay to characterize C1q-mediated production of superoxide anion (O2-) in more detail. C1q coated to microtiter wells induced O2- release, which occurred microtiter wells induced O2- release, which occurred after a lag period of 10 to 20 min, and was then sustained over approximately 1 h. O2- production could be triggered by the purified pepsin-resistant, collagen-like fragment of C1q, but not by mannose-binding protein and pulmonary surfactant protein A, proteins that also contain collagen-like domains. Concentrations of C1q which promoted a vigorous O2- generation did not induce release of neutrophil primary granules and caused little or no secondary granule release. Investigation of the biochemical events mediating C1q stimulated O2- production by neutrophils revealed that the response invoked two biochemical pathways with distinct sensitivities to previously described inhibitors. A role for Ca2+ in initiation of the response was suggested by the inhibitory effect of EGTA, the calmodulin antagonist W7, and the intracellular Ca2+ chelator BAPTA. The protein kinase inhibitor staurosporine did not inhibit the induction of the response, but did block that component of the response occurring after approximately 30 min. Neither phase of C1q-mediated O2- production was inhibited by pertussis toxin, a strong inhibitor of the G-protein-coupled FMLP-mediated response. In summary, C1q-triggered O2- production is relatively unique both in terms of the kinetics of the response and the biochemical pathways evoked. These data support the hypothesis that more than one biochemical pathway induced by ligand-receptor interaction can activate the neutrophil NADPH oxidase.

Interaction of C1q receptor with lung surfactant protein A

Earlier we reported the purification of C1q receptor (C1qR) from U937 cells and human tonsil lymphocytes (Malhotra, R. and Sim, R. B., Biochem. J. 1989. 218: 625) and showed that C1qR interacts with the ligands C1q, mannose-binding protein, conglutinin and lung surfactant protein A (SP-A) (Malhotra, R., Thiel, S., Reid, K. B. M. and Sim, R. B., J. Exp. Med. 1990. 172: 955). C1qR was characterized as an acidic glycoprotein, which, when solubilized, exists as a dimer of Mr 115,000 under non-denaturing conditions. In this article we provide evidence for binding of radioiodinated SP-A to U937 cells and show that binding of radioiodinated SP-A to U937 cells is specific, saturable, salt dependent and is inhibited by purified C1qR and by C1q. The interaction of SP-A with U937 cells was found to up-regulate the surface expression of C1qR. Incubation of SP-A with U937 cells at 37 degrees C for 80 min was found to increase the receptor number per cell. Increase in receptor number was inhibited in the presence of sodium azide and monensin. Incubation of cells with calcium ionophore A23187 induced increased surface expression in the absence of SP-A. The results indicate that interaction of SP-A with U937 cells triggers the expression of an intracellular pool of C1qR.

Reconstitution of a deficiency of AKR mouse macrophages for their response to lipid A activation for tumor cytotoxicity by complement subcomponent C1q: role of IFN-gamma

C5-deficient AKR mouse macrophages were initially found to be refractory to activation by lipid A to mediate tumor cytotoxicity for P815 mastocytoma or L1210 mouse leukemia targets as compared with responsive C3H mouse macrophages. The lower level of tumor cytotoxicity by lipid A-activated AKR macrophages correlated with lower levels of cytotoxic nitric oxide generation as measured by nitrite end product accumulation. The refractory state of AKR macrophages was unexpectedly found to be independent of their C5 deficiency in that IFN-gamma reconstituted their response to activation by lipid A coincident with an increase in C1q mRNA synthesis. AKR macrophages were augmented in their lipid A activation by exogenous soluble C1q in the absence of IFN-gamma, which corresponded with an increased production of nitric oxide by C1q-reconstituted macrophages. In contrast, responsive C3H mouse macrophages with sufficient levels of C1q synthesis were inhibited by exogenous soluble monomeric C1q in their lipid A activation. Both AKR and C3H macrophages plated over immobilized C1q were inhibited in their lipid A activation for tumor cytotoxicity and nitric oxide generation. Our results provide evidence that C1q modulates macrophage activation by lipid A for nitric oxide-mediated tumor cytotoxicity under the influence of IFN-gamma, which stimulates C1q synthesis and secretion. These findings strongly suggest that macrophage synthesis of C1q, but not C5, is a prerequisite for their activation by lipid A.

Surfactant protein A is opsonin in phagocytosis of herpes simplex virus type 1 by rat alveolar macrophages

In the present study we used flow cytometry to investigate the phagocytosis of fluorescein isothiocyanate-labeled herpes simplex virus type 1 (FITC-HSV-1) by rat alveolar macrophages and the effects of surfactant protein A (SP-A) on this process. The phagocytosis of FITC-HSV-1 by alveolar macrophages, which was studied as a model for virus phagocytosis in general, was strongly enhanced in the presence of SP-A. The SP-A-mediated phagocytosis was time and concentration dependent, reaching a maximal level after 15 min of incubation and at an SP-A concentration of 5 micrograms/ml. Using a fluorescence quenching technique, we could show that at least 65% of the viruses were indeed internalized by the macrophages. The addition of SP-A to the system was sufficient for the phagocytosis of FITC-HSV-1 by the alveolar macrophages, suggesting that SP-A acts as an opsonin. This hypothesis was further strengthened by the observation that F(ab')2 fragments of immunoglobulin G directed against SP-A could abolish FITC-HSV-1 phagocytosis by alveolar macrophages preincubated with SP-A. Comparing the opsonic capacity of serum and SP-A, SP-A proved to be twice as potent as serum in stimulating phagocytosis of FITC-HSV-1 by alveolar macrophages. Complement factor C1q, which is known to possess a similar collagen-like domain as SP-A, did not stimulate phagocytosis of FITC-HSV-1 by alveolar macrophages nor did it inhibit SP-A-mediated HSV-1 phagocytosis. This study demonstrates that SP-A may play an important role in the antiviral defenses of the lung.

Complement component C1q and its receptor are involved in the interaction of human sperm with zona-free hamster eggs

C1q is a component of the classical complement pathway that can react with the Fc-fragment of immunoglobulins and with other proteins, such as fibronectin, laminin, and a specific C1q receptor present on several cell types. Given its role in many adhesion systems, mainly related to phagocytosis, we tested the effects of C1q on the interaction between human spermatozoa and zona-free hamster eggs. The presence of C1q in the medium used for gamete coincubation resulted in promotion of sperm-oolemma adhesion and an inhibition of penetration. The number of adherent sperm per egg at 5 micrograms/ml concentration was 90 +/- 35 vs. 29 +/- 7 for the control (P less than 0.001). At 1 microgram/ml, the lower concentration at which C1q had an effect, the number of penetrating sperm/egg was 0.6 vs. 1.7 for the control without C1q (P less than 0.01), and the percent of penetrated eggs was 28% vs. 85%. At 50 micrograms/ml, the percent of penetrated eggs was 7%, with a penetration index of 0.07. The addition of C1q to the medium resulted in sperm agglutination, which varied between sperm donors. The presence of C1q receptors, as detected by anti-C1qR monoclonal antibodies (Mabs), was demonstrated both on zona-free hamster eggs by immunobead rosetting and on human spermatozoa by immunobead binding and indirect immunofluorescence. Mabs directed against different epitopes of C1qR had different effects on gamete interaction, with a partial inhibition of penetration mediated by some of them. The binding of C1q to antibody-free human spermatozoa was also demonstrated both by means of indirect immunofluorescence and utilizing 125I-C1q.(ABSTRACT TRUNCATED AT 250 WORDS)