Anaphylatoxin C3a but not C3a(desArg) is a chemotaxin for the mouse macrophage cell line J774

Varying results have been published regarding the functional reactivity of different cell types, including human monocytes, to the anaphylatoxin C3a and its degradation product C3a(desArg). To further delineate the functions of C3a and C3a(desArg) on this cell type we used the murine macrophage (Mø) cell line J774A.1 which is known to respond to the anaphylatoxin C5a. J774 cells specifically bound fluorescein isothiocyanate-labeled recombinant human C3a (rC3a). The cells migrated along rC3a concentration gradients in a dose-dependent manner with an optimal concentration of about 3 nM (rC5a: 7 nM) and a half-maximal effective concentration (EC₅₀) of about 1.2 nM (rC5a: 2 nM). The degradation product rC3a(desArg) was devoid of chemotactic activity. mRNA for the recently cloned G protein-coupled mouse high-affinity C3a receptor (C3aR) was detected in J774 cells, suggesting that this receptor represents the binding site for C3a on J774 Mø. In support of the specific nature of C3a-stimulated cellular mobility, RBL-2H3 transfectants expressing the human C3aR were also shown to migrate along gradients of rC3a (optimal concentration about 8 nM; EC₅₀ about 3.5 nM) whereas rC3a(desArg) was again inactive. In summary, our findings demonstrate for the first time a specific, receptor-mediated chemoattraction of cells of the monocytic lineage to the anaphylatoxin C3a which may contribute to the accumulation of Mø at sites of inflammation.     

血液净化材料在体外静态接触血浆时血浆中 C3a des Arg生成的研究

用放射免疫分析法研究了五种血液净化材料：磺化聚醚砜,聚醚砜,聚砜,聚甲基丙烯酸甲酯和醋酸纤维素与人血浆在37℃温育30、60、90、120min后,血浆中补体C3激活产物C3a des Arg的生成情况。结果发现：醋酸纤维素、聚甲基丙烯酸甲酯、聚醚砜、聚砜在与血浆温育后血浆中C3a des Arg的浓度依次降低;以醋酸纤维激活补体C3的情况最严重;磺化聚醚砜的最低,并且磺化度越大,C3a des Arg的浓度越小。说明聚醚砜材料为血液净化材料有较优异的补体相容性,并且通过磺化可大大提高聚醚砜的补体相容性。放免分析法检测材料激活补体C3是一种方便可靠的检测手段,可作为开发生物材料时对有应用前景的生物材料的筛选和评价。     

C3a and Its Receptor C3aR Are Detectable in Normal Human Epidermal Keratinocytes and Are Differentially Regulated via TLR3 and LL37

To study the molecular interplay between TLRs and complement representing ancient danger-sensing mechanisms, we examined the regulation of the C3a/anaphylatoxin C3a receptor (C3aR) axis in normal human epidermal keratinocytes (NHEKs) by treatment with different TLR ligands. Protein staining followed by flow cytometry revealed highly constitutive intracellular expression levels of the C3aR in NHEKs. Stimulation with Poly I:C up-regulated C3aR mRNA and intra- and extracellular expression in NHEKs which showed functional relevance by up-regulating CXCL10 and down-regulating C3 expression in response to C3a. mRNA and protein levels of C3 and protease cathepsin L (CTSL) that can cleave C3 were up-regulated by the TLR3 ligand Poly I:C. Enhanced intracellular expression levels of the biologically active C3 fragment (C3a), in response to TLR3 stimulation were also detectable in NHEKs. Cathelicidin antimicrobial peptide LL-37 potentiated Poly I:C-induced C3aR, C3, and CTSL up-regulation. In conclusion, we point to a role of TLR3 to promote up-regulation of C3aR, C3, and CTSL expression levels and generation of C3a. Our data provide evidence that local generation and activation of complement components as described for T cells or myeloid cells represent a scenario which may take place in a similar way in NHEKs.     

C3a(desArg) does not bind to and signal through the human C3a receptor

Contradictory results have been published in the past regarding the functional responses of different cell types to the anaphylatoxin C3a and its natural catabolite C3a(desArg). To elucidate the interaction of the C3a receptor (C3aR) with its ligand(s) we studied the binding of human recombinant C3a (rC3a) and rC3a(desArg) to RBL-2H3 transfectants which express the C3aR. As the addition of 11 aminoterminal amino acids did not alter the functional activity of the recombinant C3a as compared to serum-derived C3a the specific binding of rC3a and rC3a(desArg) to the transfectants could be determined by flow cytometry using a monoclonal antibody (mab) against their N-terminal histidine tag. Recombinant C3a bound to the C3aR with a half maximal concentration of about 3 nM whereas no evidence for a binding of rC3a(desArg) could be obtained. Furthermore, rC3a(desArg) did not signal through the C3aR. Neither the release of lysosomal N-acetyl-beta-D-glucosaminidase nor the directional migration of C3aR-expressing RBL-2H3 transfectants could be detected in response to rC3a(desArg) whereas rC3a was highly active in both assays. Our data demonstrate a defined ligand specificity of the C3aR for the anaphylatoxin C3a. Its natural catabolite C3a(desArg), however, does not signal through the C3aR. Modulating effects of C3a(desArg) on the synthesis of cytokines in human monocytes and B lymphocytes may therefore be induced by receptor-independent mechanisms while their in vivo relevance remains as yet undefined.     

The C terminus of the human C5a receptor (CD88) is required for normal ligand-dependent receptor internalization

The biological effects of the potent inflammatory mediator C5a, a complement split product, on human neutrophils and monocytes are limited by the rapid internalization of its specific receptor (C5aR, CD88). The C terminus of the C5aR is phosphorylated after stimulation with C5a of phorbol ester, and this phosphorylation might lead to receptor internalization. In this context, we have studied the effects on C5aR internalization of C5a, phorbol 12-myristate 13-acetate (PMA), the protein kinase inhibitor staurosporine, and pertussis toxin on rat basophilic RBL.2H3 cells stably transfected with the human wild-type or mutant C5aR. C5aR mutants lacked either part of the cytosolic C terminus, including suggested major phosphorylation sites, or a putative phosphorylation motif for protein kinase C in the third cytosolic loop. Additionally, agonist-induced internalization was analyzed on HEK293 cells co-transfected with C5aR and the pertussis toxin-resistant G protein alpha subunit, Gα16. Staurosporine-sensitive agonist-dependent C5aR internalization could be detected, suggesting that C5aR phosphorylation, most likely of the C terminus, participates in this type of internalization. In contrast, PMA-induced C5aR internalization seems to be independent of putative phosphorylation sites in either the truncated section of the C terminus or the third cytosolic loop. The phorbol ester-induced C5aR internalization may, therefore, be caused by an indirect and less specific effect of protein kinase C on the internalization machinery. Manipulation of the pertussis toxin-sensitive or -resistant G protein-dependent signal transduction had no effect on ligand-induced internalization.     

The Human Mast Cell Line HMC-1 Binds and Responds to C3a but not C3a(desArg)

Controversial results have been published in the past regarding the functional reactivity of different cell types to the anaphylatoxin C3a and its degradation product C3a(desArg). To understand better the effects of C3a and C3a(desArg) on human mast cells, the authors performed binding experiments and calcium mobilization studies on the human mast cell line HMC-1 which has been shown previously to express C3a binding sites. For this purpose, functionally active, recombinant C3a (rC3a) was constructed with an 11 amino acid peptide attached to the N-terminus of the molecule. Using a monoclonal antibody (MoAb) against this tag, binding of rC3a to HMC-1 cells could be demonstrated by flow cytometry. Its binding was specific as it could be blocked with serum-derived C3a. In contrast, no binding of rC3a(desArg) to HMC-1 cells was detectable. Recombinant C3a led to a transient mobilization of intracellular calcium [Ca²⁺]_i in HMC-1 which was inhibitable by the C3a-specific MoAb K13/16. No increase of [Ca²⁺]_i was detected when the cells were treated with C3a(desArg). The authors found C3a receptor (C3aR)-specific mRNA in HMC-1 cells indicating that this receptor represents the binding site for C3a on these cells. These results demonstrate a specific binding for C3a but not for C3a(desArg) on cells of the human mast cell line HMC-1. As a consequence, functional activity was restricted to C3a with C3a(desArg) being completely inactive. Therefore, the data strongly suggest that the recently cloned high affinity C3aR which is assumed to represent the binding site for the anaphylatoxin on HMC-1 cells is unresponsive to C3a(desArg).     

Targeting C3a/C5a receptors inhibits human mesangial cell proliferation and alleviates immunoglobulin A nephropathy in mice

Complement activation has a deep pathogenic influence in immunoglobulin (Ig)A nephropathy (IgAN). C3a and C5a, small cleavage fragments generated by complement activation, are key mediators of inflammation. The fragments exert broad proinflammatory effects by binding to specific receptors (C3aR and C5aR, respectively). However, no studies thus far have investigated the effects of C3a, C5a and their receptors on IgAN. We observed that C3aR and C5aR antagonists repressed IgA‐induced cell proliferation and interleukin (IL)‐6 and monocyte chemotactic protein 1 (MCP‐1) production in cultured human mesangial cells (HMCs). Furthermore, an IgAN mouse model induced by Sendai virus infection was employed to investigate the effects of C3aR and C5aR on IgAN in vivo for the first time. Wild‐type (WT) and several knock‐out mouse strains (C3aR^–/– or C5aR^–/–) were immunized intranasally with increasing doses of inactivated virus for 14 weeks and were subjected to two intravenous viral challenges during the time‐period indicated. In the Sendai virus‐induced IgAN model, C3aR/C5aR‐deficient mice had significantly reduced proteinuria, lower renal IgA and C3 deposition, less histological damage and reduced mesangial proliferation compared with WT mice. Both C3aR deficiency and C5aR deficiency, especially C3aR deficiency, inhibited renal tumour necrosis factor (TNF)‐α, transforming growth factor (TGF)‐β, IL‐1β, IL‐6 and MCP‐1 expression significantly. However, C3aR/C5aR‐deficient and WT mice with IgAN did not differ with respect to their blood urea nitrogen (BUN) and serum creatinine levels. Our findings provide further support for the idea that C3aR and C5aR are crucially important in IgAN, and suggest that pharmaceutically targeting C3aR/C5aR may hold promise for the treatment of IgAN.     

Stringent regulation of complement lectin pathway C3/C5 convertase by C4b-binding protein (C4BP)

The complement lectin pathway, an essential component of the innate immune system, is geared for rapid recognition of infections as each C4b deposited via this pathway is capable of forming a C3/C5 convertase. In the present study, role of C4b-binding protein (C4BP) in regulating the lectin pathway C3/C5 convertase assembled on zymosan and sheep erythrocytes coated with mannan (E_Man) was examined. While the C4BP concentration for inhibiting 50% (IC₅₀) formation of surface-bound C3 convertase on the two surfaces was similar to that obtained for the soluble C3 convertase (1.05 nM), 3- and 41-fold more was required to inhibit assembly of the C5 convertase on zymosan (2.81 nM) and E_Man (42.66 nM). No difference in binding interactions between C4BP and surface-bound C4b alone or in complex with C3b was observed. Increasing the C4b density on zymosan (14,000–431,000 C4b/Zym) increased the number of C4b bound per C4BP from 2.87 to 8.23 indicating that at high C4b density all seven α-chains of C4BP are engaged in C4b-binding. In contrast, the number of C4b bound per C4BP remained constant (3.79 ± 0.60) when the C4b density on E_Man was increased. The data also show that C4BP regulates assembly and decay of the lectin pathway C3/C5 convertase more stringently than the classical pathway C3/C5 convertase because of a 7- to 13-fold greater affinity for C4b deposited via the lectin pathway than the classical pathway. C4BP thus regulates efficiently the four times greater potential of the lectin pathway than the classical pathway in generating the C3/C5 convertase and hence production of pro-inflammatory products, which are required to fight infections but occasionally cause pathological inflammatory reactions.     

Inhibition of cleavage of the third component of human complement (C3) by its small cleavage fragment, C3a: inhibition occurs with the classical-pathway, but not the alternative-pathway, C3 convertase

Activation of the third component of complement (C), C3, is central to the functioning of the C system in inflammation. Cleavage of C3 by the C3 convertases of both the classical and alternative pathways results in the formation of two split products, C3b and C3a. C3a inhibited cleavage of C3 by the classical-pathway C3 convertase. The inhibition varied in a concn-dependent relationship, with a concn of approximately 40 micrograms/ml yielding 50% inhibition. Removal of the carboxy terminal arginine from the C3a did not alter the inhibition. C3a did not inhibit cleavage of C3 by the alternative C pathway C3 convertase, or cleavage of C5 by C5 convertase. The C3-cleaving capacity of EAC142oxy that had been previously incubated with C3a could be recovered completely by washing the cells, indicating that the C3a binding to the EAC42oxy cell must have been reversed without having had an effect on the amount of C2 bound. Ribonuclease, a molecule of similar size and charge to C3a, did not affect C3 cleavage and C3a inhibition was not reduced by providing a surface for non-specific adsorption of the C3a, suggesting that the effect of C3a on C3 cleavage was not mediated by non-specific interaction with cell surfaces. C3a inhibited the C3-cleaving capacity of the fluid-phase enzyme, C42oxy, to the same degree as it inhibited the cell-bound enzyme, EAC42oxy, indicating that the C3a must interact with the C42 complex directly. Inhibition of C3 cleavage by C3a is the first demonstration of product inhibition of a complement enzyme. It may provide another control of C3 activation.     

Complement receptors type 1 (CR1, CD35) and 2 (CR2, CD21) cooperate in the binding of hydrolyzed complement factor 3 (C3i) to human B lymphocytes

The C3b-binding receptor, CR1/CD35, supports CR2/CD21-mediated activation of complement by human B lymphocytes, possibly by associating with CR2 to promote or stabilize the binding of hydrolyzed C3 (C3i), the primary component of the AP convertase, C3i-Bb. To evaluate this hypothesis, we examined the uptake kinetics and binding equilibria for C3i dimer interaction with human blood cells in the absence and presence of CR1- and CR2-blocking mAb. C3i displayed dual uptake kinetics to B lymphocytes, comprising of rapid binding to CR1 and slower binding to CR2. The forward rate constants (k(1)) for CR1 and CR2, operating independently, differed ca. 9-fold (k(1)=193+/-9.4 and 22.2+/-6.0 x 10(3) M(-1)s(-1), respectively). Equilibrium binding of C3i to B lymphocytes was also complex, varying in strength by ca. 13-fold over the C3i concentration range examined. The maximum association constant (K(a, max)=109+/-27.2 x 10(7) l/mole) was ca. 9- and 6-fold greater, respectively, than those for CR1 or CR2 acting alone (K(a)=13.2+/-5.3 and 18.5+/-3.5 x 10(7) l/mole). The high avidity of the CR1-CR2 complex for C3i is consistent with its rates of C3i uptake and release being determined by CR1 and CR2, respectively.     

Sinomenine regulates CD14/TLR4, JAK2/STAT3 pathway and calcium signal via α7nAChR to inhibit inflammation in LPS-stimulated macrophages

Objective: To investigate the cellular mechanism that sinomenine (SIN) inhibits inflammation in macrophages induced by LPS through α7 nicotinic acetylcholine receptor (α7nAChR).

Materials and methods: RAW264.7 cells were stimulated with LPS and treated by SIN or nicotine (Nic). A selective antagonist of α7nAChR, α-bungarotoxin (BTX) was used to block α7nAChR. AG490 was used to inhibit JAK2 activation. ELISA was performed to detect the levels of TNF-α and MCP-1. Western blotting was used to analyze the expression of MIF, MMP-9, CD14, TLR4, STAT3 and p-STAT3. Intracellular-free calcium level was measured by Fluorescent probe fluo-3/AM

Results: SIN inhibited the production of TNF-α, MCP-1, MIF, and MMP-9, decreased the expression of CD14 and TLR4, and inhibited the release of intracellular-free calcium from intracellular stores in RAW 264.7 cells stimulated by LPS. JAK-specific inhibitor AG490 attenuated the inhibitory effect of SIN on TNF-α. SIN increased the phosphorylation of STAT3. And the above effects of SIN were attenuated by antagonist of α7nAChR.

Conclusions: SIN can decrease the expression of CD14/TLR4 and intracellular free calcium level, activate JAK2/STAT3 pathway to inhibit inflammatory response through α7nAChR in macrophages.     

The functional heterogeneity of murine-resident macrophages to a chemotactic signal and induction of C3b-receptor-mediated ingestion

Peritoneal macrophages (MØ) were derived from unstimulated Balb/c mice and separated into distinct subpopulations based on their buoyant densities on discontinuous gradients of Percoll. They were assayed for their relative capacities to migrate toward a chemotactic stimulant and to be induced, in vitro, to express C3b-receptor-mediated ingestion. Functional heterogeneity was demonstrated in both chemotactic responsiveness and in the induction of ingestion activity; two subpopulations representative of approximately 50% of the peritoneal MØ were both quantitatively more responsive to a chemotactic stimulus and to the induction of ingestion activity. Upon cultivation of the MØ for up to 4 days an initially unresponsive subpopulation (i.e. to the induction signal for ingestion) acquired the capacity to respond to the signal for the induction of C3b-coated erythrocytes, whereas a fourth subpopulation remained unresponsive. These findings suggest that among a population of unstimulated MØ there may exist subpopulations in different states of differentiation.     

Alternative complement pathway activation by HIV infected cells: C3 fixation does not lead to complement lysis but enhances NK sensitivity

HIV infected T and monocytic cell lines could activate and fix C3 fragments when incubated in human serum under conditions allowing for activation of the alternative complement pathway. Normal T lymphocytes incubated with HIV could also activate and fix C3. This activity was, at least in part, the property of the virus itself since cell-free HIV could efficiently activate C3. The C3 activating HIV infected cells were resistant to complement-mediated lysis, even after prolonged incubation periods. However, their sensitivity to cell-mediated natural killing increased, presumably due to their interaction with complement receptor bearing NK lymphocytes. The results suggest that the alternative complement pathway may contribute to the depletion of CD4+ T lymphocytes during HIV infection in vivo.     

Degradation of aggregates of activated C3 (C3b) by monocytes of patients with rheumatoid arthritis is related to vasculitis

We investigated whether a decreased complement receptor expression or function of monocytes isolated from peripheral blood of 52 patients with rheumatoid arthritis (RA) and rheumatoid vasculitis (RV) could account for the previously observed diminished degradation of immune complexes by monocytes of patients with RA and RV. On average, monocytes from all patients expressed significantly less CR1, and degraded significantly less AC3b when compared with monocytes of healthy controls. In addition, monocytes from RV patients degraded significantly less AC3b when compared with monocytes from patients with RA. The expression of both CR1 and CR3 on monocytes of RV patients was lower compared with RA patients but this difference was only significant for CR3. No differences were found between AC3b degradation and the expression of CR1 and CR3 between patients with active and inactive RA. Using linear discriminant analysis on the variables AC3b, CR1 and CR3, 94% of the patients could be classified correctly as healthy controls, RA or RV, suggesting a true multi-variate relationship between these parameters and patients groups. Our results suggest that the diminished capacity of monocytes from RA patients to degrade AC3b is due partly to a decreased expression of CR1 and CR3.     

Preparation of monoclonal antibodies to C3b by immunization with C3b(i)-Sepharose

We have prepared and characterized four monoclonal antibodies (MAbs) to human C3b of high specificity and affinity. Our procedure did not require a purified source of C3b for immunization. Instead, C3b and C3bi were deposited on Sepharose 4B via the alternative pathway of complement activation in normal human serum, and this C3b(i)-Sepharose served as the immunogen. C3b(i)-Sepharose was also prepared from a number of primate and non-primate sources, and this allowed us to demonstrate that the anti-human C3b MAbs cross-reacted with primate-derived C3b, but not with C3b from non-primates. The procedures we have developed may be useful in the further investigation of species-specific C3 fragment-binding proteins from both primate and non-primate sources.     

Complement activation in septic baboons detected by neoepitope-specific assays for C3b/iC3b/C3c, C5a and the terminal C5b-9 complement complex (TCC).

We have investigated the cross-reactivity of various species in neoepitope-specific methods for quantification of human complement activation products. In contrast to most other species examined, baboon showed a substantial cross-reactivity supporting a high degree of homology between human and baboon complement. An assay for C3b, iC3b and C3c (MoAb bH6) showed moderately good reactivity, in contrast to a C3a assay which did not cross-react. Excellent reactivity was found for C5a using MoAbs C17/5 and G25/2. The reactivity of an established TCC assay (MoAb aE11 to a C9 neoepitope and polyclonal antibody to C5) was improved substantially by replacing the anti-C5 antibody with a new MoAb to C6 particularly selected on the basis of baboon cross-reactivity. Plasma samples from baboons receiving 2.5 x 10(9) and 1.0 x 10(10) live Escherichia coli bacteria/kg were examined with the assays described. In vivo complement activation with the lowest dose was moderate and kept under control, in contrast to the highest dose, where an uncontrolled increase in all activation products continued throughout the infusion period. These results support the hypothesis that sufficiently high amounts of endotoxin lead to uncontrolled activation of complement as seen in irreversible septic shock. The results are discussed with particular emphasis on activation of the terminal complement pathway.     

Reconstitution of a functional interleukin (IL)-7 receptor demonstrates that the IL-2 receptor γ chain is required for IL-7 signal transduction

The interleukin (IL)-2 receptor γ chain has recently been shown to be a component of the IL-7 and IL-4 receptors. Using a transient transfection assay and the trans-activation of reporter gene constructs which are under the control of cytokine-responsive promoter elements, we have studied signal transduction through the IL-7 receptor (IL-7R). The reporter gene expression was not stimulated by receptors that contained the cytoplasmic domain of the IL-7R, either as intact IL-7R or as part of a chimeric receptor. However, co-expression of the IL-7R with the IL-2 receptor γ chain was able to stimulate gene activation. For maximal stimulation the intact cytoplasmic domains of each chain was required.     

Adenosine A2a receptor induced gliosis via Akt/NF-κB pathway in vitro

Gliosis is characterized by increased expression of glial fibrillary acidic protein (GFAP) and astroglial proliferation, although the mechanism underlying the process is still largely undefined. This study explores the role of the adenosine A2a receptor (A2aR) in gliosis after ischemia-like injury in a rat astrocyte cell line transfected with A2aR. A2aR transfection enhanced GFAP expression and cell proliferation. A2aR-selective antagonist Sch58261 decreased GFAP expression in a dose- and time-dependent manner with increased activation of Akt, and induced activation of NF-κB. An Akt inhibitor reversed the effect of Sch58261. These results suggest that the effect of A2aR on gliosis is related to the Akt/NF-κB signal pathway.     

3CD, but not 3C, cleaves the VP1/2A site efficiently during Aichi virus polyprotein processing through interaction with 2A

Picornavirus genomes are translated into a single large polyprotein, which is processed by virus-encoded proteases into individual functional proteins. 3C of all picornaviruses is a protease, and the leader (L) and 2A proteins of some picornaviruses are also involved in polyprotein processing. Aichi virus (AiV), which is associated with acute gastroenteritis in humans, is a member of the genus Kobuvirus of the family Picornaviridae. The AiV L and 2A proteins have already been shown to exhibit no protease activity. In this study, we investigated AiV polyprotein processing by 3C and 3CD using a cell-free translation system. 3C and 3CD were capable of processing the polyprotein in trans; 3C, however, cleaved the VP1/2A site inefficiently, while 3CD cleaved this site almost completely. Mammalian two-hybrid and coimmunoprecipitation assays showed an interaction between 2A and 3CD. Using a 3CD mutant and various 2A mutants of substrate proteins, we showed a clear correlation between the 2A-3CD interaction and the VP1/2A cleavage by 3CD. Thus, this study suggests that tight interaction of 3CD with the 2A region of a precursor protein is required for efficient cleavage at the VP1/2A site.