Maturation-dependent expression of C1q binding proteins on the cell surface of human monocyte-derived dendritic cells

Critical Role for a Subset of Intestinal Macrophages in Shaping Gut Microbiota in Adult Zebrafish

The gut microbiota is strongly influenced by environmental factors, although host contribution is far less understood. We leveraged macrophage-deficient interferon regulatory factor irf8 zebrafish mutants to investigate the role of macrophages in this process. In conventionally raised adult irf8-deficient mutants, we found a significant loss of intestinal macrophages associated with a strikingly altered gut microbiota when compared to co-housed siblings. The destabilization of the gut commensal microbiota was associated with a severe reduction in complement C1q genes and outgrowth of a rare bacterial species. Consistent with a critical function of irf8 in adult intestinal macrophages, irf8 is abundantly expressed in these cells normally, and restoring macrophage irf8 expression in irf8 mutants was sufficient to recover commensal microbes and C1q genes expression. This study reports an important subpopulation of intestinal macrophages that requires irf8 to establish in the gut, ensure normal colonization of gut microbes, and prevent immune dysregulation.

MafB is a critical regulator of complement component C1q

The transcription factor MafB is expressed by monocytes and macrophages. Efferocytosis (apoptotic cell uptake) by macrophages is important for inhibiting the development of autoimmune diseases, and is greatly reduced in Mafb-deficient macrophages. Here, we show the expression of the first protein in the classical complement pathway C1q is important for mediating efferocytosis and is reduced in Mafb-deficient macrophages. The efferocytosis defect in Mafb-deficient macrophages can be rescued by adding serum from wild-type mice, but not by adding serum from C1q-deficient mice. By hemolysis assay we also show that activation of the classical complement pathway is decreased in Mafb-deficient mice. In addition, MafB overexpression induces C1q-dependent gene expression and signals that induce C1q genes are less effective in the absence of MafB. We also show that Mafb-deficiency can increase glomerular autoimmunity, including anti-nuclear antibody deposition. These results show that MafB is an important regulator of C1q.

Monocyte Expressed Macromolecular C1 and C1q Receptors as Molecular Sensors of Danger: Implications in SLE

The ability of circulating blood monocytes to express C1q receptors (cC1qR and gC1qR) as well as to synthesize and secrete the classical pathway proteins C1q, C1r, and C1s and their regulator, C1-INH is very well established. What is intriguing, however, is that, in addition to secretion of the individual C1 proteins monocytes are also able to display macromolecular C1 on their surface in a manner that is stable and functional. The cell surface C1 complex is presumably formed by a Ca²⁺-dependent association of the C1r₂⋅C1s₂ tetramer to C1q, which in turn is anchored via a membrane-binding domain located in the N-terminus of its A-chain as shown previously. Monocytes, which circulate in the blood for 1–3 days before they move into tissues throughout the body, not only serve as precursors of macrophages and dendritic cells (DCs), but also fulfill three main functions in the immune system: phagocytosis, antigen presentation, and cytokine production. Since the globular heads of C1q within the membrane associated C1 are displayed outwardly, we hypothesize that their main function – especially in circulating monocytes – is to recognize and capture circulating immune complexes or pathogen-associated molecular patterns in the blood. This in turn may give crucial signal, which drives the monocytes to migrate into tissues, differentiate into macrophages or DCs, and initiate the process of antigen elimination. Unoccupied C1q on the other hand may serve to keep monocytes in a pre-dendritic phenotype by silencing key molecular players thus ensuring that unwarranted DC-driven immune response does not occur. In this paper, we will discuss the role of monocyte/DC-associated C1q receptors, macromolecular C1 as well as secreted C1q in both innate and acquired immune responses.

The role of globular heads of the C1q receptor in HPV 16 E2-induced human cervical squamous carcinoma cell apoptosis is associated with p38 MAPK/JNK activation

BACKGROUND

Human papillomavirus type 16 (HPV 16) E2 protein is a multifunctional DNA-binding protein. HPV 16 E2 regulates many biological responses, including DNA replication, gene expression, and apoptosis. The purpose of this study was to investigate the relationship among the receptor for globular heads of the human C1q (gC1qR) gene expression, HPV 16 E2 transfection and apoptosis regulation in human cervical squamous carcinoma cells (C33a and SiHa).

METHODS

gC1qR expression was examined in C33a and SiHa cells using real-time PCR and Western blot analysis. Apoptosis of C33a and SiHa cells was assessed by flow cytometry. C33a and SiHa cell viability, migration and proliferation were detected using the water-soluble tetrazolium salt (WST-1) assay, a transwell assay and 3H-thymidine incorporation into DNA (3H-TdR), respectively.

RESULTS

C33a and SiHa cells that were transfected with a vector encoding HPV 16 E2 displayed significantly increased gC1qR gene expression and p38 mitogen-activated protein kinase (p38 MAPK)/c-jun N-terminal kinase (JNK) activation as well as up-regulation of cellular apoptosis, which was abrogated by the addition of gC1qR small interfering RNA (siRNA). Furthermore, the changes in C33a and SiHa cell viability, migration and proliferation that were observed upon HPV 16 E2 transfection were abrogated by SB203580 (a p38 MAPK inhibitor) or SP600125 (a JNK inhibitor) treatment.

CONCLUSION

These data support a mechanism whereby HPV 16 E2 induces apoptosis by silencing the gC1qR gene or inhibiting p38 MAPK/JNK signalling in cervical squamous cell carcinoma.

The globular heads of the C1q receptor regulate apoptosis in human cervical squamous carcinoma cells via a p53-dependent pathway

Background

The globular heads of the human C1q receptor (gC1qR) localize predominantly to the mitochondrial matrix. gC1qR mediates many biological responses, including growth perturbation, morphological abnormalities and the initiation of apoptosis. The purpose of this study was to investigate the relationship between mitochondrial dysfunction, p53 status and gC1qR expression and the regulation of apoptosis in human cervical squamous carcinoma cells (C33a and SiHa).

Methods

Here, gC1qR expression was examined in human cervical tissues using real-time PCR and Western blot analysis. Apoptotic death of C33a and SiHa cells was assessed by flow cytometric analysis that detected the subG1 population. Mitochondrial function was assessed via ROS generation, the content of cytosolic Ca²⁺, and the change in mitochondrial membrane potential (Δψm). The viability and migration of C33a and SiHa cells were detected via the water-soluble tetrazolium salt (WST-1) assay and the transwell assay, respectively.

Results

gC1qR expression was decreased in cervical squamous cell carcinoma tissues compared with normal tissues. C33a and SiHa cells transfected with a vector encoding gC1qR displayed mitochondrial dysfunction and apoptosis, which was abrogated by the addition of a mutant form of p53 or p53 small interference RNA (siRNA). Furthermore, upon overexpression of gC1qR, cell viability and migration were significantly enhanced, and the apoptosis of C33a and SiHa cells were decreased when cells were treated with mutant p53 or p53 siRNA.

Conclusions

These data support a mechanism whereby gC1qR induces apoptosis through the mitochondrial and p53-dependent pathways in cervical squamous cell carcinoma.

Modulation of cellular immunity by antibodies against calreticulin

Although caltreticulin (CRT) is mainly a residential ER protein, it is also expressed on the membrane surface of various types of cells exhibiting multiple functions. We report here that intraperitoneal administration of a soluble recombinant CRT fragment (rCRT/39-272) led to a substantial decrease in delayed type hypersensitivity (DTH) responses in BALB/c mice and EAE in C57BL/6 mice. In the recall response against keyhole limpet hemocyanin (KLH) in vitro, draining lymph node cells from the rCRT/39-272-treated mice produced less IFN-γ but more IL-4 as compared with the cells from the control group. The immunomodulating effect of intraperitoneally administered rCRT/39-272 was attributed to anti-CRT Abs thereby induced, because, in passive transfer experiments, the CRT-specific antiserum could suppress DTH in BALB/c mice. B-cell-deficient μMT mice were not susceptible to rCRT/39-272-mediated DTH suppression. Furthermore, CRT appears on the surface of murine T cells soon after activation and remains detectable (at relatively low level) by flow cytometry for approximately 5 days in vitro. Anti-CRT Abs were able to inhibit AKT phosphorylation, proliferation, and cytokine production by activated murine T cells. We propose that cell surface CRT could play a role in the function of effector T cells and may be considered a target for immunological manipulation.

Proteomic differentiation pattern in the U937 cell line

The U937 cell line, originally established from a histiocytic lymphoma, has been widely used as a powerful in vitro model for haematological studies. These cells retain the immature cell phenotype and can be induced to differentiate by several factors, among which 12-O-tetradecanoyl-13-phorbol acetate (TPA). Fully differentiated cells acquire the adherent phenotype and exhibit various properties typical of macrophages. However, in spite of a great deal of research devoted to the U937 cellular model, the molecular basis of biological processes involved in the monocyte/macrophage differentiation remains unclear. The present study has been undertaken to contribute to this knowledge, in order to identify proteomic-based differentiation pattern for the U937 cells exposed to TPA. Present results have highlighted that the U937 cell differentiation is correlated with a significant proteomic modulation, corresponding to about 30% of the identified proteins, including both over- and down-regulated proteins. Negative modulation regarded proteins involved in the regulation of cell proliferation and in metabolic processes. Proteins appearing incremented in macrophagic phenotype include calcium- and phospholipid-binding proteins and several proteins related to the phagocytic activity. Conclusively, we suggest that this new set of differentially expressed proteins may represent meaningful myelo-monocytic differentiation markers to be applied to the study of several haematological diseases.

The non-classical functions of the classical complement pathway recognition subcomponent C1q

C1q, the ligand recognition subcomponent of the classical complement pathway has steadily been gaining recognition as a bridge between innate and adaptive immunity. C1q has been shown to be involved in the modulation of various immune cells (such as dendritic cells, platelets, microglia cells and lymphocytes), clearance of apoptotic cells, a range of cell processes such as differentiation, chemotaxis, aggregation and adhesion, and pathogenesis of neurodegenerative diseases and systemic lupus erythematosus. Recent studies have highlighted the importance of C1q during pregnancy, coagulation process and embryonic development including neurological synapse function. It is intriguing to note that a prototypical defence molecule has so many diverse functions that probably have its origin in its versatility as a potent charge pattern recognition molecule, modularity within the ligand-recognising globular domain, and the redundancy of putative C1q receptors. The range of function that C1q has been shown to perform also provides clues for the undiscovered functions of a number of C1q family members.

C1q differentially modulates phagocytosis and cytokine responses during ingestion of apoptotic cells by human monocytes, macrophages, and dendritic cells

C1q, the first component of the classical complement pathway, is also a pattern recognition receptor involved in the recognition and clearance of apoptotic cells. C1q deficiency in humans leads to development of lupus-like autoimmune disease, and it has been speculated that impaired clearance of apoptotic cells may contribute to disease development. Since phagocytes initiate specific and appropriate immune responses as a result of initial ligand-receptor interactions, regulation of gene expression by C1q may also contribute to the sculpting of an immune response to the ingested "self-Ags." In this study, the role of C1q in apoptotic cell clearance and subsequent modulation of cytokine release by phagocytes was assessed including donor matched human monocytes, monocyte-derived macrophages (HMDMs), and dendritic cells (DCs). First, C1q binding is much greater to late compared with early apoptotic cells. Second, C1q binding to apoptotic cells significantly enhanced the levels of ingestion by monocytes but had no effect on HMDM and DC uptake. Third, in the presence of serum, C1q bound to apoptotic cells, activated the complement pathway, leading to C3b deposition, and enhancement of uptake of apoptotic cells by monocytes, HMDMs, and DCs. Finally, although C1q, either immobilized on a plate or bound to apoptotic cells, modulates the LPS-induced cytokine levels released by human monocytes, HMDMs, and DCs toward a more limited immune response, both the degree and direction of modulation differed significantly depending on the differentiation state of the phagocyte, providing further evidence of the integration of these cell- and environment-specific signals in determining appropriate immune responses.

Role of the receptor for the globular domain of C1q protein in the pathogenesis of hepatitis C virus-related cryoglobulin vascular damage

Mixed cryoglobulinemia (MC) is a lymphoproliferative disorder observed in approximately 10 to 15% of hepatitis C virus (HCV)-infected patients. Circulating, nonenveloped HCV core protein, which has been detected in cryoprecipitable immune complexes, interacts with immunocytes through the receptor for the globular domain of C1q protein (gC1q-R). In this study, we have evaluated circulating gC1q-R levels in chronically HCV-infected patients, with and without MC. These levels were significantly higher in MC patients than in those without MC and in healthy controls and paralleled specific mRNA expression in PBL. Soluble gC1q-R circulates as a complexed form containing both C1q and HCV core proteins. Higher serum gC1q-R levels negatively correlated with circulating concentrations of the C4d fragment. The presence of sequestered C4d in the vascular bed of skin biopsies from MC patients was indicative of in situ complement activation. In vitro studies showed that release of soluble gC1q-R is regulated by HCV core-mediated inhibition of cell proliferation. Our results indicate that up-regulation of gC1q-R expression is a distinctive feature of MC, and that dysregulated shedding of C1q-R molecules contributes to vascular cryoglobulin-induced damage via the classic complement-mediated pathway.

C1q binds phosphatidylserine and likely acts as a multiligand-bridging molecule in apoptotic cell recognition

Efficient apoptotic cell clearance is critical for maintenance of tissue homeostasis, and to control the immune responses mediated by phagocytes. Little is known about the molecules that contribute "eat me" signals on the apoptotic cell surface. C1q, the recognition unit of the C1 complex of complement, also senses altered structures from self and is a major actor of immune tolerance. HeLa cells were rendered apoptotic by UV-B treatment and a variety of cellular and molecular approaches were used to investigate the nature of the target(s) recognized by C1q. Using surface plasmon resonance, C1q binding was shown to occur at early stages of apoptosis and to involve recognition of a cell membrane component. C1q binding and phosphatidylserine (PS) exposure, as measured by annexin V labeling, proceeded concomitantly, and annexin V inhibited C1q binding in a dose-dependent manner. As shown by cosedimentation, surface plasmon resonance, and x-ray crystallographic analyses, C1q recognized PS specifically and avidly (K(D) = 3.7-7 x 10(-8) M), through multiple interactions between its globular domain and the phosphoserine group of PS. Confocal microscopy revealed that the majority of the C1q molecules were distributed in membrane patches where they colocalized with PS. In summary, PS is one of the C1q ligands on apoptotic cells, and C1q-PS interaction takes place at early stages of apoptosis, in newly organized membrane patches. Given its versatile recognition properties, these data suggest that C1q has the unique ability to sense different markers which collectively would provide strong eat me signals, thereby allowing efficient apoptotic cell removal.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

Immune modulation of human dendritic cells by complement

Deficiency in complement proteins such as C1q is associated with the development of systemic lupus erythematosus (SLE). Here, we show that the differentiation of dendritic cells (DC) in the presence of C1q (C1qDC) gives rise to CD1a(+)/DC-SIGN(+) cells with high phagocytic capacity and low expression of CD80, CD83 and CD86. Further, when C1qDC were exposed to LPS, a significant reduction in the production of IL-6, TNF-alpha and IL-10 occurred with a limited up-regulation of CD80, CD83 and CD86. In addition, C1qDC were less responsive to activation by CD40L in terms of IL-12p70 secretion and CD86 expression. C1qDC showed an impaired ability to stimulate alloreactive T cells, with a reduced production of IFN-gamma. In conclusion, we have shown that C1q is a potent modulator of DC, resulting in cells characterized by an impaired capacity of cytokine production and an impaired up-regulation of costimulatory molecules, leading to a limited T cell response. Therefore, we hypothesize that, next to a pivotal role in the safe clearance of apoptotic cells, C1q regulates the threshold of DC activation and thereby prevents hyperactivation of the overall immune response.

C1q binding and complement activation by prions and amyloids

C1q binds to many non-self and altered-self-materials. These include microorganisms, immune complexes, apoptotic and necrotic cells and their breakdown products, and amyloids. C1q binding to amyloid fibrils found as extracellular deposits in tissues, and subsequent complement activation are involved in the pathology of several amyloid diseases, such as Alzheimer's disease. Prion diseases, such as scrapie also involve formation of amyloid by polymerization of the host prion protein (PrP). Complement activation is likely to contribute to neuronal damage in the end stages of prion diseases, but is also thought to participate in the initial infection, dissemination and replication stages. Infectious prion particles are likely to bind C1q and activate the complement system. Bound complement proteins may then influence the uptake and transport of prion particles by dendritic cells (DCs) and their subsequent proliferation at sites such as follicular DCs.

Dendritic cell surface calreticulin is a receptor for NY-ESO-1: direct interactions between tumor-associated antigen and the innate immune system

How the immune system recognizes endogenously arising tumors and elicits adaptive immune responses against nonmutated tumor-associated Ags is poorly understood. In search of intrinsic factors contributing to the immunogenicity of the tumor-associated Ag NY-ESO-1, we found that the NY-ESO-1 protein binds to the surface of immature dendritic cells (DC), macrophages, and monocytes, but not to that of B cells or T cells. Using immunoprecipitation coupled with tandem mass spectrometry, we isolated DC surface calreticulin as the receptor for NY-ESO-1. Calreticulin Abs blocked NY-ESO-1 binding on immature DC and its cross-presentation to CD8+ T cells in vitro. Calreticulin/NY-ESO-1 interactions provide a direct link between NY-ESO-1, the innate immune system, and, potentially, the adaptive immune response against NY-ESO-1.

Complement proteins C1q and MBL are pattern recognition molecules that signal immediate and long-term protective immune functions

C1q and mannose binding lectin, members of the "defense collagen" family, are pattern recognition molecules that can trigger rapid enhanced phagocytosis resulting in efficient containment of pathogens or clearance of cellular debris, apoptotic cells and immune complexes. In addition, interaction of C1q and mannose binding lectin with the phagocyte alters subsequent phagocyte cytokine synthesis, and thus may have important implications in directing acute inflammation as well as long-term protective immunity. The importance of the role of defense collagens in phagocytosis of apoptotic cells is highlighted by studies in vivo of mice deficient in C1q, pulmonary surfactant D and mannose binding lectin in which there is delayed clearance of apoptotic cells. Indeed, deficiency of C1q is a risk factor for the development of autoimmunity in both humans and mice, consistent with the hypothesis that inefficient clearance of apoptotic cells results in release of autoantigens and contributes to the pathology associated with autoimmune diseases such as systemic lupus erythematosus. Further understanding of the importance of C1q and mannose binding lectin in the clearance of apoptotic cells and regulation of cytokine synthesis and identification of the receptors implicated in mediating these processes should provide novel targets for therapeutic intervention in the control and manipulation of the immune response in terms of both host defense against infectious disease and tissue repair and remodeling.

Prospects of vaccines for invasive aspergillosis

Invasive aspergillosis is a disease of immunocompromised hosts and the pathogenesis of this disorder is heavily dependent upon the defect within a given host. Consequently, vaccine development is limited by our understanding of effective host responses and by limitations in our knowledge of fungal molecules that elicit protective immunity. Nonetheless, the past few years have witnessed advances in our understanding both of the immune response to this organism and in the relationship between antigenicity and the ability to confer protection. Manipulations that promote the development of T(H)1-associated responses correlate with increased resistance to disease, at least partly because of consequent enhancement of innate cellular effector function. Two areas of investigation most actively being pursued include the search for adjuvants that will allow products of Aspergillus fumigatus to become effective vaccine candidates, regardless of the form of immunity they ordinarily induce, and the identification of the specific antigens that will most effectively elicit beneficial responses. Strategies using antigen-exposed dendritic cells as adjuvants appear to be particularly promising. Though we currently are far away from a candidate that is applicable for human trials, recent progress is encouraging.

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.

Does Trypanosoma cruzi calreticulin modulate the complement system and angiogenesis?

Calreticulin, a calcium-binding protein that is highly conserved in its multiple functions, is present in a wide spectrum of subcellular compartments in virtually every cell of higher organisms. In this article, we propose a dual role for parasite calreticulin, with emphasis on the Trypanosoma cruzi model. By modulating the vertebrate complement system, calreticulin might provide the parasite with an effective immune-escape mechanism. Alternatively, by inhibiting angiogenesis, the parasite molecule might protect the host from ongoing neoplasic aggressions. Many questions are still unanswered, particularly those regarding the consequences that these interactions could have in vivo for both the parasite and the host.

The classical activation pathway of the human complement system is specifically inhibited by calreticulin from Trypanosoma cruzi

The high resistance of Trypanosoma cruzi trypomastigotes, the causal agent of Chagas' disease, to complement involves several parasite strategies. In these in vitro studies, we show that T. cruzi calreticulin (TcCRT) and two subfragments thereof (TcCRT S and TcCRT R domains) bind specifically to recognition subcomponents of the classical and lectin activation pathways (i.e., to collagenous tails of C1q and to mannan-binding lectin) of the human complement system. As a consequence of this binding, specific functional inhibition of the classical pathway and impaired mannan-binding lectin to mannose were observed. By flow cytometry, TcCRT was detected on the surface of viable trypomastigotes and, by confocal microscopy, colocalization of human C1q with surface TcCRT of infective trypomastigotes was visualized. Taken together, these findings imply that TcCRT may be a critical factor contributing to the ability of trypomastigotes to interfere at the earliest stages of complement activation.

Maturation of dendritic cells abrogates C1q production in vivo and in vitro

Dendritic cells (DCs) and complement are essential components of the innate immune system. Immature DCs (immDCs) and mature DCs (mDCs) can migrate to lymphoid areas inducing, respectively, tolerance and immune responses. Primary deficiency of complement component C1q (C1q) leads to autoimmunity, suggesting a role in the maintenance of tolerance. In the present study, we investigated the production of C1q by immDCs, mDCs, and macrophages. We demonstrated that monocyte-derived and CD34(+)-derived interstitial DCs are a rich source of C1q. C1q produced by immDCs is functionally active in complement activation and binding to apoptotic cells. The production of C1q is completely down-regulated upon DC maturation in vitro. Moreover, we found that DC differentiation in the presence of interferon-alpha (IFN-alpha) accelerated DC maturation and strongly impaired overall C1q production. Finally, we demonstrated the presence, in significant numbers, of DC-SIGN(+)/C1q(+) cells in T-cell areas of tonsils, next to DC-LAMP(+) mDCs lacking C1q. We conclude from these results that immDC, a cell with tolerogenic properties, is a rich source of active C1q in vitro and in vivo, which is down-regulated on maturation. Therefore, immDCs may be considered an additional source of C1q in humans.