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

Human Cord Blood Serum-Derived APP α-Secretase Cleavage Activity is Mediated by C1 Complement

Alzheimer's Disease (AD) is the leading cause of dementia in the elderly. In healthy individuals, amyloid precursor protein (APP) is cleaved by α-secretase, generating soluble α-amyloid precursor protein (sAPPα), which contributes neuroprotective functions in the neuronal environment. In contrast, in the neurodegenerative environment of AD patients, amyloid-β-peptide (Aβ) of either 40 or 42 residues are generated by increased activity of β- and γ-secretase. These proteins amalgamate in specific regions of the brain, which disrupts neuronal functions and leads to cognitive impairment. Human umbilical cord blood cells (HUCBC) have proven useful as potential immunomodulatory therapies in various models of neurodegenerative diseases, including AD. Our most recent work studied the impact of umbilical cord blood serum (CBS) on modulation of sAPPα production. Heat-sensitive CBS significantly promoted sAPPα production, indicating that heat-sensitive factor(s) play(s) a role in this process. Liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis was used to determine the molecular source of α-secretase in purified CBS and aged blood serum (AgBS) fraction. Of the proteins identified, the subunits of C1 complex (C1q, C1r, and C1s) and alpha-2-macroglobulin showed significantly greater levels in purified α-CBS fraction (α-CBSF) compared with the AgBS fraction (AgBSF). Specifically, C1 markedly increased sAPPα and alpha-carboxyl-terminal fragment (α-CTF) production in a dose-dependent fashion, whereas C1q alone only minimally increased and C3 did not increase sAPPα production in the absence of sera. Furthermore, C1q markedly increased sAPPα and α-CTF, while decreasing Aβ, in CHO/APPwt cells cultured in the presence of whole sera. These results confirm our initial assumption that APP α-secretase activity in human blood serum is mediated by complement C1, opening a potential therapeutic modality for the future of AD.

The immune response to secondary necrotic cells

When apoptotic cells are not cleared in an efficient and timely manner, they progress to secondary necrosis and lose their membrane integrity. This results in a leakage of immunostimulatory, danger associated molecular patterns (DAMPs), similar to accidental (or primary) necrosis. However, primary necrosis is a sudden event with an inadvertent release of almost unmodified DAMPs. Secondary necrotic cells, in contrast, have gone through various modifications during the process of apoptosis. Recent research revealed that the molecules released from the cytoplasm or exposed on the cell surface differ between primary necrosis, secondary necrosis, and regulated necrosis such as necroptosis. This review gives an overview of these differences and focusses their effects on the immune response. The implications to human physiology and diseases are manifold and will be discussed in the context of cancer, neurodegenerative disorders and autoimmunity.

Impaired efferocytosis by monocytes in multiple myeloma

Efficient clearance of apoptotic cells by efferocytosis is important for tissue homeostasis. Impaired efferocytosis leads to the accumulation of cell debris, which is regarded as a trigger in chronic inflammation and autoimmune diseases. Patients with hematological neoplastic disorders such as multiple myeloma (MM) exhibit high blood levels of apoptotic microparticles. The present study investigated whether these high levels of apoptotic microparticles are associated with insufficient dead cell clearance. Blood samples were collected from patients with MM immediately prior to and 3, 7 and 10 days after the initial cycle of bortezomib-based therapy. In addition, bone marrow aspirates (BMA) were collected prior to and following therapy. Prior to therapy, a 52% reduction in efferocytosis by blood monocytes was observed compared with the healthy controls (P<0.017). This was associated with an elevated number of 7-AAD⁺ dead cell remnants in the blood flow as well as in BMA. A portion of the blood samples contained active caspase 3. The subsequent bortezomib-based therapy had no effect on efferocytosis, although the quantity of dead cell remnants decreased. This reduction was associated with a decline in cluster of differentiation 8 (CD8)⁺ and CD4⁺ T cells and an increase in the number of monocytes. However, of 28 distinct soluble immune-modulating molecules (i.e. chemokines, cytokines and soluble co-stimulators) only C-C motif chemokine ligand 2 (CCL2), CCL24 and sCD27 were affected by bortezomib-based therapy. The levels of all other molecules remained unchanged or were below the detection threshold in all samples. The present study results revealed that the presence of dead cell remnants in the blood and bone morrow of patients with MM is associated with impaired efferocytosis by monocytes; however, its contribution to inflammatory events during MM remains unclear.

Proteinase 3 Interferes With C1q-Mediated Clearance of Apoptotic Cells

Proteinase 3 (PR3) is the autoantigen in granulomatosis with polyangiitis, an autoimmune necrotizing vasculitis associated with anti-neutrophil cytoplasmic antibodies (ANCAs). Moreover, PR3 is a serine protease whose membrane expression can potentiate inflammatory diseases such as ANCA-associated vasculitis and rheumatoid arthritis. During apoptosis, PR3 is co-externalized with phosphatidylserine (PS) and is known to modulate the clearance of apoptotic cells through a calreticulin (CRT)-dependent mechanism. The complement protein C1q is one mediator of efferocytosis, the clearance of altered self-cells, particularly apoptotic cells. Since PR3 and C1q are both involved in the clearance of apoptotic cells and immune response modulation and share certain common ligands (i.e., CRT and PS), we examined their possible interaction. We demonstrated that C1q binding was increased on apoptotic rat basophilic leukemia (RBL) cells that expressed PR3, and we demonstrated the direct interaction between purified C1q and PR3 molecules as shown by surface plasmon resonance. To better understand the functional consequence of this partnership, we tested C1q-dependent phagocytosis of the RBL cell line expressing PR3 and showed that PR3 impaired C1q enhancement of apoptotic cell uptake. These findings shed new light on the respective roles of C1q and PR3 in the elimination of apoptotic cells and suggest a novel potential axis to explore in autoimmune diseases characterized by a defect in apoptotic cell clearance and in the resolution of inflammation.

C1r/C1s deficiency is insufficient to induce murine systemic lupus erythematosus

C1s deficiency is strongly associated with the development of human systemic lupus erythematosus (SLE); however, the mechanisms by which C1s deficiency contributes to the development of SLE have not yet been elucidated in detail. Using ICR-derived-glomerulonephritis (ICGN) mouse strain that develops SLE and very weakly expresses C1s in the liver, we investigated the protective roles of C1s against SLE. A genetic sequence analysis revealed complete deletion of the C1s1 gene, a mouse homolog of the human C1s gene, with partial deletion of the C1ra and C1rb genes in the ICGN strain. This deletion led to the absence of C1r/C1s and a low level of C1q in the circulation. In order to investigate whether the C1r/C1s deficiency induces SLE, we produced a congenic mouse strain by introducing the deletion region of ICGN into the C57BL/6 strain. Congenic mice exhibited no C1r/C1s and a low level of C1q in the circulation, but did not have any autoimmune defects. These results suggest that C1r/C1s deficiency is not sufficient to drive murine SLE and also that other predisposing genes exist in ICGN mice.

Cellular Differentiation of Human Monocytes Is Regulated by Time-Dependent Interleukin-4 Signaling and the Transcriptional Regulator NCOR2

Human in vitro generated monocyte-derived dendritic cells (moDCs) and macrophages are used clinically, e.g., to induce immunity against cancer. However, their physiological counterparts, ontogeny, transcriptional regulation, and heterogeneity remains largely unknown, hampering their clinical use. High-dimensional techniques were used to elucidate transcriptional, phenotypic, and functional differences between human in vivo and in vitro generated mononuclear phagocytes to facilitate their full potential in the clinic. We demonstrate that monocytes differentiated by macrophage colony-stimulating factor (M-CSF) or granulocyte macrophage colony-stimulating factor (GM-CSF) resembled in vivo inflammatory macrophages, while moDCs resembled in vivo inflammatory DCs. Moreover, differentiated monocytes presented with profound transcriptomic, phenotypic, and functional differences. Monocytes integrated GM-CSF and IL-4 stimulation combinatorically and temporally, resulting in a mode- and time-dependent differentiation relying on NCOR2. Finally, moDCs are phenotypically heterogeneous and therefore necessitate the use of high-dimensional phenotyping to open new possibilities for better clinical tailoring of these cellular therapies.

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In vitro monocyte cultures model in vivo inflammatory dendritic cells and macrophages

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Monocyte-derived dendritic cells integrate interleukin-4 signaling time dependently

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NCOR2 controls differentiation of in vitro generated monocyte-derived dendritic cells

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In vitro generated monocyte-derived cells are phenotypically heterogeneous

The role of complement system in adipose tissue-related inflammation

As the common factor linking adipose tissue to the metabolic context of obesity, insulin resistance and atherosclerosis are associated with a low-grade chronic inflammatory status, to which the complement system is an important contributor. Adipose tissue synthesizes complement proteins and is a target of complement activation. C3a-desArg/acylation-stimulating protein stimulates lipogenesis and affects lipid metabolism. The C3a receptor and C5aR are involved in the development of adipocytes' insulin resistance through macrophage infiltration and the activation of adipose tissue. The terminal complement pathway has been found to be instrumental in promoting hyperglycemia-associated tissue damage, which is characteristic of the major vascular complications of diabetes mellitus and diabetic ketoacidosis. As a mediator of the effects of the terminal complement complex C5b-9, RGC-32 has an impact on energy expenditure as well as lipid and glucose metabolic homeostasis. All of this evidence, taken together, indicates an important role for complement activation in metabolic diseases.

Incorporation of a Ligand Peptide for Immune Inhibitory Receptor LAIR-1 on Biomaterial Surfaces Inhibits Macrophage Inflammatory Responses

Leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1) is an inhibitory receptor broadly expressed on immune cells, with its ligands residing within the extracellular matrix protein collagen. In this study, surfaces are modified with a LAIR-1 ligand peptide (LP), and it is observed that macrophages cultured on LAIR-1 LP-conjugated surfaces exhibit significantly reduced secretion of inflammatory cytokines in response to proinflammatory stimuli that reflect an injured environment. These downregulated mediators include TNF-α, MIP-1α, MIP-1β, MIP-2, RANTES, and MIG. Knockdown of LAIR-1 using siRNA abrogates this inhibition of cytokine secretion, supporting the specificity of the inhibitory effect to this receptor. These results are the first to demonstrate that integration of LAIR-1 ligands with biomaterials could suppress inflammatory responses.

Complement components as promoters of immunological tolerance in dendritic cells

Complement and dendritic cells (DCs) share many functional features that drive the outcome of immune-inflammatory processes. Both have a sentinel function, acting as danger sensors specialized for a rapid, comprehensive and selective action against potential threats without damaging the healthy host cells. But while complement has been considered as a "master alarm" system poised for direct pathogen killing, DCs are regarded as "master regulators" or orchestrators of a vast range of effector immune cells for an effective immune response against threatening insults. The original definition of the complement system, coined to denote its auxiliary function to enhance or assist in the role of antibodies or phagocytes to clear microbes or damaged cells, envisaged an important crosstalk between the complement and the mononuclear phagocyte systems. More recent studies have shown that, depending on the microenvironmental conditions, several complement effectors are competent to influence the differentiation and/or function of different DC subsets toward immunogenicity or tolerance. In this review we will infer about the capability of complement activators and inhibitors to "condition" a tolerogenic and anti-inflammatory immune response by direct interaction with DC surface receptors, and about the implications of this knowledge to devise new complement-based therapeutic approaches for autoimmune pathologies.

Cathepsin S inhibition suppresses autoimmune-triggered inflammatory responses in macrophages

In several types of antigen-presenting cells (APCs), Cathepsin S (CatS) plays a crucial role in the regulation of MHC class II surface expression and consequently influences antigen (Ag) presentation of APCs to CD4⁺ T cells. During the assembly of MHC class II-Ag peptide complexes, CatS cleaves the invariant chain p10 (Lip10) - a fragment of the MHC class II-associated invariant chain peptide. In this report, we used a selective, high-affinity CatS inhibitor to suppress the proteolytic activity of CatS in lymphoid and myeloid cells. CatS inhibition resulted in a concentration-dependent Lip10 accumulation in B cells from both healthy donors and patients with systemic lupus erythematosus (SLE). Furthermore, CatS inhibition led to a decreased MHC class II expression on B cells, monocytes, and proinflammatory macrophages. In SLE patient-derived peripheral blood mononuclear cells, CatS inhibition led to a suppressed secretion of IL-6, TNFα, and IL-10. In a second step, we tested the effect of CatS inhibition on macrophages being exposed to patient-derived autoantibodies against C1q (anti-C1q) that are known to be associated with severe lupus nephritis. As shown previously, those SLE patient-derived high-affinity anti-C1q bound to immobilized C1q induce a proinflammatory phenotype in macrophages. Using this human in vitro model of autoimmunity, we found that CatS inhibition reduces the inflammatory responses of macrophages as demonstrated by a decreased secretion of proinflammatory cytokines, the downregulation of MHC class II and CD80. In summary, we can show that the used CatS inhibitor is able to block Lip10 degradation in healthy donor- and SLE patient-derived B cells and inhibits the induction of proinflammatory macrophages. Thus, CatS inhibition seems to be a promising future treatment of SLE.

Calreticulin Release at an Early Stage of Death Modulates the Clearance by Macrophages of Apoptotic Cells

Calreticulin (CRT) is a well-known “eat-me” signal harbored by dying cells participating in their recognition by phagocytes. CRT is also recognized to deeply impact the immune response to altered self-cells. In this study, we focus on the role of the newly exposed CRT following cell death induction. We show that if CRT increases at the outer face of the plasma membrane and is well recognized by C1q even when phosphatidylserine is not yet detected, CRT is also released in the surrounding milieu and is able to interact with phagocytes. We observed that exogenous CRT is endocytosed by THP1 macrophages through macropinocytosis and that internalization is associated with a particular phenotype characterized by an increase of cell spreading and migration, an upregulation of CD14, an increase of interleukin-8 release, and a decrease of early apoptotic cell uptake. Importantly, CRT-induced pro-inflammatory phenotype was confirmed on human monocytes-derived macrophages by the overexpression of CD40 and CD274, and we found that monocyte-derived macrophages exposed to CRT display a peculiar polarization notably associated with a downregulation of the histocompatibility complex of class II molecules hampering its description through the classical M1/M2 dichotomy. Altogether our results highlight the role of soluble CRT with strong possible consequences on the macrophage-mediated immune response to dying cell.

Complement in removal of the dead - balancing inflammation

Recognition and removal of apoptotic and necrotic cells must be efficient and highly controlled to avoid excessive inflammation and autoimmune responses to self. The complement system, a crucial part of innate immunity, plays an important role in this process. Thus, apoptotic and necrotic cells are recognized by complement initiators such as C1q, mannose binding lectin, ficolins, and properdin. This triggers complement activation and opsonization of cells with fragments of C3b, which enhances phagocytosis and thus ensures silent removal. Importantly, the process is tightly controlled by the binding of complement inhibitors C4b-binding protein and factor H, which attenuates late steps of complement activation and inflammation. Furthermore, factor H becomes actively internalized by apoptotic cells, where it catalyzes the cleavage of intracellular C3 to C3b. The intracellularly derived C3b additionally opsonizes the cell surface further supporting safe and fast clearance and thereby aids to prevent autoimmunity. Internalized factor H also binds nucleosomes and directs monocytes into production of anti-inflammatory cytokines upon phagocytosis of such complexes. Disturbances in the complement-mediated clearance of dying cells result in persistence of autoantigens and development of autoimmune diseases like systemic lupus erythematosus, and may also be involved in development of age-related macula degeneration.

Trichinella spiralis Calreticulin Binds Human Complement C1q As an Immune Evasion Strategy

As a multicellular parasitic nematode, Trichinella spiralis regulates host immune responses by producing a variety of immunomodulatory molecules to escape from host immune attack, but the mechanisms underlying the immune evasion are not well understood. Here, we identified that T. spiralis calreticulin (Ts-CRT), a Ca²⁺-binding protein, facilitated T. spiralis immune evasion by interacting with the first component of human classical complement pathway, C1q. In the present study, Ts-CRT was found to be expressed on the surface of different developmental stages of T. spiralis as well as in the secreted products of adult and muscle larval worms. Functional analysis identified that Ts-CRT was able to bind to human C1q, resulting in the inhibition of C1q-initiated complement classical activation pathway reflected by reduced C4/C3 generation and C1q-dependent lysis of antibody-sensitized sheep erythrocytes. Moreover, recombinant Ts-CRT (rTs-CRT) binding to C1q suppressed C1q-induced THP-1-derived macrophages chemotaxis and reduced monocyte–macrophages release of reactive oxygen intermediates (ROIs). Blocking Ts-CRT on the surface of newborn larvae (NBL) of T. spiralis with anti-Ts-CRT antibody increased the C1q-mediated adherence of monocyte–macrophages to larvae and impaired larval infectivity. All of these results suggest that T. spiralis-expressed Ts-CRT plays crucial roles in T. spiralis immune evasion and survival in host mostly by directly binding to host complement C1q, which not only reduces C1q-mediated activation of classical complement pathway but also inhibits the C1q-induced non-complement activation of macrophages.

The regulatory dendritic cell marker C1q is a potent inhibitor of allergic inflammation

The complement subunit C1q was recently identified as a marker for monocyte-derived regulatory dendritic cells supporting the differentiation of interleukin (IL)-10-secreting CD4⁺ T cells with a suppressive activity. Furthermore, C1q expression is upregulated in peripheral blood mononuclear cells of allergic patients in the course of successful allergen immunotherapy. Herein, we investigated a potential direct role of C1q in downregulating allergic inflammation. In mice with ovalbumin (OVA) or birch pollen (BP)-induced allergic asthma, C1q is as efficacious as dexamethasone to reduce both airway hyperresponsiveness (AHR), eosinophil, and ILC2 infiltrates in bronchoalveolar lavages, as well as allergen-specific T helper 2 cells in the lungs. Administration of C1q does not expand IL-10⁺/Foxp3⁺ regulatory T cells in the lungs, spleen, or in the blood. Depletion of plasmacytoid dendritic cells (pDCs) abrogates the capacity of C1q to reduce AHR and eosinophilic infiltrates in OVA-sensitized mice. Also C1q treatment inhibits the activation of human and mouse pDCs by CpGs, thereby demonstrating a critical role for pDCs in the anti-inflammatory activity of C1q. We conclude that regulatory dendritic cells can mediate a potent direct anti-inflammatory activity via the expression and/or secretion of molecules such as C1q, independently of their capacity to expand the pool of regulatory T cells.

Evidence for C1q-mediated crosslinking of CD33/LAIR-1 inhibitory immunoreceptors and biological control of CD33/LAIR-1 expression

C1q collagen-like region (CLR) engaging and activating the LAIR-1 inhibitory immunoreceptor represents a non-complement mechanism for maintaining immune quiescence. Given the binding promiscuity of C1q’s globular region (gC1q), we hypothesized that C1q concurrently associates with distinct inhibitory immunoreceptors to produce C1q-mediated modulatory networking. Like LAIR-1, CD33 inhibitory immunoreceptors are highly expressed on monocytes. Binding CD33 restricts cell activation/differentiation; however, natural ligands for CD33 remain elusive. CD33 has IgC2-like domains potentially recognized by gC1q. Thus, we asked whether C1q binds to CD33 and if C1q mediates CD33/LAIR-1 crosslinking. Our findings demonstrate that C1q and gC1q interact with CD33 to activate its inhibitory motifs, while CLR does not. Whole C1q is required to crosslink CD33 and LAIR-1 and concurrently activate CD33/LAIR-1 inhibitory motifs. While C1q binds CD33C2 domains, decreased C1q-CD33 interactions resulting from sialic acid masking of CD33C2 domains suggests a process for regulating C1q-CD33 activity. Consistent with defective self-tolerance, CD33/LAIR-1 expression is reduced in systemic lupus erythematosus (SLE) myelomonocytes. The anti-inflammatory cytokine M-CSF, but not DC growth factors, sustains CD33/LAIR-1 expression on both healthy and SLE cells suggesting further biological control of C1q-CD33/LAIR-1 processes.

CD200 modulates macrophage cytokine secretion and phagocytosis in response to poly(lactic co-glycolic acid) microparticles and films

Biocompatibility is a major concern for developing biomaterials used in medical devices, tissue engineering and drug delivery. Poly(lactic-co-glycolic acid) (PLGA) is one of the most widely used biodegradable materials, yet still triggers a significant foreign body response that impairs healing. Immune cells including macrophages respond to the implanted biomaterial and mediate the host response, which can eventually lead to device failure. Previously in our laboratory, we found that CD200, an immunomodulatory protein, suppressed macrophage inflammatory activation in vitro and reduced local immune cell infiltration around a biomaterial implant. While in our initial study we used polystyrene as a model material, here we investigate the effect of CD200 on PLGA, a commonly used biomaterial with many potential clinical applications. We fabricated PLGA with varied geometries, modified their surfaces with CD200, and examined macrophage cytokine secretion and phagocytosis. We found that CD200 suppressed secretion of the pro-inflammatory cytokine TNF-α and enhanced secretion of the anti-inflammatory cytokine IL-10, suggesting a role for CD200 in promoting wound healing and tissue remodeling. In addition, we found that CD200 increased phagocytosis in both murine macrophages and human monocytes. Together, these data suggest that modification with CD200 leads to a response that simultaneously prevents inflammation and enhances phagocytosis. This immunomodulatory feature may be used as a strategy to mitigate inflammation or deliver drugs or anti-inflammatory agents targeting macrophages.

Increased activity of the complement system in cerebrospinal fluid of the patients with Non-HIV Cryptococcal meningitis

Backgrounds

Cryptococcal meningitis (CM) has been known to lead to significant morbidity and mortality. The relative contribution of the complement system in protection and pathogenesis during CM remains largely unknown. The purpose of this study was to evaluate the baseline complement component profiles in human cerebrospinal fluid (CSF) and plasma from non-HIV patients with CM, and therefore to provide insights of possible roles of the complement system in CM.

Methods

CSF and blood samples from forty two CM patients not infected with HIV and thirteen non-CM control patients (Ctrl) were retrospectively selected and evaluated from the patients admitted to the hospital with a suspected diagnosis of CM. CSF and blood samples were collected at the admission. Enzyme-linked immunosorbent assay (ELISA) for complement components, cytokine IL-12 and western blot for C3 activation were performed on CSF and plasma samples. The levels of complement C1q, factor B (FB), mannose binding lectin (MBL), C2, C3, C4, C5, C4 binding protein (C4BP), Factor I (FI), Factor H (FH), sC5b-9 in CSF and plasma samples were compared. Pearson’s correlation coefficients were calculated on variables between complement components and the levels of total protein in the CSF samples.

Results

Our data demonstrated that the CSF levels of complement components of C1q, FB, MBL as well as complement pathway factors sC5b-9 and complement regulator FH were all elevated in patients with CM as compared to the controls, CSF C3 breakdown products iC3b were found in both CSF and plasma samples of the CM patients. A positive correlation was found between the levels of CSF protein and MBL, C1q or FB.

Conclusions

The activity of the complement system in CSF was increased in non-HIV patients with CM. C1q, MBL and FB are the important participants in the complement activation in CM. The relative contribution of each of the specific complement pathways and complement cascades in protection and inflammation resolution against CM warrant further investigation.

Complement Protein C1q Enhances Macrophage Foam Cell Survival and Efferocytosis

In the atherosclerotic lesion, macrophages ingest high levels of damaged modified low-density lipoproteins (LDLs), generating macrophage foam cells. Foam cells undergo apoptosis and, if not efficiently cleared by efferocytosis, can undergo secondary necrosis, leading to plaque instability and rupture. As a component of the innate immune complement cascade, C1q recognizes and opsonizes modified forms of LDL, such as oxidized or acetylated LDL, and promotes ingestion by macrophages in vitro. C1q was shown to be protective in an atherosclerosis model in vivo. Therefore, this study aimed to investigate whether ingestion of modified LDL in the presence of C1q alters macrophage foam cell survival or function. In an unbiased transcriptome analysis, C1q was shown to modulate expression of clusters of genes involved in cell death and apoptosis pathways in human monocyte-derived macrophages ingesting modified LDL; this was validated by quantitative PCR in human and murine macrophages. C1q downregulated levels and activity of active caspase-3 and PARP-1 in human and mouse macrophages during ingestion of modified LDL. This led to a measurable increase in survival and decrease in cell death, as measured by alamarBlue and propidium iodide assays, respectively. C1q opsonization also increased phagocytosis and efferocytosis in macrophage foam cells. These data suggest that C1q promotes macrophage survival during ingestion of excess cholesterol, as well as improves foam cell efferocytic function. This may be important in slowing disease progression and provides insight into the protective role of C1q in early atherosclerosis.

The production and secretion of complement component C1q by human mast cells

C1q is the initiation molecule of the classical pathway of the complement system and is produced by macrophages and immature dendritic cells. As mast cells share the same myeloid progenitor cells, we have studied whether also mast cells can produce and secrete C1q. Mast cells were generated in vitro from CD34+ progenitor cells from buffy coats or cord blood. Fully differentiated mast cells were shown by both RNA sequencing and qPCR to express C1QA, C1QB and C1QC. C1q produced by mast cells has a similar molecular make-up as serum C1q. Reconstituting C1q depleted serum with mast cell supernatant in haemolytic assays, indicated that C1q secreted by mast cells is functionally active. The level of C1q in supernatants produced under basal conditions was considerably enhanced upon stimulation with LPS, dexamethasone in combination with IFN- γ or via FcεRI triggering. Mast cells in human tissues stained positive for C1q in both healthy and in inflamed tissue. Moreover, mast cells in healthy and diseased skin appear to be the predominant C1q positive cells. Together, our data reveal that mast cells are able to produce and secrete functional active C1q and indicate mast cells as a local source of C1q in human tissue.

Complimentary action: C1q increases ganglion cell survival in an in vitro model of retinal degeneration

Using a previously described retinal explant culture system as an acute injury model, we here explore the role of C1q, the initiator of the classical complement pathway, in neuronal cell survival and retinal homeostasis. Full-thickness adult rat retinal explants were divided into four groups, receiving the following supplementation: C1q (50nM), C1-inhibitor (C1-inh; Berinert; 500mg/l), C1q+C1-inh, and no supplementation (culture controls). Explants were kept for 12h or 2days after which they were examined morphologically and with a panel of immunohistochemical markers. C1q supplementation protects ganglion cells from degeneration within the explant in vitro system. This effect is correlated to an attenuated endogenous production of C1q, and a quiesced gliotic response.

Macrophage molecular signaling and inflammatory responses during ingestion of atherogenic lipoproteins are modulated by complement protein C1q

BACKGROUND AND AIMS

This study investigated the effect of innate immune protein C1q on macrophage programmed responses during the ingestion of atherogenic lipoproteins. C1q plays a dual role in atherosclerosis where activation of complement by C1q is known to drive inflammation and promote disease progression. However, C1q is atheroprotective in early disease using mouse models. Our previous studies have highlighted a non-complement associated role for C1q in polarizing macrophages towards an M2-like anti-inflammatory phenotype during ingestion of targets such as atherogenic lipoproteins. This study aims to investigate the molecular mechanisms involved.

METHODS

We investigated the molecular signaling mechanisms involved in macrophage polarization using an unbiased examination of gene expression profiles in human monocyte derived macrophages ingesting oxidized or acetylated low density lipoproteins in the presence or absence of C1q.

RESULTS

Expression of genes involved in Janus kinase and signal transducer and activator of transcription (JAK-STAT) signaling, peroxisome proliferator activating receptor (PPAR) signaling and toll-like receptor (TLR) signaling were modulated by C1q in this screen. C1q was also shown to significantly suppress JAK-STAT pathway activation (a maximum 55% ± 13% reduction, p = 0.044) and increase transcriptional activation of PPARs (a maximum 229% ± 54% increase, p = 0.0002), consistent with an M2-like polarized response. These pathways were regulated in macrophages by C1q bound to different types of modified atherogenic lipoprotein and led to a reduction in the release of pro-inflammatory cytokine IL-6.

CONCLUSIONS

This study identifies potential molecular mechanisms for the beneficial role C1q plays in early atherosclerosis.

Complement is activated in progressive multiple sclerosis cortical grey matter lesions

BACKGROUND

The symptoms of multiple sclerosis (MS) are caused by damage to myelin and nerve cells in the brain and spinal cord. Inflammation is tightly linked with neurodegeneration, and it is the accumulation of neurodegeneration that underlies increasing neurological disability in progressive MS. Determining pathological mechanisms at play in MS grey matter is therefore a key to our understanding of disease progression.

METHODS

We analysed complement expression and activation by immunocytochemistry and in situ hybridisation in frozen or formalin-fixed paraffin-embedded post-mortem tissue blocks from 22 progressive MS cases and made comparisons to inflammatory central nervous system disease and non-neurological disease controls.

RESULTS

Expression of the transcript for C1qA was noted in neurons and the activation fragment and opsonin C3b-labelled neurons and glia in the MS cortical and deep grey matter. The density of immunostained cells positive for the classical complement pathway protein C1q and the alternative complement pathway activation fragment Bb was significantly increased in cortical grey matter lesions in comparison to control grey matter. The number of cells immunostained for the membrane attack complex was elevated in cortical lesions, indicating complement activation to completion. The numbers of classical (C1-inhibitor) and alternative (factor H) pathway regulator-positive cells were unchanged between MS and controls, whilst complement anaphylatoxin receptor-bearing microglia in the MS cortex were found closely apposed to cortical neurons. Complement immunopositive neurons displayed an altered nuclear morphology, indicative of cell stress/damage, supporting our finding of significant neurodegeneration in cortical grey matter lesions.

CONCLUSIONS

Complement is activated in the MS cortical grey matter lesions in areas of elevated numbers of complement receptor-positive microglia and suggests that complement over-activation may contribute to the worsening pathology that underlies the irreversible progression of MS.

Complement Component C1q Programs a Pro-Efferocytic Phenotype while Limiting TNFα Production in Primary Mouse and Human Macrophages

Deficiency in complement component C1q is associated with an inability to clear apoptotic cells (efferocytosis) and aberrant inflammation in lupus, and identification of the pathways involved in these processes should reveal important regulatory mechanisms in lupus and other autoimmune or inflammatory diseases. In this study, C1q-dependent regulation of TNFα/IL-6 expression and efferocytosis was investigated using primary mouse bone marrow-derived macrophages and human monocyte-derived macrophages. C1q downregulated LPS-dependent TNFα production in mouse and human macrophages. While prolonged stimulation with C1q (18 h) was required to elicit a dampening of TNFα production from mouse macrophages, the human macrophages responded to C1q with immediate downregulation of TNFα. IL-6 production was unchanged in mouse and upregulated by human macrophages following prolonged stimulation with C1q. Our previous studies indicated that C1q programmed enhanced efferocytosis in mouse macrophages by enhancing expression of Mer tyrosine kinase and its ligand Gas6, a receptor–ligand pair that also inhibits proinflammatory signaling. Here, we demonstrated that C1q-dependent programming of human macrophage efferocytosis required protein synthesis; however, neither Mer nor the related receptor Axl was upregulated in human cells. In addition, while the C1q-collagen-like tails are sufficient for promoting C1q-dependent phagocytosis of antibody-coated targets, the C1q-tails failed to program enhanced efferocytosis or dampen TNFα production. These data further elucidate the mechanisms by which C1q regulates proinflammatory signaling and efferocytosis in macrophages, functions that are likely to influence the progression of autoimmunity and chronic inflammation.

Analysis of the Putative Role of CR1 in Alzheimer's Disease: Genetic Association, Expression and Function

Chronic activation of the complement system and induced inflammation are associated with neuropathology in Alzheimer’s disease (AD). Recent large genome wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs) in the C3b/C4b receptor (CR1 or CD35) that are associated with late onset AD. Here, anti-CR1 antibodies (Abs) directed against different epitopes of the receptor, were used to localize CR1 in brain, and relative binding affinities of the CR1 ligands, C1q and C3b, were assessed by ELISA. Most Abs tested stained red blood cells in blood vessels but showed no staining in brain parenchyma. However, two monoclonal anti-CR1 Abs labeled astrocytes in all of the cases tested, and this reactivity was preabsorbed by purified recombinant human CR1. Human brain-derived astrocyte cultures were also reactive with both mAbs. The amount of astrocyte staining varied among the samples, but no consistent difference was conferred by diagnosis or the GWAS-identified SNPs rs4844609 or rs6656401. Plasma levels of soluble CR1 did not correlate with diagnosis but a slight increase was observed with rs4844609 and rs6656401 SNP. There was also a modest but statistically significant increase in relative binding activity of C1q to CR1 with the rs4844609 SNP compared to CR1 without the SNP, and of C3b to CR1 in the CR1 genotypes containing the rs6656401 SNP (also associated with the larger isoform of CR1) regardless of clinical diagnosis. These results suggest that it is unlikely that astrocyte CR1 expression levels or C1q or C3b binding activity are the cause of the GWAS identified association of CR1 variants with AD. Further careful functional studies are needed to determine if the variant-dictated number of CR1 expressed on red blood cells contributes to the role of this receptor in the progression of AD, or if another mechanism is involved.

Anti-C1q Autoantibodies from Systemic Lupus Erythematosus Patients Induce a Proinflammatory Phenotype in Macrophages

Anti-C1q autoantibodies (anti-C1q) are frequently found in patients with systemic lupus erythematosus (SLE) and correlate with the occurrence of proliferative lupus nephritis. A previous study of anti-C1q in experimental lupus nephritis demonstrated an important role for FcγRs in the pathogenesis of lupus nephritis, suggesting a direct effect on phagocytes. Therefore, we developed an in vitro model to study the effect of SLE patient-derived anti-C1q bound to immobilized C1q (imC1q) on human monocyte-derived macrophages (HMDMs) obtained from healthy donors and SLE patients. HMDMs were investigated by analyzing the cell morphology, LPS-induced cytokine profile, surface marker expression, and phagocytosis rate of apoptotic Jurkat cells. Morphologically, bound anti-C1q induced cell aggregations of HMDMs compared with imC1q or IgG alone. In addition, anti-C1q reversed the effect of imC1q alone, shifting the LPS-induced cytokine release toward a proinflammatory response. FcγR-blocking experiments revealed that the secretion of proinflammatory cytokines was mediated via FcγRII. The anti-C1q-induced inflammatory cytokine profile was accompanied by a downregulation of CD163 and an upregulation of LPS-induced CD80, CD274, and MHC class II. Finally, HMDMs primed on bound anti-C1q versus imC1q alone displayed a significantly lower phagocytosis rate of early and late apoptotic cells accompanied by a reduced Mer tyrosine kinase expression. Interestingly, anti-C1q-dependent secretion of proinflammatory cytokines was similar in SLE patient-derived cells, with the exception that IL-10 was slightly increased. In conclusion, anti-C1q induced a proinflammatory phenotype in HMDMs reversing the effects of imC1q alone. This effect might exacerbate underlying pathogenic mechanisms in lupus nephritis.

Uncoupling complement C1s activation from C1q binding in apoptotic cell phagocytosis and immunosuppressive capacity

Complement activation contributes to inflammation in many diseases, yet it also supports physiologic apoptotic cells (AC) clearance and its downstream immunosuppressive effects. The roles of individual complement components in AC phagocytosis have been difficult to dissect with artificially depleted sera. Using human in vitro systems and the novel antibody complement C1s inhibitor TNT003, we uncoupled the role of the enzymatic activation of the classical pathway from the opsonizing role of C1q in mediating a) the phagocytosis of early and late AC, and b) the immunosuppressive capacity of early AC. We found that C1s inhibition had a small impact on the physiologic clearance of early AC, leaving their immunosuppressive properties entirely unaffected, while mainly inhibiting the phagocytosis of late apoptotic/secondary necrotic cells. Our data suggest that C1s inhibition may represent a valuable therapeutic strategy to control classical pathway activation without causing significant AC accumulation in diseases without defects in AC phagocytosis.

The role of C1q in recognition of apoptotic epithelial cells and inflammatory cytokine production by phagocytes during Helicobacter pylori infection

Background

Gastric epithelial cells (GECs) undergo apoptosis during H. pylori infection and phagocytes within the mucosa engulf these cells. The recognition and clearance of apoptotic cells is a multifactorial process, enhanced by the presence of various bridging molecules and opsonins which are abundant in serum. However, it is not clear how recognition or clearance may differ in the context of H. pylori infection induced apoptosis. In addition, efferocytosis of sterile apoptotic cells is known to confer anti-inflammatory properties in the engulfing phagocyte, however it is unknown if this is maintained when phagocytes encounter H. pylori-infected cells. Thus, the ability of macrophages to bind and engulf gastric epithelial cells rendered apoptotic by H. pylori infection and the association of these interactions to the modulation of phagocyte inflammatory responses was investigated in the absence and presence of serum with a particular focus on the role of serum protein C1q.

Methods

Control (uninfected) or H. pylori-infected AGS cells were co-cultured with THP-1 macrophages in the presence or absence of serum or serum free conditions + C1q protein (40–80 μg/mL). Binding of AGS cells to THP-1 macrophages was assessed by microscopy and cytokine (IL-6 and TNF-α) release from LPS stimulated THP-1 macrophages was quantified by ELISA.

Results

We show that macrophages bound preferentially to cells undergoing apoptosis subsequent to infection with H. pylori. Binding of apoptotic AGS to THP-1 macrophages was significantly inhibited when studied in the absence of serum and reconstitution of serum-free medium with purified human C1q restored binding of macrophages to apoptotic cells. Co-culture of sterile apoptotic and H. pylori-infected AGS cells both attenuated LPS-stimulated cytokine production by THP-1 macrophages. Further, direct treatment of THP-1 macrophages with C1q attenuated LPS stimulated TNF-α production.

Conclusions

These studies suggest that C1q opsonizes GECs rendered apoptotic by H. pylori. No differences existed in the ability of infected or sterile apoptotic cells to attenuate macrophage cytokine production, however, there may be a direct role for C1q in modulating macrophage inflammatory cytokine production to infectious stimuli.

Electronic supplementary material

The online version of this article (doi:10.1186/s12950-015-0098-8) contains supplementary material, which is available to authorized users.

Complement System Part II: Role in Immunity

The complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology. It starts with a description of complement contribution to the normal physiology (homeostasis) of a healthy organism, including the silent clearance of apoptotic cells and maintenance of cell survival. In pathology, complement can be a friend or a foe. It acts as a friend in the defense against pathogens, by inducing opsonization and a direct killing by C5b–9 membrane attack complex and by triggering inflammatory responses with the anaphylatoxins C3a and C5a. Opsonization plays also a major role in the mounting of an adaptive immune response, involving antigen presenting cells, T-, and B-lymphocytes. Nevertheless, it can be also an enemy, when pathogens hijack complement regulators to protect themselves from the immune system. Inadequate complement activation becomes a disease cause, as in atypical hemolytic uremic syndrome, C3 glomerulopathies, and systemic lupus erythematosus. Age-related macular degeneration and cancer will be described as examples showing that complement contributes to a large variety of conditions, far exceeding the classical examples of diseases associated with complement deficiencies. Finally, we discuss complement as a therapeutic target.

Protective natural autoantibodies to apoptotic cells: evidence of convergent selection of recurrent innate-like clones

During murine immune development, recurrent B cell clones arise in a predictable fashion. Among these B cells, an archetypical clonotypic set that recognizes phosphorylcholine (PC) antigens and produces anti-PC IgM, first implicated for roles in microbial protection, was later found to become expanded in hyperlipidemic mice and in response to an increased in vivo burden of apoptotic cells. These IgM natural antibodies can enhance clearance of damaged cells and induce intracellular blockade of inflammatory signaling cascades. In clinical populations, raised levels of anti-PC IgM correlate with protection from atherosclerosis and may also downmodulate the severity of autoimmune disease. Human anti-PC-producing clones without hypermutation have been isolated that can similarly discriminate apoptotic from healthy cells. An independent report on unrelated adults has described anti-PC-producing B cells with IgM genes that have conserved CDR3 motifs, similar to stereotypic clonal sets of B cell chronic lymphocytic leukemia. Taken together, emerging evidence suggests that, despite the capacity to form an effectively limitless range of Ig receptors, the human immune system may often recurrently generate lymphocytes expressing structurally convergent B cell receptors with protective and homeostatic roles.

The paradoxical roles of C1q and C3 in autoimmunity

In this review we will focus on the links between complement and autoimmune diseases and will highlight how animal models have provided insights into the manner by which C1q and C3 act to modulate both adaptive and innate immune responses. In particular we will highlight how C1q may not only act as initiator of the classical complement pathway, but can also mediate multiple immune responses in a complement activation independent manner.

Beyond apoptosis in lupus

PURPOSE OF REVIEW

Systemic lupus erythematosus (SLE) is characterized by autoantibodies directed against nuclear autoantigens normally concealed from immune recognition in healthy individuals. Here, we summarize recently identified mechanisms of abnormal cell death leading to exposure and aberrant processing of nucleoprotein self antigens, and discuss their role in the SLE pathogenesis.

RECENT FINDINGS

During the past few years, the unveiling of several new forms of cell death has expanded our understanding beyond the simple view of 'apoptotic' versus 'necrotic' cell death. SLE patients show abnormalities in cell death at several levels, including increased rates of apoptosis, necrosis, and autophagy, as well as reduced clearance of dying cells. These abnormalities lead to an increased autoantigen burden and antigen modifications, such as nucleic acid oxidation that increases the inflammatory properties of self antigens. Recent investigations have highlighted the role of opsonins in determining the immunogenic versus tolerogenic characteristics of self antigens.

SUMMARY

Dysregulation of different forms of programmed cell death contributes to increased exposure, availability, and immunogenic characteristics of intracellular self antigens, which all participate in development of lupus autoimmunity. As our understanding of abnormalities of cell death in SLE advances, potential therapeutic opportunities await human implementation.

Extracellular traps and macrophages: new roles for the versatile phagocyte

MΦ are multipurpose phagocytes with a large repertoire of well-characterized abilities and functions, including regulation of inflammation, wound healing, maintenance of tissue homeostasis, as well as serving as an integral component of the innate-immune defense against microbial pathogens. Working along with neutrophils and dendritic cells, the other myeloid-derived professional phagocytes, MΦ are one of the key effector cells initiating and directing the host reaction to pathogenic organisms and resolving subsequent responses once the threat has been cleared. ETs are a relatively novel strategy of host defense involving expulsion of nuclear material and embedded proteins from immune cells to immobilize and kill bacteria, fungi, and viruses. As research on ETs expands, it has begun to encompass many immune cell types in unexpected ways, including various types of MΦ, which are not only capable of generating METs in response to various stimuli, but recent preclinical data suggest that they are an important agent in clearing ETs and limiting ET-mediated inflammation and tissue damage. This review aims to summarize historical and recent findings of biologic research regarding ET formation and function and discuss the role of MΦ in ET physiology and associated pathologies.

Complement protein C1q bound to apoptotic cells suppresses human macrophage and dendritic cell-mediated Th17 and Th1 T cell subset proliferation

A complete genetic deficiency of the complement protein C1q results in SLE with nearly 100% penetrance in humans, but the molecular mechanisms responsible for this association have not yet been fully determined. C1q opsonizes ACs for enhanced ingestion by phagocytes, such as Mϕ and iDCs, avoiding the extracellular release of inflammatory DAMPs upon loss of the membrane integrity of the dying cell. We previously showed that human monocyte-derived Mϕ and DCs ingesting autologous, C1q-bound LALs (C1q-polarized Mϕ and C1q-polarized DCs), enhance the production of anti-inflammatory cytokines, and reduce proinflammatory cytokines relative to Mϕ or DC ingesting LAL alone. Here, we show that C1q-polarized Mϕ have elevated PD-L1 and PD-L2 and suppressed surface CD40, and C1q-polarized DCs have higher surface PD-L2 and less CD86 relative to Mϕ or DC ingesting LAL alone, respectively. In an MLR, C1q-polarized Mϕ reduced allogeneic and autologous Th17 and Th1 subset proliferation and demonstrated a trend toward increased Treg proliferation relative to Mϕ ingesting LAL alone. Moreover, relative to DC ingesting AC in the absence of C1q, C1q-polarized DCs decreased autologous Th17 and Th1 proliferation. These data demonstrate that a functional consequence of C1q-polarized Mϕ and DC is the regulation of Teff activation, thereby "sculpting" the adaptive immune system to avoid autoimmunity, while clearing dying cells. It is noteworthy that these studies identify novel target pathways for therapeutic intervention in SLE and other autoimmune diseases.

Complement modulation of T cell immune responses during homeostasis and disease

The complement system is an ancient and critical effector mechanism of the innate immune system as it senses, kills, and clears infectious and/or dangerous particles and alerts the immune system to the presence of the infection and/or danger. Interestingly, an increasing number of reports have demonstrated a clear role for complement in the adaptive immune system as well. Of note, a number of recent studies have identified previously unknown roles for complement proteins, receptors, and regulators in T cell function. Here, we will review recent data demonstrating the influence of complement proteins C1q, C3b/iC3b, C3a (and C3aR), and C5a (and C5aR) and complement regulators DAF (CD55) and CD46 (MCP) on T cell function during homeostasis and disease. Although new concepts are beginning to emerge in the field of complement regulation of T cell function, future experiments should focus on whether complement is interacting directly with the T cell or is having an indirect effect on T cell function via APCs, the cytokine milieu, or downstream complement activation products. Importantly, the identification of the pivotal molecular pathways in the human systems will be beneficial in the translation of concepts derived from model systems to therapeutic targeting for treatment of human disorders.

Complement, c1q, and c1q-related molecules regulate macrophage polarization

Complement is a critical system of enzymes, regulatory proteins, and receptors that regulates both innate and adaptive immune responses. Natural mutations in complement molecules highlight their requirement in regulation of a variety of human conditions including infectious disease and autoimmunity. As sentinels of the immune system, macrophages are specialized to respond to infectious microbes, as well as normal and altered self, and dictate appropriate immune responses. Complement components such as anaphylatoxins (C3a and C5a) and opsonins [C3b, C1q, mannan binding lectin (MBL)] influence macrophage responses. While anaphylatoxins C3a and C5a trigger inflammasome activation, opsonins such as C1q and related molecules (MBL and adiponectin) downregulate inflammasome activation and inflammation, and upregulate engulfment of apoptotic cells consistent with a pro-resolving or M2 macrophage phenotype. This review summarizes our current understanding of the influence of the complement system on macrophage polarization with an emphasis on C1q and related molecules.

Complement protein C1q promotes macrophage anti-inflammatory M2-like polarization during the clearance of atherogenic lipoproteins

OBJECTIVE

Innate immune protein C1q plays a dual role in the chronic inflammatory disease of atherosclerosis. Complement activation via C1q exacerbates pathology in the atherosclerotic lesion in later stages of the disease. However, in early stages of disease C1q is protective. We hypothesize that complement-independent activities of C1q are involved in reprogramming macrophage inflammatory polarization.

METHODS

The influence of C1q on macrophage inflammatory responses during clearance of oxLDL was examined. Changes in cytokines at the gene and protein level were measured by quantitative PCR and ELISA assay.

RESULTS

C1q modulated cytokine expression in Raw264.7 macrophages during ingestion of oxLDL. Levels of pro-inflammatory cytokines IL-1β and IL-6 were downregulated by C1q, whereas levels of the anti-inflammatory cytokine IL-10 were increased. In addition, data from an NFκB-luciferase gene reporter assay suggest that C1q suppresses activation of NFκB during lipoprotein clearance in macrophages, providing one mechanism by which C1q downregulates pro-inflammatory cytokine production.

CONCLUSIONS

C1q-polarization of macrophages toward an anti-inflammatory (M2-like) phenotype may be important in dampening inflammation in the early atherosclerotic lesion. Further investigation of molecular pathways targeted by C1q may provide novel therapeutic targets for this disease.

Anti-C1q autoantibodies from active lupus nephritis patients could inhibit the clearance of apoptotic cells and complement classical pathway activation mediated by C1q in vitro

Anti-C1q antibodies are prevalent in patients with active lupus nephritis and were found to be closely associated with renal involvement and predictive for a flare of nephritis. However, the pathogenesis of anti-C1q antibodies involved in human lupus nephritis remains unclear. C1q, which plays a key role in apoptotic cell and immune complex removal, is a very important functional molecule in the pathogenesis of SLE. The aim of this study was to investigate the influence of anti-C1q autoantibodies from active lupus nephritis patients on the bio-functions of C1q in vitro. We purified IgG autoantibodies against C1q from lupus nephritis patients, and found that they could recognize C1q bound on early apoptotic cells at 30 μg/ml, and could significantly decrease the phagocytosis by macrophages of early apoptotic cells opsonized by 50 μg/ml C1q in comparison with normal IgG. Levels of circulating immune complexes of the ten patients were measured by a circulating immune complexes (CIC)-C1q Enzyme Immunoassay Kit. Anti-C1q autoantibodies affinity purified by microtiter plates could significantly inhibit the deposition of C3c on CIC-C1q in a dose dependent manner in comparison with IgG from 10 healthy blood donors. The binding of opsonized immune complexes to RBCs was significantly inhibited by anti-C1q autoantibodies purified by microtiter plates in a dose dependent manner. Our observations suggest that serum anti-C1q autoantibodies from active lupus nephritis patients could interfere with some biological function of C1q in vitro.

Relative contribution of c1q and apoptotic cell-surface calreticulin to macrophage phagocytosis

C1q has been shown to recognize apoptotic cells, to enhance their uptake and to modulate cytokine release by phagocytes and thus promote immune tolerance. Surface-exposed calreticulin (CRT), known as a C1q receptor, is also considered to be an early eat-me signal that enhances the phagocytosis of apoptotic cells and is capable of eliciting an immunogenic response. However, the molecular mechanisms that trigger these functions are not clear. We hypothesized that CRT and C1q might act together in these processes. We first showed, by means of fluorescence resonance energy transfer (FRET), that CRT interacts with the C1q globular region at the surface of early apoptotic cells. Next, we pointed out that knockdown of CRT on early apoptotic HeLa cells impairs the enhancement effect of C1q on their uptake by THP-1 monocyte-derived macrophages. Furthermore, a deficiency of CRT induces contrasting effects on cytokine release by THP-1 macrophages, increasing interleukin (IL)-6 and monocyte chemotactic protein 1/CCL2 and decreasing IL-8. Remarkably, these effects were greatly reduced when apoptotic cells were opsonized by C1q, which counterbalanced the effect of the CRT deficiency. These results demonstrate that CRT-C1q interaction is involved in the C1q bridging function and they highlight the particular ability of C1q to control the phagocyte inflammatory status, i.e. by integrating the molecular changes that could occur at the surface of dying cells.

C1q induction and global complement pathway activation do not contribute to ALS toxicity in mutant SOD1 mice

Accumulating evidence from mice expressing ALS-causing mutations in superoxide dismutase (SOD1) has implicated pathological immune responses in motor neuron degeneration. This includes microglial activation, lymphocyte infiltration, and the induction of C1q, the initiating component of the classic complement system that is the protein-based arm of the innate immune response, in motor neurons of multiple ALS mouse models expressing dismutase active or inactive SOD1 mutants. Robust induction early in disease course is now identified for multiple complement components (including C1q, C4, and C3) in spinal cords of SOD1 mutant-expressing mice, consistent with initial intraneuronal C1q induction, followed by global activation of the complement pathway. We now test if this activation is a mechanistic contributor to disease. Deletion of the C1q gene in mice expressing an ALS-causing mutant in SOD1 to eliminate C1q induction, and complement cascade activation that follows from it, is demonstrated to produce changes in microglial morphology accompanied by enhanced loss, not retention, of synaptic densities during disease. C1q-dependent synaptic loss is shown to be especially prominent for cholinergic C-bouton nerve terminal input onto motor neurons in affected C1q-deleted SOD1 mutant mice. Nevertheless, overall onset and progression of disease are unaffected in C1q- and C3-deleted ALS mice, thus establishing that C1q induction and classic or alternative complement pathway activation do not contribute significantly to SOD1 mutant-mediated ALS pathogenesis in mice.

An alternative role of C1q in bacterial infections: facilitating Streptococcus pneumoniae adherence and invasion of host cells

Streptococcus pneumoniae (pneumococcus) is a major human pathogen, which evolved numerous successful strategies to colonize the host. In this study, we report a novel mechanism of pneumococcal-host interaction, whereby pneumococci use a host complement protein C1q, primarily involved in the host-defense mechanism, for colonization and subsequent dissemination. Using cell-culture infection assays and confocal microscopy, we observed that pneumococcal surface-bound C1q significantly enhanced pneumococcal adherence to and invasion of host epithelial and endothelial cells. Flow cytometry demonstrated a direct, Ab-independent binding of purified C1q to various clinical isolates of pneumococci. This interaction was seemingly capsule serotype independent and mediated by the bacterial surface-exposed proteins, as pretreatment of pneumococci with pronase E but not sodium periodate significantly reduced C1q binding. Moreover, similar binding was observed using C1 complex as the source of C1q. Furthermore, our data show that C1q bound to the pneumococcal surface through the globular heads and with the host cell-surface receptor(s)/glycosaminoglycans via its N-terminal collagen-like stalk, as the presence of C1q N-terminal fragment and low m.w. heparin but not the C-terminal globular heads blocked C1q-mediated pneumococcal adherence to host cells. Taken together, we demonstrate for the first time, to our knowledge, a unique function of complement protein C1q, as a molecular bridge between pneumococci and the host, which promotes bacterial cellular adherence and invasion. Nevertheless, in some conditions, this mechanism could be also beneficial for the host as it may result in uptake and clearance of the bacteria.

Nucleic acids and endosomal pattern recognition: how to tell friend from foe?

The innate immune system has evolved endosomal and cytoplasmic receptors for the detection of viral nucleic acids as sensors for virus infection. Some of these pattern recognition receptors (PRR) detect features of viral nucleic acids that are not found in the host such as long stretches of double-stranded RNA (dsRNA) and uncapped single-stranded RNA (ssRNA) in case of Toll-like receptor (TLR) 3 and RIG-I, respectively. In contrast, TLR7/8 and TLR9 are unable to distinguish between viral and self-nucleic acids on the grounds of distinct molecular patterns. The ability of these endosomal TLR to act as PRR for viral nucleic acids seems to rely solely on the mode of access to the endolysosomal compartment in which recognition takes place. The current dogma states that self-nucleic acids do not enter the TLR-sensing compartment under normal physiological conditions. However, it is still poorly understood how dendritic cells (DC) evade activation by self-nucleic acids, in particular with regard to specific DC subsets, which are specialized in taking up material from dying cells for cross-presentation of cell-associated antigens. In this review we discuss the current understanding of how the immune system distinguishes between foreign and self-nucleic acids and point out some of the key aspects that still require further research and clarification.

Protective autoantibodies in the rheumatic diseases: lessons for therapy

The adaptive immune system augments host defenses against diverse infectious threats, yet also carries intertwined risks for the development of autoimmune disease. The immune system incorporates homeostatic pathways for essential housekeeping functions that involve recognition of oxidation-modified endogenous molecules. Now, the properties of a physiological class of natural autoantibodies, which seem to modulate the severity or even prevent the onset of autoimmune disease, are beginning to be defined. Whereas disease-associated IgG autoantibodies to nuclear antigens and citrulline-modified self-proteins have been shown to activate innate pattern recognition receptors leading to increased cell death and tissue injury, a class of IgM autoantibodies to oxidation-associated neo-antigens can oppose these pathogenic effects. These naturally arising regulatory IgM autoantibodies enhance the capacity for the phagocytic clearance of host cells affected by programmed death pathways. These antibodies can also suppress key signalling pathways in the innate immune system involved in the control and resolution of inflammatory responses to Toll-like receptor agonists and disease-associated IgG autoantibodies.

Oxidative stress sensitizes retinal pigmented epithelial (RPE) cells to complement-mediated injury in a natural antibody-, lectin pathway-, and phospholipid epitope-dependent manner

Uncontrolled activation of the alternative complement pathway (AP) is thought to be associated with age-related macular degeneration. Previously, we have shown that in retinal pigmented epithelial (RPE) monolayers, oxidative stress reduced complement inhibition on the cell surface, resulting in sublytic complement activation and loss of transepithelial resistance (TER), but the potential ligand and pathway involved are unknown. ARPE-19 cells were grown as monolayers on transwell plates, and sublytic complement activation was induced with H2O2 and normal human serum. TER deteriorated rapidly in H2O2-exposed monolayers upon adding normal human serum. Although the effect required AP activation, AP was not sufficient, because elimination of MASP, but not C1q, prevented TER reduction. Reconstitution experiments to unravel essential components of the lectin pathway (LP) showed that both ficolin and mannan-binding lectin can activate the LP through natural IgM antibodies (IgM-C2) that recognize phospholipid cell surface modifications on oxidatively stressed RPE cells. The same epitopes were found on human primary embryonic RPE monolayers. Likewise, mouse laser-induced choroidal neovascularization, an injury that involves LP activation, could be increased in antibody-deficient rag1(-/-) mice using the phospholipid-specific IgM-C2. In summary, using a combination of depletion and reconstitution strategies, we have shown that the LP is required to initiate the complement cascade following natural antibody recognition of neoepitopes, which is then further amplified by the AP. LP activation is triggered by IgM bound to phospholipids. Taken together, we have defined novel mechanisms of complement activation in oxidatively stressed RPE, linking molecular events involved in age-related macular degeneration, including the presence of natural antibodies and neoepitopes.

Fundamental roles of the innate-like repertoire of natural antibodies in immune homeostasis

The composition of the early immune repertoire is biased with prominent expression of spontaneously arising B cell clones that produce IgM with recurrent and often autoreactive binding specificities. Amongst these naturally arising antibodies (NAbs) are IgM antibodies that specifically recognized amaged and senescent cells, often via oxidation-associated neo-determinants. These NAbs are present from birth and can be further boosted by apoptotic cell challenge. Recent studies have shown that IgM NAb to apoptotic cells can enhance phagocytic clearance, as well as suppress proinflammatory responses induced via Toll-like receptors, and block pathogenic IgG-immune complex (IC)-mediated inflammatory responses. Specific antibody effector functions appear to be involved, as these anti-inflammatory properties are dependent on IgM-mediated recruitment of the early recognition factors of complement. Clinical surveys have suggested that anti-apoptotic cell (AC) IgM NAbs may modulate disease activity in some patients with autoimmune disease. In mechanistic studies, anti-AC NAbs were shown to act in dendritic cells by inhibition of the mitogen-activated protein kinase (MAPK) pathway, a primary signal transduction pathway that controls inflammatory responses. This immunomodulatory pathway has an absolute requirement for the induction of MAPK phosphatase-1. Taken together, recent studies have elucidated the novel properties of a class of protective NAbs, which may directly blunt inflammatory responses through a primitive pathway for regulation of the innate immune system.

Up-regulation of the complement system in subcutaneous adipocytes from nonobese, hypertriglyceridemic subjects is associated with adipocyte insulin resistance

BACKGROUND

Dysfunctional adipose tissue plays an important role in the etiology of the metabolic syndrome, type 2 diabetes, and dyslipidemia. However, the molecular mechanisms underlying adipocyte dysfunction are incompletely understood.

AIM

The aim of the study was to identify differentially regulated pathways in sc adipocytes of dyslipidemic subjects.

METHODS

Whole-genome expression profiling was conducted on sc adipocytes from a discovery group of nine marginally overweight subjects with familial combined hyperlipidemia (FCHL) and nine controls of comparable body sizes as well as two independent confirmation groups. In this study, FCHL served as a model of familial insulin resistance and dyslipidemia, in the absence of frank obesity.

RESULTS

Functional analyses and gene set enrichment analysis using the Kyoto Encyclopedia of Genes and Genomes or a custom pathway database identified the complement system and complement regulators as one of the top up-regulated pathways in FCHL [false discovery rate (FDR) < 1E-30]. Higher adipocyte complement expression in FCHL was confirmed in the appropriate confirmation group. Higher complement gene expression was associated with lower adipocyte insulin receptor substrate-1 expression as marker of adipocyte insulin resistance, independent of age, sex, or disease status, and this association was corroborated in the two confirmation groups. Additionally, complement gene expression was associated with triglycerides in the discovery set and with triglycerides and/or waist circumference in the confirmation groups. Complement pathway up-regulation did not appear to be driven by hypertriglyceridemia because a 40% pharmacological reduction in triglycerides did not affect complement expression.

CONCLUSIONS

These findings point to an up-regulation of a complement-related transcriptome in sc adipocytes under metabolically stressed conditions, even in the absence of overt obesity. Such up-regulation may subsequently influence downstream processes, including macrophage infiltration into adipose tissue and adipocyte insulin resistance.

C1q limits dendritic cell differentiation and activation by engaging LAIR-1

C1q, the first component of complement, and leukocyte-associated Ig-like receptor 1 (LAIR-1; CD305), an inhibitory receptor expressed on hematopoietic cells, have both been associated with arrest of monocyte-derived dendritic cell (DC) differentiation and inhibition of Toll-like receptor activity in plasmacytoid DCs. Defects in both molecules have been implicated in susceptibility to, and progression of, systemic lupus erythematosus. Inhibitory signaling partners for C1q on monocytes and DCs remain undefined. Because C1q contains collagen-like motifs and LAIR-1 is a universal collagen receptor, we hypothesized that C1q is a functional ligand for LAIR-1. Binding analyses in cell-free systems and on the cell membrane demonstrate that C1q and its collagen tail associate with LAIR-1 and LAIR-2 (CD306), a soluble inhibitor of LAIR-1. Both C1q and its collagen tail trigger phosphorylation of LAIR-1 immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in monocytes. Functional analyses show that C1q-mediated inhibition of monocyte-DC differentiation and C1q-mediated inhibition of IFN-α production by plasmacytoid DCs were both reversed by LAIR-2. Moreover, C1q-mediated inhibition of DC differentiation was reversed by LAIR-1 siRNA. Thus, C1q is a functional ligand for LAIR-1 restricting immune cell differentiation and activation. The discovery of C1q interactions with LAIR-1 and LAIR-2 lends much needed insight into molecular mechanisms operating to prevent the loss of tolerance, particularly in systemic lupus erythematosus.