Complement Activation-Independent Attenuation of SARS-CoV-2 Infection by C1q and C4b-Binding Protein.

The complement system is a key component of the innate immune response to viruses and proinflammatory events. Exaggerated complement activation has been attributed to the induction of a cytokine storm in severe SARS-CoV-2 infection. However, there is also an argument for the protective role of complement proteins, given their local synthesis or activation at the site of viral infection. This study investigated the complement activation-independent role of C1q and C4b-binding protein (C4BP) against SARS-CoV-2 infection. The interactions of C1q, its recombinant globular heads, and C4BP with the SARS-CoV-2 spike and receptor binding domain (RBD) were examined using direct ELISA. In addition, RT-qPCR was used to evaluate the modulatory effect of these complement proteins on the SARS-CoV-2-mediated immune response. Cell binding and luciferase-based viral entry assays were utilised to assess the effects of C1q, its recombinant globular heads, and C4BP on SARS-CoV-2 cell entry. C1q and C4BP bound directly to SARS-CoV-2 pseudotype particles via the RBD domain of the spike protein. C1q via its globular heads and C4BP were found to reduce binding as well as viral transduction of SARS-CoV-2 spike protein expressing lentiviral pseudotypes into transfected A549 cells expressing human ACE2 and TMPRSS2. Furthermore, the treatment of the SARS-CoV-2 spike, envelope, nucleoprotein, and membrane protein expressing alphaviral pseudotypes with C1q, its recombinant globular heads, or C4BP triggered a reduction in mRNA levels of proinflammatory cytokines and chemokines such as IL-1ß, IL-8, IL-6, TNF-α, IFN-α, and RANTES (as well as NF-κB) in A549 cells expressing human ACE2 and TMPRSS2. In addition, C1q and C4BP treatment also reduced SARS-CoV-2 pseudotype infection-mediated NF-κB activation in A549 cells expressing human ACE2 and TMPRSS2. C1q and C4BP are synthesised primarily by hepatocytes; however, they are also produced by macrophages, and alveolar type II cells, respectively, locally at the pulmonary site. These findings support the notion that the locally produced C1q and C4BP can be protective against SARS-CoV-2 infection in a complement activation-independent manner, offering immune resistance by inhibiting virus binding to target host cells and attenuating the infection-associated inflammatory response.

l-ficolin-MASP arm of the complement system in schizophrenia.

The abnormal neurodevelopment secondary to in utero adversities, such as hypoxia, malnutrition and maternal infections, underlies schizophrenia (SZ) etiology. As the genes of MBL-associated serine proteases (MASP) of the complement lectin pathway, MASP1 and MASP2, are expressed in the developing cortex and are functionally important for neuronal migration, we hypothesize that the malfunction ofl-ficolin-MASP arm may also be involved in schizophrenia pathophysiology as it was shown for MBL-MASP complexes. We investigated serum l-ficolin and plasma MASP-2 levels, the activity of l-ficolin-bound MASP-2, as well as an array of the complement-related variables in chronic schizophrenic patients in the acute phase of the disease and controls without physical or mental diagnoses. The median concentration of l-ficolin in Armenian controls was 3.66 µg/ml and similar to those reported for other Caucasian populations. SZ-cases had âˆ¼40 % increase in serum l-ficolin (median 5.08 µg/ml; P < 0.0024). In the pooled sample, l-ficolin level was higher in males than in females (P < 0.0031), but this gender dichotomy was not affecting the variable association with schizophrenia (P < 0.016). Remarkably, MASP-2 plasma concentration showed gender-dependent significant variability in the group of patients but not in controls. When adjusted for gender and gender*diagnosis interaction, a significantly high MASP-2 level in female patients versus female controls was observed (median: 362 ng/ml versus 260 ng/ml, respectively; P < 0.0020). A significant increase in l-ficolin-bound MASP-2 activity was also observed in schizophrenia (on the median, cases vs controls: 7.60 vs 6.50 RU; P < 0.021). Correlation analyses of the levels of l-ficolin and MASP-2, l-ficolin-(MASP-2) activity and the demographic data did not show any significant association with the age of individuals, family history, age at onset and duration of the illness, and smoking. Noteworthy, the levels of l-ficolin and MASP-2 in circulation were significantly associated with the type of schizophrenia (paranoid SZ-cases had much higher l-ficolin (P < 0.0035) and lower MASP-2 levels than the other types combined (P < 0.049)). Correlations were also found between: (i) the classical pathway functional activity and l-ficolin level (rs = 0.19, P < 0.010); (ii) the alternative pathway functional activity and MASP-2 level (rs = 0.26, P < 0.00035); (iii) the activity of l-ficolin-bound MASP2 and the downstream C2 component haemolytic activity (rs = -0.19, P < 0.017); and (iv) l-ficolin and the upstream C-reactive protein (CRP) serum concentrations (r = 0.28, P < 0.018). Overall, the results showed l-ficolin-related lectin pathway alterations in schizophrenia pathophysiology. It is likely that in addition to the MBL-MASP component over-activity reported previously, the alterations of the lectin pathway in schizophrenia also involve variations of l-ficolin-(MASP-2) on protein concentration and activity levels.

Fusarium verticillioides NAT1 (FDB2) N-malonyltransferase is structurally, functionally and phylogenetically distinct from its N-acetyltransferase (NAT) homologues.

Fusarium endophytes damage cereal crops and contaminate produce with mycotoxins. Those fungi overcome the main chemical defence of host via detoxification by a malonyl-CoA-dependent enzyme homologous to xenobiotic metabolizing arylamine N-acetyltransferase (NAT). In Fusarium verticillioides (teleomorph Gibberella moniliformis, GIBMO), this N-malonyltransferase activity is attributed to (GIBMO)NAT1, and the fungus has two additional isoenzymes, (GIBMO)NAT3 (N-acetyltransferase) and (GIBMO)NAT2 (unknown function). We present the crystallographic structure of (GIBMO)NAT1, also modelling other fungal NAT homologues. Monomeric (GIBMO)NAT1 is distinctive, with access to the catalytic core through two "tunnel-like" entries separated by a "bridge-like" helix. In the quaternary arrangement, (GIBMO)NAT1 monomers interact in pairs along an extensive interface whereby one entry of each monomer is covered by the N-terminus of the other monomer. Although monomeric (GIBMO)NAT1 apparently accommodates acetyl-CoA better than malonyl-CoA, dimerization changes the active site to allow malonyl-CoA to reach the catalytic triad (Cys110, His158 and Asp173) via the single uncovered entry, and anchor its terminal carboxyl-group via hydrogen bonds to Arg109, Asn157 and Thr261. Lacking a terminal carboxyl-group, acetyl-CoA cannot form such stabilizing interactions, while longer acyl-CoAs enter the active site but cannot reach catalytic Cys. Other NAT isoenzymes lack such structural features, with (GIBMO)NAT3 resembling bacterial NATs and (GIBMO)NAT2 adopting a structure intermediate between (GIBMO)NAT1 and (GIBMO)NAT3. Biochemical assays confirmed differential donor substrate preference of (GIBMO)NAT isoenzymes, with phylogenetic analysis demonstrating evolutionary separation. Given the role of (GIBMO)NAT1 in enhancing Fusarium pathogenicity, unravelling the structure and function of this enzyme may benefit research into more targeted strategies for pathogen control.

Human Properdin Released By Infiltrating Neutrophils Can Modulate Influenza A Virus Infection.

The complement system is designed to recognise and eliminate invading pathogens via activation of classical, alternative and lectin pathways. Human properdin stabilises the alternative pathway C3 convertase, resulting in an amplification loop that leads to the formation of C5 convertase, thereby acting as a positive regulator of the alternative pathway. It has been noted that human properdin on its own can operate as a pattern recognition receptor and exert immune functions outside its involvement in complement activation. Properdin can bind directly to microbial targets via DNA, sulfatides and glycosaminoglycans, apoptotic cells, nanoparticles, and well-known viral virulence factors. This study was aimed at investigating the complement-independent role of properdin against Influenza A virus infection. As one of the first immune cells to arrive at the site of IAV infection, we show here that IAV challenged neutrophils released properdin in a time-dependent manner. Properdin was found to directly interact with haemagglutinin, neuraminidase and matrix 1 protein Influenza A virus proteins in ELISA and western blot. Furthermore, modelling studies revealed that properdin could bind HA and NA of the H1N1 subtype with higher affinity compared to that of H3N2 due to the presence of an HA cleavage site in H1N1. In an infection assay using A549 cells, properdin suppressed viral replication in pH1N1 subtype while promoting replication of H3N2 subtype, as revealed by qPCR analysis of M1 transcripts. Properdin treatment triggered an anti-inflammatory response in H1N1-challenged A549 cells and a pro-inflammatory response in H3N2-infected cells, as evident from differential mRNA expression of TNF-α, NF-κB, IFN-α, IFN-ß, IL-6, IL-12 and RANTES. Properdin treatment also reduced luciferase reporter activity in MDCK cells transduced with H1N1 pseudotyped lentiviral particles; however, it was increased in the case of pseudotyped H3N2 particles. Collectively, we conclude that infiltrating neutrophils at the site of IAV infection can release properdin, which then acts as an entry inhibitor for pandemic H1N1 subtype while suppressing viral replication and inducing an anti-inflammatory response. H3N2 subtype can escape this immune restriction due to altered haemagglutinin and neuraminindase, leading to enhanced viral entry, replication and pro-inflammatory response. Thus, depending on the subtype, properdin can either limit or aggravate IAV infection in the host.

Complement Proteins as Soluble Pattern Recognition Receptors for Pathogenic Viruses.

The complement system represents a crucial part of innate immunity. It contains a diverse range of soluble activators, membrane-bound receptors, and regulators. Its principal function is to eliminate pathogens via activation of three distinct pathways: classical, alternative, and lectin. In the case of viruses, the complement activation results in effector functions such as virion opsonisation by complement components, phagocytosis induction, virolysis by the membrane attack complex, and promotion of immune responses through anaphylatoxins and chemotactic factors. Recent studies have shown that the addition of individual complement components can neutralise viruses without requiring the activation of the complement cascade. While the complement-mediated effector functions can neutralise a diverse range of viruses, numerous viruses have evolved mechanisms to subvert complement recognition/activation by encoding several proteins that inhibit the complement system, contributing to viral survival and pathogenesis. This review focuses on these complement-dependent and -independent interactions of complement components (especially C1q, C4b-binding protein, properdin, factor H, Mannose-binding lectin, and Ficolins) with several viruses and their consequences.

Complement-Independent Modulation of Influenza A Virus Infection by Factor H.

The complement system is an ancient innate immune defense mechanism that can recognize molecular patterns on the invading pathogens. Factor H, as an inhibitor of the alternative pathway, down-regulates complement activation on the host cell surface. Locally synthesized factor H at the site of infection/injury, including lungs, can act as a pattern recognition molecule without involving complement activation. Here, we report that factor H, a sialic acid binder, interacts with influenza A virus (IAV) and modulates IAV entry, as evident from down-regulation of matrix protein 1 (M1) in H1N1 subtype-infected cells and up-regulation of M1 expression in H3N2-infected A549 cells. Far-western blot revealed that factor H binds hemagglutinin (HA, ~70 kDa), neuraminidase (NA, ~60 kDa), and M1 (~25 kDa). IAV-induced transcriptional levels of IFN-α, TNF-α, IL-12, IL-6, IFN-α, and RANTES were reduced following factor H treatment for the H1N1 subtype at 6 h post-infection. However, for the H3N2 subtype, mRNA levels of these pro-inflammatory cytokines were enhanced. A recombinant form of vaccinia virus complement control protein (VCP), which like factor H, contains CCP modules and has complement-regulatory activity, mirrored the results obtained with factor H. Both factor H (25%), and VCP (45%) were found to reduce luciferase reporter activity in MDCK cells transduced with H1N1 pseudotyped lentiviral particles. Factor H (50%) and VCP (30%) enhanced the luciferase reporter activity for H3N2, suggesting an entry inhibitory role of factor H and VCP against H1N1, but not H3N2. Thus, factor H can modulate IAV infection and inflammatory responses, independent of its complement-related functions.

C4b Binding Protein Acts as an Innate Immune Effector Against Influenza A Virus.

C4b Binding Protein (C4BP) is a major fluid phase inhibitor of the classical and lectin pathways of the complement system. Complement inhibition is achieved by binding to and restricting the role of activated complement component C4b. C4BP functions as a co-factor for factor I in proteolytic inactivation of both soluble and cell surface-bound C4b, thus restricting the formation of the C3-convertase, C4b2a. C4BP also accelerates the natural decay/dissociation of the C3 convertase. This makes C4BP a prime target for exploitation by pathogens to escape complement attack, as seen in Streptococcus pyogenes or Flavivirus. Here, we examined whether C4BP can act on its own in a complement independent manner, against pathogens. C4BP bound H1N1 and H3N2 subtypes of Influenza A Virus (IAV) most likely via multiple sites in Complement Control Protein (CCP) 1-2, 4-5, and 7-8 domains of its α-chain. In addition, C4BP CCP1-2 bound H3N2 better than H1N1. C4BP bound three IAV envelope proteins: Haemagglutinin (~70 kDa), Neuraminidase (~55 kDa), and Matrix protein 1 (~25kDa). C4BP suppressed H1N1 subtype infection into the lung epithelial cell line, A549, while it promoted infection by H3N2 subtype. C4BP restricted viral entry for H1N1 but had the opposite effect on H3N2, as evident from experiments using pseudo-typed viral particles. C4BP downregulated mRNA levels of pro-inflammatory IFN-α, IL-12, and NFκB in the case of H1N1, while it promoted a pro-inflammatory immune response by upregulating IFN- α, TNF-α, RANTES, and IL-6 in the case of H3N2. We conclude that C4BP differentially modulates the efficacy of IAV entry, and hence, replication in a target cell in a strain-dependent manner, and acts as an entry inhibitor for H1N1. Thus, CCP containing complement proteins such as factor H and C4BP may have additional defense roles against IAV that do not rely on the regulation of complement activation.

Secretion of functionally active complement factor H related protein 5 (FHR5) by primary tumour cells derived from Glioblastoma Multiforme patients.

The complement system is an important humoral immune surveillance mechanism against tumours. However, many malignant tumours are resistant to complement mediated lysis. Here, we report secretion of complement factor H related protein 5 (FHR5) by primary tumour cells derived from Glioblastoma multiforme (GBM) patients. We investigated whether the secreted FHR5 exhibited functional activity similar to factor H, including inhibition of complement mediated lysis, acting as a co-factor for factor I mediated cleavage of C3b, and decay acceleration of C3 convertase. Immunoblotting analysis of primary GBM cells (B30, B31 and B33) supernatant showed the active secretion of FHR5, but not of Factor H. ELISA revealed that the secretion of soluble GBM-FHR5 by cultured GBM cells increased in a time-dependent manner. Primary GBM-FHR5 inhibited complement mediated lysis, possessed co-factor activity for factor I mediated cleavage and displayed decay acceleration of C3 convertase. In summary, we detected the secretion of FHR5 by primary GBM cells B30, B31 and B33. The results demonstrated that GBM-FHR5 shares biological function with FH as a mechanism primary GBM cells potentially use to resist complement mediated lysis.

Enterococcus faecalis Escapes Complement-Mediated Killing via Recruitment of Complement Factor H.

BACKGROUND: Enterococcus faecalis is considered to be the most important species of enterococci responsible for blood stream infections in critically ill patients. In blood, the complement system is activated via the classical pathway (CP), the lectin pathway (LP), or the alternative pathway (AP), and it plays a critical role in opsonophagocytosis of bacteria including E faecalis. METHODS: In a mouse model of enterococcus peritonitis, BALB-C mice were challenged with a high dose of E faecalis 12 hours after intraperitoneal administration of anti-Factor H (FH) antibodies or isotype control. Four hours later, control mice developed higher bacterial burden in blood and organs compared with mice treated with anti-FH antibodies. RESULTS: We demonstrate that complement recognition molecules C1q, CL-11, and murine ficolin-A bind the enterococcus and drive the CP and the LP in human and mouse. We further describe that E faecalis evades the AP by recruitment of FH on its surface. Our results show a strong C3b deposition on E faecalis via both the CP and the LP but not through the AP. CONCLUSIONS: These findings indicate that E faecalis avoids the complement phagocytosis by the AP via sequestering complement FH from the host blood.

Recombinant chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) protects against LPS-induced lung injury in mice.

Acute lung injury (ALI) and/or acute respiratory distress syndrome (ARDS) are clinical conditions caused by trauma, lung infection or sepsis. ALI/ARDS is associated with massive recruitment of neutrophils into the lung with release of reactive oxygen species and excessive inflammatory response that damage alveolar tissue. Here we report the successful use of a potent recombinant chemotaxis inhibitory protein (rCHIPS) derived from Staphylococcus aureus in reducing the severity of ALI/ARDS. Treatment with rCHIPS reduces pulmonary inflammation and permeability in mice after intranasal administration of lipopolysaccharide (LPS). rCHIPS treatment significantly reduces lung myeloperoxidase (MPO) activity, pro-inflammatory cytokines, broncho-alveolar lavage (BAL) fluid protein content as well as histopathological changes. In addition, treatment with rCHIPS significantly diminishes neutrophils and leukocytes recruitment into lung tissue after LPS administration and hence protects mice from reactive oxygen species mediated lung injury. Our finding reveals potential therapeutic benefits of using rCHIPS for the treatment of ALI/ARDS.

Human Properdin Modulates Macrophage: Mycobacterium bovis BCG Interaction via Thrombospondin Repeats 4 and 5.

Mycobacterium tuberculosis can proficiently enter macrophages and diminish complement activation on its cell surface. Within macrophages, the mycobacterium can suppress macrophage apoptosis and survive within the intracellular environment. Previously, we have shown that complement regulatory proteins such as factor H may interfere with pathogen-macrophage interactions during tuberculosis infection. In this study, we show that Mycobacterium bovis BCG binds properdin, an upregulator of the complement alternative pathway. TSR4+5, a recombinant form of thrombospondin repeats 4 and 5 of human properdin expressed in tandem, which is an inhibitor of the alternative pathway, was also able to bind to M. bovis BCG. Properdin and TSR4+5 were found to inhibit uptake of M. bovis BCG by THP-1 macrophage cells in a dose-dependent manner. Quantitative real-time PCR revealed elevated pro-inflammatory responses (TNF-α, IL-1ß, and IL-6) in the presence of properdin or TSR4+5, which gradually decreased over 6 h. Correspondingly, anti-inflammatory responses (IL-10 and TGF-ß) showed suppressed levels of expression in the presence of properdin, which gradually increased over 6 h. Multiplex cytokine array analysis also revealed that properdin and TSR4+5 significantly enhanced the pro-inflammatory response (TNF-α, IL-1ß, and IL-1α) at 24 h, which declined at 48 h, whereas the anti-inflammatory response (IL-10) was suppressed. Our results suggest that properdin may interfere with mycobacterial entry into macrophages via TSR4 and TSR5, particularly during the initial stages of infection, thus affecting the extracellular survival of the pathogen. This study offers novel insights into the non-complement related functions of properdin during host-pathogen interactions in tuberculosis.

Human Properdin Opsonizes Nanoparticles and Triggers a Potent Pro-inflammatory Response by Macrophages without Involving Complement Activation.

Development of nanoparticles as tissue-specific drug delivery platforms can be considerably influenced by the complement system because of their inherent pro-inflammatory and tumorigenic consequences. The complement activation pathways, and its recognition subcomponents, can modulate clearance of the nanoparticles and subsequent inflammatory response and thus alter the intended translational applications. Here, we report, for the first time, that human properdin, an upregulator of the complement alternative pathway, can opsonize functionalized carbon nanotubes (CNTs) via its thrombospondin type I repeat (TSR) 4 and 5. Binding of properdin and TSR4+5 is likely to involve charge pattern/polarity recognition of the CNT surface since both carboxymethyl cellulose-coated carbon nanotubes (CMC-CNT) and oxidized (Ox-CNT) bound these proteins well. Properdin enhanced the uptake of CMC-CNTs by a macrophage cell line, THP-1, mounting a robust pro-inflammatory immune response, as revealed by qRT-PCR, multiplex cytokine array, and NF-κB nuclear translocation analyses. Properdin can be locally synthesized by immune cells in an inflammatory microenvironment, and thus, its interaction with nanoparticles is of considerable importance. In addition, recombinant TSR4+5 coated on the CMC-CNTs inhibited complement consumption by CMC-CNTs, suggesting that nanoparticle decoration with TSR4+5, can be potentially used as a complement inhibitor in a number of pathological contexts arising due to exaggerated complement activation.

Complement Dependent and Independent Interaction Between Bovine Conglutinin and Mycobacterium bovis BCG: Implications in Bovine Tuberculosis.

Bovine conglutinin, the first animal collectin to be discovered, is structurally very similar to Surfactant Protein D (SP-D). SP-D is known to interact with Mycobacterium tuberculosis, and the closely-related M. bovis, the causative agent of bovine tuberculosis. We speculated that due to the overall similarities between conglutinin and SP-D, conglutinin is likely to have a protective influence in bovine tuberculosis. We set out to investigate the role of conglutinin in host-pathogen interaction during mycobacterial infection. We show here that a recombinant truncated form of conglutinin (rfBC), composed of the neck and C-type lectin domains, binds specifically and in a dose-dependent manner to the model organism Mycobacterium bovis BCG. rfBC showed a significant direct bacteriostatic effect on the growth of M. bovis BCG in culture. In addition, rfBC inhibited the uptake of M. bovis BCG by THP-1 macrophages (human monocyte lineage cell line) and suppressed the subsequent pro-inflammatory response. Conglutinin is well-known as a binder of the complement activation product, iC3b. rfBC was also able to inhibit the uptake of complement-coated M. bovis BCG by THP-1 macrophages, whilst modulating the pro-inflammatory response. It is likely that rfBC inhibits the phagocytosis of mycobacteria by two distinct mechanisms: firstly, rfBC interferes with mannose receptor-mediated uptake by masking lipoarabinomannan (LAM) on the mycobacterial surface. Secondly, since conglutinin binds iC3b, it can interfere with complement receptor-mediated uptake via CR3 and CR4, by masking interactions with iC3b deposited on the mycobacterial surface. rfBC was also able to modulate the downstream pro-inflammatory response in THP-1 cells, which is important for mobilizing the adaptive immune response, facilitating containment of mycobacterial infection. In conclusion, we show that conglutinin possesses complement-dependent and complement-independent anti-mycobacterial activities, interfering with both known mechanisms of mycobacterial uptake by macrophages. As mycobacteria are specialized intracellular pathogens, conglutinin may inhibit M. bovis and M. tuberculosis from establishing an intracellular niche within macrophages, and thus, negatively affect the long-term survival of the pathogen in the host.

Serine proteases of the complement lectin pathway and their genetic variations in ischaemic stroke.

AIMS: The aim of the current study was to assess the proteolytic activities of collectin-bound MASP-1 and MASP-2 in the blood of patients with ischaemic stroke, as well as the association of their six genetic polymorphisms (rs3203210, rs28945070, rs28945073 in MASP1 gene and rs2273343, rs12711521, rs147270785 in MASP2 gene) with this pathology. METHODS: In total, 250 patients and 300 healthy subjects were involved in this study. MBL-associated serine protease (MASP)-1 and MASP-2 activities were measured using in-house developed immunofluorescent and enzyme-linked immunosorbent assays, respectively. Sequence specific primer PCR was used to study the association of MASP1 and MASP2 genetic polymorphisms with ischaemic stroke. RESULTS: The results obtained demonstrate that the activities of collectin-bound MASP-1 and MASP-2 in patients with ischaemic stroke are significantly higher than those in healthy subjects (p<0.001). According to the data obtained for genotyping, the rs3203210 polymorphism in the MASP1 gene and the rs147270785 polymorphism in the MASP2 gene are associated with ischaemic stroke (p<0.0001). CONCLUSIONS: In conclusion we suggest that the complement lectin pathway serine proteases, MASP-1 and MASP-2, can be associated with ischaemic stroke development risk and may participate in pathological events leading to post-ischaemic brain damage. Moreover rs3203210 and rs147270785 single nucleotide polymorphisms in the MASP1 and MASP2 genes, respectively, are strongly associated with ischaemic stroke, and the minor rs3203210*C and rs147270785*A alleles of these polymorphisms may be considered as protective factors for ischameic stroke, at least in the Armenian population.

Potential influences of complement factor H in autoimmune inflammatory and thrombotic disorders.

Complement system homeostasis is important for host self-protection and anti-microbial immune surveillance, and recent research indicates roles in tissue development and remodelling. Complement also appears to have several points of interaction with the blood coagulation system. Deficiency and altered function due to gene mutations and polymorphisms in complement effectors and regulators, including Factor H, have been associated with familial and sporadic autoimmune inflammatory - thrombotic disorders, in which autoantibodies play a part. These include systemic lupus erythematosus, rheumatoid arthritis, atypical haemolytic uremic syndrome, anti-phospholipid syndrome and age-related macular degeneration. Such diseases are generally complex - multigenic and heterogeneous in their symptoms and predisposition/susceptibility. They usually need to be triggered by vascular trauma, drugs or infection and non-complement genetic factors also play a part. Underlying events seem to include decline in peripheral regulatory T cells, dendritic cell, and B cell tolerance, associated with alterations in lymphoid organ microenvironment. Factor H is an abundant protein, synthesised in many cell types, and its reported binding to many different ligands, even if not of high affinity, may influence a large number of molecular interactions, together with the accepted role of Factor H within the complement system. Factor H is involved in mesenchymal stem cell mediated tolerance and also contributes to self-tolerance by augmenting iC3b production and opsonisation of apoptotic cells for their silent dendritic cell engulfment via complement receptor CR3, which mediates anti-inflammatory-tolerogenic effects in the apoptotic cell context. There may be co-operation with other phagocytic receptors, such as complement C1q receptors, and the Tim glycoprotein family, which specifically bind phosphatidylserine expressed on the apoptotic cell surface. Factor H is able to discriminate between self and nonself surfaces for self-protection and anti-microbe defence. Factor H, particularly as an abundant platelet protein, may also modulate blood coagulation, having an anti-thrombotic role. Here, we review a number of interaction pathways in coagulation and in immunity, together with associated diseases, and indicate where Factor H may be expected to exert an influence, based on reports of the diversity of ligands for Factor H.

Lectin pathway effector enzyme mannan-binding lectin-associated serine protease-2 can activate native complement C3 in absence of C4 and/or C2.

All 3 activation pathways of complement-the classic pathway (CP), the alternative pathway, and the lectin pathway (LP)- converge into a common central event: the cleavage and activation of the abundant third complement component, C3, via formation of C3-activating enzymes (C3 convertases). The fourth complement component, C4, and the second component, C2, are indispensable constituents of the C3 convertase complex, C4bC2a, which is formed by both the CP and the LP. Whereas in the absence of C4, CP can no longer activate C3, LP retains a residual but physiologically critical capacity to convert native C3 into its activation fragments, C3a and C3b. This residual C4 and/or C2 bypass route is dependent on LP-specific mannan-binding lectin-associated serine protease-2. By using various serum sources with defined complement deficiencies, we demonstrate that, under physiologic conditions LP-specific C4 and/or C2 bypass activation of C3 is mediated by direct cleavage of native C3 by mannan-binding lectin-associated serine protease-2 bound to LP-activation complexes captured on ligand-coated surfaces.-Yaseen, S., Demopulos, G., Dudler, T., Yabuki, M., Wood, C. L., Cummings, W. J., Tjoelker, L. W., Fujita, T., Sacks, S., Garred, P., Andrew, P., Sim, R. B., Lachmann, P. J., Wallis, R., Lynch, N., Schwaeble, W. J. Lectin pathway effector enzyme mannan-binding lectin-associated serine protease-2 can activate native complement C3 in absence of C4 and/or C2.

Interactions of the innate immune system with carbon nanotubes.

The therapeutic application of nanomaterials requires that they are biocompatible and can reach the desired target. The innate immune system is likely to be the first defence machinery that would recognise the nanomaterials as 'non-self'. A number of studies have addressed the issue of how carbon nanotubes (CNTs) interact with phagocytic cells and their surface receptors that can impact on their intracellular processing and subsequent immune response. In addition, soluble innate immune factors also get involved in the recognition and clearance of CNTs. The interaction of CNTs with the complement system, the most potent and versatile innate immune mechanism, has shed interesting light on how complement activation on the surface of CNTs can modulate their phagocytosis and effector cytokine response. The charge or altered molecular pattern on the surface of CNTs due to functionalization and derivatization can also dictate the level of complement activation and subsequent inflammatory response. It is becoming evident that complement deposition may facilitate phagocytic uptake of CNTs through receptor routes that leads to dampening of pro-inflammatory response by complement-receptor bearing macrophages and B cells. Thus, recombinant complement regulators decorated on the CNT surface can constructively influence the therapeutic strategies involving CNTs and other nanoparticles.

Pulmonary surfactant protein SP-D opsonises carbon nanotubes and augments their phagocytosis and subsequent pro-inflammatory immune response.

Carbon nanotubes (CNTs) are increasingly being developed for use in biomedical applications, including drug delivery. One of the most promising applications under evaluation is in treating pulmonary diseases such as tuberculosis. Once inhaled or administered, the nanoparticles are likely to be recognised by innate immune molecules in the lungs such as hydrophilic pulmonary surfactant proteins. Here, we set out to examine the interaction between surfactant protein D (SP-D), a key lung pattern recognition molecule and CNTs, and possible downstream effects on the immune response via macrophages. We show here that a recombinant form of human SP-D (rhSP-D) bound to oxidised and carboxymethyl cellulose (CMC) coated CNTs via its C-type lectin domain and enhanced phagocytosis by U937 and THP-1 macrophages/monocytic cell lines, together with an increased pro-inflammatory response, suggesting that sequestration of SP-D by CNTs in the lungs can trigger an unwanted and damaging immune response. We also observed that functionalised CNTs, opsonised with rhSP-D, continued to activate complement via the classical pathway, suggesting that C1q, which is the recognition sub-component of the classical pathway, and SP-D have distinct pattern recognition sites on the CNTs. Consistent with our earlier reports, complement deposition on the rhSP-D opsonised CNTs led to dampening of the pro-inflammatory immune response by THP-1 macrophages, as evident from qPCR, cytokine array and NF-κB nuclear translocation analyses. This study highlights the importance of understanding the interplay between innate immune humoral factors including complement in devising nanoparticle based drug delivery strategies.

Complement factor H interferes with Mycobacterium bovis BCG entry into macrophages and modulates the pro-inflammatory cytokine response.

Mycobacterium tuberculosis is an accomplished intracellular pathogen, particularly within the macrophage and this is of the utmost importance in the host-pathogen stand-off observed in the granuloma during latent tuberculosis. Contact with innate immune molecules is one of the primary interactions that can occur with the pathogen M. tuberculosis once inhaled. Complement proteins may play a role in facilitating M. tuberculosis interactions with macrophages. Here, we demonstrate that factor H, a complement regulatory protein that down-regulates complement alternative pathway activation, binds directly to the model organism M. bovis BCG. Binding of factor H reaches saturation at 5-10µg of factor H/ml, well below the plasma level. C4 binding protein (C4BP) competed with factor H for binding to mycobacteria. Factor H was also found to inhibit uptake of M. bovis BCG by THP-1 macrophage cells in a dose-dependent manner. Real-time qPCR analysis showed stark differential responses of pro- and anti-inflammatory cytokines during the early stages of phagocytosis, as evident from elevated levels of TNF-α, IL-1ß and IL-6, and a concomitant decrease in IL-10, TGF-ß and IL-12 levels, when THP-1:BCG interaction took place in the presence of factor H. Our results suggest that factor H can interfere with mycobacterial entry into macrophages and modulate inflammatory cytokine responses, particularly during the initial stages of infection, thus affecting the extracellular survival of the pathogen. Our results offer novel insights into complement activation-independent functions of factor H during the host-pathogen interaction in tuberculosis.

Complement Deposition on Nanoparticles Can Modulate Immune Responses by Macrophage, B and T Cells.

Nanoparticles are attractive drug delivery vehicles for targeted organ-specific as well as systemic therapy. However, their interaction with the immune system offers an intriguing challenge to the success of nanotherapeutics in vivo. Recently, we showed that pristine and derivatised carbon nanotubes (CNT) can activate complement mainly via the classical pathway leading to enhanced uptake by phagocytic cells, and transcriptional down-regulation of pro-inflammatory cytokines. Here, we report the interaction of complement-activating CC-CNT and RNA-CNT, and non-complement-activating gold-nickel (Au-Ni) nanowires with cell lines representing macrophage, B and T cells. Complement deposition considerably enhanced uptake of CNTs by immune cells known to overexpress complement receptors. Real-Time qPCR and multiplex array analyses showed complement-dependent down-regulation of TNF-α and IL-1ß and up-regulation of IL-12 by CMC- and RNA-CNTs, in addition to revealing IL-10 as a crucial regulator during nanoparticle-immune cell interaction. It appears that complement system can recognize molecular patterns differentially displayed by nanoparticles and thus, modulate subsequent processing of nanoparticles by antigen capturing and antigen presenting cells, which can shape innate and adaptive immune axes.