Factor B, a serine protease proenzyme and part of the complement system, binds to other complement proteins such as C3b and properdin. While it is known to be activated by factor D, other interactions that interfere with the binding between Factor B and C3b are less well-understood. We attached C3b to a thiol Sepharose via its free SH group and conducted a competition assay employing 125I-labelled factor B to study competitive binding. Two anti-C3d monoclonal antibodies (F49b and 4C2) were found to partially inhibit the binding of Factor B to C3b. The inhibitory effects of these monoclonal antibodies were found to be dose-dependent. 4C2 was found to have a maximum inhibition of 50%, while the maximum inhibition by F49b was ∼40%. In contrast, two anti-C3c monoclonal antibodies (F39b and F20b) were able to enhance the formation of the factor B-C3b complex in a dose-dependent manner. Competition binding studies with isolated C3 fragments further supported the involvement of the C3d region in factor B binding, as C3d inhibited the binding of factor B to C3b completely. Furthermore, additional competition binding studies conducted with each of the three domains of factor B (Ba, vWF and SP) demonstrated that each domain independently inhibited the binding of intact factor B to C3b by ∼50%.### Competing Interest StatementThe authors have declared no competing interest.* Abbreviations : CCP : complement control protein repeat vWF : von Willebrand Factor SP : serine protease Pipes : [(Piperazine-N,N’-bis-2-ethanesulphonic acid). FPLC

The classical pathway of the complement system is activated by the binding of C1q in the C1 complex to the target activator, including immune complexes. Factor H is regarded as the key downregulatory protein of the complement alternative pathway. However, both C1q and factor H bind to target surfaces via charge distribution patterns. For a few targets, C1q and factor H compete for binding to common or overlapping sites. Factor H, therefore, can effectively regulate the classical pathway activation through such targets, in addition to its previously characterized role in the alternative pathway. Both C1q and factor H are known to recognize foreign or altered-self materials, e.g., bacteria, viruses, and apoptotic/necrotic cells. Clots, formed by the coagulation system, are an example of altered self. Factor H is present abundantly in platelets and is a well-known substrate for FXIIIa. Here, we investigated whether clots activate the complement classical pathway and whether this is regulated by factor H. We show here that both C1q and factor H bind to the fibrin formed in microtiter plates and the fibrin clots formed under in vitro physiological conditions. Both C1q and factor H become covalently bound to fibrin clots, and this is mediated via FXIIIa. We also show that fibrin clots activate the classical pathway of complement, as demonstrated by C4 consumption and membrane attack complex detection assays. Thus, factor H downregulates the activation of the classical pathway induced by fibrin clots. These results elucidate the intricate molecular mechanisms through which the complement and coagulation pathways intersect and have regulatory consequences.

Abstract Severe cases of SARS-CoV-2 infection are characterised by an imbalanced immune response, excessive inflammation, and the development of acute respiratory distress syndrome, which can lead to multiorgan failure and death. Several studies have demonstrated dysregulated complement activity as an indicator of immunopathogenesis in the SARS-CoV-2 infection. Notably, the complement alternative pathway has been implicated in driving the excessive inflammation during severe SARS-CoV-2 infection. Reduced levels of factor H (FH), a down-regulator of the alternative pathway, and increased levels of properdin (Factor P/FP), the only known up-regulator of the alternative pathway, have been observed in individuals with severe COVID-19 infection. The present study investigated the complement activation-independent, and a more direct role of FH and FP against SARS-CoV-2 infection. Using direct ELISA, the interactions of FH and FP with the SARS-CoV-2 spike (S) and receptor binding domain (RBD) were assessed. Using S protein expressing lentiviral pseudotypes, the cell binding and luciferase-based virus entry assays were employed to assess the potential modulatory effects of FH, FP, and recombinant thrombospondin repeats 4 and 5 (TSR4+5) on SARS-CoV-2 cell entry. We also evaluated the immunomodulatory functions of FH and FP in the cytokine response triggered by SARS-CoV-2 pseudotypes via RT-qPCR. SARS-CoV-2 S and RBD proteins were found to bind both FH and FP. Treatment of A549 cells expressing human ACE2 and TMPRSS2 with FP or TSR4+5 resulted in increased cell entry and binding of SARS-CoV-2 pseudotypes. In silico studies revealed that FP increases affinity between SARS-CoV-2 and host ACE2. The impact of FP on viral cell entry and binding was reversed by anti-FP antibody treatment in A549-hACE2+TMPRSS2 cells. However, FH treatment reduced the cell entry and binding of SARS-CoV-2 lentiviral pseudotypes. Furthermore, the A549-hACE2+TMPRSS2 cells challenged with SARS-CoV-2 spike, envelope, nucleoprotein, and membrane protein expressing alphaviral pseudotypes pre-treated with FP or TSR4+5, exhibited upregulation of the transcripts of pro-inflammatory cytokines, such as IL-1β, IL-8, IL-6, TNF-α, IFN-α and RANTES (as well as NF-κB). Conversely, FH pre-treatment downregulated the expression of these pro-inflammatory cytokines. Treatment of A549-hACE2+TMPRSS2 cells with FP increased S protein-mediated NF-κB activation, while FH treatment reduced it. These findings suggest that FH may act as an inhibitor of SARS-CoV-2 cell entry and binding, thereby attenuating the infection-associated inflammatory response in a complement activation-independent manner. FP may contribute to viral cell entry, binding, and exacerbating the immune response. That may result in potentially influencing the severity of the infection.

The classical pathway of the complement system is activated by the binding of C1q in the C1 complex to the target activator including immune complexes. Factor H is regarded as the key downregulatory protein of the alternative pathway of complement. However, C1q and factor H both bind to target surfaces via charge distribution patterns. For few targets, C1q and factor H compete for binding to common or overlapping sites. Factor H, therefore, can effectively regulate the classical pathway activation by such targets, in addition to its previously characterized role in the alternative pathway. Both C1q and factor H are reported to recognize “foreign” or altered-self materials. Clots, formed by the coagulation system, are an example of altered self. Factor H is present abundantly in platelets and is a well-known substrate for FXIIIa. Here, we investigated whether clots activate the classical pathway of complement and whether this is regulated by factor H. We show here that both C1q and factor H bind to fibrin formed in microtitre plates as well as fibrin clots formed under in vitro physiological conditions. Both C1q and factor H become covalently bound to fibrin clots and this is mediated via FXIIIa. We also show that fibrin clots activate the classical pathway of complement, as demonstrated by C4 consumption and membrane attack complex detection assays. Thus, factor H downregulates the classical pathway activation induced by fibrin clots. These results elucidate the intricate molecular mechanisms through which the complement and coagulation pathways intersect and have regulatory consequences.

AbstractSevere cases of SARS-CoV-2 infection are characterised by an imbalanced immune response, excessive inflammation, and the development of acute respiratory distress syndrome, which can lead to multiorgan failure and death. Several studies have demonstrated dysregulated complement activity as an indicator of immunopathogenesis in the SARS-CoV-2 infection. Notably, the complement alternative pathway has been implicated in driving the excessive inflammation during severe SARS-CoV-2 infection. Reduced levels of factor H (FH), a down-regulator of the alternative pathway, and increased levels of properdin (Factor P/FP), the only known up-regulator of the alternative pathway, have been observed in individuals with severe COVID-19 infection. The present study investigated the complement activation-independent, and a more direct role of FH and FP against SARS-CoV-2 infection. Using direct ELISA, the interactions of FH and FP with the SARS-CoV-2 spike (S) and receptor binding domain (RBD) were assessed. Using S protein expressing lentiviral pseudotypes, the cell binding and luciferase-based virus entry assays were employed to assess the potential modulatory effects of FH, FP, and recombinant thrombospondin repeats 4 and 5 (TSR4+5) on SARS-CoV-2 cell entry. We also evaluated the immunomodulatory functions of FH and FP in the cytokine response triggered by SARS-CoV-2 pseudotypes via RT-qPCR. SARS-CoV-2 S and RBD proteins were found to bind both FH and FP. Treatment of A549 cells expressing human ACE2 and TMPRSS2 with FP or TSR4+5 resulted in increased cell entry and binding of SARS-CoV-2 pseudotypes. In silico studies revealed that FP increases affinity between SARS-CoV-2 and host ACE2. The impact of FP on viral cell entry and binding was reversed by anti-FP antibody treatment in A549-hACE2+TMPRSS2 cells. However, FH treatment reduced the cell entry and binding of SARS-CoV-2 lentiviral pseudotypes. Furthermore, the A549-hACE2+TMPRSS2 cells challenged with SARS-CoV-2 spike, envelope, nucleoprotein, and membrane protein expressing alphaviral pseudotypes pre-treated with FP or TSR4+5, exhibited upregulation of the transcripts of pro-inflammatory cytokines, such as IL-1β, IL-8, IL-6, TNF-α, IFN-α and RANTES (as well as NF-κB). Conversely, FH pre-treatment downregulated the expression of these pro-inflammatory cytokines. Treatment of A549-hACE2+TMPRSS2 cells with FP increased S protein-mediated NF-κB activation, while FH treatment reduced it. These findings suggest that FH may act as an inhibitor of SARS-CoV-2 cell entry and binding, thereby attenuating the infection-associated inflammatory response in a complement activation-independent manner. FP may contribute to viral cell entry, binding, and exacerbating the immune response. That may result in potentially influencing the severity of the infection.

ABSTRACT Due to profound heterogeneity within stromal immune tumor microenvironment (TME), pancreatic ductal adenocarcinoma (PDAC) remains a hard to treat disease, with the lowest 5-year survival below 10%. Large-scale transcriptomic analysis has revealed two main, clinically relevant PDAC signatures: therapy responsive ‘Classical’ subtype with better prognosis, and poorly-differentiated Basal-like with poor prognosis. It has also become evident that the cellular and humoral components in the immune TME considerably influence the outcome of tumorigenesis. Complement system, a potent humoral innate immune mechanism, also forms a part of this immune TME. In addition to the regular production of various complement components in the liver, certain infiltrating immune cells such as macrophages, dendritic cells and neutrophils, can produce a few complement components locally at the site of infection and inflammation including TME, and modulate tumorigenic outcomes. Neutrophils are the most prevalent innate immune cells in the PDAC TME; however, its role has been attributed as either pro-tumorigenic or anti-tumorigenic. Neutrophils, when stimulated or under stress, are capable of releasing their secretory granules that also contain the only known up-regulator of the complement alternative pathway, Complement Factor P (CFP) or properdin. Properdin can not only facilitate alternative pathway activation by stabilising the C3 convertase, but also act as a pattern recognition receptor on its own and modify inflammatory response. Here, by combining multicenter transcriptome analysis of PDAC patient tumors, single-cell-RNA-seq analysis, preclinical mouse models and human PDAC specimens, we show that properdin expression and neutrophil surveillance are linked to better prognosis in PDAC patients. Furthermore, properdin expression is substantially higher in well-to-moderately differentiated Classical subtype compared to the highly aggressive basal-like PDAC tumours. Mechanistically, exogenous properdin binds to the cell membrane and activates caspase 3/7 to induce apoptosis in basal-like PDAC cells. Together, these findings suggest that the complement protein, properdin, could be a favorable prognostic factor and exhibit anti-tumorigenic functions in PDAC.

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 of L-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 mu g/ml and similar to those reported for other Caucasian populations. SZ-cases had similar to 40 % increase in serum L-ficolin (median 5.08 mu g/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.

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-1β, 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.

Several disorders associated with the total or partial absence of components of the human complement system are known. Deficiencies of classical pathway (CP) components are generally linked to systemic lupus erythematosus (SLE) or SLE-like syndromes. However, only approximately one-third of patients who lack C2 show mild symptoms of SLE. The relatively high frequency of homozygous C2 deficiency without or with minor disease manifestation suggests that there might be a compensatory mechanism which allows the activation of the CP of complement without the absolute requirement of C2. In this study we show that factor B (FB), the C2 homologue of the alternative pathway (AP) of complement, can substitute for C2. This was confirmed by using C4b as immobilised ligand and FB as analyte in Surface Plasmon Resonance (BIACORE). C2 binding to the immobilised C3b-like molecule C3(CH3NH2) was not seen. The estimated binding constant for C4bB complex formation was 2.00 * 10-5 [M]. We were further able to demonstrate that C4b supports the cleavage of Factor B by Factor D. Finally, cleavage of 125I-C3 by C4bBb was evaluated and gave strong evidence that the "hybrid" convertase C4bBb can cleave and activate C3 in vitro. Cleavage activity is very low, but consistent with some of the "C2-bypass" observations of others.

A new type of intrinsic activity was revealed in the complement protein C3 during investigations of nucleophilic complement inhibitors. Nascent C3b molecules produced by cleavage-activation of C3 have highly reactive acyl groups which form short-lived sites for covalent binding to biological materials. Alpha-2 macroglobulin and C4 may utilise similar “autoligation” mechanisms.

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.

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-β, 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.

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.

Mannose-binding lectin (MBL) plays, as a soluble pattern recognition molecule, a central role in the innate immune response. MBL binds to the surface of carbohydrates on a wide variety of pathogens and mediates opsonophagocytosis via activation of the lectin complement pathway or by directly promoting opsonophagocytosis in a complement-independent manner, it has been reported that MBL acts as an immunomodulator and promoter of apoptosis. Additionally, MBL and the MBL-associated serine proteases-1 and -2 have been associated with the coagulation system. Therefore, it is not surprising that MBL deficiency has been associated with increased susceptibility to various infectious and autoimmune diseases. As a key component of the innate immune system, MBL is particularly important when the adaptive immune response is either immunocompromised or immunosuppressed; consequently, the majority of the reported cases of MBL deficiency associated with disease are found in infants or young children and immunocompromised patients. In this chapter, we will give a comprehensive overview of the literature on MBL by discussing its structure, function, interaction with its serine proteases, genetics and its role in association with various pathologies.

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.

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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.

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.

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.