Transient Binding Dynamics of Complement System Pattern Recognition Molecules on Pathogens.

Previous studies of pattern recognition molecules (PRMs) of the complement system have revealed difficulties in observing binding on pathogens such as Aspergillus fumigatus and Escherichia coli, despite complement deposition indicative of classical and lectin pathway activation. Thus, we investigated the binding dynamics of PRMs of the complement system, specifically C1q of the classical pathway and mannose-binding lectin (MBL) of the lectin pathway. We observed consistently increasing deposition of essential complement components such as C4b, C3b, and the terminal complement complex on A. fumigatus and E. coli. However, C1q and MBL binding to the surface rapidly declined during incubation after just 2-4 min in 10% plasma. The detachment of C1q and MBL can be linked to complement cascade activation, as the PRMs remain bound in the absence of plasma. The dissociation and the fate of C1q and MBL seem to have different mechanistic functions. Notably, C1q dynamics were associated with local C1 complex activation. When C1s was inhibited in plasma, C1q binding not only remained high but further increased over time. In contrast, MBL binding was inversely correlated with total and early complement activation due to MBL binding being partially retained by complement inhibition. Results indicate that detached MBL might be able to functionally rebind to A. fumigatus. In conclusion, these results reveal a (to our knowledge) novel "hit-and-run" complement-dependent PRM dynamic mechanism on pathogens. These dynamics may have profound implications for host defense and may help increase the functionality and longevity of complement-dependent PRMs in circulation.

Complement C7 and clusterin form a complex in circulation.

Introduction: The complement system is part of innate immunity and is comprised of an intricate network of proteins that are vital for host defense and host homeostasis. A distinct mechanism by which complement defends against invading pathogens is through the membrane attack complex (MAC), a lytic structure that forms on target surfaces. The MAC is made up of several complement components, and one indispensable component of the MAC is C7. The role of C7 in MAC assembly is well documented, however, inherent characteristics of C7 are yet to be investigated. Methods: To shed light on the molecular characteristics of C7, we examined the properties of serum-purified C7 acquired using polyclonal and novel monoclonal antibodies. The properties of serum­purified C7 were investigated through a series of proteolytic analyses, encompassing Western blot and mass spectrometry. The nature of C7 protein-protein interactions were further examined by a novel enzyme-linked immunosorbent assay (ELISA), as well as size­exclusion chromatography. Results: Protein analyses showcased an association between C7 and clusterin, an inhibitory complement regulator. The distinct association between C7 and clusterin was also demonstrated in serum-purified clusterin. Further assessment revealed that a complex between C7 and clusterin (C7-CLU) was detected. The C7-CLU complex was also identified in healthy serum and plasma donors, highlighting the presence of the complex in circulation. Discussion: Clusterin is known to dissociate the MAC structure by binding to polymerized C9, nevertheless, here we show clusterin binding to the native form of a terminal complement protein in vivo. The presented data reveal that C7 exhibits characteristics beyond that of MAC assembly, instigating further investigation of the effector role that the C7-CLU complex plays in the complement cascade.

Mannan-binding lectin serine protease-2 (MASP-2) in human kidney and its relevance for proteolytic activation of the epithelial sodium channel.

Proteolytic activation of the renal epithelial sodium channel (ENaC) is increased by aldosterone. The aldosterone-sensitive protease remains unidentified. In humans, elevated circulating aldosterone is associated with increased urinary extracellular vesicle (uEVs) excretion of mannan-binding lectin associated serine protease-2 (MASP-2). We hypothesized that MASP-2 is a physiologically relevant ENaC-activating protease. It was confirmed that MASP2 mRNA is abundantly present in liver but not in human and mouse kidneys. Aldosterone-stimulation of murine cortical colleting duct (mCCD) cells did not induce MASP-2 mRNA. In human kidney collecting duct, MASP-2 protein was detected in AQP2-negative/ATP6VB1-positive intercalated cells suggestive of MASP2 protein uptake. Plasma concentration of full-length MASP-2 and the short splice variant MAp19 were not changed in a cross-over intervention study in healthy humans with low (70 mmol/day) versus high (250 mmol/day) Na+ intake despite changes in aldosterone. The ratio of MAp19/MASP-2 in plasma was significantly increased with a high Na+ diet and the ratio correlated with changes in aldosterone and fractional Na+ excretion. MASP-2 was not detected in crude urine or in uEVs. MASP2 activated an amiloride-sensitive current when co-expressed with ENaC in Xenopus oocytes, but not when added to the bath solution. In monolayers of collecting duct M1 cells, MASP2 expression did not increase amiloride-sensitive current and in HEK293 cells, MASP-2 did not affect γENaC cleavage. MASP-2 is neither expressed nor co-localized and co-regulated with ENaC in the human kidney or in urine after low Na+ intake. MASP-2 does not mediate physiological ENaC cleavage in low salt/high aldosterone settings.

Lectin Pathway Enzyme MASP-2 and Downstream Complement Activation in COVID-19.

Mannose-binding lectin-associated serine protease 2 (MASP-2) is the main activator of the lectin complement pathway and has been suggested to be involved in the pathophysiology of coronavirus disease 2019 (COVID-19). To study a possible association between MASP-2 and COVID-19, we aimed at developing a sensitive and reliable MASP-2 ELISA. From an array of novel mouse-monoclonal antibodies using recombinant MASP-2 as antigen, two clones were selected to create a sandwich ELISA. Plasma samples were obtained from 216 healthy controls, 347 convalescent COVID-19 patients, and 147 prospectively followed COVID-19 patients. The assay was specific towards MASP-2 and did not recognize the truncated MASP2 splice variant MAP-2 (MAp19). The limit of quantification was shown to be 0.1 ng/mL. MASP-2 concentration was found to be stable after multiple freeze-thaw cycles. In healthy controls, the mean MASP-2 concentration was 524 ng/mL (95% CI: 496.5-551.6). No significant difference was found in the MASP-2 concentrations between COVID-19 convalescent samples and controls. However, a significant increase was observed in prospectively followed COVID-19 patients (mean: 834 ng/mL [95% CI: 765.3-902.7, p < 0.0001]). In these patients, MASP-2 concentration correlated significantly with the concentrations of the terminal complement complex (ρ = 0.3596, p < 0.0001), with the lectin pathway pattern recognition molecules ficolin-2 (ρ = 0.2906, p = 0.0004) and ficolin-3 (ρ = 0.3952, p < 0.0001) and with C-reactive protein (ρ = 0.3292, p = 0.0002). Overall, we developed a specific quantitative MASP-2 sandwich ELISA. MASP-2 correlated with complement activation and inflammatory markers in COVID-19 patients, underscoring a possible role of MASP-2 in COVID-19 pathophysiology.

Cell-autonomous Hedgehog signaling controls Th17 polarization and pathogenicity.

Th17 cells are key drivers of autoimmune disease. However, the signaling pathways regulating Th17 polarization are poorly understood. Hedgehog signaling regulates cell fate decisions during embryogenesis and adult tissue patterning. Here we find that cell-autonomous Hedgehog signaling, independent of exogenous ligands, selectively drives the polarization of Th17 cells but not other T helper cell subsets. We show that endogenous Hedgehog ligand, Ihh, signals to activate both canonical and non-canonical Hedgehog pathways through Gli3 and AMPK. We demonstrate that Hedgehog pathway inhibition with either the clinically-approved small molecule inhibitor vismodegib or genetic ablation of Ihh in CD4+ T cells greatly diminishes disease severity in two mouse models of intestinal inflammation. We confirm that Hedgehog pathway expression is upregulated in tissue from human ulcerative colitis patients and correlates with Th17 marker expression. This work implicates Hedgehog signaling in Th17 polarization and intestinal immunopathology and indicates the potential therapeutic use of Hedgehog inhibitors in the treatment of inflammatory bowel disease.

SARS-CoV-2 Antibodies Mediate Complement and Cellular Driven Inflammation.

The ongoing pandemic of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to constitute a serious public health threat worldwide. Protective antibody-mediated viral neutralization in response to SARS-CoV-2 infection has been firmly characterized. Where the effects of the antibody response are generally considered to be beneficial, an important biological question regarding potential negative outcomes of a SARS-CoV-2 antibody response has yet to be answered. We determined the distribution of IgG subclasses and complement activation levels in plasma from convalescent individuals using in-house developed ELISAs. The IgG response towards SARS-CoV-2 receptor-binding domain (RBD) after natural infection appeared to be mainly driven by IgG1 and IgG3 subclasses, which are the main ligands for C1q mediated classical complement pathway activation. The deposition of the complement components C4b, C3bc, and TCC as a consequence of SARS-CoV-2 specific antibodies were depending primarily on the SARS-CoV-2 RBD and significantly correlated with both IgG levels and disease severity, indicating that individuals with high levels of IgG and/or severe disease, might have a more prominent complement activation during viral infection. Finally, freshly isolated monocytes and a monocyte cell line (THP-1) were used to address the cellular mediated inflammatory response as a consequence of Fc-gamma receptor engagement by SARS-CoV-2 specific antibodies. Monocytic Fc gamma receptor charging resulted in a significant rise in the secretion of the pro-inflammatory cytokine TNF-α. Our results indicate that SARS-CoV-2 antibodies might drive significant inflammatory responses through the classical complement pathway and via cellular immune-complex activation that could have negative consequences during COVID-19 disease. We found that increased classical complement activation was highly associated to COVID-19 disease severity. The combination of antibody-mediated complement activation and subsequent cellular priming could constitute a significant risk of exacerbating COVID-19 severity.

The alpha/B.1.1.7 SARS-CoV-2 variant exhibits significantly higher affinity for ACE-2 and requires lower inoculation doses to cause disease in K18-hACE2 mice.

The alpha/B.1.1.7 SARS-CoV-2 lineage emerged in autumn 2020 in the United Kingdom and transmitted rapidly until winter 2021 when it was responsible for most new COVID-19 cases in many European countries. The incidence domination was likely due to a fitness advantage that could be driven by the receptor-binding domain (RBD) residue change (N501Y), which also emerged independently in other variants of concern such as the beta/B.1.351 and gamma/P.1 strains. Here, we present a functional characterization of the alpha/B.1.1.7 variant and show an eightfold affinity increase towards human angiotensin-converting enzyme-2 (ACE-2). In accordance with this, transgenic hACE2 mice showed a faster disease progression and severity after infection with a low dose of B.1.1.7, compared to an early 2020 SARS-CoV-2 isolate. When challenged with sera from convalescent individuals or anti-RBD monoclonal antibodies, the N501Y variant showed a minor, but significant elevated evasion potential of ACE-2/RBD antibody neutralization. The data suggest that the single asparagine to tyrosine substitution remarkable rise in affinity may be responsible for the higher transmission rate and severity of the B.1.1.7 variant.

Functional Effects of Receptor-Binding Domain Mutations of SARS-CoV-2 B.1.351 and P.1 Variants.

The recent identification and rise to dominance of the P.1 and B.1.351 SARS-CoV-2 variants have brought international concern because they may confer fitness advantages. The same three positions in the receptor-binding domain (RBD) are affected in both variants, but where the 417 substitution differs, the E484K/N501Y have co-evolved by convergent evolution. Here we characterize the functional and immune evasive consequences of the P.1 and B.1.351 RBD mutations. E484K and N501Y result in gain-of-function with two different outcomes: The N501Y confers a ten-fold affinity increase towards ACE-2, but a modest antibody evasion potential of plasma from convalescent or vaccinated individuals, whereas the E484K displays a significant antibody evasion capacity without a major impact on affinity. On the other hand, the two different 417 substitutions severely impair the RBD/ACE-2 affinity, but in the combined P.1 and B.1.351 RBD variants, this effect is partly counterbalanced by the effect of the E484K and N501Y. Our results suggest that the combination of these three mutations is a two-step forward and one step back in terms of viral fitness.

SARS-CoV-2 Neutralizing Antibody Responses towards Full-Length Spike Protein and the Receptor-Binding Domain.

Tools to monitor SARS-CoV-2 transmission and immune responses are needed. We present a neutralization ELISA to determine the levels of Ab-mediated virus neutralization and a preclinical model of focused immunization strategy. The ELISA is strongly correlated with the elaborate plaque reduction neutralization test (ρ = 0.9231, p < 0.0001). The neutralization potency of convalescent sera strongly correlates to IgG titers against SARS-CoV-2 receptor-binding domain (RBD) and spike (ρ = 0.8291 and 0.8297, respectively; p < 0.0001) and to a lesser extent with the IgG titers against protein N (ρ = 0.6471, p < 0.0001). The preclinical vaccine NMRI mice models using RBD and full-length spike Ag as immunogens show a profound Ab neutralization capacity (IC50 = 1.9 × 104 to 2.6 × 104 and 3.9 × 103 to 5.2 × 103, respectively). Using a panel of novel high-affinity murine mAbs, we also show that a majority of the RBD-raised mAbs have inhibitory properties, whereas only a few of the spike-raised mAbs do. The ELISA-based viral neutralization test offers a time- and cost-effective alternative to the plaque reduction neutralization test. The immunization results indicate that vaccine strategies focused only on the RBD region may have advantages compared with the full spike.

MASP-1 and MASP-3 Bind Directly to Aspergillus fumigatus and Promote Complement Activation and Phagocytosis.

Activation of the complement system is mediated by the interaction between pathogens and pattern recognition molecules (PRMs); mannose-binding lectin (MBL), ficolins, and collectin-10/-11 from the lectin pathway and C1q from the classical pathway. Lectin pathway activation specifically depends on proteases named MBL-associated serine proteases (MASPs) that are found in complexes with PRMs. In this study, we hypothesize that MASPs can recognize selected pathogens independently of PRMs. Using different clinical strains of opportunistic fungi, we have observed that MASPs directly recognize certain fungal pathogens in a way that can facilitate complement activation. Among these were Aspergillus fumigatus - a dangerous pathogen, especially for immunocompromised patients. In flow cytometry and fluorescence microscopy, we found that MASP-1 and -3 bound to all A. fumigatus growth stages (conidia, germ tubes, and hyphae), whereas rMASP-2 and the nonproteolytic rMAP-1 did not. Bound rMASPs could recruit rMBL and rficolin-3 to A. fumigatus conidia in a nonclassical manner and activate complement via rMASP-2. In experiments using recombinant and purified components, rMASP-1 increased the neutrophilic phagocytosis of conidia. In serum where known complement activation pathways were blocked, phagocytosis could be mediated by rMASP-3. We have encountered an unknown pathway for complement activation and found that MASP-1 and MASP-3 have dual functions as enzymes and as PRMs.

The SARS-CoV-2 Y453F mink variant displays a pronounced increase in ACE-2 affinity but does not challenge antibody neutralization.

Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 from humans to animals has been reported for many domesticated species, including farmed minks. The identification of novel spike gene mutations appearing in minks has raised major concerns about potential immune evasion and challenges for the global vaccine strategy. One genetic variant, known as "cluster five," arose among farmed minks in Denmark and resulted in a complete shutdown of the world's largest mink production. However, the functional properties of this new variant are not established. Here we present functional data on the cluster-five variant, which contains a mutation resulting in a Y453F residue change in the receptor-binding domain (RBD) of the spike protein. Using an ELISA-based angiotensin-converting enzyme-2/RBD inhibition assay, we show that the Y453F variant does not decrease established humoral immunity from previously infected individuals or affect the neutralizing antibody response in a vaccine mouse model based on the original Wuhan strain RBD or spike as antigens. However, biolayer interferometry analysis demonstrates that it binds the human angiotensin-converting enzyme-2 receptor with a 4-fold higher affinity than the original strain, suggesting an enhanced transmission capacity and a possible challenge for viral control. These results also indicate that the rise in the frequency of the cluster-five variant in mink farms might be a result of the fitness advantage conferred by the receptor adaptation rather than evading immune responses.

SARS-CoV-2 Antibody Responses Are Correlated to Disease Severity in COVID-19 Convalescent Individuals.

Globally, the COVID-19 pandemic has had extreme consequences for the healthcare system and has led to calls for diagnostic tools to monitor and understand the transmission, pathogenesis, and epidemiology, as well as to evaluate future vaccination strategies. In this study, we have developed novel, to our knowledge, flexible ELISA-based assays for specific detection of human SARS-CoV-2 Abs against the receptor-binding domain, including an Ag sandwich ELISA relevant for large population screening and three isotype-specific assays for in-depth diagnostics. Their performance was evaluated in a cohort of 350 convalescent participants with previous COVID-19 infection, ranging from asymptomatic to critical cases. We mapped the Ab responses to different areas on protein N and S and showed that the IgM, A, and G Ab responses against receptor-binding domain are significantly correlated to the disease severity. These assays and the data generated from them are highly relevant for diagnostics and prognostics and contribute to the understanding of long-term COVID-19 immunity.

Circulating Ficolin-2 and Ficolin-3 Form Heterocomplexes.

The complement system constitutes an important part of the innate immune system. The collectins and the ficolins are soluble pattern recognition molecules that contribute to complement activation via the lectin pathway. During previous experiments with ficolin-2 and ficolin-3, we have observed that the molecules may interact. We therefore hypothesized the existence of stable ficolin-2/-3 heterocomplexes. We could demonstrate ficolin-2/-3 heterocomplexes in normal human serum and plasma by ELISA using Abs specific for ficolin-2 and ficolin-3. The formation of heteromeric protein complexes were validated by coimmunoprecipitation and Western blot analysis. When recombinant ficolin-2 and recombinant ficolin-3 were mixed, no complexes were formed. However, when coexpressing ficolin-2 and ficolin-3 in Chinese hamster ovary cells, we could detect ficolin-2/-3 heterocomplexes in the supernatant. Furthermore, we measured concentration of the ficolin-2/-3 heterocomplexes in arbitrary units in 94 healthy individuals. We also established the relationship between the concentrations of ficolin-2, ficolin-3, and the ficolin-2/-3 heterocomplexes. We observed that the concentration of the ficolin-2/-3 heterocomplex correlated significantly with ficolin-2 (ρ: 0.24, p < 0.018) and ficolin-3 concentrations (ρ: 0.46, p < 0.0001). In conclusion, we describe a novel protein complex between ficolin-2 and ficolin-3 present in serum and plasma, which might be of additional biological relevance apart from the native ficolin-2 and ficolin-3 molecules.

Combining MAP-1:CD35 or MAP-1:CD55 fusion proteins with pattern-recognition molecules as novel targeted modulators of the complement cascade.

Dysregulation of the complement system is involved in the pathogenesis of several diseases, and its inhibition has been shown to be a feasible therapeutic option. Therefore, there is an interest in the development of complement modulators to treat complement activation-related inflammatory pathologies. Mannose-binding lectin (MBL)/ficolin/collectin-associated protein-1 (MAP-1) is a regulatory molecule of the lectin pathway (LP), whereas complement receptor 1 (CD35) and decay-accelerating factor (CD55) are membrane-anchored regulators with effects on the central effector molecule C3. In this study, we developed 2 novel soluble chimeric inhibitors by fusing MAP-1 to the 3 first domains of CD35 (CD351-3) or the 4 domains of CD55 (CD551-4) to modulate the complement cascade at 2 different stages. The constructs showed biologic properties similar to those of the parent molecules. In functional complement activation assays, the constructs were very efficient in inhibiting LP activation at the level of C3 and in the formation of terminal complement complex. This activity was enhanced when coincubated with recombinant LP recognition molecules MBL and ficolin-3. Recombinant MAP-1 fusion proteins, combined with recombinant LP recognition molecules to target sites of inflammation, represent a novel and effective therapeutic approach involving the initiation and the central and terminal effector functions of the complement cascade.-Pérez-Alós, L., Bayarri-Olmos, R., Skjoedt, M.-O., Garred, P. Combining MAP-1:CD35 or MAP-1:CD55 fusion proteins with pattern-recognition molecules as novel targeted modulators of the complement cascade.

The ficolin response to LPS challenge in mice.

The ficolins belong to an important family of pattern recognition molecules, which contributes to complement activation via the lectin pathway. How the ficolins respond to inflammatory stimuli remains only partly understood. In the present study, we investigated the ficolin A and ficolin B expression and protein distribution patterns in a mouse model of LPS-induced inflammation. The time- and tissue-specific expression of ficolin A and B was determined by real time PCR. Furthermore, ficolin protein levels in serum and bone marrow extracts from LPS challenged mice were determined by novel in-house developed sandwich ELISAs. Ficolin A was mainly expressed in liver and spleen. However, our data also suggested that ficolin A is expressed in bone marrow, which is the main site of ficolin B expression. The level of ficolin A and B expression was increased after stimulation with LPS in the investigated tissues. This was followed by a downregulation of expression, causing mRNA levels to return to baseline 24 h post LPS challenge. Protein levels appeared to follow the same pattern as the expression profiles, with an exception of ficolin B levels in serum, which kept increasing for 24 h. Ficolin A was likewise significantly increased in bronchoalveolar lavage fluid from mice infected with the fungi A. fumigatus, pointing towards a similar effect of the ficolins in non-sterile mouse models of inflammation. The results demonstrate that LPS-induced inflammation can induce a significant ficolin response, suggesting that the murine ficolins are acute phase reactants with increase in both mRNA expression and protein levels during systemic inflammation.

Alpha-cyclodextrin inhibits cholesterol crystal-induced complement-mediated inflammation: A potential new compound for treatment of atherosclerosis.

BACKGROUND AND AIMS: Cholesterol crystal (CC)-induced inflammation is a critical step in the development of atherosclerosis. CCs activate the complement system and induce an inflammatory response resulting in phagocytosis of the CCs, production of reactive oxygen species (ROS) and release of cytokines. The cyclodextrin 2-hydroxypropyl-ß-cyclodextrin has been found to reduce CC-induced complement activation and induce regression of established atherosclerotic plaques in a mouse model of atherosclerosis, thus inhibition of complement with cyclodextrins is a potential new strategy for treatment of inflammation during atherosclerosis. We hypothesized that other cyclodextrins, like α-cyclodextrin, may have related functions. METHODS: The effect of cyclodextrins on CC-induced complement activation, phagocytosis, and production of ROS from granulocytes and monocytes was investigated by flow cytometry and ELISA. RESULTS: We showed that α-cyclodextrin strongly inhibited CC-induced complement activation by inhibiting binding of the pattern recognition molecules C1q (via IgM) and ficolin-2. The reduced CC-induced complement activation mediated by α-cyclodextrin resulted in reduced phagocytosis and reduced ROS production in monocytes and granulocytes. Alpha-cyclodextrin was the most effective inhibitor of CC-induced complement activation, with the reduction in deposition of complement activation products being significantly different from the reduction induced by 2-hydroxypropyl-ß-cyclodextrin. We also found that α-cyclodextrin was able to dissolve CCs. CONCLUSIONS: This study identified α-cyclodextrin as a potential candidate in the search for therapeutics to prevent CC-induced inflammation in atherosclerosis.

Development of a Quantitative Assay for the Characterization of Human Collectin-11 (CL-11, CL-K1).

Collectin-11 (CL-11) is a pattern recognition molecule of the lectin pathway of complement with diverse functions spanning from host defense to embryonic development. CL-11 is found in the circulation in heterocomplexes with the homologous collectin-10 (CL-10). Abnormal CL-11 plasma levels are associated with the presence of disseminated intravascular coagulation, urinary schistosomiasis, and congenital disorders. Although there has been a marked development in the characterization of CL-11 there is still a scarcity of clinical tools for its analysis. Thus, we generated monoclonal antibodies and developed a quantitative ELISA to measure CL-11 in the circulation. The antibodies were screened against recombinant CL-11 and validated by ELISA and immunoprecipitation of serum and plasma. The best candidates were pairwise compared to develop a quantitative ELISA. The assay was validated regarding its sensitivity, reproducibility, and dilution linearity, demonstrating a satisfactory variability over a working range of 0.29-18.75 ng/ml. The mean plasma concentration of CL-11 in healthy controls was determined to be 289.4 ng/ml (range 143.2-459.4 ng/ml), highly correlated to the levels of CL/10/11 complexes (r = 0.729). Plasma CL-11 and CL-10/11 co-migrated in size exclusion chromatography as two major complexes of ~400 and >600 kDa. Furthermore, we observed a significant decrease at admission in CL-11 plasma levels in patients admitted to intensive care with systemic inflammatory response syndrome. By using the in-house antibodies and recombinant CL-11, we found that CL-11 can bind to zymosan independently of calcium by a separate site from the carbohydrate-binding region. Finally, we showed that CL-11/MASP-2 complexes trigger C4b deposition on zymosan. In conclusion, we have developed a specific and sensitive ELISA to investigate the ever-expanding roles of CL-11 in health and disease and shown a novel interaction between CL-11 and zymosan.

Chimeric Proteins Containing MAP-1 and Functional Domains of C4b-Binding Protein Reveal Strong Complement Inhibitory Capacities.

The complement system is a tightly regulated network of proteins involved in defense against pathogens, inflammatory processes, and coordination of the innate and adaptive immune responses. Dysregulation of the complement cascade is associated with many inflammatory disorders. Thus, inhibition of the complement system has emerged as an option for treatment of a range of different inflammatory diseases. MAP-1 is a pattern recognition molecule (PRM)-associated inhibitor of the lectin pathway of the complement system, whereas C4b-binding protein (C4BP) regulates both the classical and lectin pathways. In this study we generated chimeric proteins consisting of MAP-1 and the first five domains of human C4BP (C4BP1-5) in order to develop a targeted inhibitor acting at different levels of the complement cascade. Two different constructs were designed and expressed in CHO cells where MAP-1 was fused with C4BP1-5 in either the C- or N-terminus. The functionality of the chimeric proteins was assessed using different in vitro complement activation assays. Both chimeric proteins displayed the characteristic Ca2+-dependent dimerization and binding to PRMs of native MAP-1, as well as the co-factor activity of native C4BP. In ELISA-based complement activation assays they could effectively inhibit the lectin and classical pathways. Notably, MAP-1:C4BP1-5 was five times more effective than rMAP-1 and rC4BP1-5 applied at the same time, emphasizing the advantage of a single inhibitor containing both functional domains. The MAP-1/C4BP chimeras exert unique complement inhibitory properties and represent a novel therapeutic approach targeting both upstream and central complement activation.