The Lectin Complement Pathway Is Involved in Protection Against Enteroaggregative Escherichia coli Infection

C9 immunostaining as a tissue biomarker for periprosthetic joint infection diagnosis

Background

Culture-negative periprosthetic joint infections (PJI) are often false diagnosed as aseptic implant failure leading to unnecessary revision surgeries due to repeated infections. A marker to increase the security of e PJI diagnosis is therefore of great importance. The aim of this study was to test C9 immunostaining of periprosthetic tissue as a novel tissue-biomarker for a more reliable identification of PJI, as well as potential cross-reactivity.

Method

We included 98 patients in this study undergoing septic or aseptic revision surgeries. Standard microbiological diagnosis was performed in all cases for classification of patients. Serum parameters including C-reactive protein (CRP) serum levels and white blood cell (WBC) count were included, and the periprosthetic tissue was immunostained for C9 presence. The amount of C9 tissue staining was evaluated in septic versus aseptic tissue and the amount of C9 staining was correlated with the different pathogens causing the infection. To exclude cross-reactions between C9 immunostaining and other inflammatory joint conditions, we included tissue samples of a separate cohort with rheumatoid arthritis, wear particles and chondrocalcinosis.

Results

The microbiological diagnosis detected PJI in 58 patients; the remaining 40 patients were classified as aseptic. Serum CRP values were significantly increased in the PJI cohort. Serum WBC was not different between septic and aseptic cases. We found a significant increase in C9 immunostaining in the PJI periprosthetic tissue. To test the predictive value of C9 as biomarker for PJI we performed a ROC analyses. According to the Youden's criteria C9 is a very good biomarker for PJI detection with a sensitivity of 89% and a specificity of 75% and an AUC of 0.84. We did not observe a correlation of C9 staining with the pathogen causing the PJI. However, we observed a cross reactivity with the inflammatory joint disease like rheumatoid arthritis and different metal wear types. In addition, we did not observe a cross reactivity with chondrocalcinosis.

Conclusion

Our study identifies C9 as a potential tissue-biomarker for the identification of PJI using immunohistological staining of tissue biopsies. The use of C9 staining could help to reduce the number of false negative diagnoses of PJI.

Association of the FCN2 Gene Promoter Region Polymorphisms with Very Low Birthweight in Preterm Neonates

Single nucleotide polymorphisms (SNPs) localised to the promoter region of the FCN2 gene are known to influence the concentration of ficolin-2 in human serum and therefore potentially have clinical associations. We investigated the relationships between SNPs at positions −986 (A > G), −602 (G > A), −64 (A > C) and −4 (A > G) and clinical complications in 501 preterms. Major alleles at positions −986 and −64 and A/A homozygosity for both polymorphisms were less frequent among babies with very low birthweight (VLBW, ≤1500 g) compared with the reference group (OR = 0.24, p = 0.0029; and OR = 0.49, p = 0.024, respectively for A/A genotypes). A lower frequency of G/G homozygosity at position −4 was associated with gestational age <33 weeks and VLBW (OR = 0.38, p = 0.047; and OR = 0.07, p = 0.0034, respectively). The AGAG haplotype was protective for VLBW (OR = 0.6, p = 0.0369), whilst the GGCA haplotype had the opposite effect (OR = 2.95, p = 0.0249). The latter association was independent of gestational age. The AGAG/GGAA diplotype favoured both shorter gestational age and VLBW (OR = 1.82, p = 0.0234 and OR = 1.95, p = 0.0434, respectively). In contrast, AGAG homozygosity was protective for lower body mass (OR = 0.09, p = 0.0155). Our data demonstrate that some FCN2 variants associated with relatively low ficolin-2 increase the risk of VLBW and suggest that ficolin-2 is an important factor for fetal development/intrauterine growth.

Antibacterial Properties and Potential Mechanism of Serum from Chinese Alligator

The Chinese alligator (Alligator sinensis) is an ancient reptile with strong immunity that lives in wetland environments. This study tested the antibacterial ability of Chinese alligator serum (CAS) against Klebsiella pneumoniae, Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa and analyzed the potential underlying mechanisms. Results showed that the CAS had a marked antibacterial effect on K. pneumoniae, E. coli, and P. aeruginosa, while S. aureus was only mildly affected. However, these effects disappeared when Protease K was added to the serum. The serum proteome analysis revealed that the antibacterial ability of CAS was produced by interactions among various proteins and that the complement proteins played a major antibacterial role. Therefore, we made relevant predictions about the structure and function of complement component 3. In addition, sequence alignment and phylogenetic analysis of complement component 3d (C3d) in four mammalian species and two alligator species showed that the amino acids that make up the acid pocket on the concave surface of alligator C3d are not identical to those in mammals. This study provided evidence that CAS elicits significant antibacterial effects against some pathogens and provides the basis for further development of novel antibacterial drugs.

Inhibition of CL-11 reduces pulmonary inflammation in a mouse model of Klebsiella pneumoniae lung infection

Infection caused by K. pneumoniae is associated with severe inflammation due to stimulation of the innate immune components including the complement system, which is the main player of the innate immune response. Excessive complement-mediated inflammation may cause severe lung injury. Here we clearly show that K. pneumoniae binds to different lectin pathway carbohydrate recognition molecules and activates the complement cascade via the LP. Administration of anti-CL-11 antibodies 6 h before the infection impairs LP functional activity but it shows no effect on the survival time of mice infected with K. pneumoniae. Similarly, no significant difference in bacterial load in blood and lung tissues was observed between mice that received anti-CL-11 and control group treated with an isotype antibody. Interestingly, treatment of mice with anti-CL-11 prior to infection significantly improved histopathological changes and lung injury score induced by K. pneumoniae. Moreover, administration of anti-CL-11 reduced leukocytes infiltration into lung tissues and decreased the levels of the inflammatory mediators TNF-α, IL-6, and IL-1β in the infected mice. These findings indicate that inhibition of the LP could secure a significant level of protection against lung injury during the infection caused by K. pneumoniae.

Polymorphisms of the FCN2 Gene 3'UTR Region and Their Clinical Associations in Preterm Newborns

Ficolin-2 is regarded as an important innate immunity factor endowed with both lectin (carbohydrate recognition) qualities and ability to induce complement activation. The aim of this study was to investigate the association of the FCN2 3'-untranslated region (3'UTR) polymorphisms with ficolin-2 expression and perinatal complications in preterm neonates. The sequencing analysis allowed us to identify six 3'UTR polymorphisms with minor allele frequency (MAF) >1%: rs4521835, rs73664188, rs11103564, rs11103565, rs6537958 and rs6537959. Except for rs4521835, all adhered to Hardy-Weinberg expectations. Moreover, rs6537958 and rs6537959 were shown to be in perfect linkage disequilibrium (LD) with nine other genetic polymorphisms: rs7040372, rs7046516, rs747422, rs7847431, rs6537957, rs6537960, rs6537962, rs11462298 and rs7860507 together stretched on a distance of 1242 bp and very high LD with rs11103565. The 3'UTR region was shown to bind nuclear extract proteins. The polymorphisms at rs4521835 and rs73664188 were found to influence serum ficolin-2 concentration significantly. All polymorphisms identified create (together with exon 8 polymorphism, rs7851696) two haplotype blocks. Among 49 diplotypes (D1-D49) created from rs7851696 (G>T), rs4521835 (T>G), rs73664188 (T>C), rs11103564 (T>C), rs11103565 (G>A) and rs6537959 (T>A), twenty two occurred with frequency >1%. Two diplotypes: D13 (GTTTGT/GGTCGT) and D10 (GTTTGT/GGTCGA), were significantly more frequent among preterm neonates with early onset of infection and pneumonia, compared with newborns with no infectious complications (OR 2.69 and 2.81, respectively; both p<0.05). The minor (C) allele at rs73664188 was associated with an increased risk of very low (≤1500 g) birthweight (OR=1.95, p=0.042) but was associated with the opposite effect at rs11103564 (OR=0.11, p=0.005).

Emerging roles of the complement system in host-pathogen interactions

The complement system has historically been entertained as a fluid-phase, hepatically derived system which protects the intravascular space from encapsulated bacteria. However, there has been an increasing appreciation for its role in protection against non-encapsulated pathogens. Specifically, we have an improved understanding of how pathogens are recognized by specific complement proteins, as well as how they trigger and evade them. Additionally, we have an improved understanding of locally derived complement proteins, many of which promote host defense. Moreover, intracellular complement proteins have been identified that facilitate local protection and barrier function despite pathogen invasion. Our review aims to summarize these advances in the field as well as provide an insight into the pathophysiological changes occurring when the system is dysregulated in infection.

Biochemical and structural characterization of a recombinant fibrinogen-related lectin from Penaeus monodon

Fibrinogen-related lectins are carbohydrate-binding proteins of the innate immune system that recognize glycan structures on microbial surfaces. These innate immune lectins are crucial for invertebrates as they do not rely on adaptive immunity for pathogen clearance. Here, we characterize a recombinant fibrinogen-related lectin PmFREP from the black tiger shrimp Penaeus monodon expressed in the Trichoplusia ni insect cell. Electron microscopy and cross-linking experiments revealed that PmFREP is a disulfide-linked dimer of pentamers distinct from other fibrinogen-related lectins. The full-length protein binds N-acetyl sugars in a Ca²⁺ ion-independent manner. PmFREP recognized and agglutinated Pseudomonas aeruginosa. Weak binding was detected with other bacteria, including Vibrio parahaemolyticus, but no agglutination activity was observed. The biologically active PmFREP will not only be a crucial tool to elucidate the innate immune signaling in P. monodon and other economically important species, but will also aid in detection and prevention of shrimp bacterial infectious diseases.

Ficolin A derived from local macrophages and neutrophils protects against lipopolysaccharide-induced acute lung injury by activating complement

Ficolins are important and widely distributed pattern recognition molecules that can induce lectin complement pathway activation and initiate the innate immune response. Although ficolins can bind lipopolysaccharide (LPS) in vitro, the sources, dynamic changes and roles of local ficolins in LPS-induced pulmonary inflammation and injury remain poorly understood. In this study, we established a ficolin knockout mouse model by clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) technology, and used flow cytometry and hematoxylin and eosin staining to study the expressions and roles of local ficolins in LPS-induced pulmonary inflammation and injury. Our results show that besides ficolin B (FcnB), ficolin A (FcnA) is also expressed in leukocytes from the bone marrow, peripheral blood, lung and spleen. Further analyses showed that macrophages and neutrophils are the main sources of FcnA and FcnB, and T and B cells also express a small amount of FcnB. The intranasal administration of LPS induced local pulmonary inflammation with the increased recruitment of macrophages and neutrophils. LPS stimulation induced increased expression of FcnA and FcnB in neutrophils at the acute stage and in macrophages at the late stage. The severity of the lung injury and local inflammation of Fcna-/- mice was increased by the induction of extracellular complement activation. The recovery of LPS-induced local lung inflammation and injury was delayed in Fcnb-/- mice. Hence, these findings suggested that the local macrophage- and neutrophil-derived FcnA protects against LPS-induced acute lung injury by mediating extracellular complement activation.

Antibodies Specific to Membrane Proteins Are Effective in Complement-Mediated Killing of Mycoplasma bovis

The metabolic inhibition (MI) test is a classic test for the identification of mycoplasmas, used for measuring the growth-inhibiting antibodies directed against acid-producing mycoplasmas, although their mechanism still remains obscure. To determine the major antigens involved in the immune killing of Mycoplasma bovis, we used a pulldown assay with anti-M. bovis antibodies as bait and identified nine major antigens.

The metabolic inhibition (MI) test is a classic test for the identification of mycoplasmas, used for measuring the growth-inhibiting antibodies directed against acid-producing mycoplasmas, although their mechanism still remains obscure. To determine the major antigens involved in the immune killing of Mycoplasma bovis, we used a pulldown assay with anti-M. bovis antibodies as bait and identified nine major antigens. Among these antigens, we performed the MI test and determined that the growth of M. bovis could be inhibited effectively in the presence of complement by antibodies against specifically membrane protein P81 or UgpB in the presence of complement. Using a complement killing assay, we demonstrated that M. bovis can be killed directly by complement and that antibody-dependent complement-mediated killing is more effective than that by complement alone. Complement lysis and scanning electron microscopy results revealed M. bovis rupture in the presence of complement. Together, these results suggest that the metabolic inhibition of M. bovis is antibody-dependent complement-mediated killing. This study provides new insights into mycoplasma killing by the complement system and may guide future vaccine development studies for the treatment of mycoplasma infection. Furthermore, our findings also indicate that mycoplasmas may be an appropriate new model for studying the lytic activity of membrane attack complex (MAC).

Decreased Ficolin-3-mediated Complement Lectin Pathway Activation and Alternative Pathway Amplification During Bacterial Infections in Patients With Type 2 Diabetes Mellitus

Bacterial infections are frequent and severe in patients with diabetes mellitus. Whether diabetes per se induces functional alterations in the complement system hampering activation during infection is unknown. We investigated key elements of the complement system during bacterial infections in patients with type 2 diabetes mellitus (T2DM) and compared them to non-diabetic (ND) individuals. Using a prospective design, we included 197 T2DM, and 196 ND subjects, all with clinical diagnosis of acute community-acquired bacterial infections. Functional activities of the ficolin-3-mediated lectin (F3-LP), mannose binding lectin-mediated lectin- (MBL-LP), classical (CP), and alternative pathways (AP), as well as concentrations of complement activation products C4d and sC5b-9 were determined. Functional in vitro activities of F3-LP and AP were significantly higher in T2DM than in ND subjects, (median 64% vs. 45%, p = 0.0354 and 75 vs. 28%, p = 0.0013, respectively), indicating a decreased in vivo activation and lack of consumption of F3-LP and AP in T2DM patients, whereas no difference in functional capacities of CP and MBL-LP were observed between T2DM and ND subjects. Diminished F3-LP and AP activation was most pronounced in diabetic patients with urinary tract infections with positive microbiological culture results for Escherichia coli bacteria. In the T2DM group 3-months mortality significantly associated with diminished F3-LP and AP, but not with CP activation. Concentrations of C4d and sC5b-9 were significantly lower in the T2DM than in ND patients. In conclusion, we found impaired F3-LP activation and lack of AP amplification during bacterial infections in patients with type 2 diabetes, compared to non-diabetic subjects, suggesting a diminished complement mediated protection to bacterial infections in T2DM.

Ficolins and the Recognition of Pathogenic Microorganisms: An Overview of the Innate Immune Response and Contribution of Single Nucleotide Polymorphisms

Ficolins are innate pattern recognition receptors (PRR) and play integral roles within the innate immune response to numerous pathogens throughout the circulation, as well as within organs. Pathogens are primarily removed by direct opsonisation following the recognition of cell surface carbohydrates and other immunostimulatory molecules or via the activation of the lectin complement pathway, which results in the deposition of C3b and the recruitment of phagocytes. In recent years, there have been a number of studies implicating ficolins in the recognition and removal of numerous bacterial, viral, fungal, and parasitic pathogens. Moreover, there has been expanding evidence highlighting that mutations within these key immune proteins, or the possession of particular haplotypes, enhance susceptibility to colonization by pathogens and dysfunctional immune responses. This review will therefore encompass previous knowledge on the role of ficolins in the recognition of bacterial and viral pathogens, while acknowledging the recent advances in the immune response to fungal and parasitic infections. Additionally, we will explore the various genetic susceptibility factors that predispose individuals to infection.