Camilla Schjalm

Properdin binding to complement activating surfaces depends on initial C3b deposition

Significance The role of properdin in stabilization of the alternative pathway C3 convertase is indisputable, whereas its role as pattern recognition molecule remains controversial. Properdin lacks the structural homology shared by other pattern recognition molecules of the complement system, and has its major function in stabilizing the C3bBb convertase. We found that properdin binding was completely abolished by C3 inhibition after the exposure of human serum to myeloperoxidase, human umbilical vein endothelial cells, and Neisseria meningitidis, showing that properdin is not a pattern recognition molecule for these targets. We therefore challenge the view of properdin as a pattern recognition molecule, and argue that properdin typically binds a complement-activating surface subsequent to C3b to stabilize the alternative pathway C3 convertase. Two functions have been assigned to properdin; stabilization of the alternative convertase, C3bBb, is well accepted, whereas the role of properdin as pattern recognition molecule is controversial. The presence of nonphysiological aggregates in purified properdin preparations and experimental models that do not allow discrimination between the initial binding of properdin and binding secondary to C3b deposition is a critical factor contributing to this controversy. In previous work, by inhibiting C3, we showed that properdin binding to zymosan and Escherichia coli is not a primary event, but rather is solely dependent on initial C3 deposition. In the present study, we found that properdin in human serum bound dose-dependently to solid-phase myeloperoxidase. This binding was dependent on C3 activation, as demonstrated by the lack of binding in human serum with the C3-inhibitor compstatin Cp40, in C3-depleted human serum, or when purified properdin is applied in buffer. Similarly, binding of properdin to the surface of human umbilical vein endothelial cells or Neisseria meningitidis after incubation with human serum was completely C3-dependent, as detected by flow cytometry. Properdin, which lacks the structural homology shared by other complement pattern recognition molecules and has its major function in stabilizing the C3bBb convertase, was found to bind both exogenous and endogenous molecular patterns in a completely C3-dependent manner. We therefore challenge the view of properdin as a pattern recognition molecule, and argue that the experimental conditions used to test this hypothesis should be carefully considered, with emphasis on controlling initial C3 activation under physiological conditions.

Lack of the Lysosomal Membrane Protein, GLMP, in Mice Results in Metabolic Dysregulation in Liver

Ablation of glycosylated lysosomal membrane protein (GLMP, formerly known as NCU-G1) has been shown to cause chronic liver injury which progresses into liver fibrosis in mice. Both lysosomal dysfunction and chronic liver injury can cause metabolic dysregulation. Glmpgt/gt mice (formerly known as Ncu-g1gt/gtmice) were studied between 3 weeks and 9 months of age. Body weight gain and feed efficiency of Glmpgt/gt mice were comparable to wild type siblings, only at the age of 9 months the Glmpgt/gt siblings had significantly reduced body weight. Reduced size of epididymal fat pads was accompanied by hepatosplenomegaly in Glmpgt/gt mice. Blood analysis revealed reduced levels of blood glucose, circulating triacylglycerol and non-esterified fatty acids in Glmpgt/gt mice. Increased flux of glucose, increased de novo lipogenesis and lipid accumulation were detected in Glmpgt/gt primary hepatocytes, as well as elevated triacylglycerol levels in Glmpgt/gt liver homogenates, compared to hepatocytes and liver from wild type mice. Gene expression analysis showed an increased expression of genes involved in fatty acid uptake and lipogenesis in Glmpgt/gt liver compared to wild type. Our findings are in agreement with the metabolic alterations observed in other mouse models lacking lysosomal proteins, and with alterations characteristic for advanced chronic liver injury.

High serum CXCL10 in Rickettsia conorii infection is endothelial cell mediated subsequent to whole blood activation

BACKGROUND The pathophysiological hallmark of Rickettsia conorii (R. conorii) infection comprises infection of endothelial cells with perivascular infiltration of T-cells and macrophages. Although interferon (IFN)-γ-induced protein 10 (IP-10)/CXCL10 is induced during vascular inflammation, data on CXCL10 in R. conorii infection is scarce. METHODS Serum CXCL10 was analyzed in two cohorts of southern European patients with R. conorii infection using multiplex cytokine assays. The mechanism of R. conorii-induced CXCL10 release was examined ex vivo using human whole blood interacting with endothelial cells. RESULTS (i) At admission, R. conorii infected patients had excessively increased CXCL10 levels, similar in the Italian (n=32, ∼56-fold increase vs controls) and the Spanish cohort (n=38, ∼68-fold increase vs controls), followed by a marked decrease after recovery. The massive CXCL10 increase was selective since it was not accompanied with similar changes in other cytokines. (ii) Heat-inactivated R. conorii induced a marked CXCL10 increase when whole blood and endothelial cells were co-cultured. Even plasma obtained from R. conorii-exposed whole blood induced a marked CXCL10 release from endothelial cells, comparable to the levels found in serum of R. conorii-infected patients. Bacteria alone did not induce CXCL10 production in endothelial cells, macrophages or smooth muscle cells. CONCLUSIONS We show a massive and selective serum CXCL10 response in R. conorii-infected patients, likely reflecting release from infected endothelial cells characterized by infiltrating T cells and monocytes. The CXCL10 response could contribute to T-cell infiltration within the infected organ, but the pathologic consequences of CXCL10 in clinical R. conorii infection remain to be defined.

Combined Inhibition of C5 and CD14 Attenuates Systemic Inflammation in a Piglet Model of Meconium Aspiration Syndrome

Background: Meconium aspiration syndrome (MAS) is a severe lung condition affecting newborns and it can lead to a systemic inflammatory response. We previously documented complement activation and cytokine release in a piglet MAS model. Additionally, we showed ex vivo that meconium-induced inflammation was dependent on complement and Toll-like receptors. Objectives: To assess the efficacy of the combined inhibition of complement (C5) and CD14 on systemic inflammation induced in a forceful piglet MAS model. Methods: Thirty piglets were randomly allocated to a treatment group receiving the C5-inhibitor SOBI002 and anti-CD14 (n = 15) and a nontreated control group (n = 15). MAS was induced by intratracheal meconium instillation, and the piglets were observed for 5 h. Complement, cytokines, and myeloperoxidase (MPO) were measured by ELISA. Results: SOBI002 ablated C5 activity and the formation of the terminal complement complex in vivo. The combined inhibition attenuated the inflammasome cytokines IL-1β and IL-6 by 60 (p = 0.029) and 44% (p = 0.01), respectively, and also MPO activity in the bronchoalveolar fluid by 42% (p = 0.017). Ex vivo experiments in human blood revealed that the combined regimen attenuated meconium-induced MPO release by 64% (p = 0.008), but there was only a negligible effect with single inhibition, indicating a synergic cross-talk between the key molecules C5 and CD14. Conclusion: Combined inhibition of C5 and CD14 attenuates meconium-induced inflammation in vivo and this could become a future therapeutic regimen for MAS.

Ficolins do not alter host immune responses to lipopolysaccharide-induced inflammation in vivo

Ficolins are a family of pattern recognition molecules that are capable of activating the lectin pathway of complement. A limited number of reports have demonstrated a protective role of ficolins in animal models of infection. In addition, an immune modulatory role of ficolins has been suggested. Yet, the contribution of ficolins to inflammatory disease processes remains elusive. To address this, we investigated ficolin deficient mice during a lipopolysaccharide (LPS)-induced model of systemic inflammation. Although murine serum ficolin was shown to bind LPS in vitro, there was no difference between wildtype and ficolin deficient mice in morbidity and mortality by LPS-induced inflammation. Moreover, there was no difference between wildtype and ficolin deficient mice in the inflammatory cytokine profiles after LPS challenge. These findings were substantiated by microarray analysis revealing an unaltered spleen transcriptome profile in ficolin deficient mice compared to wildtype mice. Collectively, results from this study demonstrate that ficolins are not involved in host response to LPS-induced systemic inflammation.