Ole-Lars Brekke

Innate Immune Responses to Danger Signals in Systemic Inflammatory Response Syndrome and Sepsis

The systemic immune response induced by non‐infectious agents is called systemic inflammatory response syndrome (SIRS) and infection‐induced systemic immune response is called sepsis. The host inflammatory response in SIRS and sepsis is similar and may lead to multiple organ dysfunction syndrome (MODS) and ultimately death. The mortality and morbidity in SIRS and sepsis (i.e. critical illness) remain high despite advances in diagnostic and organ supporting possibilities in intensive care units. In critical illness, the acute immune response is organized and executed by innate immunity influenced by the neuroendocrine system. This response starts with sensing of danger by pattern‐recognition receptors on the immune competent cells and endothelium. The sensed danger signals, through specific signalling pathways, activate nuclear transcription factor κB and other transcription factors and gene regulatory systems which up‐regulate the expression of pro‐inflammatory mediators. The plasma cascades are also activated which together with the produced pro‐inflammatory mediators stimulate further the production of inflammatory biomarkers. The acute inflammatory response underlies the pathophysiological mechanisms involved in the development of MODS. The inflammatory mediators directly affect organ function and cause a decline in remote organ function by mediating the production of nitric oxide leading to mitochondrial anergy and cytopathic hypoxia, a condition of cellular inability to use oxygen. Understanding the mechanisms of acute immune responses in critical illness is necessary for the development of urgently needed therapeutics. The aim of this review is to provide a description of the key components and mechanisms involved in the immune response in SIRS and sepsis.

Cholesterol Crystals Induce Complement-Dependent Inflammasome Activation and Cytokine Release

Inflammation is associated with development of atherosclerosis, and cholesterol crystals (CC) have long been recognized as a hallmark of atherosclerotic lesions. CC appear early in the atheroma development and trigger inflammation by NLRP3 inflammasome activation. In this study we hypothesized whether CC employ the complement system to activate inflammasome/caspase-1, leading to release of mature IL-1β, and whether complement activation regulates CC-induced cytokine production. In this study we describe that CC activated both the classical and alternative complement pathways, and C1q was found to be crucial for the activation. CC employed C5a in the release of a number of cytokines in whole blood, including IL-1β and TNF. CC induced minimal amounts of cytokines in C5-deficient whole blood, until reconstituted with C5. Furthermore, C5a and TNF in combination acted as a potent primer for CC-induced IL-1β release by increasing IL-1β transcripts. CC-induced complement activation resulted in upregulation of complement receptor 3 (CD11b/CD18), leading to phagocytosis of CC. Also, CC mounted a complement-dependent production of reactive oxygen species and active caspase-1. We conclude that CC employ the complement system to induce cytokines and activate the inflammasome/caspase-1 by regulating several cellular responses in human monocytes. In light of this, complement inhibition might be an interesting therapeutic approach for treatment of atherosclerosis.

The role of complement C3 opsonization, C5a receptor, and CD14 in E. coli-induced up-regulation of granulocyte and monocyte CD11b/CD18 (CR3), phagocytosis, and oxidative burst in human whole blood.

The relative role of complement and CD14 in Escherichia coli‐induced leukocyte CD11b up‐regulation, phagocytosis, and oxidative burst in human whole blood was examined. The highly specific thrombin inhibitor lepirudin was used as anticoagulant, as it does not affect complement activation. Complement inhibition at the level of C3 (anti‐C2 and anti‐factor D) and C5 (C5a receptor antagonist and anti‐C5/C5a) efficiently inhibited CD11b up‐regulation, phagocytosis, and oxidative burst in granulocytes. Monocyte activation was generally less complement‐dependent, but when C3 activation was blocked, a pronounced inhibition of phagocytosis and oxidative burst was obtained. Only the combination of anti‐C2 and antifactor D blocked E. coli C3 opsonization completely. Whole E. coli, disrupted E. coli, and the C3‐convertase activator cobra venom factor up‐regulated CD11b rapidly on both cell types, proportional to their complement activation potential in the fluid phase. In comparison, purified LPS at concentrations comparable with that present in the E. coli preparations did not activate complement. Oxidative burst was induced only by whole bacteria. Finally, the combination of complement inhibition and anti‐CD14 completely blocked E. coli‐induced granulocyte and monocyte CD11b up‐regulation and quantitatively, virtually abolished phagocytosis. The results indicate that complement and CD14, despite differential effects on granulocytes and monocytes, are the two crucial, quantitative factors responsible for E. coli‐induced CD11b, phagocytosis, and oxidative burst in both cell types.

Combined inhibition of complement and CD14 abolish E. coli-induced cytokine-, chemokine- and growth factor-synthesis in human whole blood.

The relative role of complement and CD14 in E. coli-induced cytokine synthesis in an in vitro human whole blood model of sepsis was examined. Fresh lepirudin-anticoagulated whole blood was incubated with E. coli for 2h. Monoclonal antibodies or a C5a receptor antagonist were used to block complement. Inflammatory mediators (n=27) were measured by multiplex technology, selected cytokine mRNA by real time PCR, and CD11b, oxidative burst and phagocytosis by flow cytometry. E. coli significantly increased 18 of the 27 inflammatory mediators, including proinflammatory cytokines (TNF-alpha, IL-6, INF-gamma and IL-1beta), chemokines (IL-8, MCP-1, MIP-1alpha, MIP-1beta, eotaxin and IP-10), growth factors (VEGF, FGF-basic, G-CSF and GM-CSF) and other interleukins (IL-9, IL-15 and IL-17). Notably, the increases in all mediators were abolished by a combined inhibition of CD14 and complement using anti-C2 and anti-factor D in combination, whereas the relative effect of the inhibition of complement and CD14 varied. In comparison, a C5a receptor antagonist and anti-CD14 in combination reduced cytokine synthesis less efficiently. Real time PCR analysis confirmed that the cytokine synthesis was blocked at the mRNA level. Similarly, E. coli-induced CD11b up-regulation, oxidative burst and phagocytosis was totally inhibited by CD14, anti-C2 and anti-factor D in combination after 2h incubation. In conclusion, the combined inhibition of complement using anti-C2, anti-factor D and CD14 almost completely inhibits the E. coli-induced inflammatory response. The combined approach may therefore be a new treatment regimen in Gram-negative sepsis.

Eculizumab treatment during pregnancy does not affect the complement system activity of the newborn

Eculizumab is a humanized IgG2/4 chimeric anti-complement C5 antibody used to treat patients with paroxysmal nocturnal hemoglobinuria (PNH) or atypical hemolytic uremic syndrome. The aim of this study was to evaluate whether or not the complement activity in newborns from pregnant women who receive eculizumab is impaired. A novel eculizumab-C5 complex (E-C5) specific assay was developed and revealed that two newborns carried only 6-7% of the E-C5 detected in their eculizumab-treated PNH mothers. Serum from the pregnant women completely lacked terminal complement pathway activity, whereas the complement activity in the serum of the newborns was completely normal. Data from the pregnant women and their newborns were compared with that of healthy age-matched female controls and healthy newborns, as well as a non-treated pregnant woman with PNH and her newborn. These all showed normal complement activity without detectable E-C5 complexes. Furthermore, absence of eculizumab or E-C5 in the newborn could not be explained by lack of eculizumab binding to the neonatal Fc receptor (FcRn), as eculizumab bound strongly to the receptor in vitro. In conclusion, despite binding to FcRn neither eculizumab nor E-C5 accumulates in fetal plasma, and eculizumab treatment during pregnancy does not impair the complement function in the newborn.

Hypothesis: Combined Inhibition of Complement and CD14 as Treatment Regimen to Attenuate the Inflammatory Response

Pattern recognition is an essential event in innate immunity. Complement and Toll-like receptors (TLR), including the CD14 molecule, are two important upstream components of the innate immune system, recognizing exogenous structures as well as endogenous ligands. They act partly independent in the inflammatory network, but also have several cross-talk mechanisms which are under current investigation. Complement is an essential part of innate immunity protecting the host against infection. However, it is a double-edged sword since inappropriate activation may damage the host. Uncontrolled systemic activation of complement, as seen in severe sepsis, may contribute to the breakdown of homeostatic mechanisms leading to the irreversible state of septic shock. Complement inhibition is promising for protection of lethal experimental sepsis, but clinical studies are missing. Lipopolysaccharide (LPS) has been implicated in the pathogenesis of gram-negative sepsis by inducing synthesis of pro-inflammatory cytokines through binding to CD14 and the TLR4/MD-2 complex. Neutralization of LPS or blocking of CD14 has been effective in preventing LPS-induced lethal shock in animal studies, but results from clinical studies have been disappointing, as for most other therapeutic strategies. Based on some recently published data and further pilot data obtained in our laboratory, we hypothesize that inhibition of complement combined with neutralization of CD14 may attenuate the uncontrolled inflammatory reaction which leads to breakdown of homeostasis during sepsis. We further postulate this regimen as an approach for efficient inhibition of the initial innate recognition, exogenous as well as endogenous, to prevent downstream activation of the inflammatory reaction in general.

Bride and groom in systemic inflammation--the bells ring for complement and Toll in cooperation.

Attenuating the sepsis-induced systemic inflammatory response, with subsequent homeostatic imbalance, has for years been one of the main tasks in sepsis related research. Complement and the TLR family constitute two important upstream sensor and effector-systems of innate immunity. Although they act as partly independent branches of pattern recognition, recent evidence indicate a considerable cross-talk implying that they can either compensate, synergize or antagonize each other. Combined inhibition of these pathways is therefore a particularly interesting approach with a profound anti-inflammatory potential. In previous preclinical studies, we demonstrated that targeting the key molecules C3 or C5 of complement and CD14 of the TLR family had a vast anti-inflammatory effect on Gram-negative bacteria-induced inflammation and sepsis. In this review, we elucidate the significance of these key molecules as important targets for intervention in sepsis and systemic inflammatory response syndrome. Finally, we argue that a combined inhibition of complement and CD14 represent a potential general treatment regimen, beyond the limit of sepsis, including non-infectious systemic inflammation and ischemia reperfusion injury.

The induction of cytokines by polycation containing microspheres by a complement dependent mechanism.

The cytokine-inducing potential of various microspheres were evaluated in a short-time screening assay of lepirudin-anticoagulated human whole blood utilizing the Bio-Plex Human cytokine 27-plex system. The inflammatory cytokines IL-1β, TNF and IL-6; the anti-inflammatory mediators IL-1ra and IL-10; the chemokines IL-8, MIP-1α and MCP-1; and the growth factor VEGF were induced by polycation (poly-l-lysine or poly(methylene-co-guanidine)) containing microspheres. Alginate microspheres without polycations did not induce the corresponding cytokine panel, nor did soluble alginate. By inhibiting complement C3 using compstatin analog CP20, a total inhibition of complement activation as well as the inflammatory mediators was achieved, indicating that complement activation alone was responsible for the induced cytokines. A strong deposition of C3c on the poly-l-lysine containing surface, while not on the microspheres lacking polycations, also points to the formation of C3 convertase as involved in the biomaterial-induced cytokine induction. These results show that complement is responsible for the induction of cytokines by polycation containing microspheres. We point to complement as an important initiator of inflammatory responses to biomaterials and the lepirudin anticoagulated whole blood assay as an important tool to identify the most tolerable and safe materials for implantation to humans.

Differential Effect of Inhibiting MD-2 and CD14 on LPS- Versus Whole E. coli Bacteria-Induced Cytokine Responses in Human Blood

BACKGROUND Sepsis is a major world-wide medical problem with high morbidity and mortality. Gram-negative bacteria are among the most important pathogens of sepsis and their LPS content is regarded to be important for the systemic inflammatory reaction. The CD14/myeloid differentiation factor 2 (MD-2)/TLR4 complex plays a major role in the immune response to LPS . The aim of this study was to compare the effects of inhibiting MD-2 and CD14 on ultra-pure LPS - versus whole E. coli bacteria-induced responses. METHODS Fresh human whole blood was incubated with upLPS or whole E. coli bacteria in the presence of MD-2 or CD14 neutralizing monoclonal antibodies, or their respective controls, and/or the specific complement-inhibitor compstatin. Cytokines were measured by a multiplex (n = 27) assay. NFκB activity was examined in cells transfected with CD14, MD-2 and/or Toll-like receptors. RESULTS LPS-induced cytokine response was efficiently and equally abolished by MD-2 and CD14 neutralization. In contrast, the response induced by whole E. coli bacteria was only modestly reduced by MD-2 neutralization, whereas CD14 neutralization was more efficient. Combination with compstatin enhanced the effect of MD-2 neutralization slightly. When compstatin was combined with CD14 neutralization, however, the response was virtually abolished for all cytokines, including IL-17, which was only inhibited by this combination. The MD-2-independent effect observed for CD14 could not be explained by TLR2 signaling. CONCLUSION Inhibition of CD14 is more efficient than inhibition of MD-2 on whole E. coli-induced cytokine response, suggesting CD14 to be a better target for intervention in Gram-negative sepsis, in particular when combined with complement inhibition.

The key roles of complement and tissue factor in Escherichia coli-induced coagulation in human whole blood

The complement system and the Toll‐like (TLR) co‐receptor CD14 play important roles in innate immunity and sepsis. Tissue factor (TF) is a key initiating component in intravascular coagulation in sepsis, and long pentraxin 3 (PTX3) enhances the lipopolysaccharide (LPS)‐induced transcription of TF. The aim of this study was to study the mechanism by which complement and CD14 affects LPS‐ and Escherichia coli (E. coli)‐induced coagulation in human blood. Fresh whole blood was anti‐coagulated with lepirudin, and incubated with ultra‐purified LPS (100 ng/ml) or with E. coli (1 × 107/ml). Inhibitors and controls included the C3 blocking peptide compstatin, an anti‐CD14 F(ab′)2 antibody and a control F(ab′)2. TF mRNA was measured using quantitative polymerase chain reaction (qPCR) and monocyte TF surface expression by flow cytometry. TF functional activity in plasma microparticles was measured using an amidolytic assay. Prothrombin fragment F 1+2 (PTF1.2) and PTX3 were measured by enzyme‐linked immunosorbent assay (ELISA). The effect of TF was examined using an anti‐TF blocking antibody. E. coli increased plasma PTF1.2 and PTX3 levels markedly. This increase was reduced by 84–>99% with compstatin, 55–97% with anti‐CD14 and > 99% with combined inhibition (P < 0·05 for all). The combined inhibition was significantly (P < 0·05) more efficient than compstatin and anti‐CD14 alone. The LPS‐ and E. coli–induced TF mRNA levels, monocyte TF surface expression and TF functional activity were reduced by > 99% (P < 0·05) with combined C3 and CD14 inhibition. LPS‐ and E. coli–induced PTF1.2 was reduced by 76–81% (P < 0·05) with anti‐TF antibody. LPS and E. coli activated the coagulation system by a complement‐ and CD14‐dependent up‐regulation of TF, leading subsequently to prothrombin activation.