Complement activation by whole endotoxin is blocked by a monoclonal antibody to factor B

The Endotoxin Hypothesis of Parkinson's Disease

The endotoxin hypothesis of Parkinson's disease (PD) is the idea that lipopolysaccharide (LPS) endotoxins contribute to the pathogenesis of this disorder. LPS endotoxins are found in, and released from, the outer membrane of Gram-negative bacteria, for example in the gut. It is proposed that gut dysfunction in early PD leads to elevated LPS levels in the gut wall and blood, which promotes both α-synuclein aggregation in the enteric neurons and a peripheral inflammatory response. Communication to the brain via circulating LPS and cytokines in the blood and/or the gut-brain axis leads to neuroinflammation and spreading of α-synuclein pathology, exacerbating neurodegeneration in brainstem nuclei and loss of dopaminergic neurons in the substantia nigra, and manifesting in the clinical symptoms of PD. The evidence supporting this hypothesis includes: (1) gut dysfunction, permeability, and bacterial changes occur early in PD, (2) serum levels of LPS are increased in a proportion of PD patients, (3) LPS induces α-synuclein expression, aggregation, and neurotoxicity, (4) LPS causes activation of peripheral monocytes leading to inflammatory cytokine production, and (5) blood LPS causes brain inflammation and specific loss of midbrain dopaminergic neurons, mediated by microglia. If the hypothesis is correct, then treatment options might include: (1) changing the gut microbiome, (2) reducing gut permeability, (3) reducing circulating LPS levels, or (4) blocking the response of immune cells and microglia to LPS. However, the hypothesis has a number of limitations and requires further testing, in particular whether reducing LPS levels can reduce PD incidence, progression, or severity. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Local release of properdin in the cellular microenvironment: role in pattern recognition and amplification of the alternative pathway of complement

Properdin, the only positive regulatory protein of the complement system, acts as both a stabilizer of the alternative pathway (AP) convertases and as a selective pattern recognition molecule of certain microorganisms and host cells (i.e., apoptotic/necrotic cells) by serving as a platform for de novo C3b,Bb assembly. Properdin, a highly positively charged protein, normally exists as cyclic dimers (P(2)), trimers (P(3)), and tetramers (P(4)) of head-to-tail associations of monomeric 53 kDa subunits. While most complement proteins are produced mainly in the liver, properdin is synthesized primarily by various cell types, including neutrophils, monocytes, primary T cells, and shear-stressed endothelial cells resulting in properdin serum levels of 4-25 μg/ml. Multiple inflammatory agonists stimulate the release of properdin from stimulated leukocytes into the cellular microenvironment. Concentrated, focused increases in properdin levels may lead to stabilization and initiation of AP convertases, thus greatly amplifying the complement response to a local stimulus. This review highlights current knowledge related to these properties and discusses the implications of properdin production in a pro-inflammatory microenvironment.

Activation of complement components and reduced regulator expression in alcohol-induced liver injury in the rat

The purpose of this study was to evaluate the possible contribution of complement-mediated inflammation to the development of alcoholic liver disease. Male Wistar rats were fed ethanol by liquid diet in a model that results in continuous ethanol intoxication and induces early signs of alcoholic liver injury. After a six-week study period liver samples were analyzed for the deposition of complement components (C1, C3, and C8) and expression of cell membrane-bound regulators (Crry and CD59). Activation of the homologous complement system in vitro was tested by treating frozen liver sections with normal rat serum (NRS). Immunohistochemical analysis showed deposits of C8 in the liver sections of ethanol-treated rats. When frozen liver sections from these rats were treated with NRS, periportal deposition of both C3 and C8, but only slight C1 deposition, was observed. Immunohistochemical and Western blot analysis both revealed a reduced expression of the complement regulators Crry and CD59. These results suggest an induction of complement-activating capacity in the liver after chronic ethanol treatment. Lack of C1 deposition in the lesions suggests that complement activation occurs primarily via the alternative pathway. The reduced expression of the critical complement regulatory proteins Crry and CD59 may sensitize the liver to complement-mediated damage.

Treatment of sepsis in cardiac surgery: role of immunoglobulins

Cardiopulmonary bypass (CPB) is associated with an injury that may cause pathophysiological changes such as systemic inflammatory response syndrome, multiple organ dysfunction syndrome, and mediator-induced multiorgan failure. Systemic endotoxinaemia, release of proinflammatory cytokines, and interactions between neutrophils and endothelium have been reported to correlate with a high incidence of organ dysfunction, infection and sepsis following cardiac surgery. This review discusses the dysregulation of the immune response as a major reason for the higher susceptibility to infections following cardiac surgery, various treatment strategies to reduce CPB-induced inflammation, and especially the prophylactic use of immunoglobulins in cardiac surgery.

Abrogation of the alternative complement pathway by targeted deletion of murine factor B

To investigate the role of complement protein factor B (Bf) and alternative pathway activity in vivo, and to test the hypothesized potential genetic lethal effect of Bf deficiency, the murine Bf gene was interrupted by exchange of exon 3 through exon 7 (including the factor D cleaving site) with the neor gene. Mice heterozygous for the targeted Bf allele were interbred, yielding Bf-deficient offspring after the F1 generation at a frequency suggesting that Bf deficiency alone has no major effect on fertility or fetal development. However, in the context of one or more genes derived from the 129 mouse strain, offspring homozygous for Bf deficiency were generated at less than expected numbers (P = 0.012). Bf-deficient mice showed no gross phenotypic difference from wild-type littermates. Sera from Bf-deficient mice lacked detectable alternative complement pathway activity; purified mouse Bf overcame the deficit. Classical pathway-dependent total hemolytic activity was lower in Bf-deficient than wild-type mice, possibly reflecting loss of the alternative pathway amplification loop. Lymphoid organ structure and IgG1 antibody response to a T-dependent antigen appeared normal in Bf-deficient mice. Sensitivity to lethal endotoxic shock was not significantly altered in Bf-deficient mice. Thus, deficiency of Bf and alternative complement activation pathway led to a less dramatic phenotype than expected. Nevertheless, these mice provide an excellent model for the assessment of the role of Bf and the alternative pathway in host defense and other functions in vivo.

The influence of cardiopulmonary bypass on cytokines and cell-cell communication

Cardiopulmonary bypass (CPB) is characterized by systemic endotoxemia immediately after its onset as well as the systemic release of proinflammatory cytokines, including tumor necrosis factor-alpha and the interleukins 1 and 6. Recent studies document that increased morbidity and mortality rates correlate with elevated systemic concentrations of these proinflammatory cytokines during adult and neonatal sepsis, following thoracoabdominal aortic aneurysm repair, as well as following CPB. These proinflammatory cytokines induce increased neutrophil and endothelial surface adhesive molecule expression, thereby promoting enhanced neutrophil-endothelial adherence. Increased neutrophil-endothelial adherence and subsequent neutrophil organ binding are thought to be a "final common pathway" of organ injury during clinical inflammatory conditions. Proinflammatory cytokines also increase cellular expression of inducible nitric oxide synthase, thus increasing cellular production of nitric oxide, a known inflammatory mediator. This review discusses recent evidence of the adverse effects of proinflammatory cytokine release during CPB and therapeutic modalities that can reduce the systemic concentrations of these mediators of inflammation.