Septicaemia models using Streptococcus pneumoniae and Listeria monocytogenes: understanding the role of complement properdin.

Streptococcus pneumoniae and Listeria monocytogenes, pathogens which can cause severe infectious disease in human, were used to infect properdin-deficient and wildtype mice. The aim was to deduce a role for properdin, positive regulator of the alternative pathway of complement activation, by comparing and contrasting the immune response of the two genotypes in vivo. We show that properdin-deficient and wildtype mice mounted antipneumococcal serotype-specific IgM antibodies, which were protective. Properdin-deficient mice, however, had increased survival in the model of streptococcal pneumonia and sepsis. Low activity of the classical pathway of complement and modulation of FcγR2b expression appear to be pathogenically involved. In listeriosis, however, properdin-deficient mice had reduced survival and a dendritic cell population that was impaired in maturation and activity. In vitro analyses of splenocytes and bone marrow-derived myeloid cells support the view that the opposing outcomes of properdin-deficient and wildtype mice in these two infection models is likely to be due to a skewing of macrophage activity to an M2 phenotype in the properdin-deficient mice. The phenotypes observed thus appear to reflect the extent to which M2- or M1-polarised macrophages are involved in the immune responses to S. pneumoniae and L. monocytogenes. We conclude that properdin controls the strength of immune responses by affecting humoral as well as cellular phenotypes during acute bacterial infection and ensuing inflammation.

Protective role for properdin in progression of experimental murine atherosclerosis.

Genetic, dietary and immune factors contribute to the pathogenesis of atherosclerosis in humans and mice. Complement activation is an integral part of the innate immune defence but also shapes cellular responses and influences directly triglyceride synthesis. Deficiency of Factor B of the alternative pathway (AP) of complement is beneficial in LDLR(-/-) mice fed a high fat diet. The serum glycoprotein properdin is a key positive regulator of the AP but has not been studied in experimental atherosclerosis. Atherosclerosis was assessed after feeding low fat (LFD) or high fat (HFD) Western type diets to newly generated LDLR(-/-) Properdin(KO) (LDLR(-/-)P(KO)) and LDLR-/-PWT mice. Lipids, lymphocytes and monocytes were similar among genotypes, genders and diets. Complement C3, but not C3adesarg, levels were enhanced in LDLR(-/-)P(KO) mice regardless of diet type or gender. Non-esterified fatty acids (NEFA) were decreased in male LDLR(-/-)P(KO) fed a HFD compared with controls. All mice showed significant atherosclerotic burden in aortae and at aortic roots but male LDLR(-/-) mice fed a LFD were affected to the greatest extent by the absence of properdin. The protective effect of properdin expression was overwhelmed in both genders of LDLR(-/-)mice when fed a HFD. We conclude that properdin plays an unexpectedly beneficial role in the development and progression of early atherosclerotic lesions.

The role of complement in the development and manifestation of murine atherogenic inflammation: novel avenues.

Atherosclerosis is a chronic progressive inflammatory disease which manifests in the arterial vascular tree. It is a major cause of cardiovascular morbidity and contributes significantly to mortality in the developed world. Triggers for this inflammatory process are elevated levels of cholesterol, bacterial infection and obesity. The immune response in atherosclerosis is essentially pro-atherogenic, leading to lipid accumulation and cellular changes within the arterial wall. Small-animal models of atherosclerosis are used to study the relevance of candidate factors (cells, genes, diets) in the development and progression of lesions. From a multidisciplinary viewpoint, there are challenges and limitations to this approach. Activation of complement determines or modifies the outcome of acute and chronic inflammation. This review dissects the role of complement in the early development as well as the progressive manifestation of murine atherosclerosis and the advances in knowledge provided by the use of specific mouse models. It gives a critical overview of existing models, analyses seemingly conflicting results obtained with complement-deficient mouse models, highlights the importance of interrelationships between pro-coagulpant activity, adipose tissue, macrophages and complement, and uncovers exciting avenues of topical research.

Monocyte-derived macrophages matured under prolonged hypoxia transcriptionally up-regulate HIF-1α mRNA.

This study tested the hypothesis that prolonged severe hypoxia during monocyte to macrophage differentiation results in macrophages with a pattern of gene expression and phenotype distinct from those maturing in normal oxygen levels. Macrophages accumulate in hypoxic and anoxic areas within pathological sites such as tumours, wounds, and arthritic joints, and have been proposed as vehicles for gene therapy delivery to such tissues. Several non-pathological tissues are also hypoxic. We therefore argue that differentiation from monocyte to macrophage in hypoxic conditions is a common occurrence. However, the effect of long term severe hypoxia on monocyte to macrophage differentiation has not been studied. Here, using primary human peripheral blood monocytes, we show that maturation for 5 days in 0.2% oxygen results in decreased phagocytosis, and decreased CD40 and CD206 expression. Chronic hypoxia induced much higher mRNA levels of the pro-angiogenic cytokine, VEGF, in adherence-purified macrophages (27-fold), CD14-magnetic bead purified monocytes (90-fold), and PBMC (104-fold) compared to acute (24h) hypoxia (11, 17 and 9-fold, respectively). This suggests that macrophages may play an even greater role in angiogenesis than previously appreciated. Furthermore, chronic hypoxia resulted in up-regulation of HIF-1α mRNA, in all monocyte-derived macrophage types studied. Actinomycin D experiments indicate that the increases in HIF-1α mRNA were not due to increased mRNA stability. To our knowledge this is the first study demonstrating up-regulation of HIF-1α mRNA by hypoxia in macrophages. Taken together, the data support the hypothesis that hypoxia affects monocyte to macrophage maturation, resulting in a distinct gene expression pattern and phenotype.