When tissue is injured or becomes locally infected, an inflammatory reaction ensues. There are three principal components of an inflammatory response: I) increased blood flow, II) increased capillary permeability, and III) increased migration of leukocytes into the affected area. Inflammatory responses are characterized by the accumulation of neutrophils and macrophages at an inflammatory site. Where local inflammation is triggered by infection, trauma, or immune complex deposition, C5a is likely to be an important chemokine. C5a is generated by the cleavage of the fifth component of complement (C5) upon activation of the classical or alternative pathways of the complement system. The biological effects of C5a are mediated by a specific high affinity receptor (C5aR, also named CD88) that is expressed predominantly on cells of myeloid origin, such as monocytes/macrophages and other granulocytes. The stimulation of mononuclear cells with C5a, leads to increased synthesis of pro-inflammatory cytokines (IL-1, IL-6, IL8, and TNFa ). Moreover, C5a triggers degranulation (which liberate various enzymes and rapidly changes the cell’s phenotype); chemotaxis (which allows extravasation or cell infiltration); and the activation of the NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.
The regulation of macrophage activation by anaphylatoxins is not well understood. However, the significance of C5a in several inflammatory diseases is demonstrated by the fact that agents, that blocked the action of C5a, also suppressed inflammation in several animal models. Most of these studies used blocking antibodies raised against C5a or recombinant proteins that are receptor antagonist analogues of C5a. However, there are many problems associated with the use of such proteins to treat human patients. Immunogenicity is a common problem and proteins are very expensive to manufacture, very susceptible to degradation by proteases in serum or the gastrointestinal tract, and generally display poor pharmacokinetic properties.
Using a range of molecular, cellular and biochemical techniques, we plan to investigate the phospholipid signaling cascades triggered by C5a in the stimulation of neutrophils and macrophages, with particular emphasis on the role of phospholipases A2, C, and D, and of lipid kinases, such as PI3-kinase, DAG kinase, and sphingosine kinase, as well as the differntial gene expression pattern triggered by C5a in neutrophils and macrophages.
Studying the intracellular signaling cascades triggered by C5a, on neutrophils
and macrophages, will potentially lead to the identification of key molecules,
more attractive to be targeted as potential "druggable" candidates
for treating a variety of human diseases.