Several plasma proteins that recognize microbial structures and participate in innate immunity belong to the pentraxin family, which is a phylogenetically old group of structurally homologous pentameric proteins. Prominent members of this family include the short pentraxins C-reactive protein (CRP) and serum amyloid P (SAP) and the long pentraxin PTX3. Both CRP and SAP bind to several different species of bacteria and fungi. The molecular ligands recognized by CRP and SAP include phosphorylcholine and phosphatidylethanolamine, respectively, which are found on bacterial membranes and on apoptotic cells, as discussed earlier. PTX3 recognizes various molecules on fungi, selected gram-positive and gram-negative bacteria, and viruses. CRP, SAP, and PTX3 all activate complement by binding C1q and initiating the classical pathway.
Plasma concentrations of CRP are very low in healthy individuals but can increase up to 1000-fold during infections and in response to other inflammatory stimuli. The increased levels of CRP are a result of increased synthesis by the liver induced by the cytokines IL-6 and IL-1, which are produced by phagocytes as part of the innate immune response. Liver synthesis and plasma levels of several other proteins, including SAP and others unrelated to the pentraxins, also increase in response to IL-1 and IL-6, and as a group these plasma proteins are called acute-phase reactants.
PTX3 is produced by several cell types, including dendritic cells, macrophages, and endothelial cells, in response to TLR ligands and inflammatory cytokines, such as TNF, but it is not an acute-phase reactant. PTX3 is also stored in neutrophil granules and released as neu-trophils die. PTX3 recognizes apoptotic cells and certain microorganisms. Studies with knockout mice reveal that PTX3 provides protection against some microbes, including the fungus Aspergillus fumigatus.
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