CD14

CD14 is a human protein made mostly by macrophages as part of the innate immune system. It helps to detect bacteria in the body by binding lipopolysaccharide, a pathogen-associated molecular pattern.
CD14 exists in two forms, one anchored to the membrane by a glycosylphosphatidylinositol tail, the other a soluble form. Soluble CD14 either appears after shedding of mCD14 or is directly secreted from intracellular vesicles.
The x-ray crystal structure of human CD14 reveals a monomeric, bent solenoid structure containing a hydrophobic amino-terminal pocket.
CD14 was the first described pattern recognition receptor.

Function

CD14 acts as a co-receptor for the detection of bacterial lipopolysaccharide. CD14 can bind LPS only in the presence of lipopolysaccharide-binding protein.
Although LPS is considered its main ligand, CD14 also recognizes other pathogen-associated molecular patterns such as lipoteichoic acid. Cluster of differentiation CD14 is a
receptor for a very wide range of microbial products including lipopolysaccharide, peptidoglycans, and lipoteichoic acid.
Image:Toll-like receptor pathways revised.jpg|thumbnail|left|500px|Signaling pathway of toll-like receptors. Dashed grey lines represent unknown associations

Tissue distribution

CD14 is expressed mainly by macrophages and by neutrophils. It is also expressed by dendritic cells. The soluble form of the receptor is secreted by the liver and monocytes and is sufficient in low concentrations to confer LPS-responsiveness to cells not expressing CD14. mCD14 and sCD14 are also present on enterocytes. sCD14 is also present in human milk, where it is believed to regulate microbial growth in the infant gut.

Differentiation

CD14+ monocytes can differentiate into a host of different cells, including dendritic cells, a differentiation pathway encouraged by cytokines, including GM-CSF and IL-4.

Interactions

CD14 has been shown to interact with lipopolysaccharide-binding protein.