CD14 is a glycoprotein that mediates the interaction of lipopolysaccharide (LPS, endotoxin) with cells, thereby signaling the presence of gram-negative bacteria (1-3). CD14 is either soluble (CD14) (4, 5) or membrane-bound (mCD14) by a glycosylphosphatidylinositol (GPI) anchor (6, 7). mCD14 is a 55 kDa glycoprotein (1), while CD14 varies from about 43 to 53 kDa, depending on the degree of glycosylation and whether it was synthesized without the anchor or was shed by phospholipase cleavage of the anchor or by proteolysis (12-14). There is no evidence for different mRNAs for m- and CD14. There is no apparent sequence homology with other proteins. The sequence of human CD14 is 63-73% identical to that of mouse, rat, or rabbit CD14 (15).
CD14 is expressed primarily on myeloid cells, such as monocytes, macrophages and neutrophils (1-3), the cells most sensitive to LPS, and to a lesser extent on other cells, such as B cells (8) and a circulating dendritic cell progenitor (9). CD14 appears to mediate LPS stimulation of cells that do not express mCD14 (10, 11), such as endothelial, epithelial and smooth-muscle cells. CD14 is found in both serum and urine (5).
The binding of LPS to CD14 requires an acute phase protein, LPS-binding protein (LBP) (16). The relationship of mCD14, CD14, LPS and LBP is complicated. At low concentrations of LPS, LBP is essential for the binding of LPS to CD14, but at high concentrations, LBP may actually inhibit binding of LPS to CD14. In addition, CD14 may compete with mCD14 for LPS (17) and may serve to help clear LPS (18). These four factors thus appear to participate in a complex feedback mechanism of immune regulation involving both up-regulation and down-regulation of the inflammatory process triggered by LPS. It is loss of control of this mechanism that appears to lead to septic shock. LPS-bound CD14 signals production of inflammatory cytokines and other inflammatory proteins, but the mechanism of signal transduction is unclear. Since a GPI anchor is not transmembrane, there presumably is another transmembrane protein on cells through which LPS-bound CD14 transmits a signal (19).
In addition to its well known role in gram-negative infections, CD14 likely serves other functions as well. It recognizes soluble peptidylglycan from gram-positive cell walls (20), and it has been reported to bind apoptotic cells and induce their phagocytosis (21).
