THE 2 SOLUBLE FORMS OF THE LIPOPOLYSACCHARIDE RECEPTOR, CD14 - CHARACTERIZATION AND RELEASE BY NORMAL HUMAN MONOCYTES

CD14, a glycolipid-anchored membrane glycoprotein, acts as a high affi nity lipopolysaccharide receptor on leukocytes. We previously reported that the Mono-Mac-6 cell line releases two different soluble forms of CD14 (sCD14) (Labeta et al., Eur. J. Immunol. 1993. 23: 2144). Here w e show that the two sCD14,which we now refer to as sCD14 alpha (low M( r)) and sCD14 beta (high M(r)), are also synthesized and released by n ormal human monocytes and present in normal plasma. Their mechanism of release was examined by using the Mono-Mac-6 cell line, chinese hamst er ovary cell (CHO)/CD14(+) transfectants and plasma from paroxysmal n octurnal hemoglobinuria (PNH) patients. It was found that: (1) sCD14 b eta is released faster than sCD14 alpha and that the release of the la tter is a lengthy process. (2) Monensin blocked the biosynthesis of me mbrane-bound CD14 (mCD14) and sCD14, additionally, a 50-kDa CD14 polyp eptide accumulated in the cell lysate, suggesting that the different f orms of CD14 may have a common precursor. (3) Monensin also blocked th e release of sCD14 alpha from surface-labeled cells, suggesting that c onversion of mCD14 to sCD14 alpha involves a mechanism of endocytosis followed by exocytosis. Interestingly, (4) sCD14 alpha and sCD14 beta were detected in PNH plasma, indicating that sCD14 alpha may also deri ve from an endogenous pathway. (5) Phospholipase C-released CD14 was i dentical in size to mCD14, thus differed from sCD14 beta by similar to 2000, indicating that release of sCD14 beta involves further processi ng. (6) CHO cells transfected with a CD14 cDNA coding for an eight C-t erminal amino acids shorter product released an sCD14 beta-like form; thus absence of the eight C-terminal amino acids prevented mCD14 expre ssion but not the secretion of sCD14 beta. The characterization of sCD 14 alpha and sCD14 beta reported here may be useful for better underst anding of variations in sCD14 levels in pathological conditions and th e contribution of each sCD14 in sepsis and other, as yet unknown funct ions.