The two soluble forms of the lipopolysaccharide receptor, CD14: Characterization and release by normal human monocytes

Abstract

CD14, a glycolipid‐anchored membrane glycoprotein, acts as a high affinity 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 we show that the two sCD 14, which we now refer to as sCD 14α (low M_r), and sCD 14β (high M_r), are also synthesized and released by normal human monocytes and present in normal plasma. Their mechanism of release was examined by using the Mono‐Mac‐6 cell line, chinese hamster ovary cell (CHO)/CD14⁺ transfectants and plasma from paroxysmal nocturnal hemoglobinuria (PNH) patients. It was found that: (1) sCD14β is released faster than sCD 14α and that the release of the latter is a lengthy process. (2) Monensin blocked the biosynthesis of membrane‐bound CD14 (mCD 14) and sCD 14, additionally, a 50‐kDa CD14 polypeptide accumulated in the cell lysate, suggesting that the different forms of CD14 may have a common precursor. (3) Monensin also blocked the release of mCD14α from surface‐labeled cells, suggesting that conversion of mCD14 to sCD14α involves a mechanism of endocytosis followed by exocytosis. Interestingly, (4) sCD 14α and sCD 14β were detected in PNH plasma, indicating that sCD14α may also derive from an endogenous pathway. (5) Phospholipase C‐released CD 14 was identical in size to mCD 14, thus differed from sCD 14β by ˜ 2000, indicating that release of sCD 14β involves further processing. (6) CHO cells transfected with a CD14 cDNA coding for an eight C‐terminal amino acids shorter product released an sCD 14β‐like form; thus absence of the eight C‐terminal amino acids prevented mCD 14 expression but not the secretion of sCD 14β. The characterization of sCD 14α and sCD 14β reported here may be useful for better understanding of variations in sCD 14 levels in pathological conditions and the contribution of each sCD 14 in scpsis and other, as yet unknown functions.