Endotoxin receptors (CD14) are found with CD16 (Fc γ RIII) in an intracellular compartment of neutrophils that contains alkaline phosphatase

Abstract

CD14 is a glycosylphosphatidylinositol (GPI)-anchored protein on the surfaces of monocytes and polymorphonuclear leukocytes (PMN) that binds and initiates cellular responses to bacterial LPS. PMN also contain an intracellular pool of CD14 that can be deployed rapidly to the cell surface in response to stimulation with a variety of agonists. To determine which of the well-characterized subcellular compartments of PMN contains CD14, cells were cavitated and fractionated on Percoll gradients. The gradient fractions were assayed for CD14 by ELISA and Western blot and for the marker proteins β-glucuronidase (azurophil granules), vitamin B12 binding protein (specific granules), alkaline phosphatase (secretory vesicles and plasma membrane), and HLA (plasma membrane). Approximately one-half of the CD14 ran with plasma membrane fractions and one-half with intracellular membranes of light density. Both intracellular and cell surface CD14 were associated tightly with membrane, and both forms showed identical electrophoretic mobility. The intracellular CD14 was clearly not present in azurophil granules or specific granules, but ran precisely with alkaline phosphatase, a marker for secretory vesicles. Parallel studies showed that an additional GPI-linked protein, Fc γ RIII (CD16), also fractionated precisely with CD14 and alkaline phosphatase. Association of CD14 with secretory vesicles were confirmed by studies on cells stimulated with the formyl peptide fNLLP for 20 min at 37 degrees C before fractionation. This treatment caused translocation of CD14 from intracellular fractions to plasma membrane fractions. No release of the specific granule marker vitamin B12 binding protein was observed under these conditions, whereas two other GPI-anchored proteins, alkaline phosphatase and CD16, moved coincidentally with CD14 to comigrate with the plasma membrane. Time course studies of CD14 and CD16 surface expression confirmed the rapid and coordinate up-regulation of these proteins. Thus, the intracellular compartment containing CD14 and CD16 had the properties of secretory vesicles. These vesicles may represent a specialized membrane domain of PMN enriched in GPI-anchored proteins.