CD14 is a membrane protein (mCD14) found on monocytes and neutrophils that is required for the innate immune response to lipopolysaccharide (LPS) and lipoarabinomannan (LAM). CD14 can also be found in serum as soluble CD14 (sCD14) that when bound to bacterial products, enables many non-CD14 bearing cells to be activated. Lipopolysaccharide binding protein (LBP) is a plasma protein that disaggregates and catalytically transfers LPS to CD14. To examine the role of CD14 and LBP in LAM-dependent activation, we used the U373 astrocyte cell line to stably express membrane-bound CD14 (U373-CD14). In serum-free conditions, U373-CD14 cells could respond to LAM stimulation as measured by expression of intracellular adhesion molecule-1 (ICAM-1). Vector control cells (U373-RSV) could not respond to LAM or LPS; but, upon the addition of serum as a source of soluble CD14, control U373-RSV cells could respond to LPS, but not LAM. Therefore, LAM can activate U373 cells only through membrane CD14 and not soluble CD14. We also demonstrate that this membrane CD14-dependent LAM response is greatly facilitated by the addition of LBP.
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