CD14-dependent induction of protein tyrosine phosphorylation by lipopolysaccharide in murine B-lymphoma cells.

Incubation of the mouse B-lymphoma cell line 70Z/3 with bacterial lipopolysaccharide (LPS) results in the secretion of immunoglobulin M (IgM) to the cell surface. We now demonstrate that LPS rapidly induces the tyrosine phosphorylation of a 41 kDa protein in 70Z/3 cells transfected with CD14, a glycosyl phosphatidylinositol-anchored membrane receptor for complexes of LPS and LPS binding protein. There was no indication of LPS-mediated tyrosine phosphorylation in untransfected 70Z/3 cells, which do not express CD14. The 41 kDa tyrosine phosphoprotein was specifically induced by LPS, since it was not observed after incubation with another activator of IgM expression, interferon-gamma. Induction of this 41 kDa phosphoprotein was not observed when the transfected cells were treated with LPS in the absence of serum. Phosphorylation was also blocked by preincubation of the cells with an antibody to CD14. Furthermore, lipid A from Rhodobacter sphaeroides inhibited LPS-mediated tyrosine phosphorylation and surface IgM expression. Expression of CD14 in the LPS-unresponsive mutant 70Z/3 cell line 1.3E2 did not result in the secretion of IgM, although tyrosine phosphorylation was increased after incubation with LPS, suggesting that the mutation in these cells is downstream of the membrane LPS receptor.