CD14-dependent and CD14-independent signaling pathways in murine macrophages from normal and CD14 knockout mice stimulated with lipopolysaccharide or taxol.

The antitumor agent, Taxol, shares with bacterial LPS the ability to activate murine macrophages, and its LPS-mimetic effects are blocked by LPS analogue antagonists. Since CD14 is central to the recognition of LPS by macrophages, we sought to examine a role for CD14 in the response to Taxol vs LPS. A comparison of responses of macrophages from wild-type mice with those from mice lacking CD14 due to a targeted disruption of the CD14 gene (CD14-deficient knockout (CD14KO)) revealed that like LPS, Taxol induces both CD14-dependent and -independent pathways of gene activation, although the CD14 dependency of Taxol stimulation is much less striking than that observed with LPS. The macrophage interaction with low concentrations of LPS (< or = 10 ng/ml) is largely CD14 dependent, as evidenced by the lack of induction of TNF-alpha, IL-1beta, and interferon-inducible protein-10 (IP-10) genes by CD14KO macrophages cultured in the absence of soluble CD14 (i.e., in autologous CD14KO -/- mouse serum). However, at high concentrations of LPS or Taxol, a CD14-independent pathway of activation is observed: this pathway leads to minimal IP-10 gene induction, even though induction of TNF-alpha and IL-1beta occurs. Measurements of TNF secretion followed a similar pattern to that observed at the level of steady state mRNA. These data suggest the existence of two pathways of activation by both LPS and Taxol: one that is CD14 dependent and leads to induction of TNF-alpha, IL-1beta, and IP-10 gene induction, and a CD14-independent pathway that results in the induction of TNF-alpha and IL-1beta, with minimal induction of IP-10.