SPECIFIC BINDING OF SOLUBLE PEPTIDOGLYCAN AND MURAMYLDIPEPTIDE TO CD14 ON HUMAN MONOCYTES

Previously, we were able to show that soluble peptidoglycan (sPG)-indu ced monokine production in human peripheral monocytes is inhibited by anti-CD14 monoclonal antibodies and by lipid A partial structures, Thi s suggested but did not prove that monocytic surface protein CD14 is i nvolved in the activation of human monocytes not only by cell wall com ponents of gram-negative bacteria such as lipopolysaccharide (LPS) but also by cell wall components of gram-positive bacteria such as sPG, I n the present study, we provide experimental evidence that CD14 indeed constitutes a binding site for sPG recognition and activation of huma n monocytes, The results show that fluorescein isothiocyanate-sPG (FIT C-sPG) binds to human monocytes in a saturable, dose-dependent, and sp ecific manner. For maximal binding, 2 to 3 mu g of FITC-sPG per mi was sufficient, and this binding is completed within 90 min; about 40% of the binding is completed within the first 3 min. The FITC-sPG binding is considered specific because unlabeled sPG and also muramyldipeptid e (MDP), the minimal bioactive structure of sPG, inhibit the binding o f sPG to monocytes in a dose-dependent manner. This specific binding w as also inhibited by an anti-CD14 monoclonal antibody, LPS, and lipid A partial structure compound 406. Direct evidence for an interaction o f sPG with CD14 is provided by experiments involving native polyacryla mide gel electrophoresis that showed a shift of the electrophoretic mo bility of CD14 by LPS as well as by sPG, These results allow the concl usion that sPG binds directly to CD14, that MDP represents the active substructure of sPG, and that CD14 may be a lectin-like receptor which plays a key role in cellular stimulation by bioactive components of n ot only gram-negative but also gram-positive bacteria.