Endotoxins displaying differences in the chemical structure of their lipid A were used to induce the expression of chemokines in the human monocytic THP-1 cell line. LPS from two enterobacterial species such as Escherichia coli and Yersinia enterocolitica induced mRNA expression of IFN-gamma-inducible protein (IP)-10, macrophage-inflammatory protein (MIP)-1alpha, MIP-1beta, monocyte chemoattractant protein (MCP)-1 and IL-8. LPS from the non-enterobacterial genera Brucella and Ochrobactrum induced the expression of these chemokines to a lower extent. Attempts to address the signaling routes involved in these responses were carried out in transiently transfected HEK293 cells. Induction of kappaB-driven transcriptional activity by enterobacterial LPS was observed in cells transfected with TLR-4 alone, although co-transfection of TLR-4, MD-2 and CD14 provided optimal induction. The response to Brucella spp. and Ochrobactrum anthropi LPS was only significant at the concentration of 10 microg/ml. These data indicate that LPS from Brucella spp. and O. anthropi, which contain lipid A moieties with structural features different from those of Enterobacteriaceae elicit biochemical signaling via TLR-4 only at high concentrations. Neither TLR-1, TLR-2 and TLR-6 nor heterodimeric combinations of these receptor molecules are involved. Conversely, the ability of LPS to activate the TLR-4 route is a reliable molecular biomarker for endotoxicity.