Although dendritic cell (DC) activation is a critical event for the induction of immune responses, the signaling pathways involved in this process have not been characterized. In this report, we show that DC activation induced by lipopolysaccharide (LPS) can be separated into two distinct processes: first, maturation, leading to upregulation of MHC and costimulatory molecules, and second, rescue from immediate apoptosis after withdrawal of growth factors (survival). Using a DC culture system that allowed us to propagate immature growth factor–dependent DCs, we have investigated the signaling pathways activated by LPS. We found that LPS induced nuclear translocation of the nuclear factor (NF)-κB transcription factor. Inhibition of NF-κB activation blocked maturation of DCs in terms of upregulation of major histocompatibility complex and costimulatory molecules. In addition, we found that LPS activated the extracellular signal–regulated kinase (ERK), and that specific inhibition of MEK1, the kinase which activates ERK, abrogated the ability of LPS to prevent apoptosis but did not inhibit DC maturation or NF-κB nuclear translocation. These results indicate that ERK and NF-κB regulate different aspects of LPS-induced DC activation: ERK regulates DC survival whereas NF-κB is responsible for DC maturation.

The toluene/o-xylene monooxygenase cloned fromPseudomonas stutzeri OX1 displays a very broad range of substrates and a very peculiar regioselectivity, because it is able to hydroxylate more than one position on the aromatic ring of several hydrocarbons and phenols. The nucleotide sequence of the gene cluster coding for this enzymatic system has been determined. The sequence analysis revealed the presence of six open reading frames (ORFs) homologous to other genes clustered in operons coding for multicomponent monooxygenases found in benzene- and toluene-degradative pathways cloned from Pseudomonas strains. Significant similarities were also found with multicomponent monooxygenase systems for phenol, methane, alkene, and dimethyl sulfide cloned from different bacterial strains. The knockout of each ORF and complementation with the wild-type allele indicated that all six ORFs are essential for the full activity of the toluene/o-xylene monooxygenase inEscherichia coli. This analysis also shows that despite its activity on both hydrocarbons and phenols, toluene/ o-xylene monooxygenase belongs to a toluene multicomponent monooxygenase subfamily rather than to the monooxygenases active on phenols.

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