Chemokines are the main proinflammatory mediators in human monocytes activated by Staphylococcus aureus, peptidoglycan, and endotoxin.

It is widely believed that the cytokines tumor necrosis factor (TNF)-alpha, interleukin (IL)-1, and IL-6 are the main proinflammatory mediators induced in the host by bacteria and their cell wall components. To test this hypothesis, we compared the level of expression of 600 genes activated in human monocytes by Staphylococcus aureus, peptidoglycan, endotoxin, and interferon-gamma. These stimulants induced expression of over 120 genes, as identified by cDNA arrays. The highest activated genes for proinflammatory mediators induced by all three bacterial stimulants were chemokine genes (IL-8 and macrophage inflammatory protein (MIP)-1alpha), whereas cytokine genes (TNF-alpha, IL-1, and IL-6) were induced to a lower extent. Genes for other chemokines (MIP-2alpha, MIP-1beta, and monocyte chemoattractant protein-1) were also induced higher than the cytokine genes by peptidoglycan, and as high or higher than the cytokine genes by S. aureus and endotoxin. This high induction of chemokine genes was confirmed by quantitative RNase protection assay, and high secretion of chemokines was confirmed by enzyme-linked immunosorbent assays. Although genes for chemokines were the highest and genes for cytokines were the second highest induced genes by all three bacterial stimulants, each stimulus induced a unique pattern of gene expression. By contrast, expression of a completely different gene pattern was induced by a nonbacterial stimulus, interferon-gamma. These results establish chemokines as the main mediators induced by both Gram-positive and Gram-negative bacteria and are consistent with the highly inflammatory nature of bacterial infections.