A cDNA encoding the C-C chemokine MDC was isolated from a human macrophage cDNA library by differential hybridization using monocyte- and macrophage-specific cDNA probes. During monocyte to macrophage differentiation in vitro, MDC expression is first detected after 1 day of culturing and reaches maximum levels after 6 days when macrophages have fully matured, as judged from the expression of known macrophage marker genes. Exposure of macrophages to lipopolysaccharide (LPS) results in a dose-dependent increase in MDC mRNA levels, with maximum induction occurring after 6-8 h, whereas expression levels of macrophage inflammatory protein-1alpha (MIP-1alpha), MIP-2, interleukin-1beta (IL-1beta), and tumor necrosis factor alpha (TNF-alpha) respond much faster to LPS. Furthermore, MDC expression in macrophages is enhanced by the inflammatory mediators TNF-alpha and IL-1beta. Similar to other TNF-alpha/IL-1beta-inducible genes, costimulation of macrophages with both cytokines leads to higher MDC expression levels than stimulation with a single cytokine. By contrast, both resting and activated monocytes do not express MDC mRNA.