ABSTRACT
The present study was designed to investigate the capacity of human mononuclear phagocytes to produce a cytokine chemotactic for monocytes (monocyte chemotactic protein (MCP), alternative acronyms JE, monocyte chemotactic and activating factor, MCP-1, and tumor-derived chemotactic factor). Human PBMC exposed in vitro to bacterial LPS expressed high levels of MCP transcripts. Monocyte-depleted lymphoid cells were not induced to express MCP by LPS. Percoll-gradient purified monocytes were able to express high levels of MCP transcripts. In an effort to exclude a role of contaminating non-monocytic cells, mononuclear phagocytes were separated by flow cytometry and sorting: CD14+ cells exposed to LPS showed high levels of MCP mRNA. LPS-stimulated monocytes released chemotactic activity for monocytes that could be inhibited by absorption with anti-MCP antibodies. IL-1, TNF, IFN-gamma, granulocyte-macrophage-CSF and, to a lesser extent, macrophage-CSF, as well as inactivated streptococci, also induced MCP gene expression. Actinomycin D experiments indicated that induction of MCP in monocytes was gene transcription-dependent. The protein synthesis inhibitor cycloheximide (Cy) blocked IL-1-, TNF-, or LPS-induced MCP gene expression in monocytes. In contrast, expression of the structurally related chemotactic cytokine IL-8 was superinduced by Cy. Moreover, Cy superinduced MCP gene expression in cells other than monocytes, including endothelial cells, smooth muscle cell and fibrosarcoma cells, indicating different mechanisms of regulation in mononuclear phagocytes vs cells of other lineages. The capacity of cells of the monocyte-macrophage lineage to produce a cytokine that recruits and activates circulating monocytes may be of considerable importance in inflammatory and immunologic reactions. Thus, the mononuclear phagocyte system can autonomously regulate the extravasation and activation of immature elements of the same lineage, a key event in inflammation and immunity.
