Induction of chemokine gene expression from peripheral blood mononuclear cells (PBMCs) stimulated by proinflammatory cytokines plays an important role in both wound repair and response to infectious agents. In the present study, we show that the proinflammatory cytokine interleukin-6 (IL-6) potently induced mRNA expression and secretion of the CC chemokine monocyte chemotactic protein 1 (MCP-1) in PBMCs. In addition, because human immunodeficiency virus (HIV) infection in vivo and in vitro has been shown to dysregulate the production of and/or the response to cytokines, PBMCs from both healthy uninfected and HIV-infected individuals were studied for their constitutive and IL-6–induced expression of MCP-1. No substantial differences were observed between the two groups of individuals. In addition, IL-6 upregulated MCP-1 expression in the promonocytic cell line U937 and in its chronically HIV-infected counterpart, U1. In these cell lines, IL-6 selectively induced MCP-1 and not other chemokines, including regulated upon activation normal T cells expressed and secreted (RANTES), macrophage inflammatory protein-1α (MIP-1α), MIP-1β, and IL-8. IL-6 induction of MCP-1 was partially inhibited by hydrocortisone in U1 cells. Thus, IL-6 activates PBMCs to secrete MCP-1, a CC chemokine pivotal for monocyte recruitment in tissue and organs in which important inflammatory events occur.

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We have recently described a significant correlation between human immunodeficiency virus-1 (HIV-1) RNA replication and monocyte chemotactic protein-1 (MCP-1) levels in the cerebrospinal fluid (CSF) of individuals with the acquired immunodeficiency syndrome (AIDS) with HIV encephalitis (E). Because local macrophages (microglia) are the cells predominantly infected in the brain, we investigated whether in vitro HIV infection affects MCP-1 production in mononuclear phagocytes (MP). MCP-1 secretion and expression were consinstently upregulated over constitutive levels in human monocyte-derived macrophages (MDM) infected with the M-tropic R5 BaL strain of HIV-1. HIV replication was required for this effect, as demonstrated by the absence of chemokine upregulation after infection in the presence of 3’-azido-3’-deoxythimidine (AZT) or cell-exposure to heat-inactivated (▵°) virus. MCP-1 induction was not restricted to HIV-1 BaL, but was also observed during productive infection of MDM with two primary isolates differing for entry coreceptor usage and of U937 cells with the X4 HIV-1 MN strain. Based on the observation that exogenous HIV-1 Tat induced MCP-1 expression in astrocytes, we also investigated its role in MDM and U937 cells. Exogenous Tat induced MCP-1 production from MDM in a concentration-dependent manner, however, it was not effective on uninfected U937 cells or on the chronically infected U937-derived cell line U1. Transfection of Tat-expressing plasmids moderately activated HIV expression in U1 cells, but failed to induce MCP-1 expression in this cell line or in uninfected U937 cells. HIV replication-dependent expression of MCP-1 in MP may be of particular relevance for the pathogenesis of HIV infection in nonlymphoid organs such as the brain.