Background:  The pro-inflammatory CD16⁺ monocyte (MO) subset is dramatically expanded in peripheral blood during progression to AIDS. Compared to CD16^- MO, CD16⁺ MO produce higher levels of tumor necrosis factor and interleukin -1, exhibit lower phagocytic activity and higher antigen presenting capacity, and migrate preferentially in response to fractalkine and stromal cell-derived factor. In this study we investigated the role of CD16⁺ MO in promoting HIV infection of CD4⁺ T cells.

Methods:  CD16⁺ MO, CD16^- MO, and CD4⁺ T cells were isolated from peripheral blood mononuclear cells (PBMC) of healthy individuals using magnetic beads. The HIV strains used wereYU2 (R5), NDK (X4), NL4.3-BaL-GFP (R5), and HXBN10-89.6-GFP (R5X4). HIV replication was monitored by reverse transciptase (RT) assay, p24 ELISA, and FACS analysis of green fluorescent protein (GFP) and intracellular p24 expression. MO:T conjugates were visualized by confocal microscopy. Cell proliferation was assessed by bromodeoxyuridine incorporation. Screening for 120 cytokines and chemokines in cell supernatants (sup) was performed using a cytokine antibody array.

Results:  CD16^+- and CD16^- MO-bound HIV but did not support productive infection. However, co-culture of HIV-pulsed MO with phytohemagglutinin-activated CD4⁺ T cells (T_PHA) resulted in a productive infection that was significantly higher in CD16⁺ MO:T compared to CD16^- MO:T co-cultures. Conjugates formed between T_PHA and CD16⁺ MO-derived macrophages (MF), but not CD16^- MO-MF, were major sites of HIV replication. Resting T cells co-cultured with CD16⁺, but not CD16^- MO, became highly permissive to HIV infection. Among 16 soluble proteins detected, eotaxin-2 and MCP-1 were expressed exclusively in CD16⁺ MO:T co-culture sup and were produced by MO-MF and MO-MF:T conjugates, but not by T cells. T cells co-cultured with both CD16⁺ and CD16^- MO expressed CCR3 (eotaxin-2 receptor, 7-10%) and CCR4 (MCP-1 receptor, 60% to70%), but not CCR2 (MCP-1 receptor). Addition of eotaxin-2 and MCP-1 to CD16^- MO:T co-cultures or resting T cells was sufficient to increase HIV replication to levels similar to those observed in CD16⁺ MO:T co-cultures.

Conclusions:  CD16⁺ MO transmit HIV virions to activated CD4⁺ T cells following cell-to-cell contact and render resting T cells permissive to HIV infection by producing eotaxin-2 and MCP-1. Thus, expansion of CD16⁺ MO in HIV-infected individuals may contribute to HIV pathogenesis by carrying infectious virions into anatomic sites where CD16⁺ MO are preferentially recruited and producing eotaxin-2 and MCP-1, which activate resting CD4⁺ T cells and render them permissive to productive HIV infection.

Keywords: CD16+ monocytes; eotaxin-2; MCP-1