Human monocytes and macrophages regulate immune tolerance via integrin αvβ8-mediated TGFβ activation.

Monocytes are crucial immune cells involved in regulation of inflammation either directly or via differentiation into macrophages in tissues. However, many aspects of how their function is controlled in health and disease are not understood. Here we show that human blood monocytes activate high levels of the cytokine TGF beta, a pathway that is not evident in mouse monocytes. Human CD14(+), but not CD16(+), monocytes activate TGF beta via expression of the integrin alpha v beta 8 and matrix metalloproteinase 14, which dampens their production of TNF alpha in response to LPS. Additionally, when monocytes differentiate into macrophages, integrin expression and TGF beta-activating ability are maintained in anti-inflammatory macrophages but down-regulated in pro-inflammatory macrophages. In the healthy human intestine, integrin alpha v beta 8 is highly expressed on mature tissue macrophages, with these cells and their integrin expression being significantly reduced in active inflammatory bowel disease. Thus, our data suggest that integrin alpha v beta 8-mediated TGF beta activation plays a key role in regulation of monocyte inflammatory responses and intestinal macrophage homeostasis.