Intra- and Extracellular Degradation of Neutrophil Extracellular Traps by Macrophages and Dendritic Cells

Neutrophil extracellular traps (NETs) composed of nuclear DNA associated with histones and granule proteins are involved in the extracellular killing of pathogens. Excessive NET formation has been implicated in several noninfectious pathological conditions. The disposal of NETs is, therefore, important to prevent inadvertent effects resulting from the continued presence of NETs in the extracellular environment. In this study, we investigated the interaction of NETs released by freshly isolated, PMA-stimulated primary human neutrophils with primary human monocyte-derived macrophages or dendritic cells (DCs). NETs were internalized by macrophages, and removal of the protein component prevented engulfment of NETs, whereas complexation with LL-37 restored the uptake of "naked" (protein-free) NETs. NETs were also found to dampen the bacterial LPS-induced maturation of DCs. Cytokine profiling was conducted by using a multiplex array following the interaction of NETs with macrophages or DCs, and NETs alone were found to be noninflammatory, whereas immunomodulatory effects were noted in the presence of LPS with significant upregulation of IL-1β secretion, and a marked suppression of other LPS-induced factors including vascular endothelial growth factor (VEGF) in both cell types. Moreover, macrophage digestion of NETs was dependent on TREX1 (also known as DNaseIII), but not DNaseII, whereas extracellular DNase1L3-mediated degradation of NETs was observed for DCs. Collectively, these findings shed light on the interactions between NETs and phagocytic cells and provide new insights regarding the clearance of NETs, double-edged swords of innate immunity.