Neutrophil Extracellular Traps Activate Proinflammatory Functions of Human Neutrophils

Neutrophil extracellular traps (NETs) consist of decondensed nuclear chromatin that is
associated with proteins and are released by neutrophils during an inflammatory
response. Released NETs are able to capture pathogens, prevent their dissemination
and potentially kill them via antimicrobial peptides and proteins that are associated with
the decondensed chromatin. In addition to their antimicrobial functions, NETs have also
been shown to exert immunomodulatory effects by activation and differentiation of
macrophages, dendritic cells and T cells. However, the effect of NETs on neutrophil
functions is poorly understood. Here we report the first comprehensive study regarding
the effects of NETs on human primary neutrophils in vitro. NETs were isolated from
cultures of PMA-exposed neutrophils. Exposure of neutrophils to isolated NETs resulted in
the activation of several neutrophil functions in a concentration-dependent manner. NETs
induced exocytosis of granules, the production of reactive oxygen species (ROS) by the
NADPH oxidase NOX2, NOX2-dependent NET formation, increased the phagocytosis
and killing of microbial pathogens. Furthermore, NETs induced the secretion of the
proinflammatory chemokine IL-8 and the B-cell-activating cytokine BAFF. We could
show that the NET-induced activation of neutrophils occurs by pathways that involve
the phosphorylation of Akt, ERK1/2 and p38. Taken together our results provide further
insights into the proinflammatory role of NETs by activating neutrophil effector function and
further supports the view that NETs can amplify inflammatory events. On the one hand the
amplified functions enhance the antimicrobial defense. On the other hand, NET-amplified
neutrophil functions can be involved in the pathophysiology of NET-associated diseases.
In addition, NETs can connect the innate and adaptive immune system by inducing the
secretion of the B-cell-activating cytokine BAFF.