Neutrophils fine-tune the NET effects of activation Free

In this issue of Blood, Thomas et al¹ explore the nuances of one of the most striking yet poorly understood effector mechanisms of the innate immune system—the capacity for neutrophils to deploy their DNA contents in the form of neutrophil extracellular traps, known as NETs. The findings suggest that both the triggers and the fallout of NETs may be more complex than once thought.

Neutrophils are innate immune granulocytes that respond rapidly and en masse to various stimuli including pathogens, tissue injury, and inflammatory mediators. At steady state, neutrophils circulate in the bloodstream, and upon sensing a challenge, they extravasate to tissue sites where they execute effector mechanisms to protect the host. A unique effector mechanism engaged by neutrophils is the capacity to expel their DNA contents to form NETs. NETs are thought to be important for defense against bacterial pathogens, entrapping extracellular bacteria and exposing them to NET-associated bactericidal molecules like charged histones and proteases.^2-4 However, NETs have also been highlighted as proinflammatory, and potentially problematic features of autoimmune conditions such as vasculitis and lupus, and they can be particularly noxious when deployed within the circulatory system, which is thought to induce aberrant coagulation and thrombosis.^5,6 

In this study, the authors investigate a less appreciated trigger of NETosis—the complement system. Specifically, they novelly interrogate whether C1q, a molecule largely expressed by mononuclear phagocytes, can trigger neutrophil NETosis, and how this might affect blood coagulation. Initially, the authors report that, whereas C1q alone does not cause a striking NETosis response, neutrophils primed by the highly inflammatory bacterial lipopolysaccharide (LPS) are particularly primed to NETose in response to C1q. This is because LPS rapidly induced a programatic shift in the neutrophils, up-regulating transcription of complement receptor genes within just 1 to 3 hours. The fact that this priming is regulated on the transcriptional rather than the protein level illustrates that, although neutrophils are considered transcriptionally repressed, critical elements of their effector responses indeed rely on transcriptional regulation.

The connection between LPS/C1q–driven NETosis and coagulation was also investigated. Surprisingly, although LPS alone favored coagulation via neutrophil-derived tissue factor (TF), costimulation with LPS + C1q–induced coagulation to a lesser extent, raising the possibility that LPS-primed C1q–triggered neutrophils were outfitted with anticoagulatory features. Although LPS-primed C1q NETs themselves were indeed procoagulatory compared to C1q-treated cells alone, the LPS/C1q–treated neutrophils indeed produced anticoagulatory factors. These included TF pathway inhibitor (TFPI), which counteracts TF, and protein C, which counteracts factors XI/II that are features of LPS/C1q NETs. Consequently, LPS-primed C1q–treated neutrophils appear to intrinsically regulate the coagulatory effects of LPS/C1q NETs.

These findings illustrate one way in which neutrophils can balance their own effector mechanisms and regulate the sometimes-maladaptive consequences of their molecular payload. The capacity for neutrophils to tune their own inflammatory reactions via molecular feedback loops is increasingly appreciated.⁷ It is particularly notable that a NETosing neutrophil, largely considered an explosive, proinflammatory actor, is endowed with such nuanced self-regulatory mechanisms in the form of anticoagulants. Whether this is true of all NETosing neutrophils, or specific to the case of LPS/C1q-triggered neutrophils, is worthy of consideration and raises the question of whether all NETs are created equal. Further studies on this topic are merited, especially since work by some investigators has suggested differences in NETs induced by different stimuli.⁸ 

It is additionally intriguing that synergy between LPS and C1q can lead to extensive NETosis, whereas neither stimulus alone had this striking effect. The fact that LPS treatment transcriptionally unlocks a sensitized neutrophil effector state illustrates a sophisticated neutrophil program and challenges the view that neutrophils pull the trigger to degranulate and produce oxidants immediately after encountering a stimulus. Rather, perhaps neutrophils assess the local environment by detecting infection (here, sensing LPS via TLR4) and respond by optimizing their offensive (here, production of NETs). The synergistic effects of LPS + C1q were vetted in a humanized mouse system, bolstering the in vivo implications of this finding and highlighting its potential importance in the clinic. Indeed, it is natural to consider that such a primed neutrophil state might arise in diseases like sepsis or COVID-19, where neutrophils appear highly dysregulated.⁹ Whether other stimuli besides LPS and C1q could synergize similarly should be investigated. A deeper understanding of this will be crucial, since it could inform potential avenues to regulate untoward neutrophil responses in human patients.

Conflict-of-interest disclosure: The authors declare no competing financial interests.