Although neutrophil extracellular traps (NETs) were discovered only 16 years ago, they have already taken us from heaven to hell as we learned that apart from beneficial trapping of pathogens, they cause, or contribute to, numerous disorders. The latter is connected to their persistent presence in the blood or tissue, and we hardly know how they are removed in mild pathophysiological conditions and why their removal is impaired in multiple severe pathological conditions. Herein, we bring together all data available up till now on how NETs are cleared—from engaged cells, their phenotypes, to involved enzymes and molecules. Moreover, we hypothesize on why NET removal is challenged in multiple disorders and propose further directions for studies on NET removal as well as possible therapeutic strategies to have them cleared.

Systemic inflammation is a detrimental condition associated with high mortality. However, obese individuals seem to have higher chances of surviving sepsis. To elucidate what immunological differences exist between obese and lean individuals we studied the course of endotoxemia in mice fed high-fat diet (HFD) and ob/ob animals. Intravital microscopy revealed that neutrophil extracellular trap (NET) formation in liver vasculature is negligible in obese mice in sharp contrast to their lean counterparts (ND). Unlike in lean individuals, neutrophil influx is not driven by leptin or interleukin 33 (IL-33), nor occurs via a chemokine receptor CXCR2. In obese mice less platelets interact with neutrophils forming less aggregates. Platelets transfer from ND to HFD mice partially restores NET formation, and even further so upon P-selectin blockage on them. The study reveals that in obesity the overexaggerated inflammation and NET formation are limited during sepsis due to dysfunctional platelets suggesting their targeting as a therapeutic tool in systemic inflammation.

Although key for pathogen immobilization, neutrophil extracellular traps (NETs) often cause severe bystander cell/tissue damage. This was hypothesized to depend on their prolonged presence in the vasculature, leading to cytotoxicity. Imaging of NETs (histones, neutrophil elastase, and extracellular DNA) with intravital microscopy in blood vessels of mouse livers in a pathogen-replicative-free environment (endotoxemia) led to detection of NET proteins attached to the endothelium for months despite the early disappearance of extracellular DNA. Intravascular liver macrophages (Kupffer cells) and neutrophils, but not monocytes, were involved in NET removal. They used scavenger receptors (SRs; SR-A) and Toll-like receptors (TLRs; TLR2/4) to recognize NET components. Despite the absence of further stimuli, 14 days later a second wave of NET formation occurred, initiated by remnants of NETs from the first wave. The second burst of NET production was triggered by histones, which induced an inflammatory milieu interleukin-1β and activated platelets and coagulation-related events, including factor VII–activating protease activity. This, in turn, recruited and activated neutrophils to release the second wave of NETs. In peptidyl arginine deiminase–deficient mice, not forming NETs, inflammation and liver damage were reduced compared with their wild-type counterparts. When mice were challenged with methicillin-resistant Staphylococcus aureus 14 or 165 days after the second NETs, the course of infection/injury was diminished and exacerbated, respectively. Our study demonstrates that the complete removal of NETs in vivo takes much longer than hypothesized, and a vicious cycle of NET formation/disassembly affects subsequent infection, depending on the time elapsed since its occurrence.

Skin infiltration by neutrophils is a hallmark of the chronic inflammatory skin disease psoriasis, yet the mechanisms underlying neutrophil recruitment and positioning in chronically inflamed skin remain poorly understood. In this study, we demonstrate the significant impact of a total genetic deficiency of secretory leukocyte protease inhibitor (SLPI) on neutrophil migration in mouse skin. Without SLPI, neutrophils displayed an unconventional migratory pattern, characterized by altered interactions with vessel walls and reduced efficiency in extravasating from blood vessels into skin tissue during the early stages of experimental psoriasis. This was associated with changes in tissue motility, positioning neutrophils farther from the skin entry vessels and closer to the skin surface. Neutrophil diapedesis was partially dependent on SLPI within the neutrophils themselves. The impact of SLPI on neutrophil movement was further supported by the increased migration of human neutrophils in the presence of neutrophil-penetrant recombinant SLPI. Additionally, our data suggest that neutrophils with varying capacities for vessel wall interaction are released from the bone marrow into circulation in an SLPI-dependent manner. These findings establish a role for SLPI in regulating the spatiotemporal infiltration of neutrophils into the skin in psoriasis, highlighting its relevance to psoriasis pathophysiology.

Natural killer (NK) cells are central to cancer immunosurveillance and immunotherapy. Their ability to engage in killing is critical for efficient target elimination and depends on tightly regulated Ca2+ signaling controlling granzyme degranulation. Toll-like receptor (TLR) engagement has been linked to Ca2+ signaling in other immune cell types, while a specific role in NK cells remains unresolved. Given that NK cells express a broad range of TLRs, and that presence of TLR ligands—including damage- and pathogen-associated molecular patterns—can influence clinical scenarios including adoptive NK cell therapy, investigating TLR-driven Ca2+ signaling in NK cells is particularly relevant. Here, we examined how stimulation with selected TLR ligands influences Ca2+ signaling and NK cell activity. Immediate stimulation induced a rapid elevation of cytosolic Ca2+ in expanded NK cells. Functionally, TLR stimulation increased degranulation and enhanced cytotoxicity at high effector-to-target ratios. Under conditions of target excess, however, TLR-treated NK cells displayed impaired killing, likely due to unbalanced Ca2+ levels. These findings demonstrate that TLR signaling directly modulates Ca2+ flux in NK cells and can either potentiate or impair cytotoxic activity depending on context. Although clinical implications remain hypothetical, such TLR-driven dysregulation affects NK cell killing activity in the inflammatory or pathogen-rich environments frequently encountered after chemotherapy. Our findings suggest that TLR–Ca2+ impact on cytotoxicity should be considered in adoptive transfer therapies where previous treatment affects systemic levels of TLR ligands, such as in patients with acute myeloid leukemia, where NK cell immunotherapy is frequently tested in trials.