Intravascular Neutrophil Extracellular Traps Capture Bacteria from the Bloodstream during Sepsis

Abstract

During the systemic inflammatory response of severe sepsis, neutrophils accumulate in the liver microcirculation, but their functional significance is largely unknown. We show that neutrophils migrate to liver sinusoids during endotoxemia and sepsis where they exert protective effects by releasing neutrophil extracellular traps (NETs), which are DNA-based structures that capture and eliminate microbes. NETs released into the vasculature ensnare bacteria from the bloodstream and prevent dissemination. NET production requires platelet-neutrophil interactions and can be inhibited by platelet depletion or disruption of integrin-mediated platelet-neutrophil binding. During sepsis, NET release increases bacterial trapping by 4-fold (beyond the basal level provided by resident intravascular macrophages). Blocking NET formation reduces the capture of circulating bacteria during sepsis, resulting in increased dissemination to distant organs. Thus, NETs ensnare circulating bacteria and provide intravascular immunity that protects against bacterial dissemination during septic infections.

Measuring and interpreting platelet-leukocyte aggregates

“…The development of in vivo imaging techniques, such as intravital microscopy (IVM) further allows to track cells in live animals over longer time periods in organs including cremaster muscle, liver, lung, brain and kidney (Figure 1C) (56,57), and have been greatly beneficial in analysing dynamic interaction between platelets and leukocytes (e.g. duration of interactions and behavioural changes in leukocytes and/or platelets upon contact) (10,53,55,58–61). …”

“…In this context, several markers specifically label platelets, including CD41 (GPIIb) (60), CD42 (GPIb) (61) and CD49b (α2 integrin subunit) (10,59,60,65). While the use of antibodies against these proteins enables clear identification of platelets interacting with other cells, some antibodies affect platelet function under certain conditions.…”

“…Indeed, during bacterial infection, platelet-KC interactions increase in duration and result in platelet aggregates encapsulating bacteria previously trapped by KCs (59). Furthermore, TLR4-mediated platelet activation leads to profound binding of platelets to neutrophils in the liver (11), an event critical for promoting NET formation and hence bacterial elimination (10). Large platelet-neutrophil aggregates (PNAs) are formed also during viral infection within the liver microvasculature and precede NET release (98).…”

“…Platelet-leukocyte interactions are further stabilised by crosstalk of numerous additional receptor/ligand pairs that were reviewed previously (1,5) and trigger mutual activation and release of granular content by both platelets and leukocytes, thereby modulating leukocyte function and fine-tuning immune responses (5). Importantly, platelet-leukocyte interactions facilitate leukocyte recruitment and extravasation to sites of inflammation (6,7), promote leukocyte release of pro-inflammatory mediators (8,9), oxidative burst, phagocytosis, release of neutrophil extracellular traps (NETs) (10,11), and may also dampen inflammation under some pathological conditions (12–14). …”

Changing world of neutrophils

“…In a septic model, rapid platelet-dependent NET formation was observed in liver sinusoids that increased trapping of bacteria by four-fold [71]. NETs were also shown to be formed and to be effective in certain viral infections [49].…”