The modulation of neutrophil extracellular traps in chronic critical illness: results of a preliminary translational sub-study

Introduction
Neutrophil Extracellular Trap formation (NETosis) represents an alternative neutrophil function that has important roles in both defence from infection and inadvertent tissues damage in dysregulated immune responses such as sepsis1. NETs are increasingly being recognised as important in a range of critical illnesses including ARDS and sepsis. We evaluated NETosis activity in patients with established critical illness.
Methods.
Samples were acquired patients with critical illness who had received mechanical ventilation for five days, then weekly thereafter. Healthy young and elderly controls were obtained from the MARTINI trial. Neutrophils were extracted from whole blood using Percol gradients and a chemiluminescent NETosis assay was performed with vehicle control and stimulated by phorbol 12-myristate 13-acetate (PMA) to produce maximal NETosis.
Results.
Unstimulated NETosis at recruitment was suppressed in the chronically critically ill group compared to healthy young (p=0.005) and elderly (p=0.01) controls. NETosis was similar in the chronically critically unwell at recruitment and the following one and two weeks. When stimulated with PMA, healthy elderly patients increased NETosis more than healthy young controls (p=0.035). Stimulated NETosis was similar to controls in the chronically critically unwell population at all time points (figure 1).
Discussion.
The chronically critically ill have reduced baseline NETosis activity, which may contribute to the Persistent Inflammation, Immunosuppression and Catabolism syndrome 2. Neutrophil dysfunction may well play a role in both risks of nosocomial infection and deleterious tissue damage: down-regulation may represent a compensatory mechanism to limit this tissue damage in the recovery phase1. One weakness in our study is that patients with ongoing critical illness did not differentiate patients with and without ongoing inflammatory-provoking/infective stimuli and therefore may actually be a mix e of several subgroups. In conclusion, baseline NETosis is suppressed patients with chronic critical illness. Future work should focus on the effects of this related to infections and healing.