Abstract
A growing body of evidence suggests that innate immune cells can respond in a memory-like (adaptive) fashion, which is referred to as trained immunity. Only few in vivo studies have shown training effects in neutrophils; however, no in vitro setup has been established to study the induction of trained immunity or tolerance in neutrophils by microbial agents. In light of their short lifespan (up to 48 h), we suggest to use the term trained sensitivity for neutrophils in an in vitro setting. Here, we firstly describe a feasible two-hit model, using different doses of lipopolysaccharide (LPS) in bone marrow neutrophils. We found that low doses (10 pg/mL) induce pro-inflammatory activation (trained sensitivity), whereas priming with high doses (100 ng/mL) leads to suppression of pro-inflammatory mediators such as TNF-α or IL-6 (tolerance) (p < 0.05). On a functional level, trained neutrophils displayed increased phagocytic activity and LFA-1 expression as well as migrational capacity and CD11a expression, whereas tolerant neutrophils show contrasting effects in vitro. Mechanistically, TLR4/MyD88/PI3Ks regulate the activation of p65, which controls memory-like responses in mouse bone marrow neutrophils (p < 0.05). Our results open a new window for further in vitro studies on memory-like inflammatory responses of short-lived innate immune cells such as neutrophils.
Highlights
The innate immune system is continuously challenged by several intracellular and extracellular stressors, resulting in a variety of pro- or anti-inflammatory responses
Neutrophils were stimulated with increasing doses of LPS to verify the inflammatory response in a dose-dependent manner for the production of TNF-α and IL-6 (Figure 1B,C; p < 0.05)
Upon a second challenge by a fixed dose of LPS (100 ng/mL), primed neutrophils expressed a distinct biphasic dose-response pattern for the production of pro-inflammatory cytokines, TNF-α and IL-6, exhibiting significantly increased levels by low-dose priming (p < 0.05), especially 10 pg/mL LPS, whereas high-dose (≥1 ng/mL) primed neutrophils showed markedly decreased cytokine amounts compared to unprimed state of neutrophils (Figure 1D,E; p < 0.05)
Summary
The innate immune system is continuously challenged by several intracellular and extracellular stressors, resulting in a variety of pro- or anti-inflammatory responses. Studies in different myeloid cells (i.e., monocytes, macrophages, natural killer (NK) cells), microglia, or non-immune cells challenge this notion and highlight the ability of innate immune cells to develop non-specific memory-like inflammatory responses to protect against secondary infections (trained immunity) [1,2,3,4,5,6]. Pioneering studies disclosed a pathogen-associated molecular pattern (PAMP)-specific induction of trained immunity, where priming by β-glucan or bacillus Calmette–Guerin (BCG) after subsequent challenge by gram (–) bacterial lipopolysaccharide (LPS) triggers an exacerbated inflammatory response [1,12,13,14]. Various seminal studies have shown that both memory-like inflammatory responses are accompanied by epigenetic reprogramming with resulting distinct changes in metabolism [8,20,21,22]
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