Leukotriene B4 Potentiates CpG Signaling for Enhanced Cytokine Secretion by Human Leukocytes

Abstract

TLRs are known to be important in innate host defense against a variety of microbial infections. In particular, TLR9 has been associated with immune defense against different foreign organisms by recognition of unmethylated DNA sequences. In this report, we provide evidence that leukotriene B4 (LTB4) has the capacity to modulate TLR9 expression on human neutrophils. The effect of LTB4 was found to be specific, because related leukotrienes such as LTC4 and LTD4 or neutrophil agonists IL-8 and C5a failed to modulate TLR9 expression in neutrophils. Using fluorochrome-tagged CpG DNA, we observed that LTB4 treatment also increased TLR9 ligand binding in neutrophils. Moreover, LTB4 stimulation potentiates CpG-mediated signaling via an endosome-independent mechanism in human neutrophils, leading to enhanced secretion of proinflammatory cytokines. The increase in cytokine secretion by LTB4 following CpG stimulation of neutrophils was associated with the activation of TGF-β-activated kinase (TAK-1) as well as p38 and c-Jun (JNK) kinases. In contrast, in PBMC LTB4 leads to an increase in cytokine secretion following CpG stimulation but via a MyD88- and endosome-dependent mechanism. As observed in neutrophils, PBMC stimulation with LTB4 in the presence of CpG also results in enhanced TAK-1, p38, and JNK phosphorylation/activation. These data provide new evidence underlying the immunomodulatory properties of LTB4 leading to antimicrobial defense.

Multiple Phenotypic Changes Define Neutrophil Priming

“…It primes neutrophil responses to toll-like-receptor (TLR) agonists, resulting in enhanced cytokine (IL-8, TNFα) secretion (Gaudreault et al, 2012). TLR9 mRNA levels are upregulated upon priming with LTB 4 , but there is no increase in surface expression of TLR2, TLR4, or the co-receptors TLR1 and TLR6 following LTB 4 exposure (Gaudreault and Gosselin, 2009; Gaudreault et al, 2012). Instead, neutrophil LTB 4 -induced hyper-responsiveness is mediated by the potentiation of TLR-induced intracellular signaling.…”

Too much of a good thing: How modulating LTB 4 actions restore host defense in homeostasis or disease

“…TLR2 is a cell surface receptor that senses peptidoglycan molecules commonly found on gram-positive bacteria and TLR9 is an endosomal receptor that detects DNA [67, 68]. LTB 4 stimulation upregulates the expression of TLR2 and TLR9 in human neutrophils [69]. Enhanced expression of TLRs could allow for neutrophils to better sense various pathogens and induce a stronger signaling cascade, which allows for a more potent immune response.…”

Leukotriene B4 Enhances NOD2-Dependent Innate Response against Influenza Virus Infection

“…In this regard, it was demonstrated that LTB 4 treatment significantly prevents IAV replication in vivo through an up-regulated production of antimicrobial peptides including beta-defensin–3 and the mouse cathelicidin-related antimicrobial peptide by neutrophils [ 9 ]. In addition, LTB 4 has the capacity to potentiate TLR-mediated response of neutrophils to various TLR agonists [ 10 , 11 ]. Such effects of LTB 4 were relied on at least two mechanisms.…”

“…Such effects of LTB 4 were relied on at least two mechanisms. Firstly, neutrophil treatment with LTB 4 increases expression of intracellular TLR7, 8 and 9, thus enhancing the recognition of foreign RNA and DNA [ 10 , 11 ] and secondly, LTB 4 can also act on key molecules of the TLR signaling cascade, particularly TAK1, to potentiate the secretion of cytokines following recognition of TLR ligands [ 11 ].…”