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

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 B 4 (LTB 4 ) has the capacity to modulate TLR9 expression on human neutrophils. The effect of LTB 4 was found to be specific, because related leukotrienes such as LTC 4 and LTD 4 or neutrophil agonists IL-8 and C5a failed to modulate TLR9 expression in neutrophils. Using fluorochrome-tagged CpG DNA, we observed that LTB 4 treatment also increased TLR9 ligand binding in neutrophils. Moreover, LTB 4 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 LTB 4 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 LTB 4 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 LTB 4 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 LTB 4 leading to antimicrobial defense.