7b). Pro-inflammatory stimuli such as LPS[52] and the cytokines TNF-α[53] and IL-1[54] have been shown JQ1 mouse to activate NF-κB via the canonical pathway by phosphorylating serines, leading to I kappa B (IκB) degradation and translocation of the NF-κB complex into the nucleus, thereby activating gene expression
of pro-inflammatory cytokines, which augments the pro-inflammatory response in a positive feed-forward loop.[5] The assessed cytokines were also increased but to a lesser degree in the absence of NF-κB activation with LPS treatment alone (Fig. 7a,b). It is therefore plausible that LPS and Pyl A co-administration activates a strong cytokine response, which then further induces NF-κB activation via a feed-forward mechanism. Lipopolysaccharide induces a strong inflammatory response and leads
MK-8669 in vitro to the recruitment of leucocytes.[55, 56] CRTH2 agonists also chemoattract CRTH2-positive leucocytes,[19, 57] including Th2 cells,[19] eosinophils[58] and dendritic cells.[37] The increase in inflammatory cytokines seen with combined injection of both LPS and the CRTH2 agonist Pyl A may be as a result of the increase in infiltrating leucocytes rather than a direct effect on myocytes. Importantly, CRTH2 is also expressed on Th1 cells in the mouse, unlike the human, which is likely to have contributed to the unexpected pro-inflammatory response seen in the mouse. Several murine studies with CRTH2 agonists/antagonists and the use of CRTH2 knock-out mice have shown a pro-inflammatory role for the CRTH2 receptor.[36, 38, 59-63] The CRTH2 agonist DK-PGD2 causes eosinophilia in mouse lung[63] and intra-peritoneal administration of DK-PGD2 causes a two-fold induction of monocyte chemoattractant Janus kinase (JAK) protein-1 and a 25-fold induction of macrophage inflammatory protein-2.[36] Furthermore, in a murine study of FITC-induced inflammation of the skin (a model of contact hypersensitivity), a CRTH2 antagonist was found to significantly reduce the production of the pro-inflammatory cytokines
TNF-α, IL-1β and the chemokines macrophage inflammatory protein-2 and GRO-α.[64] However, no distinction between the Th1 or Th2 type cytokines being modulated was made. Similarly reduced levels of lung IFN-γ, IL-4 and IL-5 have been observed in a mouse model of airway inflammation upon administration of a CRTH2 antagonist.[62] Our finding of increased fetal viability with Pyl A in LPS-treated mice was surprising in view of the shortened time interval from injection to delivery. Although following spontaneous labour there were no surviving pups in the LPS and LPS/Pyl A treatment groups (Fig. 5b). We attribute this to the pups delivering preterm, based on unpublished data showing non-viability at E16 even in the absence of inflammation-induced preterm labour.