This indicated that mice lacking microbial flora do not have a generalized defect in the endothelial vasculature and also that neutrophils from these mice are functionally capable of migrating to the inflamed tissue upon receiving the appropriate signals. We hypothesized that the microbiota mediates

its effects on inflammatory responses by activating pattern recognition receptor-signalling pathways. To test this hypothesis, we analysed mice deficient in the known pattern recognition receptor pathways and examined their ability to mount a neutrophil response to an intraperitoneal zymosan challenge. Receptor interacting protein-2 (RIP-2) knockout mice, which are defective in nucleotide-binding, oligomerization LY2157299 domain-containing protein-1 (NOD1) or NOD2 signalling, were able to respond normally to zymosan (Fig. 4a). Similar results were obtained in mitochondrial antiviral LY2606368 in vivo signalling (MAVS) knockout mice, which were defective in retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) signalling. Also, normal neutrophil recruitment was observed in mice lacking Caspase 1, NOD-like receptor family, pyrin domain containing 3 (NLRP3), or Apoptosis-associated speck-like protein (ASC), which are defective in inflammasome activation (Fig. 4a). However, MyD88 knockout mice showed

a markedly reduced recruitment of neutrophils following zymosan stimulation (Fig. 4b), similar to that observed in the flora-deficient mice. Like the flora-deficient animals, MyD88 knockout mice did not have a statistically significant reduction in the number of neutrophils in the blood under basal conditions (see Supplementary material

Fig. S4a). Furthermore, on challenge with Chlormezanone zymosan, neutrophils in the bloodstream of Myd88−/− mice outnumbered those in wild-type mice (see Supplementary material Fig. S4b), mimicking the phenotype that was observed in mice lacking microbial flora. MyD88 is an adaptor protein for most Toll-like receptors (TLR) and some cytokine receptors, most notably the IL-1 receptor (IL-1R). IL-1 is a pro-inflammatory cytokine that plays a key role in recruiting neutrophils to sites of inflammation in response to some inflammatory stimuli. However, we had previously shown that the neutrophilic inflammatory response to zymosan does not require the IL-1R and we confirmed again that this was the case (Fig. 4b). TLR2 has been reported to be one of the receptors for zymosan and it was possible that this was why MyD88 was required for the inflammatory response to this agent.[27] However, we found that TLR2-deficient mice had a normal zymosan-induced infiltration of neutrophils in the peritoneum (Fig. 4b). This is not surprising because the major receptor for zymosan is thought to be Dectin-1, which does not signal though MyD88.