While there is a clear role for MyD88 in the ability of conventional mice to mount neutrophilic inflammation to zymosan, we found that several other innate immune signalling pathways were not required for this response. Although Clarke et al. have reported that commensal bacteria prime neutrophils via NOD1 signalling in ways that enhance their phagocytic potential to various bacteria,[16] we found FK506 in vivo that RIP2 knockout mice did not show reduced inflammation to zymosan. Since RIP2 is required for NOD1/2 signalling, this finding argued against a role for either NOD1 or NOD2 in mediating a gut flora-induced effect in our system.[32] Therefore, NOD1/2 signalling may be important for phagocytosis

but is not needed for neutrophilic inflammation to this agent. Similarly, we found no contribution of the inflammasome components (NLRP3/ASC/caspase 1) or the RNA-sensing RIG-I like receptors https://www.selleckchem.com/products/CP-690550.html in mediating zymosan-induced inflammation. Hence, we show that intestinal flora affect the ability of the immune system to mount neutrophilic inflammation

via the MyD88 pathway. To examine when the MyD88 pathway was required, we took advantage of the ROSA26-Cre system, in which the MyD88 gene could be temporally deleted by the addition of tamoxifen. We showed that for zymosan-induced peritonitis, the presence of MyD88 was not required at the time of challenge. This eliminates the possibility that zymosan Nintedanib (BIBF 1120) needs to signal through MyD88 via TLR2 or IL-1R or any other MyD88-dependent receptor. These data therefore, make a strong case for the necessity of priming by intestinal flora-induced MyD88 activation for zymosan-induced neutrophil migration, before the actual zymosan challenge. Hence a significant finding of this study is that although the MyD88 pathway is essential for creating an innate immune system

that is poised to respond to inflammatory agent, this pathway is not needed at the elicitation phase of an inflammatory response (unless of course the pro-inflammatory stimulus was using MyD88-dependent receptors such as TLRs). An implication of our study is that the set point of the naive (i.e. never exposed to microbes) innate immune system may be anti-inflammatory for many stimuli. However, in conventionally reared mice the immune system is perturbed by exposure to microbial flora in ways that alter the cytokines that are made. As part of this process MyD88-dependent pattern recognition receptor signalling by microbial flora appears to alter this set point in ways that promote inflammatory responses. In summary, we postulate that TLR ligands derived from the intestinal flora constitutively enter the blood and tissues. Here, they prime tissue-resident cells via MyD88 signalling, so that they provide appropriate stimulatory signals that condition the innate immune system to be able to respond to future inflammatory insults in ways that promote neutrophil migration into tissue sites.