Post-transcriptional control of inflammasome activation in intestinal macrophages

Colonic macrophages (cMPs) are important for intestinal homeostasis as they efficiently kill microbes yet produce low levels of inflammatory cytokines. We found high constitutive mRNA expression but low protein levels for the inflammasome proteins NLRP3 and proIL-1beta as well as for certain pro-inflammatory cytokines, including TNF-alpha and IL-6 in cMPs compared to bone marrow-derived, and peritoneal macrophages. In contrast, high mRNA but low protein production was not found for ASC, procaspase-1 or IL-10. TLR activation enhanced mRNA for NLRP3 and proIL-1beta  in cMPs, but resulted in minimal increase in protein expression, resulting in a correspondingly low production of mature IL-1beta following NLRP3 or NLRC4 activation. In contrast, during experimental colitis inflammatory monocytes and cMPs (CD64⁺F4/80⁺ cells) expressed high levels of both mRNA and protein for NLRP3, proIL-1beta and TNF-alpha, and produced high levels of IL-1beta  following inflammasome activation indicating that microenvironmental signals during steady-state but not inflammatory conditions control the expression of inflammatory molecules, including the NLRP3 inflammasomes. Finally, blocking proteasome activity, as well as IL-10 signaling in cMPs (CD64⁺F4/80⁺Ly6C^- cells) resulted in enhanced NLRP3 and proIL-1beta protein expression in cMPs from non-inflamed mice. These data indicate novel post-transcriptional mechanisms to control inflammasome activation in cMPs.