Reduced transcript stabilization restricts TNF-α expression in RAW264.7 macrophages infected with pathogenic mycobacteria: evidence for an involvement of lipomannan

Abstract Lack of p38 MAPK-dependent TNF-α mRNA stabilization in Mycobacterium avium ssp. paratuberculosis-infected macrophages. Despite the critical role that TNF-α plays in the containment of mycobacterial infection, the mechanisms involved in regulation of its expression by mycobacteria are poorly defined. We addressed this question by studying MAP, which causes a chronic enteritis in ruminants and is linked to human Crohn’s disease. We found that in MAP-infected macrophages, TNF-α gene expression was substantially lower than in macrophages infected with nonpathogenic MS or stimulated with LPS. TNF-α transcription alone could not fully explain the differential TNF-α mRNA expression, suggesting that there must be a substantial contribution by post-transcriptional mechanisms. Accordingly, we found reduced TNF-α mRNA stability in MAP-infected macrophages. Further comparison of MAP- and MS-infected macrophages revealed that lower TNF-α mRNA stability combined with lower mRNA and protein expression in MAP-infected macrophages correlated with lower p38 MAPK phosphorylation. Theses findings were independent of viability of MAP and MS. We demonstrate that the major mycobacterial cell-wall lipoglycan LM of MAP and MS induced TNF-α mRNA transcription, but only the MS-LM induced p38 MAPK-dependent transcript stabilization. Overall, our data suggest that pathogenic mycobacteria cause weak p38 MAPK activation and TNF-α mRNA stabilization as a result of their structural cell-wall components such as LM and thereby, restrict TNF-α expression in macrophages.