Mycobacterium avium subspecies paratuberculosis (MAP) causes a chronic enteritis 
in ruminants. In addition, MAP is presently the most favored pathogen linked to 
Crohn's disease. In this study, we were interested in dissecting the molecular 
mechanisms of macrophage activation or deactivation after infection with MAP. By 
subtractive hybridization of cDNAs, we identified the immune-responsive gene 1 
(IRG1), which was expressed substantially higher in lipopolysaccharide 
(LPS)-stimulated than in MAP-infected murine macrophage cell lines. A nuclear 
run-on transcription assay revealed that the IRG1 gene was activated 
transcriptionally in LPS-stimulated and MAP-infected macrophages with higher 
expression in LPS-stimulated cells. Analysis of post-transcriptional regulation 
demonstrated that IRG1 mRNA stability was increased in LPS-stimulated but not in 
MAP-infected macrophages. Furthermore, IRG1 gene expression of macrophages 
infected with the nonpathogenic Mycobacterium smegmatis differed from those of 
LPS-stimulated and MAP-infected macrophages. At 2 h postinfection, M. 
smegmatis-induced IRG1 gene expression was as low as in MAP-infected, and 8 h 
postinfection, it increased nearly to the level in LPS-stimulated macrophages. 
Transient transfection experiments revealed similar IRG1 promoter activities in 
MAP- and M. smegmatis-infected cells. Northern analysis demonstrated increased 
IRG1 mRNA stability in M. smegmatis-infected macrophages. IRG1 mRNA 
stabilization was p38 mitogen-activated protein kinase-independent. Inhibition 
of protein synthesis revealed that constitutively expressed factors seemed to be 
responsible for IRG1 mRNA destabilization. Thus, our data demonstrate that 
transcriptional and post-transcriptional mechanisms are responsible for a 
differential IRG1 gene expression in murine macrophages treated with LPS, MAP, 
and M. smegmatis.