Interleukin-32ε induces caspase-independent apoptosis mediated by N-Myc interactor in macrophages infected with Mycobacterium tuberculosis.

Mycobacterium tuberculosis (Mtb) is the pathogen responsible for tuberculosis, a leading cause of illness and death worldwide. Growing evidence suggests that the proinflammatory cytokine IL-32 plays a major role in host defences against pathogens such as Mtb. IL-32 exists in six alternatively spliced isoforms, but antituberculosis effects have been reported only for some of them. In this study, we examined the effect of all six IL-32 isoforms on Mtb replication in the murine macrophage cell line RAW 264.7. Compared with cells transfected with the other isoforms, IL-32ε-transfected cells exhibited the strongest antituberculosis effect and the highest rate of Mtb-induced apoptosis. Of note, this apoptosis pathway was independent of caspase-3 activation. Instead, N-Myc interactor (NMI), an inhibitor of Wnt signalling, was a key player in IL-32ε-mediated apoptosis by inhibiting Wnt/β-catenin signalling and thereby activating c-Myc-mediated apoptosis. Moreover, we identified two cis-acting elements that are binding sites for the transcriptional regulators paired box 6 (PAX6) and transcription factor CP2 (TFCP2) in the promoter of NMI and these elements proved essential for IL-32ε-induced upregulation of Nmi expression. Furthermore, IL-32ε-mediated activation of the mitogen-activated protein kinase p38 also contributed to NMI upregulation. In conclusion, our results demonstrate that Mtb infection-induced IL-32ε-mediated apoptosis in macrophages plays a key role in host defences against Mtb.