Differential regulation of neutrophil degranulation by p38 MAPK and mitochondrial ROS during Mycobacterium lep

Erythema Nodosum Leprosum (ENL) is an inflammatory complication of leprosy that is associated with the progression of peripheral nerve damage and the resultant physical disabilities. Neutrophil-mediated inflammation plays a crucial role in the emergence of ENL. Recent evidence suggests that Mycobacterium leprae triggers neutrophil degranulation and the generation of Low-Density Neutrophils (LDN). This study aimed to investigate whether inhibition of critical regulators of human neutrophil activation can regulate M. leprae-induced neutrophil degranulation.

To explore the mechanisms underlying inflammation in vitro, whole-blood cultures were incubated with M. leprae. The effects of selective inhibitors in M. leprae co-cultures were subsequently evaluated by measuring surface expression of granule markers, levels of inflammatory cytokines, and release of granule-related proteins, including MMP-9, PTX3, and MPO.

The authors found that at least 8 h of M. leprae incubation was necessary to trigger primary and secondary granule degranulation, while tertiary granules began to degranulate within 4 h. At this 4-h mark, LDNs were observed in cell cultures. By 24 h, we confirmed neutrophil degranulation and LDN generation. We observed a correlation between the release of pro-inflammatory cytokines and degranulation markers. The secretion of IL-8 and IL-6 was associated with secondary and tertiary granule degranulation. Selective inhibition of p38 MAPK affected the release of tertiary and secondary granules as well as IL-6 secretion. Moreover, mito-TEMPO treatment showed that mitochondrial ROS production seems to be crucial for M. leprae-induced primary and secondary granule degranulation. Finally, the inhibition of TGF-β receptor 1 indicates that TGF-β modulates M. leprae-induced neutrophil degranulation and LDN generation through binding to the receptor.

These results indicate that p38 MAPK pathway and mitochondrial ROS production are important contributors to M. leprae-induced neutrophil degranulation, while TGF-β/TGF-βR1 signaling exerts an important regulatory effect. These findings advance our knowledge of the molecular mechanisms underlying M. leprae-induced degranulation and raise the possibility of new potential therapeutic strategies for targeting ENL-related degranulation.