Strain-dependent disease progression and necrotizing granuloma formation induced by virulent Mycobacterium avium complex strains in a murine model

Mycobacterium avium complex (MAC) is the leading cause of non-tuberculous mycobacterial pulmonary disease (NTM-PD), a chronic infection with a heterogeneous clinical course. Although murine models of MAC-PD have been developed, reproducing the key features of progressive human disease, including diverse pathological features and therapeutic sensitivities, remains challenging. In this study, we evaluated five clinical MAC strains, including a newly identified highly virulent isolate, NBRC112750, in immunocompetent BALB/c mice. Among these, FKJ-1 and NBRC112750 induced progressive pulmonary infection with increasing bacterial burdens and extensive lung involvement by 25 weeks post-infection. Notably, both strains led to the formation of necrotizing granulomas resembling those observed in M. tuberculosis -infected C3HeB/FeJ mice. These lesions featured neutrophilic infiltration, foamy macrophages, and collagen encapsulation. Using FKJ-1 strain, we also established an inhalation infection model, in which low-dose exposure reproduced necrotizing granulomas. Despite in vitro drug susceptibility, FKJ-1 infection exhibited poor response to standard therapy, highlighting strain-dependent variability in treatment efficacy. These findings establish a murine model that reflects both key pathological and therapeutic aspects of MAC-PD and provides a valuable platform for investigating MAC pathogenesis and evaluating novel therapies.