Pathogenicity of Mycolicibacterium phlei, a non-pathogenic nontuberculous mycobacterium in an immunocompetent host carrying anti-interferon gamma autoantibodies: a case report

BACKGROUND: Mycolicibacterium phlei (M. phlei) is known to be a non-pathogenic nontuberculous mycobacterium (NTM) which rarely causes diseases in humans. A disseminated NTM infection is mostly caused by the Mycobacterium avium complex (MAC) and is known to develop in immunocompromised hosts, like those with acquired immune deficiency syndrome (AIDS). Here, we report a case of disseminated M. phlei infection in an immunocompetent host carrying anti-interferon gamma (IFN-γ) autoantibodies. CASE PRESENTATION: We detected M. phlei in multiple organs of an elderly woman with no significant medical history except positivity for anti-IFN-γ autoantibodies. She tested negative for human immunodeficiency virus (HIV)-1, 2/ Human T-cell leukemia virus type 1 (HTLV-1) antibody. High-resolution computed tomography (HRCT) of the chest demonstrated a nodule in the left S1 + 2 segment, interlobular septal thickening, multi lymphadenopathy, and osteolysis. A maximum intensity projection image following fluorodeoxyglucose-positron emission tomography (FDG-PET) revealed multifocal hypermetabolic lesions in the nodule and all the swollen lymph nodes seen in HRCT. FDG also accumulated in multiple bones. Advanced primary lung cancer was suspected, and biopsies of each lesion were performed. The pathology revealed caseating granuloma, positive for acid-fast bacteria, and DNA sequencing of the acid-fast bacteria confirmed the organism to be M. phlei. The patient also tested positive for anti-IFN-γ autoantibodies. Based on these findings, she was diagnosed with disseminated M. phlei infection, with anti-IFN-γ autoantibodies. CONCLUSIONS: Though known to be non-pathogenic, we show that M. phlei can be pathogenic like the MAC in immunocompetent individuals carrying anti-IFN-γ autoantibodies.