Pathological and Immune Features of Non-Tuberculous Mycobacteria and Mycobacterium tuberculosis Cutaneous/Mucosa Infections of Fifty-Four Biopsies

The histopathologic changes associated with cutaneous non-tuberculous mycobacterial (NTM) infections closely resemble those seen in Mycobacterium tuberculosis (MTB) infections, often leading to confusion. This study aimed to enhance clinicopathologic diagnosis and clarify the pathogenesis of NTM and MTB infections by comparing the clinicopathologic features and immunohistochemistry. We conducted a detailed observation and comparative analysis of histomorphological features in 27 biopsies of cutaneous/mucosa NTM infections and 27 biopsies of MTB infections, focusing on macrophage distribution and the infiltration of various macrophage subpopulations. Our findings revealed that NTM disease was more prone to developing small vessel hyperplasia, dilation, congestion, and interstitial edema compared to tuberculosis (TB). Additionally, the counts of lymphocytes, plasma cells, and neutrophils were significantly higher in NTM infections than in TB. NTM disease was primarily characterized by non-necrotizing granulomas, whereas TB was mainly associated with caseous necrotizing granulomas. Distinct macrophage subpopulations were observed in different lesion regions. granuloma epithelioid macrophages induced by NTM infections primarily expressed CD68 and CD206, while macrophages in non-granulomatous regions predominantly expressed CD86 and CD163. This suggests that these macrophages belong to different subpopulations with distinct roles. Moreover, the positivity rate of PD-L1 in mononuclear inflammatory cells was notably elevated in both NTM and MTB infections. The similarities and differences in histopathological features, macrophage responses, and immune markers between NTMD and TB provide valuable insights into their pathogenic mechanisms. Understanding these variations could inform better diagnostic and therapeutic strategies for managing NTM infections.