A granulin-positive macrophage subtype in mycobacterial granulomas alleviates tissue damage by limiting excessive inflammation.

Abstract

Granulomas play a crucial role in the pathology of tuberculosis, but the immune environment governing their formation remains largely unknown. To explore the dynamic changes in the immune microenvironment during the formation of tuberculous granulomas, we infected adult zebrafish with <i>Mycobacterium marinum</i> and then examined uninfected and infected kidneys, as well as large and small granulomas in the kidneys. Using single-cell RNA sequencing technology, we identified two major macrophage subpopulations in the hematopoietic tissue (kidney) of zebrafish under uninfected physiological conditions: monocyte derived and tissue-resident macrophages. Interestingly, the infection induced the emergence of epithelioid cells and a previously undescribed <i>grna.2</i>⁺ macrophage subpopulation. Depletion of <i>grna.2</i>⁺ macrophages with the nitroreductase-metronidazole ablation system resulted in shortened zebrafish survival after infection, increased bacterial load, and more granulomas, especially necrotic granulomas. Depletion of <i>grna.2</i>⁺ macrophages also produced a denser granuloma structure with fewer T cells. RNA-seq and flow cytometry analysis revealed that depletion of <i>grna.2</i>⁺ macrophages led to upregulated inflammatory signaling pathways, including <i>tnfα</i> and <i>il1β</i>, and increased macrophage lytic cell death. Similarly, in samples from tuberculosis patients, we also identified GRN-positive macrophages, which exhibit similar anti-inflammatory functions. This subset of <i>grna.2</i>⁺ macrophages present in developing granulomas can suppress excessive inflammatory responses to alleviate macrophage lytic death, reduce tissue damage, promote T cell infiltration and ultimately help control mycobacterial growth in vivo.