Different macrophages, different fortunes in tuberculosis granulomas

Tuberculosis (TB) remains defined by its signature lesions, called granulomas, where immune cells gather around Mycobacterium tuberculosis (Mtb) and either restrain or permit bacterial growth. Within those aggregates, macrophages are central players: they can engulf and kill bacteria, but they can also become niches where Mtb survives and replicates. Despite their importance, the range of macrophage identities and behaviors inside granulomas has not been fully mapped. Joshua T. Mattila and colleagues set out to fill that gap using a nonhuman primate model of early TB. To capture the variety of macrophage types and their local context, the team examined lung tissue and individual granulomas with single-cell RNA sequencing and immunofluorescence. Those complementary approaches let them read the gene activity of thousands of individual cells while also visualizing where different macrophages sit inside lesions. The work focused on the early stages of infection, aiming to reveal which macrophage programs are present when granulomas are forming and when Mtb first establishes itself in the lung.

The study combined single-cell RNA sequencing and immunofluorescence to profile macrophages taken from lung tissue and the granulomas of infected nonhuman primates. Analysis of the single-cell gene-expression profiles revealed distinct macrophage subsets rather than a single uniform population. These included embryonic-origin tissue-resident alveolar macrophages and monocyte-derived alveolar and interstitial macrophages. Immunofluorescence showed that these subsets occupy different spatial niches within granulomas. Crucially, the researchers found that tissue-resident alveolar macrophages and a subset of macrophages undergoing epithelial-to-mesenchymal transition accounted for the highest frequency of Mtb-infected cells. Within infected cells, patterns of gene expression differed from uninfected counterparts, with changes in immune- and migration-associated genes. The authors interpret these differences as either host responses to infection or pathways that Mtb induces or exploits to survive, highlighting that infection status correlates with distinctive molecular programs in specific macrophage types.

These findings emphasize that macrophage heterogeneity and spatial arrangement inside granulomas matter for how TB develops. By showing that certain resident macrophages and a particular transitioning subset are more frequently infected, the study suggests that not all macrophages are equally likely to harbor Mtb. The observation that infected cells express different immune- and migration-associated genes implies that the bacteria and host cell programs interact in ways that could favor bacterial persistence. For researchers and clinicians, this means that strategies aimed at altering macrophage behavior will need to account for both the origin of macrophages and their position within lesions. The work therefore provides a map of cell types and molecular states that could guide more targeted immunomodulatory approaches in future TB research, helping to focus efforts on the macrophage populations that most influence bacterial survival and lesion outcome.