Tuberculosis bacteria trigger unusual macrophage death with DNA release

Mycobacterium tuberculosis (Mtb) lives inside human lung macrophages, the very cells that are supposed to defend the body. Rather than simply killing those cells outright, Mtb can manipulate the ways they die, a strategy that helps the bacterium survive and spread. Volker Briken and his team set out to identify which programmed cell death pathways are actually activated when human macrophages are infected with Mtb. To do this they compared a lab-grown macrophage model, the THP-1 cell line, with primary human monocyte-derived macrophage (hMDM) cells over the first three days (days 1–3) after infection. Using multiple imaging and molecular techniques, the researchers looked for hallmarks of apoptosis, pyroptosis, necroptosis, ferroptosis and other death programs. The goal was to pin down whether a specific necrotic pathway is responsible for the tissue-damaging cell death seen in tuberculosis, or whether Mtb triggers a different, less well-characterized route to kill macrophages.

The team used confocal microscopic analysis, immunoblotting, time-lapse live-cell imaging and high-resolution confocal imaging to follow infected cells. Confocal microscopic analysis showed that Mtb-infected THP-1 cells or hMDMs rarely exhibited apoptosis. Immunoblotting revealed that Mtb induces significant CASP3 and GSDME activation in THP-1 cells, but not in hMDMs. In THP-1 cells, Mtb also produced a significant increase in GSDMD cleavage, a hallmark of pyroptosis, whereas hMDMs did not show this change. MLKL phosphorylation was not observed in either THP-1 cells or hMDMs during Mtb infections, indicating an absence of necroptosis. Markers of ferroptosis such as GPX4 expression and lipid peroxidation levels showed no changes. Time-lapse live-cell imaging revealed no lysosomal membrane permeabilization prior to plasma membrane rupture (PMR). After PMR, both THP-1 cells and hMDMs released DNA; that DNA from THP-1 cells contained low levels of myeloperoxidase and histone H3 citrullination. High-resolution confocal imaging showed Mtb associated with the released DNA, and the authors found that pyroptosis induction in THP-1 cells was dispensable for the DNA release and cell death induction.

Putting these observations together, the study concludes that Mtb-triggered cell death in primary human macrophages (hMDMs) does not follow the canonical programmed cell death pathways commonly studied—apoptosis, pyroptosis, necroptosis or ferroptosis. Instead, the defining feature in both the THP-1 model and hMDMs was the release of DNA after plasma membrane rupture, a pattern that the authors say correlates with a process potentially similar to NETosis in neutrophils. This shifts attention away from single, well-known death pathways and toward alternative mechanisms by which Mtb damages and escapes from host cells. For researchers, the findings highlight the need to consider DNA-associated release events and NETosis-like processes when studying how Mtb interacts with macrophages and how infected cells might influence inflammation or bacterial spread. The work frames new questions about how released DNA and associated proteins influence immune responses in tuberculosis and where therapeutic or diagnostic opportunities might lie.