New research on how infected immune cells die reveals potential targets to protect lung tissue from severe respiratory infection.
RT’s Three Key Takeaways:
- Unexpected Cell Death Pathway: Researchers discovered that the widely used research compound z-VAD-FMK triggers programmed macrophage death during mycobacterial infection through an off-target mechanism independent of caspase-8.
- Underlying Immune Signaling: The observed immune cell destruction relies on specific molecular signaling pathways, including tumor necrosis factor-alpha (TNF-α), type I interferon, and a specific regulatory protein cascade.
- Therapeutic Implications: Blocking key necroptotic proteins reduced lung pathology and lowered bacterial levels in preclinical models, suggesting new avenues for host-directed therapies against respiratory infections.
Tuberculosis (TB) remains a major global healthcare threat, and the premature destruction of infected immune cells called macrophages serves as a primary driver of pulmonary disease pathology, according to a study published in mLife.
Identifying the cellular pathways responsible for macrophage death is critical for developing new host-directed treatments. In cell death research, investigators routinely use z-VAD-FMK, a pan-caspase inhibitor, under the standard assumption that it induces a form of programmed cell death known as necroptosis by blocking caspase-8. However, a research team led by Li Zhang from Southern University of Science and Technology discovered that z-VAD-FMK triggers macrophage necroptosis during Mycobacterium tuberculosis infection via an unexpected, non-canonical off-target mechanism, according to the study.
An Off-Target Signaling Pathway
Through genetic and pharmacological evaluations, the researchers demonstrated that this form of macrophage death depended on the receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3), and mixed lineage kinase domain-like protein (MLKL) axis, as well as Toll-like receptor 2 (TLR2), TNF-α, and type I interferon signaling.
Knocking down mouse caspase-8 and knocking out human caspase-8 in macrophages failed to prevent z-VAD-FMK from triggering necroptosis during mycobacterial infection, according to study data. Furthermore, other selective caspase inhibitors did not reproduce this cellular death, confirming that the process operates through a caspase-8-independent off-target mechanism. The authors also noted that RIPK3 became functionally active without typical phosphorylation at its canonical T231/S232 sites, indicating alternative modes of activation.
The off-target necroptosis was also seen in macrophages infected with other related bacteria, including Mycobacterium kansasii and Rhodococcus equi, according to the study.
Preclinical Findings and Treatment Potential
When evaluated in vivo, deleting MLKL showed minimal effect on disease outcomes in standard wild-type laboratory mice. However, in specialized Sp140-deficient mice that develop hyperactivated type I interferon responses and necrotic granulomas mimicking severe human illness, MLKL deficiency significantly mitigated pulmonary pathology, reduced bacterial burden, and alleviated weight loss following high-dose aerosol infection, according to the researchers.
The authors noted that identifying the precise targets involved in this pathway will advance scientific understanding of necrotic granuloma formation during TB, which may inform the development of future host-directed therapies for respiratory care.
