A recent study by Ritacco et al from the Gaudet Lab published in Molecular Cell, “The human antibacterial factor APOL3 couples lysosomal damage to mitochondrial DNA efflux and type I IFN induction”, showed that in living cells, APOL3 couples lysosomal damage to mtDNA efflux and type I IFN production. Their study focuses on studying how sub-lethal lysosomal damage during infection or cellular injury engages the innate immune system. They discovered that transient lysosomal damage inflicted by sterile or infectious insults triggered APOL3 recruitment to BAX/BAK-permeabilized mitochondria in which it selectively enhanced inner mitochondrial membrane (IMM) permeability to mtDNA by solubilizing cardiolipin. Moreover, they presented evidence that transient lysosomal damage induces sub-lethal mitochondrial outer membrane permeabilization (MOMP); however, in resting cells, IMM integrity maintains a barrier against wholesale mtDNA release. Furthermore, they demonstrated that IFN-γ priming elicited APOL3, which negated this barrier by permeabilizing the IMM, thereby facilitating a physical interaction between mtDNA and cyclic GMP-AMP synthase (cGAS), leading to type I IFN induction without cell death. Thus, altogether, their results illustrate how cells use an immune-inducible antibacterial protein to expedite the selective breakdown of endosymbiosis and facilitate a heightened response to injury and infection.
SUMMARY
Lysosomal damage is an endogenous danger signal, but its significance for innate immunity and the specific signaling pathways it engages remain unclear. Here, we uncover an immune-inducible pathway that connects lysosomal damage to mitochondrial DNA (mtDNA) efflux and type I IFN production. We find that transient lysosomal damage elicits sub-lethal mitochondrial outer membrane permeabilization (MOMP) via BAK/BAX macropores; however, the inner mitochondrial membrane (IMM) maintains a barrier against wholesale mtDNA release. Priming with type II IFN (IFN-γ) induced the antibacterial factor APOL3, which, upon sensing lysosomal damage, targets mitochondria undergoing MOMP to selectively permeabilize the IMM, enhance mtDNA release, and potentiate downstream cGAS signaling. Biochemical and cellular reconstitution revealed that, analogous to its bactericidal detergent-like mechanism, APOL3 permeabilized the IMM by solubilizing cardiolipin. Our findings illustrate how cells enlist an antibacterial protein to expedite the breakdown of endosymbiosis and facilitate a heightened response to injury and infection.
GRAPHICAL ABSTRACT
Ryan Gaudet, PhD, is an Assistant Professor of Microbiology & Immunology, at Columbia University and his research seeks to understand the molecular mechanisms that underpin the ability of immune cytokines to transform seemingly vulnerable tissue cells into potent effectors of immune system. Dominic Ritacco, co-first author of the article, is a PhD student in the Gaudet Lab in the Microbiology and Immunology Department at Columbia University Irving Medical Center. His PhD work revolves around the idea that non-immune cells can protect themselves against pathogens through effector interferon-stimulated genes (ISGs). He recently won the ICIS-Pfizer Junior Investigator Award in Cytokines 2025, held in Seattle (USA). We have interviewed them about their latest article.
What was the key question you addressed with this paper, and what led you to ask it?
The key question addressed in this paper was how sub-lethal lysosomal damage during infection or cellular injury engages the innate immune system. Lysosomes are constantly under threat from both infectious and non-infectious insults, and damage can lead to the leakage of the acidic contents into the cell. Widespread lysosomal destabilization triggers cell death, but little is known about the mechanisms by which transient lysosomal damage triggers signaling pathways such as interferon production, inflammasome activation, and metabolic reprogramming.
Could you tell us how the initial idea and/or observation led to the major discovery?
Previously, we had found that the interferon-induced effector APOL3 dissolves the bacterial inner membrane after first trafficking to bacterial-damaged lysosomes. Surprisingly, sterile lysosomal injury is also sufficient to recruit APOL3, prompting us to question whether APOL3 plays a role in the response to cellular injury beyond infection. Additionally, for reasons previously unknown, we found that after this initial recruitment to damaged lysosomes, APOL3 would localize to mitochondria. The goal of this study was to identify the significance and cellular consequences of this response.
Which of your findings was the most unexpected and/or exciting to you?
The most unexpected and exciting finding for us was the remarkable extent to which the cell can leverage the endosymbiotic history of mitochondria to alter cellular signaling via context-dependent, regulated release of DAMPs. Here, we show that cells in IFN-gamma-inflamed conditions that undergo transient lysosomal damage, whether infectious or non-infectious, breach mitochondrial tolerance, releasing inflammatory mitochondrial DNA into the cytosol, which is recognized as a danger signal and initiates type I interferon production.
If people take away only three things from this paper, what do you want them to be?
- Interferons affect responses to cellular damage, not just infection.
- The mitochondria can act as a signaling hub, interpreting and propagating specific cellular responses to insults.
- Type I and type II interferons elicit distinct transcriptional programs, often with different kinetics, yet both converge on a shared goal of host-defense. In this case, we show that type II IFN is needed for robust type I IFN production following lysosomal damage.
Why is this discovery of particular significance to the Cytokines community?
In this study, we demonstrate that type II IFN enhances the cellular response to transient lysosomal damage by enlisting the antibacterial effector APOL3, which disrupts mitochondrial tolerance and induces type I IFN production. This is particularly significant for the Cytokines community because it highlights that cytokine-driven stimulatory conditions can fundamentally alter how cells interpret and respond to cellular insult. More broadly, it underscores a major mechanistic knowledge gap in our understanding of how cytokines tune or redirect cellular responses to diverse stimuli.
Citation of the article:
Ritacco D., Shahnawaz H., Oduguwa A. et al. The human antibacterial factor APOL3 couples lysosomal damage to mitochondrial DNA efflux and type I IFN induction. Molecular Cell, 2026; 86, 1116-1133.e8. https://doi.org/10.1016/j.molcel.2026.01.029
20th April 2026
by Maialen Sebastian-delaCruz, PhD