Every year, sepsis kills roughly 11 million people worldwide, more than lung cancer and heart attacks combined. Despite decades of research, no drug exists that directly strengthens the immune system’s ability to destroy the bacteria driving the condition. Now, a study published in the Proceedings of the National Academy of Sciences in 2025 points to a specific molecular player that could change that: a chloride channel called PACC1, encoded by the gene TMEM206, which macrophages rely on to acidify the internal compartments where trapped bacteria are killed.
Researchers found that mice engineered to lack the Pacc1 gene could not clear bacteria effectively and were far more vulnerable to lethal infection. The results position PACC1 as one of the most clearly defined immune targets to emerge from sepsis research in years, though significant hurdles remain before the finding could benefit patients.
A channel built for killing
Macrophages are the immune system’s first responders. They swallow invading bacteria whole, sealing them inside membrane-bound compartments called phagosomes. To finish the job, those compartments must become intensely acidic, fusing with enzyme-packed lysosomes to form phagolysosomes that function as molecular killing chambers.
PACC1 sits at the center of that process. First identified through genome-wide screens and published in Science in 2019, the channel is evolutionarily conserved and opens specifically in response to drops in pH. Structural studies by Ruan et al., published in Nature in 2020, later revealed how the protein senses acidity at the atomic level, explaining why it activates precisely where macrophages need it most. Separate work in Cell Reports confirmed that PACC1 regulates endosomal acidification and transferrin receptor-mediated endocytosis, connecting the channel to the broader machinery cells use to process internalized cargo.
A focused study on peritoneal macrophages then showed that PACC1 localizes directly to phagosomes. When the channel was disrupted, those compartments failed to acidify properly, and the macrophages’ capacity to kill bacteria dropped measurably. The finding tied the channel’s biophysics to a concrete antimicrobial outcome for the first time.
From ion channel to sepsis survival
The newest and most consequential evidence comes from the PNAS study. Researchers created Pacc1 knockout mice and tested phagolysosome development and acidification under conditions modeling systemic infection. Without PACC1, macrophages could not properly mature their phagolysosomes. The study also found that PACC1 expression is enriched in macrophage and mononuclear phagocyte populations, suggesting the channel plays a specialized role in frontline immune cells rather than acting as a generic housekeeping protein.
In septic challenge experiments, mice lacking the channel succumbed more readily to infection. The authors concluded that PACC1 is essential for host defense against bacterial sepsis.
That conclusion did not appear overnight. An earlier meeting abstract published in The Journal of Immunology had already signaled the research group’s direction, describing conditional knockout and overexpression approaches linking PACC1 to bacterial pneumonia and sepsis. The progression from basic ion-channel characterization to in vivo infection models reflects a deliberate, multi-year experimental program.
Why the gap to treatment is still wide
All direct evidence for PACC1’s role in sepsis comes from mouse models. No human clinical data exist showing that modulating this channel improves outcomes for patients with sepsis or severe bacterial infections. Human sepsis is notoriously heterogeneous, shaped by differences in pathogens, underlying health conditions, and timing of care. No regulatory body has evaluated PACC1 as a therapeutic target.
Several specific questions remain unanswered:
Interaction with antibiotics. No experiments have tested whether activating or enhancing PACC1 could work alongside standard antibiotic therapy, particularly against antibiotic-tolerant bacterial strains that survive treatment by entering dormant states inside host cells. The hypothesis that boosting phagolysosomal chloride influx could restore killing of tolerant pathogens is plausible but untested.
Off-target effects. PACC1 is expressed beyond macrophages. Its roles in endosomal acidification and receptor-mediated endocytosis mean it participates in normal cellular housekeeping in other tissues. Any pharmacological activator would need to avoid disrupting those baseline functions in epithelial, neuronal, or other non-immune cells. Systemic modulation could carry side effects invisible in immune-focused mouse studies.
How much activation is too much. The current literature emphasizes loss of function: removing PACC1 impairs bacterial killing and worsens outcomes. That makes a conceptual case for pharmacological activation, but drug developers would need to define how much additional channel activity is beneficial. Overly aggressive phagolysosome acidification could damage host cells, alter antigen presentation, or trigger unintended inflammatory cascades.
Human genetic evidence. It is not yet clear whether naturally occurring variants in TMEM206 correlate with sepsis susceptibility in human populations. Genome-wide association data linking the gene to infection outcomes would strengthen the case considerably but have not been reported.
What sets this target apart from failed sepsis drugs
Sepsis drug development has a grim track record. Broad anti-inflammatory strategies, including therapies targeting TNF-alpha and activated protein C, have repeatedly failed in large clinical trials, often because dampening inflammation across the board left patients unable to fight infection. Other immunomodulatory approaches targeting PD-1/PD-L1 checkpoints, IL-7, and GM-CSF remain in various stages of investigation but have not yet delivered approved therapies.
PACC1 offers something different: a defined molecular target with a specific, well-characterized mechanism tied directly to bacterial killing rather than to the inflammatory response itself. The channel’s enrichment in macrophage populations and its direct role in phagolysosome acidification distinguish it from the broader immune-modulation strategies that have struggled in trials. Whether that distinction translates into a viable drug program will depend on future work testing pharmacological activators, mapping the channel’s functions in human immune cells, and determining how modulation interacts with standard-of-care antibiotics.
As of May 2026, PACC1 should be viewed as a promising mechanistic insight rather than a near-term therapy. The existing studies explain why macrophages fail to kill efficiently when the channel is missing and show that this deficit can tip the balance in mouse models of sepsis. They do not yet answer how safely or effectively PACC1 can be tuned in humans. As with many discoveries in innate immunity, the path from ion channel to bedside will be long. But the clarity of the underlying biology, and the sheer unmet need in sepsis care, make this a target worth watching closely.
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*This article was researched with the help of AI, with human editors creating the final content.
