Immune Enzyme PTPN2 Emerges as Master Switch in Sepsis Inflammation

Sepsis remains one of the most feared conditions in modern medicine, a runaway inflammatory storm triggered when the body’s response to infection turns against its own tissues. Now, a team of researchers at McGill University has identified a previously unappreciated molecular conductor of that storm: an enzyme called PTPN2, also known as TC-PTP. In a study published in Cellular and Molecular Life Sciences, the group shows that this protein tyrosine phosphatase is essential for sustaining a specific arm of innate immune signaling known as the non-canonical inflammasome pathway, and that its absence profoundly dampens the inflammatory response in a mouse model of acute sepsis. The discovery not only fills a long-standing gap in immunology but also points to a potential therapeutic lever for one of the deadliest medical emergencies worldwide.

To understand why the finding matters, it helps to revisit how the innate immune system detects bacterial invaders. Immune cells called macrophages patrol tissues armed with pattern recognition receptors, molecular sensors that spot conserved microbial signatures. Among the most potent of these signatures is lipopolysaccharide, or LPS, a component of the outer membrane of Gram-negative bacteria. When bacteria multiply, they shed LPS-rich outer membrane vesicles, tiny blebs of membrane that can travel to distant sites in the body, sneak inside immune cells, and ignite inflammation far from the original infection. This dissemination of inflammatory triggers is a hallmark of systemic inflammatory conditions, sepsis chief among them.

Once intracellular LPS is detected, the cell deploys an inflammasome, a multiprotein machine that acts as both alarm and weapon. Inflammasomes come in two flavors. The canonical pathway relies on well-studied sensors such as NLRP3, while the non-canonical pathway is triggered directly by intracellular LPS through the inflammatory caspase-11 in mice, and caspase-4 and caspase-5 in humans. Activation of either pathway leads caspase-1 to process pro-inflammatory cytokines, most notably interleukin-1 beta, and to cleave Gasdermin-D, the executioner protein that punches holes in the cell membrane and drives inflammatory cell death. While decades of research have mapped how post-translational modifications tune the canonical pathway, the modifications governing the non-canonical route have remained largely mysterious. That gap is precisely where the McGill team, led by Michel L. Tremblay, focused its attention.

The researchers zeroed in on PTPN2, an enzyme that removes phosphate groups from tyrosine residues on target proteins, thereby switching signaling circuits on or off. To probe its role, they generated mice lacking Ptpn2 specifically in myeloid cells, the lineage that includes macrophages, and challenged these animals with LPS-rich outer membrane vesicles and with acute sepsis models. The results were striking. In macrophages deficient in Ptpn2, the transcription of interleukin-1 beta, the flagship cytokine of inflammasome biology, was markedly impaired. The defect traced back to dysregulated signaling through TLR4, the cell-surface LPS receptor, and the p38 MAPK pathway, a cascade that normally drives the priming phase of inflammasome activation by ramping up cytokine gene expression.

Intriguingly, the compromised priming did not translate into wholesale collapse of the inflammasome machinery. Levels of caspase-11, caspase-1, and Gasdermin-D remained unchanged in the knockout cells, indicating that the enzyme’s influence is exerted upstream, at the level of inflammatory gene transcription and signal tuning, rather than by altering the abundance of the core inflammasome components themselves. This distinction is biologically important: it suggests PTPN2 shapes the intensity of the inflammatory output rather than the mere presence of the detonator, a nuance that could matter greatly when designing drugs that target the pathway.

The study also uncovered a novel substrate for PTPN2, adding a fresh branch to the enzyme’s known interaction map. The team found that Lyn, a member of the Src-family of kinases, becomes hyperphosphorylated in the absence of Ptpn2. Because phosphatases and kinases operate in opposing pairs, this observation identifies Lyn as a direct or proximal target whose phosphorylation state PTPN2 normally restrains. Src-family kinases sit at hubs connecting receptor activation to downstream signaling, so their dysregulation in Ptpn2-deficient macrophages provides a mechanistic thread linking the phosphatase to the altered TLR4 and p38 MAPK signaling observed in the knockout animals.

Adding a second layer of complexity, the researchers found that loss of PTPN2 unleashes a STAT1-mediated program that boosts production of nitric oxide, a reactive molecule with a double-edged role in immunity. While nitric oxide can help kill pathogens, the study showed that in this context it undermines inflammasome activity by compromising mitochondrial fitness. Mitochondria are not merely cellular power plants; they supply signals and metabolites that feed inflammasome activation, and their dysfunction can blunt the cell’s inflammatory capacity. In Ptpn2-deficient macrophages, therefore, excessive nitric oxide erodes the very organelles the inflammasome depends upon, further dampening the response.

The in vivo consequences of these cellular defects became clear when the mice were subjected to acute sepsis. Animals lacking Ptpn2 in myeloid cells exhibited reduced numbers of small peritoneal macrophages, a key resident immune population, and lower serum levels of interleukin-1 beta following the septic challenge. Together, these findings indicate a dampened pro-inflammatory response in the absence of the phosphatase. In the context of sepsis, where excessive inflammation drives organ damage and death, a blunted response might at first glance seem protective. But the picture is nuanced: interleukin-1 beta also mobilizes host defenses, and the reduced macrophage population suggests that PTPN2 supports aspects of immune competence as well as inflammation. The enzyme, in other words, is not simply a brake or an accelerator but a fine-tuner of the response.

That dual role is what makes PTPN2 an attractive therapeutic candidate. Phosphatases were long considered undruggable, but recent advances in medicinal chemistry have brought PTPN2 inhibitors into clinical consideration, particularly in oncology, where the enzyme’s effects on T cells and tumor immunity have drawn intense interest. The McGill study adds a new dimension to that conversation: in sepsis and systemic inflammation, modulating PTPN2 activity could, in principle, recalibrate the non-canonical inflammasome axis and the cytokine output that fuels inflammatory injury. The authors highlight this potential, positioning the enzyme as a target for managing systemic inflammation. Any such strategy would need to account for the pathway’s complexity, since the same enzyme that supports cytokine production also, through its restraint of STAT1 and nitric oxide, protects mitochondrial fitness and inflammasome function.

The work also carries broader conceptual weight for immunology. By demonstrating that a tyrosine phosphatase governs the non-canonical inflammasome pathway, the study establishes that this arm of innate immunity, often treated as a parallel, LPS-triggered circuit, is subject to the same intricate layers of post-translational regulation that shape its canonical counterpart. It connects outer membrane vesicle biology, TLR4 signaling, Src-family kinase activity, STAT1 activation, mitochondrial health, and cytokine secretion into a single regulatory network centered on one enzyme. For clinicians confronting sepsis, a syndrome that kills millions each year and for which treatment remains largely supportive, the identification of a tunable node in the inflammatory circuitry offers a genuine reason for optimism. For basic researchers, it opens a set of new questions: how PTPN2 is itself regulated during infection, whether the Lyn and STAT1 branches operate independently, and whether human patients with altered PTPN2 activity show different sepsis trajectories. The answers could shape the next generation of anti-inflammatory therapies.

Subject of Research: Role of the phosphatase PTPN2 in regulating non-canonical inflammasome activation and inflammatory responses in sepsis

Article Title: PTPN2 sustains non-canonical inflammasome activation and shapes the inflammatory response to sepsis

Article References: Colalillo, B., Aubry, I., Aumont, P., Poirier, A. J., Hincapie, A. M., Wu, C., St-Laurent, E., Martinez Cordova, Z., & Tremblay, M. L. (2026). PTPN2 sustains non-canonical inflammasome activation and shapes the inflammatory response to sepsis. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06469-7

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06469-7

Keywords: PTPN2, sepsis, inflammasome, non-canonical pathway, interleukin-1 beta, LPS, macrophages, TLR4, p38 MAPK, Lyn kinase, STAT1, nitric oxide