Ozone Promotes Macrophage Efferocytosis in Neuropathic Pain

Neuropathic pain is not only a disorder of neuronal excitability. Tissue injury also produces apoptotic cells, damage-associated signals, and inflammatory mediators that can sustain abnormal sensory processing. In their open-access study, Ruan and colleagues examined whether improving the removal of dying cells could reduce pain-related inflammation. The reference paper, published in Frontiers in Immunology, focuses on ozone-induced activation of the AMPK/Gas6-MerTK/SOCS3 signaling pathway in macrophages.

Study Background and Research Question

Neuropathic pain arises after nerve injury or disease and is commonly expressed as spontaneous pain, allodynia, hyperalgesia, and sensory abnormalities. The study describes neuropathic pain as affecting approximately 7%–10% of the population, according to the reference paper. Existing pharmacological options can be limited by adverse effects, inadequate efficacy, tolerance, or dependence-related concerns. This creates a need for interventions that target the inflammatory and cellular processes maintaining pain rather than only dampening neuronal transmission.

One such process is efferocytosis, the regulated engulfment and disposal of apoptotic cells by phagocytes. Efficient efferocytosis limits the persistence of cellular debris and can promote resolution-associated immune responses. By contrast, uncleared apoptotic cells may contribute to the release or persistence of inflammatory signals. In the setting of peripheral nerve injury, this failure of clearance could amplify neuroinflammation and reinforce hypersensitivity.

The central question was therefore whether ozone could enhance macrophage clearance of apoptotic cells and, if so, whether this effect was responsible for reduced neuropathic pain. The authors specifically tested the roles of AMP-activated protein kinase (AMPK), growth arrest-specific 6 (Gas6), the Mer tyrosine kinase receptor (MerTK), and suppressor of cytokine signaling 3 (SOCS3).

Key Innovation from the Reference Study

The main innovation is the connection of ozone treatment to an efferocytosis-centered mechanism of analgesia. Rather than presenting ozone solely as a general anti-inflammatory intervention, the study proposes that it improves the macrophage response to apoptotic neutrophils. This places apoptotic-cell clearance upstream of changes in inflammatory cytokines and pain behavior.

The proposed sequence is coherent: ozone activates AMPK; AMPK increases Gas6 expression; Gas6 engages MerTK; and MerTK-dependent signaling enhances macrophage uptake of apoptotic cells. The authors report that Protein S, another ligand associated with apoptotic-cell recognition, was not similarly upregulated. The downstream increase in SOCS3 was associated with lower expression of the pro-inflammatory mediators IL-1β, IL-6, and TNF-α.

This distinction between Gas6 and Protein S is important because it narrows the proposed mechanism. It suggests that the response is not simply a nonspecific increase in all engulfment-associated proteins, but may involve selective regulation of the Gas6-MerTK axis. The work also uses pharmacological blockade to move beyond correlation: inhibition of AMPK or MerTK eliminated the protective effect of ozone on pain behavior.

Methods and Experimental Design Insights

The in vivo component used mice subjected to chronic constriction injury, or CCI, as a model of peripheral neuropathic pain. Ozone was administered at 15, 30, or 45 mg, and the investigators evaluated mechanical hypersensitivity together with apoptotic-cell accumulation, macrophage-related responses, signaling proteins, and inflammatory mediators. The dose comparison identified 30 mg as the concentration producing a significant improvement in mechanical hypersensitivity in the reported experiments.

The complementary in vitro component used bone marrow-derived macrophages (BMDMs) exposed to apoptotic neutrophils. This reductionist system allowed the authors to assess macrophage phagocytic activity without the full complexity of injured nerve tissue. It also provided a direct test of whether ozone changes the capacity of macrophages to engulf dying cells.

Protocol Parameters

• Neuropathic pain model: Use CCI mice when testing the relationship between peripheral nerve injury, apoptotic-cell accumulation, neuroinflammation, and mechanical hypersensitivity, as in the reference study.
• Ozone dose comparison: The reported in vivo comparison used 15, 30, and 45 mg; the 30 mg condition produced the strongest reported behavioral improvement.
• Macrophage assay: Evaluate BMDM uptake of apoptotic neutrophils to isolate the efferocytosis component of the response.
• Mechanistic inhibition: Include AMPK inhibition with CC and MerTK inhibition with UNC2541 when testing pathway dependence. These inhibitors were used in the reference study, but pharmacological specificity and off-target effects should be considered in replication.
• Outcome integration: Interpret mechanical hypersensitivity together with pathway markers, macrophage uptake, and IL-1β, IL-6, and TNF-α measurements rather than treating behavioral change as a standalone mechanistic endpoint.

A useful design feature is the combination of behavioral and cellular readouts. Pain relief alone would not establish improved efferocytosis, while an in vitro uptake result alone would not demonstrate relevance to neuropathic pain. The paired model allows the authors to connect macrophage function with the animal phenotype. Nevertheless, the study summary does not establish whether every intermediate event occurs in the same macrophage population or with the same temporal sequence in vivo.

Core Findings and Why They Matter

Ozone significantly reduced CCI-associated mechanical hypersensitivity at the reported 30 mg dose. In parallel, ozone enhanced the phagocytic activity of BMDMs toward apoptotic neutrophils. These observations support a relationship between improved clearance of dying cells and mitigation of pain-related behavior.

At the signaling level, ozone activated AMPK and increased Gas6 expression, while MerTK activation and SOCS3 expression were also promoted. Protein S was not reported to show the same increase as Gas6. The treatment was accompanied by suppression of IL-1β, IL-6, and TNF-α, consistent with a shift away from sustained inflammatory signaling.

The inhibitor experiments are particularly informative. The AMPK inhibitor CC and the MerTK inhibitor UNC2541 abolished the pain-relieving effect of ozone in the CCI model. This result supports the interpretation that AMPK and MerTK are functionally necessary for the observed response, although inhibitor studies alone cannot exclude all off-target mechanisms. Genetic perturbation or cell-specific pathway manipulation would provide stronger causal validation.

Conceptually, the study broadens the therapeutic logic of neuropathic pain research. Instead of focusing exclusively on neurons or soluble cytokines, it identifies the handling of apoptotic cells by macrophages as a potentially actionable checkpoint. If confirmed, this framework could help explain how changes in innate immune-cell behavior influence the transition from tissue injury to persistent pain.

Comparison with Existing Internal Articles

The internal article Ozone Stimulates Macrophage Efferocytosis to Relieve Neuropathic Pain provides a concise, reader-oriented summary of the same AMPK/Gas6-MerTK/SOCS3 concept. The reference paper is more useful for researchers because it reports the CCI model, the ozone dose comparison, the BMDM–apoptotic neutrophil experiment, and the inhibitor-based mechanistic tests. The internal overview can serve as a navigation point, but it should not replace the original article when assessing experimental details, controls, or translational limitations.

Limitations and Transferability

Several limitations affect interpretation. First, the work is centered on a mouse CCI model, which reproduces selected features of peripheral neuropathic pain but cannot represent the clinical heterogeneity of human disease. Neuropathic pain caused by diabetes, chemotherapy, spinal pathology, or other conditions may involve different distributions of macrophages, apoptotic cells, and inflammatory mediators.

Second, ozone exposure is biologically complex. The study supports a pathway involving AMPK, Gas6, MerTK, and SOCS3, but it does not establish that this is the only relevant pathway or identify the precise ozone-derived chemical intermediates responsible for the response. Dose translation, administration conditions, tissue exposure, and treatment timing will require independent optimization.

Third, pharmacological inhibition provides useful pathway evidence but has inherent specificity limitations. The loss of ozone efficacy after CC or UNC2541 treatment is consistent with pathway dependence, yet genetic depletion of AMPK, Gas6, or MerTK in defined macrophage populations would strengthen the causal model. Additional experiments could also determine whether enhanced efferocytosis is necessary for cytokine suppression or whether both effects arise in parallel.

Finally, the study does not establish a role for extracellular pH measurement or fluorescent pH imaging. Any pH assay should therefore be regarded as a complementary measurement of the experimental microenvironment, not as a demonstrated component of the paper’s mechanism.

Research Support Resources

Why this cross-domain matters, maturity, and limitations

Macrophage efferocytosis, ion transport, and local acid-base conditions can intersect in injured or inflamed tissues, so extracellular pH monitoring may help characterize a surrounding microenvironment in related experiments. However, this is a supporting analytical direction rather than a validated conclusion from the ozone study. Researchers should not infer AMPK, Gas6, MerTK, or SOCS3 activity from a pH signal alone; pathway proteins, efferocytosis measurements, cytokines, and pain behavior remain necessary endpoints.

For experiments that add quantitative extracellular or accessible-compartment pH measurements, researchers can use BCECF (2',7'-bis(carboxyethyl)-5(6)-Carboxyfluorescein), SKU C5694. This cell-impermeant, dual-excitation ratiometric pH fluorescent dye can support workflows described as a fluorescent pH probe for ion transport studies, a probe for cellular metabolism pH monitoring, a microenvironmental pH regulation assay, or an acid-base homeostasis research tool. Its reported signal uses the emission intensity at 535 nm after excitation at 490 nm relative to 440 nm; consult the product information for preparation and assay-specific optimization. Because BCECF is not part of the reference study, it should be used to contextualize extracellular pH rather than to substitute for the paper’s macrophage and signaling readouts.