The team, led by researchers at Sun Yat-sen University in Guangzhou, China, focused on tissue-resident alveolar macrophages, or TR-AMs, the long-lived sentinels that populate the alveoli, the tiny air sacs where oxygen exchange takes place. These cells originate from fetal liver monocytes and sustain themselves largely without replacement from the bloodstream, outnumbering interstitial macrophages in the lung by roughly eight to one. Under healthy conditions, they patrol nearly continuously between alveoli, engulfing inhaled bacteria, particles, and dying cells, while also clearing and catabolizing excess pulmonary surfactant, the lipid-protein film that keeps the air sacs from collapsing. Nearly ninety percent of them actively migrate as part of routine immune surveillance, making them both the lung’s first responders and its housekeeping crew.

Previous work on particulate pollution and macrophages had largely relied on monocyte-derived cell lines such as THP-1 and U937, which model recruited immune cells rather than the resident alveolar lineage. The new study set out to close that gap by examining how diesel particulate matter, or DPM, affects the two physiological functions that define TR-AM biology: chemotactic movement and phagocytosis. DPM is a major contributor to fine particulate pollution in urban and industrialized environments, and roughly half to ninety percent of it consists of ultrafine particles smaller than 0.1 micrometers, small enough to penetrate deep into the alveolar spaces where these macrophages live.

The researchers began with the murine alveolar macrophage cell line MH-S, exposing cells to a standardized DPM preparation for 24 hours and then performing RNA sequencing. The exposure produced sweeping transcriptional changes, with 287 genes upregulated and 219 downregulated. Pathway analysis revealed disruption in phagosome and actin cytoskeleton regulation, bacterial recognition, and cytokine signaling. Genes encoding integrins and complement receptors essential for adhesion, migration, and bacterial uptake, including Itgal, Itgam, Fpr1, Fpr2, C5ar1, C1qa, and C1qc, were significantly suppressed, and flow cytometry confirmed reduced surface expression of the CD11a and CD11b integrin proteins.

Functional testing followed. In transwell migration assays, DPM-treated macrophages showed a dramatic loss of their ability to migrate toward chemoattractants produced at sites of bacterial stimulation. Notably, the cells’ secretion of chemokines, the signals used to recruit other immune cells, was largely unchanged, suggesting that diesel particles primarily cripple the macrophages’ own motility and receptor signaling rather than their communication role. Phagocytosis assays using fluorescently labeled Escherichia coli and Staphylococcus aureus bioparticles showed sharply reduced uptake and impaired phagosome acidification in DPM-exposed cells, indicating that both particle engulfment and the maturation of the digestive compartment were compromised.

To confirm these findings in living animals, the team exposed mice to DPM via intratracheal instillation and then delivered fluorescent bacterial bioparticles directly into the airways. Primary TR-AMs, identified by their characteristic CD45-high SiglecF-positive surface profile, showed a marked reduction in the ability to phagocytose both bacterial species, and the proportion of highly phagocytic cells fell substantially. The deficit proved durable: phagocytic capacity remained suppressed for weeks after exposure, with only a gradual recovery trend over an eight-week observation period. Conditioned-medium experiments showed that the effect was driven mainly by direct contact between particles and macrophages, though epithelial cells exposed to DPM released soluble mediators that added a smaller, indirect layer of inhibition.

The mechanistic core of the study lies in the actin cytoskeleton. Both migration and phagocytosis depend on the polymerization of filamentous actin, or F-actin, which provides the mechanical force for cell deformation, filopodia extension, and the engulfment of particles larger than half a micrometer. DPM exposure significantly reduced phalloidin staining of F-actin in macrophages, and the loss of polymerization tracked closely with a more than tenfold decrease in bacterial uptake. When the researchers treated cells with cytochalasin D, a drug that disrupts filamentous actin, phagocytosis collapsed in the same way, confirming that actin remodeling is indispensable for the process. DPM also reduced the expression of the upstream regulators of actin nucleation, including the GTPases Rac1 and Cdc42, the nucleation-promoting factors WASP, N-WASP, and WAVE, and the Arp2 and Arp3 components of the actin-nucleating complex, pointing to suppression of the Rac1/Cdc42-WASP-Arp2/3 signaling cascade as a likely driver of the functional failure.

The downstream consequences for lung physiology were equally striking. Because TR-AMs are responsible for clearing excess surfactant, their paralysis led to measurable accumulation in the alveoli. Mice exposed to DPM developed Periodic acid-Schiff-positive granules in the alveolar spaces, a histological signature of accumulated surfactant glycoproteins and lipoprotein deposits. Bronchoalveolar lavage fluid showed sharply elevated total protein, a roughly 120-fold increase in surfactant protein D, elevated lactate dehydrogenase indicating cell death, and a slight decline in surfactant protein A, the collectin that normally facilitates lipid clearance. Targeted lipidomics revealed excessive accumulation of phosphatidylcholine species that normally constitute the bulk of surfactant lipids, along with ceramides and free cholesterol. In vitro, DPM-treated macrophages showed significantly reduced uptake of fluorescent cholesterol, linking the cellular defect directly to the lipid buildup observed in the animals.

That pattern, the authors note, closely resembles pulmonary alveolar proteinosis, a disease in which surfactant accumulates, gas exchange fails, and infection risk rises. While primary forms of the disease stem from defects in granulocyte-macrophage colony-stimulating factor signaling, secondary alveolar proteinosis arises from external insults that deplete or disable alveolar macrophages, and inhaled particles including indium and gallium compounds have already been implicated. The new findings suggest that diesel particulate exposure may represent another route to this pathology, offering a mechanistic bridge between everyday air pollution and a condition that is frequently underdiagnosed.

The researchers are careful to acknowledge the limits of their model. The acute murine exposure protocol compresses what would be months of real-world inhalation into concentrated instillations, and although the authors estimate that the cumulative dose corresponds to roughly six days of occupational exposure in a high-concentration underground mining environment, long-term inhalation studies will be needed to confirm relevance to ordinary urban exposure. The evidence linking the Rac1/Cdc42-WASP-Arp2/3 pathway to the observed dysfunction remains correlative, and direct measurement of chemotaxis in primary TR-AMs was not performed. Even so, the study delivers a clear and consequential message: the same soot that darkens city air can quietly disarm the lung’s resident defenders, weakening immune surveillance, delaying inflammation resolution, and allowing surfactant to accumulate where it can do real harm. Restoring macrophage actin dynamics, the authors suggest, may one day offer a therapeutic strategy for pollution-related respiratory disease.

Subject of Research: Impairment of tissue-resident alveolar macrophage phagocytosis by diesel particulate matter and its role in pulmonary surfactant accumulation

Article Title: Phagocytotic impairment of tissue-resident alveolar macrophages by diesel particulates drives pulmonary surfactant accumulation

Article References: Chen, R., Zhou, Z., Wang, J., Zhang, Y., Li, P., & Wu, H. (2026). Phagocytotic impairment of tissue-resident alveolar macrophages by diesel particulates drives pulmonary surfactant accumulation. Advanced Biotechnology, 4(2), Article 21. https://doi.org/10.1007/s44307-026-00113-y

Image Credits: AI Generated

DOI: 10.1007/s44307-026-00113-y

Keywords: diesel particulate matter, alveolar macrophages, phagocytosis, chemotaxis, F-actin, pulmonary surfactant, pulmonary alveolar proteinosis, air pollution, PM2.5, innate immunity, surfactant protein D, lipidomics

Pulmonary alveolar proteinosis, or PAP, is an exceptionally rare condition in which a soap-like substance called surfactant accumulates in the lung’s air sacs. Normally, immune cells known as alveolar macrophages clear away used surfactant with the help of a signalling protein called granulocyte-macrophage colony-stimulating factor, or GM-CSF. In the autoimmune form of the disease, the body produces antibodies that block GM-CSF, leaving macrophages unable to do their cleanup work. The result is a slow suffocation of the alveoli by lipoproteinaceous debris, producing progressive breathlessness and a dry cough.

The newly reported case, published in Respirology Case Reports, follows a 73-year-old Hispanic man whose PAP was first diagnosed in 2001 through a surgical lung biopsy after an abnormal chest x-ray. His anti-GM-CSF antibodies were documented in 2002. Over nearly two decades, he cycled through the disease’s limited therapeutic arsenal: inhaled bronchodilators, two whole-lung lavage procedures—in which clinicians physically flush miles of surfactant-laden fluid out of the lungs in a single sitting—courses of pulmonary rehabilitation, and six months of inhaled sargramostim, a synthetic GM-CSF designed to kick-start macrophage function. Nothing held the disease at bay for long. After a bout of pneumonia followed by a Mycobacterium avium-intracellulare infection triggered a steep decline in 2020, he was listed for transplantation and received two new lungs in November 2021.

What happened next is the heart of the report. Transplantation, in theory, replaces the diseased lungs with healthy ones and, with them, a fresh population of donor-derived macrophages capable of clearing surfactant. Recurrence of PAP in a transplanted lung is considered vanishingly rare, and this case is unusual both for how long it took to appear and for how stubbornly it resisted detection. Initially, the transplant was a triumph. Surveillance bronchoscopies showed no rejection, and spirometry was remarkable—the patient’s forced expiratory volume in one second, a standard measure of airflow, reached 110% of predicted values, with an FEV1-to-FVC ratio of 91%, comfortably in supranormal territory.

The trouble began subtly. Mild dyspnea in 2022 prompted imaging that revealed new opacities, and anti-GM-CSF antibodies—negative just before transplant—reappeared in his blood. But because he remained clinically stable, his team chose careful observation. Then, in October 2023, a CT scan disclosed a cavitary nodule in the right upper lobe, and bronchoscopy confirmed an infection with Nocardia abscessus, an opportunistic bacterium that thrives in immunosuppressed patients. He was treated with prolonged multidrug therapy and placed on lifelong doxycycline prophylaxis. Over the following months, additional opportunistic pathogens emerged: Mycobacterium avium complex and Aspergillus sydowii, a mold, were both eventually recovered from lavage fluid.

Herein lies the diagnostic dilemma the case illustrates so vividly. Each of these infections can produce diffuse opacities and cavitary lesions on chest imaging that look strikingly similar to recurrent PAP. Meanwhile, the diagnostic gold standard for PAP—periodic acid–Schiff, or PAS, staining of bronchoalveolar lavage fluid to reveal the characteristic lipoproteinaceous material—came back negative, not once but repeatedly, in both lavage samples and transbronchial biopsies performed in August 2024. High-resolution CT, however, kept telling a different story: evolving ground-glass opacities and a “crazy-paving” pattern, a distinctive lattice of thickened interlobular septa overlying hazy ground glass that is classic for PAP but not exclusive to it. Anti-GM-CSF antibodies were positive, yet titers had never been quantified, making it impossible to track whether the autoimmune process was waxing or waning. And the patient’s lung function added another layer of confusion: his spirometry shifted from supranormal airflow to a mildly obstructive pattern—an atypical evolution for PAP alone and one that could just as easily be blamed on infection or chronic lung allograft dysfunction.

Rather than abandon the PAP hypothesis, the clinical team pursued a strategy of longitudinal, multimodal surveillance—repeating imaging, serology, bronchoscopy, and pulmonary function testing over months. That persistence paid off. In February 2025, given persistent symptoms, progressive imaging abnormalities, and consistently positive anti-GM-CSF antibodies, clinicians initiated inhaled sargramostim and performed a whole-lung lavage. This time, the lavage fluid was unmistakable: abundant PAS-positive lipoproteinaceous material, confirming that PAP had indeed returned in the transplanted lungs. A second lavage followed in April 2025, and the response was dramatic. Ground-glass and crazy-paving changes cleared radiographically, symptoms improved markedly, and spirometry partially recovered.

Why would PAP recur in a transplanted lung? The report’s authors propose a compelling mechanism: over time, the allograft is progressively repopulated by recipient-derived alveolar macrophages, which carry the same underlying GM-CSF signalling defect that caused the original disease. In other words, transplantation replaces the soil but not the seed—the systemic autoimmune process that disables macrophage surfactant clearance remains in place, and as the donor macrophage population is gradually supplanted, the stage is reset for disease recurrence. The authors caution that neither macrophage chimerism nor donor GM-CSF signalling status was directly assessed in this patient, so the mechanism remains a hypothesis, but it fits the delayed timeline of more than three years.

Just as important is the therapeutic lesson. Despite the diagnostic detours, once recurrent PAP was confirmed, it responded robustly to conventional PAP-directed treatment—inhaled sargramostim combined with sequential whole-lung lavages. The authors note that the patient’s apparent improvement with these therapies after transplantation, when the same measures had offered only transient benefit before, should be interpreted cautiously; differences in disease burden, remaining lung reserve, timing, and the burden of concurrent infection could all have contributed. But the core message stands: the transplanted lung is not immune to PAP, yet it remains fully amenable to PAP therapy when the disease returns.

For clinicians, the case carries a clear set of implications. A single negative PAS stain does not rule out recurrent PAP, particularly when high-resolution CT shows progressive ground-glass and crazy-paving changes and anti-GM-CSF antibodies are positive. Concomitant opportunistic infections—Nocardia, mycobacteria, Aspergillus—can mask the underlying process and must be treated in parallel rather than accepted as the sole explanation for a patient’s decline. And for lung transplant recipients with a history of PAP, the authors argue, structured long-term surveillance—periodic HRCT, serial pulmonary function testing, and repeat bronchoscopy with lavage for at least the first three years—may be warranted to catch recurrence before it becomes incapacitating.

In an era when lung transplantation is increasingly offered for rare diffuse lung diseases, this single patient’s journey—two decades of PAP, a transformative transplant, a maze of opportunistic infections, and ultimately the return of the very disease the new lungs were meant to escape—serves as a reminder that some immunological problems travel with us, no matter what organs we carry them in. It is also a testament to diagnostic persistence: the answer was there all along, waiting in a wash of PAS-positive fluid.