Coelomocytes are the cellular workhorses of the echinoderm immune system. They circulate in the coelomic fluids that fill the body cavity, performing functions that in vertebrates are distributed among blood cells and immune cells of many kinds. For decades, biologists have classified these cells by looking at their shapes under the microscope, and sea cucumbers, or holothuroids, stand out among echinoderms for displaying the greatest variety of coelomocyte morphotypes of any class in the phylum. Yet morphology alone has proved a poor guide to function. While the overall immune role of these cells is broadly accepted, the specific jobs performed by each cell type have remained murky, and molecular data specific to the different morphotypes have been scarce in the literature.

Single-cell RNA sequencing, or scRNA-seq, has transformed how biologists untangle cellular heterogeneity, allowing researchers to profile the gene expression of thousands of individual cells and group them into transcriptionally distinct populations. The technique, however, has been largely underutilised in studies of non-model organisms, particularly among invertebrates. The new study set out to close that gap for sea cucumbers by applying the method to coelomocytes harvested from the perivisceral fluid of Holothuria forskali, a species collected with the help of the collection service of the Roscoff Biological Station in France. Sequencing was carried out at the Genomic Platform of the GIGA Institute in Liège.

The analysis identified ten distinct clusters of cells, each assumed to correspond to a separate transcriptional coelomocyte population. The clustering revealed a striking architecture: one cluster, designated cluster 0, occupied a central position relative to all the others on the two-dimensional map of gene expression, suggesting that it may represent undifferentiated cells, a reservoir of progenitors from which more specialised populations arise. At the opposite extreme, cluster 6 sat markedly apart from every other group, hinting at a fundamentally different biological identity and function.

To work out what each cluster might actually do, the team performed functional enrichment analyses and hunted for immune marker genes among the transcripts enriched in each population. The results showed that several clusters carry out key immune functions, including the recognition of pathogens, phagocytosis, the engulfing and destruction of foreign particles, the activation of complement-like pathways, and the regulation of redox balance, the delicate chemistry of oxidation and reduction that cells must manage to survive an immune response. These findings provide the first tentative clues about which transcriptional populations correspond to which defensive duties, offering a molecular handle on a system that has until now been described mostly by cell shape.

One of the most intriguing results concerns a recently discovered cell type called the carotenocyte. These cells, rich in carotenoids, the pigments responsible for many of the reds, oranges and yellows in nature, were only recently described in sea cucumbers, and their presence in the perivisceral fluid was confirmed when the researchers examined their processed samples under the microscope. By drawing on transcriptomic data previously generated for this cell type using bulk RNA sequencing, the team was able to confidently assign cluster 6, the most divergent group on the map, to the carotenocyte lineage, and to deepen the picture of what these pigment-laden cells express.

The convergence of two independent datasets proved decisive. Marker genes supported by both the bulk RNA sequencing of carotenocyte-enriched samples and the new single-cell data gave the researchers confidence in the identity of cluster 6, an approach that illustrates how single-cell and bulk methods can be combined to anchor cell identities in non-model organisms where no reference atlas exists. The study’s supplementary tables catalogue the marker genes for each cluster, their functional annotations against multiple databases, and the results of enrichment analyses against KEGG pathways and gene ontology categories, providing a resource for other researchers working on echinoderm immunity.

Rigorous quality control underpinned the analysis. The authors filtered cells on the basis of the number of unique molecular identifiers and detected genes per cell, examined the expression of mitochondrial genes to flag stressed or damaged cells, and used the DoubletFinder tool to detect and remove doublets, artefacts in which two cells are captured and sequenced as one. Sensitivity analyses testing different filtering parameters showed that while individual clusters could merge or split depending on the settings, the overall configuration of the map remained stable, with cluster 0 staying central and cluster 6 remaining the most divergent population, a reassuring sign that the biological signal is robust.

The implications reach well beyond sea cucumbers. Echinoderms are deuterostomes, the same major lineage as chordates, which makes their immune cells valuable comparators for understanding how immune cell lineages evolved on the branch of the tree that eventually produced our own adaptive immune system. A better grasp of holothuroid coelomocyte diversity could also help interpret stress responses in these animals, which are ecologically important grazers and the target of a rapidly expanding aquaculture industry in Asia. Knowing which cells respond to pathogens or environmental stressors, and through which genes, could inform disease management in farmed populations and shed light on how wild sea cucumbers cope with a changing ocean.

The authors are careful to frame the study as a pioneering first step. Linking the remaining transcriptional clusters to the morphotypes described in the historical literature will require further analyses, and the functional assignments drawn from enrichment analyses remain hypotheses to be tested experimentally. Still, the study demonstrates that single-cell transcriptomics can be successfully deployed on a non-model invertebrate with no reference-grade tools, and it delivers a preliminary but detailed map of the functional diversity of holothuroid coelomocytes. For a phylum whose immune system has been studied mostly through a microscope lens, the jump to single-cell genomics marks the beginning of a new era, one in which the hidden division of labour among sea cucumber immune cells can finally be read directly from their genes.

Subject of Research: Single-cell transcriptomic profiling of coelomocyte immune cell diversity in the sea cucumber Holothuria forskali

Article Title: Single-cell transcriptomics reveals transcriptional diversity of sea cucumber perivisceral fluid coelomocytes

Article References: Wambreuse, N., Lavergne, A., Fievez, L., Bureau, F., Zhang, L., Deng, B., Caulier, G., Eeckhaut, I., & Delroisse, J. (2026). Single-cell transcriptomics reveals transcriptional diversity of sea cucumber perivisceral fluid coelomocytes. BMC Genomics. https://doi.org/10.1186/s12864-026-13356-9

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13356-9

Keywords: sea cucumber, coelomocytes, single-cell RNA sequencing, echinoderm immunity, carotenocytes, Holothuria forskali, transcriptomics, immune cells, deuterostomes, phagocytosis, BMC Genomics, non-model organisms

The central challenge the researchers set out to address is a familiar one in human genetics. Most of the thousands of genetic variants identified by genome-wide association studies as risk factors for disease do not lie within protein-coding genes. Instead, they sit in regulatory regions of the genome, the stretches of DNA that control when, where, and how strongly genes are switched on. Decades of work have established this pattern, but translating a regulatory variant into a concrete molecular mechanism has remained difficult, because the field lacked an empirical framework for characterizing the downstream consequences of regulatory variation at population scale.

Single-cell molecular quantitative trait locus mapping, a technique that links genetic variants to gene regulation measured in individual cells, has emerged as a promising approach. However, previous efforts suffered from limited statistical power and, crucially, from the inability to measure chromatin state and gene expression simultaneously in the same cells. Without both layers of information, researchers could observe that a variant was associated with changes in gene expression or with changes in chromatin accessibility, but they could not reliably connect the two steps into a single causal chain running from DNA accessibility through to transcript output.

To overcome these limitations, the consortium performed paired single-nucleus RNA sequencing and single-nucleus assay for transposase-accessible chromatin sequencing on peripheral blood mononuclear cells from FinnGen donors, capturing roughly ten million nuclei. The resulting atlas identified 51,083 cis-expression quantitative trait loci affecting 20,829 genes, 338,100 cis-chromatin accessibility quantitative trait loci affecting 210,584 open chromatin peaks, 119,094 putative causal variants, and 593,765 statistical links between regulatory peaks and the genes they control. These numbers represent a resource of extraordinary depth, spanning the major classes of immune cells found in blood and providing cell-type-resolved maps of genetic regulation.

A key analytical insight came from classifying variants according to whether they completed the full regulatory cascade. Variants that altered chromatin accessibility and were also linked, through a peak-gene connection, to altered gene expression showed twice the rate of colocalization with disease associations compared with variants that affected chromatin alone. In other words, the variants most likely to be genuinely causal for disease are those whose effects can be traced through the entire chain, from open chromatin to enhancer activity to target gene expression. This finding provides a practical filter for prioritizing candidate causal variants in complex disease loci.

To validate these statistical findings experimentally, the team turned to massively parallel reporter assays, a high-throughput technique that tests the regulatory activity of thousands of DNA sequences simultaneously. The assays confirmed the regulatory effects of 10,428 fine-mapped molecular quantitative trait loci, providing direct experimental support for a substantial fraction of the atlas’s predictions and demonstrating that the computational framework captures biologically real regulatory variation rather than statistical artifacts.

Perhaps the most conceptually significant discovery concerns genes under strong evolutionary constraint, those whose sequences change rarely because mutations are harmful. Previous studies had noted a paradox: disease variants preferentially target constrained genes, yet constrained genes appear depleted of detectable expression quantitative trait loci. The new atlas resolves this contradiction by revealing a phenomenon the authors call multilayered regulatory buffering. At constrained genes, chromatin accessibility changes caused by variants occur with normal effect sizes, but their transmission to gene expression is attenuated because these genes are regulated through weaker and more numerous enhancer-gene links. The redundancy of many weak regulatory connections dampens the expression consequence of any single regulatory perturbation.

Crucially, the reporter assay experiments confirmed that this buffering operates downstream of the regulatory element itself. Constraint acts at the interface between chromatin and expression rather than on the intrinsic activity of the cis-regulatory DNA. This distinction matters because it explains why the paradox exists in the first place: standard expression quantitative trait locus studies measure only the final output, and buffering at the chromatin-to-expression step suppresses that output signal even when the underlying chromatin effects are intact. The finding reconciles apparently conflicting observations across the field and suggests that disease risk at constrained genes may accumulate through subtle, distributed effects that individual expression studies systematically miss.

The atlas also delivers concrete mechanistic hypotheses for specific diseases. At autoimmune loci, the researchers traced complete regulatory cascades at TICAM1, an adaptor protein in Toll-like receptor signaling, and RHOH, a small GTPase involved in T cell receptor signaling, both linked to autoimmune hypothyroidism. They similarly dissected Finnish-enriched variants at TNRC18 and IL21R, the latter a receptor for an interleukin with established roles in T cell biology. Beyond blood-related traits, the framework explained associations in other tissues, including loci connected to Alzheimer’s disease such as PILRB and TYROBP, genes involved in microglial signaling, illustrating that regulatory mechanisms mapped in immune cells can illuminate neurological and other complex diseases.

The resource is being made broadly available to the research community. Summary statistics for chromatin accessibility quantitative trait loci, expression quantitative trait loci, and peak-gene links are publicly accessible and browsable through an interactive web interface, and the analysis pipelines, including the CASCADE classification framework and supporting software packages, have been released on GitHub. Individual-level data remain protected under Finnish and European data regulations but can be accessed by approved researchers through established application channels. As the field moves toward functional interpretation of the ever-growing catalog of disease-associated variants, atlases of this kind, which connect genetic variation to regulatory architecture across millions of cells and hundreds of donors, are likely to become foundational references for drug target discovery and precision medicine in immunology and beyond.

Subject of Research: Population-scale single-cell multiomic mapping of regulatory genetic variation in human immune cells

Article Title: Population-scale immune multiome atlas reveals regulatory disease mechanisms

Article References: Kanai, M., Delorey, T. M., Honkanen, J., Rodosthenous, R. S., Juvila, J., Murphy, S., Teixeira-Soldano, I., Hwang, H. S., Karjalainen, J., Halonen, J., Panagiotaropoulou, G., Zhang, Y., McCabe, C., Chen, E., Nanki, K., Yoshida, T., Liu, K., Glean, M., Mehrotra, N., … Xavier, R. J. (2026). Population-scale immune multiome atlas reveals regulatory disease mechanisms. Nature. https://doi.org/10.1038/s41586-026-11078-2

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11078-2

Keywords: single-cell multiomics, chromatin accessibility, gene expression, quantitative trait loci, FinnGen, immune cells, regulatory variants, enhancer-gene links, autoimmune disease, genetic fine-mapping, reporter assays, evolutionary constraint

The answer, according to the new work, centers on monocytes, a population of white blood cells that circulates in the bloodstream and is best known as an emergency responder dispatched to sites of infection or injury. The research reveals that in the uterus, monocytes play a far more nuanced role than simply arriving to fight threats. They act as a continuous supply line, replenishing distinct subsets of uterine macrophages, the tissue-resident immune cells that orchestrate tissue remodeling, clear cellular debris and calibrate inflammatory responses throughout the reproductive cycle.

Macrophages are not a uniform population. In tissues across the body, they exist as specialized subsets, each tuned to particular functions. Some are heavily phagocytic, engulfing dead cells and remodelled extracellular matrix. Others are regulatory, producing signals that suppress damaging immune reactions and promote tissue tolerance. In the uterus, this specialization is especially pronounced, because the organ must alternately support inflammatory processes, such as the tissue breakdown that accompanies menstruation or the onset of labor, and anti-inflammatory states, such as the immune tolerance required during pregnancy.

One of the central discoveries of the study is that these uterine macrophage subsets are not self-sustaining. Unlike macrophages in some other tissues, which can persist for long periods by dividing locally, many uterine macrophage populations depend on continual replenishment from blood-borne monocytes. As the reproductive cycle progresses and the uterine landscape shifts from proliferative to secretory to menstrual phases, monocytes are recruited into the tissue and differentiate into the specific macrophage subsets required at each stage. This means the composition of the uterine immune system is actively rebuilt across every cycle rather than being fixed at steady state.

That arrangement is elegant in its responsiveness, but it also creates vulnerability. Because the system depends on a constant influx and correct differentiation of monocytes, any disruption to monocyte recruitment, survival or maturation can ripple outward, altering the balance of macrophage subsets and, with it, the integrity of the tissue environment. The researchers found evidence that such disruptions occur in reproductive disease, where the normal homeostatic loop between monocytes and macrophages breaks down, contributing to maladaptive inflammation and impaired tissue repair.

The concept of dual roles is key to interpreting these findings. On one hand, monocytes serve as builders and maintainers, supplying fresh macrophages that support regeneration and keep inflammation in check. On the other hand, monocytes are the raw material of inflammation itself. When recruited in excessive numbers or instructed inappropriately, they can differentiate into macrophage states that amplify inflammatory damage rather than resolve it. The same cellular pathway that sustains health can, when dysregulated, drive pathology. This duality helps explain why the uterus, with its repeated cycles of wounding and repair, is susceptible to inflammatory and remodeling disorders.

Technically, the study relied on a combination of modern single-cell approaches and lineage tracing, tools that allow researchers to follow individual cells and their descendants over time. By profiling the transcriptomes of uterine immune cells across different phases of the reproductive cycle, the team could map which macrophage subsets were present at each stage and infer their origins. Fate-mapping experiments then demonstrated directly that newly arriving monocytes, rather than long-lived resident cells dividing in place, gave rise to the replenished macrophage pools. Functional analyses connected these cellular dynamics to the tissue-level processes of remodeling and inflammation regulation.

The implications extend across a broad range of reproductive health conditions. Abnormal menstrual bleeding, endometriosis, adenomyosis, recurrent pregnancy loss, preterm birth and poor wound healing after childbirth have all been linked, to varying degrees, with altered macrophage activity in the uterus. The new framework provides a unifying explanation: if monocytes fail to replenish the right macrophage subsets at the right time, or if they replenish the wrong ones, the result can be tissue that remodels abnormally, inflammation that lingers when it should resolve, or repair that proceeds incompletely.

There is also a broader significance beyond reproductive biology. The finding that a major tissue macrophage compartment depends on continuous monocyte supply adds to a growing appreciation that tissue-resident macrophages in many organs occupy a spectrum, with some established before birth and self-maintained, and others continually renewed from blood. Where a given tissue sits on that spectrum likely shapes its disease susceptibility, its regenerative capacity and its response to therapy. The uterus, with its naturally recurring cycles of breakdown and rebuilding, offers an unusually accessible window into these dynamics.

For clinicians and drug developers, the work suggests potential avenues for intervention. If reproductive diseases arise from imbalances in the monocyte-to-macrophage pipeline, then strategies that modulate monocyte recruitment, guide their differentiation toward restorative macrophage states, or correct the signals that misdirect them could offer new treatments. Such approaches remain distant, and the researchers caution that the mechanisms linking monocyte-derived macrophages to specific disease processes still require detailed elaboration. But the study provides a clear cellular target and a conceptual map for future investigation into conditions that affect millions of people worldwide.

Subject of Research: The roles of uterine monocytes in replenishing uterine macrophages and maintaining tissue homeostasis across reproductive cycles in health and disease.

Article Title: The dual roles of uterine monocytes in regulation of tissue homeostasis throughout reproductive cycles in health and disease

Article References: Shahzad, A., Moran, O., Alebrahim, Y., Colombo, S., Lacerda Mariano, L., Shorthouse, O., Morgan, H., Bhatt, D., Scott, N., Hunter, F. K., Laverty, C., Ruane, P., Graham, G., Paterson, I., Kaur, N., Amin, Z., Lokman, M., Lin, I.-H., Murtuza-Baker, S., … Mann, E. R. (2026). The dual roles of uterine monocytes in regulation of tissue homeostasis throughout reproductive cycles in health and disease. Nature Immunology. https://doi.org/10.1038/s41590-026-02651-y

Image Credits: AI Generated

DOI: 10.1038/s41590-026-02651-y

Keywords: uterine monocytes, macrophages, tissue homeostasis, reproductive cycle, endometrium, inflammation, tissue remodeling, immunology, monocyte differentiation, reproductive disease, immune cells, uterus

Working with embryos of the African clawed frog, Xenopus, a long-standing model system for developmental biology, the researchers discovered that local flows of ectodermal tissue serve as a directional cue that polarizes embryonic myeloid cells, the earliest immune cells to arise in vertebrate development, and guides their migration across the embryonic surface. The finding, rooted in live imaging of intact embryos, suggests that physical movement of the tissue itself can act as a compass for patrolling immune cells, a concept that substantially broadens how scientists think about cell guidance during development.

For decades, the dominant framework for understanding directed cell migration has been chemotaxis, in which cells detect spatial gradients of diffusing signaling molecules and reorganize their internal machinery to move toward or away from the source. In immune surveillance, chemoattractants released at sites of injury or infection recruit myeloid cells with remarkable precision. But chemotaxis has never been the whole story. Cells also respond to the stiffness of their surroundings, to adhesive patterns laid down in the extracellular matrix and to mechanical forces transmitted through neighboring cells. The new study adds a striking item to this list of physical guidance mechanisms: the bulk flow of the tissue in which the cells are embedded.

During morphogenesis, sheets of embryonic cells do not sit still. They intercalate, converge, extend and stream past one another as the body plan takes shape, generating large-scale tissue flows that reshape the embryo. Le and colleagues found that these flows, far from being mere background motion, actively inform immune cell behavior. Myeloid cells residing in or on the flowing ectoderm aligned their polarity with the direction of local tissue movement, extending their leading edge downstream of the flow and committing to persistent, directional migration along the current of cells.

The technical achievement behind this discovery lies in the combination of live embryonic imaging with quantitative analysis of both cell trajectories and tissue velocity fields. By tracking individual myeloid cells at high spatial and temporal resolution while simultaneously mapping the displacements of the surrounding ectodermal cells, the researchers could correlate immune cell orientation and migration direction with the local flow vector of the tissue. The statistical coupling between flow and migration was strong and consistent, indicating that the cells were genuinely reading mechanical information from their moving environment rather than drifting passively or following unrelated chemical gradients.

Crucially, the mechanism operates through cell-on-cell interactions. As the ectodermal cells stream past, they exert forces on, and exchange adhesive contacts with, the myeloid cells in their midst. These mechanical and adhesive interactions appear to bias the internal cytoskeletal polarity of the immune cell, coordinating the actin dynamics that drive protrusion at the cell front and contraction at the rear. In other words, the flowing tissue does not carry the immune cells like debris in a river; it actively instructs them, polarizing their intracellular machinery so that they migrate directionally in response to the motion they experience.

This mode of guidance is conceptually distinct from chemotaxis in an important way. A chemical gradient provides positional information through the concentration of a molecule in space, whereas a tissue flow provides information through movement in time. An immune cell embedded in a flowing sheet effectively samples the relative motion of its substrate across its own surface, converting a dynamic mechanical field into an intracellular polarity axis. Such a mechanism has the advantage of being self-organizing: wherever morphogenetic flows occur, guidance comes for free, without the need for pre-patterned chemoattractant sources.

The implications reach well beyond Xenopus. Morphogenetic tissue flows are a universal feature of animal development, driving gastrulation, neurulation and organ formation across species. If migrating cells generally couple their polarity to such flows, then the same guidance principle could steer a wide range of cell types, from neural crest cells that populate the face to primordial germ cells that journey to the gonad, and it could operate in mammalian embryos as well. The finding also suggests a way in which immune surveillance might be automatically coordinated with tissue construction, since the very movements that build the embryo would simultaneously distribute its patrolling immune cells to where they are needed.

There are also implications for regenerative medicine and immunology. Wound healing, inflammation and fibrosis all involve both immune cell migration and tissue remodeling, and the new work suggests these processes may be mechanically intertwined in ways not previously appreciated. If tissue motion can direct immune cells, then therapies or engineered tissues might one day be designed to steer immune responses by controlling the mechanical environment, guiding anti-inflammatory cells into damaged tissue or redirecting cells away from sites where their accumulation causes harm.

Many questions remain. The molecular machinery that translates tissue flow into intracellular polarity, likely involving mechanosensitive adhesion receptors and cytoskeletal regulators, has yet to be fully identified. Whether the same mechanism functions in later developmental stages, in adult tissues or during disease remains to be tested. But the core message of the study is already clear and, for many in the field, eye-opening: developing tissues do not merely tolerate the immune cells traveling through them. They actively carry information, and the flowing fabric of the embryo itself helps guide the sentries of the immune system to their destinations.

Subject of Research: Mechanical guidance of embryonic myeloid cell migration by local tissue flow

Article Title: Tissue flow acts as a guidance cue for immune cell polarization and directional migration

Article References: Le, H. A., Hartmann, J., Alert, R., & Mayor, R. (2026). Tissue flow acts as a guidance cue for immune cell polarization and directional migration. Nature Cell Biology. https://doi.org/10.1038/s41556-026-02058-9

Image Credits: AI Generated

DOI: 10.1038/s41556-026-02058-9

Keywords: tissue flow, immune cells, cell migration, cell polarization, Xenopus embryos, myeloid cells, ectoderm, mechanotransduction, developmental biology, chemotaxis, cell-on-cell interactions, Tissue

The new study, published in the open-access journal Microplastics and Nanoplastics, focused on Eisenia fetida, the composting earthworm that has become the standard model organism for terrestrial ecotoxicology. Rather than examining gross measures of animal health such as weight change, reproduction or survival, the researchers zoomed in on the cellular level, specifically on coelomocytes, the immune cells that circulate in the coelomic fluid of earthworms and perform functions analogous to those of white blood cells in humans. These cells come in two principal flavors: amoebocytes, which patrol tissues and engulf foreign material, and eleocytes, which are derived from chloragocytes and contribute to immunity and nutrient transport.

To test how plastic ingestion reshapes this immune cell population, the team exposed earthworms outside of soil for six days to food pellets under three conditions. One group received pellets with no microplastic particles at all, serving as the mock-treated control. A second group received food containing 10 percent by weight of pure polystyrene microplastic particles ranging from 25 to 75 micrometers in size. A third group received the same polystyrene diet but supplemented with 0.5 percent by weight of Irgafos 168, a phosphite antioxidant widely used in plastic manufacturing to prevent polymer degradation during processing. This additive-design decision is what gives the study its sharpest edge, because most laboratory toxicity tests rely on pristine, additive-free plastic spheres that bear little resemblance to the weathered, chemically loaded particles found in real environments.

One of the study’s most notable methodological achievements was the use of a non-invasive technique to harvest the earthworms’ immune cells. Instead of sacrificing the animals, the researchers induced them to expel coelomic fluid containing coelomocytes, allowing repeated sampling from the same individuals and reducing experimental variability. The recovered cells were then analyzed by flow cytometry, a laser-based technique that can distinguish and count thousands of individual cells per second based on their size, internal complexity and fluorescent labeling. This allowed the team to quantify not just the total number of immune cells, but their viability and the relative proportions of amoebocytes and eleocytes within each sample.

The first key finding was deceptively reassuring: the total number of cells isolated from the worms was not significantly affected by microplastic ingestion. In other words, earthworms eating polystyrene did not simply produce fewer coelomocytes overall. But when the researchers looked at cell viability, a different picture emerged. Earthworms that had ingested pure polystyrene particles showed significantly reduced coelomocyte viability compared with the mock-treated controls. The cells were still there, but a larger fraction of them were dead or dying, a sign that something in the plastic-exposed animals was harming the immune cells themselves.

The damage grew worse when the plastic carried its industrial additive. In worms fed the Irgafos 168-containing polystyrene, the number of living cells per milligram of body fresh weight dropped even further than in the pure-polystyrene group, indicating that the additive amplified the toxicity of the particles. The researchers backed up this in-vivo result with ex vivo assays, exposing isolated coelomocytes directly to the two particle types in the laboratory. Those experiments independently confirmed that particles supplemented with Irgafos 168 were more toxic to the cells than the pure polystyrene particles, strengthening the argument that the chemical additive, not merely the plastic polymer, drives part of the harm.

Why would an antioxidant designed to protect plastics from degradation be harmful to living cells? Irgafos 168 belongs to a class of phosphite compounds that can oxidize over time into phosphate derivatives, and laboratory studies have suggested that both the parent compound and its degradation products can interact with cell membranes and intracellular signaling. Because additives are not covalently bound to the polymer matrix, they can leach out of plastic particles once the material enters the environment and encounters warmth, digestive fluids or microbial action. When an earthworm swallows a contaminated particle, its gut becomes a reaction vessel in which these compounds can be released at close range to the very tissues responsible for defense and nutrient absorption.

Beyond the viability data, the study documented that ingestion of microplastic particles and their additives caused measurable shifts in the distribution of immune cell subpopulations compared with mock-treated worms. Changes in the balance between amoebocytes and eleocytes are more than a bookkeeping detail; they point to a reprogramming of the immune system itself. Amoebocytes are the earthworm’s first line of cellular defense against pathogens, phagocytosing bacteria and encapsulating foreign bodies, while eleocytes participate in immune regulation and reflect the metabolic state of the coelomic cavity. A skewed ratio between these populations could impair an earthworm’s ability to fight off infections, respond to other pollutants, or maintain normal physiological function, even in the absence of visible illness.

The ecological implications extend well beyond a single species in a laboratory feeding trial. Eisenia fetida serves as a surrogate for the broader community of soil-dwelling organisms that face chronic exposure to plastic contamination. Microplastic particles are now documented in agricultural soils across the globe, introduced through sewage sludge, plastic mulch films, irrigation water and the atmospheric deposition of fragmenting debris. Earthworms ingest soil particles indiscriminately as they feed, which means that plastic fragments in the 25 to 75 micrometer range fall squarely within the size class these animals routinely consume. If chronic exposure erodes immune competence in wild populations, soils could become more vulnerable to pathogen outbreaks, and the decomposition processes that underpin nutrient cycling could slow.

The study also carries a broader warning for how microplastic toxicity research is conducted. A growing body of literature has argued that testing pristine, spherical, additive-free particles systematically understates the risks posed by environmental plastics, which arrive pre-loaded with stabilizers, plasticizers, pigments and flame retardants. By deliberately including Irgafos 168 in their experimental design and demonstrating enhanced toxicity, the Bayreuth team has provided concrete experimental support for that argument. The findings suggest that regulatory assessments of microplastic risk, which often focus on the polymer alone, may need to account for the full chemical package that real-world particles carry. For now, the image that emerges from this research is a sobering one: the earthworms that quietly sustain the world’s soils are swallowing our plastic waste, and the chemical hitchhikers riding on that waste appear to reach deep into their cellular defenses, killing the very immune cells that would normally keep them safe.

Subject of Research: Effects of microplastic particle ingestion and plastic additives on earthworm immune cells

Article Title: Microplastic ingestion induces changes in coelomocyte composition of Eisenia fetida

Article References: Fritsche, J. K., Döring, M. V. R., Mauel, A., Senker, J., Feldhaar, H., Freitag, R., & Jérôme, V. (2026). Microplastic ingestion induces changes in coelomocyte composition of Eisenia fetida. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00224-2

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00224-2

Keywords: microplastics, polystyrene, Irgafos 168, Eisenia fetida, coelomocytes, immune cells, flow cytometry, ecotoxicology, soil health, plastic additives, earthworms, environmental toxicity