The team’s first clue came from combination experiments in living animals. When the researchers paired anti-platelet agents with an antibody that blocks PD-L1—a cornerstone immune checkpoint target exploited by tumors to disarm T cells—they observed a synergistic anti-tumor effect that neither treatment achieved alone. That synergy hinted that platelets were actively contributing to the tumor’s defenses rather than merely bystanders in the tumor microenvironment. When the investigators looked more closely at tumors, they found that activated platelets were adhering directly to tumor-infiltrating CD8-positive T cells. Critically, when platelet activity was suppressed alongside PD-L1 blockade, this adhesion was reduced, and the T cells showed enhanced activation. In other words, stripping platelets away from the immune cells appeared to unleash at least some of the anti-tumor firepower that checkpoint inhibitors are designed to restore.

To determine whether this was a direct cause-and-effect relationship rather than an indirect consequence of the tumor environment, the team built a controlled laboratory system that isolated the players. They used cytotoxic T lymphocytes derived from induced pluripotent stem cells—a line known as CTL3-3—engineered to specifically recognize the WT1 antigen, a protein expressed on many cancer cells and a well-established tumor-associated target. When intact platelets were added to the co-culture of these antigen-specific killer cells and cancer cells, the platelets suppressed the T cells’ cytotoxic activity. The suppression was unmistakable: T cells that would otherwise have efficiently killed their WT1-expressing targets became measurably less lethal in the presence of platelets.

One obvious question was whether platelets were exerting this effect through soluble factors—chemical signals released into the surrounding fluid—rather than through physical contact. To test this, the researchers collected supernatants, the liquid fractions from cultures of activated platelets, and applied them to the T cells. These supernatants did partially suppress cytotoxicity, suggesting that platelet-derived soluble mediators contribute something to the inhibitory effect. But the suppression achieved by soluble factors alone was consistently weaker than the suppression caused by intact platelets. That difference was telling. It implied that the dominant mechanism required the platelets themselves to be physically present, in contact with the T cells, rather than merely whispering chemical instructions from a distance.

Flow cytometry analysis then revealed the nature of that physical contact. Activated platelets express P-selectin, also known as CD62P, a transmembrane adhesion molecule stored in platelet granules and rapidly displayed on the cell surface when platelets are activated. The analysis showed that these P-selectin-bearing platelets directly interact with the CTL3-3 T cells. The team confirmed the interaction with two independent imaging approaches: immunofluorescence staining, which visualized the molecular partners at the interface between the cells, and electron microscopy, which captured the ultrastructural detail of platelets clinging to T cell surfaces. Together, these techniques established that the platelet-T cell adhesion was not an artifact of the assay but a reproducible, physically verifiable phenomenon.

The molecular identity of the T cell-side partner proved to be PSGL-1, the canonical ligand for P-selectin. PSGL-1 is a glycoprotein best known for its role in guiding leukocytes through the bloodstream and into tissues during inflammation, where it engages selectins on vascular endothelial cells. The researchers used neutralizing antibodies to block either P-selectin on the platelets or PSGL-1 on the T cells. Either intervention inhibited the adhesion between the two cell types and, crucially, restored the cytotoxic activity of the T cells in the presence of platelets. This bidirectional blockade experiment is the strongest evidence in the study that the P-selectin–PSGL-1 axis is the functional conduit through which platelets suppress tumor killing. When the handshake is interrupted, the T cells recover their ability to destroy their targets.

Equally important is what the platelets did not do. The researchers found that the interaction suppressed the antigen-specific killing activity of the T cells without affecting their proliferation. The T cells continued to divide and expand normally; they simply became less effective executioners while in contact with platelets. This distinction matters for therapeutic thinking. A mechanism that throttles the cytotoxic function of T cells while leaving their numbers intact suggests a reversible, contact-dependent form of immune regulation—one that could potentially be switched off pharmacologically without causing the broader immune dysfunction that might accompany agents that deplete or globally suppress T cells. It also distinguishes this mechanism from classical checkpoint pathways such as PD-1/PD-L1, which alter T cell activation states through signaling rather than through physical sequestration.

The broader context of this work sits at the intersection of hematology and tumor immunology, a crossroads that has attracted growing attention in recent years. Platelets are known to interact with cancer cells in ways that facilitate tumor progression and enhance malignancy—they shield circulating tumor cells from immune surveillance in the bloodstream, promote metastatic seeding, and release a cargo of growth factors and immunomodulatory molecules. Cancer patients frequently exhibit platelet activation and thrombocytosis, and high platelet counts have been associated with poorer prognosis across multiple tumor types. What has remained murky is the direct effect of platelets on the anti-tumor activity of CD8-positive T cells, the population that immunotherapies from checkpoint inhibitors to engineered T cell therapies ultimately depend upon. By demonstrating a direct, receptor-defined inhibitory interaction, this study fills a conceptual gap and provides a mechanistic explanation for why anti-platelet strategies might synergize with immunotherapy.

The therapeutic implications are tantalizing but must be tempered with caution. The combination of anti-platelet agents and PD-L1 blockade produced synergistic anti-tumor effects in the preclinical setting, and the mechanistic work identifies P-selectin and PSGL-1 as druggable nodes—both are accessible to neutralizing antibodies, and agents targeting the selectin pathway already exist in other clinical contexts. Yet translating these findings into patients will require careful navigation of platelet biology’s essential role in hemostasis. Systemic platelet inhibition carries bleeding risks, and any strategy would need to selectively disrupt the platelet-T cell interaction without compromising clotting. Blocking the P-selectin–PSGL-1 axis specifically, rather than platelet function broadly, may offer a narrower therapeutic window, but this remains to be tested in clinical studies. The current work was conducted in animal models and in vitro systems, and tumor immunology is notorious for promising mechanisms that fail to survive translation.

Nevertheless, the study reframes platelets from passive clotting agents into active participants in the immune battlefield of cancer, and it does so with unusual mechanistic precision—identifying the cell types, the contact dependence, the receptor-ligand pair, and the functional consequence, all within a single coherent experimental arc. For a field racing to explain why immunotherapies work spectacularly in some patients and fail in others, the discovery that activated platelets can handcuff CD8-positive T cells through P-selectin binding offers a new variable to measure, a new biomarker to explore, and a new combination to test. If subsequent studies confirm that the same interaction operates in human tumors and that disrupting it safely enhances checkpoint inhibitor responses, the humble platelet—long celebrated for saving lives one clot at a time—may find itself recast as an accomplice of cancer, and its inhibition as an unexpected weapon in the immunotherapy arsenal.

Subject of Research: The inhibition of CD8-positive T cell anti-tumor activity by platelet P-selectin binding to PSGL-1

Article Title: P-Selectin on activated platelets inhibits the anti-tumor activity of CD8+ T cells through PSGL-1 binding

Article References: Nishiguchi, S., Yokomura, M., Gomibuchi, Y., Nagano, S., Yasunaga, T., Hirohashi, Y., Kawamoto, H., Takagi, S., & Katayama, R. (2026). P-Selectin on activated platelets inhibits the anti-tumor activity of CD8+ T cells through PSGL-1 binding. Cancer Immunology, Immunotherapy. https://doi.org/10.1007/s00262-026-04578-7

Image Credits: AI Generated

DOI: 10.1007/s00262-026-04578-7

Keywords: platelets, P-selectin, PSGL-1, CD8-positive T cells, cancer immunotherapy, PD-L1 blockade, WT1 antigen, cytotoxic T lymphocytes, tumor microenvironment, immune checkpoint inhibitors, iPSC-derived T cells, tumor immunology

The strategy grew out of a long-standing interest in cellular senescence, a stress response that permanently halts cell division. Senescent cells are famously double-edged. When a cell accumulates dangerous damage, senescence shuts it down before it can become cancerous, and during wound healing, senescent cells coordinate repair by broadcasting signals that recruit other cells to the injury site. The trouble begins when these cells overstay their welcome. Instead of quietly disappearing once their job is done, lingering senescent cells keep emitting inflammatory and profibrotic signals that drive chronic inflammation and scarring. Scott Lowe, who chairs the Cancer Biology and Genetics Program at MSK’s Sloan Kettering Institute and is a Howard Hughes Medical Institute investigator, has spent years probing this duality. The therapeutic dream, he explains, is to clear the harmful senescent cells while sparing the helpful ones—a goal that has proven difficult because senescent cells in different contexts can look deceptively similar.

The breakthrough came when the researchers examined macrophages, the scavenger immune cells that patrol tissues, in fibrotic environments. Within these populations, they identified a small subset expressing a surface protein called P-selectin alongside multiple molecular hallmarks of senescence. The team dubbed these cells MΦP+sen+—macrophages that are both P-selectin positive and senescence positive. What struck the scientists was their consistency: the same distinctive population appeared in fibrotic tumor niches in liver and lung cancers and in heavily scarred liver and lung tissue, and its presence correlated with poorer responses to immunotherapy in patients. Clemens Hinterleitner, a postdoctoral researcher and co-first author of the study, notes that finding this specific macrophage population so reliably in fibrotic tissue was remarkable, and that its presence signaled a compromised local immune environment.

These cells are far more than innocent bystanders. According to the study, MΦP+sen+ macrophages contribute to the fibrosis that forms a physical barrier around tumors, and they simultaneously dampen the activity of neighboring immune cells by sending out profibrotic and immunosuppressive signals. Co-first author Valentin Barthet describes them as actively erecting a wall that prevents the immune system from doing its job. The cells make up only about 3 to 5 percent of fibrotic tissue, yet experiments in mouse models of liver and lung fibrosis and fibrotic liver and lung cancers showed that selectively killing them or silencing their signals had a profound effect on fibrosis. The team validated the key findings in tumor samples from lung cancer patients treated at MSK and in human lung and liver cancer datasets, lending clinical weight to the mouse data.

The consequences of eliminating or disabling these macrophages were dramatic. The scar tissue shielding the tumor began to dissolve, and the suppression of other immune cells eased. Freed from both the physical and chemical barriers, checkpoint inhibitor immunotherapy—the class of drugs that unleashes T cells against cancer—suddenly worked in mice, shrinking tumors that had previously been resistant to treatment. In other words, the intervention did not attack the cancer cells directly. It removed the ecosystem support that allowed the tumor to hide, converting a cold, immune-excluded tumor microenvironment into one that existing therapies could penetrate.

Delivering a lethal payload to such a small and specific cell population posed a formidable engineering challenge, and this is where the collaboration with Daniel Heller’s biomedical engineering lab proved decisive. Heller’s group had previously developed a nanoparticle platform that targets P-selectin, and the team adapted it with new drugs and a new mission. The particles are roughly a thousand times smaller than the width of a human hair and are coated with fucoidan, a naturally occurring sugar molecule found in brown seaweed and sold as an anti-inflammatory supplement. Fucoidan binds specifically to P-selectin, ensuring that the nanoparticles are taken up not by just any macrophage but by the precise P-selectin-positive subset driving the fibrosis. When the targeted macrophages ingest and digest a particle, the drug inside is released—an effect Heller likens to soldiers springing from a Trojan Horse to launch a precision attack from within.

The researchers loaded the nanoparticles with two drugs that are potent against senescent cells but have been hampered by toxicity in clinical use. The first, Navitoclax, exploits a vulnerability in senescent cells’ survival machinery: these stressed cells block their own natural self-destruct program, and Navitoclax removes that block, causing them to die. The second, a BET inhibitor called dBET6, takes a different tack—it silences the inflammatory signals that senescent cells broadcast into surrounding tissue. With the alarm switched off, the cells stop producing the signals that drive fibrosis and immune suppression, even though the cells themselves survive. Both drugs had shown promise against cellular senescence in the laboratory but failed clinically because, when administered conventionally, they damage healthy cells and tissues throughout the body. In the mouse experiments, the nanoparticle-packaged versions appeared safe and did not produce the same toxicity, raising the possibility that targeted delivery could give both drugs a second chance in the clinic.

The implications may extend well beyond liver and lung cancer. Searching a large database of immune cell profiles, the researchers found the same harmful macrophage population associated with breast and colon cancers as well as chronic inflammatory diseases including osteoarthritis and rheumatoid arthritis. Aveline Filliol, a senior scientist in the Lowe Lab and co-corresponding author, points out that these macrophages also share important features with lipid-associated macrophages, a cell type found in the atherosclerotic plaques that cause heart disease—suggesting the platform could one day be adapted to deliver drugs to those cells as well. If MΦP+sen+ cells prove to be a common culprit across many diseases, a single nanoparticle platform, retargeted with different drug cargoes, could address an unusually wide range of conditions united by a shared cellular mechanism.

Beyond the specific findings, the work exemplifies an increasingly influential strategy in oncology: targeting the cancer ecosystem rather than the tumor cells alone. Cancer cells, Lowe argues, do not act in isolation; building a tumor-supportive environment requires coordinated interactions between malignant cells and the surrounding tissue, and disrupting those interactions represents a significant new frontier. The study also showcases the kind of interdisciplinary collaboration—between biologists and engineers, and between bench scientists and clinicians—that allowed the team to move from a biological observation to an engineering solution to validation in human tissue within a single institution. MSK is now seeking commercialization partners to advance the research into clinical trials, first in cancer and potentially in other fibrotic and inflammatory diseases. A patent application covering the use of fucoidan nanoparticles to treat senescence-associated pathologies has been published, and related intellectual property has been granted, signaling that the road from laboratory insight to patient benefit is already under construction. If the approach survives the rigors of human testing, it could offer a way to make immunotherapy work for the many patients it currently fails—and to treat the scarred, inflamed tissue that underlies diseases far beyond cancer.

Subject of Research: Senescence-directed nanoparticle targeting of fibrosis-driving macrophages to overcome immune exclusion in cancer

Article Title: Using nanoparticles to target fibrosis in cancer and other diseases

Article References: Using nanoparticles to target fibrosis in cancer and other diseases. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: nanoparticles, fibrosis, cellular senescence, macrophages, immunotherapy, cancer, P-selectin, fucoidan, Navitoclax, BET inhibitor, tumor microenvironment, checkpoint inhibitors