The obstacle that has defeated photodynamic therapy, or PDT, in pigmented melanoma is fundamentally optical and chemical. PDT relies on photosensitizing drugs that, when illuminated with light of a specific wavelength, generate reactive oxygen species, including excited-state singlet oxygen, that kill tumor cells directly or shut down the blood vessels feeding the tumor. Melanin undermines this at every step: it absorbs and scatters light so effectively that very little reaches deeper tissue, and it chemically neutralizes reactive oxygen species before they can inflict lethal damage. Many melanoma cells also actively pump photosensitizers back out of the cell. In measurements on excised S91 murine melanoma reported in the new study, transmission of 690-nanometer treatment light fell by four orders of magnitude through just 2 millimeters of tissue, and light became undetectable beyond 3 millimeters.

The research team, led by L. Pires and B. C. Wilson of Texas A&M University and the Princess Margaret Cancer Center in Toronto, did not try to blast through this barrier with ever-stronger light. Instead, they paired two photosensitizers already in clinical use: Photodithazine, a chlorin e6 derivative that concentrates in tumor cells, and Visudyne, a liposomal benzoporphyrin formulation that targets the tumor microvasculature. Each drug was illuminated at its own red absorption peak, delivered through an optical fiber, and a topically applied optical clearing agent made of PEG-400 and propanediol was rubbed into tape-stripped skin to reduce light scattering and improve penetration. The protocol was first shown to eradicate thin, pigmented tumors, but in immunodeficient mice the animals still succumbed to metastatic spread.

The decisive change in the new work was testing the identical protocol in mice with intact immune systems, and against tumors deliberately grown to 3 to 4 millimeters thick, several times deeper than any effective light dose could reach. In immunocompetent DBA mice, the treated tumor site became intensely inflamed within hours, ulcerated within days, and healed completely by about three weeks, leaving no trace of tumor. Nineteen of 20 treated mice, 95 percent, survived to the 90-day endpoint without disease. In nude mice lacking T cells, the same treatment produced only superficial damage, and every animal died of tumor overgrowth with a median survival of just 21 days, indistinguishable from untreated controls. The dramatic split between the two groups is, the authors argue, only explicable if the immune system finished what the light started.

How could light that penetrates only the top millimeter of a tumor destroy the entire mass? The answer, according to the study’s mechanistic analysis, is that the photodynamic damage to the superficial layer acts as a biological alarm. Dying cancer cells release damage-associated molecular patterns such as calreticulin, ATP and HMGB1, molecules that alert dendritic cells and other antigen-presenting cells. These cells ingest tumor debris, carry tumor antigens to the draining lymph nodes, and prime cytotoxic CD8-positive T cells that can recognize and kill melanoma cells anywhere in the body, including regions the light never touched. The researchers have named this cascade photodynamic immune stimulation, or PDIS.

The immune signatures measured after treatment were striking in their breadth and speed. Within three to six hours, CD3-positive T cells and CD19-positive B cells flooded into the tumor, while populations of Ly-6G-positive, Ly-6C-positive myeloid cells associated with immunosuppression fell. By 12 hours, immunohistochemistry showed T cells distributed throughout the tumor parenchyma, not merely clustered at the margins. In blood, eosinophils rose sharply while circulating lymphocytes dropped, consistent with migration toward the tumor, and activated CD69-positive T cells increased in both blood and lymph nodes. Plasma levels of inflammatory messengers including IL-2, IL-6, IL-18, ENA-78 and GRO-alpha surged, returning toward baseline by two weeks.

Gene expression profiling of treated tumors revealed a wholesale transcriptional shift, most pronounced at 24 hours. Gene Ontology analysis showed enrichment of pathways governing lymphocyte differentiation, cytokine signaling, leukocyte proliferation and myeloid cell differentiation, while KEGG pathway analysis highlighted IL-17 signaling, chemokine signaling, natural killer cell-mediated cytotoxicity, T-cell and B-cell receptor signaling, Toll-like receptor signaling and pathways of apoptosis and cellular senescence. Most tellingly, T cells in the draining lymph nodes shifted over time from naive and central-memory phenotypes toward effector-memory cells, the population capable of rapid cytotoxic attack on re-encountering tumor antigens, the cellular signature of durable immune memory.

A series of functional experiments made the causal case. When CD8-positive T cells were depleted with antibodies before treatment, five of six mice died of tumor overgrowth within 34 days, collapsing the 95 percent disease-free survival down toward control levels and proving that cytotoxic T cells are the dominant effectors. In adoptive transfer experiments, splenocytes from treated mice mixed with melanoma cells and injected into healthy recipients slowed tumor growth dramatically, without any attack on healthy tissue, indicating tumor-specific memory. In mice bearing tumors on both flanks, treating only one tumor completely eradicated it and significantly delayed growth of the untreated opposite tumor, an abscopal effect that extended median survival from 6 to 15 days. Finally, when mice cured 30 days earlier were injected intravenously with a million melanoma cells, all remained tumor-free, while every control mouse developed bulky lung and peritoneal metastases.

The authors are careful about the limits of the work. All experiments used male mice and a single murine melanoma cell line, so the findings must be validated across genetically diverse models before clinical translation. They also note that the light gradient through pigmented tissue may generate different cell death pathways at different depths, potentially shaping the strength of the immune response, a relationship they are now investigating to optimize dosing. Even so, male mice are known to mount weaker immune responses than females, making the observed effect particularly notable, and the fact that melanin’s barrier could be sidestepped rather than defeated challenges two decades of assumptions in the field.

The clinical possibilities outlined by the team are tantalizing. PDIS could be delivered shortly before surgical resection, exploiting the perioperative window to counter the immune suppression that surgery itself induces and to attack unsuspected micrometastases. Its ability to convert immunologically cold tumors into hot ones suggests rational combination with immune checkpoint inhibitors, preclinical studies of which are planned. The measured immune biomarkers could guide patient selection and real-time monitoring, extending dosimetry beyond drug uptake and light dose to include the tumor’s immune landscape. The same principle may extend to uveal melanoma, where the group’s multiphoton PDT work has already shown lymphocyte activation with partial tumor targeting, and to immunotherapy-resistant cancers such as pancreatic and lung tumors. For a disease whose five-year survival collapses to roughly 30 percent once it metastasizes, a low-toxicity treatment that both destroys the primary tumor and vaccinates the body against recurrence would represent a genuinely new chapter in melanoma care.

Subject of Research: Photodynamic immune stimulation as an immunotherapy strategy for pigmented melanoma

Article Title: Photodynamic immune stimulation eradicates primary pigmented melanoma and induces long-lasting anti-tumor immunity

Article References: Pires, L., Calcada, C., Cerqueira-Rodrigues, B., Fan, Y., George, J. T., McGaha, T., & Wilson, B. C. (2026). Photodynamic immune stimulation eradicates primary pigmented melanoma and induces long-lasting anti-tumor immunity. British Journal of Cancer. https://doi.org/10.1038/s41416-026-03585-w

Image Credits: AI Generated

DOI: 10.1038/s41416-026-03585-w

Keywords: melanoma, photodynamic therapy, immunotherapy, photosensitizers, CD8 T cells, abscopal effect, immune memory, tumor microenvironment, optical clearing, immunogenic cell death, preclinical study, British Journal of Cancer