Evidence of immunogenic cell death (ICD) and ICD-dependent dendritic cell activation induced by extracorporeal photopheresis in patients with leukaemic forms of cutaneous T-cell lymphoma.

Background: Despite novel therapeutic options, the long-term management of cutaneous T-cell lymphoma (CTCL) remains challenging. Extracorporeal photopheresis (ECP) is an immunomodulating photochemotherapy associated with higher overall survival when used for the treatment of leukaemic forms of CTCL. Its exact mode of action has not been fully elucidated. Immunogenic cell death (ICD) is pivotal in cancer immunotherapy, marked by the release of damage-associated molecular patterns that enhance dendritic cell (DC) maturation and cytotoxic T-lymphocyte responses.

Objectives: To explore ICD in patients with leukaemic forms of CTCL during ECP and its effect on DC activation.

Methods: We conducted in vitro studies with peripheral blood mononuclear cells (PBMCs) from healthy donors and ex vivo experiments with white blood cells (WBCs) from patients with leukaemic forms of CTCL undergoing ECP. We assessed cell viability, apoptosis and ICD markers [ATP, high mobility group box 1 protein (HMGB1), calreticulin] using flow cytometry, enzyme-linked immunosorbent assay and quantitative polymerase chain reaction. Engulfment assays evaluated DC activation by ECP-treated CD4+ T cells.

Results: ECP-treated healthy PBMCs and WBCs from patients with leukaemic forms of CTCL showed a significant induction of ICD hallmarks, including ATP release, HMGB1 secretion and calreticulin surface exposure. In patients with leukaemic forms of CTCL, calreticulin exposure was mainly present in CD4+CD26- T cells, indicating greater ICD susceptibility of malignant T cells. ECP-treated CD4+ T cells were phagocytosed by DCs, a process that was found to be dependent on ICD signals.

Conclusions: ECP induces ICD in malignant T cells and, to a lesser extent, in healthy T cells, facilitates DC activation. These findings suggest that ECP enhances targeted immune responses against malignant T cells in leukaemic forms of CTCL, offering new insights into its therapeutic mechanisms and potential applications in cancer immunotherapy.