ABSTRACT: Background: Electrochemotherapy (ECT) utilizes reversible electroporation for localized chemotherapeutic delivery, where beyond local cytotoxicity, ECT acts as an in situ immunomodulator. However, the dynamic spatial orchestration of the TME that translates this localized therapy into potential abscopal-like responses in untreated lesions remains poorly understood. Methods: To uncover the mechanisms of evolving tumor microenvironment (TME) that dictate this immune activation, we applied Xenium spatial transcriptomics to longitudinally sampled in-transit cutaneous melanoma metastases from three patients, capturing baseline immune states and two distinct post-ECT intervals (two-to-three and nine-to-ten days). Furthermore, to explicitly characterize potential abscopal-like effects, biopsies were collected also from non-treated lesions. Following sample integration, we annotated cell types and identified functional subpopulations of key compartments, which we compared across treatment timepoints and patients. Further, we resolved functional spatial niches and identified spatially localized immune front, which we characterized and linked to patients' clinical responses. Results: Our analysis uncovered heterogeneous patient trajectories: a robust local and regional responder, a localized-only responder, and a delayed regional responder. These divergent outcomes were linked to baseline TME heterogeneity that was reflected by diverging proportions of functional spatial niches. While the partial responder exhibited an immune-cold architecture, complete responders were characterized by an immune-permissive TME with a prominent immune front at the interface of malignant and immune-active niches. Molecular profiling of these spatial boundaries exposed patient-specific signatures of activation and suppression within infiltrating T cells, myeloid cells, and fibroblasts. Although the states of malignant cells remained predominantly unchanged in the partial responder, the malignant cells of complete responders transitioned into a unique SOX2-OT-expressing dormant state, sequestered within keratinocyte-rich zones and characterized by an immunosuppressive signature post-treatment. Conclusions: Together, our findings suggest that beside local ablation, ECT might actively instigate regional antitumor immunity, a therapeutic effect that we observed in two patients harboring a permissive TME. Further, our deep spatial TME profiling identifies several exploratory observations on the effects of ECT that should be addressed in future studies.
