Interactions Between Tryptase-Positive Mast Cells and Melanin-A⁺ Cells in the Microenvironment of Cutaneous Melanoma

Cutaneous melanoma remains one of the most aggressive tumors, yet the role innate immunity plays in its progression remains poorly understood. Effector elements with high regulatory potential, capable of both promoting and inhibiting tumor growth-mast cells (MCs), are of particular interest. This includes quantitatively characterizing the interactions between tryptase-positive mast cells (MCs) with atypical Melanin-A⁺ cells and describing their spatial phenotype, in relation to the stage of cutaneous melanoma. A retrospective analysis was carried out on samples retrieved from 128 patients with cutaneous melanoma (AJCC 8th edition: IA-IIID). Histological analysis, histochemistry (toluidine blue, Giemsa), and diplex /multiplex IHC for tryptase and Melan-A were performed; as well as Fluorescence imaging, 3D reconstructions and quantitative mapping in QuPath v 0.6.0. Proximity was assessed by the nucleus-to-nucleus distance: <10 μm (contact), 10-20 μm (paracrine zone), >20 μm (out of interaction). The relative amount of MCs in the intratumoral zone was lower than in the intact dermis, with a simultaneous increase in their absolute density per mm² in the melanoma microenvironment, maximum in the peritumoral area and most pronounced at stage II. Three types of interactions were identified: (i) juxtaposition without secretion, (ii) degranulation of MCs directed to tumor cells, (iii) melanosecretion of Melanin-A⁺ cells directed towards MCs, followed by phagocytosis of melanocores. An inverse intratumoral connection between the number of MCs and the number of Melanin-A⁺ cells was noted; MCs with elongated forms, extensive contacts and polarized tryptase secretion, including granule localization near/at the nuclei of adjacent cells, were frequently observed. The obtained data indicate stage-region-dependent bidirectional cross-talk between melanin and MCs, forming tissue spatial signals, potentially useful as biomarkers and targets for personalized therapy.