Spontaneous Regression of Swine Melanoma: The Role of Tumour-infiltrating T and NK Cells

Abstract

Background/Aim: Melanoma is a skin cancer without effective therapy, showing high immunogenicity and mostly partial spontaneous regression (SR). The exact mechanisms of SR are still not well understood; therefore, the use of animal melanoma models is necessary to unravel the immunological processes during SR. Materials and Methods: Skin melanoma samples (n=57) and peripheral blood samples (n=57) from the same animals were collected. Melanoma-bearing Libechov Minipigs (MeLiM) aged 3, 4, 6, 8, 10, 12, 20, and 32 weeks were used, and samples were analysed by flow cytometry for detection of immune cell subpopulations. Results: The proportion of CD3–CD8+ (NK) cells in melanoma samples was found to be higher compared to blood samples at 6-8 weeks of age and then at 12 weeks of age. The population of CD4+CD8+ (effector/memory T helper) cells and CD4–CD8+ (cytotoxic T and NK) cells was also increased in melanoma compared to blood samples in 10-32-week-old pigs. The proportion of CD4–CD8+ cells in melanoma samples, then augmented until the 32nd week. On the contrary, the proportion of CD4+CD8– (naive T helper) cells was lower in melanoma samples versus blood samples in 6-32-week-old animals. Conclusion: Cytotoxic T cells were the most abundant population of tumour infiltrating immune cells found in MeLiM melanomas of animals aged 10-32 weeks, probably causing the destruction of melanoma cells. Furthermore, the development of specific (adaptive) immune response represented mainly by cytotoxic T cells seems to be crucial for the successful SR of porcine melanoma.