Role of interleukin (IL)-2 and IL-15 in the tumour progression of a melanoma cell line MELP, derived from an IL-2 progressor patient.

MELP is an interleukin (IL)-2 receptor (IL-2R; alpha+ beta+ gamma-) melanoma cell line that was derived, before the beginning of the immunotherapy, from a patient whose metastasis increased in size during treatment with IL-2/interferon-alpha. In these cells, continuous culture in the presence IL-2 (1000 UI/ml) causes the selection of a cell sub-line (termed MILG) expressing the gamma-chain which is tumorigenic in nude mice. Here, we further analysed the characteristics of MELP and MILG cells as well as clones selected at limiting dilution in the presence of high concentrations of IL-2 or IL-15, or those selected after transfection for the expression of a human IL-2 transgene (MELP-CL1). MELP cells, but not six other melanomas cell lines, shed two soluble immunosuppressive molecules, CD25 and intercellular adhesion molecule-1, whose levels also strongly increase in vivo during immunotherapy. In vitro MELP cells express transcripts for IL-6, transforming growth factor, basic fibroblast growth factor and vascular-endothelial growth factor. Cloning at limiting dilution was obtained in culture fed with IL-2 or IL-15. All these clones, as MILG cells, express the transcript for the IL-2R gamma chain. This could favour improved interactions with cytokines using this chain. By contrast, MELP-CL1 cells, which secrete low amounts of biologically active IL-2 (200 UI/10(6) cells) exhibit a phenotype and growth characteristics similar to those of the parental MELP cells. Indeed, a crosslinking experiment with 125I-IL-2, has showed that MELP and MELP-CL1 cells display a scant IL-2 binding ability that is strongly increased in MELP cells fed for 1 week with 1000 UI/ml IL-2. These cells, as well as MILG cells express a betagamma-complex which can also bind IL-15. IL-2 induces a rapid tyrosine phoshorylation in MILG cells, which is followed by a prolonged induction of c-fos and c-jun genes. By contrast, in MELP cells IL-2 only causes a delayed induction of c-myc gene. All MELP derivatives, but not MILG cells, express the transcripts for IL-15, which is not secreted but is present as an intracellular protein. All MELP cells express the transcript for the IL-15R alpha chain. MELP-CL1 cells are not tumorigenic in nude mice, whereas MILG cells form rapidly growing tumours in 75% of the mice. Coinjection at the same site of MILG and MELP-CL1 cells causes the rapid regression of MILG tumours in 80% of the mice, whereas their bilateral injection causes the rapid development of MILG tumours in 100% of the nude mice. Finally, treatment in nude mice of MILG cells with low amounts of IL-2 (1000 UI per mouse) and IL-15 (50 ng per mouse) induces the development of much more aggressive tumours.The expression of functional IL-2Rs in a subset of human melanomas could be responsible for tumour progression.