EXPRESSION OF 2 INTERLEUKIN-15 MESSENGER-RNA ISOFORMS IN HUMAN TUMORSDOES NOT CORRELATE WITH SECRETION - ROLE OF DIFFERENT SIGNAL PEPTIDES

Interleukin (IL)-15 is a four-helix bundle cytokine sharing several bi ological properties with IL-2. By reverse transcriptase-polymerase cha in reaction analysis, human cancer cell lines of different histotypes are shown to express two IL-15 amplification products: a 524-bp band c orresponding to the IL-15 mRNA found in macrophages, and another of 64 3 bp corresponding to an alternatively spliced mRNA including a 119-bp alternative exon. IL-15 was undetectable in the supernatant of tumor cell lines expressing either one or both of the mRNA isoforms as evalu ated by a bioassay or by ELISA, indicating that IL-15 is not secreted. However, IL-15 could be detected intracellularly in some tumor cells by confocal microscopy analysis. Since the pre-proteins encoded by the two mRNA isoforms differ in the signal peptide sequence, we have anal yzed the characteristics of these signal peptides and their possible r ole in controlling secretion. The two IL-15 cDNA isoforms, expressed i n COS-7 cells, induced very low levels of IL-15 secretion. However, su bstitution of the sequence encoding natural signal peptide(s) with the one from IgV kappa chain in the IL-15 cDNA results in a significantly higher secretion of biologically active IL-15-(15-30-fold) upon cDNA transfection. A poor efficiency of natural signal peptides may represe nt one of the mechanisms involved in the control of IL-15 secretion.