Differential intracellular trafficking, secretion and endosomal localization of two IL-15 isoforms

To analyze the intracellular trafficking of two IL-15 isoforms bearing 48- or 21-amino acid leader peptides (L), we have generated cDNA encoding the two proteins fused at the C terminus with green fluorescent protein (GFP). Confocal microscopy analyses showed that, when transfected in CHO cells, 48L IL-15/GFP was localized in the Golgi apparatus and in early endosomes, while 21L IL-15/GFP was detectable only in the cytosol. The presence of 48L IL-15/GFP in endosomes was confirmed by enzyme-linked immunosorbent assay on endosome-enriched subcellular fractions. Exogenous IL-15 was bound and taken up in endosomes by untransfected CHO cells, indicating that endosomal localization was, at least in part, related to a receptor-mediated uptake. The 48L IL-15/GFP fusion protein was efficiently secreted by COS-7 or CHO cell transfectants, while IL-15 secretion was less efficient in transfectants expressing 21L IL-15/GFP or untagged 48L or 21L IL-15. Treatment with brefeldin A or with inhibitors of N-linked glycosylation further indicated that the 48L IL-15/GFP is secreted through the endoplasmic reticulum/Golgi pathway. Our data suggest a different trafficking of the two IL-15 isoforms and multiple mechanisms controlling IL-15 secretion.