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 t wo proteins fused at the C terminus with green fluorescent protein (GFP). C onfocal microscopy analyses showed that, when transfected in CHO cells, 48L IL-15/GFP was localized in the Golgi apparatus and in early endosomes, whi le 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 take n up in endosomes by untransfected CHO cells, indicating that endosomal loc alization 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 e xpressing 21L IL-15/GFP or untagged 48L or 21L IL-15. Treatment with brefel din A or with inhibitors of N-linked glycosylation further indicated that t he 48L IL-15/GFP is secreted through the endoplasmic reticulum/Golgi pathwa y. Our data suggest a different trafficking of the two IL-15 isoforms and m ultiple mechanisms controlling IL-15 secretion.