Nucleic acid molecules that encode IL-15 or fragments thereof, which expressprotein at a higher level than nucleic acid molecules with native codingsequences for IL-15 are disclosed. Nucleic acid molecules with additionalmodifications such as the absence of coding sequences for IL-15 signalsequences and/or the absence of IL-15 untranslated sequences and/or inclusionof non-IL-15 signal sequences are also disclosed. Vectors, including plasmidsand viral vectors, comprising such nucleic acid molecules； and to host cellscomprising such nucleic acid molecules are disclosed as well as methods ofusing such nucleic acid molecules alone or in combination with nucleic acidsequences encoding immunogens which are part of the nucleic acid moleculesand/or part of a different nucleic acid molecule. Recombinant vaccines andlive attenuated pathogens encoding fusion proteins, and methods of using thesame, are disclosed.
