Nucleic acid molecules that encode IL-15 or fragments thereof, which express protein at a higher level than nucleic acid molecules with native coding sequences for IL-15 are disclosed. Nucleic acid molecules with additional modifications such as the absence of coding sequences for IL-15 signal sequences and/or the absence of IL-15 untranslated sequences and/or inclusion of non-IL-15 signal sequences are also disclosed. Vectors, including plasmids and viral vectors, comprising such nucleic acid molecules; and to host cells comprising such nucleic acid molecules are disclosed as well as methods of using such nucleic acid molecules alone or in combination with nucleic acid sequences encoding immunogens which are part of the nucleic acid molecules and/or part of a different nucleic acid molecule. Recombinant vaccines and live attenuated pathogens encoding fusion proteins, and methods of using the same, are disclosed.