The present invention relates to agents that modulate interleυkin-15 (“IL-15”) signal transduction or function (“Therapeutic Agents”) and the use ol” those agents to modulate immune function. The Therapeutic Agents target the interaction between IL-15 and its receptor and modulate IL-15-induced signal transduction. The Therapeutic Agents may be formulated with polymers, such as poly-β-1-♦4-N-acetylglucosamine. for administration to a human subject to modulate IL-15-mediated immune function.