Interleukin-15 (IL-15) signaling is heavily regulated by a high specificity IL-15 binding protein known as interleukin-15 receptor alpha (IL-15R alpha). In-vivo disruption of IL-15R alpha in the constitutive IL-15R alpha knockout (IL-15R alpha KO) mouse results in a shift towards an oxidative muscle phenotype characterized by dramatic increases in mitochondrial density. The IL-15R alpha KO mouse displays elevated levels of IL-15 transcript in muscle tissue, along with increased circulating levels of IL-15. As a result, it has been suggested that loss of IL-15R alpha from skeletal muscle enhances muscle IL-15 secretion, and that muscle-derived IL-15 acts in an autocrine fashion to elicit pro-oxidative effects. However, this proposed mechanism of IL-15/IL-15R alpha action in skeletal muscle is based primarily on in-vivo associative observations, and has yet to be explored in a direct manner. Thus, our purpose was to assess the immediate influence of IL-15R alpha on the capacity of skeletal muscle to secrete and respond to IL-15, and also to determine whether IL-15 has the ability to act directly on skeletal muscle to induce pro-oxidative changes. These aims were addressed in-vitro using primary myogenic cultures derived from IL-15R alpha KO mice and B6129 controls, as well as cultures of the C2C12 immortalized myogenic cell line. Cultures obtained from IL-15R alpha KO mice displayed a diminished capacity to secrete IL-15 in relation to 86129 controls. Acute treatment of B6129-derived cultures with recombinant IL-15 increased transcriptional expression of the pro-oxidative genes PGC1 alpha and PPARS. IL-15 treatment failed to elicit a similar response in cultures generated from IL-15R alpha KO mice. Chronic treatment of C2C12 cultures with IL-15 during myogenic differentiation resulted in mature myocytes with greater mitochondrial density in relation to vehicle treated controls. Collectively, these results provide evidence that IL-15 has the capacity to act directly on skeletal muscle in a pro-oxidative manner, and that disruption of IL-15R alpha ablates the ability of skeletal muscle to secrete and respond to IL-15. (C) 2015 Elsevier Inc. All rights reserved.