Serum and muscle interleukin-15 levels decrease in aging mice: Correlation with declines in soluble interleukin-15 receptor alpha expression

Interleukin-15 (IL-15) is a skeletal muscle-derived cytokine with favorable effects on muscle mass and body composition. Modulation of IL-15 levels has been suggested as a treatment for sarcopenia and age-associated increases in adiposity. However, it is unclear whether IL-15 levels change during aging, as measurement of IL-15 at physiological concentrations in mice has been technically difficult, and translational regulation of IL-15 is complex. Moreover, the IL-15 receptor alpha (IL-15Rα) can comprise part of a membrane-associated receptor complex, or appear as a soluble form which stabilizes IL-15 and facilitates IL-15 secretion. Here, we report measurement of physiological levels of murine IL-15, and determine that muscle and serum IL-15 levels decline progressively with age. However, expression of IL-15 mRNA and membrane-associated subunits of the IL-15 receptor did not change with age in muscle. Expression of soluble IL-15Rα (sIL-15Rα) mRNA declined 5-fold with age, and serum IL-15 levels correlated highly with muscle sIL-15 mRNA expression, suggesting declines in sIL-15Rα expression lead to decreased circulating IL-15 levels during aging. These findings complement studies which described several single-nucleotide polymorphisms in the human IL-15Rα gene which impact muscularity and adiposity, and provide a technical basis for further investigation of IL-15 and the sIL-15Rα in determining body composition in aging mice, as a model for humans.