IL-2 complex rapidly expands the NK compartment from immature precursors in IL-15^−/− mice. (A) B6 or IL-15^−/− mice received injections of IL-2 complex or IL-15 on day 0 and day 2 and spleens were analyzed on day 3. (B) Mice received congenically marked, NK1.1⁺CD3⁻-enriched splenocytes on day 0 and IL-2 complex injections on day 0 and day 2. Proliferation was measured by CFSE dilution on day 3.