C57BL/6 mice genetically deficient in IL-15 were generated by homologous recombination in embryonic stem (ES) cells. The structure of the murine IL-15 genomic locus has been described 21. A gene targeting vector was constructed in which a 7.5-kb SpeI-EcoRV fragment containing IL-15 exons 3–5, encoding amino acids 1–65 of the primary translation product, was replaced with a PGK-neo cassette. A thymidine kinase cassette (MC-TK) was inserted into the 5′ end of the vector. C57BL/6-derived ES cells were electroporated with the IL-15 targeting vector and selected in G418 and ganciclovir as described 22. ES cell clones carrying a targeted IL-15 gene were identified by a combination of PCR and genomic Southern blot analyses and were injected into BALB/c blastocysts. Resulting male chimeras were bred to C57BL/6 females. The offspring were analyzed for germline transmission of the mutant IL-15 allele by PCR and genomic Southern blot analyses. Mice heterozygous for the IL-15 mutation (IL-15+/−) were intercrossed to generate IL-15–deficient (IL-15−/−) mice.
The IL-15−/− mice used throughout these studies were bred at Immunex and maintained on a C57BL/6 genetic background. All mice were housed under specific pathogen-free conditions and were used between 9 and 20 wk of age. Age- and sex-matched littermates (IL-15+/+ or IL-15+/−) or C57BL/6 mice (Taconic Farms, Inc.) were used as controls as indicated. Initial studies revealed no apparent difference between IL-15+/+ and IL-15+/− mice in cellularity of their secondary lymphoid tissues, phenotype of spleen or LN cells, or splenic NK cell responses.