STAT1 Isoforms Differentially Regulate NK Cell Maturation and Anti-tumor Activity

Natural killer (NK) cells are important components of the innate immune defense against infections and cancers. Signal transducer and activator of transcription 1 (STAT1) is a transcription factor that is essential for NK cell maturation and NK cell-dependent tumor surveillance. Two alternatively spliced isoforms of STAT1 exist: a full-length STAT1 alpha and a C-terminally truncated STAT1 beta isoform. Aberrant splicing is frequently observed in cancer cells and several anti-cancer drugs interfere with the cellular splicing machinery. To investigate whether NK cell-mediated tumor surveillance is affected by a switch in STAT1 splicing, we made use of knock-in mice expressing either only the STAT1 alpha (Stat1(alpha/alpha)) or the STAT1 beta (Stat1(beta/beta)) isoform. NK cells fromStat1(alpha/alpha)mice matured normally and controlled transplanted tumor cells as efficiently as NK cells from wild-type mice. In contrast, NK cells fromStat1(beta/beta)mice showed impaired maturation and effector functions, albeit less severe than NK cells from mice that completely lack STAT1 (Stat1(-/-)). Mechanistically, we show that NK cell maturation requires the presence of STAT1 alpha in the niche rather than in NK cells themselves and that NK cell maturation depends on IFN gamma signaling under homeostatic conditions. The impaired NK cell maturation inStat1(beta/beta)mice was paralleled by decreased IL-15 receptor alpha (IL-15R alpha) surface levels on dendritic cells, macrophages and monocytes. Treatment ofStat1(beta/beta)mice with exogenous IL-15/IL-15R alpha complexes rescued NK cell maturation but not their effector functions. Collectively, our findings provide evidence that STAT1 isoforms are not functionally redundant in regulating NK cell activity and that the absence of STAT1 alpha severely impairs, but does not abolish, NK cell-dependent tumor surveillance.