Ku70 and Ku80 form Ku, a ring-shaped complex that initiates the non-homologous end-joining (NHEJ) DNA repair pathway.1 Ku binds to double-stranded DNA (dsDNA) ends and recruits other NHEJ factors (e.g., LIG4, DNA-PKcs). While Ku can bind to double-stranded RNA (dsRNA)2 and trap mutated-DNA-PKcs on ribosomal RNA (rRNA),3,4 the physiological role on Ku-RNA interaction in otherwise wildtype cells remains unclear. Intriguingly, Ku is dispensable for murine development5,6 but essential in human cells.7 Despite similar genome sizes, human cells express ~100-fold more Ku than mouse cells, implying functions beyond NHEJ - possibly through a dose-sensitive interaction with dsRNA, which binds Ku 10~100 times weaker than dsDNA.2,8 Investigating Ku's essentiality in human cells, we found that Ku-depletion - unlike LIG4 - induces profound interferon (IFN) and NF-kB signaling via dsRNA-sensor MDA5/RIG-I and MAVS. Prolonged Ku-degradation further activates other dsRNA sensors, especially PKR (suppressing translation) and OAS/RNaseL (cleaving rRNA), leading to growth arrest and cell death. MAVS, RIG-I, or MDA5 knockouts suppressed IFN signaling and, like PKR knockouts, all partially rescued Ku-depleted human cells. Ku-irCLIP analyses revealed Ku binding to diverse dsRNA, predominantly stem-loops in primate-specific antisense Alu elements9 in introns and 3'-UTRs. Ku expression rose sharply in higher primates, correlating tightly with Alu-expansion (r = 0.94/0.95). Thus, Ku plays a vital role in accommodating Alu-expansion in primates by limiting dsRNA-induced innate immunity, explaining both Ku's elevated expression and its essentiality in human cells.