Microbiota-derived inosine programs protective CD8 T cell responses against influenza in newborns.

Early-life susceptibility to respiratory viral infections remains a major public health concern, yet the underlying mechanisms are poorly understood. We demonstrate that antibiotic-induced dysbiosis impairs influenza-specific CD8 T cell immunity in infant mice and humans through the disruption of nuclear factor interleukin 3 (NFIL3)-dependent T cell programming. Mechanistically, we show that dysbiosis reduces intestinal and circulating inosine levels, disrupting NFIL3's epigenetic regulation of T cell factor 1 (TCF1) expression. This leads to intrinsic defects in CD8 T cell proliferation and differentiation, diminished effector responses, and impaired formation of tissue-resident memory cells. Bifidobacterium colonization restores intestinal and pulmonary inosine levels, establishing a specific pathway of gut-lung metabolic communication. Notably, inosine supplementation rescues NFIL3-dependent regulation of TCF1, enhancing CD8 T cell responses and protection against influenza infection in dysbiotic infants. Our findings reveal how early-life microbial communities shape antiviral immunity and identify inosine as a therapeutic target for enhancing respiratory defenses in infants.