A new IRF-1–driven apoptotic pathway triggered by IL-4/IL-13 kills neonatal Th1 cells and weakens protection against viral infection

Early life immune responses are deficient in Th1 lymphocytes that compromise neonatal vaccination. We found that IL-4 and IL-13 engage a developmentally expressed IL-4Ra/IL-13Ra1 heteroreceptor to endow IFN regulatory factor 1 (IRF-1) with apoptotic functions, which redirect murine neonatal Th1 reactivation to cell death. IL-4/IL-13–induced STAT6 phosphorylation serves to enhance IRF-1 transcription and promotes its egress from the nucleus. In the cytoplasm, IRF-1 can no longer serve as an anti-viral transcription factor but, instead, colocalizes with Bim and instigates the mitochondrial, or intrinsic, death pathway. The new pivotal function of IRF-1 in the death of neonatal Th1 cells stems from the ability of its gene to bind STAT6 for enhanced transcription and the proficiency of its protein to precipitate Bim-driven apoptosis. This cytokine-induced, IRF-1–mediated developmental death network weakens neonatal Th1 responses during early life vaccination and increases susceptibility to viral infection.