STAT6 tunes maximum T cell IL-4 production from stochastically regulated Il4 alleles.

T helper 2 (Th2) cells stochastically express from the Il4 locus but it has not been determined whether allelic expression is linked or independent. Here, we provide evidence that alleles are independently activated and inactivated. We compared Il4 locus expression in T cells from hemizygous IL-4 reporter mice in culture and in vivo following exposure to type 2 immunogens. In culture, Il4 alleles had independent, heritable expression probabilities. Modeling showed that in co-expressors, dual allele transcription occurs for only short periods, limiting per-cell mRNA variation in individual cells within a population of Th2 cells. In vivo profiles suggested that early in the immune response, IL-4 output was derived predominantly from single alleles, but co-expression became more frequent over time and were tuned by STAT6, supporting the probabilistic regulation of Il4 alleles in vivo among committed IL-4 producers. We suggest an imprinted probability of expression from individual alleles with a short transcriptional shutoff time controls the magnitude of T cell IL-4 output, but the amount produced per allele is amplified by STAT6 signaling. This form of regulation may be a relevant general mechanism governing cytokine expression.