Probabilistic regulation of IL-4 production in Th2 cells: accessibility at the Il4 locus.

IL-4 secreting and nonsecreting cells from Th2 cultures have a similar probability of producing IL-4 upon subsequent stimulation, implying that there is stochastic element in IL-4 production by stimulated Th2 cells. Purified IL-4 producers and nonproducers have similar Gata3 and c-maf mRNA expression. Il4 gene accessibility, analyzed by restriction enzyme accessibility (REA) at sites in the promoter, in the second intron (DNase I hypersensitivity sites HSII and HSIII) and in CNS-1 in the two populations was also similar. However, upon TCR stimulation, site VA, which is 5 kB 3' of exon 4, displayed a striking increase in accessibility but REA was 2- to 3-fold greater in producers than nonproducers. Cyclosporin A treatment inhibited VA opening, implying the involvement of NFAT in increased VA accessibility. Induction of VA accessibility is sensitive to cycloheximide, suggesting an additional factor(s) is needed. Thus, opening of VA is a probabilistic event determining which Th2 cells transcribe Il4.