REGULATION OF INTERLEUKIN-2 GENE-EXPRESSION BY CD28 COSTIMULATION IN MOUSE T-CELL CLONES - BOTH NUCLEAR AND CYTOPLASMIC RNAS ARE REGULATED WITH COMPLEX KINETICS

T-cell receptor (TCR) signalling is required to induce expression of t he interleukin 2 (IL-2) gene in mouse T cells. Additional costimulatio n through CD28 augments IL-2 production by 30- to 100-fold. Using IL-2 RNA accumulation and transcription reporter assays, we have addressed potential mechanisms of CD28 regulation at various time points of sti mulation. The kinetic regulation of IL-2 mRNA by TCR and CD28 signals is complex: (i) at the earliest detectable time point, CD28 signalling causes a 20-fold increase compared with TCR signalling alone; (ii) bo th groups rapidly accumulate mRNA for the first 4 h; (iii) IL-2 mRNA t hen disappears from cells stimulated through the TCR alone but plateau s or increases slightly in cells costimulated through CD28; and (iv) a fter 8 h, the mRNA disappears in cultures with the anti-CD28 antibody. Transcription reporter assays did not show a specific effect of CD28 signalling on IL-2 enhancer driven transcription. This was true for ei ther a 353- or a 1.9-kb enhancer, over a broad range of kinetics and T CR occupancy, and with several TCR signal mimics. The early component of CD28 costimulation is nuclear, however, since the initial enhanceme nt of mRNA is also found in unspliced IL-2 RNA. Between 2 and 6 h, the re is a marked difference in the rates of decay of IL-2 mRNA in the pr esence and absence of the CD28 signalling. Rapid decay of IL-2 mRNA co mmences after 8 h even in the presence of CD28 signals, although the d ecay occurs at a rate slower than that seen after 4 h of anti-TCR stim ulation alone. This complexity suggests the existence of two interesti ng molecular mechanisms by which CD28 costimulates lymphokine gene exp ression.