Interleukin 2 (IL2) is the primary growth hormone used by mature T cellsand this lymphokine plays an important role in the magnification of cell-mediatedimmune responses. Under normal circumstances its expression is limited toantigen-activated type 1 helper T cells (TH1) and the ability to transcribe thisgene is often regarded as evidence for commitment to this developmentallineage. There is, however, abundant evidence than many non-TH1 T cells, underappropriate conditions, possess the ability to express this gene. Of paramountinterest in the study of T-cell development is the mechanisms by whichdifferentiating thymocytes are endowed with particular combinations of cellsurface proteins and response repertoires. For example, why do most helper Tcells express the CD4 differentiation antigen?
As a first step in understanding these developmental processes the geneencoding IL2 was isolated from a mouse genomic library by probing with aconspecific IL2 cDNA. The sequence of the 5' flanking region from + 1 to -2800was determined and compared to the previously reported human sequence.Extensive identity exists between +1 and -580 (86%) and sites previously shown tobe crucial for the proper expression of the human gene are well conserved in bothsequence location in the mouse counterpart.
Transient expression assays were used to evaluate the contribution ofvarious genomic sequences to high-level gene expression mediated by a cloned IL2promoter fragment. Differing lengths of 5' flanking DNA, all terminating in the 5'untranslated region, were linked to a reporter gene, bacterial chloramphenicolacetyltransferase (CAT) and enzyme activity was measured after introduction intoIL2-producing cell lines. No CAT was ever detected without stimulation of therecipient cells. A cloned promoter fragment containing only 321 bp of upstreamDNA was expressed well in both Jurkat and EL4.El cells. Addition of intragenicor downstream DNA to these 5' IL2-CAT constructs showed that no obviousregulatory regions resided there. However, increasing the extent of 5' DNA from-321 to -2800 revealed several positive and negative regulatory elements. Onenegative region that was well characterized resided between -750 and -1000 andconsisted almost exclusively of alternating purine and pyrimidines. There is nosequence resembling this in the human gene now, but there is evidence that theremay have once been.
No region, when deleted, could relax either the stringent induction-dependenceon cell-type specificity displayed by this promoter. Reagents thatmodulated endogenous IL2 expression, such as cAMP, cyclosporin A, and IL1,affected expression of the 5' IL2-CAT constructs also. For a given reagent,expression from all expressible constructs was suppressed or enhanced to the sameextent. This suggests that these modulators affect IL2 expression throughperturbation of a central inductive signal rather than by summation of the effectsof discrete, independently regulated, negative and positive transcription factors.
