AN ALTERNATIVE SPLICE VARIANT OF HUMAN IL-4, IL-4-DELTA-2, INHIBITS IL-4-STIMULATED T-CELL PROLIFERATION

Alternative splicing of mRNA can generate protein isoforms that are pr eferentially expressed in different tissues or during different states of cell differentiation or activation. Protein isoforms may have diff erent functions, In this study, we cloned, expressed, and tested funct ional effects of a naturally occurring splice variant of human IL-4, c alled IL-4 delta 2. In IL-4 delta 2, the second exon of IL-4 is omitte d by alternative splicing, with exons 1, 3, and 4 joined in an open re ading frame. We found that IL-4 delta 2 RNA is expressed in the PBMC o f all donors tested, usually in lower amounts than IL-4 RNA. In contra st, IL-4 delta 2 RNA is expressed in much higher levels than IL-4 RNA in thymocytes and bronchoalveolar lavage cells, suggesting tissue spec ificity of expression. IL-4 delta 2 cDNA was expressed in yeast. Recom binant human (rh) IL-4 delta 2 was partially purified and found to be glycosylated, with a protein core of 13 to 15 kDa, Unlike rhlL-4, rhlL -4 delta 2 did not act as a costimulator for T cell proliferation, How ever, rhlL-4 delta 2 inhibited the ability of rhlL-4 to act as a T cel l costimulator. Inhibition was independent of glycosylation and was no t mediated by toxicity. lodinated IL-4 delta 2 was found to bind speci fically to human PBMC and tumor lines known to express IL-4 receptors. Excess unlabeled IL-4 inhibited cellular binding of labeled IL-4 delt a 2. Thus, rhIL-4 delta 2 is a naturally occurring splice variant of I L-4 that is preferentially expressed in the thymus and airways and inh ibits function of complete IL-4. The balance between IL-4 and IL-4 del ta 2 may be important in the regulation of IL-4 effects.