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Expression of an alternative spliced IL4 mRNA was found in in vitro activated T cells. In this study we show that the expression of IL4 mRNA, as well as the expression of this alternatively spliced form of IL4 mRNA, is not restricted to these cells. We analyzed different human lymphoid tissues and cell lines of different origin and found that the alternatively spliced IL4 transcripts are also expressed in human lymphoid tissues, in purified B cells, in the various B cell-derived cell lines, and even in nonlymphoid cell lines. Stimulation with the phorbol ester PMA enhanced expression of both transcripts in all cells studied. The two IL4 transcripts were cloned and sequenced from the B cell line Namalwa. In the alternatively spliced form, the same exon 2 is deleted as has been observed in in vitro activated T cells. In principle, such alternatively spliced mRNA may give rise to a truncated IL4 protein, as the deletion does not result in a frameshift. We tested supernatants of activated PBMC, cell lines, and cell extracts for the presence of IL4 protein. We found IL4 protein expression in activated PBMC, but not in any of the stimulated cell lines or in the purified B cells. Using a modified in situ hybridization method with Dig-labeled PCR products, however, these cells did express IL4 mRNA. This shows that transcription of both IL4 forms is not restricted to T cells and can be induced in other cell types as well. Using these non-T cells, no protein has been found in the supernatant, however. It is possible that transcription of the IL4 gene is not necessarily followed by translation and that translation into the IL4 protein requires an additional signal.