THE ALTERNATIVE SPLICING OF THE CD45 TYROSINE PHOSPHATASE IS CONTROLLED BY NEGATIVE REGULATORY TRANS-ACTING SPLICING FACTORS

CD45, a receptor-type protein tyrosine phosphatase involved in lymphoc yte activation, consists of five isoforms generated by alternative mRN A splicing. The pattern of CD45 splicing depends upon cell type, state of activation, and thymic development. We previously showed that mous e B and thymocyte cell lines transfected with a human CD45 minigene co nstruct appropriately splice the alternative exons, demonstrating the presence of trans-acting tissue-specific splicing factors. To study th e regulation of CD45 alternative splicing, cells having different spli cing patterns were transiently fused, and mRNA was analyzed using reve rse transcription-polymerase chain reaction. Human B cells, normally p roducing only the largest CD45 mRNAs, could splice out the alternative exons after fusion with mouse thymocytes. In contrast, the splicing p attern of human T cell lines (smaller CD45 isoforms) was unaltered by fusion with mouse B cells. This suggests that cells expressing the sma llest CD45 isoform contain negatively acting trans-factor(s) that allo w the alternative exons to be skipped, and that the full length isofor m containing all three alternative exons is the default pattern of spl icing. In agreement, incubation of thymocytes or T cell subsets with c ycloheximide increased mRNA corresponding to the larger isoforms. CD45 isoform expression can now be interpreted in terms of the presence or absence of negative regulatory trans-acting splicing factors.