Transitions from high to low molecular weight isoforms of CD45 (T200) involve rapid activation of alternate mRNA splicing and slow turnover of surface CD45R.

The expression of CD45 isoforms and of CDw29 has been analyzed as a function of time by correlating cell surface phenotype with mRNA synthesis. After activation, T cells lose CD45R and acquire a high density of CD45 p180 and of CDw29. Throughout this transition the density of CD45 common determinants steadily increases, resulting in a net gain of surface CD45. The gradual loss of CD45R could reflect a rapid switch in CD45 mRNA splicing patterns followed by a slow loss of surface CD45R. Alternatively it could reflect a delayed activation of splicing to produce the 4.8-kb CD45 mRNA, or long lived 5.4-kb CD45 mRNA. Analysis of CD45 mRNA indicated that at 24 h postactivation, 5.4-kb CD45 mRNA is lost and only 4.8-kb mRNA is detectable. The amount of 4.8-kb mRNA increases until day 3 and then decreases somewhat. Thus our results support the interpretation that transitions in CD45 isoform mRNA expression occur within the first 24 h after activation and that the persistence of CD45R+ T cells until day 3 to 4 of culture results from slow turnover of surface CD45R glycoprotein.