MEMBRANE CD45R ISOFORM EXCHANGE ON CD4 T-CELLS IS RAPID, FREQUENT ANDDYNAMIC IN-VIVO

CD4 T cells bearing high (240-190 kDa) and low (180 kDa) molecular mas s isoforms of the leukocyte common antigen CD45 define functionally di stinct subsets which have been equated with naive and memory T cells. In the rat, CD4 T cells expressing a high molecular mass isoform [iden tified by monoclonal antibody MRC-OX22 (anti-CD45RC)] exchange this fo r the 180 kDa molecule (CD45RC(-)) when stimulated by antigen. Here we show, by transferring mature allotype-marked CD45RC(-) CD4 T cells (d epleted of immature Thy-1(+) CD45RC(-) recent thymic emigrants) into n ormal euthymic recipients, that many T cells re-express the high molec ular mass isoform in less than 6 h. By 24 h, 30-60% of CD45RC(-) CD4 T cells became CD45RC(+); within a week the entire cohort appeared to e xchange the low for the high molecular mass isoform. Isoform exchange was dynamic and many CD4 T cells returned once again to the CD45RC(-) state. CD45RC(-) CD4 T cells declined in number more rapidly than the CD45RC(+) subset after transfer. The results suggest that CD45R isofor ms distinguish between resting T cells (CD45RC(+)) and those which hav e encountered antigen in the recent past. CD45R isoforms would appear to be unsuitable markers of naive and memory T cells.