CD45 isoform phenotypes of human T cells: CD4(+)CD45RA(-)RO(+) memory T cells re-acquire CD45RA without losing CD45RO

We have studied the alterations in CD45R phenotypes of CD4(+)CD45RA(-)RO(+) T cells in recipients of T cell-depleted bone marrow grafts. These patient s are convenient models because early after transplantation, their T cell c ompartment is repopulated through expansion of mature T cells and contains only cells with a memory phenotype. In addition, re-expression of CD45RA by former CD4(+)CD45RA(-) T cells can be accurately monitored in the pool of recipient T cells that, in the absence of recipient stem cells, can not be replenished with CD45RA(+) T cells through the thymic pathway. We found tha t CD4(+)CD45RA(-)RO(+) recipient T cells could re-express CD45RA but never reverted to a genuine CD4(+)CD45RA(+)RO(-) naive phenotype. Even 5 years af ter transplantation, they sl:ill co-expressed CD45RO. In addition, the leve l of CD45RA and CD45RC expression was tower (similar to 35%) than that of n aive cells. In contrast, the level of CD45RB expression was comparable to t hat of naive cells. We conclude that CD4(+)CD45RA(-)RO(+) T cells may re-ex press CD45(high) isoforms but remain distinguishable from naive cells by th eir lower expression of CD45RA/RC and co-expression of CD45RO. Therefore, i t is likely that the long-lived memory T cell will be found in the populati on expressing both low and high molecular CD45 isoforms.