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

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Abstract
We have studied the alterations in CD45R phenotypes of CD4+CD45RA–RO+ T cells in recipients of T cell-depleted bone marrow grafts. These patients are convenient models because early after transplantation, their T cell compartment 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 that CD4+CD45RA–RO+ recipient T cells could re-express CD45RA but never reverted to a genuine CD4+CD45RA+RO– naive phenotype. Even 5 years after transplantation, they still co-expressed CD45RO. In addition, the level of CD45RA and CD45RC expression was lower (∼ 35 %) than that of naive cells. In contrast, the level of CD45RB expression was comparable to that of naive cells. We conclude that CD4+CD45RA–RO+ T cells may re-express CD45high isoforms but remain distinguishable from naive cells by their lower expression of CD45RA / RC and co-expression of CD45RO. Therefore, it is likely that the long-lived memory T cell will be found in the population expressing both low and high molecular CD45 isoforms.