Function of CD4 T cell subsets in vivo: expression of CD45R isoforms

Currently high (hi) and low (lo) molecular weight isoforms of CD45R are used to identify naive and memory T cells. However, CD45R isoforms were found to be expressed cyclically on CD4 T cells in vivo. A switch in isoform paralleled a change of function. During early post-thymic development, a switch from CD45Rlo to CD45Rhi occurred prior to antigen encounter. Naive CD45Rhi T cells when stimulated by antigen became CD45Rlo but reverted to CD45Rhi with time. It is argued that CD45Rlo T cells functionally resemble effector T cells and are at a pivotal stage of differentiation. Many are equipped to migrate into sites of inflammation. Removed from antigen, antigen-primed CD45Rlo T cells become CD45Rhi (memory) and thus adopt the same phenotype as naive T cells.