Murine CD4+ T cells can be separated into two distinct populations on the basis of their levels of expression of the CD45RB antigen (CD45Rhi and CD45Rlo). Murine CD45Rlo cells arise from CD45Rhi cells after antigenic exposure and provide antigen-specific help to B cells in a secondary immune response. In the present study, the ability of CD45Rhi and CD45Rlo cells to proliferate in response to either soluble antigen or allogeneic cells was examined by limiting dilution analysis. CD45Rhi cells were the major responding cells in unprimed animals; priming caused a large increase in the frequencies of responding CD45Rlo cells and this increase was evident 11 months later. These results further support the notion that CD4+ CD45Rlo cells are long-term memory cells.
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