Limiting dilution analysis of CD45R^hi and CD45R^lo T cells: Further evidence that CD45R^lo cells are memory cells

Murine CD4⁺ T cells can be separated into two distinct populations on the basis of their levels of expression of the CD45R_B antigen (CD45R^hi and CD45R^lo). Murine CD45R^lo cells arise from CD45R^hi cells after antigenic exposure and provide antigen-specific help to B cells in a secondary immune response. In the present study, the ability of CD45R^hi and CD45R^lo cells to proliferate in response to either soluble antigen or allogeneic cells was examined by limiting dilution analysis. CD45R^hi cells were the major responding cells in unprimed animals; priming caused a large increase in the frequencies of responding CD45R^lo cells and this increase was evident 11 months later. These results further support the notion that CD4⁺ CD45R^lo cells are long-term memory cells.