T cell proliferative response to interleukin 2: different frequency of responders among CD45R0 and CD45RA subsets

A proportion of human blood T cells is known to proliferate in cultures containing 10 or more units/ml of interleukin (IL)2, in the absence of exogenously added antigen. A possible explanation for this phenomenon is that the IL2 serves to maintain the proliferation of memory T cells which have recently been activated by antigen in vivo. To test this possibility we therefore separated T cells according to their CD45 (common leukocyte antigen) phenotype into the CD45RO and CD45RA subsets which are associated with memory and naive status, respectively. The frequency of responder cells in limiting dilution analysis of adult MNC was 1:1625 CD45RO cells, 1:2405 for CD45RA cells, and 1:1642 for unseparated cells. CD45RA cells from newborns had a 1:1850 frequency of responders. The IL2 responder cells were predominantly CD3+ TcR alpha beta +, with a minor component of TcR delta cells. There were no differences in the frequency of T cells using V beta 5 and V beta 8 between responding and nonresponding cells. The data argue against a requirement for prior sensitization for a proliferative response to IL2 and give no evidence that the subset of T cells which responds to IL2 differs in phenotype from nonresponsive cells.