Following Antigen Challenge, T Cells Up-Regulate Cell Surface Expression of CD4 In Vitro and In Vivo

The low precursor frequency of Ag-specific T cells has raised significant barriers to studying the T cell response in vivo. We demonstrate that T cells up-regulate the cell surface expression of CD4 following Ag recognition, which identifies Ag-specific T cells in vitro and in vivo and allows their characterization. The CD4high cell subpopulation contains the Ag-specific population as indicated by Ag-induced proliferation and limiting dilution analyses. The use of the CD4high marker will allow analysis of the dynamics of the T cell immune response in vivo, the study of the suboptimal T cell response to Ag, and the identification of T cells which are reactive to known and unknown autoantigens.