A SUBSET OF MEMORY CD 4 + HELPER T LYMPHOCYTES IDENTIFIED BY EXPRESSION OF Pgp - 1 By KAREN BUTTERFIELD , C . GARRISON FATHMAN , AND RALPH

The phenotypic distinction between naive T lymphocytes, before primary activation by antigen, and previously stimulated memory T cells has remained elusive until recently. Although several surface markers phenotypically subdivide CD4+ or CD8+ cells (1-6), or are transiently expressed by T cells upon activation (7-12), none ofthese determinants has been formally shown to identify a subset ofmurine memory T cells . We recently observed that the cell surface determinant Pgp-1 (Ly-24) is heterogeneously expressed by murine peripheral T lymphocytes (13) . Pgp-1 is a transmembrane 95-kD glycoprotein found on a wide variety of cell types besides lymphocytes, including bone marrow, lung, brain, and liver (14, 15) . The molecule appears to be highly conserved among species (15), although its function remains unknown . Cell surface expression of Pgp-1 by T cells has been shown to be acquired at the time ofprimary antigenic stimulation and constitutively expressed thereafter (13) . A prediction from this observation was that T cells responding to immunizing antigen in vivo would be found within the Pgp-1 + subset . This was confirmed for the CD8+ subset of murine T cells ; after immunization with either of two antigens, specific cytolytic T lymphocytes were found nearly exclusively within the Pgp-1 + subset (13, 16) . The current study extends these observations to CD4+ helper T lymphocytes (Th) by showing that the minor Pgp-1+ subset of CD4+ cells also contains the antigen-specific Th after immunization with either keyhole limpet hemocyanin (KLH) or sperm whale myoglobin (SWM).