Distinction of virgin and memory T lymphocytes. Stable acquisition of the Pgp-1 glycoprotein concomitant with antigenic stimulation.

The Pgp-1 glycoprotein was identified on a minor (27%) subset of peripheral Lyt-2+ or L3T4+ T cells. In contrast, mature medullary-type thymocytes (Lyt-2+ L3T4-, Lyt-2- L3T4+) were nearly devoid of cells expressing detectable surface Pgp-1. The appearance of peripheral Pgp-1- T cells was found to be thymus dependent, as demonstrated by the diminished proportion of Pgp-1- T cells after thymectomy and their virtual absence in athymic nude mice. The subsequent acquisition of surface Pgp-1 was found to be a stable differentiation event occurring concomitantly with primary antigenic stimulation; selected Pgp-1- mature T cells from thymus or periphery acquired constitutive expression of Pgp-1 after stimulation in vitro with alloantigen or mitogens. These observations were extended by studies in vivo showing that immunization with various antigens augmented the percentage of Pgp-1+ spleen cells within the Lyt-2+ subset. Furthermore, the frequencies of antigen-specific CTLp, after immunization by any of three different antigens tested, were greatly enriched in the Pgp-1+ compared with the Pgp-1- subpopulations. Peritoneal exudate Lyt-2+ cells, after a localized allograft rejection, demonstrated a particularly prominent Pgp-1+ subpopulation (78%) that contained virtually all the allospecific cytolytic activity. A model consistent with all of these data proposes that mature thymocytes lacking surface Pgp-1 upon emigration to the periphery acquire its expression at the time of primary antigenic stimulation. Hence, expression of Pgp-1 among peripheral T cells is an important differentiation marker for identifying antigen-stimulated memory T cells.