The surface phenotype of activated T lymphocytes.

The surface phenotype of T cells reflects both their relative maturity and their activation state. To determine the pattern of surface markers characteristic of activated T cells, purified mature T cells were stimulated in vitro for periods of 0.5-5 days with Concanavalin-A (Con-A) or phorbol myristic acetate (PMA) and ionomycin, fluorescein-labelled with monoclonal antibodies, then analysed by flow cytometry. The level of expression of the function-associated antigens CD4 (L3T4) and CD8 (Ly-2) decreased transiently early after activation with PMA/ionomycin, but not after stimulation with Con-A. Both stimuli caused a small drop in the level of CD3 and the T cell antigen receptor (TCR). At no time was CD3, CD4 or CD8 completely lost from the surface. Following activation Pgp-1, the interleukin-2 (IL-2) receptor (as detected by the monoclonal antibody 7D4) and the peanut agglutinin (PNA) receptor were gained by a proportion of cells, MEL-14 was lost by a proportion of cells, and no change was observed in the expression of heat stable antigen. Thy-1 or Ly-1. From the present data no evidence has been found for the generation of the 'immature', CD4- CD8- phenotype found in the thymus by activation of mature T cells. During T cell development, however, changes in expression of Pgp-1, MEL-14, the IL-2 receptor and the PNA receptor may be associated with activation, rather than differentiation per se.