Co-cultivation of CD4+ and CD8+ human T-cells leads to the appearance of CD4 cells expressing CD8 through de novo synthesis of the CD8 alpha-subunit.

The appearance of a population of dual-staining CD4+CD8+ cells in human T-lymphocyte cultures has been reported by various authors, including our own observation that they are always seen in simple phytohaemagglutinin-stimulated cultures from several different donors. The purpose of the present study was to investigate factors involved in the dual-staining (DS) phenotype, and to clarify some apparent inconsistencies between published observations. Our findings can be summarised as follows. 1. A population of DS CD4+CD8+ cells always appears in PHA-stimulated T-cell cultures if they contain both CD4 and CD8 subsets. The incidence of DS cells is related to PHA concentration, but other factors are involved since DS cells are not seen in PHA-stimulated cultures of purified CD4+ or CD8+ cells. Stimulation with PHA is not a prerequisite since very similar results are seen with ConA. 2. Direct physical contact between CD4+ and CD8+ cells is required for the appearance of the DS phenotype; soluble factors alone, including IL-4, appear nor to be responsible. 3. The DS phenotype in these conditions is always CD4+ cells weakly expressing CD8 and is a consequence of de novo synthesis of the CD8alpha molecule by the CD4+ cells.