Upregulation of CD4 on CD8⁺ T cells: CD4^dimCD8^bright T cells constitute an activated phenotype of CD8⁺ T cells

Aside from an intermediate stage in thymic T-cell development, the expression of CD4 and CD8 is generally thought to be mutually exclusive, associated with helper or cytotoxic T-cell functions, respectively. Stimulation of CD8⁺ T cells, however, induces the de novo expression of CD4. We demonstrate that while superantigen (staphylococcal enterotoxin B, SEB) and anti-CD3/CD28 costimulation of purified CD8⁺ T cells induced the expression of CD4 on CD8⁺ T cells by 30 and 17%, respectively, phytohaemagglutinin (PHA) stimulation did not induce CD4 expression on purified CD8⁺ T cells but significantly induced the expression of both CD4 on CD8 (CD4^dimCD8^bright) and CD8 on CD4 (CD4^brightCD8^dim) T cells in unfractionated peripheral blood mononuclear cells (PBMC). The level of the PHA-mediated induction of CD4^dimCD8^bright and CD4^brightCD8^dim was at 27 and 17%, respectively. Depletion of CD4⁺ T cells from PBMC abrogated this PHA-mediated effect. Autologous CD4⁺ and CD8⁺ T-cell co-cultures in the presence of PHA induced this CD4^dimCD8^bright T-cell expression by 33%, demonstrating a role for CD4 cells in the PHA-mediated induction of the double positive cells. The induction of CD4^dimCD8^bright was independent of a soluble factor(s). Phenotypic analysis of CD4^dimCD8^bright T cells indicated significantly higher levels of CD95, CD25, CD38, CD69, CD28, and CD45RO expression than their CD8⁺CD4^- counterparts. CD4^dimCD8^bright T cells were also negative for CD1 a expression and were predominately T-cell receptor (TCR) αβ cells. Our data demonstrate that CD4^dimCD8^bright T cells are an activated phenotype of CD8⁺ T cells and suggest that CD4 upregulation on CD8⁺ T cells may function as an additional marker to identify activated CD8⁺ T cells.