Phenotypic characteristics of double-positive T cells in peripheral blood of healthy adults

Recent studies have shown that healthy adults have circulating double-positive (DP) T cells at small concentrations. These T cells develop from CD4⁺ and CD8⁺T cell populations and co-express CD4 and CD8 on their surface. According to the expression of CD4 and CD8 markers, the DP T cells can be divided into two subpopulations: CD4^brightCD8^dim and CD4^dimCD8^bright. The phenotypic and functional features of these subpopulations are currently not fully understood. The aim of the study was to identify the phenotypic characteristics of CD4^brightCD8^dim and CD4^dimCD8^brightT cells in peripheral blood from healthy adults depending on the expression of maturation antigens. The cell phenotyping was performed by flow cytometry using antibodies against the following human surface antigens: CD57 (FITC-conjugated), CD62L (ECD-conjugated), CD28 (PerCP/Cy5.5-conjugated), CD27 (Pe/Cy7-conjugated), CD4 (APC-conjugated), CD8 (APC-AF700-conjugated), CD3 (APC-AF750-conjugated), CD45RA (Pacific Blue-conjugated), and CD45 (Krome Orange-conjugated). The differences between the groups were estimated using Mann–Whitney U test. Results were presented as median and interquartile range – Me (Q_0.25-Q_0.75). A total pool of DP T cells was detected, constituting 0.73% (0.42-1.61) of the total CD3⁺T cell population, at a total concentration of 11 cells/μL (6-20). The percentage of CD4^brightCD8^dim and CD4^dimCD8^brightT cells was within 0.25% (0.18-0.44), and 0.29% (0.20-0.98) at concentrations of 4 cells/1μL (2-7) and 5 cells/1μL (2-12), respectively. Effector memory cells (CD45RA^-CD62L^-) and effector memory cells re-expressing CD45RA (CD45RA⁺CD62L^-) were predominant in CD4^brightCD8^dim T cells, whereas central memory phenotype (CD45RA⁺CD62L⁺) prevailed among CD4^dimCD8^brightT cells. Moreover, a reduced level of the “naïve” phenotype (CD45RA⁺CD62L⁺) was noted in CD4^brightCD8^dim and CD4^dimCD8^brightT cell populations. An increase in CD57 expression on the surface of CD4^brightCD8^dimT cells with a simultaneous decrease in CD27 and CD28 was also detected as compared to CD4⁺T cells. In turn, CD4^dimCD8^brightT cells increase the expression of CD28 and decrease the expression of CD57 on their surface if compared to CD8⁺T cells. DP T cells have a more mature phenotype being often represented by memory cells.