Characterization of CD4+ Single Positive Cells That Lack CD3 in the Human Thymus

In the thymus of experimental animals, single positive cells that lack surface CD3 expression (CD3-CD4+CD8- or CD3-CD4-CD8+) have been reported to have intermediate characteristics between the CD4-CD8- cells and CD4+CD8+ cells. Here we show the presence of single positive cells in the human thymus that lack CD3 by means of three-color flow cytometric analysis. CD3- cells constitute 12 ± 7.5% of CD4 single positive cells in 10 normal thymus samples. CD8 single positive cells, on the other hand, contained only 1.5 ± 2.2% CD3- cells. Seventeen percent of these CD3-4+8- thymocytes were in S + G2/M phase, while only 4.3% of CD4+CD8- thymocytes were in S + G2/M. The surface antigen phenotype of CD3-CD4+CD8- thymocytes was CD1low, CD2+, αβTCR-, CD29high, LFA-1+, ICAM-1-, IL-2R-, CD45lowcommon, CD45RA-, and CD45R0low. They expressed cytoplasmic CD3+ and 28% of them expressed cytoplasmic CD3 and cytoplasmic αβTCR. The levels of CD2, CD29, CD45R0, cytoplasmic CD3, and cytoplasmic αβTCR expression were intermediate between the CD3-4-8- and CD4+8+ populations. These results indicate that CD3-CD4+CD8- thymocytes constitute a minor but significant population in the human thymus. Their surface antigen phenotype and the high incidence of dividing cells suggest that they are immature and probably on their way to the CD4+8+ double positive thymocytes.