Surface antigens of human thymocyte populations defined by CD3, CD4 and CD8 expression: CD1a is expressed by mature thymocytes but not peripheral T cells.

Three colour flow cytometric analysis has been used to analyze the expression of a series of surface antigens on human thymic and peripheral T-cell populations. CD4, CD8 and CD3 were used to divide the populations into the conventional major categories, and the distribution of CD1a, CD2, CD7, CD34, CD44, class I MHC and class II MHC was then determined. Some characteristics of 'single positive' (CD4+8- and CD4-8+) T-lineage cells were unexpected. Amongst thymocytes, some 'immature single positives' were delineated as larger sized cells lacking cell surface CD3 and expressing low levels of class I MHC; however, in contrast with murine thymocytes, these were all CD4+8-, rather than being predominantly CD4-8+. Amongst peripheral T cells, a small proportion of CD7- cells were detected, within both the CD4+8-3+ and the CD4-8+3+ categories. Finally, in contrast to previous conclusions, CD1a was expressed at high levels on mature (CD4+8-3+ and CD4-8+3+) human thymocytes, although in agreement with previous reports it was absent from peripheral T cells. CD1a is therefore a useful marker of post-selection, post-thymic T-cell maturation.