CD69 CELL-SURFACE EXPRESSION IDENTIFIES DEVELOPING THYMOCYTES WHICH AUDITION FOR T-CELL ANTIGEN RECEPTOR-MEDIATED POSITIVE SELECTION

CD69, an 'activation marker' that is rapidly induced on mature T cells after stimulation through the T cell antigen receptor (TCR) was found to be expressed on approximately 10% of normal thymocytes. All of these CD69+ thymocytes express alphabeta TCR, and they include both TCR(low)CD4+CD8+ and TCR(high)CD4+CD8- or CD4-CD8+ thymocytes. The CD69+ cells can be further segregated into heat-stable antigen (HSA)+TCR(low), HSA+TCR(high) and HSA-TCR(high) thymocyte populations. None of CD69+ cells express the mature T cell marker Qa-2. Thus CD69+ cells present in vivo appear phenotypically to represent transitional cell populations between immature TCR(low)HSA+Qa-2-double-positive cells and mature TCR(high)HSA-QA-2+ single-positive cells. In addition, TCR engagement by MHC molecules is required for CD69 expression in the thymus. Taken together, the CD69+ thymocytes appear to represent the cells auditioning in positive selection process or they are the cells that have been positively selected recently. Analysis of a TCR transgenic mouse model revealed an increased number of CD69+ thymocytes in a positively selecting thymus, whereas no CD69+ transgenic TCR+ thymocytes were observed in the non-selecting thymus. Based on the results of this study, we suggest that the surface expression of CD69 serves as a useful marker to identify and trace those thymocytes that are engaged in the TCR-mediated positive selection process in the thymus.