A novel mouse thymocyte antigen (F3Ag): down-regulation during the CD4+CD8+ double-positive stage indicates positive selection.

We describe a novel mAb (F3) which reacts with a 65 kDa thymocyte surface protein, expressed on approximately 80% of thymocytes, referred to as F3Ag. In ontogeny, F3Ag expression begins in the CD4(-)CD8(-) double-negative (DN) CD25(+) population and is maintained through approximately 85% of the CD4(+)CD8(+) double-positive (DP) stage. DP cells with high TCR expression and CD4(+) single-positive (SP) cells are predominantly negative for F3Ag, whereas many CD8(+) SP thymocytes express F3Ag. F3Ag-DP thymocytes show a reduced expression of RAG-1 and RAG-2 compared with F3Ag+ DP cells. The shutdown of F3Ag expression during the DP stage is related to positive selection: mice deficient for MHC class I and class II molecules maintain F3Ag expression in almost all DP cells. Transgenic (tg) mice carrying TCR restricted for MHC class II show a more pronounced down-regulation of F3Ag in the DP compartment than normal mice, depending on the presence of a positively selecting MHC. The size of the F3Ag- DP subset is positively correlated with the efficacy of positive selection into the CD4(+) SP compartment. Because some CD8(+) SP cells express F3Ag, the relationship between F3Ag down-regulation and positive selection is less obvious in DP cells of mice carrying MHC class I-restricted tg TCR. However, in reaggregate thymic organ cultures, sorted F3Ag- DP cells differentiate into CD8(+) SP cells more rapidly than do F3Ag+ DP cells. Thus, after down-regulation in the DP stage, a proportion of CD8(+) SP cells appears to re-express F3Ag. In addition, the proportion of F3Ag-CD8(+) SP cells depends on the efficacy of positive selection into the CD8 lineage. Taken together, the regulation of the expression of the F3Ag appears to be associated with signals that control thymic repertoire selection.