Characterization of thymocytes expressing the common acute lymphoblastic leukemia antigen.

We studied the relationship between CALLA + thymocytes and two known markers of T-lymphocyte differentiation, Tdt and the sheep erythrocyte receptor. Thymocytes were studied using double fluorochrome analysis (with monoclonal anti-CALLA antibody and anti-Tdt) before and after E rosette separation. We found that approx. 4% of unseparated thymocytes were CALLA + and that most CALLA + cells were also Tdt +. After E rosettes separation CALLA + Tdt + cells were found mostly in the ER- fraction (20% of ER- cells) while only 1.0% of ER + cells were CALLA +. The expression of CALLA on ER- Tdt + thymocytes suggests that CALLA may define cells early in T-cell differentiation.