Age‐related expression of TL antigen in AKR/J mice

A survey of age‐related expression of thymus‐leukemia (TL) alloantigens (TL 1,2,4) among bone marrow, spleen and thymus cells of grossly normal and leukemic AKR/J mice is presented. The response of the stained cells to antisera directed against TL antigens was analysed by means of the fluorescence‐activated cell sorter (FACS) apparatus. A transient expression of TL antigens on cells among the bone marrow population was obsered in I‐ to 20‐day‐old AKR/J mice, followed by an undetectable level up to 3 months and its reappearance thereafter. Thymocytes expressed TL from the age of 4 months onwards, reaching a transient maximal level at the age of 6 months in females and 8 months in males. Subsequently, an agerelated decrease took place. In spleen cells from newborn mice TL expression was seen, followed by a rapid decrease to undetectable levels up to the age of 5‐6 months. In most tests the expression of TL.4 preceded the TL 1,2 phenotype. The frequency of TL+ leukemias was about 50% among the early‐occurring spontaneous leukemias (in 5‐ to 7‐month‐old mice) and decreased to 20% with age increase. Leukemia development following treatment with methyl‐nitrosourea (MNUA) or exposure to X‐rays increased the frequency of TL+ tumors to 75‐100%. These results suggest that heterogeneous target cells are involved in AKR leukemogenesis.