EXPRESSION OF THE BCL-2 PROTEIN IN MURINE AND HUMAN THYMOCYTES AND INPERIPHERAL T-LYMPHOCYTES

Bcl-2, a proto-oncogene that can block apoptosis, was found to be expr essed throughout the thymic medulla, but in only scattered cells in th e thymic cortex. In order to determine the precise distribution of Bcl -2 protein during thymocyte development, we utilized mAb specific for either mouse or human Bcl-2. Thymocyte subpopulations were assessed us ing three-color flow cytometry and a saponin-permeabilization method. Staining of adult mouse and human thymocytes was comparable, with 20 t o 35% of cells expressing Bcl-2. Bcl-2 was expressed in nearly all CD4 + and CD8+, and CD3hi cells, but in only 5 to 10% of CD4+8+ cells. The CD4-8-population was more variable, with 25 to 40% of human cells and 65 to 80% of murine cells expressing Bcl-2. In sorted adult murine CD 4-8- cells, the very immature Pgp-1+/IL-2Ralpha- subset had a high per centage of Bcl-2+ cel Is. Bcl-2 expression was also examined during mu rine fetal development. At fetal day 15.5 to 16.5, 60 to 70% of total thymocytes expressed Bcl-2. By fetal day 17.5, overall Bcl-2 expressio n fell to adult levels of 20 to 30%. Bcl-2 was present in peripheral T cells from lymph node, spleen, and peripheral blood at uniformly high levels. In vitro stimulation with anti-CD3 or anti-TCR antibodies inc reased Bcl-2 expression in total thymocyte cultures, but could not ind uce Bcl-2 expression in CD4+8+ cells, even with the addition of a vari ety of cytokines. These data suggest that early double negative thymoc ytes express Bcl-2 but lose Bcl-2 with differentiation to the double p ositive stage. Thymocytes regain Bcl-2 during selection to a single po sitive state and retain Bcl-2 in the periphery.