Presence of B220 within thymocytes and its expression on the cell surface during apoptosis

B220 is the full-length splicing isoform of a tyrosine phosphatase CD45 and is predominantly expressed as a transmembrane protein on B cells. Other sp licing isoforms of CD45 are yielded by alternative splicing of exons 4, 5 a nd 6. Recently, the expression of B220 on peripheral T cells during activat ion-induced cell death has been reported. To investigate whether B220 is im plicated in apoptosis of immature T cells, we analysed (by flow cytometry u sing the anti-B220 monoclonal antibody, RA3-6B2) the expression of B220 on mouse thymocytes undergoing X-irradiation- and dexamethasone (DEX)-induced apoptosis. The expression of B220 on thymocytes positive for Thy-1 was indu ced by X-irradiation or DEX treatment and increased with length of incubati on. The expression of B220 was pronounced on the apoptotic hypodiploid cell s in the fraction showing lower forward scattering values. Reverse transcri ption-polymerase chain reaction detected mRNA containing exons 4, 5 and 6 o f CD45 in normal thymocytes as well as those exposed to X-rays or DEX. Surp risingly, cytoplasmic B220 antigens were detected in a considerable fractio n of normal thymocytes. Moreover, the expression level of the 220 000-MW pr otein in normal thymocytes was similar to that in the thymocytes undergoing apoptosis. During apoptosis, the expression level of B220 antigen was redu ced in the cytoplasm but, conversely, up-regulated on the surface of thymoc ytes. These results suggest that B220 is constitutively expressed as a cyto plasmic form within thymocytes and possibly translocated to the cell membra ne during apoptosis.