B220 is expressed on apoptotic thymocytes induced by X-irradiation

CD45 is cell surface glycoprotein and expressed on all haematopoietic cells except mature erythrocytes and platelets. Eight isoforms of CD45 are gener ated by alternative splicing of exons 4-6. B220 including all three exons i s expressed specifically on pan-B cell lineage. Recently, it was reported t hat B220 was expressed on apoptotic T cells induced by staphylococcal enter otoxin B (SEB). In the present study, we investigated the expression of B22 0 on murine thymocytes after whole-body X-irradiation. We used the forward light scattering of flow cytometry as a parameter of cell size, and defined two populations; FSChigh (normal cell size) and FSClow (correspond to apop totic cell in size) fraction. B220(+) cells in FSChigh fraction reached a m aximum value (35%) at 18 hr after irradiation. In FSClow fraction, 40-60% c ells were positive for B220 at any time points. These results suggest that B220 is expressed on thymocytes in the pre-apoptotic stage, because B220 wa s expressed on not only FSClow cells but also FSChigh cells.