EXPRESSION OF B220 ON ACTIVATED T-CELL BLASTS PRECEDES APOPTOSIS

Superantigens are bacterial or viral products that polyclonally activa te T cells bearing certain TCR beta chain variable elements. For insta nce, V beta 8(+) T cells proliferate in response to staphylococcal ent erotoxin B (SEE) in vivo and then undergo Fas- and/or TNF-mediated apo ptosis. We have recently shown that apoptotic SEE-reactive T cells exp ress the B cell marker B220. Here we report the identification of a no vel subset of CD4(+)B220(+) T cell blasts that are the precursors of t hese apoptotic cells in SEE-immunized mice. Moreover, we show that the CD4(-)CD8(-)B220(+) T cells that accumulate in the lymphoid organs of Fas ligand-defective gld mice stably express a form of the B220 molec ule which exhibits biochemical similarities to that expressed by activ ated wild-type T cells, but is distinct from that displayed on the sur face of B cells. Surprisingly, we also find a population of CD4(+)B220 (+) pre-apoptotic T cells in Fast-defective gld mice, arguing that the se cells can be generated in a Fas-independent fashion. Collectively, our data support a general model whereby upon activation, T cells up-r egulate B220 before undergoing apoptosis. When the apoptotic mechanism s are defective, T cells presumably down-regulate their coreceptor mol ecules but retain expression of B220 as they accumulate in lymphoid or gans.