The avian ChB6 alloantigen triggers apoptosis in a mammalian cell line

Many developing B lymphocytes are deleted by apoptosis. However, the mechanism signaling their demise remains poorly understood. Like mammals, chicken B cells are selected during their development; >95% of the cells in the bursa of Fabricius die without entering the secondary immune system. The molecule chB6 (Bu-1) has been used as a marker to identify B cells in the chicken. ChB6 is a type I transmembrane glycoprotein whose function is enigmatic. We have provided evidence that chB6 can induce a rapid form of cell death exhibiting characteristics of apoptosis. Here we further examine cell death induced by chB6 in a transfected mouse cell line. ChB6 is shown to cause apoptosis in this cell line as detected by a TUNEL assay for DNA fragmentation. This apoptosis is subject to regulation by signals from growth factor or by Bcl-x(L). Furthermore, we show that Ab binding to chB6 leads to cleavage of caspase 8, caspase 3, and poly(ADP ribose) polymerase. Overall, these data support the hypothesis that chB6 is a novel death receptor on avian B cells.