Membrane localization of the avian chB6 alloantigen

Chickens are one of the classic models of vertebrate immunity. We have been interested in the role of the alloantigen chB6 (formerly called BU1) in the development of B cells within the bursa and have presented evidence that chB6 can trigger apoptosis. Furthermore, the pathway that triggers apoptosis is similar to that triggered by FAS, with caspase 8 as an initiator, caspases 9 and 3 as later caspases, and bcl-xl functioning as an inhibitor of apoptosis. We have been unable to demonstrate a steady state association of chB6 and caspase 8 in DT40, so the mechanism of caspase 8 activation remains unclear. Other investigators have shown that FAS segregates into lipid rafts in type I cells and this is correlated with death signaling. In addition, FAS ligation leads to the formation of distinct signaling complexes termed SPOTS. Using immunofluorescence, we observed that chB6, when cross-linked by secondary antibodies, is found in patchy areas on DT40 cells reminiscent of SPOTS. By double labeling with fluorescent cholera toxin B, we show that chB6 co-localizes with lipid raft areas of the DT40 membrane and will be internalized by the cells. This suggests an active distribution of chB6 in the membrane and it may be preferentially loaded into lipid rafts along with BCR in avian cells. This work offers a new insight into the signaling via chB6. This work supported by a grant from the DePaul University Research Council.