The Immunomodulator Enterotoxin Influences BCR Signaling by Stabilizing Lipid Domains

Adjuvants may potentiate immune response at the cell level by either enhancing receptor activation at the membrane or by increasing uptake of antigen. To investigate the mechanism behind the adjuvancy of LTIIb, a toxin that binds GM3, we studied its effects on the membrane structure, the mobility of BCR and B cell membrane signaling. bimFCS, a novel technique for probing interaction between membrane molecules and membrane domains, confirms that LTIIb pre-clusters cholesterol-stabilized domains. LTIIb, or its binding subunit alone, is found to reduce the mobility of BCRs, as measured by FRAP measurements and induce membrane signaling, as confirmed by calcium imaging. The calcium signaling pathway, however, seems to compete with that of BCR activation through IgM crosslinking. The B subunit of Choleratoxin (CTB), commonly used for labeling GM1, though also modulates cholesterol-stabilized domains, does not affect BCR mobility or trigger calcium signaling. These results show that GM1 and GM3, though both enriched in cholesterol-stabilized domains, behave differently, at least in CH27 B cells, upon crosslinking.