Role of B7 costimulatory molecules in the adjuvant activity of the heat-labile enterotoxin of Escherichia coli

Much interest has been directed at understanding the adjuvant properties of the heat-labile enterotoxin of Escherichia coli (LT). In this study, we have assessed how LT compared with the nonenzymatic mutant LT (E112K) affect the level of B7-1 and B7-2 expression on APCs, and we determined how these costimulatory molecules influence their adjuvant properties. Analysis of B7-1 and B7-2 expression on B cells revealed that LT enhanced B7-2 but not B7-1, while LT (E112K) had no effect on the expression of either costimulatory molecule. Treatment of macrophage or dendritic cells with LT resulted in a predominant enhancement of B7-2, while LT (E112K) induced mainly B7-1 expression. Analysis of LT- and LT (E112K)-treated B cells, macrophage, and dendritic cells also revealed significant differences in their ability to enhance anti-CD3-stimulated CD4(+) T cell proliferative responses via B7-1 and B7-2. Furthermore, the ability of LT to enhance both Ab and CD4(+) T cell responses to a coadministered Ag was severely abrogated in B7-2- but not B7-1-deficient mice. In contrast, the in vivo adjuvant properties of LT (E112K) appeared to be mediated by both B7-1 and B7-2 for optimal CD4(+) T cell responses, while B7-1 appeared to be the predominant B7 molecule involved in the ability of LT (E112K) to augment Ab responses to a coadministered Ag. These findings demonstrate distinct differences in the ability of LT and LT (E112K) to enhance B7-1 and B7-2 on APC, as well as a dependence upon these costimulatory molecules for their adjuvant properties.