B cells capturing antigen conjugated with CpG oligodeoxynucleotides induce Th1 cells by elaborating IL-12.

APCs initiate T cell-mediated immune responses against foreign Ags. Dendritic cells are professional APCs that play unique roles, including Ag-nonspecific capture, priming of naive T cells, and Th1 induction, whereas B cells generally lack these functions. In this study we uncovered novel aspects of murine B cells as APCs using CpG oligodeoxynucleotides (CpG) conjugated with an Ag. B cells served as efficient APCs independently of surface Igs. This characteristic was underlaid by the CpG-mediated Ag uptake and presentation, which were functional only when CpG were covalently conjugated to Ag. The B cells cultured with CpG-conjugated Ag not only enhanced IFN-gamma formation by Th1 cells, but also induced Th1 differentiation from unprimed T cells. These effects paralleled with the increase in the expression of CD40, CD86, and class II molecules on B cells and the coordinated production of IL-12 by the cells. To our knowledge this is the first report revealing that B cells share with dendritic cells common intrinsic characteristics, such as the Ag-nonspecific capture and presentation, and the induction of Th1 differentiation from unprimed T cells.