Facilitated Antigen Presentation by B Cells Expressing IgD when Responding T Cells Express IgD-Receptors

In vitrostudies have confirmed that cognate interactions between T and B cells are required to demonstrate enhanced helper activity using T cells with upregulated IgD-receptors (IgD-Rs). We studied the mechanism by which IgD-R+T cells facilitate antibody responses by examining whether T cells also benefit from their expression of IgD-R. Experiments were designed to determine whether upregulation of IgD-R on T cells facilitates antigen presentation by IgD+B cells. Goat Ig-primed splenic T cells from BALB/c mice were tested for their ability to respond to antigen-presenting B cells treated with goat anti-mouse (GAM) IgM or GAM IgD. T cell responses to GAM IgM and GAM IgD presented by B cells were significantly higher when goat Ig-primed cells were induced to express IgD-R by exposure to oligomeric IgD compared with goat Ig-primed control T cells. This effect was inhibited when monomeric IgD was added to the cultures. No differences in T and IgD-R+T cell responses were seen using adherent cells as APCs. B cells from IgD−/−mice were also tested. Such B cells present antigen to IgD-R+T cells without promoting enhanced responses compared with B cells from heterozygous IgD+/−mice. These studies suggest that IgD may play a costimulatory role during antigen presentation. We conclude that when T cells are induced to express IgD-R, these lectin-like receptors can ligate B cell membrane IgD during antigen presentation to facilitate responses of each of the cells engaged in cognate interaction, yielding enhanced antigen-specific T cell and B cell responses.