IgD Receptor-Mediated Signal Transduction in T Cells

Upregulation of immunoglobulin D-specific receptors (IgD-R) on CD4+ T cells may facilitate their interaction with specific carbohydrate moieties uniquely associated with membrane IgD on B cells. Previous studies have shown that upregulation of IgD-R facilitates cognate T–B cell interactions by mediating bidirectional signaling resulting in increased antibody responses and clonal expansion of antigen-specific T cells. Murine T hybridoma cells, 7C5, constitutively express IgD-R, as has been confirmed by staining with biotinylated IgD. Earlier studies have shown that inhibitors of protein tyrosine kinase (PTK) completely prevented upregulation of IgD-R in response to oligomeric IgD, suggesting that cross-linking of IgD-R may induce signal transduction and functional consequences through one or more PTK activation pathways, leading to upregulation of IgD-R. In the present study we show that cross-linking of IgD-R by oligomeric IgD indeed results in (a) T cell activation as seen by tyrosine phosphorylation of several intracellular proteins, (b) tyrosine phosphorylation of p56 Lck and PLC-γ in 7C5 T hybridoma cells, and (c) phosphorylation of an ∼29-kDa band that exhibits strong affinity for IgD. We analyzed tyrosine phosphorylation of p56 Lck and PLC-γ in BALB/c splenic T cells that were exposed to oligomeric IgD both in vivo and in vitro. In vitro cross-linking as well as in vivo followed by in vitro cross-linking of IgD-R resulted in enhanced phosphorylation of p56 Lck and moderate tyrosine phosphorylation of PLC-γ. These results suggest that interactions between IgD-R and IgD mediate signal transduction and support our previous findings that IgD-R+ T cells enhance cognate T cell–B cell interactions and antibody production.