Recruitment of the cytoplasmic adaptor Grb2 to surface IgG and IgE provides antigen receptor-intrinsic costimulation to class-switched B cells

B cell antigen receptors containing immunoglobulin G (IgG) or IgE, but not those containing IgM, show enhanced signaling. Wienands and colleagues trace this boosted signaling capacity to conserved IgG and IgE cytoplasmic tyrosine residues that recruit the adaptor Grb2. The improved antibody responses of class-switched memory B cells depend on enhanced signaling from their B cell antigen receptors (BCRs). However, BCRs on both naive and antigen-experienced B cells use the canonical immunoglobulin-associated α and β-protein signaling subunits. Here we identified a BCR isotype–specific signal-amplification mechanism. Whereas immunoglobulin M (IgM)-containing BCRs initiated intracellular signals exclusively through immunoglobulin-associated α- and β-proteins, IgG- and IgE-containing BCRs also used a conserved tyrosine residue in the cytoplasmic segments of immunoglobulin heavy chains. When phosphorylated, this tyrosine recruited the adaptor Grb2, resulting in sustained protein kinase activation and prolonged generation of second messengers, which together culminated in enhanced B cell proliferation. Hence, membrane-bound IgG and IgE exert antigen recognition as well as costimulatory functions, thereby rendering memory B cells less dependent on T cell help.