New-Delhi immunologists have discovered a new B-cell signalling pathway. Akanksha Chaturvedi and co-workers, reporting in Nature Immunology, show that the glycosylphosphatidylinositol (GPI)-linked form of immunoglobulin D initiates cyclic-AMP-dependent signalling that promotes the differentiation of resting B cells to germinal-centre (GC) B cells.

B cells that are activated by antigen in the T-cell areas of lymphoid tissues have two possible fates. They can either differentiate locally to antibody-secreting plasmablasts or migrate to a B-cell follicle to proliferate and form a GC. It is not clear how this cell-fate decision is regulated, but the authors speculate that the tuning of B-cell-receptor signalling might be important.

To determine which signalling pathways promote the differentiation of resting B cells to GC B cells, the authors activated mouse B cells with a surrogate antigen, anti-IgD Fab, and treated them with inhibitors of various signalling pathways. The cells were then assessed for phenotypical changes that are associated with GC B cells and for their ability to form GCs when injected into an antigen-primed mouse.

Surprisingly, the inhibition of protein kinase A (PKA), which is a cAMP-dependent signalling molecule, resulted in a failure to induce the expression of GC markers, such as GL7 and PNA-R, and the ability of these B cells to form GCs was impaired. This result was unexpected, because cAMP-dependent signalling had not been implicated in B-cell activation previously. Cross-linking of IgD resulted in a tenfold increase in the level of intracellular cAMP (cAMPi), which further supports the idea that this is a new pathway of B-cell signalling.

Membrane-lipid microdomains known as rafts have been implicated in the control of B-cell activation — so, does the IgD–cAMPi pathway involve rafts? Disruption of rafts in anti-IgD antibody-stimulated B cells had no effect on the classical Ca2+ flux, but the cAMPi response was markedly impaired. This indicates that, unlike Ca2+ signalling, IgD-induced cAMPi signalling is initiated in rafts.

GPI-linked proteins are enriched in rafts, so the authors wondered if the GPI-linked form of IgD might be involved. Resting B cells were treated with PI-PLC, an enzyme that cleaves GPI-linked proteins from the cell surface. Whereas the amount of IgD in the non-raft membrane fraction was not affected by treatment with PI-PLC, the amount of IgD in rafts was decreased by approximately 60%. Strikingly, IgD-dependent cAMPi responses were wiped out in PI-PLC-treated cells.

Finally, to show that optimal GC responses require GPI-linked IgD, resting B cells were treated with PI-PLC then activated with anti-IgD Fab. The upregulation of expression of GC phenotypic markers was inhibited and the B cells had a reduced capacity to form GCs. So, the GPI-linked form of IgD, first described ten years ago, finally has a function.