Germinal Center Active B Cell Receptor Signaling in the Cutting Edge: An In Vivo Reporter Reveals

Long-lasting Ab responses rely on the germinal center (GC), where B cells bearing high-affinity Ag receptors are selected from a randomly mutated pool to populate the memory and plasma cell compartments. Signaling downstream of the BCR is dampened in GC B cells, raising the possibility that Ag presentation and competition for T cell help, rather than Ag-dependent signal-ing per se, drive these critical selection events. In this study we use an in vivo reporter of BCR signaling, Nur77-eGFP, to demonstrate that although BCR signal-ing is reduced among GC B cells, a small population of cells exhibiting GC light zone phenotype (site of Ag and follicular helper T cell encounter) express much higher levels of GFP. We show that these cells exhibit somatic hypermutation, gene expression characteristic of signaling and selection, and undergo BCR signaling in vivo. A ffinity maturation of BCRs is vital for optimizing Ag-specific protective Abs. Upon Ag recognition in the context of cognate T cell help, a subset of activated B cells is recruited into the germinal center (GC) (1–5). GC B cells undergo random somatic hypermutation (SHM) to diversify their BCR repertoires and subsequently undergo selection and expansion that are regulated by resident follic-ular dendritic cells and recruited T follicular helper (Tfh) cells. GC B cells with highest affinity BCRs survive to populate the memory and long-lived plasma cell (LLPC) pools, thereby conferring potent and lasting humoral immunity. Ag capture by the BCR and recruitment of Tfh help regulate GC B cell selection. However, it is not known whether BCR sig-naling contributes to affinity maturation. These two models are not mutually exclusive, and data in favor of both have been reported (2–4, 6, 7). The GC has been histologically divided into light and dark zones (LZ and DZ), both populated by GC B cells. Importantly , Tfh and follicular dendritic cells are enriched in the LZ, suggesting that this may be the site of Ag-driven selection (1, 4). GC B cells shuttle rapidly (on the order of hours) between these two zones in a chemokine-dependent manner (8, 9). This is thought to be the dynamic spatiotemporal correlate of iterative rounds of SHM and selection in the GC reaction. LZ and DZ GC B cells can be identified reliably according to expression of the chemokine receptor CXCR4 and the co-stimulatory ligand CD86 (7, 10). Gene expression profiling of these subsets has revealed a signature of both BCR …