Human germinal center B cells are intrinsically distinct from naive B cells

Abstract

Abstract In germinal centers (GCs) B cells undergo cycles of proliferation and somatic hyper mutation followed by affinity discrimination. Selection of GC B cells expressing high affinity B cell receptors (BCRs) is driven by the ability of B cells to signal through the BCR and extract antigen to present to follicular helper T cells. Here we provide evidence that human tonsillar GC B cells, as compared to naïve tonsillar B cells, may have a higher threshold for affinity selection but be better able to interact with T cells and, once triggered through the BCR, may form more stable signaling complexes. A comprehensive quantitative analysis of surface proteins showed distinct profiles for naïve and GC B cells. In particular, GC B cells express less of the integrin VLA-4 and as a consequence were less able to engage beads coated with its ligand, VCAM-1. Given that VLA-4 tethers B cells to VCAM-expressing cell surfaces and facilitates BCR-dependent activation, a reduction in VLA-4 expression would be predicted to raise the threshold for affinity selection in GC B cells. GC B cells also tended to have increased surface expression of proteins that mediate T-B-cell interactions, such as CD80 and CD86, suggesting that once antigen is captured and presented GC B cells would be better equipped to engage helper T cells. To explore BCR signaling we imaged synapses formed between B cells with anti-Ig-containing planar lipid bilayers via total internal reflection fluorescence microscopy. GC B cells tended to form more stable signaling complexes showing greater co-localization between their BCRs and components of their signaling pathway. Taken together these data point toward intrinsic differences between GC and naïve B cells that may contribute to the outcome of GC responses.