Unique properties of GC B cells permit optimal BCR affinity discrimination

Abstract

Abstract Upon encountering pathogens, B cells within lymphoid organs enter specialized micro-environments of germinal centers (GCs). In the dark zones (DZ) of GCs, B cells proliferate and undergo somatic hypermutation, and subsequently migrate to the light zone (LZ) of the GC where selection of B cells that express the highest affinity B cell receptors (BCRs) occurs. Affinity selection is driven by the ability of BCRs to signal and extract antigen from the surfaces of follicular dendritic cells. Here we provide evidence that human tonsil LZ GC B cells are inherently better able to undergo affinity selection as compared to naïve tonsil B cells. LZ GC B cells engaged membranes that contained F(ab’)2 anti-light chain as a surrogate antigen through highly dynamic F-actin-containing pod-like structures that concentrated BCRs at their contact points. Using DNA-based mechanical force nano-sensors we observed robust pulling forces localized to the contact sites of the pods with the membrane. In contrast naïve B cells formed flat stable contacts with the membranes and showed only defuse weak pulling forces. Using antikappa antibodies of low (KD=3.9×10−7) versus high (KD=2.4×10−9) affinities as surrogate antigens, we show that LZ GC B cells were able to discriminate between these and responded only to the high affinity antigen. In contrast, naïve B cells responded equally to both antigens. Both LZ GC B cells and naive B cells were able to extract and internalize membrane-bound antigen, however, only in LZ GC B cells was the subsequent trafficking of the antigen to acidic intracellular compartments for processing highly dependent on the amount of antigen internalized. Thus, LZ GC B cells appear to be intrinsically programmed for optimal affinity selection.