Sensing of antigen affinity and kinetic rates by B cells expressing receptors of HIV-1 broadly neutralizing antibodies

Abstract B cell activation is dependent on the productive engagement of an antigen (Ag) with the B cell receptor (BCR). How complex protein Ags that bind with different affinities and kinetic rates affect the strength of B cell activation is not fully understood. We used a panel of HIV-1 Env proteins of varying affinities and each expressed in either monomeric or multimeric forms to investigate the role of binding rates on B cell activation and Ag-induced BCR down-modulation. Ramos cells expressing the CD4 binding-site CH31 IgM BCRs were functional for Ag-specific activation and gave Ca-flux responses that was not dependent on the overall affinity or dissociation rates (off-rates) but on the association rates (on-rates) of Ag binding. Monomeric Ags did not induce flux responses and required multimerization, and the strength of activation was dependent on the on-rate of the tetrameric Ags. Comparison of half-life of the tetrameric Ags indicated a requirement of a half-life threshold for both activation and BCR down-modulation. In contrast, trimeric Ags that bound with faster on-rates (ka>104 M−1s−1) did not require higher-order multimerization (6-mer or 20-mer) for Ca-flux responses. These results provide support to a kinetic model in which B cell activation is dependent on the rates of receptor occupancy, and an above threshold half-life of the Ag-BCR complex.