Mechanosensing through the B cell antigen receptor

About This Grant

Project Summary/Abstract Signaling through the B cell antigen receptor (BCR) is indispensable for B lymphocyte maturation, activation and differentiation as it governs the initiation of transcriptional programs associated with B cell activation and fate decisions, as well as the BCR-dependent processing of antigen and presentation of antigen to T cells. The dysregulation of BCR signaling results in immunodeficiency, autoimmunity or tumorigenesis. However, despite its pivotal relevance to health and disease, the mechanisms by which antigen binding initiates BCR signaling are still incompletely understood. Upon maturation, naïve B cells express both IgM and IgD isotypes of BCR, which have identical specificity and affinity for antigen. Yet, the functional advantage to co-express two BCR isotypes with equal affinities on every mature B cell remains puzzling. After antigen stimulation in the germinal center (GC), B cells differentiate and undergo somatic hypermutation as well as class switch recombination, by which their BCRs greatly increase affinity for antigen and change isotypes to IgG, IgE, and IgA. It has been shown that GC B cells have intrinsically higher-affinity thresholds for BCR signaling and pull on BCRs to extract from dendritic cells bound antigens mechanically. However, it is not known how the affinity threshold translates into force threshold and how mechanical force modulates BCR signaling. Using a set of mechanoimmunological concepts, ideas, approaches, and tools, we propose to investigate these fundamental questions under two specific aims, Aim 1 will test the B cell mechanosensory hypothesis at the single-cell and single-bond level by characterizing the Ig isotype-dependent in situ affinities and force-dependent lifetimes of BCR–antigen bonds and by correlating concurrently measured calcium (Ca2+) fluxes in B cells with the biophysical measurements. Aim 2 will test the B cell mechanosensory hypothesis at the cell population level by characterizing the Ig isotype-dependent B cell endogenous forces on BCR–antigen bonds, by measuring the Ig isotype-dependent calcium fluxes in B cells induced by BCR–antigen engagement, and by determining the force requirement for Ig isotype-dependent calcium fluxes in B cells. The proposed research holds the promise to provide the conceptual framework, design principles, and engineering strategies to harness the power of the B cell adaptive immune system to combat diseases ranging from viral infections, auto-immune disorders, to cancer.