Tunable B cell response through allosteric modulation of Bruton's tyrosine kinase

Abstract The B cell receptor (BCR) is essential for both the initial burst of B cell proliferation in response to antigens, and for the differentiation of plasma cells and memory B cells during germinal center reactions. However, the BCR’s precise role in germinal center selection has been difficult to assess: mechanistic investigations into its modulation during the decision of B cell fate suffer from experimental limitations due to the fundamental role of the BCR in B cell biology. Bruton’s tyrosine kinase (BTK), a member of the TEC family, is a key component of BCR signaling. Using a computational screening method based on small molecule docking outside of the BTK active site, we have identified putative allosteric modulators of BTK. In vitro primary B cell Nur77 upregulation, calcium flux, proliferation and plasma cell differentiation confirmed that these molecular probes alter B cell activation strength, kinetics, and fate only when the BCR is engaged. In vivo immunization using an antigen model reveled that BTK agonism strongly favors B cell differentiation into switched plasma cells. BTK agonism, either at the time of the immunization or at the peak of germinal center formation, enhanced humoral immunity, proving for the first time that transient, augmented BCR signaling during germinal center reaction has a direct impact on B cell differentiation ability. In addition to being powerful tools for basic B cell biology investigation, allosteric agonists of BTK could be developed into antigen-licensed B cell adjuvants for vaccines in at risk populations, as well as into cancer immunotherapy drugs which hyperactivate B cells in tumor microenvironments during checkpoint inhibitor blockade therapies.