Mechanical Extraction of Antigen from B Cell Immune Synapses: A Unique Way to Sense Ligand Affinity

Abstract

The generation of high affinity antibodies depends on the ability of B cells to acquire and internalize foreign antigens from the surface of antigen-presenting cells (APCs). This process starts by binding of antigens to the B cell receptors, which triggers intracellular signaling resulting in B cell spreading, antigen clustering and eventually antigen internalization. We show that to internalize the antigen clusters, B cells use dynamic pulling mediated by contractions of the non-muscle myosin IIa. Myosin contractions lead to invagination of the antigen-presenting membrane and pinching off of the antigen by clathrin-mediated endocytosis. Interestingly, atomic force microscopy measurements showed that the BCR was unusually sensitive to force. Consequently, myosin IIa contractions quickly ruptured many BCR-antigen bonds, particularly within small microclusters. However, larger microclusters sustained the forces and were internalized. Thus, myosin contractility mechanically tests stability of the microclusters, imposing a dynamic threshold on strength of the BCR-antigen bonds necessary for antigen extraction. Consistently with this idea, the efficiency of internalization was highly sensitive to the affinity of the BCR for the antigen, and reducing the strength of myosin contractility abrogated discrimination between high and low affinity antigens. We also show that the intensity and timing of B cell spreading, antigen clustering and myosin contractility differ in B cell subsets, possibly explaining different sensitivity of B cell subsets to antigen affinity. These results show that affinity discrimination by B cells is a mechano-active process that is regulated by coordination of signaling, actomyosin contractility and endocytosis.