Membrane Mechanics of B Lymphocyte Activation

Abstract

B lymphocytes are a critical component of the immunological machinery whose primary role is to produce and secrete antibodies that detect foreign antigens. When stimulated by their corresponding antigen, B lymphocytes are triggered to differentiate into antibody secreting Plasma cells. It has been shown that this differentiation requires the transcription factor XBP-1. The mechanism by which XBP-1 deficiency deters Plasma cell differentiation is not understood. XBP-1 regulates lipid synthesis in B lymphocytes, and hence in activated cells, the lipid composition of the cell membrane in XBP-1 deficient cells is different. The aim of this work is to mechanically characterize the B lymphocyte membrane and quantify the consequences of XBP-1 deficiency on the membrane mechanical properties of activated B lymphocytes. We probed the mechanical properties of the cell membrane using optical tweezers. Membrane tethering experiments were performed by locally dissociating the lipid membrane from the underlying cytoskeleton and extending a tube of lipid bilayer from the cell surface while measuring the force of extension, and then the relaxation of the force after extension. Wild type B lymphocytes exhibit three stages of tethering: 1) a linearly increasing force due to local cell stiffness 2) an approximately constant force (plateau force) regime after the membrane locally dissociates from the cytoskeleton, and 3) force relaxation after the tether extension is stopped. Tethering experiments were performed on wildtype (WT) and XBP-1 deficient B lymphocytes activated by bacteria derived lipopolysaccharide. Experimental results show that activated XBP-1 deficient cells have a lower membrane viscosity indicated by a lower plateau force and a faster tether force relaxation. Additionally, a micromechanical model is developed to describe the force of tether extension.