Structurally distinct endocytic pathways for B cell receptors in B lymphocytes

Abstract

B lymphocytes play a critical role in adaptive immunity. Upon antigen binding, B cell receptors (BCR) cluster on the plasma membrane and are internalized by endocytosis. B cells capture diverse antigens in various contexts and concentrations. However, it is unclear whether the mechanism of BCR endocytosis changes in response to these factors. Here, we studied the concentration dependence of anti-human IgM Fab'2-induced BCR clustering and internalization in the human IgM+ DG-75 B cell line using advanced imaging methods, including correlative super resolution fluorescence and electron microscopy. By directly visualizing nanoscale structures associated with BCR clusters, we provide evidence that BCR cluster size increases with Fab'2 concentration and the mechanism of internalization switches in response to BCR cluster size. At low concentrations of Fab'2, B cells internalize BCR clusters by classical clathrin-mediated endocytosis. However, at high Fab'2 concentrations, B cells retrieve clathrin-bound BCR clusters using large invaginations of the plasma membrane.

Clathrin-mediated endocytosis cooperates with bulk endocytosis to generate vesicles

“…A positive membrane curvature may thus facilitate clathrin-coated pit generation, because artificially generated positive membrane curvatures are preferred for CCS generation (Zhao et al, 2017). Likewise, in immune B cells and macrophages, clathrin has been shown to form on large invaginations of the plasma membrane (Roberts et al, 2020). We propose that plasma membrane invaginations are a major platform for clathrin-coated pit formation by providing positive curvature, where unique proteins, lipids, tensions, adhesion energies, phases, or cytoskeletal structures may enhance clathrin lattice growth and development.…”

“…This finding suggests that two seemingly independent forms of endocytosis, clathrin-mediated endocytosis, and bulk endocytosis, work together to enhance endocytic capacity. Such a collaboration is of broad importance, given that clathrin-mediated endocytosis is essential for the life of most eukaryotic cells (Mettlen et al, 2018), and bulk endocytosis is a major form of endocytosis in secretory cells and beyond (Clayton et al, 2008;Kononenko et al, 2014;Richards et al, 2000;Roberts et al, 2020;Watanabe et al, 2013;Wu et al, 2014a;Wu and Wu, 2007).…”

Bidirectional feedback between BCR signaling and actin cytoskeletal dynamics

“…FCH and double SH3 domain‐containing protein 2 (FCHSD2), a protein that generates F‐actin at sites of clathrin‐mediated endocytosis, selectively colocalizes with relatively larger nanoscale BCR‐Antigen clusters, generating F‐actin around them, which enhances their endocytosis. In contrast, relatively smaller nanoscale BCR clusters (induced by lowering the concentration of soluble antigen) were only associated with classical clathrin‐mediated endocytic pits [93]. B cells can also internalize antigen through clathrin‐independent pathways (Fig.…”