B cells extract antigens at Arp2/3-generated actin foci interspersed with linear filaments

Abstract

Antibody production depends on B cell internalization and presentation of antigens to helper T cells. To acquire antigens displayed by antigen-presenting cells, B cells form immune synapses and extract antigens by the mechanical activity of the acto-myosin cytoskeleton. While cytoskeleton organization driving the initial formation of the B cell synapse has been studied, how the cytoskeleton supports antigen extraction remains poorly understood. Here we show that after initial cell spreading, F-actin in synapses of primary mouse B cells and human B cell lines forms a highly dynamic pattern composed of actin foci interspersed with linear filaments and myosin IIa. The foci are generated by Arp2/3-mediated branched-actin polymerization and stochastically associate with antigen clusters to mediate internalization. However, antigen extraction also requires the activity of formins, which reside near the foci and produce the interspersed filaments. Thus, a cooperation of branched-actin foci supported by linear filaments underlies B cell mechanics during antigen extraction.

The cell pushes back: The Arp2/3 complex is a key orchestrator of cellular responses to environmental forces

“…Arp2/3-dependent branched actin polymerisation is also important for generation of actin foci that are interspersed with linear filaments and myosin-IIa in B cells. These Arp2/3-dependent structures drive antigen internalisation implicating the mechanical properties of branched actin in immunity [ 24 ]. Specifically, both the Arp2/3 complex and formins appeared to be needed for antigen uptake by B cells by the creation of a focal filament network complex that through local bursts of actin polymerisation generates an inward force driving antigen internalisation.…”

Physical ‘strength’ of the multi‐protein chain connecting immune cells: Does the weakest link limit antibody affinity maturation?

“…[ 29–32 ] The BCR is subjected to a mechanical pulling force by the contractile motion of the B cell surface. [ 19,27,33–38 ] This force is transmitted through the chain of protein complexes that links the B cell to the FDC. This chain involves the following complexes: BCR–Ag, Ag–Ab (IC) and Ab–Fc γ RIIB receptor (or Ab–CR2 receptor).…”

“…B cells are known to exert a pulling force for Ag acquisition. [ 19,27,33–38 ] Mechanical forces can change the on and off rates in complex ways. [ 39,51–54 ] In ‘slip bonds’, tensile forces hasten dissociation, whereas in ‘catch bonds’, they delay dissociation; some bonds can exhibit both behaviours.…”

Endophilin A2 regulates B cell protein trafficking and humoral responses

“…Substrate-bound antigen internalization was quantified by detecting antigen clusters extracted from the PMSs inside the B cells identified using B220 or CD19 surface staining as described previously (Nowosad et al 2016). Endophilin A2-GFP and clathrin-mCherry spots were detected as described (Roper et al 2019). Briefly, Endophilin A2 or clathrin TIRF images were enhanced by convolution with a 325 nm-sized bandpass filter and spots above threshold were tracked and analyzed for fluorescence intensities in all relevant channels.…”