WASp Is Crucial for the Unique Architecture of the Immunological Synapse in Germinal Center B-Cells.

Yanan Li,Arpita Upadhyaya,Anshuman Bhanja,Xiaodong Zhao,Wenxia Song

doi:10.3389/fcell.2021.646077

Abstract

B-cells undergo somatic hypermutation and affinity maturation in germinal centers. Somatic hypermutated germinal center B-cells (GCBs) compete to engage with and capture antigens on follicular dendritic cells. Recent studies show that when encountering membrane antigens, GCBs generate actin-rich pod-like structures with B-cell receptor (BCR) microclusters to facilitate affinity discrimination. While deficiencies in actin regulators, including the Wiskott-Aldrich syndrome protein (WASp), cause B-cell affinity maturation defects, the mechanism by which actin regulates BCR signaling in GBCs is not fully understood. Using WASp knockout (WKO) mice that express Lifeact-GFP and live-cell total internal reflection fluorescence imaging, this study examined the role of WASp-mediated branched actin polymerization in the GCB immunological synapse. After rapid spreading on antigen-coated planar lipid bilayers, GCBs formed microclusters of phosphorylated BCRs and proximal signaling molecules at the center and the outer edge of the contact zone. The centralized signaling clusters localized at actin-rich GCB membrane protrusions. WKO reduced the centralized micro-signaling clusters by decreasing the number and stability of F-actin foci supporting GCB membrane protrusions. The actin structures that support the spreading membrane also appeared less frequently and regularly in WKO than in WT GCBs, which led to reductions in both the level and rate of GCB spreading and antigen gathering. Our results reveal essential roles for WASp in the generation and maintenance of unique structures for GCB immunological synapses.

Highlights

B-cell-mediated antibody responses provide essential immune protection against infectious diseases
Phosphorylated CD79a was detected in the contact zone of both wild type (WT) and Wiskott-Aldrich syndrome protein (WASp) knockout (WKO) light zone (LZ) Germinal center B-cells (GCBs) (Figure 1A) but not in the contact zone of GCBs interacting with Tf-Planar Lipid Bilayers (PLBs) (Supplementary Figure 4)
Both the mean fluorescence intensity (MFI) and the total fluorescence intensity (TFI) of phosphorylated CD79a (pCD79a) in the contact zone of individual LZ GCBs increased over time, peaked after ∼5 and ∼9 min incubation, respectively, and persisted at least for 20 min (Figures 1A,B)

Summary

Introduction

B-cell-mediated antibody responses provide essential immune protection against infectious diseases. WASp in Germinal Center B-Cells (DZ) of GCs. Germinal center B-cells (GCBs) with mutated BCRs migrate to the light zone (LZ), where GCBs compete with each other to engage and capture antigen through their clonalspecific BCRs (Chan and Brink, 2012; Shlomchik and Weisel, 2012; Victora and Nussenzweig, 2012). BCR’s ability to transduce signals and capture antigens is essential for GCBs to survive and differentiate into memory B-cells that mediate rapid and robust recall antibody responses and long-lived plasma cells that maintain the levels of protective antibodies (Gitlin et al, 2014; Turner et al, 2018; Shlomchik et al, 2019)

Methods

Results

Conclusion