Distinct Roles for Bruton's Tyrosine Kinase in B Cell Immune Synapse Formation.

Sara Roman-Garcia,Sara V Merino-Cortes,Sofia R Gardeta,Marjolein J W De Bruijn,Rudi W Hendriks,Yolanda R Carrasco

doi:10.3389/fimmu.2018.02027

Sara Roman-Garcia, Sara V Merino-Cortes + Show 4 more

Open Access

https://doi.org/10.3389/fimmu.2018.02027

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Abstract

Bruton's tyrosine kinase (Btk) has a key role in the signaling pathways of receptors essential for the B lymphocyte response. Given its implication in B cell-related immunodeficiencies, leukemias/lymphomas and autoimmunity, Btk is studied intensely and is a target for therapy. Here, using primary B cells from distinct mouse models and the pharmacological inhibitors ibrutinib and acalabrutinib, we report distinct roles for Btk in antigen-triggered immune synapse (IS) formation. Btk recruitment to the plasma membrane regulates the B cell ability to trigger IS formation as well as its appropriate molecular assembly; Btk shuttling/scaffold activities seem more relevant than the kinase function on that. Btk-kinase activity controls antigen accumulation at the IS through the PLCγ2/Ca2+ axis. Impaired Btk membrane-recruitment or kinase function likewise alters antigen-triggered microtubule-organizing center (MTOC) polarization to the IS, B cell activation and proliferation. Data also show that, for B cell function, IS architecture is as important as the quantity of antigen that accumulates at the synapse.

Highlights

Bruton’s tyrosine kinase (Btk) belongs to the Tec family of non-receptor tyrosine kinases
Quantification of the area and total quantity of su-Ag aggregated at the cSMAC indicated that ibrutinibtreated WT (Ibru)-B cells had smaller cSMAC and accumulated less suAg at the immune synapse (IS) than controls; values for area and total suAg aggregation in Xid B cells were similar to those for wild type (WT) (Figures 1A,E)
pleckstrin homology (PH) domain-mediated Btk recruitment to the plasma membrane is required for the B cell capacity to trigger IS formation and for appropriate pSMAC assembly; the shuttling/scaffold activities and the regulation of PIP2 production at the antigen recognition site appeared to be involved

Summary

Introduction

Bruton’s tyrosine kinase (Btk) belongs to the Tec family of non-receptor tyrosine kinases. For activation downstream of the BCR, Btk is recruited from cytosol to the plasma membrane by phosphatidylinositol [3,4,5]-trisphosphate (PIP3) through the PH domain, followed by phosphorylation at residue Y551 by Lyn/Syk, and auto- or transphosphorylation at Y223 [3]. Btk binds to phosphorylated SLP65 and activates phospholipase Cγ2 (PLCγ2), which triggers Ca2+ mobilization and diacylglycerol (DAG)-dependent pathways [3]. Btk has scaffold/shuttling functions; it brings phosphatidylinositol-4-phosphate 5-kinase (PIP5KI) from the cytosol to the plasma membrane to produce phosphatidylinositol [4,5]-bisphosphate (PIP2), a substrate of PLCγ2, and of PI3K to generate PIP3 [4]. Btk is implicated in integrin activation downstream of the BCR [5], and in actin dynamics to promote BCR microcluster assembly, cell spreading and antigen processing [6,7,8]

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