FcαRI Dynamics Are Regulated by GSK-3 and PKCζ During Cytokine Mediated Inside-Out Signaling

Toine Ten Broeke,Henk Honing,Shamir Jacobino,Leo Koenderman,Jeanette H W Leusen,Jan A M Van Der Linden,Madelon Bracke,Deon Kanters,Jantine E Bakema,Arianne M Brandsma

doi:10.3389/fimmu.2018.03191

Toine Ten Broeke, Henk Honing + Show 8 more

Open Access

https://doi.org/10.3389/fimmu.2018.03191

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Abstract

IgA binding to FcαRI (CD89) is rapidly enhanced by cytokine induced inside-out signaling. Dephosphorylation of serine 263 in the intracellular tail of FcαRI by PP2A and PI3K activation are instrumental in this process. To further investigate these signaling pathways, we targeted downstream kinases of PI3K. Our experiments revealed that PI3K activates PKCζ, which subsequently inhibits GSK-3, a constitutively active kinase in resting cells and found here to be associated with FcαRI. We propose that GSK-3 maintains FcαRI in an inactive state at homeostatic conditions. Upon cytokine stimulation, GSK-3 is inactivated through a PI3K-PKCζ pathway, preventing the maintenance of phosphorylated inactive FcαRI. The concomitantly activated PP2A is then able to dephosphorylate and activate FcαRI. Moreover, FRAP and FLIP studies showed that FcαRI activation coincides with an increased mobile fraction of the receptor. This can enhance FcαRI valency and contribute to stronger avidity for IgA immune complexes. This tightly regulated inside-out signaling pathway allows leukocytes to respond rapidly and efficiently to their environment and could be exploited to enhance the efficacy of future IgA therapeutics.

Highlights

Transmembrane receptors specific for the Fc-portion of immunoglobulins, Fc-receptors (FcR), play an important role in leukocyte activation by recognizing and binding of opsonized targets during inflammatory processes [1]
We have previously demonstrated that IgA binding to FcαRI on human eosinophils is modulated by cytokine induced activation of phosphatidylinositol 3-kinase (PI3K) [17]
To further investigate which pathways could be involved in cytokine induced FcαRI activation, we used pharmacological inhibitors of known targets of PI3K, like p70 S6 Kinase (p70S6K), Protein Kinase B (PKB), and Protein Kinase C (PKC) isoforms, in a rosette assay

Summary

Introduction

Transmembrane receptors specific for the Fc-portion of immunoglobulins, Fc-receptors (FcR), play an important role in leukocyte activation by recognizing and binding of opsonized targets during inflammatory processes [1]. Specific FcR exist for all 5 classes of human immunoglobulins of which those specific for IgG and IgE are best studied. Less is known about the receptor specific for monomeric IgA, FcαRI (CD89). FcR play a major role in effector mechanisms induced by many IgG therapeutics currently used in the clinic [4, 5]. To IgG, promising potential of therapeutic IgA monoclonals found in preclinical studies implicate that their efficacy largely depend on FcαRI, validating the need for more knowledge on FcαRI function [6,7,8,9]

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