CD22 ligation inhibits downstream B cell receptor signaling and Ca2+ flux upon activation

Authors

  • N. Sieger,

    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
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  • S. J. Fleischer,

    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
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  • H. E. Mei,

    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
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  • K. Reiter,

    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
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  • A. Shock,

    1. UCB Pharma, Slough, UK
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  • G. R. Burmester,

    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
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    • Dr. Burmester has received consulting fees, speaking fees, and/or honoraria from UCB (consulting, scientific grants, and honoraria for lectures) and Immunomedics (clinical trial) (less than $10,000 each).

  • C. Daridon,

    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
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    • Drs. Daridon and Dörner contributed equally to this work.

  • T. Dörner

    Corresponding author
    1. Charité University Medicine Berlin and the German Rheumatism Research Center, Berlin, Germany
    • Department of Medicine/Rheumatology and Clinical Immunology, Charité Berlin, Charitéplatz 1, 10098 Berlin, Germany
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    • Drs. Daridon and Dörner contributed equally to this work.

    • Dr. Dörner has received consulting fees, speaking fees, and/or honoraria from UCB Pharma, Roche, and Sanofi-Aventis (less than $10,000 each) and research support from Immunomedics.


Abstract

Objective

CD22 is a surface molecule exclusively expressed on B cells that regulates adhesion and B cell receptor (BCR) signaling as an inhibitory coreceptor of the BCR. Central downstream signaling molecules that are activated upon BCR engagement include spleen tyrosine kinase (Syk) and, subsequently, phospholipase Cγ2 (PLCγ2), which results in calcium (Ca2+) mobilization. The humanized anti-CD22 monoclonal antibody epratuzumab is currently being tested in clinical trials. This study was undertaken to determine the potential mechanism by which this drug regulates B cell activation.

Methods

Purified B cells were preincubated with epratuzumab, and the colocalization of CD22 and CD79α, without BCR engagement, was assessed by confocal microscopy. The phosphorylation of Syk (Y348, Y352) and PLCγ2 (Y759) as well as the Ca2+ flux in the cells were analyzed by flow cytometry upon stimulation of the BCR and/or Toll-like receptor 9 (TLR-9). The influence of CD22 ligation on BCR signaling was assessed by pretreating the cells with epratuzumab or F(ab′)2 fragment of epratuzumab, in comparison with control cells (medium alone or isotype-matched IgG1).

Results

Epratuzumab induced colocalization of CD22 and components of the BCR independent of BCR engagement, and also reduced intracellular Ca2+ mobilization and diminished the phosphorylation of Syk and PLCγ2 after BCR stimulation in vitro. Inhibition of kinase phosphorylation was demonstrated in both CD27− and CD27+ B cells, and this appeared to be independent of Fc receptor signaling. Preactivation of the cells via the stimulation of TLR-9 did not circumvent the inhibitory effect of epratuzumab on BCR signaling.

Conclusion

These findings are consistent with the concept of targeting CD22 to raise the threshold of BCR activation, which could offer therapeutic benefit in patients with autoimmune diseases.

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