Elsevier

Journal of Autoimmunity

Volume 53, September 2014, Pages 78-84
Journal of Autoimmunity

The Mertk receptor tyrosine kinase promotes T–B interaction stimulated by IgD B-cell receptor cross-linking

https://doi.org/10.1016/j.jaut.2014.03.004Get rights and content

Highlights

  • Mertk-KO mice exhibit reduced IgG and IgE responses to anti-mouse IgD.

  • Mertk expression is required for optimal T–B interaction after B-cell IgD crosslinking.

  • Mertk may modulate the immunological synapse during T–B interaction.

Abstract

The Mertk receptor tyrosine kinase facilitates macrophage and DC apoptotic-cell clearance and regulates immune tolerance. Mertk may also contribute to B-cell activation, because Mertk-KO mice fail to develop autoantibodies when allo-activated by T cells. We investigated this possibility with a well-characterized model in which injection of mice with goat anti-IgD antibody causes membrane IgD cross-linking that induces T-independent B cell activation and antigen presentation to T cells. Goat anti-mouse IgD antibody-injected C57BL/6 Mertk-KO mice had normal initial B cell activation and proliferation, but significantly lower T cell activation and proliferation, as well as lower IgE and IgG anti-goat IgG responses, as compared to C57BL/6 WT controls. B cell antigen processing, analyzed by evaluating B cell fluorescence following injection of monoclonal anti-IgD antibody labeled with biotin or FITC, was comparable between Mertk-KO mice and WT mice. IgD Ab primed B cells from Mertk-KO mice exhibited significantly lower ability in activating memory T cells isolated from WT mice injected with the same antigen 10 days before. These observations suggest that Mertk expression is required for optimal B-cell antigen presentation, which is, in turn, required in this model for optimal T cell activation and subsequent T cell-dependent B cell differentiation.

Introduction

Goat IgG anti-mouse IgD antibody (Ab) (GαmD) binds to B-cell membrane IgD and cross-links this B cell receptor (BCR), inducing T cell-independent B-cell activation, GamD internalization and processing, and presentation of Class II MHC-associated GamD-derived peptides to CD4+ T cells [1], [2]. The goat IgG-specific CD4+ T cells primed or activated by these B cells then secrete cytokines, including IL-4, and express surface molecules that activate newly generated B cells to proliferate and differentiate into Ab secreting cells [3]. Most initially activated B cells undergo apoptosis 2–5 days after injection [4]. Major findings that have been reported with this well-characterized model are summarized in Fig. 1 [5], [6], [7]. The IL-4 produced by T cells in this response is required for Ig isotype switching to IgE and suppresses complement-fixing isotypes (IgG2a and IgG3) [8], [9]. Studies with univalent F(ab') fragments of anti-IgD Ab and with mAbs that poorly crosslink IgD demonstrate that T-independent B cell activation is required to stimulate T cell activation and B cell antibody production [10]. Indeed, failure to activate B cells by this mechanism can induce Ag specific T-cell tolerance [11].

Mertk belongs to the TAM (Tyro-3, Axl, and Mertk) subfamily of receptor tyrosine kinases [12]. Mertk mediates the engulfment of apoptotic cells through the bridging molecules (growth-arrest-specific gene 6 (Gas6) or Protein S (ProS)), and promotes maturation of macrophages to a non-inflammatory phenotype [13], [14], [15]. Without Mertk, mice develop lupus-like autoimmune manifestations and retinal degeneration-associated blindness [13], [16]. Mertk is expressed on subpopulations of macrophages and dendritic cells [17] and is also found on certain B cells, where it is tightly regulated [18]. Mertk-deficient B cells are unresponsive to T-cell help in a chronic graft-versus-host disease (cGVHD) model [18], [19], yet the mechanisms whereby Mertk regulates B-cell function remain obscure.

To understand better how Mertk regulates B-cell responses, we chose the anti-IgD model because it allows in vivo evaluation of T-independent B cell activation by membrane IgD cross-linking, B cell Ag processing, B cell-dependent T cell activation, and T cell-dependent Ag production. We cross-linked B-cell IgD receptors with GαmD and studied the subsequent B-cell activation-induced immune responses. We report here that B cells lacking Mertk have a defect in activating T cells, and are associated with lower Ab production compared to B cells from WT mice.

Section snippets

Mice and reagents

C57BL/6J (B6; 6–8 weeks old; WT controls) and C57BL/6J Mertk deficient (Mertk-KO; 6–8 weeks old) mice were bred and maintained in our mouse colony. Experimental mice were sex and age matched. All mouse procedures followed the guidelines for the use of animals in research and were approved by the Institutional Animal Care and Use Committee of Temple University. Affinity-purified goat Ab specific for mouse IgD (GαmD) and monoclonal mouse IgG1 anti-mouse IgD (clone 1.3.7) were produced and tested

Mertk-KO mice exhibit significantly reduced responses to goat anti-mouse IgD cross-linking

We previously reported an intrinsic B-cell unresponsiveness to bm12 induced chronic GVHD from Mertk-KO mice [18], [19]. To further explore the function of Mertk on B cells, we injected Mertk-KO mice with GαmD and measured immunoglobulin levels compared to WT mice undergoing the same treatment. We therefore measured the serum level of total IgG with untreated mice serum as control. As expected, WT mice showed a dramatic increase of total IgG in the serum 10 days after GαmD injection. Mertk-KO

Discussion

Studies on the function of Mertk receptor tyrosine kinase have focused on apoptotic cell clearance and immune regulation through Mertk-bearing macrophages and DCs [12], [13], [21], [22], [23], [24]. We have noted expression and function of Mertk on certain B cells [18], [19]. In this report, we describe deficient antigen presentation by B cells from Mertk deficient mice. Similar to B cells from WT mice, B cells from Mertk-KO mice were activated and proliferated normally in response to membrane

Acknowledgments

This work was supported by grants from the NIAID (IU19AI082726) and NIH (AI097758) to PLC and by Arthritis Foundation Postdoctoral Fellowship Award and NIDDK (1K01DK095067-01A1) to WHS. FDF receives support from the U.S. Department of Veterans Affairs.

References (26)

  • F.D. Finkelman et al.

    Lymphokine control of in vivo immunoglobulin isotype selection

    Annu Rev Immunol

    (1990)
  • S.C. Morris et al.

    IL-4 suppression of in vivo T cell activation and antibody production

    J Immunol

    (2000)
  • D.K. Goroff et al.

    Polyclonal activation of the murine immune system by an antibody to IgD. XI. Contribution of membrane IgD cross-linking to the generation of an in vivo polyclonal antibody response

    J Immunol

    (1991)
  • Cited by (7)

    • Immunological role of TAM receptors in the cancer microenvironment

      2020, International Review of Cell and Molecular Biology
      Citation Excerpt :

      The role of TAM receptors on B cells has not been well characterized. It is known that B cells express Mertk and it has also been reported that Mertk depletion in mouse B cells led to decreased antibody production and reduced T cell activation, pointing toward a costimulatory role of Mertk on B cells (Shao et al., 2014). Although Axl and Tyro3 expression have been correlated with B cell leukemia (Ghosh et al., 2011), the specific roles of TAM receptors on B cells are yet to be studied.

    View all citing articles on Scopus
    View full text