experimental-analysis-of-apoptotic-thymocyte-engulfment-by-macrophages

Experimental Analysis of Apoptotic Thymocyte Engulfment by Macrophages

Cell apoptosis is a natural process and plays a critical role in embryonic development, homeostatic regulation, immune tolerance induction, and resolution ...

University of Cincinnati

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. 

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

Glomerulonephritis is one of the most severe manifestations of systemic lupus erythematosus, with considerable morbidity and mortality. There remains a major unmet need for successful management of lupus nephritis. TAM family receptor tyrosine kinases (Mer and Axl) play an important role in the maintenance of immune homeostasis in the kidney. Mer is constitutively expressed in the glomeruli; Axl expression is inducible in glomeruli under inflammatory conditions. To investigate the distinct functions of Axl and Mer in lupus nephritis, we compared the severity of nephrotoxic serum glomerulonephritis in wild-type (WT), Axl-knockout (KO), Mer-KO, and Axl/Mer-KO mice. Mer-KO mice developed severe glomerulonephritis, with significantly decreased survival and increased blood urea nitrogen levels compared with WT mice given the same treatment. However, nephrotoxic serum-treated Axl-KO mice had significantly increased survival rates and improved renal function compared with similarly treated WT, Mer-KO, and Axl/Mer-KO mice. Interestingly, mice lacking both Axl and Mer developed kidney inflammation comparable to WT mice. Western blot analysis revealed significantly increased Stat3 phosphorylation and caspase-1 activation in the kidneys of nephritic Mer-KO mice. In contrast, Axl-deficient nephrotoxic serum-injected mice showed decreased Akt phosphorylation and Bcl-xL upregulation. Thus, the reciprocal activation of Axl and Mer receptor tyrosine kinases has a major impact on the outcome of renal inflammation.

Opposing Roles of Tyrosine Kinase Receptors Mer and Axl Determine Clinical Outcomes in Experimental Immune-Mediated Nephritis.

Endotoxin induces a variety of proinflammatory mediators and plays a crucial role in kidney inflammation. The receptor tyrosine kinase, Mer, diminishes renal inflammation by attenuating inflammatory responses. We previously reported that Mer is predominantly expressed on glomerular endothelial cells (GECs) and that Mer deficiency is associated with increased renal inflammation when mice are challenged with nephrotoxic serum. We consequently hypothesized that Mer signaling down-regulates LPS-driven inflammatory responses in GECs. To test this hypothesis, primary GECs were isolated from the kidneys of Mer-KO and wild-type (WT) control mice. LPS treatment induced Akt and STAT3 activation along with Bcl-xl up-regulation in WT GECs; these responses were all increased in Mer-deficient GECs. In addition, STAT1 and ERK1/2 up-regulation and activation were observed in Mer-KO GECs exposed to LPS. In contrast, expression of the inhibitory signaling molecule, suppressor of cytokine signaling-3 (SOCS-3), was much higher in LPS-stimulated WT than Mer-deficient GECs. Deficiency of Mer was also associated with significantly increased NF-κB expression and activation. These observations indicate that Mer functions as an intrinsic feedback inhibitor of inflammatory mediator-driven immune responses in GECs during kidney injury and suggest a new therapeutic strategy for glomerular diseases.

Mechanism of Mer receptor tyrosine kinase inhibition of glomerular endothelial cell inflammation.

Glomerulonephritis (GN) is a typical lesion in autoantibody and immune complex disorders, including SLE. Because the Gas6/Axl pathway has been implicated in the pathogenesis of many types of GN, targeting this pathway might ameliorate GN. Consequently, we have studied the efficacy and mechanism of R428, a potent selective Axl inhibitor, in the prevention of experimental anti-GBM nephritis. Axl upregulation was investigated with Sp1/3 siRNA in the SV40-transformed mesangial cells. For Axl inhibition, a daily dose of R428 (125 mg/kg) or vehicle was administered orally. GN was induced with anti-GBM sera. Renal disease development was followed by serial blood urine nitrogen (BUN) determinations and by evaluation of kidney histology at the time of sacrifice. Axl-associated signaling proteins were analyzed by Western blotting and inflammatory cytokine secretion was analyzed by Proteome array. SiRNA data revealed the transcription factor Sp1 to be an important regulator of mesangial Axl expression. Anti-GBM serum induced severe nephritis with azotemia, protein casts and necrotic cell death. R428 treatment diminished renal Axl expression and improved kidney function, with significantly decreased BUN and glomerular proliferation. R428 treatment inhibited Axl and significantly decreased Akt phosphorylation and renal inflammatory cytokine and chemokine expression; similar effects were observed in anti-GBM antiserum-treated Axl-KO mice. These studies support a role for Axl inhibition in glomerulonephritis.

Targeted inhibition of Axl receptor tyrosine kinase ameliorates anti-GBM-induced lupus-like nephritis.

Microparticles (MPs) play important roles in intercellular communication, including adhesion, signal transduction, cell activation, and apoptosis. They possess a wide spectrum of biological effects in the immune responses. MPs could be immunotolerogenic or immunogenic depending on the contents and composition. Elevated levels of MPs have been reported in many forms for rheumatic diseases. This review focuses on the immunopathogenic and therapeutic role of MPs in rheumatic diseases.

Article	Year
The Mertk receptor tyrosine kinase promotes T-B interaction stimulated by IgD B-cell receptor cross-linking. Journal of autoimmunity\| PubMed ID: 24768065	2014
Opposing Roles of Tyrosine Kinase Receptors Mer and Axl Determine Clinical Outcomes in Experimental Immune-Mediated Nephritis. Journal of immunology (Baltimore, Md. : 1950)\| PubMed ID: 27527599	2016
Mechanism of Mer receptor tyrosine kinase inhibition of glomerular endothelial cell inflammation. Journal of leukocyte biology\| PubMed ID: 29350876	2018
Targeted inhibition of Axl receptor tyrosine kinase ameliorates anti-GBM-induced lupus-like nephritis. Journal of autoimmunity\| PubMed ID: 29895432	2018
The Role of Microparticles in Rheumatic Diseases and their Potentials as Therapeutic Tools. Journal of molecular immunology\| PubMed ID: 29984366	2016

Wen-Hai Shao

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