Serum amyloid A inhibits dendritic cell apoptosis to induce glucocorticoid resistance in CD4+ T cells

J L Ather,R C Budd,V Anathy,M E Poynter,K A Fortner

doi:10.1038/cddis.2013.327

Abstract

Mediators produced by the airway epithelium control the activation, recruitment, and survival of pulmonary dendritic cells (DC) that present antigen to CD4+ T cells during the genesis and exacerbation of allergic asthma. The epithelial-derived acute phase protein, serum amyloid A (SAA), induces DC maturation and TH17 polarization. TH17 responses are associated with severe forms of allergic asthma that are poorly controlled by corticosteroids. We sought to determine whether SAA would enhance the survival of DC during serum starvation and could then contribute to the development of a glucocorticoid-resistant phenotype in CD4+ T cells. Bone marrow-derived dendritic cells (BMDC) that were serum starved in the presence of SAA were protected from activation of caspase-3 and released less lactate dehydrogenase. In comparison with untreated serum-starved BMDC, treatment with SAA downregulated mRNA expression of the pro-apoptotic molecule Bim, increased production of the pro-survival heat shock protein 70 (HSP70), and induced secretion of pro-inflammatory cytokines. SAA-treated BMDC that were serum starved for 48 h remained capable of presenting antigen and induced OTII CD4+ T cells to secrete IL-17A, IL-17F, IL-21, IL-22, and IFNγ in the presence of ovalbumin. IL-17A, IL-17F, IL-21, and IFNγ production occurred even when the CD4+ T cells were treated with dexamethasone (Dex), whereas glucocorticoid treatment abolished cytokine secretion by T cells cocultured with untreated serum-starved BMDC. Measurement of Dex-responsive gene expression demonstrated CD4+ T cells as the target of glucocorticoid hyperresponsiveness manifest as a consequence of BMDC stimulation by SAA. Finally, allergic airway disease induced by SAA and antigen inhalation was unresponsive to Dex treatment. Our results indicate that apo-SAA affects DC to both prolong their viability and increase their inflammatory potential under apoptosis-inducing conditions. These findings reveal mechanisms through which SAA enhances the CD4+ T-cell-stimulating capacity of antigen-presenting cells that may actively participate in the pathogenicity of glucocorticoid-resistant lung disease.

Highlights

serum amyloid A (SAA)-treated Bone marrow-derived dendritic cells (BMDC) that were serum starved for 48 h remained capable of presenting antigen and induced OTII CD4 þ T cells to secrete IL-17A, IL-17F, IL-21, IL-22, and IFNc in the presence of ovalbumin
We investigated the OTII CD4 þ T-cell responses to BMDC that had been serum starved for 48 h in the presence or absence of apo-SAA. apo-SAA-treated BMDC induced CD4 þ T cells to secrete enhanced amounts of the TH17 cytokines IL-17A, IL-17F, IL-21, and IL-22, whereas they did not enhance the production of the TH2 cytokine IL-13, and only marginally increased the levels of the TH1 cytokine IFNg (Figure 3)
Wild type BMDC were serum starved for 48 h in the presence or absence of the pan-caspase inhibitor zVAD, prior to coculture with OTII CD4 þ T cells and OVA. zVAD-treated cells upregulated IL-17A, IL-21, and IL-22 (Figure 7b)

Summary

Introduction

Our results indicate that apo-SAA affects DC to both prolong their viability and increase their inflammatory potential under apoptosis-inducing conditions These findings reveal mechanisms through which SAA enhances the CD4 þ T-cell-stimulating capacity of antigen-presenting cells that may actively participate in the pathogenicity of glucocorticoid-resistant lung disease. The acute phase protein serum amyloid A (SAA) is produced by a variety of cells in response to inflammatory insult and has been linked to a number of diseases, including Alzheimer’s disease, rheumatoid arthritis, In the present study, we investigated the effect of apo-SAA on BMDC under conditions of serum starvation, which would normally induce apoptosis mediated by mitochondrial outer membrane permeabilization and caspase-3 activation.[6] Our results demonstrate that apo-SAA treatment interferes with the induction of Bim, inhibits caspase-3 activation, and induces expression of the chaperone protein and cytokine, heat shock protein 70 (HSP70). SAA represents an endogenous mediator of DC lifespan and function that both quantitatively and qualitatively dictates the CD4 þ T-cell response

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Conclusion