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Treatment of patients with glucocorticoids can result in an increased risk of infection with pathogens such as fungi. Dectin-1 is a member of the C-type lectin receptor superfamily and was shown to be one of the major receptors for fungal beta-glucans. Activation of Dectin-1 increases the production of cytokines and chemokines and T-cell stimulatory capacity of DC and mediates resolution of fungal infections.
Here we show that antigen-presenting cells generated in the presence of dexamethasone (Dex-DC) have a reduced capacity to stimulate T-cell proliferation and decreased expression of costimulatory molecules, that can not be enhanced upon stimulation with Dectin-1 ligands. Stimulation of Dex-DC with beta-glucans induced a strong upregulation of Syk phosphorylation and increased secretion of IL-10, while the production of IL-12, IL-23 and TNF-alpha was reduced. Downstream of Syk stimulation of Dectin-1 on Dex-DC resulted in phosphorylation of STAT3 and reduced nuclear localization of transcription factors involved in DC activation and function. Defects in the Dectin-1 molecule can result at least in increased mucosal infections with fungi in affected individuals. In our study we analyzed Dectin-1 expression and signaling in Dex-DC and compared it to immature dendritic cells (iDC).
First, we analyzed whether DC that were generated in the presence of dexamethason (Dex-DC) express Dectin-1 on their surface. Dectin-1 was highly expressed on Dex-DC cells compared to iDC. Stimulation of iDC with zymosan or curdlan increased the expression of the maturation markers CD80, CD83 and CD86. In contrast, Dex-DC show a reduced upregulation of these markers. CD11b was expressed at lower levels on Dex-DC as compared to iDC and was downregulated on both subsets after stimulation with beta-glucans. CD11b is part of CR3, which was found to be another beta-glucan and collaborating receptor of Dectin-1.
As exprected, dexamethasone-treatment of DC results in reduced capacity to stimulate the proliferation of allogeneic T-cells in a MLR- assay, that could not be increased by stimulation with Dectin-1 ligand curdlan or the TLR-4 ligand LPS.
DC generated in the presence of dexamethasone secreted lower amounts of TNF-alpha, IL-23 and IL-12p70 upon stimulation with Curdlan. Interestingly, secretion of IL-10 was increased by Dex-DC. Secretion of cytokines by Dex-DC was Syk-dependent as shown by incubation of cells with the syk-inhibitor R406 prior to Dectin-1 stimulation with curdlan.
Incubation of Dex-DC with beta-glucans resulted in increased phosphorylation of Syk as compared to iDC and strongly increased generation of superoxide-anions. Downstream of Syk, we observed a highly increased activation of STAT3 signaling while the induction of STAT3 signaling was absent in iDC. STAT3 mediates immune inhibitory effects on DC function by promoting expression and secretion of anti-inflammatory cytokines like IL-10 and subsequent inhibition of Th1- and Th17-mediated response. In nuclei of Dex-DC transcription factors which are important for dendritic cell activation and cytokine secretion were not detected. In contrast, p50 was found to accumulate in nuclei of Dex-DC which might contribute to IL-10 expression.
The phosphatase SHP-1, which is thought to regulate Syk-activitiy, was found to be expressed at lower levels in Dex-DC. CD45, another Syk-regulating phosphatase was expressed at similar levels as compared to iDC.
Dexamethasone inhibited the function and differentiation of monocyte derived DC and abrogated the immunological effects induced by interaction of fungal beta-glucans with Dectin-1. It inhibited the secretion of cytokines and chemokines by antigen-presenting cells such as TNF-alpha, IL-12 and IL-23, which are important for T-cell activation and reduced the upregulation of costimulatory molecules on the cell surface upon interaction with beta-glucans. This is probably due to a diminished nuclear expression of several transcription factors involved in DC differentiation and function. Furthermore, dexamethasone increased the expression of Dectin-1 and Syk-phosphorylation but redirected downstream signaling towards STAT-3 that results in production of IL-10, that further contributes to the inhibition of anti-fungal immune responses. Finally the phosphatase SHP-1 was expressed at lower levels in Dex-DC and might further be affected by Dectin-1 mediated oxidative stress.
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