Plasmacytoid dendritic cells (pDC) have a pivotal role in clearing viral infections, and can regulate tolerance or immunity depending on their activation status. We have previously demonstrated a strong influx of pDC in the tubulointerstitium of human renal allograft rejection biopsies. In experimental transplant models, pDC were demonstrated to be activators of indirect alloreactivity, suggesting they should be able to take up donor antigens. In this study, we investigated the capacity and requirements of human pDC to ingest and present donor antigen.

In absence of any stimulus pDC were unable to ingest apoptotic cells (AC) (mean 2%), which was only slightly increased after activation using TLR9 ligand CpG (mean 15%), in consensus with previous reports. However, priming pDC by conditioned medium (CM) derived from human kidney cell lines, as well as primary tubular epithelial cells, and subsequent TLR9 ligation using CpG or cytomegalovirus (CMV), strongly induced the capacity to ingest AC (mean 44%). Priming by CM led to phosphorylation of the key transcription factor Interferon Regulatory Factor-7, in absence of IFNα production. Importantly, upon priming 10-fold lower concentrations of CpG were required to get optimal TLR9 activation. Consequently, activated primed pDC produced vast amounts of IFNα (mean 5,354 vs 475 pg/mL) and the chemokines CCL4 and CXCL10. Moreover, priming increased pDC phenotypic maturation (CD40/80/83/86 and CCR7) and TLR7/9 expression, compared to non-primed pDC. As a functional consequence, primed pDC induced a vigorous allogeneic T cell proliferation as compared to their non-primed counterparts (mean 60% vs 4%), as well as inducing a strong TH1 skewing (e.g. IFNγ mean 2,268 vs 60 pg/mL). Using donor HLA antigen and a CD4 T cell clone with indirect specificity, pDC were shown to be cells with efficient indirect antigen presentation capacity.

In conclusion, we show that factors produced by renal epithelial cells enable the phagocytic capacity of pDC following TLR9 ligation. Moreover, this tissue priming lowers the TLR9 activation threshold by one order of magnitude. Subsequently, TLR9 ligation by CpG or CMV strongly enhances their function as antigen presenting cells, and could explain the observed relation between viral and allogeneic immunity.

CITATION INFORMATION: Ruben J, Garcia-Romo G, Breman E, van der Kooij S, Redeker A, Arens R, van Kooten C. Tissue Priming of Plasmacytoid Dendritic Cells Enhances Their Phagocytosis and Lowers the Threshold for Subsequent Toll-Like Receptor 7/9 Activation. Am J Transplant. 2016;16 (suppl 3).

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