Role of Ca in toll-like receptor 9 activation in human plasmacytoid dendritic cells.

Plasmacytoid dendritic cells (pDCs) are major producers of type I interferons in response to activation of endosomal toll-like receptors (TLRs), e.g. TLR9. While a number of cell biological and intracellular signaling events associated with TLR9 activation in pDCs have been studied, role of free calcium (Ca) is not clear. We found that influx of extracellular Ca is crucial for TLR9 mediated IFNα production by human pDCs. We also unraveled a role of Ca in potentiating cellular uptake of self-DNA in complex with the cathelicidin antimicrobial peptide, LL37, an endogenous ligand for human TLR9 in autoimmune contexts. IFNα in response to TLR9 activation, by CpG oligonucleotides, is tuned within a window of Ca concentration, through a bimodal regulatory switch, by differential engagement of Ca/calmodulin-dependent protein kinase II (CAMKII) and calcineurin phosphatase (CALN). Ca signaling for TLR9 activation at physiologic calcium concentrations depends on CAMKII recruitment, while inhibition of TLR9 activation at supraphysiologic calcium concentrations is mediated by CALN. This bimodal regulation was masked in response to physiological peptide-DNA complexes, presumably due to potentiation of complex formation and increased cellular uptake in higher Ca concentrations. Thus infection susceptibility associated with relevant clinical contexts as well as role of Ca signaling in autoimmune diseases warrant further investigations for novel pathogenetic cues involving pDC function.