Role of Ca2+ 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 (Ca2+) is not clear. We found that influx of extracellular Ca2+ is crucial for TLR9 mediated IFN alpha production by human pDCs. We also unraveled a role of Ca2+ in potentiating cellular uptake of self-DNA in complex with the cathelicidin antimicrobial peptide, LL37, an endogenous ligand for human TLR9 in autoimmune contexts. IFNa in response to TLR9 activation, by CpG oligonucleotides, is tuned within a window of Ca2+ concentration, through a bimodal regulatory switch, by differential engagement of Ca2+/calmodulin-dependent protein kinase II (CAMKII) and calcineurin phosphatase (CALN). Ca2+ 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 Ca2+ concentrations. Thus infection susceptibility associated with relevant clinical contexts as well as role of Ca2+ signaling in autoimmune diseases warrant further investigations for novel pathogenetic cues involving pDC function.