It is well known that interferon (IFN)-alpha is important to the pathogenesis of systemic lupus erythematosus (SLE). However, several reports have indicated that the number of IFN-alpha producing cells are decreased or that their function is defective in patients with SLE. We studied the function of plasmacytoid dendritic cells (pDCs) under persistent stimulation of Toll-like receptor (TLR)9 via a TLR9 ligand (CpG ODN2216) or SLE serum.
The concentrations of IFN-alpha were determined in serum and culture supernatant of peripheral blood mononuclear cells (PBMCs) from SLE patients and healthy controls after stimulation with CpG ODN2216 or SLE serum. The numbers of circulating pDCs were analyzed by fluoresence-activated cell sorting analysis. pDCs were treated with CpG ODN2216 and SLE serum repeatedly, and levels of produced IFN-alpha were measured. The expression of IFN-alpha signature genes and inhibitory molecules of TLR signaling were examined in PBMCs from SLE patients and healthy control individuals.
Although there was no significant difference in serum concentration of IFN-alpha and number of circulating pDCs between SLE patients and healthy control individuals, the IFN-alpha producing capacity of PBMCs was significantly reduced in SLE patients. Interestingly, the degree which TLR9 ligand-induced IFN-alpha production in SLE PBMCs was inversely correlated with the SLE serum-induced production of IFN-alpha in healthy PMBCs. Because repeated stimulation pDCs with TLR9 ligands showed decreased level of IFN-alpha production, continuous TLR9 stimulation may lead to decreased production of IFN-alpha in SLE PBMCs. In addition, PBMCs isolated from SLE patients exhibited higher expression of IFN-alpha signature genes and inhibitory molecules of TLR signaling, indicating that these cells had already undergone IFN-alpha stimulation and had become desensitized to TLR signaling.
We suggest that the persistent presence of endogenous IFN-alpha inducing factors induces TLR tolerance in pDCs of SLE patients, leading to impaired production of IFN-alpha.