Plasmacytoid dendritic cells (pDCs) produce large amounts of type I interferons (IFN-␣/␤) in response to viral or endogenous nucleic acids through activation of their endosomal Toll-like receptors (TLR-7and TLR-9). Enhanced TLR-7-mediated IFN-␣ production by pDCs in women, compared with men, has been reported, but whether sex hormones, such as estrogens, are involved in this sex-based difference is unknown. Here we show, in humanized mice, that the TLR-7-mediated response of human pDCs is increased in female host mice relative to male. In a clinical trial, we establish that treatment of postmenopausal women with 17␤-estradiol markedly enhances TLR-7-and TLR-9-dependent production of IFN-␣ by pDCs stimulated by synthetic ligands or by nucleic acid-containing immune complexes. In mice, we found exogenous and endogenous estrogens to promote the TLR-mediated cytokine secretion by pDCs through hematopoietic expression of estrogen receptor (ER) ␣. IntroductionDendritic cells (DCs) are specialized sentinels in the immune system that detect invading pathogens and play a crucial role in orchestrating the immune responses. In response to viral infection, a specialized DC subset, plasmacytoid DCs (pDCs), produces a large amount of type I IFNs (IFN-␣/␤), which are potent anti-viral and immunostimulatory cytokines. 1 pDCs become activated to produce IFN-␣/␤ through Toll-like receptors (TLR-7 and TLR-9) within endosomal compartments that can sense viral nucleic acids. In the context of autoimmune diseases, such as systemic lupus erythematosus (SLE), these TLRs can also be inappropriately activated by self-nucleic acids complexed with autoreactive antibodies, resulting in IFN-␣ production by pDCs. 2 Activation of pDCs by endogenous DNA and RNA has been suggested to play a critical role in promoting and exacerbating SLE. 2-4 SLE patients show increased serum levels of IFN-␣ and overexpression of IFN-␣-regulated genes in blood cells, suggesting a central role for type I IFNs in disease pathogenesis. [5][6][7][8] This is supported by the observation that antinuclear antibody and SLE syndrome can develop during IFN-␣ treatment in patients with nonautoimmune disorders. 9 Likewise, IFN-␣ administration accelerates disease development and enhances disease severity in lupus-prone mouse strains. 10,11 In addition to IFN-␣, TNF-␣ has been shown to be increased in the serum of patients with active SLE disease and correlates with IFN-␣ levels. 12,13 Although pDCs can also produce TNF-␣, it is not clear whether they represent the unique source of this cytokine in SLE. 14 Cumulative evidence supports a role for sex-based differences in the pathogenesis of autoimmune and infectious diseases, which may be the result of sex hormones through their effects on innate and adaptive immunity. 15,16 A strong sex bias is observed in SLE, whose incidence is approximately 9 times higher in women relative to men. 15 Because disease onset is much more frequent in women of childbearing age, it has been hypothesized that sex steroid hormones, such as estro...

Human plasmacytoid dendritic cells (pDCs) play a major role in innate immunity through the production of type I IFNs after TLR engagement by pathogens. Sex-based differences in the innate function of human pDCs have been established, with pDCs from women exhibiting enhanced TLR7-mediated IFN-α production as compared with pDCs from males. In mice, we recently provided evidence for a role of estrogens as a positive regulator of pDC innate functions through cell-intrinsic estrogen receptor α signaling, but did not exclude a role for other X-linked factors, particularly in human pDCs. In this study, we investigated the respective contribution of X chromosome dosage and sex hormones using a humanized mouse model in which male or female NOD-SCID-β2m−/− were transplanted with human progenitor cells purified from either male or female cord blood cells. We showed that, in response to TLR7 ligands, the frequency of IFN-α– and TNF-α–producing pDCs from either sex was greater in female than in male host mice, suggesting a positive role for estrogens. Indeed, blockade of estrogen receptor signaling during pDC development in vitro inhibited TLR7-mediated IFN-α production by human pDCs, which expressed both ESR1 and ESR2 genes. Interestingly, we also found that X chromosome dosage contributed to this sex bias as female pDCs have an enhanced TLR7-mediated IFN-α response as compared with male ones, irrespective of the sex of the recipient mice. Together, these results indicate that female sex hormones, estrogens, and X chromosome complement independently contribute to the enhanced TLR7-mediated IFN-α response of pDCs in women.