Plasmacytoid dendritic cells promote systemic sclerosis with a key role for TLR8

Abstract

Systemic sclerosis (SSc) is a multisystem life-threatening fibrosing disorder that lacks effective treatment. The link between the inflammation observed in organs such as the skin and profibrotic mechanisms is not well understood. The plasmacytoid dendritic cell (pDC) is a key cell type mediating Toll-like receptor (TLR)-induced inflammation in autoimmune disease patients, including lupus and skin diseases with interface dermatitis. However, the role of pDCs in fibrosis is less clear. We show that pDCs infiltrate the skin of SSc patients and are chronically activated, leading to secretion of interferon-α (IFN-α) and CXCL4, which are both hallmarks of the disease. We demonstrate that the secretion of CXCL4 is under the control of phosphatidylinositol 3-kinase δ and is due to the aberrant presence of TLR8 on pDCs of SSc patients, which is not seen in healthy donors or in lupus pDCs, and that CXCL4 primarily acts by potentiating TLR8- but also TLR9-induced IFN production by pDCs. Depleting pDCs prevented disease in a mouse model of scleroderma and could revert fibrosis in mice with established disease. In contrast, the disease was exacerbated in mice transgenic for TLR8 with recruitment of pDCs to the fibrotic skin, whereas TLR7 only partially contributed to the inflammatory response, indicating that TLR8 is the key RNA-sensing TLR involved in the establishment of fibrosis. We conclude that the pDC is an essential cell type involved in the pathogenesis of SSc and its removal using depleting antibodies or attenuating pDC function could be a novel approach to treat SSc patients.

Fig. 1.. pDCs infiltrate the skin of SSc patients and spontaneously secrete IFN-α and CXCL4.

Skin biopsies from (A) control subjects or (B) systemic sclerosis (SSc) patients were immunostained for CD123 (red signal; original magnification, ×200; inset, ×400), and (C) quantification of the number of CD123⁺ counted in five high-power microscopic fields (based on five bioptic samples of patients with SSc and of five controls) is shown. (D) Representative dot plots of plasmacytoid dendritic cells (pDCs), B cells, conventional DCs (cDCs), and monocytes from peripheral blood mononuclear cells (PBMCs) from healthy donors (HDs) or SSc patients analyzed by flow cytometry. (E) Percentages of pDCs, monocytes, B cells, and cDCs within PBMC of either HDs (open circle, n = 10 to 15) or SSc patients (n = 14 to 33) defined as early diffuse (closed circle), late diffuse (closed square), or limited (closed triangle) were quantified by flow cytometry. ns, not significant. (F) Purified pDCs from HDs (n = 8) or SSc patients (n = 20) were cultured for 24 hours without stimuli. CXCL4 and interferon-α (IFN-α) were quantified by enzyme-linked immunosorbent assay (ELISA). Results were normalized to 10,000 cells to account for the differences between donors in the number of cells put in culture. (G) The mean fluorescence intensity (MFI) of the costimulatory molecules CD83 and CD86 expressed on pDCs (n = 10 for HDs; n = 33 for SSc patients) was quantified by flow cytometry. Statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 2.. TLR8 is aberrantly expressed on pDCs from SSc patients.

(A) TLR8 expression levels [relative Ct (Rel Ct)] were quantified in purified pDCs [n = 18 HDs; n = 29 SSc patients; n = 6 systemic lupus erythematosus (SLE) patients], B cells (n = 14 HDs; n = 8 SSc patients), or monocytes (n = 17 HDs; n = 11 SSc patients) from PBMCs as indicated. (B to D) Gene expression level of TLR7, TLR9, and TLR10 was quantified in (B) purified pDCs (n = 17 to 18 HDs; n = 29 to 31 SSc patients; n = 6 SLE patients), (C) purified B cells (n = 12 to 14 HDs; n = 6 to 8 SSc patients), or (D) purified monocytes (n = 16 to 18 HDs; n = 11 SSc patients) by quantitative polymerase chain reaction (qPCR). All results are represented as means ± SEM. Statistical significance was evaluated using a Mann-Whitney U test, and only comparisons that are significant are shown. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 3.. TLR8 signaling induces CXCL4 and IFN-α secretion by SSc PDCs.

(A) Gene expression levels of CXCL4 were quantified in total PBMC or pDC–depleted (dep) PBMC prepared from either HDs (n = 6) or SSc patients (n = 23) by qPCR. Statistical significance was evaluated using the unpaired Mann-Whitney test between HD and SSc PBMC and with a paired t test between PBMC and pDC-depleted PBMC from SSc patients. (B) Purified pDCs from HDs (n = 4 to 10) were cultured either with media (Med) alone as a control or in the presence of a TLR7 agonist [heat-inactivated VR95 influenza virus at a multiplicity of infection (MOI) of 2], a TLR8 agonist (ORN-8L at 200 μg/ml), or a TLR9 agonist (CpG-C274 at 0.5 μM), and 24 hours later, CXCL4 in the supernatants was measured by ELISA. Results were normalized to the value of media alone and represented as means ± SEM. (C and D) pDCs purified from SSc patients (n = 15 and 12, respectively) were cultured for 24 hours either in media alone or with either (C) ORN-8L or (D) CpG-C274, and CXCL4 production was analyzed by ELISA. Results were normalized to control and represented as means ± SEM. (E) HD (n = 10) and SSc pDCs (n = 20) were stimulated with ORN-8L for 24 hours, and supernatants were collected for measurement of IFN-α, IP-10, IL-6, and TNF by multiplex bead assay. Results were normalized to 10,000 cells. (F) Purified pDCs from SSc patients (n = 12) were cultured either with media alone or in the presence of a TLR9 agonist for 24 hours, and supernatants were collected for measurement of IFN-α, IP-10, IL-6, and TNF by multiplex bead assay. Results were normalized to 10,000 cells. All results are represented as means ± SEM. For (B) to (E), statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 4.. CXCL4 potentiates TLR8-mediated activation of SSc pDCs.

(A and B) Purified pDCs from SSc patients (n = 8) were cultured in media alone (control), with ORN-8L (200 μg/ml), or with ORN-8L and CXCL4 (10 μg/ml) for 24 hours, and (A) IFN-α and IP-10 or (B) IL-6 and TNF were quantified in the supernatants by ELISA. Values were normalized to ORN-8L alone and represented as means ± SEM. (C to E) Purified pDCs from HDs were cultured alone, and purified pDCs from SSc patients were cultured for 24 hours either alone or in the presence of the PI3Kδ inhibitor CAL-101 (10 μM); (C) CXCL4, (D) IFN-α, or (E) IL-6 levels were quantified by ELISA. Results are normalized to 10,000 cells. All results are represented as means ± SEM, and statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 5.. CXCL4 potentiates TLR9-mediated activation but has minimal effect on TLR7-mediated activation of pDCs purified from SSc or HDs.

Purified pDCs from HDs (A to C) or SSc patients (D and E) were cultured in media alone (control), either (A and D) with a CpG-C TLR9 agonist (0.1 μM) or with CpG-C and CXCL4 (10 μg/ml), or (B) with a CpG-B TLR9 agonist (0.25 μM) or with CpG-B and CXCL4, and (C and E) with a TLR7 agonist [heat-inactivated VR95 influenza virus at a MOI of 0.5 (FLU)] or with FLU and CXCL4 for 24 hours (n = 6 to 18 as indicated). IFN-α was quantified in the supernatants by ELISA. Values were normalized to 10,000 cells. All results are represented as means ± SEM, and statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 6.. pDCs infiltrate the skin of BLM-treated mice, and their depletion attenuates skin fibrosis.

pDC depletion was achieved as described in Materials and Methods. (A to C) Representative images of Siglec-H⁺ cells (red signal; original magnification, ×400) and (D) average (value based on five skin sections with n = 6 to 13 mice per group) in wild-type (WT) mice injected with (A) PBS and (B) with BLM and of (C) CLEC4C-DTR mice injected with DT and BLM. (E) Representative images of Masson’s trichrome–stained skin sections at a ×10 magnification, arrow indicates the area used to quantify skin thickness, and (F) average of skin thickness of WT mice receiving PBS [left panel in (E); open circle in (F)], WT mice injected with DT and treated with BLM [middle panel in (E); closed circle in (F)], and CLEC4C-DTR mice injected with DT and BLM [right panel in (E); closed square in (F)] is shown. (G) Collagen content in the skin expressed as a ratio of collagen per total protein (tot prot.) is shown. (H and I) Representative images of α–smooth muscle actin (α-SMA) immunohistochemistry of the dermis at a ×40 magnification and averages of the number of α-SMA–positive cells per surface area in dermis are shown. (J to L) Relative gene expression of the indicated genes using the NanoString technology. All values represent means ± SEM; n = 6 to 14 mice for each group. Data are cumulative of three independent experiments, and statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 7.. pDCs are critical for the maintenance of skin fibrosis and for the presence of CXCL4 in the skin.

Fibrosis was induced by injection of BLM in WT mice for either 3 or 5 weeks (wks) or in CLEC4C-DTR mice for 5 weeks as described in Materials and Methods. At 3 weeks of BLM treatment, pDC depletion was achieved in CLEC4C-DTR mice by DT injection (in red) while fibrosis induction was sustained for 2 more weeks. (A) Average of skin thickness and (B) of the number of Siglec-H⁺ cells in the skin. (C) Representative images of the cxcl4 transcript by RNA in situ hybridization (brown signal; original magnification, ×400; inset, ×630) and (D) average of the number of CXCL4⁺ cells. Experiment was performed twice (n = 4 to 10 per group). All results are represented as means ± SEM, and statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 8.. TLR8 exacerbates disease in the BLM-induced fibrosis model.

(A) Representative images of Masson’s trichrome–stained skin sections at a magnification of ×10 of WT mice injected with PBS or WT mice, Tg8 mice, TLR7KO mice, and Tg8 × TLR7KO (Tg8 × 7KO) mice injected with BLM. (B) Average of skin thickness (n = 11 to 18 per group). (C) Representative images of Siglec-H⁺ cells in the skin of WT mice injected with PBS or with BLM and of Tg8 mice, TLR7KO mice, and Tg8 × TLR7KO mice injected with BLM. (D) Histograms represent number of Siglec-H⁺ cells and are cumulative of at least 10 mice from three independent experiments. (E and F) Relative expression of indicated genes regulated by TLR8 and/or TLR7 using the NanoString technology. (G and H) Gene expression levels of huTLR8 by qPCR for (G) macrophages (F4/80⁺, CD11b⁺) or (H) pDCs (F4/80⁻, CD11b⁻ and B220⁺, Siglec-H⁺, BST2⁺) purified from WT or Tg8 mice (n = 3 to 4) that received PBS or 4 weeks of BLM as indicated (see Materials and Methods). All results are represented as means ± SEM from three to six independent experiments for each group, and statistical significance was evaluated using a Mann-Whitney U test. *P < 0.05, **P < 0.01, ***P < 0.001.