TWEAK/Fn14 Signaling Involvement in the Pathogenesis of Cutaneous Disease in the MRL/lpr Model of Spontaneous Lupus

Abstract

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK, TNFSF12) and its sole receptor Fn14, belonging to the TNF ligand and receptor superfamilies respectively, are involved in cell survival and cytokine production. The role of TWEAK/Fn14 interactions in the pathogenesis of cutaneous lupus has not been explored. TWEAK treatment of murine PAM212 keratinocytes stimulated the secretion of RANTES via Fn14 and promoted apoptosis. Parthenolide, but not wortmanin or the MAPK inhibitor PD98059, significantly decreased production of RANTES, indicating that this effect of TWEAK is mediated via NF-κB signaling. UVB irradiation significantly upregulated the expression of Fn14 on keratinocytes in vitro and in vivo and increased RANTES production. MRL/lpr Fn14 knockout (KO) lupus mice were compared with MRL/lpr Fn14 wild-type (WT) mice to evaluate for any possible differences in the severity of cutaneous lesions and the presence of infiltrating immune cells. MRL/lpr Fn14 KO mice had markedly attenuated cutaneous disease as compared with their Fn14 WT littermates, as evidenced by the well-maintained architecture of the skin and significantly decreased skin infiltration of T cells and macrophages. Our data strongly implicate TWEAK/Fn14 signaling in the pathogenesis of the cutaneous manifestations in the MRL/lpr model of spontaneous lupus and suggest a possible target for therapeutic intervention.

Figure 1. Fn14 expression and RANTES production in skin cells

(a) RT-PCR for Fn14 expression, with fold change calculated versus an irrelevant hybridoma set at 1. (b) Fn14 expression by flow cytometry (red=unstained, blue=control, orange=Fn14). (c) PAM212 keratinocytes were untreated, or incubated with TWEAK or control Ig (0.1 μg/ml) for 48 hours. (d) PAM212 keratinocytes were pretreated with wortmanin, parthenolide, or PD98059, followed by Fc-TWEAK or control Ig for 48 hours. (e) PAM212 were treated for 48 hours (1 μg/ml Fc-TWEAK, 1 μg/ml control Ig, 1 μg/ml Fc-TWEAK+80 IU/ml IFN-γ, and 1 μg/ml control Ig+80 IU/ml IFN-γ), and stained with annexin-V and 7-AAD. The figure shows the number of early apoptotic keratinocytes with each treatment. The figures shown are representative of 2 or 3 independent experiments. **=p<0.01, ***=p<0.001, ****=p<0.0001.

Figure 2. Effects of TWEAK and UV

(a) PAM212 were UV irradiated for 30 seconds and/or treated with TWEAK, and Fn14 measured by PCR. (b) PAM212 were treated as specified, and analyzed for Fn14 receptor expression by flow cytometry. Cells were gated in two populations, and Fn14+ cells from P1 and P2 of each group were compared. (c) Cells were UVB irradiated for 10 seconds, and treated with 1 μg/ml Fc-TWEAK or control Ig. (d) Cells were treated with Fc-TWEAK or control Ig (0.1 μg/ml) or UVB irradiated. Irradiated cells were also treated with TWEAK pre-incubated or not with an anti-TWEAK mAb. In panels (c) and (d), supernatants at 48 hours were analyzed by ELISA. The figures shown are representative of at least 2 or 3 independent experiments. *=p<0.05, **=p<0.01, ***=p<0.001, ****=p<0.0001.

Figure 3. Induction of apoptosis by TWEAK+/−UVB

(a) PAM212 keratinocytes were subjected to UVB irradiation for 10 seconds. Cells were then treated Fc-TWEAK or control Ig (0.1 μg/ml) for 48 hours, and the degree of apoptosis analyzed by flow cytometry. The figures above are representative of 2 or 3 independent experiments. (b) Representative FACS plots of the experiment shown in (a). *=p<0.05, **=p<0.01.

Figure 4. MRL/lpr Fn14 KO mice have attenuated skin disease

(a) Left and middle panels show skin scores from male and female mice at earlier (26–31 weeks) and later (34–39 weeks) time points (earlier: MRL/MpJ, n=10; MRL/lpr Fn14 WT, n=19; MRL/lpr Fn14 KO, n=13; later: MRL/lpr Fn14 WT, n=22; MRL/lpr Fn14 KO, n=15). Right panels show representative photographs of macroscopic skin lesions in MRL/lpr Fn14 WT (34 weeks) and MRL/lpr Fn14 KO mice (42 weeks). The left and right panels in each row of photographs depict the same mouse, before and after hair removal with Nair cream to better demonstrate the skin lesions. (b) Representative images of H&E stained sections from MRL/lpr Fn14 WT and MRL/lpr Fn14 KO mice (scale bar = 7 mm). Asterisk indicates hypergranulosis. Arrowhead indicates parakeratosis. Arrow points towards an area of acanthotic epidermis. (c) Fn14 WT mice had significantly higher scores in the trunk region. **=p<0.01, ***=p<0.001.

Figure 5. MRL/lpr Fn14 KO have fewer skin infiltrating cells and less epidermal apoptosis

(a) Representative images of CD3 stained sections from randomly selected MRL/lpr Fn14 WT (n=7) and Fn14 KO (n=5) mice (39 weeks) (scale bar = 7 mm). Cell counts are in the right panel. (b) Representative images of IBA-1 stained sections from randomly selected MRL/lpr Fn14 WT (n=4) and MRL/lpr Fn14 KO (n=8) mice (scale bar = 7 mm). Images were analyzed for staining density using ImageJ. (c) Lesional and unaffected skin was taken from randomly selected MRL/lpr Fn14 WT (n=6) and MRL/lpr Fn14 KO (n=6) mice. TUNEL assay was performed on paraffin embedded skin sections. Shown here is a representative image from 1 mouse in each group (scale bar = 14 mm). Fluorescence intensity was measured using ImageJ (right panel). *=p<0.05, ***=p<0.001.

Figure 6. Human lupus skin biopsies stain positive for Fn14

Paraffin tissue sections were stained for Fn14 by immunohistochemistry, as described in the Materials and Methods. The top left panel shows the negative control (primary antibody withheld, normal skin) (scale bar = 14 mm). The top middle and right panels show stained normal skin, displaying minimal Fn14 positivity (scale bar = 14 mm). The bottom left (scale bar = 9 mm) and middle panels (scale bar = 14 mm) show two magnifications of a stained section from a lupus discoid skin lesion. There is marked dermal Fn14 staining, which is representative of the pattern seen in two additional lupus discoid skin lesion biopsies. The bottom right panel shows Fn14 staining in a non-lesional lupus skin biopsy (scale bar = 14 mm).