Increased MxA Protein Expression and Dendritic Cells in Spongiotic Dermatitis Differentiates Dermatomyositis from Eczema

Summary: Background/Purpose: Dermatomyositis (DM) is conventionally characterized by interface dermatitis (ID) on skin histopathology. A subset of patients with clinically diagnosed DM have skin biopsies showing spongiotic dermatitis (SD), a histopathology more commonly seen in eczema. Diagnosis of DM is challenging, with significant delays following initial presentation. Our goals were as follows: 1) to identify the percentage of clinically diagnosed DM patients with skin biopsies showing SD, 2) to identify cytokine and cell markers that can determine if a skin biopsy showing SD is consistent with DM in a patient with clinical DM. Methods: Biopsies from 10 DM patients with SD histopathology (DM-SD) were compared to biopsies from 10 healthy controls, 10 patients with eczema, and 12 DM patients with ID histopathology (DM-ID). Skin biopsies were stained by H&E and by immunohistochemistry for MxA, IFN-β, CD11c (mDC), and BDCA2 (pDC). Mucin expression was assessed using Hale’s iron colloidal stain. Cytokines and mucin were quantified as area percent and mean intensity. Cells were quantified as mean number of cells per high power field. Fisher’s exact test was used to compare baseline patient characteristics. One-way ANOVA with Bonferroni’s multiple comparison test was used to compare protein expression between groups. Results: 11 of 164 (6.7%) patients with a clinical diagnosis of DM at our tertiary care center were identified as having SD. MxA, IFN-β, CD11c (mDC), and BDCA2 (pDC) protein expression was significantly higher in DM-SD compared to eczema (p < 0.01) and healthy controls (p < 0.0001) (Fig. 1, 2). Expression of MxA, IFN-β, and BDCA2 were not significantly different between DM-SD and DM-ID (Fig. 1, 2). Eosinophils identified on H&E were not significantly different between DMSD and eczema patients, although both were significantly higher compared to healthy controls and DM-ID (p < 0.01) (Fig. 2). Mucin was not significantly different between eczema, DM-SD, and DM-ID, although all were significantly elevated compared to healthy controls (p < 0.0001). Conclusion: MxA, IFN-β, CD11c (mDC), and BDCA2 (pDC) protein expression were significantly higher in DM-SD compared to eczema skin lesions. These markers can therefore be helpful to distinguish between DM-SD versus eczema.