Background/Purpose: The Idiopathic Inflammatory Myopathies (IIM) are the largest number of acquired and potentially treatable muscle disorders. Nevertheless, there are currently no FDA or EMA-approved medications apart from corticosteroids, and the mortality and morbidity of these diseases pairs the ones of rheumatoid arthritis in the pre-biologic era. In the last 10 years, interest has been raised about the roles of dendritic cells (DCs) in the pathogenesis of IIM. In this pilot study, we aim to apply previously validated approach of multi-channel confocal microscopy using fluorescent antibodies for identification and characterization of DC populations in biopsies of patients with Dermatomyositis (DM) and Inclusion Body Myositis (IBM).

Methods: This study was performed with deindentified samples from patients with IIM. This study was approved by the University of Chicago institutional IRB. A total of 3 DM and 3 IBM samples were stained for myeloid dendritic cells (mDCs) with BDCA1 and CD11c; plasmacytoid DCs (pDCs) with BDCA 2 and CD123; and cell nuclei with DAPI (Hoechst 33342). These slides were imaged with the SP8 3D 3-color STED laser scanning confocal microscope with time gating at a magnification of 630x, using a pixel size of 1024×1024 and 12 bit depth. Single fluorochrome controls were utilized to ensure no cross-bleeding was present in between fluorescent channels. Given the large amount of tissue, pertaining to each biopsy, and to minimize potential bias, we performed a random acquisition protocol by means of tiling Using a mechanized stage, the entire tissue section of interest was mapped, automatically segmented into Regions of Interest (ROIs), corresponding to individual High Power Fields (HPFs), and acquired by random means. An average of 50 ROIs per biopsy was acquired in this manner. In addition, manual imaging was performed for areas of significant inflammation, identified during the above process. The resulting image data was reviewed and manually analyzed for number of DCs by a blinded observer (IBV) using Fiji Software. Mann-Whitney U test was used to calculate statistical significance for all analyses.

Results: Subjects with IBM and DM had high counts of mDCs, particularly in DM, with an average of 14 mDCs in DM and 7 mDCs in IBM per biopsy. The difference between DM and IBM in regards to mDCs was decreased after normalization, maybe reflecting the rich inflammatory milieu of DM, in comparison to IBM. The counts of pDCs were also high in both diseases, and significantly higher compared to the ones of mDCs in IBM (p=.02), even when normalized by cell density (p=.032).

Conclusion: Our findings challenge the classic correlation between DM, IBM, pDCs and mDCs. This pilot study illustrates the complexity behind the cellular drives of IIM in regards to DCs. Priorly established patterns of inflammation used to describe these diseases must be revisited, as new therapeutic targets are urged in IIM.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/in-situ-dendritic-cell-characterization-in-idiopathic-inflammatory-myopathies/