Background/Purpose: Idiopathic inflammatory myopathies (IIMs) are muscle diseases, characterized by inflammatory infiltration and increased expression of MHC class I molecules on myofibers. Immunoproteasome, as a proteolytic complex that shapes the repertoire of antigenic peptides, has been previously demonstrated to be over-expressed in IIMs at mRNA level. In this study, we aim to investigate the functional role of immunoproteasome in the pathogenesis of IIMs.

Methods: Skeletal muscle biopsy specimens from 45 individuals with well diagnosed (ENMC criteria) inclusion body myositis (IBM, n=12), immune-mediate necrotizing myopathy (IMNM, n=12) and dermatomyositis (DM, n=12), in addition to non-IIMs (nIIMs, n=3) and healthy controls (HC, n=6) were included in this study. Immunoproteasome expressions within the muscle fibers and cellular infiltrates were examined by dual immunofluorescence (IF) and immunoblot. Proteasomal activity was measured in the patient muscle biopsies. Primary human myoblasts were used to determine the most effective cytokine in the induction of immunoproteasome expression. Additionally, the role of immunoproteasome during the in vitro inflammatory conditions was investigated in myoblasts by using shRNA based gene silencing and specific chemical inhibitors. The effect of both approaches in MHC-I expression and myokine production was measured by FACS and real time PCR, respectively.

Results: Immunoblots showed significant increase in the expression of relevant players of the immunoproteasome, β1i or β5i in IBM (n=9) and DM (n=9) muscle biopsies compared to HC (n=6). However, the expression in IMNM (n=9) was moderate. In addition, the chymotrypsin-like activity of proteasome reflected the expression of β1i and β5i in the muscle biopsies. Dual IF revealed that both myofibers and muscle infiltrating cells including CD8+ T-cells and CD68+ macrophages in IIMs (n=6 for IBM, IMNM and DM) expressed β1i or β5i. In fact, the expression of β1i and β5i co-localized with the MHC class I expressing myofibers. In contrast, the muscle fibers of HC (n=4) and nIIMs (n=3) were negative for both β1i and β5i. Our in vitro study in the cultures of human primary myoblasts showed that pro-inflammatory cytokines, TNF-α, IFN-α, IFN-β and IFN-γ are able to upregulate β1i and β5i expression. Selective inhibition or depletion of β5i amplified the TNF-α or IFN-γ mediated expression of myokines in myoblasts. Furthermore, we found that specific inhibitors of β1i or β5i reduced the cell surface expressions of MHC class I in myoblasts induced by IFN-γ.

Conclusion: We demonstrated that the immunoproteasome expression and activity are increased in skeletal muscle and could involve in pathologic MHC class I expression in IIMs. Therefore, the suppression of the immunoproteasome expression could be an approach to reduce antigen presentation by skeletal muscles and consequently the cytotoxic effect of T-cells. However, functional analyses revealed that immunoproteasome is also important to maintain the myokines that mediate attraction of immune cells in muscles fibers. Thus, the imbalance between two functions may have an impact on the disease phenotype or severity in IIMs.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-immunoproteasomes-are-essential-for-maintaining-myokine-production-and-mhc-class-i-expression-in-idiopathic-inflammatory-myopathies/