Human Skeletal Muscle Satellite Cells (HSkMSC)

TLR7/8 Activation in Immune Cells and Muscle by RNA-Containing Immune Complexes: Role in Inflammation and the Pathogenesis of Myositis

Arthritis & Rheumatology | DOI | Paper 2025 | By Yin Wu, Aditee Deshpande, Nicholas Geraci, Petra Budde, Vera Sellers, Phanindra Velisetty, Chia-Chi Sun, Fatima Strand, Carmina Bhavsar, Timothy B. Niewold, Mark A. Jensen, Irina Kalatskaya, Kavita Y. Sarin, David Fiorentino, Andrew T. Bender

Background

This study investigated whether endosomal TLR7/8 activation contributes to idiopathic inflammatory myopathies (IIM). Immune complexes from patients with IIM and lupus, plus autoantibody-associated RNAs, were tested on healthy donor PBMCs. TLR7/8 activation induced IFN-α and IL-6 and was blocked by enpatoran. Autoantibody profiling linked canonical and novel reactivities to TLR7/8 stimulation. Supernatants from activated PBMCs altered human myoblast and satellite cell gene expression, and mouse muscle scRNA-seq showed endothelial and immune cell activation after R848. The findings support TLR7/8 inhibition as a potential IIM therapy.

Key Findings

Immune complexes from patients with IIMs and lupus activated healthy donor PBMCs to produce IFN-α, and this was inhibited by the TLR7/8 inhibitor enpatoran. Autoantibody profiling identified reactivities including Ro60, Ro52, HIST1H4A, and IL-36RN that correlated with TLR7/8 activation; Jo-1 was prominent in dermatomyositis. RNAs associated with autoantibodies (His-tRNA, Y1, Y4, U1) stimulated TLR7/8 and induced IFN-α2, IL-6, IL-1β, and IFN-γ, with enpatoran reducing cytokine production. Supernatants from TLR7/8-activated PBMCs increased inflammatory genes and decreased muscle marker genes (MSTN, MYOD1, MYF5) in human myoblasts and satellite cells. In mouse muscle, R848 activated endothelial cells and macrophages/neutrophils, inducing interferon and NF-κB signatures, without direct activation of myocytes. These results suggest autoantibody-mediated TLR7/8 activation drives inflammation and muscle cell dysfunction in myositis.

Contribution

Creative Bioarray supplied human skeletal myoblasts and satellite cells, plus SuperCult Skeletal Muscle Satellite Cell Medium, used to test whether supernatants from TLR7/8-stimulated PBMCs affect muscle cells. These primary cells enabled measurement of inflammatory gene induction and reduced muscle marker expression, supporting the study's conclusion that TLR7/8-driven inflammation may impair muscle regeneration.

Cultured Human Skeletal Muscle Satellite Cells Exhibit Characteristics of Mesenchymal Stem Cells

Skeletal muscle satellite cells (SkMSCs) play crucial roles in muscle fiber maintenance, repair, and remodeling; however, it remains unknown if these properties are preserved in cultured SkMSCs. In this study, Kim et al. investigated the characteristics of cultured SkMSCs and their ability to regulate the activity of M1 macrophages.

To verify if the cultured SkMSCs retained satellite cell characteristics, the expression of myogenic markers CD56, Pax7, and MyoD, as well as proliferation potential, were assessed. In serial subcultures up to P10, except for P2, the population doubling time remained around 20-30 h, with a total of 32.7 population doublings (Fig. 1a). At P5, SkMSCs showed low expression of Pax7 and MyoD (Fig. 1b) and did not express CD56. Instead, they expressed CD15 and CD140, markers of FAPs, in 9.5 ± 0.8% and 42.8 ± 11.5% of cells, respectively (Fig. 1c). This indicates that SkMSCs lost satellite cell characteristics and became fibroblast-like cells distinct from FAPs. Next, the differentiation capacity of SkMSCs and MSCs into adipocytes and osteoblasts was compared. SkMSCs differentiated more efficiently than MSCs (Fig. 1d). At P5, SkMSCs expressed CD73 (97.1 ± 4.5%), CD90 (38.1 ± 18.4%), and CD105 (99.0 ± 0.2%), but not CD34 or HLA-DR (Fig. 1e). These findings suggest that SkMSCs have similar differentiation and surface marker expression to MSCs.

Quercetin Induces a Slow Myofiber Phenotype in Engineered Human Skeletal Muscle Tissues

Slow and fast myofibers coexist in skeletal muscle, and slow myofibers are more adept at aerobic metabolism and endurance. Quercetin is a polyphenol compound that has been demonstrated to induce slow myofibers in rodent skeletal muscle. However, whether quercetin also elicits slow myofibers in human myofibers is not known. Here, Nagai et al. evaluated the effects of quercetin on the induction of slow myofibers in human skeletal muscle satellite cells.

Muscle regulatory factors (MRFs) are involved in muscle differentiation and are used as markers for myofiber differentiation. Figure 2 depicts how quercetin alters the expression patterns of MRF genes within human muscle satellite cells by the eighth day. Quercetin had no effect on Myf5 gene expression (Fig. 2A), but it increased MyoD1 gene expression in a dose-dependent manner at 1, 3, and 10 μM (Fig. 2B). The expression of Myogenin (Fig. 2C) and Myf6 (Fig. 2D) genes was also significantly increased in a dose-dependent manner by quercetin at 10 μM and 3 and 10 μM, respectively. On Day 8, immunostaining of MF-20 (total MyHC) and slow or fast myofibers was performed (Fig. 3A, B). Quercetin significantly increased the area of MF-20-positive cells (Fig. 3C) and slow myofiber-positive cells (Fig. 3D), but not fast myofiber-positive cells (Fig. 3E). The area ratio of slow myofiber-positive cells to MF-20-positive cells rose noticeably following quercetin administration at 3 and 10 μM (Fig. 3F) and yet the fast myofiber-positive cells to MF-20-positive cells area ratio remained unaffected (Fig. 3G).