TNF-α Induces Macroautophagy and Regulates MHC Class II Expression in Human Skeletal Muscle Cells

Abstract

Macroautophagy, a homeostatic process that shuttles cytoplasmic constituents into endosomal and lysosomal compartments, has recently been shown to deliver antigens for presentation on major histocompatibility complex (MHC) class II molecules. Skeletal muscle fibers show a high level of constitutive macroautophagy and express MHC class II molecules upon immune activation. We found that tumor necrosis factor-␣ (TNF-␣), a monokine overexpressed in inflammatory myopathies, led to a marked up-regulation of macroautophagy in skeletal myocytes. Furthermore, TNF-␣ augmented surface expression of MHC class II molecules in interferon-␥ (IFN-␥)-treated myoblasts. The synergistic effect of TNF-␣ and IFN-␥ on the induction of MHC class II surface expression was not reflected by higher intracellular human leukocyte antigen (HLA)-DR levels and was reversed by macroautophagy inhibition, suggesting that TNF-␣ facilitates antigen processing via macroautophagy for more efficient MHC class II loading. Muscle biopsies from patients with sporadic inclusion body myositis, a well defined myopathy with chronic inflammation, showed that over 20% of fibers that contained autophagosomes costained for MHC class II molecules and that more than 40% of double-positive muscle fibers had contact with CD4 ؉ and CD8 ؉ immune cells. These findings establish a mechanism through which TNF-␣ regulates both macroautophagy and MHC class II expression and suggest that macroautophagy-mediated antigen presentation contributes to the immunological environment of the inflamed human skeletal muscle.Autophagy is a homeostatic process that enables eukaryotic cells to deliver cytoplasmic constituents for lysosomal degradation, for example to recycle nutrients for survival during starvation. In addition to this original function, autophagy has emerged as a key mechanism in orchestrating innate and adaptive immune responses. During macroautophagy, the major route of degradation of cytoplasmic constituents, proteins, and organelles are sequestered inside double-membrane vesicles (autophagosomes) that fuse with lysosomes/late endosomes. In the fusion vesicles, often multivesicular and multilamellar amphisomes, the captured material is degraded, and antigenic fragments are loaded onto major histocompatibility complex (MHC) class II molecules for presentation to CD4 ϩ T cells. In turn, innate and adaptive immune signals are capable of regulating macroautophagy via cytokine secretion and cell contact-dependent mechanisms (1-3).In vivo analysis of macroautophagy in transgenic mice expressing the GFP-coupled specific autophagosome marker light chain 3 (LC3), one mammalian homologue of yeast autophagy-related gene 8 (Atg8), demonstrated that the regulation of macroautophagy is organ-dependent and that some tissues produce LC3-positive autophagosomes even in the absence of nutrient starvation. Such constitutive autophagosomal activity has been observed in metabolically active tissues such as liver, thymus, and skeletal muscle (4).Skeletal myocytes are facultative antigen-p...

Immune and myodegenerative pathomechanisms in inclusion body myositis

“…Early studies have previously revealed that the antigen‐presenting properties of human myocytes exceed the mere bearing of MHC class I molecules. In fact, muscle fibers can be categorized as facultative antigen‐presenting cells, that in a proinflammatory milieu can upregulate MHC class II molecules, express intercellular adhesion molecule (ICAM)‐1 and ICOS ligand (ICOS‐L) 108, 109, 110. In line with this, MHC class II expressing muscle fibers are found in IBM 108.…”

“…Aside from the previously described cytotoxic CD8 + T cells, inflammatory infiltrates in IBM additionally harbor myeloid cells,105 plasma cells,106 and CD4 + T cells 44, 48, 82, 83, 107, 108. Early studies have previously revealed that the antigen‐presenting properties of human myocytes exceed the mere bearing of MHC class I molecules.…”

“…In fact, muscle fibers can be categorized as facultative antigen‐presenting cells, that in a proinflammatory milieu can upregulate MHC class II molecules, express intercellular adhesion molecule (ICAM)‐1 and ICOS ligand (ICOS‐L) 108, 109, 110. In line with this, MHC class II expressing muscle fibers are found in IBM 108. In fact, up to 66.7% of muscle fibers in IBM show high positivity for MHC class II as opposed to lower counts in other IIMs (PM: 23.7%; DM: 20%) 111.…”

“…Similar to CTLs, they depict a strong proinflammatory phenotype and cytotoxic properties which might be executed toward MHC class II‐bearing muscle fibers 83. Additionally, local presentation of antigen via professional antigen presenting cells (APCs) or MHC class II bearing muscle fibers has been suggested 105, 108. The pathological role of CD4 + CD28 − T cells during IBM, therefore, might have been underappreciated so far.…”

“…In addition to FYCO1 , missense pathogenic variants responsible for autophagosome maturation and degradation (VCP and p62/SQSTM1) have been found in patients with IBM 264, 265. We could previously show that autophagy is constitutively active in human myocytes and can be upregulated via the proinflammatory cytokines TNF‐ α 108 or IFN γ together with IL‐1 β 266. Interestingly, composite exposure to TNF‐ α and IFN γ leads to significant autophagy‐dependent translocation of intracellular MHC class II to the cell surface in myocytes and more than 40% of muscle fibers in IBM that costain for autophagosomes and MHC class II have contact to CD4 + and CD8 + infiltrating T cells 108.…”