In Myositis, Autoantibodies Get Inside Muscle Cells and Disrupt the Function of the Proteins they Bind to.

Image

Iago Pinal-Fernandez, M.D., Ph.D., a Staff Clinician in the NIAMS Muscle Disease Unit, and a lead author on the paper.

Overview

An international team of researchers, led by the NIAMS Intramural Research Program (IRP)'s Muscle Disease Section, has discovered a new mechanism through which myositis autoantibodies -antibodies produced by the immune system that mistakenly target the body's own components- contribute to the disease. Myositis encompasses a group of rare autoimmune diseases where the immune system mistakenly attacks healthy muscle tissue. The researchers discovered that autoantibodies can get inside muscle cells and disrupt the normal function of the proteins they bind to. This new discovery could have implications for how we understand not only myositis, but also other autoimmune diseases.

The study, published in Annals of the Rheumatic Diseases, goes against the previously held belief that autoantibodies cannot enter living cells.

Background

Myositis is a family of rare autoimmune muscle diseases. In people with myositis, their immune system attacks healthy muscle, causing inflammation and weakness. Myositis can also affect other organs, such as the lungs, skin, and joints. People with myositis can produce various types of autoantibodies, each associated with a distinct combination of clinical features.

In this study, the NIAMS researchers and their colleagues aimed to determine if autoantibodies do in fact enter muscle cells, reach their target proteins, and affect the protein's normal function.

Select Research Highlights

First, the scientists analyzed muscle biopsy tissue samples from patients with different myositis autoantibodies. They found that the autoantibodies were located in the same cellular location as their target proteins. For example, CHD4 is a protein found in the nucleus of cells. Anti-CHD4 autoantibodies were also found in the cell nucleus of these patients.

Autoantibodies in myositis affect proteins that play essential roles in cellular processes. The researchers performed RNA sequencing analyses on the tissue samples to identify gene expression patterns associated with the different autoantibodies. They discovered specific patterns of gene activity linked to each type of autoantibody, consistent with disruptions in the function of the target protein.

Finally, the scientists wanted to confirm that the autoantibodies were responsible for the results they observed in tissue samples. To do this, they isolated and purified autoantibodies from the serum of myositis patients. They used a technique called electroporation to get the purified autoantibodies inside cultures of human muscle cells that were grown in the lab. The scientists performed RNA sequencing analyses and found gene expression patterns that were the same as those observed in the muscle biopsies in patients with the corresponding type of autoantibody. This data confirmed that autoantibodies disrupt the normal function of their target proteins, thus contributing to the development and progression of myositis.

"This study shows that autoantibodies get inside live muscle cells where they disrupt the normal function of their target proteins," concluded Iago Pinal-Fernandez, M.D., Ph.D., who is a staff clinician in the NIAMS Muscle Disease Section and one of the study's lead authors. "This mechanism through which autoantibodies enter the cells may be relevant in other tissues affected by myositis, as well as in other autoimmune diseases such as systemic sclerosis, vasculitis, and lupus."

^{A model of pathogenic myositis autoantibody internalization. In myositis, autoantibodies get into the muscle fibers, disrupting the normal biological function of their autoantigen, which affects the development and progression of the disease.}

Reference:

Pinal-Fernandez I, Muñoz-Braceras S, Casal-Dominguez M, et al. Pathological autoantibody internalisation in myositis. Annals of the Rheumatic Diseases. Published Online First: 20 June 2024. doi: 10.1136/ard-2024-225773