Characterizing local antibody responses in the muscle of inclusion body myositis patients

Sporadic inclusion body myositis (IBM) is the most common adult idiopathic inflammatory myopathy. IBM etiology has been elusive, due to both degenerative and autoimmune disease features found on muscle biopsy, and significant disease heterogeneity. Investigating the role of antibodies in the muscle of IBM patients may improve our understanding of disease pathogenesis.

We used an IBM xenograft mouse model in which muscle biopsy tissue from IBM patients (n = 98) and controls (n = 131; including 54 from other types of myopathy) were implanted into immunodeficient mice (NOD-Rag1^null-IL2rγ^null). We quantified the amount of human IgG, IgA, and IgM in xenografted mouse sera using MesoScale Diagnostics (MSD) assay. We detected donor-derived antibody reactivities targeting autoantigens and infectious agents using Phage ImmunoPrecipitation Sequencing (PhIP-Seq). Finally, we used FR3 AmplifiKation Sequencing (FR3AK-Seq) to sequence the antibody mRNAs from a separate cohort of 146 patient biopsies (14 IBM, 22 healthy controls, 110 other myositis subtypes).

With the MSD assay we found human IgG, IgA, and IgM in a larger percentage of IBM xenografted mice versus controls. Using PhIP-Seq, we found anti-microbial reactivities secreted from IBM muscle are prevalent amongst a healthy control population but autoantigen reactivities in IBM are more unique at the peptide and protein level. Additionally, NT5C1A (IgG/IgA and IgM) and TIF1γ (IgG/A) autoantibodies are secreted from muscle tissues of 4/18 and 10/18 IBM xenograft donors, respectively.

Our characterization of antibody responses within the muscle of IBM patients reveals that muscle-infiltrating B cells produce both disease-associated autoantibodies and a broad spectrum of antibodies targeting non-self antigens.