MDA5 multimerization on LINE RNA drives pathogenic extracellular immune complexes in autoimmunity

Technical Summary: MDA5 Multimerization on LINE RNA Drives Pathogenic Extracellular Immune Complexes in Autoimmunity

Problem Statement
Anti-MDA5 dermatomyositis (anti-MDA5 DM) is a severe, often fatal autoimmune disease characterized by autoantibodies against MDA5, an intracellular innate immune receptor that normally detects viral double-stranded RNA (dsRNA) by forming filaments. While the presence of these autoantibodies is a diagnostic hallmark, the precise molecular mechanism by which they drive pathogenesis remains debated. Specifically, it is unclear whether the autoantibodies alone are sufficient to trigger disease or if they require specific structural contexts (e.g., filamentous vs. monomeric antigen) and extracellular scaffolds to activate innate immune pathways. Furthermore, the identity of the endogenous RNA species that scaffold these pathogenic complexes in patients has not been definitively established.

Methodology
The study employed an integrated approach combining structural biology, biochemistry, cell biology, and clinical genomics:

• Antibody Isolation and Characterization: Four patient-derived monoclonal autoantibodies (mAbs) were isolated from two anti-MDA5 DM patients (NP6, NP8 from a Japanese patient; F01, F12 from a Swedish patient). These were classified into two distinct groups (Class I and Class II) based on heavy and light chain gene usage.
• Structural Analysis: Cryo-electron microscopy (cryo-EM) was used to determine the atomic structures of MDA5 filaments (stabilized by the G495R mutation) in complex with Fab fragments or single-chain variable fragments (scFvs) of all four mAbs.
• In Vitro Reconstitution and Functional Assays: Pathogenic immune complexes (ICs) were reconstituted using purified recombinant MDA5, in vitro transcribed dsRNA, and patient mAbs. These complexes were applied to THP-1-derived macrophages and primary monocyte-derived macrophages without transfection reagents to assess immune activation (RNA-seq, ELISA for IFN-β and IL-1β).
• Cell Death and NETosis Assays: The ability of ICs to induce pyroptosis (ASC speck formation, IL-1β release) and neutrophil extracellular trap (NET) formation was evaluated using immunofluorescence and flow cytometry.
• Clinical Sample Analysis: A TRIM65-based sandwich ELISA was developed to selectively detect filamentous MDA5 in patient plasma. Plasma and peripheral blood mononuclear cell (PBMC) RNA from anti-MDA5 DM patients, anti-MDA5-negative DM patients, and healthy controls were subjected to stranded RNA-seq. TRIM65 pull-downs were performed to identify RNA species associated with MDA5 filaments in vivo.
• Bioinformatic Analysis: RNA-seq data were analyzed for differential expression of transposable elements (LINEs, SINEs, ERVs), A-to-I editing frequency, and genomic localization of induced transcripts.

Key Contributions and Results

1. Distinct Antibody Binding Modes and Epitope Convergence:

   • The study identified two distinct binding modes for patient mAbs. Class I antibodies (NP6, F01) bind to a rigid epitope on the Hel2i domain (residues 670–690) of MDA5, recognizing a conserved lysine (K685) via germline-encoded residues in the IgLV3-1 light chain. Class II antibodies (NP8, F12) bind to a flexible Hel2i loop and the C-terminal domain of an adjacent protomer, also utilizing germline-encoded residues.
   • Despite divergent heavy chain CDR3 sequences and somatic hypermutations, both classes exhibit "epitope convergence," relying on germline-encoded residues for antigen recognition.

2. Antibody-Driven Filament Crosslinking:

   • Structural modeling and negative-stain EM revealed that intact IgGs cannot bind two MDA5 protomers within the same filament simultaneously due to steric constraints. Instead, they bridge distinct filaments.
   • Class I antibodies efficiently crosslink MDA5 filaments into large, higher-order aggregates (microns in size). Class II antibodies show less efficient crosslinking. This crosslinking is dependent on antibody bivalency and the filamentous state of MDA5.

3. Stoichiometry-Dependent Immune Activation:

   • Reconstituted ICs containing filamentous MDA5, but not monomeric MDA5, potently activated type I and II interferon signaling in macrophages.
   • Activation required all three components: MDA5, dsRNA, and antibody.
   • The magnitude of activation was determined by the antibody binding mode and IC stoichiometry. Class I antibodies (efficient crosslinkers) were more potent than Class II when filaments were abundant. However, in conditions of excess monomeric MDA5, Class II antibodies were more effective because Class I antibodies were sequestered by monomers, preventing filament crosslinking.
   • Uptake of these ICs was mediated by Fcγ receptors (specifically FcγRI/CD64), delivering dsRNA to endosomal TLR3 and cytosolic RLR/MAVS pathways.

4. Induction of Inflammatory Cell Death:

   • MDA5 filament ICs triggered inflammasome activation (ASC specks, IL-1β release) and NETosis in neutrophils. This suggests a mechanism for the tissue damage and ulceration seen in anti-MDA5 DM.

5. Identification of LINE-Derived dsRNA as the Pathogenic Scaffold:

   • Analysis of patient plasma revealed elevated levels of extracellular MDA5 filaments, which correlated with disease activity and were reduced by glucocorticoid treatment.
   • RNA-seq of TRIM65-purified plasma fractions identified Long Interspersed Nuclear Elements (LINEs), specifically L1HS, as the predominant RNA species associated with MDA5 filaments.
   • Unlike other retroelements, LINEs in patient plasma showed a balanced sense-to-antisense ratio and high levels of A-to-I editing, indicating the presence of stable dsRNA duplexes formed by sense:antisense hybrids.
   • These LINE RNAs were not enriched in anti-MDA5-negative DM patients, suggesting specificity to the anti-MDA5 DM pathology.

6. Self-Amplifying Inflammatory Cycle:

   • Stimulation of macrophages with MDA5 filament ICs upregulated the transcription of host genes, many of which are near LINE loci. This led to increased production of LINE-derived sense and antisense transcripts, which formed new dsRNA duplexes.
   • These new dsRNAs scaffolded additional MDA5 filament formation, which were released via inflammatory cell death (pyroptosis/NETosis), creating a feed-forward loop of inflammation.

Significance
The paper defines a molecular architectural principle for autoimmunity targeting intracellular proteins: the pathogenicity of autoantibodies is not solely a function of affinity or the presence of nucleic acids, but is governed by the higher-order organization of the immune complex. Specifically, the ability of autoantibodies to crosslink antigen filaments into higher-order aggregates dictates the potency of innate immune activation.

The study identifies a self-amplifying pathogenic cycle in anti-MDA5 DM where:

1. Initial triggers (potentially viral) lead to extracellular MDA5 filaments.
2. Autoantibodies crosslink these filaments, activating innate immunity and cell death.
3. Inflammatory signaling induces the transcription of LINE retroelements.
4. Sense:antisense LINE hybrids form dsRNA scaffolds for new MDA5 filaments.
5. These new filaments are released, perpetuating the cycle.

This work provides a mechanistic explanation for the fulminant nature of anti-MDA5 DM, linking adaptive immune tolerance breaches (autoantibody generation) with innate immune dysregulation (LINE RNA induction) and highlighting the critical role of immune complex architecture in driving disease.