Precise identification and tracking of HMGCR-reactiveCD4+T cells in the target tissue of patients with anti-HMGCR immune-mediated necrotising myopathy

Background:Anti-3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR)-positive immune-mediated necrotising myopathy (IMNM) is characterised by the presence of IgG autoantibodies against HMGCR and a strong association with specific HLA-DR alleles. Although these findings implicate HMGCR-specific CD4+T-cells in the disease's pathogenesis, no such cells have been described. In this study, we aimed to identify and characterise HMGCR-reactiveCD4+T-cells and assess their presence in affected muscle tissue from patients with anti-HMGCR+IMNM. Methods: Peripheral blood mononuclear cells from patients with anti-HMGCR+IMNM (n=10)and dermatomyositis (DM; n=10) were stimulated with HMGCR protein and peptides identified using a natural antigen processing assay (NAPA; n=6). CD4+T-cell activation was assessed byCD154 upregulation via flow cytometry. T-cell receptor beta(TCR) sequencing was performed on paired HMGCR-reactive T-cells and muscle biopsy tissue (n=5).Results:CD4+T-cell responses to HMGCR protein were higher in patients with anti-HMGCR+IMNM compared with DM (median 0.06 vs 0.00, p=0.0059). These responses were enriched in Th1-Th17 cells, and when present, they positively correlated with anti-HMGCR antibody lev-els (r2=0.89, p=0.0012). NAPA revealed convergent presentation of seven HMGCR core peptides, with substantial overlap in the peptide repertoires between patients. These HMGCR peptides elicited robust CD4+T-cell responses, with 9/10 anti-HMGCR+IMNM patients responding to at least one peptide, compared with 1/10 DM (p=0.0003). Analysis of HMGCR-reactive TCRs beta yielded antigen-reactive motifs that were enriched in muscle biopsies (projection score 0.03 vs 0.63, p=0.007)