Single‐Cell RNA Seq in Sydenham Chorea Shows B Cell HLA‐DR/DQ Upregulation and Plasma Cell Proteasomal Activation

The pathogenesis of Sydenham chorea remains unclear. We report a 10-year-old girl presenting with subacute chorea and mild carditis following Streptococcal throat infection. Single-cell RNA sequencing on 30,794 peripheral immune cells from the patient and two sex-matched controls revealed nine immune cell clusters. We found up-regulated antigen processing pathways in B cells, monocytes, and eosinophils, enriched by HLA genes, particularly HLA-DRB5 and HLA-DQB1, and immunoglobulin (IGH) genes in B cells. Up-regulated proteasomal pathways in plasma cells were enriched by PSM genes. These insights support the B cell-mediated autoantibody hypothesis in Sydenham chorea. We highlight the potential of scRNA-seq in rare diseases to inform targeted therapies.