ABSTRACT: Background: Cardiac sarcoidosis (CS) is an enigmatic disorder characterized by unexplained patchy, sterile granulomas intermixed with preserved myocardium, and fibrotic regions without granuloma, that causes arrhythmias, sudden cardiac death, and heart failure. The mechanisms producing this remarkable histopathology and disease progression remains unexplained. Methods: Using comprehensive single cell and spatial transcriptomic analyses, we characterized the cellular composition and gene expression in preserved, granulomatous, and fibrotic regions of human CS hearts. From the identification of clonally expanded cardiac B cells with rearranged immunoglobulin sequences, we reconstructed antibodies and screened libraries containing the human peptidome or microbial and allergen peptides to define reactive epitopes in CS heart. Results: Ventricular regions with preserved, granulomatous, or fibrotic histopathology exhibited marked differences in cell composition and gene expression. Cardiomyocytes upregulated arrhythmogenic and inflammasome transcripts that would drive pyroptosis, and along with fibroblasts, activated chemoattractant cytokines that sustained myeloid and lymphoid infiltration. Within granulomas, abundant macrophages expressed modulators of cell-cell fusion and with Th17-skewed T cells upregulated B cell activating factor. Fibrotic regions, without active granulomas, exhibited tertiary lymphoid structures, with clonal expansion of mature B and plasma cells. Reconstructed antibodies derived from expanded B cell clones were inert to microbial and allergen peptides, but strongly reacted to periplakin, a desmosome protein, defining a target autoantigen. Conclusions: CS is an inflammatory, autoimmune disorder that activates proarrhythmic and chemoattractant genes in cardiomyocytes within preserved myocardium, cell-cell fusion modulators in macrophages within granulomatous regions, and tertiary lymphoid structures in fibrotic regions that produce autoimmune antibodies. The identification of a target desmosome epitope, periplakin, links CS to arrhythmic desmosome cardiomyopathies, disorders with shared clinical manifestations. The centrality of humoral autoimmunity in perpetuating CS provides specific therapeutic opportunities to limit granuloma formation and B-cell activation. Parallel analytic strategies have potential to define autoantigens in other enigmatic cardiac immune disorders.
