Disease-associated HLA-DRB1 alleles are thought to confer risk of developing autoimmunity by favoring the presentation of select autoantigenic epitopes. However, identification of these epitopes and the principles governing their presentation has been hindered by the imprecision of currently available methods, which cannot fully recapitulate the complexity of human pathophysiology. We present a natural antigen processing assay (NAPA), which overcomes these limitations by studying the presentation of autoantigenic CD4+ T cell epitopes by monocyte-derived dendritic cells (mo-DCs) from patients. We applied this strategy to study the processing and presentation of topoisomerase-1 (TOP1), a prevalent autoantigen in scleroderma that is associated with lung fibrosis and high mortality. We found that a common set of 10 epitopes was presented by mo-DCs from patients with diverse HLA-DR variants, including those not previously associated with the disease. Sequence analysis revealed a shared peptide-binding motif within the HLA-DR peptide binding grooves of patients who developed anti-TOP1 autoantibodies. In addition, a subset of naturally presented TOP1 peptides were characterized by immunological promiscuity, as they could bind to diverse HLA-DR peptide binding grooves. NAPA epitopes were immunorelevant: they could stimulate autoreactive CD4+ T cells in patients, and the number of epitopes recognized correlated with lung disease severity. These findings mechanistically implicate presentation of a convergent set of TOP1 epitopes in the development of scleroderma lung disease. Precise identification of autoantigenic epitopes is key to understanding the primordial mechanisms for the loss of tolerance, studying disease-propagating autoreactive T cells, and developing antigen-specific immunotherapy.