Somatic IRF4 mutations and thymic tropism in primary mediastinal large B-cell lymphoma.

Disease-defining signatures in lymphomas, driven by intricate molecular mechanisms, have advanced molecular taxonomies, refined classification, and may guide clinical management; however, the role of these signatures in driving disease hallmarks, including subtype-specific organotropism, remains largely unexplored. Primary mediastinal large B-cell lymphoma (PMBCL) is an exemplary lymphoma characterized by disease manifestations in the thymic niche, unique genetic alterations, and immune escape. Here, we identified interferon regulatory factor 4 (IRF4)-C99R mutations uniquely occurring in PMBCL through mutational meta-analysis of large-scale data sets. By integrating multiomics approaches with genome editing in PMBCL cells, we revealed that IRF4-C99R contributes to a differentiation block phenotype. Specifically, we showed that IRF4-C99R reduces its binding to the interferon-stimulated response element (ISRE) motif within PRDM1, encoding a key transcriptional regulator of B-cell differentiation, resulting in decreased PRDM1 expression. Additionally, IRF4-C99R suppresses Traf2 and Nck-interacting kinase, a key interferon gamma (IFN-γ) pathway regulator, by impairing ISRE motif binding, thereby reducing IFN-γ signaling and increasing thymus and activation-regulated chemokine (TARC) expression, which drives TARC-mediated chemotaxis of T regulatory cells. We also revealed that IRF4-C99R upregulates ephrin type-B receptor 1 (EPHB1) through noncanonical activating protein 1-IRF composite motif binding and showed that overexpression of EPHB1 in an immunocompetent syngeneic lymphoma model influenced organotropism to favor thymic localization, without affecting overall tumor burden. IRF4-C99R mutation-induced phenotypes were validated in primary PMBCL tissues using single-nuclei RNA sequencing, confirming that the molecular mechanisms observed in vitro align with the pathophysiology of PMBCL in patients. Together, these findings demonstrate how a single genetic mutation orchestrates the coordinated regulation of hallmark traits including thymus-specific tropism in PMBCL.