Thymic epithelial tumors (TETs) are rare and heterogeneous tumors that include thymic carcinoma and five thymoma subtypes (A, AB, B1, B2, B3). Thymomas are characterized by an active intratumor thymopoiesis and by a strong association with autoimmune disorders (AD). Notably, ADs are more frequent in patients with type B thymomas, further suggesting a potential link between specific hihistotypes and immune dysregulation. However, the precise pathophysiological mechanisms underlying this association and the involvment of B-cells are currently poorly understood.

To investigate B-cell involvement, we performed histopathological and immunohistochemical analyses on 126 TET cases. Hematoxylin and eosin staining revealed the presence of tertiary lymphoid structures (TLSs) in a significant proportion of cases, particularly among type B thymomas. The presence of these structures was further confirmed by staining these sections with the B-cell marker CD20. Overall, TLS-like structures were identified in 83% of type B thymomas.

To characterize these lymphoid structures at single-cell resolution, we analyzed the tumor microenvironment of 19 TETs using single-cell RNA sequencing. Unsupervised clustering identified three principal B-cell populations, defined by canonical B-cell genes MS4A1, CD19, and SDC1. Among the different clusters, one exhibited a transcriptional profile resembling pre-pro B-cells, expressing early developmental genes including RAG1, RAG2, and VPREB1, suggesting the presence of developing B-cells within the tumor. Another B-cell cluster displayed a germinal center-like phenotype, characterized by the expression of BCL6 and AICDA, indicative of local B-cell activation and maturation. These populations were enriched in B2/B3 thymomas, providing molecular evidence for the presence of organized TLSs within the tumor microenvironment.

These preliminary findings support a potential link between B-cell organization in thymomas and autoimmune manifestations.

To further dissect the spatial architecture and cellular interactions within TLS-like structures, we plan to perform spatial transcriptomic analyses. This approach may provide novel insights into the comprehension of the immunopathogenic mechanisms underlying thymoma-associated autoimmunity.