STABILIZED MYCT58A BYPASSES T-CELL HELP TO FUEL GERMINAL CENTER B-CELL LYMPHOMAGENESIS

B. Chen and A. Q. Xu equally contributing author.

Introduction: Constitutive MYC overexpression is a hallmark of germinal center (GC) B-cell-derived lymphomas. Stabilizing mutations in MYC, such as T58A, are frequently found in highly aggressive subtypes, including Burkitt’s lymphoma and Double-Hit lymphomas. Under physiological conditions, MYC is transiently upregulated in positively selected GC B-cells within the light zone (LZ) following T-cell help (TCH). However, the precise mechanisms by which aberrant MYC expression drives GC B-cell lymphomagenesis remain unclear. Here, we propose that aberrant MYC levels diminish GC B-cell dependence on TCH, thereby promoting lymphomagenesis.

Methods: We generated novel mouse models in which either wildtype MYC (MYC^WT) or MYC stabilizing mutant (MYC^T58A) was overexpressed in GC B-cells using the Cɣ1-Cre system. To assess the phenotypic and functional consequences, we performed flow cytometry and immunohistochemistry. To investigate MYC-driven alterations in GC dynamics and metabolism, we conducted CITE-seq analysis on MYC^WT, MYC^T58A, and control GC B-cells. Additionally, we modulated TCH and its downstream mTOR signaling pathway in GC B-cells using CD40L blockade and rapamycin treatment.

Results: MYC^WT overexpression induced GC B-cell hyperplasia and promoted the expansion of a dark zone (DZ)-like phenotype. This was accompanied by increased cell cycle entry and progression in the LZ, albeit with a concurrent rise in apoptosis. CITE-seq analysis revealed that MYC^WT overexpression led to a reduction in activation signatures in positively selected LZ GC B-cells, suggesting that these cells become less dependent on TCH. Functionally, MYC^WT overexpression conferred an advantage under conditions of limited TCH. However, MYC^WT-overexpressing GC B-cells remained sensitive to sustained CD40L blockade or mTOR inhibition, indicating that MYC^WT activity is still regulated by TCH in a premalignant state.

In contrast, MYC^T58A overexpression did not induce short-term GC hyperplasia but instead prolonged GC B-cell retention and exacerbated lymphomagenesis. CITE-seq analysis revealed that MYC^T58A GC B-cells not only exhibited a consistent reduction in activation signatures but also enriched for an intermediate LZ/DZ "Grey Zone" phenotype characterized by a metabolic profile resembling lymphoma. Functionally, MYC^T58A overexpression conferred GC B-cells with the ability to survive even under inhibition of key TCH-dependent signaling pathways, most notably mTOR signaling.

Conclusion: These findings suggest thatMYC overexpression, particularly MYC^T58A, reprograms GC B-cells to bypass TCH dependency, supporting their transition toward a lymphomagenic state. This work provides mechanistic insights into how MYC mutations drive GC B-cell transformation and highlights potential vulnerabilities for therapeutic intervention.

Keywords: non-Hodgkin; microenvironment; metabolism

No potential sources of conflict of interest.