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Gain-of-function mutations of the TLR adaptor and oncoprotein MyD88 drive B cell lymphomagenesis via sustained NF-κB activation. In myeloid cells, both short and sustained TLR activation and NF-κB activation lead to the induction of inhibitory MYD88 splice variants that restrain prolonged NF-κB activation. We therefore sought to investigate whether such a negative feedback loop exists in B cells. Analyzing MYD88 splice variants in normal B cells and different primary B cell malignancies, we observed that MYD88 splice variants in transformed B cells are dominated by the canonical, strongly NF-κB-activating isoform of MYD88 and contain at least three novel, so far uncharacterized signaling-competent splice isoforms. Sustained TLR stimulation in B cells unexpectedly reinforces splicing of NF-κB-promoting, canonical isoforms rather than the 'MyD88s', a negative regulatory isoform reported to be typically induced by TLRs in myeloid cells. This suggests that an essential negative feedback loop restricting TLR signaling in myeloid cells at the level of alternative splicing, is missing in B cells when they undergo proliferation, rendering B cells vulnerable to sustained NF-κB activation and eventual lymphomagenesis. Our results uncover MYD88 alternative splicing as an unappreciated promoter of B cell lymphomagenesis and provide a rationale why oncogenic MYD88 mutations are exclusively found in B cells., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Cardona Gloria, Bernhart, Fillinger, Wolz, Dickhöfer, Admard, Ossowski, Nahnsen, Siebert and Weber.)

Oncogenic MYD88 mutations, most notably the Leu 265 Pro (L265P) mutation, were recently identified as potential driver mutations in various B-cell non-Hodgkin Lymphomas (NHLs). The L265P mutation is now thought to be common to virtually all NHLs and occurs in between 4 and 90% of cases, depending on the entity. Since it is tumor-specific, the mutation, and the pathways it regulates, might serve as advantageous therapeutic targets for both conventional chemotherapeutic intervention, as well as immunotherapeutic strategies. Here, we review recent progress on elucidating the molecular and cellular processes affected by the L265P mutation of MYD88, describe a new in vivo model for MyD88 L265P-mediated oncogenesis, and summarize how these findings could be exploited therapeutically by specific targeting of signaling pathways. In addition, we summarize current and explore future possibilities for conceivable immunotherapeutic approaches, such as L265P-derived peptide vaccination, adoptive transfer of L265P-restricted T cells, and use of T-cell receptor-engineered T cells. With clinical trials regarding their efficacy rapidly expanding to NHLs, we also discuss potential combinations of immune checkpoint inhibitors with the described targeted chemotherapies of L265P signaling networks, and/or with the above immunological approaches as potential ways of targeting MYD88-mutated lymphomas in the future.