Remodeling of the Immune Microenvironment By Oncogenic MYD88 Dictates Immunotherapy Responses across Indolent and Aggressive B-Cell Lymphomas

The MYD88^L265P mutation is detected in a variety of lymphoma subtypes, ranging from indolent MALT or lymphoplasmacytic lymphoma (LPL) to aggressive DLBCL with activated B-cell (ABC) phenotype. Whether oncogenic MYD88^L265P constitutes a common founder mutation and how it drives lymphomas with different clinical, histopathological, and immunological characteristics, remains unknown. To address this fundamental question, we screened mice genetically engineered to express an orthologous MYD88^L252P mutation crossed with animals carrying secondary genetic lesions common in MYD88^L265P lymphomas, triggered at two B-cell differentiation stages. Several mouse strains developed indolent or aggressive lymphomas depending on the cell of origin and the secondary genetic lesion. In two of the models, human-like MALT or LPL were progressively driven by mutant MYD88 activated from immature pre-B lymphocytes with either constitutive BCR signaling or BCL2 expression. High-throughput cellular and molecular characterization of the tumor immune microenvironment (TME) by single-cell RNA sequencing coupled with BCR/TCRseq revealed bi-directional B-cell interactions with CD4⁺ Tfh and Th1 cells from early disease state, promoting survival of B-cell clones through CD40L:CD40 signaling. During progression, IL10-secreting regulatory T (Treg) cells were recruited into the tumor niche through a Ccl5-Cxcr3 axis to suppress CD8⁺ T-cell cytotoxicity. In patients with MYD88^L265P LPL characterized at progressive disease states, CD40:CD40L interaction between tumor and immune cells took place early while Tregs accumulated at late stage. In the LPL model and in human MYD88^L265P-LPL cells, CD40 signaling decreased effects of standard-of-care drugs including BTK inhibitors. Accordingly, anti-CD40 therapy disrupted B/T-cell interactions and increased ibrutinib activity. In conclusion, oncogenic MYD88 triggered at early B-cell stages promotes indolent lymphomas that are sustained by CD40 signaling, which restricts therapy responses that can be restored upon CD40 blockade.

When MYD88^L265P was activated in mature germinal-center B cells together with either Prdm1 or Trp53 deletion, mice developed extranodal ABC-DLBCL (MCD/C5 genomic class). IHC, flow cytometry and single-cell RNAseq revealed that DLBCL with Prdm1 loss showed morphologically heterogeneous B cells with abundant immune and T cells with activated/exhausted phenotypes, which recapitulated the inflammatory/immunosuppressive TME category in DLBCL patients (IN-LME, Kotlov- Cancer-Discovery-2021). In contrast, DLBCL with Tp53 deletion exhibited monotonous B-cell infiltrates with high proliferation index and a TME with depletion of T cells with effector/memory phenotypes and CD4⁺CD25⁺Foxp3⁺ Tregs, consistent with the immune depleted TME category (DP-LME). Analysis of 1,039 DLBCL patients demonstrated that majority of MCD cases with MYD88^L265P showed either an IN-LME (43%) or DP-LME (44%). Consistent with the models, ABC-DLBCL patients with MYD88^L265P and TP53 loss/mutation exhibited highly proliferative disease with a DP-LME, while ABC-DLBCLs with MYD88^L265P but without TP53 abnormalities showed an IN-LME. Notably, the number of infiltrating CD4⁺ T cells correlated with TP53 activity score (PROGeNY) across MCD-DLBCLs. Overall, the presence or absence of TP53 alterations in MYD88^L265P ABC-DLBCL induces TME characterized as DP-LME or IN-LME, respectively. Since DP-LME carries poor response to immune-chemotherapy (Kotlov-Cancer-Discovery-2021), we tested anti-CD19 CAR T-cells in the models. While CAR T-cells were initially effective in the two DLBCL models, progression occurred earlier in DLBCL with DP-LME than in IN-LME, which remained in remission 100 days after cell infusion. Responses correlated with persistence of circulating CD4⁺ CAR T cells. These data highlight that a TP53-driven DP-LME may limit immunotherapy activity in DLBCL, providing a scientific explanation to the poor outcome of DLBCL patients with TP53 alterations to CD19-directed CAR T cells (Shouval-JCO-2021). Collectively, our study defines the cellular origins and the stepwise genetic alterations of indolent and aggressive lymphomas with MYD88^L265P, dissects distinct immunological paths underlying progression driven by oncogenic MYD88, and builds proof-of-concept for advancing precision immunotherapy in B-cell lymphomas.