B-cell lymphomas are a group of heterogeneous neoplasms resulting from the clonal expansion of mature B cells arrested at various stages of differentiation. Specifically, two lymphoma subtypes arise from germinal centers (GCs), namely follicular lymphoma (FL) and GC B-cell diffuse large B-cell lymphoma (GCB-DLBCL). In addition to recent advances in describing the genetic landscape of FL and GCB-DLBCL, tumor microenvironment (TME) has progressively emerged as a central determinant of early lymphomagenesis, subclonal evolution, and late progression/transformation. The lymphoma-supportive niche integrates a dynamic and coordinated network of immune and stromal cells defining microarchitecture and mechanical constraints and regulating tumor cell migration, survival, proliferation, and immune escape. Several questions are still unsolved regarding the interplay between lymphoma B cells and their TME, including the mechanisms supporting these bidirectional interactions, the impact of the kinetic and spatial heterogeneity of the tumor niche on B-cell heterogeneity, and how individual genetic alterations can trigger both B-cell intrinsic and B-cell extrinsic signals driving the reprogramming of non-malignant cells. Finally, it is not clear whether these interactions might promote resistance to treatment or, conversely, offer valuable therapeutic opportunities. A major challenge in addressing these questions is the lack of relevant models integrating tumor cells with specific genetic hits, non-malignant cells with adequate functional properties and organization, extracellular matrix, and biomechanical forces. We propose here an overview of the 3D in vitro models, xenograft approaches, and genetically-engineered mouse models recently developed to study GC B-cell lymphomas with a specific focus on the pros and cons of each strategy in understanding B-cell lymphomagenesis and evaluating new therapeutic strategies.

Bone marrow (BM) involvement is a common feature of germinal center–derived B-cell lymphomas and is associated with a poor prognosis. In particular, follicular lymphoma (FL) infiltrates the BM in 70% of cases, and analysis of in vitro–expanded FL BM mesenchymal stromal cells (MSCs) has revealed an extensive alteration of BM stromal cell phenotypic, transcriptomic, and functional profiles. However, the mechanisms underlying the direct interplay between lymphoma B cells and their permissive stromal niche in situ have not yet been identified. In this study, we identified a significant remodeling of extracellular matrix (ECM) composition and organization in the BM of patients with FL and in a murine model of lymphoma B-cell BM xenograft. In particular, murine leptin receptor (LepR+) MSCs were identified by single-cell RNA sequencing as engaged in a bidirectional cross talk with malignant B cells, triggering their specific and progressive reprogramming and commitment toward a phenotype resembling that of human ECM/transforming growth factor β (TGFβ) myofibroblastic cancer–associated fibroblasts (CAFs) and FL-CAFs. Kinetic analysis of FL BM samples showed that ECM and TGFβ deregulation persisted after treatment, suggesting it may contribute to disease persistence and relapse. Overall, this work sheds new light on the kinetics and mechanisms of BM stromal niche reshaping in B-cell lymphomas.

Upregulated expression of the anti-apoptotic BCL2 oncogene is a common feature of various types of B-cell malignancies, from lymphoma to leukemia or myeloma. It is currently unclear how the various patterns of deregulation observed in pathology eventually impact the phenotype of malignant B cells and their microenvironment. Follicular lymphoma (FL) is the most common non-Hodgkin lymphoma arising from malignant germinal center (GC) B-cells, and its major hallmark is the t(14:18) translocation occurring in B cell progenitors and placing the BCL2 gene under the control of the immunoglobulin heavy chain locus regulatory region (IgH 3′RR), thus exposing it to constitutive expression and hypermutation. Translocation of BCL2 onto Ig light chain genes, BCL2 gene amplification, and other mechanisms yielding BCL2 over-expression are, in contrast, rare in FL and rather promote other types of B-cell lymphoma, leukemia, or multiple myeloma. In order to assess the impact of distinct BCL2 deregulation patterns on B-cell fate, two mouse models were designed that associated BCL2 and its full P1-P2 promoter region to either the IgH 3′RR, within a “3′RR-BCL2” transgene mimicking the situation seen in FL, or an Ig light chain locus context, through knock-in insertion at the Igκ locus (“Igκ-BCL2” model). While linkage to the IgH 3′ RR mostly yielded expression in GC B-cells, the Igκ-driven up-regulation culminated in plasmablasts and plasma cells, boosting the plasma cell in-flow and the accumulation of long-lived plasma cells. These data demonstrate that the timing and level of BCL2 deregulation are crucial for the behavior of B cells inside GC, an observation that could strongly impact the lymphomagenesis process triggered by secondary genetic hits.

Le lymphome folliculaire (FL) est une hémopathie maligne et représente 20-25% des cas de lymphomes non-hodgkinien (NHL). Le FL se caractérise par une accumulation de lymphocytes B tumoraux issus des centres germinatifs (GC) des ganglions. Le FL est un cancer hématologique qui dépend fortement d’un microenvironnement de soutien (dont les cellules stromales lymphoïdes, LSC). Dans 70% des cas, les B de FL colonisent la moelle osseuse, entraînant une modification des cellules stromales de la moelle osseuse (BM-SC) vers un phénotype pouvant soutenir la croissance tumorale. Nous avons donc développé un modèle de xénogreffe intra-fémorale de lignées de FL au sein de souris immunodéprimées. Ceci dans le but d’étudier la polarisation des BM-SC au contact des B tumoraux. Ainsi nous avons observé que les B tumoraux entraînent une polarisation de ces BM-SC en un phénotype similaire aux LSC, qui sécrètent des facteurs permettant la migration et la survie des B. Ces BM-SC réorganisent également la matrice extracellulaire permettant le développement des B tumoraux. Le développement du FL étant long et dépendant de certaines altérations génétiques clés, nous avons également caractérisé un modèle de lymphomagénèse transgénique. Au sein de ce modèle nous avons observé que la mutation perte-de-fonction Hvem coopère avec la surexpression de BCL2 au sein des B dans le développement de tumeurs de FL. Les B Hvem- montrent une suractivation du BCR et une tendance à l’accumulation de T « follicular helper ». De plus, les LSC semblent fortement impliquées dans la croissance tumorale par leur évolution vers un phénotype fibroblastes associés aux cancers de FL (FL-CAF) et la potentielle sécrétion d’angiopoïétine. De plus les voies de signalisation de l’IL-31 et des angiopoïétines semblent impliquées dans la croissance tumorale des B de FL Hvem-. Ce travail présente donc deux modèles murins d’études du FL permettant de soulever des questions complémentaires portant sur l’impact des B de FL sur les cellules stromales lymphoïdes (et sur les BM-SC).

Bone marrow (BM) involvement is a common feature of lymphomas deriving from germinal-center B cells and is associated with a bad prognosis. In particular, follicular lymphoma (FL) infiltrates the BM in 70% of cases, in association with a remodeling of surrounding tumor microenvironment. Analysis of in vitro -expanded FL mesenchymal stromal cells (MSCs) revealed an extensive alteration of BM stromal cell phenotypic, transcriptomic, and functional profiles. However, the mechanisms supporting the direct interplay between lymphoma B cells and their permissive stromal niche in situ have not been yet identified. In the current work, we identified in the BM milieu of FL patients a deregulation of soluble and extracellular matrix (ECM) components reflecting inflammation and ectopic differentiation into lymphoid-like stromal cells. We reproduced the same alterations in a murine model of lymphoma B-cell xenograft where a scRNAseq approach identified LepR ^pos MSCs as specifically and progressively reprogramed by tumor B-cell invasion. Analysis of FL BM collected before and after treatment confirmed that BM niche was partly dependent on the continuous contact with tumor B cells. Altogether, this work shed new lights on the kinetic and mechanisms of BM stromal niche reshaping in B-cell lymphoma.