Similar to resting mature B cells, where the B-cell antigen receptor (BCR) controls cellular survival, surface BCR expression is conserved in most mature B-cell lymphomas. The identification of activating BCR mutations and the growth disadvantage upon BCR knockdown of cells of certain lymphoma entities has led to the view that BCR signalling is required for tumour cell survival. Consequently, the BCR signalling machinery has become an established target in the therapy of B-cell malignancies. Here we study the effects of BCR ablation on MYC-driven mouse B-cell lymphomas and compare them with observations in human Burkitt lymphoma. Whereas BCR ablation does not, per se, significantly affect lymphoma growth, BCR-negative (BCR) tumour cells rapidly disappear in the presence of their BCR-expressing (BCR) counterparts in vitro and in vivo. This requires neither cellular contact nor factors released by BCR tumour cells. Instead, BCR loss induces the rewiring of central carbon metabolism, increasing the sensitivity of receptor-less lymphoma cells to nutrient restriction. The BCR attenuates glycogen synthase kinase 3 beta (GSK3β) activity to support MYC-controlled gene expression. BCR tumour cells exhibit increased GSK3β activity and are rescued from their competitive growth disadvantage by GSK3β inhibition. BCR lymphoma variants that restore competitive fitness normalize GSK3β activity after constitutive activation of the MAPK pathway, commonly through Ras mutations. Similarly, in Burkitt lymphoma, activating RAS mutations may propagate immunoglobulin-crippled tumour cells, which usually represent a minority of the tumour bulk. Thus, while BCR expression enhances lymphoma cell fitness, BCR-targeted therapies may profit from combinations with drugs targeting BCR tumour cells.

Altered macrophage infiltration upon tissue damage results in inadequate healing due to inappropriate remodeling and stem cell recruitment and differentiation. We investigated in vivo whether cells of endothelial origin phenotypically change upon heterotopic ossification induction and whether infiltration of innate immunity cells influences their commitment and alters the ectopic bone formation. Liposome-encapsulated clodronate was used to assess macrophage impact on endothelial cells in the skeletal muscle upon acute damage in the ECs specific lineage-tracing Cdh5CreER T2 :R26REYFP/dtTomato transgenic mice. Macrophage depletion in the injured skeletal muscle partially shifts the fate of ECs toward endochondral differentiation. Upon ectopic stimulation of BMP signaling, monocyte depletion leads to an enhanced contribution of ECs chondrogenesis and to ectopic bone formation, with increased bone volume and density, that is reversed by ACVR1/SMAD pathway inhibitor dipyridamole. This suggests that macrophages contribute to preserve endothelial fate and to limit the bone lesion in a BMP/injury-induced mouse model of heterotopic ossification. Therefore, alterations of the macrophage-endothelial axis may represent a novel target for molecular intervention in heterotopic ossification.

The ACVR1 gene encodes a type I receptor of bone morphogenetic proteins (BMPs). Activating mutations in ACVR1 are responsible for fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by congenital toe malformation and progressive heterotopic endochondral ossification leading to severe and cumulative disability. Until now, no therapy has been available to prevent soft-tissue swelling (flare-ups) that trigger the ossification process. With the aim of finding a new therapeutic strategy for FOP, we developed a high-throughput screening (HTS) assay to identify inhibitors of ACVR1 gene expression among drugs already approved for the therapy of other diseases. The screening, based on an ACVR1 promoter assay, was followed by an in vitro and in vivo test to validate and characterize candidate molecules. Among compounds that modulate the ACVR1 promoter activity, we selected the one showing the highest inhibitory effect, dipyridamole, a drug that is currently used as a platelet anti-aggregant. The inhibitory effect was detectable on ACVR1 gene expression, on the whole Smad-dependent BMP signaling pathway, and on chondrogenic and osteogenic differentiation processes by in vitro cellular assays. Moreover, dipyridamole reduced the process of heterotopic bone formation in vivo. Our drug repositioning strategy has led to the identification of dipyridamole as a possible therapeutic tool for the treatment of FOP. Furthermore, our study has also defined a pipeline of assays that will be useful for the evaluation of other pharmacological inhibitors of heterotopic ossification.

Mature B cells recognize and respond in a highly-specific fashion to a multitude of environmental antigens through membrane-bound immunoglobulins forming together with the Igα and Igβ proteins a functional unit called the B cell antigen receptor (BCR). Through a complex network of effector molecules, the BCR transforms environmental signals into biochemical reactions which are responsible for highly codified cellular responses affecting survival, proliferation, migration and terminal differentiation of B cells. Surface BCR expression is conserved in most types of B cell malignancies arising from mature B cells. This observation, together with genetic and biochemical evidence pointing to sustained BCR signaling in different types of B cell neoplasms represents the rationale for the current use of pharmacological inhibitors of BCR signaling to treat several forms of B lymphoproliferative disorders. Nevertheless, our understanding of how the BCR influences malignant B cell behavior remains poorly understood. In an attempt to fill this knowledge gap, we engineered a mouse model to monitor the effects of acute ablation of the BCR in highly-aggressive MYC-driven lymphomas. Inducible BCR ablation did not, per se, prevent the outgrowth of receptor-less MYC lymphoma cells both in vitro and in vivo. Instead, BCR loss weakened the fitness of the malignant B cells leading to the rapid elimination of BCR-less tumor cells in the presence of their BCR-expressing counterparts (Varano et al., 2017). Through the integration of data generated from genomics, metabolomics and bulk/single cell transcriptomics analyses, comparing BCR-deficient lymphoma cells to their proficient counterparts, we have started to elucidate the gene networks and metabolic pathways influenced by BCR expression that sustain competitive fitness of MYC-transformed lymphoma B cells. Data from CRISPR/Cas9-mediated disruption of candidate fitness genes in primary malignant B cells will be presented. In support of the findings in the mouse model, we will provide evidence that BCR-less malignant B cells are spontaneously generated during tumor progression in several forms of human B cell lymphoproliferative disorders, establishing a possible Achilles heel of anti-BCR therapies. Finally, we will report possible strategies enabling the clearance of BCR-less lymphoma cells, taking advantage of their acquired addiction to specific signaling and metabolic pathways. Our results shed light on the coordinated regulation of signaling and metabolism imposed on malignant B cells by BCR expression/signaling and provide indications for improved treatment options to fight several forms of mature B cell malignancies. Reference: Varano G, Raffel S, Sormani M, Zanardi F, Lonardi S, Zasada C, Perucho L, Petrocelli V, Haake A, Lee AK, Bugatti M, Paul U, Van Anken E, Pasqualucci L, Rabadan R, Siebert R, Kempa S, Ponzoni M, Facchetti F, Rajewsky K, Casola S. Nature. 2017; 546:302-306. Disclosures No relevant conflicts of interest to declare.

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