Müschen Laboratory

Less than 1 in 2,000 cells in the human body are B-cells. Once activated, B-cells divide at a faster rate than any other cell type and produce antibodies to neutralize foreign pathogens. Unlike other cell types, B-cells undergo multiple rounds of somatic gene recombination and hypermutation of immunoglobulin genes to evolve antibodies that bind to antigen with high affinity. Hence, adaptive immune protection by B-cells comes with an approximately 300-fold increased risk of malignant transformation compared to other cell types. B-cell leukemia/lymphoma represent the most frequent type of cancer in children (31%) and account for 10% of all cancers in adults.

To prevent the production of harmful autoantibodies and autoimmune disease, autoreactive B-cells and pre-malignant clones are eliminated by a process termed negative selection. Despite strict and rigorous negative selection, B-cells frequently give rise to autoimmune diseases and B-cell malignancies such as leukemia and lymphoma. Since humans can live without B-cells for extended periods of time, the Müschen laboratory systematically investigated lineage-specific vulnerabilities that are common in B-cell leukemia/lymphoma but not any other cell type. Contrary to established dogma, these mechanisms are not only active in preventing autoimmune disease but also represent a novel class of therapeutic oncogenic targets in malignant B-cell tumors. Over the past five years, the Müschen Laboratory established innovative conceptual frameworks for the understanding of B-cell signaling mechanisms and negative selection, some of which are summarized below:

• Energy abundance sets thresholds for negative B-cell selection
  Regulation of energy-abundance is the central determinant of negative B-cell selection: Hyperactivation of kinases downstream of an autoreactive B-cell receptor induces ATP depletion and energy stress (Chen et al., 2015; Shojaee et al., 2016).
• Metabolic gatekeepers
  B-cells not only have the smallest cytoplasmic volume but also fewer mitochondria than any other cell type. The new paradigm on ‘metabolic gatekeeper functions’ is based on B-lymphoid transcription factors repressing or limiting glucose-uptake and energy-supply to set low thresholds for negative selection of autoreactive and premalignant B-cells (Chan et al., 2017; Pan et al., 2021).
• Diverting glucose flux
  Glucose carbon-flux during B-cell transformation is diverted in a way that leaves transformed B-cells uniquely vulnerable to inhibition of PP2A, an enzyme that coordinates glycolytic flux with antioxidant protection (Xiao et al., 2018).
• Diversity of signaling input and malignant transformation
  Only mutations that converge on one central pathway promote leukemia-progression. Genetic reactivation of divergent (suppressed) pathways engage conflicting biochemical and transcriptional programs and subvert leukemia development. Pharmacological pathway-reactivation to create a diverse signaling environment can be leveraged as a new therapeutic to prevent leukemia progression (Chan et al, 2020).
• A new feedback loop of PI3K-amplification
  The endosomal protein IFITM3 was found to play a role as a central scaffold for lipid-raft assembly and surface-expression of rafts-associated receptors during B-cell activation. Recruitment of IFITM3 is critical for the initiation of PI3K-signaling, antibody affinity maturation and oncogenic B-cell transformation (Lee et al., 2020).
• Goldilocks principle of B-cell selection
  Thresholds of B-cell selection are based on signaling strength of the B-cell receptor (BCR). While B-cells with a non-functional BCR lack critical BCR-signals for survival, autoreactive B-cells elicit overwhelming strong BCR-signals. According to a Goldilocks principle of B-cell selection, only clones with intermediate BCR-signaling strength (“just right”) are positively selected to survive and proliferate. Targeted hyperactivation of BCR-downstream kinases mimics excessive signaling strength from autoreactive BCRs, thus triggering negative selection. Traditional cancer therapy is focused on kinase inhibitors to suppress oncogenic signaling so kinase hyperactivation is a novel approach for targeted therapy in B-cell malignancies (Müschen 2018; Müschen 2019).