Despite the unprecedented clinical activity of the Bruton’s tyrosine kinase inhibitor ibrutinib in MCL, acquired-resistance is common. By longitudinal integrative whole-exome and whole-transcriptome sequencing and targeted sequencing, we identified the first relapse-specific C481S mutation at the ibrutinib-binding site of BTK in MCL cells at progression following a durable response. This mutation enhanced BTK and AKT activation and tissue-specific proliferation of resistant MCL cells driven by CDK4 activation. It was absent, however, in patients with primary-resistance or progression following transient response to ibrutinib, suggesting alternative mechanisms of resistance. Through synergistic induction of PIK3IP1 and inhibition of PI3K-AKT activation, prolonged early G1 arrest induced by PD 0332991 (palbociclib) inhibition of CDK4 sensitized resistant lymphoma cells to ibrutinib killing when BTK was unmutated, and to PI3K inhibitors independent of C481S mutation. These data identify a genomic basis for acquired-ibrutinib resistance in MCL and suggest a strategy to override both primary- and acquired-ibrutinib resistance.

IntroductionThe human immune system is continuously challenged by commensal microflora as well as invasive infectious agents. The decision to mount a rapid and protective immune response to a pathogen is a consequence of the activation of the innate immune system via pattern recognition receptors, such as Toll-like receptors (TLRs), that sense microbial products. 1 TLR polymorphisms have been implicated in increased severity and predisposition to infection and septic shock 2 in both mice and humans. TLRs recognize highly conserved structures of viral (TLR3,7,8,and 9) and bacterial (TLR1,2,4,5,6,7,8,and 9) origin, known as pathogen-associated molecular patterns (PAMPs; Figure 1). TLR2 heterodimerization with TLR1 or TLR6 is triggered by bacterial lipopeptides, whereas TLR3 is activated by double-stranded RNA, TLR4 is activated by lipopolysaccharide (LPS), TLR5 is activated by flagellin, TLR7 and TLR8 are activated by single-stranded RNA (ssRNA), and TLR9 is activated by unmethylated CpG DNA motifs. Moreover, endogenous ligands released during cellular stress or matrix degradation (eg, heat-shock proteins, fibronectin, heparan sulfates) are thought to activate TLRs. 3 Numerous reports have described how TLRs orchestrate the immune response to pathogens in dendritic cells (DCs) and macrophages. 4 Much less is known about the effects of TLR-mediated B-cell activation, although the design of vaccines could benefit from a more detailed understanding of this process.Leukemic B cells often retain the expression of markers specific for their cellular origin (eg, CD5, CD10, CD138). Furthermore, several reports have demonstrated TLR expression and function in neoplastic B cells. Because DCs can be activated and matured upon triggering of TLRs, immunotherapeutic protocols in leukemia have recently included TLR agonists to improve tumor antigen presentation and subsequent T-cell activation. 5 However, recent reports have indicated that leukemic cells could hijack the TLR machinery to their own benefit. A better understanding of the effects of TLR ligands on normal B cells and their leukemic counterparts could therefore help avoid adverse vaccination effects. In this review, we will discuss the role of TLRs in generating the humoral immune response and their dual effects on different leukemic cell types. TLR expression in normal and neoplastic B cells Normal B-cell subsets and modulation of expressionThe TLR expression pattern is specific for each cell type and is summarized in Figure 2 for human B cells. TLRs expression in human B cells is characterized by high expression of TLR1, 6, 7, 9, and 10. 6-8 Low expression of TLR2 allows for the formation of the functional heterodimers TLR1/2 and TLR2/6, which are required to respond to diacylated and triacylated lipoproteins. The inability to be activated by LPS is a hallmark of human B cells because they lack TLR4, in contrast to mouse B cells. However, human B cells are well equipped to recognize nucleic acids given their expression of TLR7 and TLR9. This profile allows them to ...

Mantle cell lymphoma (MCL) accumulates in lymphoid organs, but disseminates early on in extranodal tissues. Although proliferation remains located in lymphoid organs only, suggesting a major role of the tumor ecosystem, few studies have assessed MCL microenvironment. We therefore cocultured primary circulating MCL cells from 21 patients several weeks ex vivo with stromal or lymphoid-like (CD40L) cells to determine which interactions could support their proliferation. We showed that coculture with lymphoid-like cells, but not stromal cells, induced cell-cycle progression, which was amplified by MCL-specific cytokines (insulin-like growth factor-1, B-cell activating factor, interleukin-6, interleukin-10). Of interest, we showed that our model recapitulated the MCL in situ molecular signatures (ie, proliferation, NF-κB, and survival signatures). We further demonstrated that proliferating MCL harbored an imbalance in Bcl-2 family expression, leading to a consequent loss of mitochondrial priming. Of interest, this loss of priming was overcome by the type II anti-CD20 antibody obinutuzumab, which counteracted Bcl-x induction through NF-κB inhibition. Finally, we showed that the mitochondrial priming directly correlated with the sensitivity toward venetoclax and alkylating drugs. By identifying the microenvironment as the major support for proliferation and drug resistance in MCL, our results highlight a selective approach to target the lymphoma niche.