CD49d Expression Identifies a Biologically Distinct Subtype of Chronic Lymphocytic Leukemia with Inferior Progression-Free Survival on BTK Inhibitor Therapy

BTK inhibitors resistance in B cell malignancies: Mechanisms and potential therapeutic strategies

Bruton tyrosine kinase inhibitors (BTKi) have shown prominent clinical efficacy in patients with B cell malignancies, such as chronic lymphocytic leukemia, mantle cell lymphoma, diffuse large B cell lymphoma, and Waldenström's macroglobulinemia. Nevertheless, numerous factors contribute to BTKi resistance, encompassing genetic mutations, chromosomal aberrations, dysregulation of protein expression, tumor microenvironment, and metabolic reprogramming. Accordingly, potential therapeutic strategies have been explored to surmount BTKi resistance, including noncovalent BTKi, BTK proteolysis-targeting chimeras, and combination therapies. Herein, we summarize the mechanisms responsible for BTKi resistance as well as the current preclinical and clinical strategies to address BTKi resistance in B cell malignancies treatment.

The cytoskeletal control of B cell receptor and integrin signaling in normal B cells and chronic lymphocytic leukemia

B cells migrate within lymphoid organs during maturation and activation, processes orchestrated by the interplay between B cell receptor (BCR) signaling and microenvironmental cues. Integrins act as mechanoreceptors, linking BCR activation to cytoskeletal remodeling, facilitating immune synapse formation, antigen recognition, and extraction. BCR activation models describe receptor clustering and mechanical changes within the antigen-BCR complex. Upon activation, immune synapses form, enabling antigen extraction and downstream signaling. Integrins stabilize these synapses, amplify BCR signaling, and modulate BCR positioning via actin reorganization. In chronic lymphocytic leukemia (CLL), aberrant BCR signaling and integrins are major players in leukemic cell homing, prognosis, and therapy resistance. In this review, we summarize the current understanding of the interplay of BCR mechanics and B cell localization, with a particular focus on communication between BCR signaling and integrin-mediated processes via actin dynamics. We give insights into normal B cell biology and then outline aspects typical to CLL.

Resistance mechanisms and approach to chronic lymphocytic leukemia after BTK inhibitor therapy

Bruton tyrosine kinase (BTK), an essential component of the B-cell receptor (BCR) signaling pathway, is a validated target in chronic lymphocytic leukemia. Ibrutinib, acalabrutinib, and zanubrutinib are covalent BTK inhibitors (cBTKi) that bind to residue C481, leading to sustained target inhibition. A significant proportion of patients develop resistance to continuous cBTKi therapy, predominantly via mutations in BTK and its immediate downstream effector, PLCG2. The noncovalent BTKi pirtobrutinib does not require binding to C481 and can restore BTK inhibition after progression on a cBTKi. However, non-C481 BTK mutations conferring resistance to pirtobrutinib have been identified. Furthermore, the scaffolding function of BTK, activation of bypass signaling pathways, and the tumor microenvironment may contribute to BTKi resistance. Targeting BTK for degradation is an emerging strategy that appears effective against multiple BTK mutations, and inhibitors of downstream BCR signaling proteins are under development. This review addresses BTKi resistance mechanisms and therapeutic approaches after cBTKi failure.

Co-expression of CD69, CD49d, CD279 and CD20 in chronic lymphocytic leukemia cells is a new biomarker of active disease before or under therapy

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Evaluation of Constitutive or Induced Activation State of the VLA-4 Integrin in Human and Murine Samples

The integrin heterodimer CD49d/CD29 (a.k.a. Very Late Antigen-4, VLA-4) mediates cell-cell and cell-matrix interaction through binding to its specific ligands. VLA-4 can be present on the cell surface at different conformation states that affect the binding affinity for the ligands. In chronic lymphocytic leukemia (CLL), higher VLA-4 levels have been demonstrated to be associated with a worse prognosis both in the chemo-immunotherapy era and in the BCR inhibitor setting, in keeping with the role of VLA-4 as a key molecule favoring CLL cell localization in protective niches of bone marrow and lymph nodes. Here, we describe functional flow cytometry-based methods to assess the activation state of the VLA-4 integrin, applicable in both human and murine settings, as well as in fresh or thawed samples. A specific "R" script for analyzing flow cytometry data is also provided.

The survival grip-how cell adhesion promotes tumor maintenance within the microenvironment

Cell adhesion is warranted by proteins that are crucial for the maintenance of tissue integrity and homeostasis. Most of these proteins behave as receptors to link adhesion to the control of cell survival and their expression or regulation are often altered in cancers. B-cell malignancies do not evade this principle as they are sustained in relapsed niches by interacting with the microenvironment that includes cells and their secreted factors. Focusing on chronic lymphocytic leukemia and mantle cell lymphoma, this Review delves with the molecules involved in the dialog between the adhesion platforms and signaling pathways known to regulate both cell adhesion and survival. Current therapeutic strategies disrupt adhesive structures and compromise the microenvironment support to tumor cells, rendering them sensitive to immune recognition. The development of organ-on-chip and 3D culture systems, such as spheroids, have revealed the importance of mechanical cues in regulating signaling pathways to organize cell adhesion and survival. All these elements contribute to the elaboration of the crosstalk of lymphoma cells with the microenvironment and the education processes that allow the establishment of the supportive niche.

CD38/NAD+ glycohydrolase and associated antigens in chronic lymphocytic leukaemia: From interconnected signalling pathways to therapeutic strategies

Chronic lymphocytic leukaemia (CLL) is a heterogenous disease characterized by the accumulation of neoplastic CD5+/CD19+ B lymphocytes. The spreading of the leukaemia relies on the CLL cell's ability to survive in the blood and migrate to and proliferate within the bone marrow and lymphoid tissues. Some patients with CLL are either refractory to the currently available therapies or relapse after treatment; this emphasizes the need for novel therapeutic strategies that improving clinical responses and overcome drug resistance. CD38 is a marker of a poor prognosis and governs a set of survival, proliferation and migration signals that contribute to the pathophysiology of CLL. The literature data evidence a spatiotemporal association between the cell surface expression of CD38 and that of other CLL antigens, such as the B-cell receptor (BCR), CD19, CD26, CD44, the integrin very late antigen 4 (VLA4), the chemokine receptor CXCR4, the vascular endothelial growth factor receptor-2 (VEGF-R2), and the neutrophil gelatinase-associated lipocalin receptor (NGAL-R). Most of these proteins contribute to CLL cell survival, proliferation and trafficking, and cooperate with CD38 in multilayered signal transduction processes. In general, these antigens have already been validated as therapeutic targets in cancer, and a broad repertoire of specific monoclonal antibodies and derivatives are available. Here, we review the state of the art in this field and examine the therapeutic opportunities for cotargeting CD38 and its partners in CLL, e.g. by designing novel bi-/trispecific antibodies.

The VLA-4 integrin is constitutively active in circulating chronic lymphocytic leukemia cells via BCR autonomous signaling: a novel anchor-independent mechanism exploiting soluble blood-borne ligands

In chronic lymphocytic leukemia (CLL), survival of neoplastic cells depends on microenvironmental signals at lymphoid sites where the crosstalk between the integrin VLA-4 (CD49d/CD29), expressed in ~40% of CLL, and the B-cell receptor (BCR) occurs. Here, BCR engagement inside-out activates VLA-4, thus enhancing VLA-4-mediated adhesion of CLL cells, which in turn obtain pro-survival signals from the surrounding microenvironment. We report that the BCR is also able to effectively inside-out activate the VLA-4 integrin in circulating CD49d-expressing CLL cells through an autonomous antigen-independent BCR signaling. As a consequence, circulating CLL cells exhibiting activated VLA-4 express markers of BCR pathway activation (phospho-BTK and phospho-PLC-γ2) along with higher levels of phospho-ERK and phospho-AKT indicating parallel activation of downstream pathways. Moreover, circulating CLL cells expressing activated VLA-4 bind soluble blood-borne VCAM-1 leading to increased VLA-4-dependent actin polymerization/re-organization and ERK phosphorylation. Finally, evidence is provided that ibrutinib treatment, by affecting autonomous BCR signaling, impairs the constitutive VLA-4 activation eventually decreasing soluble VCAM-1 binding and reducing downstream ERK phosphorylation by circulating CLL cells. This study describes a novel anchor-independent mechanism occurring in circulating CLL cells involving the BCR and the VLA-4 integrin, which help to unravel the peculiar biological and clinical features of CD49d+ CLL.

Role of the tumor microenvironment in CLL pathogenesis

Chronic lymphocytic leukemia (CLL) cells extensively interact with and depend on their surrounding tumor microenvironment (TME). The TME encompasses a heterogeneous array of cell types, soluble signals, and extracellular vesicles, which contribute significantly to CLL pathogenesis. CLL cells and the TME cooperatively generate a chronic inflammatory milieu, which reciprocally reprograms the TME and activates a signaling network within CLL cells, promoting their survival and proliferation. Additionally, the inflammatory milieu exerts chemotactic effects, attracting CLL cells and other immune cells to the lymphoid tissues. The intricate CLL-TME interactions also facilitate immune evasion and compromise leukemic cell surveillance. We also review recent advances that have shed light on additional aspects that are substantially influenced by the CLL-TME interplay.

CXCR4 overexpression in chronic lymphocytic leukemia associates with poorer prognosis: A prospective, single-center, observational study

Controversial data have been reported on the prognostic value of C-X-C motif chemokine receptor 4 (CXCR4) in chronic lymphocytic leukemia (CLL). This prospective, single-center, observational study aimed to evaluate the role of CXCR4 in the pathophysiology of CLL and its prognostic role. A total of 158 patients of CLL were enrolled, and CXCR4 expression on CLL cells was detected by flow cytometry (FCM) at initial diagnosis. The patients were divided into 2 groups according to the CXCR4 mean fluorescence intensity (MFI) median. Also, four patient specimens from the CXCR4low and CXCR4high groups were selected for RNASeq analysis. The progression-free survival (PFS) of CLL patients in the CXCR4high group was significantly shorter than the CXCR4low group, with a median follow-up time of 27 months (log-rank P < 0.001). Moreover, CXCR4 overexpression (MFI > 3376) was an independent marker of poor PFS in CLL patients (P < 0.001). Analysis of RNASeq results revealed that CXCR4 plays an important role in the migration of CLL. Collectively, CXCR4 expression levels on leukemia cells can be detected rapidly by FCM. CXCR4 overexpression was significantly associated with poorer prognosis in CLL patients within a shorter follow-up time.

Prognostication in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common type of leukemia in Western countries. CLL is a highly heterogeneous disease: some patients may never require therapy and others relapse several times after different therapeutic strategies. Therefore, in CLL, prognostic markers are essential to capture high-risk patients for different clinical endpoints including early treatment requirement, early progression after BTK or BCL2 inhibitors and Richter transformation. In early stage CLL, different biological and clinical biomarkers have been identified to predict time to treatment requirement that could be used to identify the most appropriate population for early intervention clinical trial. However, at the moment, the standard of care for early stage CLL remains watch & wait since no survival benefit has been identified in clinical trials with chemoimmunotherapy and with BTK inhibitors. In patients requiring treatment TP53 disruptions identify high-risk patients who benefit the most from long-term continuous therapy with BTKi. On the opposite side of the spectrum, IGHV mutated patients devoid of TP53 disruption benefit the most from fixed-duration therapy with venetoclax-obinutuzumab. In between, the highly heterogenous subgroup of patients with IGHV unmutated genes represents the group in which further efforts are needed to identify additional prognostic biomarkers aimed at selecting patients who can benefit from fixed-duration and patients who can benefit from long term BTKi therapy. In the context of the aggressive transformation of CLL, namely Richter syndrome, the clonal relationship to the CLL counterpart represents the strongest prognostic biomarker. Clonally related Richter syndrome still represents an unmet clinical need which requires further efforts to identify new therapeutic strategies.

The Role of the Microenvironment and Cell Adhesion Molecules in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is a B-cell malignancy whose progression largely depends on the lymph node and bone marrow microenvironment. Indeed, CLL cells actively proliferate in specific regions of these anatomical compartments, known as proliferation centers, while being quiescent in the blood stream. Hence, CLL cell adhesion and migration into these protective niches are critical for CLL pathophysiology. CLL cells are lodged in their microenvironment through a series of molecular interactions that are mediated by cellular adhesion molecules and their counter receptors. The importance of these adhesion molecules in the clinic is demonstrated by the correlation between the expression levels of some of them, in particular CD49d, and the prognostic likelihood. Furthermore, novel therapeutic agents, such as ibrutinib, impair the functions of these adhesion molecules, leading to an egress of CLL cells from the lymph nodes and bone marrow into the circulation together with an inhibition of homing into these survival niches, thereby preventing disease progression. Several adhesion molecules have been shown to participate in CLL adhesion and migration. Their importance also stems from the observation that they are involved in promoting, directly or indirectly, survival signals that sustain CLL proliferation and limit the efficacy of standard and novel chemotherapeutic drugs, a process known as cell adhesion-mediated drug resistance. In this respect, many studies have elucidated the molecular mechanisms underlying cell adhesion-mediated drug resistance, which have highlighted different signaling pathways that may represent potential therapeutic targets. Here, we review the role of the microenvironment and the adhesion molecules that have been shown to be important in CLL and their impact on transendothelial migration and cell-mediated drug resistance. We also discuss how novel therapeutic compounds modulate the function of this important class of molecules.