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.

在慢性淋巴细胞白血病 (CLL) 中，肿瘤细胞的存活取决于淋巴部位的微环境信号，其中约 40% 的 CLL 中表达的整合素 VLA-4 (CD49d/CD29) 与 B 细胞受体 (BCR) 之间的串扰）发生。在这里，BCR 从内到外的结合激活了 VLA-4，从而增强了 VLA-4 介导的 CLL 细胞粘附，进而从周围微环境中获取促生存信号。我们报告说，BCR 还能够通过自主抗原独立的 BCR 信号传导，有效地从内到外激活循环表达 CD49d 的 CLL 细胞中的 VLA-4 整合素。因此，表现出激活的 VLA-4 的循环 CLL 细胞表达 BCR 途径激活的标记物（磷酸化 BTK 和磷酸化 PLC-γ2）以及较高水平的磷酸化 ERK 和磷酸化 AKT，表明下游途径平行激活。此外，表达活化VLA-4的循环CLL细胞结合可溶性血源性VCAM-1，导致VLA-4依赖性肌动蛋白聚合/重组和ERK磷酸化增加。最后，有证据表明，依鲁替尼治疗通过影响自主 BCR 信号传导，损害 VLA-4 的组成型激活，最终减少可溶性 VCAM-1 结合并减少循环 CLL 细胞的下游 ERK 磷酸化。这项研究描述了循环 CLL 细胞中发生的一种新颖的锚独立机制，涉及 BCR 和 VLA-4 整合素，这有助于揭示 CD49d+ CLL 的独特生物学和临床特征。