Ibrutinib Inhibits VLA-4–Dependent Adhesion in CLL—Letter
Authors: Antonella Zucchetto, Erika Tissino, Tanja Nicole Hartmann, Alexandre Chigaev, Giovanni Del Poeta, Alfonso Colombatti, Valter Gattei
Journal: Clinical Cancer Research (2016)

We read with interest the article by Herman and colleagues (1), where the Authors demonstrate that in vitro and in vivo inhibition of the B-cell receptor (BCR) pathway by ibrutinib in chronic lymphocytic leukemia (CLL) impairs the adhesive properties of CLL cells via inhibition of the very late antigen-4 (VLA-4) integrin function.VLA-4, a heterodimer composed of the α4/CD49d and β1/CD29 integrin subunits, is a key molecule mediating both cell–matrix and cell–cell adhesion through interaction with fibronectin and the vascular cell adhesion molecule-1 (VCAM-1), respectively (2). On resting lymphocytes, VLA-4 is present in an inactive conformation, while upon lymphocyte activation by different stimuli, including those mediated by BCR triggering (2), VLA-4 conformation rapidly changes and increases its ligand-binding capacity. It is highly conceivable that inhibition of the BCR pathway by ibrutinib, by negatively affecting this inside-out VLA-4 activation, also impairs the VLA-4–mediated adhesive properties of cells.To demonstrate this line of reasoning, the Authors show adhesion assays, in which CLL cells, exposed or not to ibrutinib, are left to adhere onto VLA-4 substrates in the presence or not of bona fide blocking Ab against the α4 and β1 chains of VLA-4. Ibrutinib or anti-α4 Abs similarly hampered CLL cell adhesion, whereas anti-β1 Abs seemed ineffective in this setting (see Fig. 5B in ref. 1).By performing similar adhesion assays, we found, compared with Herman and colleagues (1), the following differences (Fig. 1): (i) pretreatment of VLA-4–expressing primary CLL cells with anti-β1 blocking Abs completely inhibited adhesion onto VLA-4 substrates; (ii) ibrutinib exposure reduced, but did not abolish, the adhesive capability of VLA-4–expressing CLL cells.It is well known that integrins, including VLA-4, functionally operate as heterodimers and that the β1 chain plays a key role in substrate recognition (see example in ref. 3). In this regard, the direct involvement of the VLA-4 β1 chain in adhesion may have practical implications, as the modulation of VLA-4 activity can be easily monitored by flow cytometry by using a conformation sensitive anti-β1 Ab (HUTS21) employed in conjunction with small peptides (e.g., an LDV-containing peptide) mimicking the binding site of the VLA-4–specific ligand VCAM-1 (4).Moreover, our observation that ibrutinib did not completely block the adhesion of VLA-4–expressing CLL cells, in keeping with a recent study (5), may provide the rationale for a combined use of ibrutinib with other BCR pathway inhibitors, such as idelalisib, to totally inhibit the inside-out integrin activation in CLL.See the Response, p. 3412No potential conflicts of interest were disclosed.This study was supported in part by Progetto Giovani Ricercatori no. GR-2011-02346826, Ministero della Salute (Rome, Italy) and the Associazione Italiana Ricerca Cancro (AIRC), Investigator Grant IG-17622 (Milan, Italy).