BCR-Induced VLA-4 Activation in the TCL1 Transgenic Mouse Model for Chronic Lymphocytic Leukemia

Introduction. Ibrutinib, a small molecule inhibitor of Bruton's tyrosine kinase (BTK), has proven to be an efficient treatment for chronic lymphocytic leukemia (CLL). A distinct characteristic of ibrutinib therapy is transient lymphocytosis. Recently, we have demonstrated that CLL patients with high levels of CD49d show reduced lymphocytosis and inferior nodal response under ibrutinib due to residual activity of BCR-induced inside-out activation of the CD49d/CD29 integrin VLA-4 (Tissino E et al. J Exp Med. 2018;215(2):681-697). Here, we used Tcl1 transgenic (tg) mice as a model to further validate the observation of VLA-4 activation under ibrutinib and to study involved signaling pathways and the effect of VLA-4 inhibition in vivo. Methods. Surface receptor expression analysis of various receptors was performed by flow cytometry. The phosphorylation of signaling molecules was measured by phosflow and western blotting. VLA-4 affinity state was determined by a real-time kinetic assay described in Chigaev A et al. J Biol Chem. 2001;276(52):48670-8. To analyze the distribution of individual VLA-4 molecules on the cell surface, immunofluorescence approaches and superresolution microscopy (STORM, Abbelight) were employed. Mouse treatment studies were performed upon transplantation of TCL1-tg splenocytes to wild-type C57BL/6J mice using the small molecule VLA-4 inhibitor firategrast in drinking water. Tumor infiltration of different organs was measured by flow cytometry. Results. Analyzing the surface expression of CD49d and other homing receptors, we found that TCL1-tg mice correspond with the CD49d-high CLL cohort. We found that both CLL cells from TCL1-tg mice and human CD49d-high CLL show similar CD49d expression levels as the corresponding healthy B cells (human: N = 116 CD49d-high CLL and 32 healthy donor, P = 0.8717; mouse: N = 12 per group, P = 0.6845). Next, we analyzed the impact of BCR pathway inhibitors on the phosphorylation of signaling molecules involved in the BCR pathway after activation by anti-IgM (aIgM) in TCL1-tg leukemic cells. Ibrutinib and idelalisib showed specific patterns of inhibition of BTK and PI3K, respectively. The combination of ibrutinib and idelalisib proved to be the most efficient in reducing the phosphorylation of BTK, SYK, ERK1/2 and Akt upon IgM activation, compared to the phosphorylation of stimulated cells without inhibition (N = 6, P = 0.0003, 0.0305, 0.0039, 0.0019, respectively). IgM stimulation induced VLA-4 high affinity as well as a reorganization of VLA-4 molecules on the cell surface, forming areas of high VLA-4 density. BCR-induced inside-out activation of VLA-4 remained functional upon treatment with ibrutinib (N = 5, cnt vs aIgM P = 0.0017, cnt vs ibrutinib+aIgM P = 0.0499), while idelalisib reduced VLA-4 activation more effectively (N = 5, cnt vs aIgM P = 0.0014, cnt vs idelalisib+aIgM P = 0.0803), suggesting a pivotal role of PI3K in the transmission of the exogenous antigen signal to the integrin. Finally, to analyze the potential of VLA-4 blockage in a tumor setting similar to VLA-4-high CLL patients, we treated wild-type C57BL/6J mice (N = 6 mice per group), which were transplanted with TCL1-tg splenocytes, with the CD49d inhibitor firategrast. This treatment reduced the tumor load in spleen and bone marrow. Conclusion. We found that the TCL1-tg mouse model is adequate to study the activity of the BCR-VLA-4 axis in CLL. Using this model, we show that a) BCR stimulation induces both, an increase in VLA-4 affinity as well as avidity (clustering), b) that PI3K is an essential transmitter between BCR and VLA-4, and c) that VLA-4 inhibition alters tumor infiltration patterns in vivo. Synergies of VLA-4 blockage with established therapy options as a possible way of reducing microenvironment-induced resistance development are currently been investigated. Disclosures Egle: Celgene: Honoraria, Other: Advisory board and Travel support. Greil: Eisai: Honoraria; Daiichi Sankyo: Consultancy, Honoraria; Sandoz: Honoraria; Genentech: Honoraria, Research Funding; Cephalon: Consultancy, Honoraria, Research Funding; Sanofi Aventis: Honoraria; Janssen-Cilag: Honoraria; AstraZeneca: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; GSK: Research Funding; Boehringer Ingelheim: Honoraria; AbbVie: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; Pfizer: Honoraria, Research Funding; Bristol-Myers-Squibb: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; Merck: Consultancy, Honoraria, Research Funding; Mundipharma: Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; Celgene: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; Novartis: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; MSD: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding; Takeda: Consultancy, Honoraria, Research Funding; Ratiopharm: Research Funding; Gilead: Consultancy, Honoraria, Other: Travel/accomodation expenses, Research Funding.