Functional Modulation of VLA6 in BCR-ABL1+ Pre-B Acute Lymphoblastic Leukemia.

Abstract 2565

Chemotherapy drug resistance in acute lymphoblastic leukemia (ALL) remains a major problem, resulting in reduced treatment efficacy and relapse. The bone marrow environment (BME) has been shown to promote resistance of leukemia cells towards chemotherapy, which has been attributed to several proteins, including integrins. Our analysis of 207 children with high-risk (BCR/ABL1⁻) pre-B ALL revealed that high expression of the laminin-binding integrin VLA6 (alpha6beta1) portends poor clinical outcomes in patients with minimal residual disease (MRD⁺) on day 29 of induction. In addition, our comparative analysis of pre-B leukemia and normal B-cells revealed that VLA6 is preferentially upregulated on BCR/ABL1⁺ pre-B ALL blasts. Alterations in adhesion properties have been described for BCR/ABL1⁺ (p210) chronic myeloid leukemia. The role of integrins and integrin VLA6 in particular for cell adhesion-mediated drug resistance (CAM-DR) in BCR/ABL1⁺ (p210) ALL has not been addressed. With respect to its role for normal immature hematopoietic cells, contradictory observations have been reported. Therefore, we hypothesized that VLA6-mediated adhesion of ALL cells to the bone marrow stromal niche contributes to drug resistance. We evaluated the role of VLA6 in BCR-ABL1⁺ leukemia using two of our established models of leukemia, a conditional knockout model of VLA6 in murine BCR-ABL1⁺ leukemia and a xenograft model of human BCR-ABL1⁺ leukemia. VLA6^fl/fl cells were oncogenically transformed using BCR-ABL1 (p210) and cultured under lymphoid-skewing conditions. Induction of pre- B (B220⁺ CD19⁺) ALL was confirmed by flow cytometry. Subsequent transduction with CreER^T2 or EmptyER^T2 generated leukemia cells in which VLA6 ablation could be induced (CreER^T2) or not (EmptyER^T2) by addition of Tamoxifen. Conditional ablation of VLA6 in vitro decreased adhesion significantly compared to undeleted controls (19.7%±8.1% vs. 87.7%±6.0%; p=0.00041) and increased apoptosis of murine BCR-ABL1⁺ leukemia cells as determined by analysis of Annexin V⁻/7-AAD⁻ viable cells by flow cytometry (VLA6 deleted vs. undeleted: 35.3%±1.1% vs. 75.1%±1.2%; p=0.0001). Moreover, VLA6 deletion sensitized murine ALL to a tyrosine kinase inhibitor (TKI), Nilotinib (p=0.022, 45.6%±2.4% vs. 73.3%±13.0%). To test the effect of VLA6 deletion on leukemic progression in vivo, VLA6 BCR/ABL1⁺ pre-B (B220⁺ CD19⁺) CreER^T2+ or control transduced ALL cells were transferred into NOD/SCID mice. 3 days thereafter, VLA6 deletion was induced by Tamoxifen administration to all animals in 2 cycles for 5 days. In vivo deletion of VLA6 in delayed leukemia progression compared to VLA6 competent controls from a median survival time (MST) of 30 days post-leukemia injection to a MST of 43 days post-leukemia injection (p=0.008 Log-rank test). In vivo deletion of VLA6 in combination with Nilotinib treatment delayed leukemia progression compared to VLA6 competent, as Nilotinib-treated control animals have uniformly died of leukemia with a MST of 39.5 days, however the Nilotinib treated VLA6 deleted group is completely alive and is still being monitored (p=0.0025). When VLA6 was ablated before transfer and recipients were observed for leukemia progression, the recipients of VLA6–deficient murine leukemia cells also showed attenuated leukemia progression compared to recipients of VLA6 competent cells. Moreover, we show that VLA6 blockade de-adheres primary ALL cells from their cognate counter receptor laminin in vitro, and sensitizes primary ALL cells to TKI Taken together, modulating the function of VLA6 in ALL offers a new approach to overcome drug resistance in ALL. Given that VLA6 is probably largely redundant for normal immature hematopoiesis, this approach may be preferable over targeting of other integrins in BCR/ABL1⁺ leukemias on which VLA6 is expressed.