Expression of ROR1 in Chronic Lymphocytic Leukemia Cells Enhances Cells Survival by Wnt5a Signaling.

ROR1 is a type I membrane-receptor tyrosine kinase with intracellular tyrosine kinase domains and extracellular Frizzled-like cysteine-rich domains (CRDs) and kringle domains, which are common to receptors of members of the Wnt-family. We found that ROR1 might serve as an orphan receptor for Wnt5a, which can activate NF-κB via a non-canonical Wnt-signaling pathway. In prior studies we found that CLL cells expressed ROR1 and high-levels of Wnt3, Wnt5b, Wnt6, Wnt10a, Wnt14, and Wnt16, but not Wnt5a, which is found expressed on dendritic cells and other stromal cells that can support leukemia-cell survival in vitro, and presumably in vivo. As such, we examined whether cells could support the survival of CLL cells in a Wnt5a-dependent manner. For this we cultured CLL cells alone or together with Chinese Hamster Ovary (CHO) cells or CHO transduced to express Wnt5a (CHO-Wnt5a) and monitored the viability of CLL cells over time. We found that the viability of CLL cells co-cultured with CHO cells or CHO-Wnt5a cells was greater than that of CLL cells cultured alone. However the viability of CLL cells co-cultured for two days with CHO-Wnt5a cells (71% ± 18% S.D) was significantly greater than that of CLL cells co-cultured with CHO cells (45% ± 17% S.D) (N=10, P<0.01). The capacity of CHO-Wnt5a to enhance the viability of CLL cells relative to that of CHO cells could be neutralized by anti-ROR1 antisera generated in patients who received autologous CLL cells transduced to express the CD40-ligand (CD154), but not by normal human serum or pre-treatment human serum. CHO cells transduced to express ROR1, but not CHO cells transduced with a control vector, could absorb the capacity of the anti-ROR1 antisera to bind ROR1 and removed its capacity to neutralize the capacity of CHO-Wnt5a cells to provide for a protecive advantage over CHO cells for CLL cell survival in vitro. These studies provide the first evidence that the survival of CLL cells can be enhanced by cells expressing Wnt5a, let alone Wnt factors in general, and might account in part for the enhanced survival of CLL cells in tissue microenvironments containing cells that express Wnt5a. Conceivably, the anti-ROR1 antibodies induced by autologous CLL cells transduced to express CD154, or the administration of antibodies or antagonists that can inhibit Wnt5a-ROR1 signaling, could have therapeutic activity in patients with CLL.