ROR1 is a receptor tyrosine kinase (RTK) recently identified to be overexpressed at the gene and protein levels in chronic lymphocytic leukemia (CLL). Monoclonal antibodies (MAbs) against RTKs have been successfully applied for therapy of solid tumors. We generated five MAbs against the Ig (n ¼ 1), cysteine-rich (CRD) (n ¼ 2) and kringle (KNG) (n ¼ 2) domains, respectively, of the extracellular part of ROR1. All CLL patients (n ¼ 20) expressed ROR1 on the surface of the leukemic cells. A significantly higher frequency of ROR1 expression was found in patients with progressive versus non-progressive disease, and in those with unmutated versus mutated IgVH genes. All five MAbs alone induced apoptosis in the absence of complement or added effector cells (Annexin-V and MTT, as well as cleavage of poly-(ADP ribose)-polymerase, caspase-8 and caspase-9) of CLL cells but not of normal B cells. Most effective were MAbs against CRD and KNG, significantly superior to rituximab (Po0.005). Cross-linking of anti-ROR1 MAbs using the F(ab 0 ) 2 fragments of anti-Fc antibodies significantly augmented apoptosis. Two of the MAbs induced complement-dependent cytotoxicity (CDC) similar to that of rituximab and one anti-ROR1 MAb (KNG) (IgG1) showed killing activity by antibody-dependent cellular cytotoxicity. The identified ROR1 epitopes may provide a basis for generating human ROR1 MAbs for therapy.

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Phosphorylation of receptor tyrosine kinases (RTKs) has a key role in cellular functions contributing to the malignant phenotype of tumor cells. We and others have previously demonstrated that RTK ROR1 is overexpressed in chronic lymphocytic leukemia (CLL). Silencing siRNA downregulated ROR1 and induced apoptosis of CLL cells. In the present study we analysed ROR1 isoforms and the phosphorylation pattern in CLL cells (n=38) applying western blot and flow-cytometry using anti-ROR1 antibodies and an anti-phospho-ROR1 antibody against the TK domain. Two major ROR1 bands with the size of 105 and 130 kDa respectively were identified, presumably representing unglycosylated (immature) and glycosylated (mature) ROR1 respectively as well as a 260 kDa band which may represent dimerized ROR1. A ROR1 band of 64 kDa that may correspond to a C-terminal fragment was also noted, present only in the nucleus. The 105 kDa ROR1 isoform was more frequently expressed in non-progressive as compared to progressive CLL patients (p=0.03). The 64, 105, 130 and 260 kDa bands were constitutively phosphorylated both at tyrosine and serine residues. Phosphorylation intensity of the mature (130 kDa) isoform was significantly higher in progressive than in non-progressive disease (p<0.001). Incubation of CLL cells with a mouse anti-ROR1 KNG or an anti-ROR1 CRD mAb respectively induced dephosphorylation of ROR1 before entering apoptosis. In conclusion CLL cells expressed different isoforms of ROR1 which were constitutively phosphorylated. The mature, phosphorylated ROR1 isoform was associated with a progressive disease stage. Targeting ROR1 by mAbs induced specific dephosphorylation and leukemic cell death. ROR1 might be an interesting therapeutic target.

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