Characterization of Peripheral B Cells Expressing CLL-Associated Receptor Tyrosine Kinase ROR1 in Healthy Donors

Abstract 1787

Recent studies have described an extensive expression of the receptor tyrosine kinase-like orphan receptor 1 (ROR1) on the surface of malignant B cells in patients suffering from chronic lymphoid leukemia (CLL). ROR1 expression has also been detected in ovarian cancer, renal cancer, melanoma, and lung adenocarcinoma, suggesting a general role of ROR1 in cancer genesis and/or maintenance. However, low levels of ROR1 mRNA or protein expression have also been found on undifferentiated embryonic stem cells, adipose tissue and on early stage of B-cells in bone marrow, but not in major adult tissues and peripheral B cells.

In contrast, our results describe for the fist time expression of ROR1 on rare B cells in peripheral blood of healthy donors. Using magnetic cell sorting techniques we were able to enrich ROR1 positive B cells from PBMCs of healthy donors. The average frequency of ROR-1⁺ B cells in PBMCs ranged from 1×10⁻³ to 1×10⁻⁴. Interestingly, ROR1⁺ B cells revealed a heterogeneous phenotype and could further be classified into a CD19⁺/CD27⁺/CD38⁻ and a CD19⁺/CD27⁻/CD38⁺ subpopulation as assessed by flow cytometry. Remarkably, in contrast to the CD27⁺ subpopulation the CD38⁺ subpopulation showed higher levels of CD5 and CD23 expression, which are both markers known to be upregulated on B cells of CLL patients. Both populations do not show elevated expression of transcription factor ZAP70 and Bcl-2 but expressed higher level of Bcl-6 compared to CD19 B cells. Bcl-6 is known to be required for pre-B cell self-renewal but has also been described to play an essential role in resistance of leukemic cells to therapeutic tyrosine kinase inhibitors. Moreover ROR1⁺ B cells revealed an immature and non-activated sate as evidenced by the expression of IgD and IgM, and the lack of IgG and CD69. Further functional examination and immunophenotyping of these rare cells are under investigation.

We have identified rare B cells expressing the tumor-associated receptor tyrosine kinase ROR1, which are heterogenic in CD27 and CD38 expression and show an immature phenotype. We are currently assessing a potential relationsphip ROR1⁺ peripheral B cells and ROR1 expressing B cells in malignancies like CLL, mantel cell (MCL) and marginal zone lymphoma (MZL).