Many tumor cells become resistant to commonly used cytotoxic drugs due to the overexpression of ATP-binding cassette (ABC) transporters. Specifically, p-glycoprotein (MDR-1) is frequently up-regulated in chemotherapy-resistant tumor cells, which is associated with poor prognosis. On the other hand, human leukocyte antigen (HLA) class I molecules are known to be significantly down-regulated in advanced tumor cells to escape from immune surveillance. However, the relationship between MDR-1 expression and HLA expression is not fully understood. Recently, we have developed a recombinant single-chain Fv diabody specific to HLA-A and demonstrated that this agent mediates cell death in HLA-overexpressing lymphoid tumor cells but not in normal cells (

Cancer Res
). Here, we investigated the expression levels of HLA class I in chemo-resistant leukemic cells and evaluated the therapeutic potential of single-chain Fv diabody specific to HLA-A, C3B3-DB (Chugai Pharmaceutical Co. Ltd., Tokyo, Japan). Chemotherapy-resistant cells were established by subculturing of myeloid leukemia cell line HL60 and Burkitt’s lymphoma cell line BL-TH in increasing doses of vincristine (VCR), and named HL60/VCR and BL-TH/VCR, respectively. MDR-1 is strongly expressed in HL60/VCR and BL-TH/VCR cells both at the mRNA and protein levels, but not in the parental cells. Interestingly, expression levels of HLA class I molecules are 8 times higher in HL60/VCR and BL-TH/VCR cells than in the parental cells, suggesting that MDR-1 modulates cell surface expression of HLA by its transporter function. Next, we examined the cytotoxic activity of C3B3-DB on these chemo-resistant cell lines. C3B3-DB induced apoptosis in HL60/VCR and BL-TH/VCR cells and these chemo-resistant cell lines were more sensitive to C3B3-DB than the parent cells. Combination of C3B3-DB with chemotherapeutic agents such as VCR and daunorubicin (DNR) resulted in enhanced cytotoxicity against HL60/VCR and BL-TH/VCR cells. Importantly, pretreatment of these chemo-resistant cell lines with C3B3-DB reduced expression levels of MDR-1 and increased drug retention in these cells as detected by flow cytometry and confocal microscopy. Furthermore, combination of C3B3-DB with VCR significantly blocked the cell cycle at the G2 phase compared with VCR alone. Similar results were obtained with primary acute myeloid leukemia cells from 2 patients, resulting in up-regulation of both HLA class I and MDR-1 molecules at relapse phase compared at diagnosis. These results suggest that C3B3-DB enhances cytotoxicity of chemotherapeutic agents and provides a novel approach for overcoming drug resistance in hematological malignancies.

Author notes

Disclosure: No relevant conflicts of interest to declare.