Clonogenic assays measure leukemia stem cell killing not detectable by chromium release and flow cytometric cytotoxicity assays.
OBJECTIVE
NK-92, a permanent natural killer (NK) cell line, shows cytotoxicity against a variety of tumors and has been tested in a phase I trial. We tested the toxicity of NK-92 and chemotherapy drugs against the stem cell capacity of the acute leukemia cell line, KG1. While the chromium-release assay is the most common method for assessing cytotoxicity of immune effectors, and flow cytometry is increasingly used, the relationship of either assay to clonogenic readouts remains unknown.


METHODS
KG1 was assessed for stem cell frequency by serial dilution, single-cell sorting and colony growth in methylcellulose. KG1 was sorted into CD34(+) CD38(+) and CD34(+) CD38⁻ populations and recultured in liquid medium or methylcellulose to determine the proliferative capacity of each fraction. Cytotoxicity of NK-92, daunorubicin and cytarabine against KG1 was measured using the chromium-release assay, flow cytometry and clonogenic assays.


RESULTS
The culture-initiating cell frequency of whole KG1 was between 1 in 100 to 1000 by serial dilution and single-cell sorting. Although a rare (1-3%) CD34(+) CD38⁻ population could be demonstrated in KG1, both fractions had equivalent proliferative capacity. The cumulative flow cytotoxicity assay was more sensitive than the chromium-release assay in detecting target cell killing. At a 10:1 ratio NK-92 eliminated the clonogenic capacity of KG1, which was not predicted by the chromium-release assay.


CONCLUSIONS
Clonogenic assays provide a more sensitive means of assessing the effect of a cytotoxic agent against putative cancer stem cells within cell lines, provided that they grow well in liquid culture medium or methylcellulose.
Cell Therapy Program, Princess Margaret Hospital, University of Toronto, Canada. brentw@uhnres.utoronto.ca
Brent A
BA
Williams
Xing-Hua
XH
Wang
Armand
A
Keating
D051997:ADP-ribosyl Cyclase 1 / Q000096:biosynthesis; D018952:Antigens, CD34 / Q000096:biosynthesis; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use; D018929:Cell Culture Techniques; D045744:Cell Line, Tumor; D002469:Cell Separation; D002859:Chromium Isotopes / Q000378:metabolism; D003561:Cytarabine / Q000494:pharmacology / Q000627:therapeutic use; D003601:Cytotoxicity Tests, Immunologic / Q000379:methods; D003602:Cytotoxicity, Immunologic; D003630:Daunorubicin / Q000494:pharmacology / Q000627:therapeutic use; D005434:Flow Cytometry; D006801:Humans; D007694:Killer Cells, Natural / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D007938:Leukemia / Q000175:diagnosis / Q000276:immunology* / Q000473:pathology / Q000628:therapy; D014411:Neoplastic Stem Cells / Q000378:metabolism* / Q000473:pathology; D012680:Sensitivity and Specificity; D014410:Tumor Stem Cell Assay
D018952:Antigens, CD34; D002859:Chromium Isotopes; D003561:Cytarabine; D051997:ADP-ribosyl Cyclase 1; D003630:Daunorubicin