A semi-automated fluorescent (SAF) assay using viable, whole cells for screening hybridoma supernatants

Cellular enzyme-linked immunocircle assay. A rapid assay of hybridomas produced against cell surface antigens

A novel method has been developed for the initial screening of hybridomas produced against cell surface antigens. Glutaraldehyde-fixed cells were immobilized as targets on the lid of a 96-well tissue culture plate which had been precoated with poly-L-lysine. Antibody binding was determined by an immunoenzymatic method in an arrangement permitting both macro- and microscopic examination. After optimization with mouse thymus cells using existing rat monoclonal antibodies, new rat-mouse hybridoma cell lines against mouse thymocytes and bone marrow cells were screened. The antibodies could be characterized immediately both by the localization of the immune reaction (surface or intracellular) or as estimated by the frequency of positive cells recognized by the antibody in the sample.

Detection of low numbers of lymphocyte surface membrane molecules using concentration immunofluorescence analysis (CIA)

Using viable lymphocytes in a concentration immunofluorescence assay (CIA), conditions were ascertained which allow detection of low numbers of surface membrane molecules. Utilizing 3 layer labeling [monoclonal antibody (MAb) specific for the membrane molecule, biotin-conjugated antibody specific for the MAb, and R-phycoerythrin-strepavidin], large numbers of lymphocytes in the assay wells, and the Pandex Fluorescence Concentration Analyzer, a fluorescence signal significantly above background was generated by as few as 8 x 10(7) molecules among 4 x 10(5) lymphocytes. Experiments using membrane molecules (Fc gamma R II, Ia antigens, Ly-39) which differ considerably in their level of expression indicated that comparable signals were generated by equivalent numbers of labeled molecules in a cell population irrespective of the number of molecules on an individual cell. Thus, CIA is theoretically capable of detecting membrane molecules whose expression is as low as 2 x 10(2) per cell. CIA should be useful in the assay of cytokine receptors and other lymphocyte membrane molecules expressed at low levels, and in the development of MAb specific for these molecules.

Comparison of fluorochrome-labeled and 51Cr-labeled targets for natural killer cytotoxicity assay

An alternative method for measuring in vitro cellular cytotoxicity has been developed utilizing the carboxyfluorescein derivative 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) as the target cell label. Target cells labeled with the fluorescent dye are incubated with effector cells, if killing of targets occurs, BCEDF is released analogous to 51Cr release. Measurement of specific lysis in this assay is based on the direct measurement of dye retained by the remaining viable target cells using the Pandex FCA. In paired experiments we have compared the fluorochrome assay to the standard 51Cr release assay in measuring porcine natural killer cytotoxicity. The target labeling time with BCECF is 30 min as opposed to 1 h with 51Cr; and there is no significant dye reincorporation after release. The optimal target number per incubation well for the BCECF assay is 5 X 10(3) cells. In both the BCECF and 51Cr release assays, maximum percent specific lysis is reached after 3-4 h incubation. By 2 h incubation, the BCECF assay reaches the maximum seen with 51Cr and in a 4 h assay the maximum NK activity measured with BCECF labeled targets is always higher than that measured with 51Cr-labeled targets. In paired experiments, we have shown the reproducibility of the BCECF assay and that the BCECF assay measures NK enhancement by NK enhancing monoclonal antibody and inhibition by NK inhibiting monoclonal antibody as good as the 51Cr release assay, if not better. In conclusion, the BCECF assay is a reliable and reproducible measure of in vitro cellular cytotoxicity, eliminates the use of radioisotopes and is cost efficient.

Fluorescence-based viability assay for studies of reactive drug intermediates

Studies of drug toxicity, toxicologic structure-function relationships, screening of idiosyncratic drug reactions, and a variety of cytotoxic events and cellular functions in immunology and cell biology require the sensitive and rapid processing of often large numbers of cell samples. This report describes the development of a high-sensitivity, high-throughput viability assay based on (a) the carboxyfluorescein derivative 2'-7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) as a vital dye, (b) instrumentation capable of processing multiple small (less than 100 cells) samples, and (c) a 96-well unidirectional vacuum filtration plate. Double staining of cultured peripheral blood mononuclear cells with BCECF and propidium iodide (PI) showed no overlap between PI+ (nonviable) and BCECF+ (viable) cells by flow cytometric analysis. Optimal conditions were developed for dye loading and minimizing physical cell damage and fluorescence quench during the assay procedure. The ratio of BCECF fluorescence to internal standard fluorescent particles was linear from 40 to greater than 20,000 cells with a signal:noise ratio of approximately 3 at 40 cells/well. Sulfamethoxazole hydroxylamine (SMX-HA) was used as a model toxic drug metabolite to explore the validity of the BCECF procedure. SMX-HA, but not its parent compound sulfamethoxazole, resulted in a dose dependent loss of cellular fluorescence and the parallel accumulation of PI+ nonviable cells. When compared to the currently used tetrazolium dye reduction viability assay, the BCECF method was 3-fold more sensitive, greater than 10-fold faster, and required 1/10-1/100 the cell numbers.

Fluorescent EIA screening of monoclonal antibodies to cell surface antigens

We have developed a sensitive enzyme immunoassay (EIA) that is useful for detecting antibodies directed against antigenic sites on the surface of mammalian cells. Approximately 100 antigen-bearing cells were trapped in the wells of 96-well Durapore membrane filter plates (Millititer GV plates, Millipore, Bedford, MA). Antibody binding was detected with alkaline phosphatase conjugated goat anti-mouse IgG + IgM. Conjugate alkaline phosphatase activity was detected with a fluorogenic substrate in the presence of levamisole, an inhibitor of endogenous cellular alkaline phosphatase using a Fluoroskan microtiter plate reader (Flow Laboratories). The method permits accurate and reproducible screening of hybridoma supernatants using a minimal number of antigen-bearing cells.

Characterization of a method using viable human target cells as the solid phase in a cell concentration fluorescence immunoassay (CCFIA) for screening of monoclonal antibodies and hybridoma supernatants

A new method has been characterized for the use of viable target cells as the solid phase for screening of hybridoma supernatants in a cell concentration fluorescence immunoassay (CCFIA). Briefly, the specific target antigen on the cells is bound by the monoclonal antibodies and revealed by use of a fluoresceinated second antibody. Separation of free from bound antibody is accomplished by filtration in the 0.2 micron filter-bottom wells of specialized assay plates. Processing is automated in a Pandex screen machine, resulting in numerical fluorescence values for each well. This method is rapid (under 1 h per 96-well plate), highly sensitive (down to 0.2 ng/ml) and sparing of target cells (0.3-2.5 X 10(4) cells per assay well). It has been applied to 37 different varieties of human solid tumor cells, as well as to human peripheral blood mononuclear cells. The cells used as targets for the characterization of this method were still capable of attachment and growth when recovered post-assay. This method was compared with a viable cell enzyme-linked immunosorbent assay (ELISA) method, showing similar sensitivity and greatly shortened assay time. Comparison of the results from this method with those obtained from flow cytometric analysis performed on viable cells showed close correlation, whereas a lower correlation was seen with immunohistochemical methods using acetone-fixed cells. Development of this method made it possible to rapidly screen many thousands of hybridoma supernatants and successfully select those which were specific for surface antigens on viable cells.