Quantification of EGFP expression on Molt-4 T cells using calibration standards

Enhanced green fluorescent protein (EGFP) is used extensively to assess gene expression on cells; however, quantification of this expression by flow cytometry has been limited by the unavailability of calibration standards. Thus, we characterized the response of an experimental set of EGFP calibration standards to environmental changes and then quantitate the expression of EGFP, in molecules of equivalent soluble fluorochrome (MESF) units, of a transfected Molt-4 T cell line by flow cytometry.

Characterization of the EGFP standards: EGFP standards were equilibrated in suspension solutions having a pH range of 5.0-9.0, temperatures of 37-80 degrees C, and osmolalities of 100-600 mOsm/kg. Quantification of EGFP on cells: For transfections, Molt-4 T cells were incubated with two different concentrations (0.2 microg and 0.4 microg) of pEGFP-N2 vector and the EGFP expression was quantified after 48 h by flow cytometry using the EGFP standards and by the cytofluor technique using a standard curve of known EGFP solutions.

The fluorescence intensity of the EGFP standards increased from pH 5.0 to 9.0 and remained relatively constant from 37 degrees C to 65 degrees C, and from 100 to 600 mOsm/kg. After transfection, the expression of the populations with high and low EGFP expression averaged 8,098 +/- 584 MESF and 3,808 +/- 375 MESF respectively. No significant differences were observed after comparing the MESF values obtained by flow cytometry and the values obtained by Cytofluor technique (high: 8,791 +/- 492 MESF; low: 4,082 +/- 398 MESF).

Our data demonstrate the feasibility of using calibration standards to quantify EGFP expression on cells. Our results emphasize the importance of monitoring the effects of environmental changes in the fluorescence intensity of both standards and samples when quantifying the expression of EGFP on living cells.