Simultaneous flow cytometric analysis of IFN-γ and CD4 mRNA and protein expression kinetics in human peripheral blood mononuclear cells during activation.

The application of fluorescently-labeled antibodies for flow cytometric identification and characterization of specific cell types within heterogeneous populations by their protein expression profile is well established. While detection of proteins is informative, concomitant transcript analysis in the same cells would provide a more complete and comprehensive view of intracellular signaling events. We recently reported on the efficient detection of RNA in suspension cells for flow cytometric analysis. The improved RNA flow cytometry procedure described here allows for the specific labeling of multiple RNA species, and is compatible with antibody-based targeting of extracellular and intracellular antigens for multiplexing purposes. To show proof of concept, human peripheral blood mononuclear cells were stimulated with phorbol 12-myristate 13-acetate and ionomycin for a maximum of 5 h, during which their CD4 and interferon-gamma (IFN-γ) transcript and protein levels were monitored. Substantial and increasing numbers of IFN-γ mRNA+ cells were detected within 30 min after initiation of induction, while IFN-γ protein+ cells could only be discerned at 1 h and beyond. Surprisingly, resting lymphocytes contained less CD4 mRNA but more of the protein per cell compared with monocytes, revealing a difference in the relationship of transcript and protein levels in these two cell types. We additionally applied monensin, which is commonly used to block cytokine secretion, and found that IFN-γ mRNA can still be analyzed consistently using the improved RNA flow cytometry staining method. Notably, a subset of IFN-γ mRNA(-)/protein+ cells that were not observed in the absence of monensin became apparent at the 5-h mark. This subset probably represents cells that have accumulated IFN-γ protein, but no longer transcribe mRNA. Collectively, the results described here exemplify how the improved RNA flow cytometry labeling procedure can be applied to simultaneously assess mRNA and protein dynamics to gain insight into the regulation of gene transcription and translation in individual cells.