The DNA-binding, fluorescent dye 7-amino-actinomycin D (7AAD) is efficiently excited by the 488 nm laser line commonly used in flow cytometry, but yields fluorescence emission further into the red spectrum than alternative DNA-specific fluorochromes. In this report, we show that the spectral properties of 7AAD allow single-laser analysis of DNA content and cell cycle simultaneously with two cell surface markers labeled with fluorescein (green)-and phycoerythrin (orange)-conjugated antibodies. The use of 7AAD makes three-color analysis practical and feasible, using the most widely available flow cytometric instruments. The power of this technique was demonstrated in two systems. Staining of human peripheral blood lymphocytes (PBL) with 7AAD was demonstrated to be dependent on cell activation and chromatin conformation; PHA-stimulated cells which have become activated and express IL 2 receptors had greater 7AAD fluorescence than nonactivated, IL 2 receptor-negative cells. Cell cycle analysis of mouse splenocytes stained with fluorescent antibodies to IgM and to Ly-1 demonstrated that the proportion of S and G2 phase cells in native spleen varies strongly among the subsets of cells identified with these markers. Of particular interest was the striking finding that the Ly-1+/IgM+ subset (Ly-1 B cells) is greatly enriched for cells in the S phase fraction. This is important because Ly-1 B cells have been associated with the production of autoantibodies, and is consistent with reports that these cells have a lymphoblastoid or a plasmablast morphology. We hypothesize that Ly-1 B cells may belong to a subset of in vivo activated cells which are either rapidly proliferating or are arrested in S phase.

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