Characterization of cells taking up β-galactosidase-coding mRNA in vivo. Mouse ear pinnae were injected with 100 μl of Ringer lactate solution containing 20 μg of RNActive-encoding β-galactosidase. Eighteen hours after injection, ears were frozen in embedding medium and cut perpendicularly to the mouse ear symmetry axis. Individual sections were stained overnight with a solution containing X-gal (a) or magenta red (c). The diagram in (b) shows the number of β-galactosidase-expressing cells visible on the stained sections prepared from the injected area located between 3 and 4.3 mm from the base of the ear. The sections shown in (a and c) are indicated by α, β and χ, respectively, in diagram (b). (c) shows two different sections (in three series) from the same ear stained with magenta red and further incubated sequentially with biotinylated anti-MHC class II molecules and streptavidin-Alexa 546. All images were acquired on a fluorescence microscope. The left panels were made using visible light. The middle panels with the fluorescence channel (a purple trace reports the position of β-galactosidase-expressing cells). The right panels are superpositions of both magenta red and antibody staining using artificial colors. Most β-galactosidase-expressing cells appear MHC class II negative, but some such as β(2/8) or β(7/8) are possibly MHC class II positive. MHC, major histocompatibility complex; mRNA, messenger RNA.