PTPROt expression in normal naive quiescent, germinal center, and memory B cells. Whole-cell lysates (75 μg) from unsorted tonsillar B cells and highly purified naive (CD19⁺sIgD⁺ CD38⁻), germinal center (CD19⁺ sIgD⁻ CD38⁺), and memory (CD19⁺ sIgD⁻ CD38⁻) B cells were immunoblotted and analyzed with the antiserum directed against the murine PTPROt homologue, PTPØ. The abundance of PTPROt in naive, germinal center, and memory B cells was compared by scanning densitometry. Autoradiograms were scanned with a CCD camera linked to a frame grabber (Alpha Imager 2000; Alpha Innotec Corp, San Leandro, CA) and band intensities were quantified by using Image Quant Software (Molecular Dynamics, Sunnyvale, CA). The filters were also probed with an antitubulin antibody to assure equal loading.