Quantification of dendritic cell staining.

Immunohistochemical staining of cryosections was performed simultaneously in batches for both WT and PU.1^−/−embryos or cultured thymic lobes. Sections were stained for HSA, CD11c, or DEC-205 expressions. A minimum of 3 sections (average of 5 sections) from different embryos were digitized (Leica Photomicroscopy system, Leica) for each gestational age and stain. Staining intensity was quantified with analysis software (NIH-Image). Background staining from muscle was subtracted, and WT sections were assigned a value of 100%. Mean values and standard deviations for relative staining intensity are shown to the nearest 5%.