MHCII expression in thymus and spleen of MHCII-deficient mouse strains. (A) Thymic sections from Aα^-/- (i), CIITA^-/- (ii), RFX5^-/- (iii), C/R^-/- (iv), and C57BL/6 (v) mice were analyzed by 3-color immunofluorescence staining and confocal microscopy. Medullary regions of the sections are shown. The sections were stained for I-A^b (red; top left quadrants), CD11c (blue; top right quadrants), and UEA-1 (green; bottom left quadrants); bottom right quadrants show overlay of all 3 stains (UEA⁺I-A^b+ cells = yellow; CD11c⁺I-A^b+ cells = pink; triple-positive cells = white). (B) Immunohistochemical analysis of I-A^b (red) and B220 (blue) expression on splenic sections of the different mouse strains. Staining for I-A^b (red) in splenic T-cell zones was consistently stronger in RFX5^-/- and CR^-/- mice than in the C57BL/6 control (see also Clausen et al³³ ).