Circulating mature T cells are present in the periphery of CB/Thy-NOD/SCID chimeras. Splenic sections from a CB/Thy-NOD/SCID mouse at 6 months post-CB reconstitution (a) and a control Thy-NOD/SCID mouse (b) stained with an anti-human–specific CD45 MoAb using a peroxidase detection method. Human CD45⁺ cells occupying the periarteriolar areas are observed in the spleen of CB/Thy-NOD/SCID mouse (a). The brown staining detected in the spleen of the control Thy-NOD/SCID mouse (b) corresponds to the background levels of staining obtained with the isotype control antibody (not shown). The staining is representative of 3 CB/Thy-NOD/SCID and 4 Thy-NOD/SCID chimeras generated in independent experiments. Percentage of human TCR /β⁺ lymphocytes (c) in the periphery of individual CB/Thy-NOD/SCIDs as detected by flow cytometry of splenocytes or PBL. The proportion of CD4⁺ (closed bars) and CD8⁺ (open bars) lymphocytes within the human TCR /β⁺ population is represented. The NOD/SCID chimeras are numerated as in Fig 1 to allow direct comparison of levels of reconstitution in the BM and thymic compartments. (d) Confocal microscopy of a splenic section from a CB/Thy-NOD/SCID mouse reconstituted with male CB stained by 2-color FISH with human Y chromosome-specific probes (green fluorescence) and anti-CD4 and CD8 MoAbs (red fluorescence). This staining is representative of 3 CB/Thy-NOD/SCID chimeras generated in independent experiments. The inset in (d) shows a magnified detail of Y chromosome-positive cells identified in the spleen. Bar in (a and b) = 125 μm; bar in d = 63.5 μm.