LAG-3 is required CD8 T-cell extrinsically for achievement of peripheral T-cell tolerance and is expressed at low levels on recipient CD4 T cells and B cells. (A) K^bD^b MHC class I–deficient animals were used as recipients of 3 Gy TBI/anti-CD40L/allo-BMT with B10.A BMCs. Adoptive transfer of 6-9 × 10⁶ purified recipient-type CD8 T cells was performed by intravenous injection at the time of allo-BMT. Dashed gray lines represent LAG-3 KO CD8 T cells, while solid black lines represent WT CD8 T cells. The percentage of donor chimerism in the B-cell lineage of the peripheral blood is shown. The dotted line at 5% donor chimerism represents our cutoff for calling an animal a chimera. N = 6-8 mice per group. (B) Expression of surface LAG-3 was detected using flow cytometry to examine gated populations of CD4⁺ T cells (left) and IgM⁺ B cells (right). N = 3-4 mice per group.