Long-term T-cell repopulation of T-lymphopenic rats by CD28 superagonist–driven expansion of a small T-cell inoculum. Lethally irradiated PVG (CD45^a) rats received syngeneic bone marrow and 5 million CD45^b T cells. (A) Recovery of transferred CD4⁺ T cells in rats after 2 injections (1 mg on days 1 and 10) of conventional or superagonistic CD28–specific mAbs. (B) Regeneration of CD4⁺ and CD8⁺ T cells in rats reconstituted with unseparated lymph node T cells and treated with superagonistic CD28–specific mAbs (•) or vehicle (PBS; ○) (1 mg on days 1 and 13). (C) Expression of TCR V segments by syngeneic (CD45^a) and transferred CD4 T cells (CD45^b) in spleens of rats 2 months after treatment with isotype control or CD28 superagonist. Means and SD are shown. *P < .05; **P < .01.