Restoration of hematopoiesis in athymic mice depends on CD4⁺ but not CD8⁺ T-cell activity. BALB/c^nu/nu mice received 10⁷ mature CD4⁺ (CD4 T, light gray bars), CD8⁺ (CD8 T, dark gray bars) T cells, or phosphate-buffered saline (PBS) (filled bars) intravenously. Control is represented by open bars. After 8 to 12 weeks, hematopoiesis was assessed. (A, B) Differential cell counts in reconstituted nude mice (A) peripheral blood and (B) bone marrow. (C) Flow cytometry analysis. SCA-1 × CD11b (top) and CD8 × CD4 (bottom) staining of control, CD4⁺, and CD8⁺ T-cell–reconstituted nude mice bone marrow. Gated populations show SCA-1⁺CD11b⁺ or CD3⁺CD4⁺ cells, with the indicated mean ± SD. (D) Relationship between bone marrow CD4⁺ T-cell compartment and restoration of normal segmented granulocyte counts in the PB (top) or SCA-1⁺CD11b⁺ population in the BM (bottom). Each symbol represents one mouse. Data shown are representative of 3 independent experiments. (E) Differential cell counts in TAP^–/– (▪) peripheral blood (left) and bone marrow (right). Control (WT) is represented by □. Error bars mean SEM. (F) Distribution of SCA-1 populations in TAP^–/– bone marrow (n = 5). *P < .001; **P < .01.