Multiple factors restrict the size of the peripheral CD4⁺T-cell compartment. (A-B) The peripheral CD4⁺ T-cell pool is increased in CD8⁺ T-cell-free mice. (A) Lymph nodes and spleen from 8-week-old male C57BL/6 β_2m^-/- and C57BL/6 mice were recovered and pooled, and single-cell suspensions were prepared. Absolute numbers of recovered CD4⁺ TCR^hicells were calculated. (B) C57BL/6 mice were injected intraperitoneally with 50 μg of purified anti-CD8 Ab (53-6.7) or with PBS every 2 days for 4 weeks. Absolute numbers of CD4⁺ TCR^hicells are shown at various times during the treatment period. (C) Continuous thymic output restricts the survival of peripheral CD4⁺ T cells in MHC class II molecule-expressing mice. Seven-week-old C57BL/6 CD45.2 mice were thymectomized or sham thymectomized. Thirty-five days after surgery, the mice were injected with 5 × 10⁶CFSE-labeled lymph node CD4⁺ T cells from C57BL/6 CD45.1 mice. CFSE fluorescence histograms and absolute numbers of recovered CD45.1 CD4⁺ TCR^hicells are shown 28 days after transfer. (A-C) Data represent mean ± SD.