Frequency of CD4⁺ T cells undergoing spontaneous proliferation in lymphopenic C57BL/6 mice. Twenty-five to 1000 CD4⁺ T cells from pooled superficial cervical, axillary, brachial, inguinal, and mesenteric LNs of CD45.1 C57BL/6 mice were injected into CD45.2 C57BL/6 CD3ϵ^−/− mice. One month after transfer, the presence of CD45.1⁺CD4⁺CD8⁻ TCR⁺ cells in the periphery (pooled LN and spleen cells) of recipient mice was individually tested. (A) Absolute numbers of recovered CD45.1⁺CD4⁺CD8⁻ TCR⁺ cells are shown as a function of the number of injected CD4⁺ T cells. Each point represents an individual mouse, and horizontal lines represent the mean number of CD45.1⁺CD4⁺CD8⁻ TCR⁺ cells recovered from positive mice. (B) Numbers of injected CD4⁺ T cells are plotted against the log frequency of negative mice (ie, injected mice in which CD4⁺ T cells were not detected 1 month after transfer). The frequency of C57BL/6 autoreactive CD4⁺ T cells was estimated at 1/214 (95% confidence limits, 1/163 to 1/281). (C) TCR/forward light scatter (FSC), Vβ8/FSC, and Vβ4/FSC dot plots for gated CD45.1⁺CD4⁺CD8⁻ T cells are shown for 1 control C57BL/6 mouse and 3 representative recipient mice injected with 100 CD4⁺ T cells.