Control of homeostatic proliferation correlates with the presence of Tregs in MBP/R⁺ and OVA/R⁺ mice. (A) Presence of naturally occurring Tregs in OVA/R⁺ and MBP/R⁺ mice but not in OVA/R^– and MBP/R^– mice. Lymph node cells from 4- to 7-week-old OVA/R^–, OVA/R⁺, MBP/R^–, and MBP/R⁺ mice were stained with anti-CD4 and anti-Foxp3, gated on live lymphocytes. Numbers indicate frequency among total live lymphocytes. Data are representative of 10 mice. (B) OVA/R^–, but not OVA/R⁺ mice, support homeostatic proliferation of MBP-specific T cells, whereas MBP/R^–, but not MBP/R⁺ mice, support homeostatic proliferation of OVA-specific T cells. Top row: 1 × 10⁷ CFSE-labeled splenocytes from H-2^d/u Thy1.1 MBP/R^– donor mice were transferred into H-2^d/u Thy1.2 OVA/R^– (left) or H-2^d/u Thy1.2 OVA/R⁺ (right) recipient mice; 12 days after transfer, recipients were sacrificed and single-cell suspensions were made from brachial, inguinal, and axillary lymph nodes. Plots show cells gated on Thy1.1⁺ anti–MBP TCR (3H12⁺) cells. Data are representative of 4 mice. Bottom row: 1 × 10⁷ CFSE-labeled splenocytes from H-2^d/u Thy1.1 OVA/R^– donor mice were transferred into H-2^d/u Thy1.2 MBP/R^– (left) or H-2^d/u Thy1.2 MBP/R⁺ (right) recipient mice; 12 days after transfer, recipients were sacrificed, and single-cell suspensions were made from brachial, inguinal, and axillary lymph nodes. Plots show cells gated on Thy1.1⁺KJ1.26⁺ cells. Data are representative of 4 mice.