<jats:title>Abstract</jats:title> <jats:p>NTregs exhibit higher homeostatic proliferation (HP) than Tconv which may provide a competitive advantage during Treg adoptive immunotherapy. Increased Treg HP could be 2° to a self-reactive TCR repertoire and/or Foxp3-mediated increase in survival/fitness. To address this, CFSE-labeled CD4 populations from Foxp3-reporter mice, were compared 10d after transfer into wt mice. We generated Foxp3+ iTregs by activating Tconv and adding TGFβ, or transducing with Foxp3-retrovirus (RV). iTregs did not increase HP over Tconv controls (no TGFβ; RV-vector). Thus, Foxp3 itself is not sufficient to augment HP. To determine the role of Foxp3, we examined GFP+ nTreg from Filig mice (F-nTreg; Foxp3 ~10% wt levels due to unstable mRNA). F-nTreg exhibited reduced HP (10%) vs. wt nTregs (50%). Foxp3-RV transduction increased HP of F-nTregs, while RV-control had no effect (40 vs. 12%). Thus, Foxp3 is necessary for high HP by nTreg. We also examined role of Foxp3 during thymic ontogeny, irradiated Thy1.1 hosts received Filig (Thy1.2) bone marrow that was transduced with Foxp3-RV vs. RV-control. Surprisingly, in this setting, Tconv cells (no endogenous Foxp3) constitutively expressing Foxp3, exhibited increased HP vs. Foxp3- RV-controls (32 vs. 12%). Thus, Foxp3 increases HP in nTreg and when expressed during thymic ontogeny but not in mature Tconv. Our findings provide new insight into the role of Foxp3 expression in Treg homeostasis and highlight additional distinctions between iTreg and nTreg.</jats:p>