BACKGROUND: Generation of functional (CD4+)(CD8-)CD25+ regulatory T cells (Treg) 
in the murine thymus depends on FoxP3. Removal of the thymus from neonatal mice 
has been shown to result in a multiple organ autoimmune disease phenotype that 
can be prevented by introducing the FoxP3+ Treg population to the animal. It has 
therefore, been proposed that functional FoxP3+ Treg cells are not made in the 
neonatal thymus; however, it remains unclear when and where functional 
(FoxP3+)(CD4+)(CD8-)CD25+ thymocytes are generated in postnatal thymus.
RESULTS: We report that neither FoxP3 mRNA nor protein is expressed in 
(CD4+)(CD8-)CD25+, or (CD4+)(CD8-)CD25- thymocytes until 3-4 days post birth, 
despite the presence of mature (CD4+)(CD8-)CD25+/- thymocytes in the thymus by 
1-2 days after birth. (FoxP3-)(CD4+)(CD8-)CD25+ thymocytes from day 2 newborn 
mice show no Treg activity. Interestingly, we are able to detect low numbers of 
FoxP3+ thymocytes dispersed throughout the medullary region of the thymus as 
early as 3-4 days post birth. Expression of FoxP3 is induced in embryonic day 17 
fetal thymus organ culture (FTOC) after 4-6 days of in vitro culture. Treatment 
of FTOCs with thymic stromal derived lymphopoietin (TSLP) enhanced expression of 
FoxP3, and blocking the TSLP receptor reduces FoxP3 expression in FTOC. 
Furthermore, TSLP stimulates FoxP3 expression in purified (CD4+)CD8- thymocytes, 
but not in (CD4+)CD8+, (CD4-)CD8+ and (CD4-)CD8- thymocytes.
CONCLUSION: Expression of FoxP3 or Treg maturation is ontogenically distinct and 
kinetically delayed from the generation of (CD4+)(CD8-)CD25+ or 
(CD4+)(CD8-)CD25- thymocytes in the postnatal thymus. TSLP produced from 
medullary thymic epithelia cells (mTEC) contributes to the expression of FoxP3 
and the maturation of natural regulatory T cells. Overall, these results suggest 
that the development of Treg cells requires paracrine signaling during late 
stages of thymocyte maturation that is distinct from signaling during positive 
or negative selection.