Significant expression of Foxp3 in murine extrathymic CD4+CD8+ double positive T cells.

Forkhead box protein 3 (Foxp3) is a specific marker and the key factor in the development and function of regulatory T (Treg) cells. The present study investigates Foxp3 expression in murine head and neck lymph node (HNLN) and peripheral blood (PB) CD4+CD8+ double positive (DP) T cells and compares it with that in CD4+ or CD8+ single positive (SP) T cells. Here we provide evidence that murine extrathymic DP cells express Foxp3. We determined that the mean percentage of Foxp3-expressing cells within HNLN and PB DP cells was, respectively, 22.4 and 16.2. The obtained results clearly indicate that DP cells are very similar to CD4+ SP cells in respect of Foxp3 expression, and thus considerably deviate from CD8+ SP cells. It was found that Foxp3 expression in DP cells is positively correleted with CD25 expression. These results suggest that murine extrathymic Foxp3+ DP T cells may represent a unique regulatory T cell subset.