Differentiation of CD4⁺ T cells. In the thymus, a subpopulation of CD4⁺ cells become positive for the transcription factor Foxp3 and develop into regulatory T (Treg) cells.⁶  CD4⁺Foxp3⁻ cells leave the thymus and give rise to Th0 cells. Depending on the type of antigen-dependent signals, Th0 cells develop into Th1, Th2, follicular T helper (Tfh), or Th17 cells.⁵