Clonal Bifurcation of Foxp3 Expression Visualized in Thymocytes and T Cells

Regulatory T cells (Tregs) are crucial for suppressing autoimmunity and inflammation mediated by conventional T cells. To be useful, some Tregs should have overlapping specificity with relevant self-reactive or pathogen-specific clones. Whether matching recognition between Tregs and non-Tregs might arise through stochastic or deterministic mechanisms has not been addressed. We tested the hypothesis that some Tregs that arise in the thymus or that are induced during Ag-driven expansion of conventional CD4(+) T cells might be clonally related to non-Tregs by virtue of asymmetric Foxp3 induction during cell division. We isolated mouse CD4(+) thymocytes dividing in vivo, wherein sibling cells exhibited discordant expression of Foxp3 and CD25. Under in vitro conditions that stimulate induced Tregs from conventional mouse CD4(+) T cells, we found a requirement for cell cycle progression to achieve Foxp3 induction. Moreover, a substantial fraction of sibling cell pairs arising from induced Treg stimulation also contained discordant expression of Foxp3. Division-linked yet asymmetric induction of Treg fate offers potential mechanisms to anticipate peripheral self-reactivity during thymic selection as well as produce precise, de novo counterregulation during CD4(+) T cell–mediated immune responses.