The skin contains a substantial number of Foxp3-expressing regulatory T cells (Tregs) even in a steady state in both mice and human. Previous studies collectively suggest that Tregs activated in response to skin-related antigens in the skin-draining lymph nodes (dLNs) accumulate and reside in the skin, thereby preventing the spontaneous development of skin inflammation. However, it remains unclear whether and how Tregs distributed in the steady-state skin regulate immune responses to percutaneous sensitization with foreign antigens.
To address this issue, we established an experimental procedure for selectively eliminating cutaneous Tregs. Intradermal injection with low-dose diphtheria toxin (DT) into the ear skin of Foxp3DTR mice, in which Foxp3+ Tregs specifically express the diphtheria toxin receptor, significantly reduced Tregs only in the treated skin. Sensitization with hapten through Treg-deficient skin resulted in increased lymph node migration and enhanced T-cell stimulatory capacity of antigen-bearing dendritic cells (DCs) as well as enhanced development of IFN-gamma-producing CD8+ T cells in the dLNs. Cutaneous Treg depletion at the sensitization site also enhanced immune responses to NP-OVA including differentiation of follicular helper T cells and NP-specific germinal center B cells and plasma cells. We found that about half of Treg cells expressed Il10 in the skin under homeostatic condition while few Foxp3- conventional T cells did. Thus, next we investigated the role for IL-10 in cutaneous Treg-mediated suppression by generating mixed bone marrow chimeras in which only skin-resident Tregs are deficient in IL-10. Those chimeras showed increased DC migration into the dLN after sensitization with hapten, indicating that IL-10 derived from cutaneous Tregs was essential for suppression of DC maturation.
In conclusion, Tregs-residing in the skin are essential for preventing excessive immune responses to percutaneous sensitization by suppressing dermal DC migration and maturation via IL-10.