Involvement of dectin-2 in ultraviolet radiation-induced tolerance.

Hapten sensitization through UV-exposed skin induces hapten-specific tolerance which can be adoptively transferred by injecting T cells into naive recipients. The exact phenotype of the regulatory T cells responsible for inhibiting the immune response and their mode of action remain largely unclear. Dectin-2 is a C-type lectin receptor expressed on APCs. It was postulated that dectin-2 interacts with its putative ligands on T cells and that the interaction may deliver costimulatory signals in naive T cells. Using a soluble fusion protein of dectin-2 (sDec2) which should inhibit this interaction, we studied the effect on contact hypersensitivity (CHS) and its modulation by UV radiation. Injection of sDec2 affected neither the induction nor the elicitation phase of CHS. In contrast, UV-induced inhibition of the CHS induction was prevented upon injection of sDec2. In addition, hapten-specific tolerance did not develop. Even more importantly, injection of sDec2 into tolerized mice rendered the recipients susceptible to the specific hapten, indicating that sDec2 can break established tolerance. FACS analysis of spleen and lymph node cells revealed a significantly increased portion of sDec2-binding T cells in UV-tolerized mice. Furthermore, transfer of UV-mediated suppression was lost upon depletion of the sDec2-positive T cells. Taken together, these data indicate that dectin-2 and its yet unidentified ligand may play a crucial role in the mediation of UV-induced immunosuppression. Moreover, sDec2-reactive T cells appear to represent the regulatory T cells responsible for mediating UV-induced tolerance.