Oral tolerance to nickel requires CD4⁺ invariant NKT cells for the infectious spread of tolerance and the induction of specific regulatory T cells

Previously, oral administration of nickel to C57BL/6 wild-type (WT) mice was shown to render both their splenic T cells and APCs (i.e., T cell-depleted spleen cells) capable of transferring nickel tolerance to naive syngeneic recipients. Moreover, sequential adoptive transfer experiments revealed that on transfer of tolerogenic APCs and immunization, the naive T cells of the recipients differentiated into regulatory T (Treg) cells. Here, we demonstrate that after oral nickel treatment Jα18^-/- mice, which lack invariant NKT (iNKT) cells, were not tolerized and failed to generate Treg cells. However, transfer of APCs from those Jα18^-/- mice did tolerize WT recipients. Hence, during oral nickel administration, tolerogenic APCs are generated that require iNKT cell help for the induction of Treg cells. To obtain this help, the tolerogenic APCs must address the iNKT cells in a CD1-restricted manner. When Jα18^-/- mice were used as recipients of cells from orally tolerized WT donors, the WT Treg cells transferred the tolerance, whereas WT APCs failed to do so, although they proved tolerogenic on transfer to WT recipients. However, Jα18^-/- recipients did become susceptible to the tolerogenicity of transferred WT APCs when they were reconstituted with IL-4- and IL-10-producing CD4⁺ iNKT cells. We conclude that CD4⁺ iNKT cells are required for the induction of oral nickel tolerance and, in particular, for the infectious spread of tolerance from APCs to T cells. Once induced, these Treg cells, however, can act independently of iNKT cells.