Phenotypic alterations of dendritic cells are involved in suppressive activity of trichosanthin-induced CD8⁺CD28^- regulatory T cells

The nature and differentiation of regulatory CD8⁺CD28 ^- T cells are poorly understood. In this study, we demonstrate that native Ag trichosanthin (Tk), a highly purified linear peptide isolated from a Chinese medicinal herb, is able to induce strong suppression of OVA-specific lymphoproliferation at low concentrations via activation of IL-4/IL-10-secreting CD8⁺CD28^- regulatory T cells (Tregs). To elucidate the underlying mechanisms, we firstly identified two types of mouse inbred strains, high susceptible (HS) and low susceptible, for the Tk-related suppression. They are H-2^d (or H-2^b) and H-2^k, respectively. The suppression is evoked only if bone marrow-derived dendritic cells (BDCs) instead of purified T cells are treated with Tk in an OVA-specific T-BDC interaction. Moreover, a special pattern of cytokine/transcription factors (IL-4 ⁺IL-10⁺IFN-γ^-Gata3⁺T-bet ^-) during suppressed OVA-specific T cell proliferation was observed in HS C57BL/6 but not in low-susceptible C3H/He mice. Consistently, the percentage of CD8⁺ CD28^- Tregs preferentially expanded from 5.5 to 26.1% in the presence of Tk, an occurrence that was also detected only in HS C57BL/6 mice. These expanded Tregs were able to induce a strong inhibition of one-way MLCs, which indicated that the Tk-induced hyporeaction and the activation of CD8⁺CD28^- Tregs might be under the influence of different genetic backgrounds. Additionally, obvious alterations of phenotypic parameters of BDCs after Tk stimulation were also identified, including enhanced production of IL-10, decreased secretion of IL-12, and detection of Jagged1, a Notch ligand on BDCs. Collectively, our data suggest that the changed APC-related factors are essential, at least in part, for the activation and differentiation of Tk-induced CD8⁺CD28^- Tregs.