Antigen specificity acquisition of CD4+ regulatory T cells via acquired pMHC I complexes

Antigen‐specific CD4 + regulatory T (Tr) cells have important roles in preventing autoimmune diseases, but they also suppress effective anti‐tumor immunity. However, the molecular mechanisms regulating their acquisition of antigen specificity, particularly with respect to CD8 + T cells, are as yet unclear. In this study, we generated CD4 + Tr cells by stimulating naïve T cells derived from ovalbumin (OVA)‐specific T cell receptor transgenic OT II mice with OVA‐pulsed dendritic cells (DC OVA ) that had been transduced with an IL‐10‐expressing adenoviral vector (DC OVA/IL‐10 ). These DC OVA/IL‐10 secreting IL‐10 induced CD4 + Tr cells that secreted IFN‐γ ‐and IL‐10, but not IL‐4. We showed that these CD4 + Tr cells acquired OVA peptide/major histocompatibility complex class I (pMHC I) following either in vitro or in vivo stimulation with DC OVA/IL‐10 . We also demonstrated that IL‐10 secretion by these pMHC I‐carrying CD4 + Tr cells was critical to their suppression of OVA‐specific CD8 + T cell responses and antitumor immunity. However, the pMHC I complexes acquired by the CD4 + Tr cells also importantly affected for their activity, enhancing their abilities to suppress OVA‐specific anti‐tumor cytotoxic T lymphocyte responses in vivo by ≈ 700% relative to analogous CD4 + Tr cells incapable of expressing pMHC I complexes (i.e., K b−/− Tr). The above conclusion has also been confirmed by the evidence that the non‐specific CD4 + 25 + Tr cells become antigen specific and contribute enhanced suppressive effect via acquired pMHC I by uptake of DC OVA ‐released exosomes carrying pMHC I. Taken together, our data suggest that CD4 + Tr cells that have acquired pMHCI complexes much more efficiently target antigen‐specific CD8 + T cells in vivo , and that this may be an important means of augmenting the antigen‐specificity of CD4 + Tr cells in general.