PD-L1 on mast cells suppresses effector CD8⁺ T-cell activation in the skin in murine contact hypersensitivity.

Abstract

The programmed cell death-1 (PD-1)/programmed death ligand 1 (PD-L1) pathway is known to inhibit the activation of effector CD8⁺ T cells. However, just how this regulatory pathway is involved in the pathophysiology of CD8⁺ T-cell-mediated inflammatory skin diseases remains unclear. Our aim was to elucidate the mechanisms by which the PD-1/PD-L1 pathway exerts its regulatory roles in CD8⁺ T-cell-mediated cutaneous immune responses. PD-L1-deficient (Pdl1^-/-) mice were used for the murine contact hypersensitivity model. Inflammatory responses such as IFN-γ production from CD8⁺ T cells in the skin was evaluated by flow cytometry. Compared with wild-type mice, Pdl1^-/- mice exhibited exacerbated ear swelling and increased numbers of IFN-γ⁺ CD8⁺ T cells in the skin. Adoptive T-cell transfer experiments revealed the involvement of the PD-1/PD-L1 pathway in the elicitation phase of contact hypersensitivity. Bone marrow chimera experiments showed that PD-L1 on radioresistant cells was responsible for this regulatory pathway. Flow cytometric analysis revealed that among the radioresistant cells in the skin, PD-L1 was most highly expressed on mast cells (MCs) before and after elicitation. Administration of anti-PD-L1 blocking antibody during the elicitation phase significantly enhanced ear swelling responses and increased the number of IFN-γ⁺CD8⁺ T cells in the skin of wild-type mice, whereas no significant effects were observed in MC-deficient (WBB6F1/J-Kit^W/Kit^W-v/J and C57BL/6-KitW^-sh/W^-sh) mice. The high level of expression of PD-L1 on human skin MCs was confirmed by database analysis and immunohistochemical analysis. PD-L1 on MCs negatively regulates CD8⁺ T-cell activation in the skin.