Prostaglandin I₂-IP signaling promotes Th1 differentiation in a mouse model of contact hypersensitivity

PGI₂, which exerts its actions via its specific Gs-coupled I prostanoid receptor (IP), is known to be present in the lymph nodes, but its roles in acquired cutaneous immune responses remain unclear. To investigate the role of PGI₂-IP signaling in cutaneous immune responses, we applied IP-deficient (Ptgir^-/-) mice to contact hypersensitivity as a model of acquired immune response and found that Ptgir^-/- mice exhibited a significantly decreased contact hypersensitivity response. Lymph node cells from sensitized Ptgir^-/- mice exhibited decreased IFN-γ production and a smaller T-bet⁺ subset compared with control mice. PGI synthase and IP expression were detected in dendritic cells and T cells, respectively, by quantitative real-time PCR analysis, suggesting that PGI₂ produced by dendritic cells acts on IP in T cells. In fact, in vitro Th1 differentiation was enhanced by an IP agonist, and this enhancement was nullified by protein kinase A inhibitor. These results suggest that PGI2-IP signaling promotes Th1 differentiation through a cAMP-protein kinase A pathway and thereby initiates acquired cutaneous immune responses.