The Nuclear Receptor PPARγ and Immunoregulation: PPARγ Mediates Inhibition of Helper T Cell Responses

Abstract

The peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors belonging to the nuclear receptor superfamily. Until recently, the genes regulated by PPARs were those believed to be predominantly associated with lipid metabolism. Recently, an immunomodulatory role for PPARγ has been described in cells critical to the innate immune system, the monocyte/macrophage. In addition, evidence for an antiinflammatory role of the PPARγ ligand, 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2) has been found. In the present studies, we demonstrate, for the first time, that murine helper T cell clones and freshly isolated splenocytes express PPARγ 1. The PPARγ expressed is of functional significance in that two ligands for PPARγ, 15d-PGJ2 and a thiazolidinedione, ciglitazone, mediate significant inhibition of proliferative responses of both the T cell clones and the freshly isolated splenocytes. This inhibition is mediated directly at the level of the T cell and not at the level of the macrophage/APC. Finally, we demonstrate that the two ligands for PPARγ mediate inhibition of IL-2 secretion by the T cell clones while not inhibiting IL-2-induced proliferation of such clones. The demonstration of the expression and function of PPARγ in T cells reveals a new level of immunoregulatory control for PPARs and significantly increases the role and importance of PPARγ in immunoregulation.