The regulatory role of PPAR-α and -γ on allergic lung inflammation and eosinophilia was investigated in vivo using a classical animal model for human asthma. This model, in which both CD4+
T lymphocytes play an important role (43
), is characterized by increased airway hyperreactivity, eosinophilia, and high IgE concentrations.
Use of PPAR-α–deficient mice allowed us to unequivocally demonstrate that the absence of PPAR-α led to increased lung eosinophilia and AHR, which were accompanied by increased antigen-specific IgE concentrations compared with WT animals. This increased functional, cellular, and humoral response was correlated to increased lung expression of GATA-3, associated with Th2 polarization (39
), and to increased levels of the proinflammatory cytokine IL-6 as well as IL-13 and eotaxin, two major factors promoting AHR, eosinophilia, and IgE production (1
, a natural PPAR-α ligand (5
), is abundantly produced, among other cell types, by mast cells in inflammatory reactions such as asthma (46
) and is highly chemotactic for eosinophils that express a specific membrane LTB4
receptor: BLTR (35
). Because PPAR-α activation by LTB4
leads to an anti-inflammatory response, the absence of PPAR-α expression (in eosinophils and other cell types from PPAR-α–deficient mice) would exacerbate the chemotactic response by creating an imbalance between stimulatory and inhibitory effects.
PPAR-α has been recently shown to negatively regulate the Th1-inducing transcription factor T-bet (47
). It might also affect directly or indirectly GATA-3, the T-bet Th2 counterpart, which is not only expressed in Th2 cells (39
) but also in lung eosinophils (40
). Thus, GATA-3 decreased expression could be due to an effect on both cell types. Although an inhibitory effect of PPAR-α has been reported for IL-6 production by vascular tissues (28
), this represents the first paper demonstrating a regulation by PPAR-α of eotaxin, an eosinophil chemotactic factor, and IL-13, an IgE-inducing factor for B cells. IL-13 is produced mainly by T cells, which express PPAR-α (13
), and by eosinophils themselves (48
). This production is regulated by both IL-5, another cytokine preferentially produced by Th2 cells, and by eotaxin (49
). Besides B cells, IL-13 affects airway epithelial cells, where it is able to induce eotaxin production (50
) and to regulate AHR and mucus overproduction (51
), smooth muscle cells, and fibroblasts, which can also be triggered to produce eotaxin, IL-6, and VCAM-1 (52
). Additionally, PPAR-α might regulate IgE production through a direct action on B cells, in which it is expressed (13
). Finally, eotaxin is produced by endothelial cells (53
), in which PPAR-α is expressed (54
). Hence, PPAR-α agonists, at least in this latter cell type, might be able to modulate its expression.
In the same model of antigen sensitization and airway challenge, we demonstrated that delivery of a PPAR-γ agonist to the lung, through nebulization, profoundly decreased AHR, IgE (and IgG1) production, and eosinophilia, with a concomitant decrease of IL-4, IL-5, IL-6, IL-13, and GATA-3 expression. According to the parameter measured, this inhibitory effect was totally or partially abrogated by coadministration of a GW9662 an irreversible PPAR-γ antagonist, thus demonstrating that most ciglitazone-mediated effects indeed occur through PPAR-γ stimulation in this model.
PPAR-γ appears to regulate IL-4 and IL-5, which promote AHR in conjunction with IL-13 (1
), but not eotaxin production. PPAR-γ activation might also influence lung infiltration by inflammatory cells and, among them, eosinophils, which are responsive to IL-5. Furthermore, in addition to T cell function, PPAR-γ is expressed in airway epithelial, submucosal and smooth muscle cells and regulates their function (26
). Other likely targets for PPAR-γ are dendritic cells, whose activity is modulated by PPAR-γ (19
), and which play a key role in the induction of the pulmonary response to antigen inhalation (58
). Inhibition of dendritic cell migration by PPAR-γ agonists, as reported previously for prostaglandin D2
), a precursor of 15d-PGJ2
(a natural PPAR-γ ligand), would contribute to decreased eosinophilia as well as to decreased T cell proliferation, which is also directly inhibited by PPAR-γ (24
). Finally, it has been shown in an arthritis model, that PPAR-γ hemizygous mice displayed increased B cell and T cell proliferation and IgM production (24
). A regulation by PPAR-γ agonists of IgE germline transcript synthesis in vitro by a human B cell line was also reported (61
). In our in vivo model, PPAR-γ agonists also regulate IgE and IgG1
production in a secondary immune response.
The further demonstration that both PPAR-α and -γ are expressed by mouse, human, and rat eosinophils and the ability for specific PPAR-α and -γ agonists to regulate two of their physiological functions in vitro strongly support the hypothesis that their inhibitory effects on lung inflammation observed in vivo in an asthma model might, in part, be due to a direct action on eosinophils. Indeed, PPAR-α and -γ agonists induced a dose-dependent inhibition of IL-5 or eotaxin-mediated eosinophil chemotaxis, whereas the effects of the PPAR-γ agonist were fully reversed by cotreatment in vitro with a specific antagonist. Eosinophil chemotaxis requires signaling through IL-5R (βc
chain) or CCR3, the G protein-coupled seven transmembrane domain receptor for eotaxin, and leads to MAP kinase (ERK, p38, and JNK) as well as c-jun and c-fos activation (62
).These factors are known targets for PPAR-α and -γ regulation (9
). In a similar way, ADCC reactions require activation of some Fc receptors, which vary according to the immunoglobulin isotype and the cell type. In eosinophil-mediated ADCC toward S. mansoni
larvae, IgE and IgA are mainly involved. Fc
RI and FcαRI/CD89, their cognate receptors, share a common subunit, FcRγ, whose complex signal transduction cascade leads to MAP kinase, c-jun, c-fos (64
), and NF-κB activation (67
), which are potential PPAR targets (9
). It is perhaps not surprising that inhibition of ADCC by PPAR agonists is weaker than for chemotaxis. Indeed, cytotoxicity is a more complex biological process not only involving chemotaxis but also cell–cell contacts and granule release, each of which would only be partially inhibited by agonists.
Together, our results demonstrate that both PPAR-α and -γ directly affect eosinophil functions in vitro and are able to regulate eosinophilia in vivo, in a murine model of asthma, where complex interactions occur. Due to recently reported failure of monotherapies targeted to eosinophils, such as anti–IL-5, in the treatment of eosinophilia, asthma, and AHR (68
), these nuclear receptors, which are expressed in many immune (including T cells) and nonimmune cell types in the inflammatory airways, might thus represent attractive alternative therapeutic targets for these pathologies.