Subcutaneous late phase responses are augmented during local inhalational tolerance in a murine asthma model

Differential Expression and Function of PDE8 and PDE4 in Effector T cells: Implications for PDE8 as a Drug Target in Inflammation

Abolishing the inhibitory signal of intracellular cAMP is a prerequisite for effector T (Teff) cell function. The regulation of cAMP within leukocytes critically depends on its degradation by cyclic nucleotide phosphodiesterases (PDEs). We have previously shown that PDE8A, a PDE isoform with 40–100-fold greater affinity for cAMP than PDE4, is selectively expressed in Teff vs. regulatory T (Treg) cells and controls CD4⁺ Teff cell adhesion and chemotaxis. Here, we determined PDE8A expression and function in CD4⁺ Teff cell populations in vivo. Using magnetic bead separation to purify leukocyte populations from the lung draining hilar lymph node (HLN) in a mouse model of ovalbumin-induced allergic airway disease (AAD), we found by Western immunoblot and quantitative (q)RT-PCR that PDE8A protein and gene expression are enhanced in the CD4⁺ T cell fraction over the course of the acute inflammatory disease and recede at the late tolerant non-inflammatory stage. To evaluate PDE8A as a potential drug target, we compared the selective and combined effects of the recently characterized highly potent PDE8-selective inhibitor PF-04957325 with the PDE4-selective inhibitor piclamilast (PICL). As previously shown, PF-04957325 suppresses T cell adhesion to endothelial cells. In contrast, we found that PICL alone increased firm T cell adhesion to endothelial cells by ~20% and significantly abrogated the inhibitory effect of PF-04957325 on T cell adhesion by over 50% when cells were co-exposed to PICL and PF-04957325. Despite its robust effect on T cell adhesion, PF-04957325 was over two orders of magnitude less efficient than PICL in suppressing polyclonal Teff cell proliferation, and showed no effect on cytokine gene expression in these cells. More importantly, PDE8 inhibition did not suppress proliferation and cytokine production of myelin-antigen reactive proinflammatory Teff cells in vivo and in vitro. Thus, targeting PDE8 through PF-04957325 selectively regulates Teff cell interactions with endothelial cells without marked immunosuppression of proliferation, while PDE4 inhibition has partially opposing effects. Collectively, our data identify PF-04957325 as a novel function-specific tool for the suppression of Teff cell adhesion and indicate that PDE4 and PDE8 play unique and non-redundant roles in the control of Teff cell functions.

Long-Term Exposure to House Dust Mite Leads to the Suppression of Allergic Airway Disease Despite Persistent Lung Inflammation

BACKGROUND

Allergic asthma is a major cause of worldwide morbidity and results from inadequate immune regulation in response to innocuous, environmental antigens. The need exists to understand the mechanisms that promote nonreactivity to human-relevant allergens such as house dust mite (HDM) in order to develop curative therapies for asthma. The aim of our study was to compare the effects of short-, intermediate- and long-term HDM administration in a murine asthma model and determine the ability of long-term HDM exposure to suppress allergic inflammation.

METHODS

C57BL/6 mice were intranasally instilled with HDM for short-term (2 weeks), intermediate-term (5 weeks) and long-term (11 weeks) periods to induce allergic airway disease (AAD). The severity of AAD was compared across all stages of the model via both immunological and pulmonary parameters.

RESULTS

Short- and intermediate-term HDM exposure stimulated the development of AAD that included eosinophilia in the bronchoalveolar lavage fluid (BALF), pronounced airway hyperreactivity (AHR) and evidence of lung inflammation. Long-term HDM exposure promoted the suppression of AAD, with a loss of BALF eosinophilia and AHR despite persistent mononuclear inflammation in the lungs. Suppression of AAD with long-term HDM exposure was associated with an increase in both Foxp3+ regulatory T cells and IL-10-positive alveolar macrophages at the site of inflammation.

CONCLUSIONS

This model recapitulates the key features of human asthma and may facilitate investigation into the mechanisms that promote immunological tolerance against clinically relevant aeroallergens.

Regulatory B cells in allergic airways disease and asthma

B cells have long been known to participate in both innate and adaptive immune responses by contributing to antigen presentation and by producing antigen-specific antibodies. Recent evidence shows that certain B-cell subsets can also inhibit T-cell immune responses. Like regulatory T cells (Treg), these regulatory B cells (Breg) appear to comprise several subpopulations. How Breg cells are generated and how they control immune responses in vivo are just beginning to be elucidated. Here, we provide detailed instructions for the identification, isolation, and functional characterization of Breg cells in a murine model of allergic airway disease.

Regulatory B cells from hilar lymph nodes of tolerant mice in a murine model of allergic airway disease are CD5+, express TGF-β, and co-localize with CD4+Foxp3+ T cells

In a biphasic, ovalbumin (OVA)-induced murine asthma model where allergic airway disease is followed by resolution and the development of local inhalational tolerance (LIT), transforming growth factor (TGF)-β-expressing CD5(+) B cells were selectively expanded locally in hilar lymph nodes (HLN) of LIT mice. LIT HLN CD5(+) B cells, but not LIT HLN CD5(-) B cells, induced expression of Foxp3 in CD4(+)CD25(-) T cells in vitro. These CD5(+) regulatory B cells (Breg) and CD4(+)Foxp3(+) T cells demonstrated similar increases in expression of chemokine receptors (CXCR4 and CXCR5) and co-localized in HLN B cell zones of LIT mice. The adoptive transfer of LIT HLN CD5(+) B cells, but not LIT HLN CD5(-) B cells, increased the number of CD4(+)Foxp3(+) T cells in the lung and inhibited airway eosinophilia in this OVA model. Thus, Breg in HLNs of LIT mice reside in a CD5(+) TGF-β-producing subpopulation and co-localize with CD4(+)Foxp3(+) T cells.