Impact of Regulatory T Cells on Type 2 Alveolar Epithelial Cell Transcriptomes during Resolution of Acute Lung Injury and Contributions of IFN-γ.

By enhancing tissue repair and modulating immune responses, Foxp3+ regulatory T cells (Tregs) play essential roles in resolution from lung injury. The current study investigated the effects that Tregs exert directly or indirectly on the transcriptional profiles of type 2 alveolar epithelial (AT2) cells during resolution in an experimental model of acute lung injury. Purified AT2 cells were isolated from uninjured mice or mice recovering from LPS-induced lung injury, either in the presence of Tregs or in Treg-depleted mice, and transcriptome profiling identified differentially expressed genes. Depletion of Tregs resulted in altered expression of 49 genes within AT2 cells during resolution, suggesting that Tregs present in this microenvironment influence AT2-cell function. Biological processes from Gene Ontology enriched in the absence of Tregs included those describing responses to IFN. Neutralizing IFN-γ in Treg-depleted mice reversed the effect of Treg depletion on inflammatory macrophages and B cells by preventing the increase in inflammatory macrophages and the decrease in B cells. Our results provide insight into the effects of Tregs on AT2 cells. Tregs directly or indirectly impact many AT2-cell functions, including IFN type I and II-mediated signaling pathways. Inhibition of IFN-γ expression and/or function may be one mechanism through which Tregs accelerate resolution after acute lung injury.

Effects of IFN-γ on immune cell kinetics during the resolution of acute lung injury.

The immunologic responses that occur early in the acute respiratory distress syndrome (ARDS) elicit immune-mediated damage. The mechanisms underlying the resolution of ARDS, particularly the role of signaling molecules in regulating immune cell kinetics, remain important questions. Th1-mediated responses can contribute to the pathogenesis of acute lung injury (ALI). Interferon-gamma (IFN-γ) orchestrates early inflammatory events, enhancing immune-mediated damage. The current study investigated IFN-γ during resolution in several experimental models of ALI. The absence of IFN-γ resulted in altered kinetics of lymphocyte and macrophage responses, suggesting that IFN-γ present in this microenvironment is influential in ALI resolution. Genetic deficiency of IFN-γ or administering neutralizing IFN-γ antibodies accelerated the pace of resolution. Neutralizing IFN-γ decreased the numbers of interstitial and inflammatory macrophages and increased alveolar macrophage numbers during resolution. Our results underline the complexity of lung injury resolution and provide insight into the effects through which altered IFN-γ concentrations affect immune cell kinetics and the rate of resolution. These findings suggest that therapies that spatially or temporally control IFN-γ signaling may promote ALI resolution. Identifying and elucidating the mechanisms critical to ALI resolution will allow the development of therapeutic approaches to minimize collateral tissue damage without adversely altering the response to injury.

Foxp3+ Regulatory T Cell Expression of Keratinocyte Growth Factor Enhances Lung Epithelial Proliferation.

Repair of the lung epithelium after injury is a critical component for resolution; however, the processes necessary to drive epithelial resolution are not clearly defined. Published data demonstrate that Foxp3+ regulatory T cells (Tregs) enhance alveolar epithelial proliferation after injury, and Tregs in vitro directly promote type II alveolar epithelial cell (AT2) proliferation, in part by a contact-independent mechanism. Therefore, we sought to determine the contribution of Treg-specific expression of a growth factor that is known to be important in lung repair, keratinocyte growth factor (kgf). The data demonstrate that Tregs express kgf and that Treg-specific expression of kgf regulates alveolar epithelial proliferation during the resolution phase of acute lung injury and in a model of regenerative alveologenesis in vivo. In vitro experiments demonstrate that AT2 cells cocultured with Tregs lacking kgf have decreased rates of proliferation compared with AT2 cells cocultured with wild-type Tregs. Moreover, Tregs isolated from lung tissue and grown in culture express higher levels of two growth factors that are important for lung repair (kgf and amphiregulin) compared with Tregs isolated from splenic tissue. Lastly, Tregs isolated from human lung tissue can be stimulated ex vivo to induce kgf expression. This study reveals mechanisms by which Tregs direct tissue-reparative effects during resolution after acute lung injury, further supporting the emerging role of Tregs in tissue repair.