Repair of the Lung by Regulatory T Cells

Acute lung injury (ALI) is a significant cause or morbidity and mortality worldwide. After injury, the lungs undergo a process of repair via the regeneration of lung epithelial cells. This regeneration process is imperative to restore adequate levels of lung function, and failure to repair can result in the acceleration toward acute respiratory distress syndrome. Understanding the cell types that interact with each other and influence lung-tissue repair and function is imperative to the design of future therapeutics for lung diseases. A decade ago, a critical role of regulatory T cells (Tregs) in the resolution phase from ALI was demonstrated using the LPS model (1). Rag1 (recombination activating 1) mice that lack B and T cells showed similar ALI, but impaired recovery compared with control animals. Adoptive transfer of Tregs to Rag1 mice rescued recovery via promotion of neutrophil clearance from the lung. Furthermore, antibody depletion of Tregs in wild-type mice delayed lung repair similarly to T-cell knockout. Increased lung injury in Treg-depleted mice was shown to be effector T cell–dependent, and increased proliferation of inflammatory T cells was demonstrated (2). Several studies have shown that Tregs are elevated in human blood and BAL from patients with ALI (1, 3, 4). ALI survivors have greater numbers of Tregs than those who succumb (3, 4). Human ALI Tregs have been shown to be capable of inhibiting the proinflammatory cytokine IFN-g. In addition, Tregs from ALI survivors express elevated TIM-3 (T-cell immunoglobulin and mucin–domain containing–3), an immune checkpoint inhibitor, compared with samples from fatal cases. These data implicate Tregs as playing a critical prosurvival and antiinflammatory role in ALI. In this issue of the Journal, Mock and colleagues (pp. 464–477) describe use of the LPS ALI model to examine cross-talk between Tregs and alveolar epithelial cells in their article entitled “Impact of Regulatory T Cells on Type II Alveolar Epithelial-Cell Transcriptomes during Resolution of Acute Lung Injury and Contributions of IFN-g” (5). Previous studies by this group have shown a role for Tregs in promoting tissue repair. Treg depletion resulted in decreased alveolar epithelial cell proliferation, and in vitro coculture showed that Tregs directly promote alveolar type II (AT2) proliferation (6). Furthermore, Tregs were shown to produce keratinocyte growth factor and amphiregulin, two epithelial proliferative cytokines, in an ALI model (7). Therefore, the main focus of the current article was to elucidate the effects that Tregs have on AT2 cells at the repair stage of lung injury. In particular, the authors were interested in studying in vivo changes in AT2 transcriptional profiles in the absence or presence of Tregs using Foxp3 and Foxp3 mice during both the injury and repair stages of inflammation. The authors hypothesized that AT2 transcription is altered by Tregs during the resolution of ALI. AT2 cells were sorted from uninjured mice and mice with or without Tregs that had been given LPS to induce ALI. Transcriptomic analysis of AT2 cells from injured versus uninjured mice identified 49 unique genes in the Treg-depleted group. Hierarchical clustering showed that groups clustered together, and most changed genes were upregulated when Tregs were absent. Gene-set enrichment analysis after resolution with or without Tregs identified eight sets that were upregulated, five of which described IFN-response gene pathways. The authors concluded that Treg depletion has a substantial effect on AT2 cells, mainly focused on altered IFN responses. The authors then sought to identify any changes in signaling molecules in BAL during LPS-induced injury in the presence or absence of Tregs. BAL was sampled at Days 1, 3, and 7 after LPS challenge. It was determined that on Day 3 at peak infection, IFNg concentrations were increased in the absence of Tregs. Using flow cytometry, the authors determined that CD4 and CD8 T cells had significantly increased levels of IFNg. It was concluded that without Tregs, IFNg responses are intensified in AT2 cells, which was supported by the fact that 42 of the 49 genes they determined to be unique in their Treg-depleted group were regulated by type I or type II IFNs. Lastly, the authors sought to determine how IFNg may alter Treg function during ALI. To do this, the authors used an IFNgneutralizing antibody in the absence or presence of Tregs and observed changes in immune-cell subsets. IFNg neutralization resulted in faster recovery and less weight loss independently of Tregs. This finding was consistent with a report that showed that IFNg neutralization in an LPS and natural killer T–cell ALI model resulted in increased Treg accumulation in the lung and elevated IL-10 production resulting in decreased lung injury (8). When Tregs were absent and IFNg was neutralized, there were more B lymphocytes and epithelial cells when compared with control animals. The increase in inflammatory macrophages that was observed when Tregs were depleted was lost after IFNg neutralization, suggesting that these changes in cell populations are mediated by both IFNg and Tregs. Neutralization of IFNg also resulted in less activated CD4 T cells in the presence of Tregs. Regulation of Tregs by IFNg has been suggested in cancer models in which IFNg promoted Treg fragility (9). Lastly it was concluded that the reduction in Tregs led to a decrease in proliferation of both endothelial and epithelial cells, consistent with other recent findings (10). The study findings can be summarized in Figure 1. Although both Treg depletion and IFNg neutralization altered repair after ALI, there were distinct effects of each, suggesting additional pathway involvement. The direct link between the two was not fully demonstrated, and the effect of IFNg on Tregs or AT2 cells in this context remains to be investigated. The mechanism by which Tregs and/or IL-10 alter AT2 IFN responses also remains unclear.