There is a paucity of information known about how the lung recovers from acute respiratory distress syndrome (ARDS) and pneumonia, and this knowledge gap has contributed to the continued high morbidity and mortality of these diseases. A subset of immune cells, Foxp3+ regulatory T lymphocytes (Tregs), is thought to be important in resolution in several experimental models of acute lung injury (ALI). Furthermore, Tregs are present in the lung of patients with ARDS, suggesting that they contribute to ARDS recovery. Tregs can arise from either thymic or extra-thymic origins. Extra-thymic, peripherally-induced Tregs (iTregs) are converted from nave T cells and serve distinct and essential functions in controlling adaptive immunity to restrain immune cell infiltrates in the bronchial and bronchiolar walls. Furthermore, our published studies show that during the resolving phase of lung injury, Tregs expand in number and change their gene expression profiles compared to Tregs in uninjured control lungs. Treg transcriptome profiling identifies several genes that shine a light on novel functions of Tregs during ALI resolution. One transcript that is markedly upregulated 23-fold in Tregs isolated from resolving lung tissue is matrix metalloproteinase 12 (Mmp12). Importantly, mice lacking endogenous Tregs and repleted with Mmp12-/- Tregs by adoptive transfer have elevated inflammatory cells and less epithelial proliferation during resolution than mice repleted with Mmp12+/+ Tregs. The Treg transcriptome also suggests that downregulated expression of Treg transcripts Kdm6b and Sik1, both of which act to regulate gene transcription, may function to regulate Treg homing, retention, stability, and survival.
The Aims seek to test the central hypothesis that Tregs are critical to resolution and that by optimizing Treg responses, ALI severity can be reduced, and resolution hastened.
Aim 1 investigates the role of iTregs during resolution of ALI resolution. The hypothesis is that iTregs are required for optimal resolution of inflammation and lung repair after ALI induced by LPS, S. pneumoniae, and influenza A. The direct effects of Tregs and iTregs will be determined using in vitro co-cultures of Tregs and AT2 cells.
Aim 2 determines the impact of Treg-expressed MMP12 during the resolution of ALI. The hypothesis is that Treg expression of MMP12 exerts multiple roles in orchestrating and facilitating the resolution of ALI. Substrates of Treg MMP12 will be identified in BAL fluid and lung tissue.
Aim 3 determines if the differentially regulated transcripts, Kdm6b and Sik1, regulate Treg-promoted resolution of ALI. We will test the hypothesis that Tregs deficient in Kdm6b have decreased Foxp3 expression levels and delayed, inadequate resolution and that Tregs deficient in Sik1 have improved quality of resolution. We anticipate that the proposed studies will determine the roles iTregs play in ALI resolution and identify mechanisms by which Tregs mediate resolution and repair. Identifying and elucidating these mechanisms will allow the development of therapeutic approaches to minimize collateral tissue damage without adversely altering the beneficial response to injury.
Acute Respiratory Distress Syndrome (ARDS) is a common pulmonary disease with high morbidity and mortality, and specific therapies to treat ARDS are lacking. Regulatory T Cells (Tregs) promote lung resolution after acute injury; however, the mechanisms involved in Treg-directed repair are not well characterized. We propose to identify the mechanisms through which Tregs promote reparative processes, which may uncover new targets to help accelerate lung repair and recovery for ARDS patients.