Amphiregulin-producing regulatory T cells guide alveolar regeneration during influenza infection

The hematopoietic system has long been charactered for its essential function in protecting against pathogens, but it is increasingly established that immune cells play integral roles in resolving inflammation and driving tissue repair. While many cell types are recruited to the site of injury and participate in coordinated immune responses, regulatory T (Treg) cells have emerged as key players of tissue protection by limiting damage and promoting regeneration in multiple organ systems. A conserved feature of “pro-repair” Treg cells is their expression of amphiregulin (Areg), an epidermal growth factor (EGFR) ligand associated with many formative processes in organismal development, tissue regeneration, and cancer. Many hematopoietic and non-hematopoietic cells produce Areg, yet Treg–specific expression has been found to be uniquely important and non-redundant in a number of damage models such as ischemic stroke, muscle injury, and influenza infection. In the lung, re-establishing epithelial barrier integrity is essential for recovery after acute viral injury. Rapid activation of renewal pathways preserves respiratory function during active inflammation and prevents against secondary infections and sequela. It has been previously reported that during influenza virus infection, Treg cell-production of Areg supports host resilience and thwarts severe alveolar damage. Animals that genetically lack Areg from Treg sources suffer a sharp loss of blood oxygenation and worse pathology. Although this growth factor signaling heavily influences disease outcome, the mechanisms by which Areg signals and how Treg cells engage with parenchymal and stromal cells within the alveolar niche are poorly understood. Given that Treg cells constitute only a small fraction of Areg-producing cells in the lung, we hypothesized that spatially restricted signaling and local tissue interactions enable this minority population to exert a major impact on organ function. Here, I used a multidisciplinary approach to interrogate the ability of lung Treg cells to promote alveolar lung repair during H1N1 influenza infection in a murine system. Through high-resolution immunofluorescence imaging, I characterize the unique distribution of Treg cells within lung tissues and their rapid recruitment to sites of active viral replication. Treg cells co-localize with a distinct population of Collagen-14+ EGFR+ mesenchymal cells (Col14+) that are Areg–responsive and robustly promote alveolar epithelial cell development. In the absence of Treg–derived Areg, Col14+ cells exhibit aberrant transcriptional programming, reduced expression of important alveolar growth factors, Fgf7 and Fgf10, and a dramatic increase in apoptotic cell death that together results in impaired alveolar epithelial progenitor cell differentiation. Following genetic ablation of stromal Egfr expression, mice experience a stark decline in blood oxygen saturation and dysplastic alveolar repair similar to loss of Areg from Treg cells, providing evidence that Areg from Treg cells instead signals through Col14+ cell intermediates. These findings underscore that localized delivery of distinct growth factors within tissue stem niches profound impacts whole organ physiology and regeneration. Lastly, I developed a novel Areg reporter mouse strain to better understand Areg producing cells in vivo. Through multiplexed, gene expression and TCR single-cell RNA sequencing, I identified the distinct factors and TCR repertoire that distinguishes “pro-repair” Treg cells in both influenza and bleomycin-induced lung injury. This system can be used as a platform for investigating the unique mechanisms by which reparative Treg cells and other Areg-producing immune cells migrate within tissues and deliver context-specific signals that orchestrate regenerative programming