Structured Abstract
INTRODUCTION
Fibroblasts are central regulators of organ health and disease, although it remains unclear how different fibroblast subsets are controlled within tissues. Recently, both murine and human fibroblast atlases have revealed the presence of a conserved fibroblast subset that is present in many organs and disease states, marked by Pi16+Dpp4+Ly6c+ gene expression, and computationally predicted to have progenitor capacity. Notably, these fibroblasts are also reported to express high Il33, encoding the alarmin IL-33, which can stimulate Group 2 innate lymphoid cells (ILC2s). ILC2s are tissue-resident immune cells that can promote fibroblast activation and are also emerging as regulators of epithelial stem cell niches.
RATIONALE
The exocrine pancreas contains endogenous fibroblast populations and although fibroblasts play key roles in pancreatic inflammation or neoplasia, little is known about their spatial organization or ontogeny. Moreover, ILC2s can influence pancreatic endocrine function and pancreatic cancer. We asked where ILC2s localized in the pancreas in relation to different fibroblast subsets, and how their potential interactions influenced fibroblast biology during homeostasis, inflammation, and carcinogenesis.
RESULTS
We found that ILC2 colocalized with fibroblasts expressing the genes Pi16+Dpp4+Ly6c+ in an interstitial niche of the exocrine pancreas, which encapsulates the organ parenchyma. Activation of ILC2 through alarmin IL-33 resulted in the specific activation of Pi16+Dpp4+Ly6c+ fibroblasts, which had progenitor capacity and could differentiate into intraparenchymal Col15a1+ fibroblasts. In healthy mice, ILC2s were important for establishing baseline number of fibroblasts in the pancreas, which represented an inflammatory set point during acute pancreatitis. Moreover, the release of IL-33 during acute pancreatitis triggered the ILC2–Pi16+Dpp4+Ly6c+ fibroblast axis to repopulate and repair the organ after injury. We found that ILC2-derived IL-13 was important for stimulating Pi16+Dpp4+Ly6c+ fibroblast proliferation. However, ILC2-extrinsic negative feedback mechanisms also constrained fibroblast numbers during IL-33–driven inflammation. For instance, using fibroblast-targeted in vivo proximity labeling reagents, we found that Tim4+ tissue-resident macrophages may regulate Pi16+Dpp4+Ly6c+ fibroblast population dynamics through direct phagocytosis. Last, we found that the ILC2–Pi16+Dpp4+Ly6c+fibroblast niche expanded early during pancreatic carcinogenesis and locally enveloped pancreatic intraepithelial neoplastic (PanIN) lesions. Using lineage-trace mice, we found that Pi16+Dpp4+Ly6c+fibroblasts represent a potent precursor pool for specific cancer-associated fibroblast (CAF) subtypes. Moreover, ILC2 deficiency resulted in reduced CAF density in an orthotopic model of pancreatic ductal adenocarcinoma (PDAC).
CONCLUSION
We found that ILC2s are important regulators of fibroblast biology in the pancreas, and both colocalize and control Pi16+Dpp4+Ly6c+ fibroblasts, which represent an important source of Col15a1+ fibroblasts or specific CAF subtypes. Hence, tissue-resident ILC2s and other immune cells are important choreographers of local fibroblast topography.
ILC2 regulate fibroblast biology in the pancreas. ILC2s colocalize with Pi16+Dpp4+Ly6c+ fibroblasts in interstitial and capsular niches of the exocrine pancreas. ILC2-derived IL-13 stimulates Pi16+Dpp4+Ly6c+ fibroblasts, which can differentiate into intraparenchymal Eng+Col15a1+ fibroblasts; this ILC2–fibroblast axis sets tissue fibroblast densities at steady-state, reinforces fibroblast repopulation after acute injury, and influences CAF development and abundance in cancer.
