Mobilization of Circulating Progenitor Epithelial Cells with Keratinocyte Growth Factor Aids in Airway Repair.

Rationale: Circulating epithelial progenitor cells (CEPC) are important for repair of the airway epithelium after significant injury. Keratinocyte growth factor (KGF) is a fibroblast growth factor that is important for wound healing and tissue repair and ameliorates mucositis in human bone marrow transplant patients. The receptor for KGF (KGFR) is almost exclusively expressed on epithelial cells, whereas KGF is secreted by fibroblasts. We therefore hypothesized that KGF could mobilize CEPC and that this could be one mechanism for the role of KGF in epithelial repair.

Methods: Bone marrow and buffy coat from uninjured mice were examined by FACS analysis for KGFR, cytokeratin 5 (CK5) and CXCR4 expression. IHC for the KGFR was performed on tissue from syngeneic tracheal transplants to determine the time course of expression of the KGFR in repairing proximal airway epithelium. Dual immunofluorescence for the KGFR and CK5 was used to identify the cell types in the repairing proximal airway which express the KGFR. KGF (1g/kg) or vehicle control was administered to mice intraperitoneally. Bone marrow and buffy coat were obtained from the mice at time points 6 and 24 hours after injection and examined by FACS analysis for mobilized circulating progenitor epithelial cells (CK5+, CXCR4+). Recipient GFP mice were injected daily for three days with KGF or vehicle control and then received wild-type syngeneic tracheal transplants. The airway repair was scored and the number of CEPC in the repairing tracheal transplants was examined.

Results: The KGFR expressing cells in the circulation represented 2.9% of nucleated cells with 1.9±0.2% of these cells also expressing CD45. The percentage of CD45+, KGFR+ nucleated cells that also expressed CK5 was 74.4±7.4%. The percentage of CD45+, KGFR+ cells that also expressed the chemokine receptor CXCR4 was 16.6±2.6%. In the bone marrow, 1.2% of nucleated cells expressed the KGFR and 1.5±0.4% of the nucleated cells co-expressed CD45 and the KGFR. Of these CD45+, KGFR+ bone marrow cells, 22±3.4% also expressed CK5. Of the CD45+, CK5+ cells, 12±5.5% also expressed CXCR4. The KGFR was found on the submucosal gland duct and basal cells in the repairing proximal airway. Administration of KGF resulted in a statistically significant increase in CK5+CXCR4+ cells in the circulation at the 6 hour time point, but not at 24 hours post KGF. No significant increase in CK5+CXCR4+ cells was seen in the bone marrow at these time points. An epithelial repair score demonstrated a significant improvement in repair of the proximal airway after tracheal transplantation in the recipient mice that received KGF (p=0.001). The number of GFP+, CK5+ cells in the tracheal transplants was then compared and was found to be significantly increased (p=0.01) in the tracheal transplants from mice that received KGF.

Conclusions: The KGFR is present on cells in the bone marrow and circulation of mice and subpopulations of these cells also express CK5, CXCR4 and CD45. KGF mobilizes CK5+CXCR4+ progenitor epithelial cells to the circulation of mice. The KGFR is present on many cell types in the repairing proximal airway, including the progenitor basal cells. Administration of KGF improves proximal airway epithelial repair and increases the number of CEPC in the tracheal transplant model. This may be one mechanism whereby KGF improves airway repair.