Background: Obliterative bronchiolitis (OB), a major cause of morbidity and mortality following lung transplantation, is characterized by progressive fibrosis of distal airways. We hypothesized that OB results from airway stem cell depletion and destruction of their niche. We focused on the fate of K5+p63+ basal stem cells in allograft airways and submucosal gland (SMG) structure[mdash]a key cartilaginous airway stem cell niche.

Methods: Ferret left lung transplantation was performed as recently described by our laboratory as an OB model (n=5). Human OB specimens were from autopsies or surgically removed allografts (n=5). Airway histology was assessed at various times post-transplantation. Using MetaMorph Software, SMGs in cartilaginous airways were quantified in human and ferret tissue, both native lungs and allografts. Basal cells positive for markers of multipotent (K5,p63) and unipotent (K14) precursor cells were also quantified.

Results: Ferret allografts revealed an intense inflammatory infiltrate with gland atrophy and a post-transplant decrease in SMGs per cartilaginous airway (g/a) over time: native lung controls=8g/a, early OB=6g/a, late OB=2g/a (p<0.0001). There was a progressive decline in the number of acini per gland with increasing severity of OB (p=0.0035). Evidence of SMG destruction was observed in late OB human allografts. Immunofluorescence analysis of basal stem cells in early OB ferrets revealed a progressive decline in the number of multipotent, K5+/p63+ basal cells by 97.8% (p=0.004) of total basal cells. There was a concomitant increase in the number of K14+K5-p63-, unipotent cells by 97.8% (p=0.029). Defects were present in distal and proximal airways. Analysis of lungs from human late OB samples revealed similar changes in basal cell phenotypes. Notably, SMG destruction and basal cell phenotype changes appear to precede OB in the distal airways.

Conclusions: SMGs and surface airway K5+p63+ basal cells are accepted stem cell compartments in proximal and distal airways. Our findings suggest that these compartments undergo progressive immune destruction and exhaustion in the transplanted lung. We hypothesize that distal airways are first to exhaust their stem cells and develop fibrosis in OB allografts as they have fewer K5+p63+ basal cells and lack SMGs that house a facultative stem cell niche.

CITATION INFORMATION: Swatek A, Crooke A, Anderson P, Lynch T, Ivanovic M, Klesney-Tait J, Eberlein M, Pena T, Engelhardt J, Parekh K. Obliterative Bronchiolitis Following Lung Transplantation Is Preceded by Depletion of Airway Submucosal Glands and Basal Stem Cells. Am J Transplant. 2016;16 (suppl 3).

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