Acute rejection, a vascular-based disorder, has been identified as the major risk factor for obliterative bronchiolitis (OB), an airway-based pathology. This study investigated the hypothesis that changes to the microvascular blood supply of small airways were associated with the development of OB, thus providing a possible link between an acute vascular insult (acute rejection) and chronic airway changes (OB).

Microvasculature of 695 small airways (99 patients) was assessed in post-mortem lung allograft specimens using monoclonal antibodies for von Willebrand factor and CD31. Group A consisted of 343 small airways from 58 patients with no evidence of OB. The remaining 41 patients had histological evidence of OB in some of their small airways and grouped as B, C, and D with some patients contributing to all three groups ie, their lung specimen had some small airways which were completely obliterated with OB, some airways which were partially obliterated and some small airways without any histological evidence of OB development. Thus group B consisted of 145 small airways (34 patients) without OB. Group C consisted of 171 small airways with partial luminal obstruction (36 patients). Group D consisted of 36 small airways (14 patients) with complete luminal obliteration.

Airway circumference (mean ± standard deviation) was 2.36 ± 0.37, 2.41 ± 0.51, 2.49 ± 0.51, and 2.57 ± 0.79 mm, respectively (p = 0.40). Mean number of blood vessels per unit length of airway circumference was 4.12 ± 1.1, 1.58 ± 0.61, 2.42 ± 1.06, and 4.42 ± 1.46 vessels/mm, respectively (p < 0.001). Blood vessels with circumference greater than 0.2 mm were present in 100%, 64%, 39%, and 7% of small airways, respectively (p < 0.001). Univariate and multivariate analyses (donor and recipient age, sex, and cytomegalovirus status, recipient pretransplant diagnosis, ischemic times, acute rejection and infective episodes, postoperative survival days, recipient group [A to D], blood vessels per unit length, and airway circumference) confirmed that reduction in blood vessels per unit length was associated with the development of OB and was time-independent.

Obliterative bronchiolitis was preceded by a decrease in microvascular supply to the small airways (group B). The subsequent onset of airway scarring (groups C and D) was associated with an increased number of significantly smaller vessels, suggestive of neovascularization.