Currently, the bronchiolitis obliterans syndrome (BOS) of chronic airway rejection represents the most significant obstacle to the long-term function of isolated pulmonary allografts in humans. Between 20% and 30% of recipients are affected by this condition. To define the possible pathogenetic role of altered expression of class II major histocompatibility complex antigens (ie, HLA-DR) in BOS, the authors studied well-characterized examples of this process immunohistologically.
Eleven BOS specimens were compared with seven controls, represented by allografts with no pathologic abnormalities taken from patients with normal posttransplant respiratory function, as well as 14 biopsies showing acute rejection. In addition, immunophenotypic subtyping of lymphocytes in all specimens was undertaken.
Control tissues exhibited variable but weak expression of HLA-DR in bronchiolar epithelium and alveolar pneumocytes. In comparison, immunostaining for class II major histocompatibility complex antigens in BOS showed no statistically significant differences, whereas the 14 examples of acute rejection manifested intense HLA-DR expression in epithelia and endothelial cells. The numbers of intrabronchiolar and peribronchiolar lymphocytes were clearly higher in both acute rejection and BOS than in controls, but these cells differed in lineage in the two rejection states. Acute rejection showed an obvious preponderance of CD43-positive T lymphocytes, whereas lymphoid cells in BOS were a relatively equal mixture of CD20-positive B cells and CD43-positive T cells. Moreover, incipient peribronchiolar B-cell follicles were observed in BOS. Natural killer (CD57-positive) lymphocytes were rare in all specimens.
These data suggest that alterations in HLA-DR expression probably do not play a central role in the genesis of BOS, as they do in acute rejection. In contrast, the results of lymphocyte immunophenotyping and correlative histologic findings in BOS suggest that both T cells and B lymphocytes may be involved in the mechanism of chronic airway rejection.