In lung‑transplant patients who develop a lung‑scarring condition called bronchiolitis obliterans syndrome, the innate‑immune peptide LL‑37 and related neutrophil peptides are higher in the airways, even when no infection is detected, while another protective protein (SLPI) is lower.

Mechanisms other than classical alloimmunity are implicated in the pathogenesis of bronchiolitis obliterans syndrome (BOS). It was hypothesised that antimicrobial peptides (AMPs), elements of the innate immune response, have a role in BOS pathogenesis. Pulmonary expression of the neutrophil-derived AMPs human cathelicidin (hCAP)-18/LL-37 and alpha-defensins (human neutrophil peptides (HNP) 1-3), and the epithelial cell-derived AMPs human beta-defensin (hBD)-2, elafin and secretory leukoprotease inhibitor (SLPI) were measured in stable lung transplant recipients and those with BOS. The relationship between airway pathogens and AMP levels was examined. Bronchoalveolar lavage (BAL) was performed on 44 lung transplant recipients (30 stable, 14 with BOS). BAL was cultured for pathogens and ELISA for AMPs was performed. The presence of airway pathogens was associated with significantly increased levels of neutrophil-derived and epithelial-derived AMPs. When patients without pathogens in BAL fluid were analysed, eight recipients with BOS had elevated hCAP-18/LL-37 and HNP 1-3 compared with 25 stable recipients. hBD-2 and elafin levels were comparable in BOS and stable recipients, but SLPI levels were reduced in BOS. Bronchiolitis obliterans syndrome is associated with elevated airway human cathelicidin 18/LL-37 and human neutrophil peptides 1-3 from activated neutrophils, even in the absence of pathogens. Together with reduced airway secretory leukoprotease inhibitor this may favour nonalloimmune airway injury with reduced antiprotease defence and increased neutrophil degranulation.