Identification of a Rejection Signature and Distinct Inflammatory Alveolar Macrophage Populations Post–lung Transplantation

Background. The long-term outcome of lung transplantation remains inferior to that of other solid organ transplants, primarily due to rejection/infection. This pilot study investigates longitudinal changes in the bronchoalveolar lavage (BAL) leukocyte transcriptomes and phenotypes, and their impact on clinical outcomes. Methods. BAL samples were collected at baseline and 1–12 mo posttransplantation. Leukocyte phenotypes, activation status, and biomarkers were characterized alongside their gene expression profiles. Results. We identified distinct BAL leukocyte transcriptional signatures associated with allograft rejection across 2 independent cohorts. Alveolar macrophages (AMs) predominated after transplantation, whereas granulocytes increased during the first year. This was associated with decreased expression of CD163, an anti-inflammatory marker, and increased expression of proinflammatory markers CD80 and CD86 on AMs. In a patient subgroup, a unique foamy macrophage (FM) subset with distinct cytology, featuring cytoplasmic lipid-laden vacuoles, was identified and confirmed in lung biopsies. FMs were accompanied by a unique non-FM (no-FM) population and were associated with elevated BAL levels of interleukin (IL)-8, IL-1β, and IL-10 and more frequent hospital readmissions. BAL transcriptomic analysis for patients carrying both FMs and no-FMs showed upregulation of genes linked to lipid metabolism, leukocyte chemotaxis, and inflammatory response pathways. Conclusions. We identified a rejection gene signature and proinflammatory shift in BAL leukocyte phenotypes after lung transplantation. The presence of FMs/no-FMs was associated with proinflammatory changes and worse clinical outcomes as per our data analysis, unadjusted for confounders due to relatively limited sample size. These findings could facilitate further investigations into early detection of lung allograft rejection and help focus on AM-targeted interventions.