Mononuclear phagocyte populations in the transplanted human lung

Background. Dendritic cells (DC) are essential for the development of alloreactivity, however, little has been published regarding the distribution and phenotype of these and related mononuclear cells in human lung transplantation. Methods. Lung frozen sections were examined for the presence of CD1a(+) DC and for mononuclear cells and alveolar macrophages expressing CD11b and CD68. The effects of transplantation and immunosuppression were assessed by comparison of normal transplant transbronchial biopsy specimens to specimens from unused donor lungs; the normal transbronchial biopsy specimens also were compared with those showing rejection or obliterative bronchiolitis. Results. All biopsy specimens, including those with obliterative bronchiolitis, showed a marked depletion of CD1a(+) DC in lung allografts. This has not been described previously. In addition, transplantation and immunosuppression reduced alveolar macrophage coexpression of CD68 and CD11b, and this was reversed in acute rejection. Conclusion. The roles of pulmonary DC and other mononuclear phagocyte subpopulations need to be further defined, and data from animal models of lung transplantation should be interpreted with caution.