Background. Dendritic cells (DC) are essential for the development of
alloreactivity, however, little has been published regarding the distr
ibution 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 macr
ophages expressing CD11b and CD68. The effects of transplantation and
immunosuppression were assessed by comparison of normal transplant tra
nsbronchial 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 s
pecimens, including those with obliterative bronchiolitis, showed a ma
rked depletion of CD1a(+) DC in lung allografts. This has not been des
cribed previously. In addition, transplantation and immunosuppression
reduced alveolar macrophage coexpression of CD68 and CD11b, and this w
as reversed in acute rejection. Conclusion. The roles of pulmonary DC
and other mononuclear phagocyte subpopulations need to be further defi
ned, and data from animal models of lung transplantation should be int
erpreted with caution.