Background.

The role of bone marrow-derived" passenger" leukocytes in the outcome of solid organ transplantation remains controversial. This study tested the relationship between high levels of donor-derived leukocytes within the transplanted organ and clinical outcome after lung transplantation.

Methods.

Sequential bronchoalveolar lavage samples were obtained from human lung allograft recipients. Leukocytes of donor origin in the bronchoalveolar lavage fluid were detected using two-color immunofluorescence, and the results were correlated with multiple clinical parameters.

Results.

Mean donor leukocytes levels for the first 200 days after transplantation were higher in patients with a good transplantation outcome compared with those patients who lost their grafts due to acute rejection (AR) or developed bronchiolitis obliterans syndrome. The presence of low numbers of donor-derived leukocytes for the first 200 days after transplantation was found to be a significant risk factor for graft loss due to either acute or chronic rejection (P= 0.032). Nearly all patients (85%) experienced AR episodes. However, the time to onset of severe AR episodes was significantly longer (P= 0.049), and the incidence of these episodes reduced, in patients who maintained high numbers of donor-derived leukocytes for the first 200 days after transplantation.

Conclusions.

The presence of high numbers of donor-derived leukocytes, particularly macrophages, in the transplanted lung in the first 200 days after transplantation was associated with stable graft function. Donor-derived leukocytes were reduced or absent in patients with a poor transplantation outcome. These findings rule out a negative influence of persisting donor leukocytes and are consistent with the emerging two-way models of transplant tolerance.