Rate of recipient-derived alveolar macrophage development and major histocompatibility complex cross-decoration after lung transplantation in humans.

Alveolar macrophages (AM) play critical roles in lung tissue homeostasis, host defense, and modulating lung injury. The rate of AM turnover (donor AM replacement by circulating monocytes) after transplantation has been incompletely characterized. Furthermore, the anatomic pattern of recipient-derived lung macrophages repopulation has not been reported, nor has their ability to accumulate and present donor major histocompatibility complex (a process we refer to as MHC cross-decoration). We longitudinally characterized the myeloid content of bronchoalveolar lavage (BAL) and biopsy specimens of lung transplant recipients and found a biphasic rate in AM turnover in the allograft, with a rapid turnover perioperatively, accelerated by both the type of induction immunosuppression and the presence of primary graft dysfunction. We found that recipient myeloid cells with cell surface AM phenotype repopulated the lung in a disorganized pattern, comprised mainly of large clusters of cells. Finally, we show that recipient AM take up and present donor peptide-MHC complexes yet are not able to independently induce an in vitro alloreactive response by circulating recipient T cells.