Introduction: The liver is an immune-rich organ, comprised of a diverse repertoire of leucocytes. In the steady state multiple subpopulations of myeloid cells reside within the liver including macrophages, monocytes, dendritic cells, eosinophils and neutrophils. During transplantation, these myeloid populations are transferred from the donor to the recipient. However, the fate and response of donor and recipient myeloid cell populations following transplantation have not been described in detail.
Materials and Methods: Orthotopic liver transplants were performed between MHC matched (congenic) and mismatched (allogeneic) mice, with differential expression of leucocyte marker CD45 to track donor (CD45.1) and recipient (CD45.2) cells. At days 1, 7 and 28 post-transplantation, mice were euthanized and their livers, spleens, lymph nodes, bone marrow and blood harvested. Organs were processed and stained with myeloid cell markers for flow cytometry analysis.
Results: Within one day of congenic liver transplantation, more than 65% of donor myeloid cells present at baseline were depleted. By day 28, of all myeloid cells in the graft, only CD11b^- dendritic cells and patrolling monocytes expressed donor marker CD45.1. In contrast, over 95% of donor myeloid cells were depleted within one day of allogeneic liver transplantation, and donor cells were undetectable by day 7. Recipient myeloid cells rapidly infiltrated both congenic and allogeneic grafts. At day 1 post-transplantation the recipient myeloid response to the congenic transplants was three-fold greater and more phenotypically diverse than that of the allogeneic transplants. This recipient response to congenic transplantation was maintained until day 7, with greater than 80% of recipient macrophages, neutrophils and eosinophils within the graft expressing proliferative marker Ki67. By day 28 the proportions and overall number of myeloid cell populations in the congenic graft had decreased to reflect those present at baseline. In contrast, by day 7 in allogeneic grafts the number of myeloid cells was 100 times that of a baseline liver, and all cells were recipient in origin. This infiltrate was made up predominately of macrophage populations, which were phenotypically distinct from those present in the congenic grafts at the same time.
Discussion: Significant differences in the phenotypes and kinetics of myeloid cell response between congenic and allogeneic grafts were identified. Donor myeloid cells were retained and established chimerism in the congenic but not allogeneic model. In each model, a phenotypically distinct subpopulation of recipient macrophages was the predominate infiltrating myeloid cell subset.
Conclusions: Distinct myeloid cell responses to MHC matched and mismatched transplantation may be significant to graft function and rejection responses.