Detection of donor-derived Langerhans cells in MHC class II immunodeficient patients after allogeneic bone marrow transplantation

T-cell dependent immune response is initiated by dendritic cells, which are the only leucocytes able to prime naive CD4-positive T cells. Langerhans cells (LC) are dendritic cells characterized by their localization within the epidermis, their dendritic shape, and their expression of specific markers such as major histocompatibility complex (MHC) class II molecules, CD1a and S100 protein. We retrospectively studied the phenotype of LC in the skin of eight children with MHC class II deficiency (bare lymphocyte syndrome) after allogeneic bone marrow transplantation (BMT). The presence of donor-derived MHC class II positive LC within the epidermis was studied by immunohistochemistry on skin biopsies performed for the determination of graft-versus-host disease. MHC class II positive LC were undetectable in the epidermis of a child who did not engraft and of three children 13–18 d after HLA-mismatched BMT, despite engraftment. However, donor-derived MHC class II positive LC were detected in four children 9–43 d after HLA-identical BMT. Our results demonstrate that LC can differentiate or expand very quickly, as early as within 9 d after BMT.