Macrophage-mediated complications after stem cell transplantation

Complications after hematopoietic stem cell transplantation (HSCT) especially graft versus host disease (GVHD) are serious events and the keys to success of HSCT. Pathological diagnosis of complications is critical but sometimes not definite in GVHD diagnosis. In this review, we focus on the role of macrophages in HSCT complications. In the early period after HSCT, residual recipient macrophages have important roles for engraftment and rejection. Hemophagocytosis is the main feature of activated recipient macrophages. Skin lesions after HSCT are usually skin GVHD but mimicking histology of interface dermatitis is hardly differentiated. Macrophages and lymphocytes of these lesions were studied and increased numbers of macrophages were significantly associated with overall survival of HSCT. It could be suggested macrophages were good predictive markers for skin GVHD-like lesions. Intestinal type transplantation-associated microangiopathy (i-TAM) is an independent pathogenesis from GVHD with serious prognosis. Activated macrophages in these lesions are important and key to the pathogenesis of i-TAM. These activated macrophages are predominantly residual recipient tissue-resident macrophages. Cryptogenic organizing pneumonitis is a lung complication around day 100 after HSCT caused by activated alveolar macrophages. They are donor-derived tissue-resident macrophages. Residual recipient and donor-derived macrophages work in different ways in HSCT complications.