ABSTRACT
Transplant rejection involves natural immune cells and acquired immune cells. For decades, acquired immune cells have been dominating the study of transplant immunity. Researchers have found the surprising new features of innate immune cells, including immune memory, which may be of great significance to further improve graft survival. The short-term survival rate of grafts is very good, but the long-term graft outcomes are less so and most transplants are eventually lost to chronic rejection in the clinic. In animal models and clinical studies, innate immune cells, especially macrophages and natural killer cells, often predominate the chronic rejection process which lead grafts lost. Recent studies suggest that innate immune cells are capable of acquiring adaptive features in that they either directly recognize the allografts or become "trained" in the allogeneic milieu to further acquire features of memory and donor specificity. In selected transplant models, targeting the adaptive features of innate immune cells has been shown to promote long-term graft survival. Clearly, these findings highlight new therapeutic opportunities in further improvement of transplant outcomes as well as in treatment of cancers and autoimmune diseases in the clinic. The authors summarize the literature reports, introduce the recent acquired response characteristics of natural immune cells, and stimulate researchers to carry out more exploration in this field by fully discussing the heterogeneity and plasticity of natural immune cell types and the outstanding problems in related field.