ABSTRACT
The thymus is a vital primary lymphoid organ that provides unique microenvironments for the proliferation, differentiation, and maturation of T cells. With advancing age, however, the thymus gradually undergoes age-related involution and reduction in immune function, which are characterized by decreases in tissue size, cellularity, and naïve T cell output. This dynamic process leads to the reduced efficacy of the immune system with age and contributes to the increased susceptibility to infection, autoimmune disease, and cancer. In addition, bone marrow transplantation, radio-chemotherapy and virus infection also impair the thymus and give rise to the decline in immune function. Therefore, understanding the molecular mechanisms involved in age-related thymic involution and development of novel therapeutic strategies for thymic rejuvenation have gained considerable interests in recent years. This review emphasizes thymic microenvironments and thymocyte-stromal cell interactions and summarizes our current knowledge about thymic rejuvenation in terms of sex steroid, cytokines, growth factors, hormones, transcription factors, cell graft, and microRNAs. At the end of each discussion, we also highlight unanswered issues and describe possible future research directions.