CD4 and graft versus host disease: Findings of Fujii and coworkers indicated that the numbers of CD4+ and CD8+ T cells were decreased, the differentiation of naive T cells to an effector phenotype was suppressed, and the pathologic damage of GVHD-targeted organ was alleviated in MSC-exosome-treated GVHD mice while the normal fibroblasts-derived exosome treatment did not ameliorate the pathological manifestations [27], which mean the unique immunoregulatory function of MSC-exosomes.