Thrombotic thrombocytopenic purpura (TTP) is a rare life-threaten vascular autoimmune disease. There is no effective method to treat it in clinical trials since pathogenesis of TTP has not been fully elucidated. Here we investigate the role of NK cells in relapse development of TTP. Our results showed that the frequencies of CD3−CD56dimCD16− and CD3−CD56briCD16− NK cells are increased in TTP patients with a history of relapse. Expression of CD107a, granzyme A and IFN-γ by CD3−CD56dim NK cells following in vitro stimulation with PMA/ionomycin / monensin is improved in the relapse group, compared with those on NK cells derived from TTP patients without relapse development. NK cells isolated from TTP patients with a history of relapse indicated stronger cytotoxicity to target K562 cells than those of NK cells derived from TTP patients without relapse development, suggesting prior activation of NK cells in vivo. Treatment with anti-human CD16 antibody up-regulates cytotoxicity of NK cells derived from TTP patients without relapse development. However, Anti-human CD16 antibody treatment does not affect cytotoxicity of NK cells isolated from TTP patients with a history of relapse, suggesting inability of CD16-mediated signaling in NK cells derived from TTP patients with relapse development. These data provide evidence of altered NK cell activation and/or licensing in TTP patients with a history of relapse modulated by CD16-mediated signaling and a new avenue of investigation into mechanisms of TTP immunopathogenesis.