IntroductionPrevious studies have reported that impaired cytotoxic activity of natural killer (NK) cells in peripheral blood is associated with multiple sclerosis (MS) activity. Furthermore, NK cell phenotype could be associated with new lesions on MRI. Cytotoxic activity of NK cells is determined by their phenotype and surface antigens. However, investigators are yet to comment on NK cell phenotype and cytotoxicity in MS patients with stable disease or recent disease activity compared to healthy controls (HC). This project investigates NK cell phenotype and cytotoxicity in MS patients with active and stable disease and in HC.MethodsSeven patients with relapsing remitting MS who have been stable on alemtuzumab for at least 6 months, five patients with active MS not on any medication and five HCs were recruited in this study. Peripheral blood mononuclear cells were isolated by centrifugation over Ficoll-Paque density gradient medium. Then NK cells were isolated using immune-magnetic negative selection. Isolated NK cells were labelled with antibodies to determine CD56Dim and CD56Bright NK cells and cytotoxic function determined using target cells (K562) and flow cytometry.ResultsOur study showed that there is no significant difference between phenotype and cytotoxicity in three groups of stable RRMS, active RRMS and HC.ConclusionIn previous studies, it has been suggested that CD56Bright NK cells are associated with stable disease and patients with large MRI lesions had reduced NK cell cytotoxicity. This finding raised the possibility of using NK cell as an indicator for disease activity. This study identified no significant difference between NK cell cytotoxicity or phenotypes between HC and MS patients with different disease activity. Given the small number of patients in this study, there remains a need for further studies on larger population to assess phenotype, cytotoxicity, cytokines and cell surface expression of NK cells.