Natural killer cell subsets in cerebrospinal fluid of patients with multiple sclerosis

Summary Changes in blood natural killer ( NK ) cells, important players of the immune innate system, have been described in multiple sclerosis ( MS ). We studied percentages and total cell counts of different effector and regulatory NK cells in cerebrospinal fluid ( CSF ) of MS patients and other neurological diseases to gain clearer knowledge of the role of these cells in neuroinflammation. NK cell subsets were assessed by flow cytometry in CSF of 85 consecutive MS patients (33 with active disease and 52 with stable MS ), 16 with other inflammatory diseases of the central nervous system ( IND ) and 17 with non‐inflammatory neurological diseases ( NIND ). MS patients showed a decrease in percentages of different CSF NK subpopulations compared to the NIND group. However, absolute cell counts showed a significant increase of all NK subsets in MS and IND patients, revealing that the decrease in percentages does not reflect a real reduction of these immune cells. Remarkably, MS patients showed a significant increase of regulatory/effector ( CD56 bright / CD56 dim ) NK ratio compared to IND and NIND groups. In addition, MS activity associated with an expansion of NK T cells. These data show that NK cell subsets do not increase uniformly in all inflammatory neurological disease and suggest strongly that regulatory CD56 bright and NK T cells may arise in CSF of MS patients as an attempt to counteract the CNS immune activation characteristic of the disease.