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Natural killer (NK) cells are activated early during inflammatory events and contribute to the shaping of the ensuing adaptive immune response. To further understand the role for NK cells in inflammation, we investigated the phenotype and function of synovial fluid (SF) NK cells from patients with chronic joint inflammation, as well as from patients with transient inflammation of the knee following trauma. We confirm that synovial NK cells are similar to the well-characterized CD56(bright) peripheral blood (PB) NK-cell subset present in healthy individuals. However, compared to this PB subset the synovial NK cells express a higher degree of activation markers including CD69 and NKp44, the latter being up-regulated also on CD56(bright) NK cells in the PB of patients. Activated synovial NK cells produced interferon-gamma and tumour necrosis factor, and the production was further up-regulated by antibody masking of CD94/NKG2A, and down-regulated by target cells expressing human leucocyte antigen-E in complex with peptides known to engage CD94/NKG2A. We conclude that synovial NK cells have an activated phenotype and that CD94/NKG2A is a key regulator of synovial NK-cell cytokine synthesis.

Original publication




Journal article



Publication Date





291 - 301


Department of Microbiology, Tumour and Cell Biology, Karolinska Institutet, Stockholm, Sweden.


Synovial Fluid, Knee Joint, Killer Cells, Natural, Cells, Cultured, Humans, Osteoarthritis, Knee, Knee Injuries, Receptors, Immunologic, Histocompatibility Antigens Class I, HLA Antigens, Cytokines, Coculture Techniques, Immunophenotyping, Adult, Aged, Middle Aged, NK Cell Lectin-Like Receptor Subfamily D, Receptors, Natural Killer Cell, NK Cell Lectin-Like Receptor Subfamily C