Natural killer (NK) cells are large granular lymphocytes that lyse a wide variety of transformed and virally-infected target cells without prior exposure to antigen, and without restriction by major histocompatability complex antigens. Although NK cells have been identified in a variety of mammalian species, how NK cells recognize antigen and trigger lysis is unknown. Recently, monoclonal antibodies made against NK-like cells from teleost fish were shown to react with NK cells from humans and rats, and to inhibit their cytolytic activity. The role of this apparently evolutionarily conserved function-associated molecule (FAM) has been further investigated utilizing a variety of domesticated farm animal species. It was observed that the anti-FAM mAb reacted specifically with peripheral blood lymphocytes isolated from sheep, horses and cattle. Further, the anti-FAM mAb inhibited NK cell lytic activity in each of these species. Finally, the anti-FAM mAb was found to inhibit conjugate formation between NK and target cells, implying that the FAM was involved in antigen recognition by NK cells in each of these species. In conclusion, it appears that NK cell function is mediated by an evolutionarily conserved FAM in a wide variety of species.
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