All about Natural Killer Cells (NKs)

Montreux, Switzerland | March 31, 2021 – NK cells were first identified in the 1960s and within 10 years researchers began to explore this previously unknown population of innate lymphocytes, now referred to as natural killer (NK) cells. The existence of natural cytotoxic lymphocytes, also known as natural killer (NK) cells, with intrinsic and innate cell killing properties was discovered in experiments with T cells. As their name suggests, NK cells are constitutively cytotoxic and, unlike cytotoxic T cells, do not require prior exposure to an antigen to activate their cell killing effects. NK cell activity was first observed in the mononuclear cell population of human peripheral blood; however, this cell type resides in several lymphoid and non-lymphoid tissues, including bone marrow, lymph nodes, skin, intestine, tonsils, liver, and lung. In terms of cytotoxicity, the fundamental difference between cytotoxic (CD8+) T cells and NK cells is that the CD8+ T cell requires activation by a helper (CD4+) T cell to express its cytotoxicity. NK cell receptors are synthesised during cell development and maturation. NK cells use innate receptors (such as immunoglobulin receptors) to grow, mature, and recognise their environment. NK cells are large granular lymphocytes (as opposed to “small lymphocytes”), they do not express the markers characteristics of T or B cells (CD3 and CD19), in humans NK cells are characterised by the markers CD56, CD16, and of course NK. These cells account for approximately 5-20% of human lymphocytes and belong to the innate immune system due to their invariant non-adaptive receptors. It was originally thought that NK cells develop exclusively in the bone marrow. However, recent evidence in humans and mice suggests that they can also develop and mature in secondary lymphoid tissues including the tonsils, spleen, and lymph nodes. The cell progenitors and intermediate populations that give rise to NK cells are defined by the differential expression of lineage-specific surface markers. Natural killer cells mediate their effects through two essential effector functions. NK cells are cytotoxic lymphocytes that can directly lyse cells that have undergone malignant transformation or that have been infected by a virus or other intracellular pathogen.