Natural killer cells, also known as NK cells, are a type of cytotoxiclymphocyte critical to the innate immune system. They are a kind of large granular lymphocytes[1][2] (LGL), and belong to the rapidly expanding family of known innate lymphoid cells (ILC) and represent 5–20% of all circulating lymphocytes in humans.[3] The role of NK cells is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. NK cells provide rapid responses to virus-infected cells, stressed cells, tumor cells, and other intracellular pathogens based on signals from several activating and inhibitory receptors. Most immune cells detect the antigen presented on major histocompatibility complex I (MHC-I) on infected cell surfaces, but NK cells can recognize and kill stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction. They were named "natural killers" because of the notion that they do not require activation to kill cells that are missing "self" markers of MHC class I.[4] This role is especially important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells.
NK cells can be identified by the presence of CD56 and the absence of CD3 (CD56+, CD3−).[5] NK cells differentiate from CD127+ common innate lymphoid progenitor,[6] which is downstream of the common lymphoid progenitor from which B and T lymphocytes are also derived.[6][7] NK cells are known to differentiate and mature in the bone marrow, lymph nodes, spleen, tonsils, and thymus, where they then enter into the circulation.[8] NK cells differ from natural killer T cells (NKTs) phenotypically, by origin and by respective effector functions; often, NKT cell activity promotes NK cell activity by secreting interferon gamma. In contrast to NKT cells, NK cells do not express T-cell antigen receptors (TCR) or pan T marker CD3 or surface immunoglobulins (Ig) B cell receptors, but they usually express the surface markers CD16 (FcγRIII) and CD57 in humans, NK1.1 or NK1.2 in C57BL/6mice. The NKp46 cell surface marker constitutes, at the moment, another NK cell marker of preference being expressed in both humans, several strains of mice (including BALB/c mice) and in three common monkey species.[9][10]
Outside of innate immunity, both activating and inhibitory NK cell receptors play important functional roles in self tolerance and the sustaining of NK cell activity. NK cells also play a role in the adaptive immune response:[11] numerous experiments have demonstrated their ability to readily adjust to the immediate environment and formulate antigen-specific immunological memory, fundamental for responding to secondary infections with the same antigen.[12] The role of NK cells in both the innate and adaptive immune responses is becoming increasingly important in research using NK cell activity as a potential cancer therapy and HIV therapy.[13][14]
^Iannello A, Debbeche O, Samarani S, Ahmad A (July 2008). "Antiviral NK cell responses in HIV infection: I. NK cell receptor genes as determinants of HIV resistance and progression to AIDS". Journal of Leukocyte Biology. 84 (1): 1–26. CiteSeerX10.1.1.619.9639. doi:10.1189/jlb.0907650. PMID18388298. S2CID26975415.
^Arina A, Murillo O, Dubrot J, Azpilikueta A, Alfaro C, Pérez-Gracia JL, et al. (May 2007). "Cellular liaisons of natural killer lymphocytes in immunology and immunotherapy of cancer". Expert Opinion on Biological Therapy. 7 (5): 599–615. doi:10.1517/14712598.7.5.599. PMID17477799. S2CID43003664.