NK cell trafficking in health and autoimmunity:a comprehensive review.

Natural killer (NK) cells represent an important subpopulation of lymphocytes that are distributed throughout the body. The development of NK cells primarily occurs in the bone marrow during adult life, involving several putative intermediate stages that finally result in functional maturation. At steady state, NK cell egress from the bone marrow to various peripheral areas is controlled by a network of adhesion molecules, including integrins, selectins, and chemokine receptors and their corresponding ligands. NK cells at different developmental stages express distinct repertoire of adhesion molecules and can therefore be recruited to different sites of the body, including lymphoid and non-lymphoid tissues, and NK cells undergo further differentiation driven by local microenvironmental signals, resulting in unique tissue-specific NK cells. Through their abilities of cytotoxicity and cytokine production, NK cells not only play key roles in the innate immune system, but also participate in shaping adaptive immune responses. On the basis of their heterogeneity in phenotype, function, and tissue distribution, NK cells can be further subdivided into distinct subsets. Under pathological conditions, such as in autoimmune, inflammatory, and infectious diseases, as local microenvironment changes, NK cell subsets would redistribute between tissues and organs and rapidly accumulate at the local pathological sites to exert their effector functions. Here, we describe the development and tissue distribution of NK cell subsets in mice and humans. We focus on the trafficking of NK cell subsets within the bone marrow and emigration into periphery at steady state, and molecular mechanisms involved in their trafficking in autoimmune diseases.