Natural killer cells: biology, physiology and medicine - part 2.

the molecular specificity of NK cells, including studies of their receptors and receptor signaling. Other reviews discussed NK cell interactions with other immune cells and their responses to infections. In Part 2 (this issue), we publish reviews related to the life span of NK cells, as well as their development, differentiation and senescence. Furthermore, a series of reviews on NK cell interactions with tumor cells, as well as reviews describing new insights into hematopoietic stem cell transplantation (HSCT) where NK cells may contribute to human tumor eradication, are published. Finally, articles that cover the role of NK cells in organ transplant rejection and inflammatory disease settings are included. In the first review, Sitnicka [11] nicely presents the progress made thus far in the dissection of the developmental stages of mouse NK cells, from HSC in bone marrow to lineage-committed NK cells. Linked to this topic, Gayoso et al. [12] discuss how aging in humans affects NK cells. They interestingly describe altered proportions of NK cell subsets in elderly people and discuss how these alterations may explain the functional changes of NK cells associated with aging [12] . As outlined above, one of the best-described functions of NK cells is the ability to kill virus-infected and transformed cells. Hence, evolutionary pressures on viruses over million of years as well as on tumor cells in the local host-tumor ‘microenvironment’ have led to mechanisms promoting escape from NK cells. In this context, Groth et al. [13] nicely review such immune escape mechanisms serving to overcome NK cell tumor and viral immunosurveillance. In the next As we described in an editorial in the last issue of the Journal of Innate Immunity (Vol. 3, No. 3, 2011), a number of groups in the beginning of the 1970s observed unexpected spontaneous cytotoxic activities among lymphocytes. This ‘annoying’ background phenomenon led to the identification of a new subpopulation of lymphocytes [1, 2] named ‘natural killer’ or briefly ‘NK’ cells by Kiessling et al. [2] . While initially engaging only a small community of scientist, the first insights into the molecular specificity of NK cells [3–5] , insights into their role in controlling virus and intracellular bacteria infections as well as certain tumors and insights into their ability to regulate other immune cells [6] have put NK cells in the front seat of modern immunology. New functions of the cells are now rapidly being described (e.g. their possible role in controlling T-cell and macrophage homeostasis [7] and the possibilities of using them in therapeutic settings of human cancer [8] ). As stated, the recent identification of adaptive immune cell features of NK cells is a ‘hot topic’ at the moment in the NK cell community [9] . Clearly, the study of NK cells now engages a large community of scientists working in diverse areas of NK cell research in experimental model systems as well as in humans [10] . In this issue of the Journal of Innate Immunity , as well as in the last issue of the journal, a series of updated review articles on NK cells are published. They summarize some of the most recent activities within this growing field of research. Several leading scientists have made seminal contributions to these two issues. In Part 1 (Vol. 3, No. 3, 2011), we published a series of reviews related to Published online: May 25, 2011 Journal of Innate Immunity