IntroductionNatural killer (NK) cells are present in the bloodstream, spleen, bone marrow, and in nonlymphoid organs and represent one of the main effectors of the immunosurveillance against tumors, by exerting 2 major effector functions, cytolysis of target cells and production of cytokines and chemokines. 1,2 The activity of NK cells depends on the interplay between inhibitory receptors for major histocompatibility complex (MHC) class I molecules and activating receptors, which operate in concert to induce the elimination of tumor cells. 3,4 Among the activating receptors particularly relevant for tumor cell recognition and killing is NKG2D, the receptor for the MHC I-related molecules MICA/B, and ULBPs (UL16-binding proteins), belonging to the C-type lectin-like receptor family. 3,[5][6][7] The NKG2D activating receptor is expressed not only on NK cells, but also on ␥␦ T cells, CD8 ϩ T cells, and a subset of CD4 ϩ T cells. The expression of NKG2D ligands is largely confined to virus-infected, tumor, and stressed cells. 7 To promote escape of tumors from NKG2D-mediated immunosurveillance, NKG2D ligands undergo proteolytic shedding. Soluble NKG2D ligands (NKG2DLs) have been shown to down-regulate the cell surface NKG2D expression on NK cells, resulting in impaired killing of tumor cells. 8,9 Another activating receptor involved in NK-cell-mediated tumor cell killing is DNAX accessory molecule-1 (DNAM-1), a transmembrane glycoprotein constitutively expressed on the majority of T cells, NK cells, and macrophages; its ligands are Nectin-2 (Nec-2, CD112) and the poliovirus receptor (PVR, CD155), which belong to the nectin/nectin-like family. 10-12 DNAM-1 ligands have been initially described as adhesion molecules mainly regulating trans-endothelial migration 13 and only recently they have been found on a variety of tumor cells. 12,14 Both DNAM-1 and NKG2D cooperate in the induction of NK-cell killing against tumor cells of different histotypes, including those of hematopoietic origin. 14,15 Similarly to the NKG2D ligands, soluble isoforms of PVR have also been found in human serum and in the culture supernatant of tumor cell lines, and their role in tumor immunoevasion has been considered. 12 It has recently been demonstrated that agents that produce a genotoxic stress or DNA-replication inhibitors up-regulate NKG2D ligand expression through the activation of ATM (ataxia telangiectasia mutated) and ATR (ATM-and Rad3-related) protein kinases on human fibroblasts and on mouse tumor cell lines, and enhance their destruction by NK cells. 16,17 Increased ligand expression is regulated by the activation of the DNA damage response (DDR). This is a cellular program devoted to the maintenance of genome integrity through the inhibition of cell cycle and activation of the DNA repair systems, or by the induction of apoptosis or a protracted cell-cycle arrest known as cellular senescence. 17,18 No information is so far available on the regulation of DNAM-1 ligand expression through the DDR pathway.Submitted August 11, 2008; acc...

We have, for the first time, ordered a pathway from agonist stimulation to integrin activation and established the Rap1-induced formation of an "integrin activation complex," containing RIAM and talin, that binds to and activates the integrin.

During early pregnancy, uterine mucosa decidualization is accompanied by a drastic enrichment of CD56 high CD16 ؊ natural killer (NK) cells. Decidual NK (dNK) cells differ from peripheral blood NK (pbNK) cells in several ways, but their origin is still unclear. Our results demonstrate that chemokines present in the uterus can support pbNK cell migration through human endothelial and stromal decidual cells. Notably, we observed that pregnant women's pbNK cells are endowed with higher migratory ability compared with nonpregnant women's or male donors' pbNK cells. Moreover, NK cell migration through decidual stromal cells was increased when progesterone-cultured stromal cells were used as substrate, and this correlated with the ability of progesterone to up-regulate stromal cell chemokine expression. Furthermore, we demonstrate that dNK cells migrate through stromal cells using a distinct pattern of chemokines. Finally, we found that pbNK cells acquire a chemokine receptor pattern similar to that of dNK cells when they contact decidual stromal cells. Collectively these results strongly suggest that pbNK cell recruitment to the uterus contributes to the accumulation of NK cells during early pregnancy; that progesterone plays a crucial role in this event; and that pbNK cells undergo reprogramming of their chemokine receptor profile once exposed to uterine microenvironment. IntroductionNatural killer (NK) cells represent a distinct population of circulating and tissue-resident lymphocytes that play an important role in the early phases of immune responses against microbial pathogens by exhibiting cytotoxic functions and secreting a number of cytokines and chemokines. NK cells develop from a lymphoid precursor resident in the bone marrow (BM), considered the main site of NK cell generation, however, the existence of a pathway of NK cell development in the thymus has been recently suggested and evidence also indicates that final maturation of NK cell precursors can occur in the periphery. [1][2][3] During development and activation, NK cells acquire a multiple cell surface receptor system including both activating and inhibitory receptors that finely control their functional activation. 4 Some of these receptors are oligoclonally distributed and/or are expressed at different densities on circulating NK cells. Based on cell surface density of these receptors, phenotypically distinct peripheral blood NK (pbNK) cell populations have been identified and suggested to represent specialized subsets capable of performing different functions and endowed with distinct migratory properties. 5 Mature NK cells circulate mainly in the peripheral blood, but are also present in several lymphoid and nonlymphoid organs such as spleen, lymph nodes, tonsils, liver, lungs, intestine, and uterus. 1,[6][7][8] Interestingly, NK cells are the most abundant class of lymphocytes found in the mucosal tissues of maternal uterus where their number reaches 70% to 80% of the total leukocytes in the first trimester of pregnancy, then start to decline,...

The activating receptor NKG2D is peculiar in its capability to bind to numerous and highly diversified MHC class I-like self-molecules. These ligands are poorly expressed on normal cells but can be induced on damaged, transformed or infected cells, with the final NKG2D ligand expression resulting from multiple levels of regulation. Although redundant molecular mechanisms can converge in the regulation of all NKG2D ligands, different stimuli can induce specific cellular responses, leading to the expression of one or few ligands. A large body of evidence demonstrates that NK cell activation can be triggered by different NKG2D ligands, often expressed on the same cell, suggesting a functional redundancy of these molecules. However, since a number of evasion mechanisms can reduce membrane expression of these molecules both on virus-infected and tumor cells, the co-expression of different ligands and/or the presence of allelic forms of the same ligand guarantee NKG2D activation in various stressful conditions and cell contexts. Noteworthy, NKG2D ligands can differ in their ability to down-modulate NKG2D membrane expression in human NK cells supporting the idea that NKG2D transduces different signals upon binding various ligands. Moreover, whether proteolytically shed and exosome-associated soluble NKG2D ligands share with their membrane-bound counterparts the same ability to induce NKG2D-mediated signaling is still a matter of debate. Here, we will review recent studies on the NKG2D/NKG2D ligand biology to summarize and discuss the redundancy and/or diversity in ligand expression, regulation, and receptor specificity.