One of the most prominent differences between the CD56bright and the CD56dim NK subsets is their intrinsic cytotoxic capabilities. As mentioned above, resting CD56dim NK cells are much more cytotoxic than resting CD56bright NK cells.7 The molecular mechanisms responsible for this are not fully understood. CD56dim NK cells are more granular than CD56bright NK

cells13 and differences in see more the intracellular signaling pathways between the two subsets may also account for their cytotoxic capabilities. Indeed, it was demonstrated by gene expression profiling that compared with CD56dim NK cells, CD56bright NK cells express lower levels of the CD3ζ adaptor molecule, which mediates some of the natural cytotoxicity receptor signaling.14 Importantly, CD56dim NK cells exhibit high

expression levels of FcγRIII (CD16), whereas CD56bright NK cells do not express CD16 or express only low levels of it and therefore, cannot perform antibody-dependent cellular cytotoxicity (ADCC). CD16 is a unique receptor not only check details because of its late function when the adaptive immune response is already activated, but also because among almost all NK cell receptors tested, it is the only receptor that could function independently without the help of other NK cell receptors.8 It is now well established that NK cells can act as major regulators of the immune response, in addition to their ‘classical’ role of killing much hazardous cells. The CD56bright CD16− NK subset is considered as the regulatory subset and a prominent example for its regulatory role is the function of these NK cells in the uterine mucosa prior to and during pregnancy, in the endometrium and decidua tissues, respectively. The data on mouse endometrial NK (eNK) cells are quite limited. It is known that mouse eNK cells

are first found in 2-week-old mice as small and agranular cells.15 Recently, it has been suggested that B220+CD11c+NK1.1+ cells may be analogous to human CD56bright NK cells16 and a recent study indeed identified these cells in the uterus of virgin mice.17 In this study, the phenotype of mouse eNK cells was examined and it was demonstrated that eNK cells are B220+CD11c+NK1.1+ DX5+ (a phenotype that is similar to that of mouse peripheral blood and spleen NK cells18). These eNK cells also express CD122 (the IL-2/IL-15 receptor common β subunit), NKp46 (which is considered the most specific NK marker across species), CD11b (an integrin subunit), CD27 (TNF receptor family member), and CD69 (an activation marker which is also expressed on human eNK cells). It is important to note that mouse eNK cells do not stain for DBA,17–19 which binds N-acetyl-d-glalctosamine conjugates and is considered a selective marker of mouse uterine NK cells.