Education of hyporesponsive NK cells by cytokines

Abstract

NK-cell tolerance to self is mediated via engagement of inhibitory receptors by cognate MHC molecules. This event is critical for NK-cell education to achieve functional competence. Thus, NK cells expressing self-MHC-specific inhibitory receptors are responsive to activating stimuli while those lacking such receptors are hyporesponsive. Nevertheless, the mechanisms underlying NK-cell education are still poorly understood. Here, we show that after stimulation with cytokines, hyporesponsive NK cells acquire stable expression of killer Ig-like receptors (KIR) as reflected by DNA hypomethylation of their KIR locus. Remarkably, only hyporesponsive NK cells that acquire KIR in the presence of their cognate MHC molecule gain functional competence and this process can occur in the absence of any accessory cells. Acquisition of competence does not result in autoreactivity, since acquired KIR are functional and therefore able to inhibit NK-cell cytotoxicity. Our data demonstrate that competent NK cells can be generated by cytokine stimulation, suggesting that NK-cell education might not only be an early event which takes place during NK-cell development but might also occur in the periphery during an immune response.Key words: Education . Killer Ig-like receptors . MHC . NK cells . Tolerance Supporting Information available onlineIntroduction NK cells are bone-marrow derived lymphocytes which play an important role in mediating innate immune responses to viruses, tumors and MHC-mismatched bone marrow grafts. The main effector functions of NK cells, such as cytokine production and cytotoxicity, are tightly regulated by a balance between activating and inhibitory signals. After engagement of multiple activating receptors expressed on their surface, NK cells can produce IFN-g and kill target cells. In most cases, ligands for activating receptors are pathogen-encoded molecules or self-proteins whose expression is up-regulated in transformed or infected cells. However, even normal cells can be lysed by NK cells if they fail to express a full set of self-MHC class I proteins [1]. These observations highlight NK-cell potential autoreactivity and raise the question of how NK cells discriminate diseased cells from healthy ones. NK cells are self-tolerant because they express inhibitory receptors specific for distinct polymorphic variants of MHC class I molecules which counteract the activating signals. Inhibitory receptors include the killer Ig-like receptors (KIR) in humans, the lectin-like Ly49 dimers in mice and the lectin-like CD94/NKG2A heterodimers in both species. Interestingly, the genes for inhibitory receptors and their cognate MHC class I ligands segregate independently, so genetic mechanisms cannot ensure that each NK-cell inhibitory receptor encounters its specific self-MHC class I molecule [2]. In line with this fact, some NK cells exist, which do not express any self-MHC class I-specific inhibitory receptor. However, 2548such NK cells are self-tolerant because they are hyporesponsive after engagement of activating ...

CD62L expression identifies a unique subset of polyfunctional CD56dim NK cells

“…It has been previously shown that CD56 dim KIR Ϫ (hyporesponsive) NK cells can up-regulate KIR after cytokine stimulation 14,23 and that only those cells that de novo express self MHC-specific KIR are licensed to kill. 29 Percentage of KIR up-regulation on proliferating cells is comparable between KIR Ϫ CD62L ϩ and KIR Ϫ CD62L Ϫ NK cells (supplemental Figure 2E). However, due to their extensive proliferation, a consistently higher proportion of KIR ϩ cells were generated from CD56 dim CD62L ϩ compared with CD62L Ϫ NK cells, suggesting that cells endowed with strong proliferative ability might have a higher chance to be licensed.…”

Impaired cytotoxicity associated with defective natural killer cell differentiation in myelodysplastic syndromes

“…CD56 dim NK cells can be separated on the basis of KIR/NKG2A expression into four consecutive maturation stages. 15 MDS patients had an unusually high frequency of NKG2A -KIR -CD56 dim NK cells ( Figure 6A). This subset represents an immature differentiation stage that is functionally not educated and is generally hyporesponsive to various stimuli.…”

Microchip-Based Single-Cell Imaging Reveals That CD56dimCD57−KIR−NKG2A+ NK Cells Have More Dynamic Migration Associated with Increased Target Cell Conjugation and Probability of Killing Compared to CD56dimCD57−KIR−NKG2A− NK Cells

“…Uneducated KIR 2 NKG2A 2 NK cells are present in human peripheral blood and have been found to be hyporesponsive to various stimuli while displaying a mature phenotype (9)(10)(11). The phenotypic stability of hyporesponsive NK cells may, however, be debated as cytokine stimulation can induce uneducated KIR 2 NKG2A 2 NK cells to acquire effector functions and expression of KIR and/or NKG2A (9,12). In addition, uneducated NK cells have been found to be equally functional as educated NK cells in secretion of IFN-g during Listeria monocytogenes infection and are the primary mediators in suppression of mouse CMV infection (13,14).…”