Activating KIRs and NKG2C in viral infections: Toward NK cell memory?

Natural killer (NK) cells are important players in the immune defense against viral infections. The contribution of activating killer immunoglobulin-like receptors (KIRs) and CD94/NKG2C in regulating anti-viral responses has recently emerged. Thus, in the hematopoietic stem cell transplantation setting, the presence of donor activating KIRs (aKIRs) may protect against viral infections, while in HIV-infected individuals, KIR3DS1, in combination with HLA-Bw4-I80, results in reduction of viral progression. Since, studies have been performed mainly at the genetic or transcriptional level, the effective size, the function, and the "licensing" status of NK cells expressing aKIRs, as well as the nature of their viral ligands, require further investigation. Certain viral infections, mainly due to Human cytomegalovirus (HCMV), can deeply influence the NK cell development and function by inducing a marked expansion of mature NKG2C+ NK cells expressing self-activating KIRs. This suggests that NKG2C and/or aKIRs are involved in the selective proliferation of this subset. The persistent, HCMV-induced, imprinting suggests that NK cells may display unexpected adaptive immune traits. The role of aKIRs and NKG2C in regulating NK cell responses and promoting a memory-like response to certain viruses is discussed

Multidirectional interactions are bridging human NK cells with plasmacytoid and monocyte-derived dendritic cells during innate immune responses

AbstractDuring innate immune responses, natural killer (NK) cells may interact with both plasmacytoid dendritic cells (pDCs) and monocyte-derived dendritic cells (MDDCs). We show that freshly isolated NK cells promote the release by pDCs of IFN-α, in a CpG-dependent manner, whereas they induce IL-6 production in a CpG-independent manner. In turn pDC-derived IFN-α up-regulates NK-mediated killing, whereas IL-6 could promote B-cell differentiation. We also show that exposure to exogenous IL-12 or coculture with maturing MDDCs up-regulates the NK-cell–dependent IFN-α production by pDCs. On the other hand, NK cells cocultured with pDCs acquire the ability to kill immature MDDCs, thus favoring their editing process. Finally, we show that activated NK cells are unable to lyse pDCs because these cells display an intrinsic resistance to lysis. The exposure of pDCs to IL-3 increased their susceptibility to NK-cell cytotoxicity resulting from a de novo expression of ligands for activating NK-cell receptors, such as the DNAM-1 ligand nectin-2. Thus, different cell-to-cell interactions and various cytokines appear to control a multidirectional network between NK cells, MDDCs, and pDCs that is likely to play an important role during the early phase of innate immune responses to viral infections and to tumors

Transplantation and innate immunity: the lesson of natural killer cells

Natural killer cells have been demonstrated to play a major role in mediating an anti-leukemia effect in patients given a T-cell depleted allogeneic hematopoietic stem cell transplantation from an HLA-haploidentical family donor. In particular, donor-derived natural killer cells, which are alloreactive (i.e. KIR/HLA mismatched) towards recipient cells, significantly contribute to the eradication of leukemia blasts escaping the preparative regimen to transplantation. A recent study on high-risk pediatric acute lymphoblastic leukemia refractory to chemotherapy further highlighted the importance of donors with alloreactive natural killer cells in haploidentical hematopoietic stem cell transplantation, as it demonstrated that these cells can emerge starting from the fourth-fifth month after the allograft and persist for many months. This study represents a major breakthrough in the cure of otherwise fatal leukemias, providing information on the best criteria for choosing the optimal donor

Human dendritic cells activate resting natural killer (NK) cells and are recognized via the NKp30 receptor by activated NK cells

During the innate response to many inflammatory and infectious stimuli, dendritic cells (DCs) undergo a differentiation process termed maturation. Mature DCs activate antigen-specific naive T cells. Here we show that both immature and mature DCs activate resting human natural killer (NK) cells. Within 1 wk the NK cells increase two- to fourfold in numbers, start secreting interferon (IFN)-γ, and acquire cytolytic activity against the classical NK target LCL721.221. The DC-activated NK cells then kill immature DCs efficiently, even though the latter express substantial levels of major histocompatibility complex (MHC) class I. Similar results are seen with interleukin (IL)-2-activated NK cell lines and clones, i.e., these NK cells kill and secrete IFN-γ in response to immature DCs. Mature DCs are protected from activated NK lysis, but lysis takes place if the NK inhibitory signal is blocked by a human histocompatibility leukocyte antigen (HLA)-A,B,C-specific antibody. The NK activating signal mainly involves the NKp30 natural cytotoxicity receptor, and not the NKp46 or NKp44 receptor. However, both immature and mature DCs seem to use a NKp30 independent mechanism to act as potent stimulators for resting NK cells. We suggest that DCs are able to control directly the expansion of NK cells and that the lysis of immature DCs can regulate the afferent limb of innate and adaptive immunity

Dendritic Cell Editing by Activated Natural Killer Cells Results in a More Protective Cancer-Specific Immune Response

Over the last decade, several studies have extensively reported that activated natural killer (NK) cells can kill autologous immature dendritic cells (DCs) in vitro, whereas they spare fully activated DCs. This led to the proposal that activated NK cells might select a more immunogenic subset of DCs during a protective immune response. However, there is no demonstration that autologous DC killing by NK cells is an event occurring in vivo and, consequently, the functional relevance of this killing remains elusive. Here we report that a significant decrease of CD11c+ DCs was observed in draining lymph nodes of mice inoculated with MHC-devoid cells as NK cell targets able to induce NK cell activation. This in vivo DC editing by NK cells was perforin-dependent and it was functionally relevant, since residual lymph node DCs displayed an improved capability to induce T cell proliferation. In addition, in a model of anti-cancer vaccination, the administration of MHC-devoid cells together with tumor cells increased the number of tumor-specific CTLs and resulted in a significant increase in survival of mice upon challenge with a lethal dose of tumor cells. Depletion of NK cells or the use of perforin knockout mice strongly decreased the tumor-specific CTL expansion and its protective role against tumor cell challenge. As a whole, our data support the hypothesis that NK cell-mediated DC killing takes place in vivo and is able to promote expansion of cancer-specific CTLs. Our results also indicate that cancer vaccines could be improved by strategies aimed at activating NK cells

ILC3s: Rhythmic Keepers of Gut Integrity at Mealtime

Cyclically, during the day, increased permeability of the intestinal epithelial barrier, allowing nutrient uptake, must be compensated for, to achieve increased protection against potentially harmful components. Seillet et al. demonstrate that, upon food intake, enteric neuron-derived VIP promotes anticipatory mucosal immunity by inducing ILC3s to produce protective IL-22

NK-dependent DC maturation is mediated by TNFα and IFNγ released upon engagement of the NKp30 triggering receptor

Abstract Natural killer (NK) cells were recently shown to play a relevant role in the process of dendritic cell (DC) maturation. This function is exerted either by direct DC stimulation or through killing those DCs that did not properly acquire a mature phenotype. While killing of immature DCs is dependent on the function of the NKp30 triggering receptor, the mechanism by which NK cells induce DC maturation is still unclear. In this study, we show that also the NK-mediated induction of DC maturation is dependent on NKp30. Upon NK/DC interaction, resulting in NKp30 engagement, NK cells produced tumor necrosis factor α (TNFα) (and interferon γ [IFNγ]) that, in turn, promoted DC maturation. Masking of NKp30 with specific monoclonal antibodies (mAbs) strongly reduced maturation of DCs cocultured with NK cells. In addition, supernatant from NK cells stimulated via NKp30 induced DC maturation, and this effect was neutralized by anti-TNFα antibodies (Abs). This NKp30 function is controlled by the HLA-specific inhibitory NK receptors. Accordingly, the ability to promote maturation was essentially confined to NK cells expressing the killer immunoglobulin-like receptor–negative (KIR–) NKG2Adull phenotype. Finally, the analysis of perforin-deficient NK cells allowed the dissection of the 2 NKp30-mediated NK-cell functions, since NKp30 could induce cytokine-dependent DC maturation in the absence of NK-mediated DC killing

KIR3DS1-Mediated Recognition of HLA-*B51: Modulation of KIR3DS1 Responsiveness by Self HLA-B Allotypes and Effect on NK Cell Licensing

Several studies described an association between killer-cell immunoglobulin-like receptor (KIR)/HLA gene combinations and clinical outcomes in various diseases. In particular, an important combined role for KIR3DS1 and HLA-B Bw4-I80 in controlling viral infections and a higher protection against leukemic relapses in donor equipped with activating KIRs in haplo-HSCT has been described. Here, we show that KIR3DS1 mediates positive signals upon recognition of HLA-B*51 (Bw4-I80) surface molecules on target cells and that this activation occurs only in Bw4-I80neg individuals, including those carrying particular KIR/HLA combination settings. In addition, killing of HLA-B*51 transfected target cells mediated by KIR3DS1+/NKG2A+ natural killer (NK) cell clones from Bw4-I80neg donors could be partially inhibited by antibody-mediated masking of KIR3DS1. Interestingly, KIR3DS1-mediated recognition of HLA-B*51 could be better appreciated under experimental conditions in which the function of NKG2D was reduced by mAb-mediated blocking. This experimental approach may mimic the compromised function of NKG2D occurring in certain viral infections. We also show that, in KIR3DS1+/ NKG2A+ NK cell clones derived from an HLA-B Bw4-T80 donor carrying 2 KIR3DS1 gene copy numbers, the positive signal generated by the engagement of KIR3DS1 by HLA-B*51 resulted in a more efficient killing of HLA-B*51-transfected target cells. Moreover, in these clones, a direct correlation between KIR3DS1 and NKG2D surface density was detected, while the expression of NKp46 was inversely correlated with that of KIR3DS1. Finally, we analyzed KIR3DS1+/NKG2A+ NK cell clones from a HLA-B Bw4neg donor carrying cytoplasmic KIR3DL1. Although these clones expressed lower levels of surface KIR3DS1, they displayed responses comparable to those of NK cell clones derived from HLA-B Bw4neg donors that expressed surface KIR3DL1. Altogether these data suggest that, in particular KIR/HLA combinations, KIR3DS1 may play a role in the process of human NK cell educatio

NKp44, a Novel Triggering Surface Molecule Specifically Expressed by Activated Natural Killer Cells, Is Involved in Non–Major Histocompatibility Complex–restricted Tumor Cell Lysis

After culture in interleukin (IL)-2, natural killer (NK) cells acquire an increased capability of mediating non–major histocompatibility complex (MHC)–restricted tumor cell lysis. This may reflect, at least in part, the de novo expression by NK cells of triggering receptors involved in cytolysis. In this study we identified a novel 44-kD surface molecule (NKp44) that is absent in freshly isolated peripheral blood lymphocytes but is progressively expressed by all NK cells in vitro after culture in IL-2. Different from other markers of cell activation such as CD69 or VLA.2, NKp44 is absent in activated T lymphocytes or T cell clones. Since NKp44 was not detected in any of the other cell lineages analyzed, it appears as the first marker specific for activated human NK cells. Monoclonal antibody (mAb)–mediated cross-linking of NKp44 in cloned NK cells resulted in strong activation of target cell lysis in a redirected killing assay. This data indicated that NKp44 can mediate triggering of NK cell cytotoxicity. mAb-mediated masking of NKp44 resulted in partial inhibition of cytolytic activity against certain (FcγR-negative) NK-susceptible target cells. This inhibition was greatly increased by the simultaneous masking of p46, another recently identified NK-specific triggering surface molecule. These data strongly suggest that NKp44 functions as a triggering receptor selectively expressed by activated NK cells that, together with p46, may be involved in the process of non-MHC-restricted lysis. Finally, we show that p46 and NKp44 are coupled to the intracytoplasmic transduction machinery via the association with CD3ζ or KARAP/DAP12, respectively; these associated molecules are tyrosine phosphorylated upon NK cell stimulation