Maria Cristina Mingari

Major histocompatibility complex class I-specific receptors on human natural killer and T lymphocytes

Summary: Human NK cells express several specialized inhibitory receptors that recognize major histocompatibility complex (MHC) class I molecules expressed on normal cells. The lack of expression of one or more HLA CLASS I alleles leads to NK‐mediated target cell lysis. Receptors specific for groups of HLA‐C (p58), HLA‐B (p70) and HLA‐A (p140) alleles belong to the Ig superfamily with two or three Ig‐like domains in their extracellular portion, and a long cytoplasmic tail containing ITIM motifs and associated with a non‐polar transmembrane portion. In contrast, the CD94/NKG2‐A receptor complex is composed of type II proteins with a C‐type lectin domain which displays a more broad specificity for different class I alleles. Recently, activatory forms of the HLA‐C‐specific receptors have been identified in some donors. They are virtually identical to the inhibitory forms in their extracellular portions, but display a short cytoplasmic tail lacking ITIM motifs associated with a Lys‐containing transmembrane portion (p50). A subset of activated T‐lymphocytes. primarily CD8+ and oligoclonal or monoclonal in nature, express NK‐type class I‐specific receptors. These receptors exert an inhibitory activity on T‐cell receptor‐mediated functions and may provide an important mechanism of down‐regulation of T‐cell responses.

Natural cytotoxicity receptors that trigger human NK-cell-mediated cytolysis

Natural killer (NK) cells can detect whether cells have undergone tumour transformation or viral infection. The discovery of specific inhibitory receptors for major histocompatibility complex class I molecules clarified the basis of this discrimination. However, the receptors responsible for NK-cell triggering in the process of natural cytotoxicity remained elusive until recently. Here, Alessandro Moretta and colleagues describe the identification and characterization of several such receptors.

What is a natural killer cell

Natural killer cells have been arbitrarily defined using a number of different phenotypic and functional criteria. We asked Lorenzo Moretta if we have truly discovered the core identity of this critical player in eliciting immune responses.

Anti-leukemia activity of alloreactive NK cells in KIR ligand-mismatched haploidentical HSCT for pediatric patients: evaluation of the functional role of activating KIR and redefinition of inhibitory KIR specificity

We analyzed 21 children with leukemia receiving haploidentical hematopoietic stem cell transplantation (haplo-HSCT) from killer immunoglobulin (Ig)-like receptors (KIR) ligand-mismatched donors. We showed that, in most transplantation patients, variable proportions of donor-derived alloreactive natural killer (NK) cells displaying anti-leukemia activity were generated and maintained even late after transplantation. This was assessed through analysis of donor KIR genotype, as well as through phenotypic and functional analyses of NK cells, both at the polyclonal and clonal level. Donor-derived KIR2DL1(+) NK cells isolated from the recipient displayed the expected capability of selectively killing C1/C1 target cells, including patient leukemia blasts. Differently, KIR2DL2/3(+) NK cells displayed poor alloreactivity against leukemia cells carrying human leukocyte antigen (HLA) alleles belonging to C2 group. Unexpectedly, this was due to recognition of C2 by KIR2DL2/3, as revealed by receptor blocking experiments and by binding assays of soluble KIR to HLA-C transfectants. Remarkably, however, C2/C2 leukemia blasts were killed by KIR2DL2/3(+) (or by NKG2A(+)) NK cells that coexpressed KIR2DS1. This could be explained by the ability of KIR2DS1 to directly recognize C2 on leukemia cells. A role of the KIR2DS2 activating receptor in leukemia cell lysis could not be demonstrated. Altogether, these results may have important clinical implications for the selection of optimal donors for haplo-HSCT.

Effector and regulatory events during natural killer–dendritic cell interactions

Summary:  The different cell types of the innate immune system can interact with each other and influence the quality and strength of an immune response. The cross talk between natural killer (NK) cells and myeloid dendritic cells (DCs) leads to NK cell activation and DC maturation. Activated NK cells are capable of killing DCs that fail to undergo proper maturation (‘DC editing’). Encounters between NK cells and DCs occur in both inflamed peripheral tissues and lymph nodes, where both cell types are recruited by chemokines released in the early phases of inflammatory responses. Different NK cell subsets (CD56brightCD16− versus CD56+CD16+) differ in their homing capabilities. In particular, CD56brightCD16− NK cells largely predominate the lymph nodes. In addition, these two subsets display major functional differences in their cytolytic activity, cytokine production, and ability to undergo proliferation. NK cell functions are also greatly influenced by the presence of polarizing cytokines such as interleukin (IL)‐12 and IL‐4. The cytokine microenvironment reflects the presence of different cell types that secrete such cytokines in response to microbial products acting on different Toll‐like receptors (TLRs). Moreover, NK cells themselves can respond directly to microbial products by means of TLR3 and TLR9. Thus, it appears that the final outcome of a response to microbial infection may greatly vary as a result of the interactions occurring between different pathogen‐derived products and different cell types of the innate immunity system. These interactions also determine the quality and strength of the subsequent adaptive responses. Remarkably, NK cells appear to play a key role in this complex network.

Regulation of KIR expression in human T cells: a safety mechanism that may impair protective T-cell responses

Killer-cell inhibitory Receptors (KIRs) are a new family of major histocompatibility complex (MHC) class I-specific receptors. KIRs allow natural killer cells to identify and lyse self cells that do not express sufficient amounts of MHC class I molecules. Here, Maria Cristina Mingari and colleagues view the expression of KIRs by cytolytic T lymphocytes and their regulation by certain cytokines as a double-edged sword.

Human natural killer cells: Origin, clonality, specificity, and receptors

Publisher Summary This chapter focuses on human natural killer (NK) cells. NK cells are considered as effector cells that lyse tumor cells or virally infected cells via nonspecific mechanisms. Both their cytolytic effect and the spectrum of their antitumor activity are greatly increased after culture of NK cells in interleukin- 2 (IL-2). IL-2-activated NK cells represent the most potent effector cells among the lymphokine-activated killer (LAK) cells. Several of the generally accepted theories concerning NK cells have been challenged by evidence based on a variety of new experimental approaches and technological breakthroughs. NK cells may originate from precursors present in early fetal liver and even in immature postnatal thymocytes. They specifically recognize class I molecules. Different from T cells, binding of NK cells to class I molecules appears to result in a “negative signal” leading to target cell protection. Many tumor cells that are susceptible to lysis mediated by NK/LAK cells express normal amounts of surface class I antigens. This phenomenon is explained in that (1) the tumor cells express class I antigens that do not function as specific protective elements for the allogeneic NK cells most frequently used in the cytolytic assays and (2) the tumor cells may lack the expression of single class I alleles functioning as protective elements. The ability of activated NK cells to lyse efficiently a wider spectrum of tumor cells can be interpreted as the expression of appropriate activation receptors on these cells as a consequence of cell activation.

Natural killer cells infiltrating human nonsmall‐cell lung cancer are enriched in CD56brightCD16− cells and display an impaired capability to kill tumor cells

Despite natural killer (NK) cells being originally identified and named because of their ability to kill tumor cells in vitro, only limited information is available on NK cells infiltrating malignant tumors, especially in humans.

Increased natural cytotoxicity receptor expression and relevant IL-10 production in NK cells from chronically infected viremic HCV patients.

Hepatitis C virus (HCV) readily establishes high‐level lifelong persistent infection in the majority of immunocompetent adults with failure of HCV‐specific CD8+ CTL to clear viral replication. Virus‐induced conditioning of innate immune responses is a possible mechanism that may contribute to the impairment of virus‐specific CD8+ CTL responses. Here, we analyzed whether triggering of NK cell receptor expression and function is affected during chronic viremic HCV infection. Flow cytometric analysis of purified resting peripheral NK cells showed no evidence of NK cell activation, while analysis of natural cytotoxicity receptors (NCR) showed that NK cells from HCV‐infected patients had selective increased expression of NKp30 and NKp46. NK cells had corresponding conserved cytotoxic activity against all targets with the exception of HepG2 hepatoma cells. Freshly separated NK cells from HCV patients showed significant production of IL‐10 and normal concentrations of IFN‐γ upon cell‐mediated triggering. Thus, increased expression of NKp30 during HCV infection with increased IL‐10 production could contribute, once NK cells localize in the liver, to a NK‐DC crosstalk leading to skewing of subsequent adaptive immune responses and lack of virus control.

Melanoma Cells Inhibit Natural Killer Cell Function by Modulating the Expression of Activating Receptors and Cytolytic Activity

Natural killer (NK) cells play a key role in tumor immune surveillance. However, adoptive immunotherapy protocols using NK cells have shown limited clinical efficacy to date, possibly due to tumor escape mechanisms that inhibit NK cell function. In this study, we analyzed the effect of coculturing melanoma cells and NK cells on their phenotype and function. We found that melanoma cells inhibited the expression of major NK receptors that trigger their immune function, including NKp30, NKp44, and NKG2D, with consequent impairment of NK cell-mediated cytolytic activity against various melanoma cell lines. This inhibitory effect was primarily mediated by indoleamine 2,3-dioxygenase (IDO) and prostaglandin E2 (PGE2). Together, our findings suggest that immunosuppressive barriers erected by tumors greatly hamper the antitumor activity of human NK cells, thereby favoring tumor outgrowth and progression.