Cytotoxic lymphocyte recognition of HLA-E: utilizing a nonclassical window to peer into classical MHC

Monalizumab efficacy correlates with HLA-E surface expression and NK cell activity in head and neck squamous carcinoma cell lines

PURPOSE

NKG2A, an inhibitory receptor expressed on NK cells and T cells, leads to immune evasion by binding to HLA-E expressed on cancer cells. Here, we investigated the relationship between HLA-E surface expression on head and neck squamous cell carcinoma (HNSCC) cell lines and the efficacy of monalizumab, an NKG2A inhibitor, in promoting NK cell activity.

METHODS

Six HNSCC cell lines were used as target cells. After exposure to IFN- γ, HLA-E surface expression on HNSCC cell lines was measured by flow cytometry. Peripheral blood mononuclear cells (PBMCs) from healthy donors and isolated NK cells were used as effector cells. NK cells were stimulated by treatment with IL-2 and IL-15 for 5 days, and NK cell-induced cytotoxicity was analyzed by CD107a degranulation and 51Cr release assays.

RESULTS

We confirmed that HLA-E expression was increased by IFN-γ secreted by NK cells and that HLA-E expression was different for each cell line upon exposure to IFN-γ. Cell lines with high HLA-E expression showed stronger inhibition of NK cell cytotoxicity, and efficacy of monalizumab was high. Combination with cetuximab increased the efficacy of monalizumab. In addition, stimulation of isolated NK cells with IL-2 and IL-15 increased the efficacy of monalizumab, even in the HLA-E low groups.

CONCLUSION

Monalizumab efficacy was correlated with HLA-E surface expression and was enhanced when NK cell activity was increased by cetuximab or cytokines. These results suggest that monalizumab may be potent against HLA-E-positive tumors and that monalizumab efficacy could be improved by promoting NK cell activity.

Class Ib MHC-Mediated Immune Interactions Play a Critical Role in Maintaining Mucosal Homeostasis in the Mammalian Large Intestine

Lymphocytes within the intestinal epithelial layer (IEL) in mammals have unique composition compared with their counterparts in the lamina propria. Little is known about the role of some of the key colonic IEL subsets, such as TCRαβ⁺CD8⁺ T cells, in inflammation. We have recently described liver-enriched innate-like TCRαβ⁺CD8αα regulatory T cells, partly controlled by the non-classical MHC molecule, Qa-1^b, that upon adoptive transfer protect from T cell-induced colitis. In this study, we found that TCRαβ⁺CD8αα T cells are reduced among the colonic IEL during inflammation, and that their activation with an agonistic peptide leads to significant Qa-1^b-dependent protection in an acute model of colitis. Cellular expression of Qa-1^b during inflammation and corresponding dependency in peptide-mediated protection suggest that Batf3-dependent CD103⁺CD11b^- type 1 conventional dendritic cells control the protective function of TCRαβ⁺CD8αα T cells in the colonic epithelium. In the colitis model, expression of the potential barrier-protective gene, Muc2, is enhanced upon administration of a Qa-1b agonistic peptide. Notably, in steady state, the mucin metabolizing Akkermansia muciniphila was found in significantly lower abundance amid a dramatic change in overall microbiome and metabolome, increased IL-6 in explant culture, and enhanced sensitivity to dextran sulfate sodium in Qa-1b deficiency. Finally, in patients with inflammatory bowel disease, we found upregulation of HLA-E, a Qa-1^b analog with inflammation and biologic non-response, in silico, suggesting the importance of this regulatory mechanism across species.

Antiviral NK cell responses in HIV infection: I. NK cell receptor genes as determinants of HIV resistance and progression to AIDS

NK cells play an important role in controlling viral infections. They can kill virus-infected cells directly as well as indirectly via antibody-dependent, cell-mediated cytotoxicity. They need no prior sensitization and expansion for this killing. NK cells are also considered as important regulators of antiviral immune responses. They do so by secreting a multitude of soluble mediators and by directly interacting with other immune cells, e.g., dendritic cells. NK cells do not possess a single well-defined receptor to recognize antigens on target cells. Instead, they express an array of inhibitory and activating receptors and coreceptors, which bind to their cognate ligands expressed on the surface of target cells. These ligands include classical and nonclassical MHC class I antigens, MHC-like proteins, and a variety of other self- and virus-derived molecules. They may be expressed constitutively and/or de novo on the surface of virus-infected cells. NK cell receptors (NKRs) of the killer-cell Ig-like receptor (KIR) family, like their MHC class I ligands, are highly polymorphic. Several recent studies suggest that epistatic interactions between certain KIR and MHC class I genes may determine innate resistance of the host to viral infections, including HIV. In the first part of this review article, we provide an overview of the current state of knowledge of NK cell immunobiology and describe how NKR genes, alone and in combination with HLA genes, may determine genetic resistance/susceptibilty to HIV infection and the development of AIDS in humans.

Increased NKG2A found in cytotoxic natural killer subset in HIV-1 patients with advanced clinical status

OBJECTIVES

To investigate the expression of NKG2A on cytotoxic natural killer (NK) cells in HIV-1-infected patients.

DESIGN

A cross-sectional study was conducted to investigate the NKG2A expression on NK cell subsets among HIV-infected individuals at different clinical stages and HIV negative controls.

METHODS

113 HIV-1 infected and 43 uninfected individuals were enrolled in this study. The HIV-1 infected patients were further categorized into three groups, CD4 cell count > 500 cells/microl (n = 44), CD4 cell count of 200-500 cells/microl (n = 49) and CD4 cell count < 200 cells/microl (n = 20). Flow cytometry was used to determine the expression of NKG2A on NK cell subsets. A flow-based assay was employed to quantify the NK cytotoxicity.

RESULTS

Fewer cytotoxic NK cells and more dysfunctional NK cells were observed in patients with advanced clinical conditions. Higher NKG2A expression level in cytotoxic NK subset were found in later stages HIV infection, 25.6% in groups with CD4 cell count > 500 cells/microl, 40.9% in groups with CD4 cell count 200-500 cells/microl and 48.3% in groups with CD4 cell count < 200 cells/microl. Lower NK mediated cytotoxicity, which was associated with the decrease in cytotoxic NK cell number and higher NKG2A expression on cytotoxic NK subsets, was found in AIDS patients compared with patients at early stage of infection. A reverse association between the percentage of NKG2A positive cells in cytotoxic NK subset and CD4 cell count was observed in all HIV-1 infected groups.

CONCLUSION

Fewer cytotoxic NK cells and higher NKG2A expression in cytotoxic NK subset was found in patients in late stage HIV infection. Such a phenomenon may relate to the escape of HIV-1-infected CD4+ T cells from being attacked by NK cells.

The immunophenotypic attributes of NK cells and NK-cell lineage lymphoproliferative disorders

Natural killer (NK) cells are a distinct lymphocyte subset that, like cytotoxic T cells, recognize and destroy abnormal self-cells. Unlike T cells, however, NK cells lack a uniquely rearranged antigen receptor gene or a single defining immunophenotypic attribute. Furthermore, there is considerable functional and phenotypic overlap between NK cells and cytotoxic T cells, and the latter are induced by cellular activation to express many NK-associated antigens. These factors all contribute to difficulties in recognizing abnormal NK-cell expansions and distinguishing them from cytotoxic T cells in the clinical laboratory. Recently, new classes of NK-associated major histocompatibility complex receptors have been described that can be of value in evaluating NK-cell and cytotoxic T-cell populations. In this review, immunophenotyping approaches using antibodies to traditional and novel T- and NK- associated antigens that can be used to evaluate these cell types and diagnose NK-cell and cytotoxic T-cell disorders in the clinical laboratory setting are discussed.

Expression and release of soluble HLA-E is an immunoregulatory feature of endothelial cell activation

Human leukocyte antigen (HLA)-E belongs, with HLA-G and HLA-F, to the non-classic major histocompatibility complex (MHC) class I (Ib) molecules, broadly defined by a limited polymorphism and a restricted pattern of cellular expression. In contrast to HLA-G, the expression and function of HLA-E and HLA-F in physiologic and pathologic processes remain poorly established. In the present study, we show that HLA-E protein expression in normal human nonlymphoid organs is mainly restricted to endothelial cells (ECs). HLA-E is also basally expressed by B and T lymphocytes, natural killer (NK) cells and by macrophages. We demonstrate that tumor necrosis factor alpha (TNFalpha), interleukin-1beta (IL-1beta), and interferon gamma (IFNgamma) up-regulate the cell-surface expression of HLA-E on ECs in vitro and induce the release of soluble HLA-E (sHLA-E). HLA-E up-regulation protects IFN-gamma-activated ECs from NK-mediated cell lysis, while sHLA-E protects bystander cells. Finally, sHLA-E is not detected in normal sera, and increased serum levels correlate with disease activity in patients with antineutrophil cytoplasmic antibody-associated systemic vasculitis. Thus, HLA-E expression and release of sHLA-E are features of EC activation and emphasize immunoregulatory functions of the endothelium. The present identification of soluble HLA-E molecules may have important implications in understanding the pathogenesis of immune-mediated vascular diseases and for the diagnosis and monitoring of patients.

Signal transducer and activator of transcription 1 activation in endothelial cells is a negative regulator of angiogenesis

To determine the role of the transcription factor signal transducer and activator of transcription (STAT) 1 on endothelial cell function, human umbilical vein endothelial cells (HUVEC) were treated with IFN-gamma, a potent activator of STAT1. IFN-gamma inhibited cell growth and tube formation of HUVECs. Although the potent proangiogenic protein vascular endothelial growth factor (VEGF) stimulated cell growth and tube formation, IFN-gamma could suppress these effects of VEGF. Transfection of HUVECs with short interfering RNA targeting STAT1 abrogated IFN-gamma-induced inhibition of HUVEC growth and tube formation, and suppressed the inhibition of VEGF-induced tube formation by IFN-gamma, indicating that STAT1 is critical for this process. IFN-gamma blocks the biological activity of VEGF through inhibition of genes necessary for the VEGF response, including angiopoietin-2, urokinase plasminogen activator, tissue inhibitor of matrix metalloproteinase-1, cyclooxygenase-2, and VEGF receptor 2. To extend these findings in vivo, the role of STAT1 in angiogenesis was examined in STAT1-deficient mice using the Matrigel in vivo angiogenesis assay. Substantial cellular infiltration and formation of vascular structures occurred in STAT1-/- mice compared with wild-type controls. These data indicate that STAT1 plays a key role in the inhibition of angiogenesis through its action within endothelial cells, and exploiting this process may be useful in treating cancers and vascular tumors.

Altered IFNγ Signaling and Preserved Susceptibility to Activated Natural Killer Cell–Mediated Lysis of BCR/ABL Targets

Previous studies have shown that BCR/ABL oncogene, the molecular counterpart of the Ph1 chromosome, could represent a privileged target to natural killer (NK) cells. In the present study, we showed that activated peripheral NK cells killed high-level BCR/ABL transfectant UT-7/9 derived from the pluripotent hematopoietic cell line UT-7 with a high efficiency. To further define the mechanisms controlling BCR/ABL target susceptibility to NK-mediated lysis, we studied the effect of IFNgamma, a key cytokine secreted by activated NK cells, on the lysis of these targets. Treatment of UT-7, UT-7/neo, and low BCR/ABL transfectant UT-7/E8 cells with IFNgamma resulted in a dramatic induction of human leukocyte antigen class I (HLA-I) molecules and subsequently in their reduced susceptibility to NK-mediated cytolysis likely as a consequence of inhibitory NK receptors engagement. In contrast, such treatment neither affected HLA-I expression on transfectants expressing high level of BCR/ABL (UT-7/9) nor modulated their lysis by NK cells. Our data further show that the high-level BCR/ABL in UT-7/9 cells display an altered IFNgamma signaling, as evidenced by a decrease in IFN regulatory factor-1 (IRF-1) and signal transducers and activators of transcription (STAT) 1 induction and activation in response to IFNgamma, whereas this pathway is normal in UT-7 and UT-7/E8 cells. A decreased HLA-I induction and nuclear phospho-STAT1 nuclear translocation were also observed in blasts from most chronic myelogenous leukemia patients in response to IFNgamma. These results outline the crucial role of IFNgamma in the control of target cell susceptibility to lysis by activated NK cells and indicate that the altered response to IFNgamma in BCR/ABL targets may preserve these cells from the cytokine-induced negative regulatory effect on their susceptibility to NK-mediated lysis.

Evolutionary relationships of major histocompatibility complex class I genes in simian primates

New World monkeys (NWMs) occupy a critical phylogenetic position in elucidating the evolutionary process of major histocompatibility complex (MHC) class I genes in primates. From three subfamilies of Aotinae, Cebinae, and Atelinae, the 5'-flanking regions of 18 class I genes are obtained and phylogenetically examined in terms of Alu/LINE insertion elements as well as the nucleotide substitutions. Two pairs of genes from Aotinae and Atelinae are clearly orthologous to human leukocyte antigen (HLA) -E and -F genes. Of the remaining 14 genes, 8 belong to the distinct group B, together with HLA-B and -C, to the exclusion of all other HLA class I genes. These NWM genes are classified into four groups, designated as NWM-B1, -B2, -B3, and -B4. Of these, NWM-B2 is orthologous to HLA-B/C. Also, orthologous relationships of NWM-B1, -B2, and -B3 exist among different families of Cebidae and Atelidae, which is in sharp contrast to the genus-specific gene organization within the subfamily Callitrichinae. The other six genes belong to the distinct group G. However, a clade of these NWM genes is almost equally related to HLA-A, -J, -G, and -K, and there is no evidence for their orthologous relationships to HLA-G. It is argued that class I genes in simian primates duplicated extensively in their common ancestral lineage and that subsequent evolution in descendant species has been facilitated mainly by independent loss of genes.

Successful adoptive transfer and in vivo expansion of human haploidentical NK cells in patients with cancer

We previously demonstrated that autologous natural killer (NK)-cell therapy after hematopoietic cell transplantation (HCT) is safe but does not provide an antitumor effect. We hypothesize that this is due to a lack of NK-cell inhibitory receptor mismatching with autologous tumor cells, which may be overcome by allogeneic NK-cell infusions. Here, we test haploidentical, related-donor NK-cell infusions in a nontransplantation setting to determine safety and in vivo NK-cell expansion. Two lower intensity outpatient immune suppressive regimens were tested: (1) low-dose cyclophosphamide and methylprednisolone and (2) fludarabine. A higher intensity inpatient regimen of high-dose cyclophosphamide and fludarabine (Hi-Cy/Flu) was tested in patients with poor-prognosis acute myeloid leukemia (AML). All patients received subcutaneous interleukin 2 (IL-2) after infusions. Patients who received lower intensity regimens showed transient persistence but no in vivo expansion of donor cells. In contrast, infusions after the more intense Hi-Cy/Flu resulted in a marked rise in endogenous IL-15, expansion of donor NK cells, and induction of complete hematologic remission in 5 of 19 poor-prognosis patients with AML. These findings suggest that haploidentical NK cells can persist and expand in vivo and may have a role in the treatment of selected malignancies used alone or as an adjunct to HCT.

Specific recognition of the viral protein UL18 by CD85j/LIR-1/ILT2 on CD8+ T cells mediates the non-MHC-restricted lysis of human cytomegalovirus-infected cells

Immune evasion mechanisms of human CMV are known; however, the immune control of infection remains poorly elucidated. We show that interaction between the viral protein UL18 on infected cells and the invariant receptor CD85j/LIR-1/ILT2 expressed on CTL is relevant for the control of infection. Resting and activated CD8(+) T cells lysed UL18 expressing cells, whereas cells infected with CMV defective for UL18 were not killed. Lysis was not dependent on CD8(+) T cell Ag specificity, MHC-unrestricted and specifically blocked by anti-CD85j and anti-UL18 mAb. Moreover, soluble recombinant UL18Fc immunoprecipitated CD85j from T cells. Activation is mediated by CD85j and its pathway is unrelated to CD3/TCR engagement. UL18 is detected in immunocompromised patients with productive infection and the mechanism used in vivo by human CMV to ensure survival of the immunocompetent host may be mediated by activation signals delivered by infected cells to T lymphocytes via UL18/CD85j interactions.

Role of NK and NKT cells in the immunopathogenesis of HCV-induced hepatitis

Natural killer (NK) cells constitute the first line of host defense against invading pathogens. They usually become activated in an early phase of a viral infection. Liver is particularly enriched in NK cells, which are activated by hepatotropic viruses such as hepatitis C virus (HCV). The activated NK cells play an essential role in recruiting virus-specific T cells and in inducing antiviral immunity in liver. They also eliminate virus-infected hepatocytes directly by cytolytic mechanisms and indirectly by secreting cytokines, which induce an antiviral state in host cells. Therefore, optimally activated NK cells are important in limiting viral replication in this organ. This notion is supported by the observations that interferon treatment is effective in HCV-infected persons in whom it increases NK cell activity. Not surprisingly, HCV has evolved multiple strategies to counter host's NK cell response. Compromised NK cell functions have been reported in chronic HCV-infected individuals. It is ironic that activated NK cells may also contribute toward liver injury. Further studies are needed to understand the role of these cells in host defense and in liver pathology in HCV infections. Recent advances in understanding NK cell biology have opened new avenues for boosting innate and adaptive antiviral immune responses in HCV-infected individuals.

Evolutionary Relationships of Major Histocompatibility Complex Class I Genes in Simian Primates

New World monkeys (NWMs) occupy a critical phylogenetic position in elucidating the evolutionary process of major histocompatibility complex (MHC) class I genes in primates. From three subfamilies of Aotinae, Cebinae, and Atelinae, the 5′-flanking regions of 18 class I genes are obtained and phylogenetically examined in terms of Alu/LINE insertion elements as well as the nucleotide substitutions. Two pairs of genes from Aotinae and Atelinae are clearly orthologous to human leukocyte antigen (HLA) -E and -F genes. Of the remaining 14 genes, 8 belong to the distinct group B, together with HLA-B and -C, to the exclusion of all other HLA class I genes. These NWM genes are classified into four groups, designated as NWM-B1, -B2, -B3, and -B4. Of these, NWM-B2 is orthologous to HLA-B/C. Also, orthologous relationships of NWM-B1, -B2, and -B3 exist among different families of Cebidae and Atelidae, which is in sharp contrast to the genus-specific gene organization within the subfamily Callitrichinae. The other six genes belong to the distinct group G. However, a clade of these NWM genes is almost equally related to HLA-A, -J, -G, and -K, and there is no evidence for their orthologous relationships to HLA-G. It is argued that class I genes in simian primates duplicated extensively in their common ancestral lineage and that subsequent evolution in descendant species has been facilitated mainly by independent loss of genes.

Inability of natural killer cells to destroy autologous HIV-infected T lymphocytes

OBJECTIVE

Determine whether natural killer (NK) cells are capable of killing HIV-infected autologous primary T-cell blasts.

DESIGN

The ability of NK cells to kill HIV-infected primary T-cell blasts, whose cell surface major histocompatibility complex (MHC) class I molecules was decreased, was evaluated in a lytic assay.

METHODS

Phytohemagglutinin-treated CD4+ T cells were infected with HIV-1. Infected cells were separated from uninfected cells by removal of CD4+ T cells. The NK cells were isolated from peripheral blood mononuclear cells (PBMC) of the same donor as the CD4+ T cells by immunomagnetic bead separation. The NK cells isolated from PBMC were then used as effector cells and the HIV-infected T-cell blasts were used as target cells in a lytic assay.

RESULTS

It was demonstrated that HIV infection of primary CD4+ T cells results in a 61-68% reduction in surface expression of MHC class I molecules. Despite the decreased MHC class I expression the NK cells were incapable of lysing autologous HIV-infected T-cell blasts, yet were effective in the lysis of the NK cell sensitive cell line, K562. The inability of NK cells to lyse HIV-infected T-cell blasts is not dependent on the strain of HIV used to infected the CD4+ T cell

CONCLUSION

These studies indicate that despite drastic decreases in MHC class I molecule expression, HIV-infected T-cell blasts can evade destruction by autologous NK cells.

Retroviral proteins that target the major histocompatibility complex class I

The human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) retroviruses are two evolutionary distinct human pathogens. HTLV-1 is the etiologic agent of two diverse diseases: adult T-cell leukemia/lymphoma, as well as the neurologic disorder tropical spastic paraparesis/HTLV-1-associated myelopathy. HTLV-1 is the only retrovirus known to be the etiologic agent of human cancer. HTLV-2, the other known oncovirus, is not apparently associated with human cancer. While HTLV-1 transforms T-cells in vitro, HIV kills CD4+ T-cells and is the etiological agent of human acquired immunodeficiency syndrome, characterized by a progressive loss of CD4+ cells, weakening of the immune system, and susceptibility to opportunistic infections and cancer. HTLV-1 and HIV-1 both cause lifelong infections, which suggests that they have evolved mechanism(s) to evade detection by the host's immune response; particularly to evade cytotoxic T-lymphocytes, which play a major role in cellular immunity against viruses and will be the focus of this review.

Biology of natural killer cells in cancer and infection

NK cells are the important cells of the immune system derived from stem cells in the marrow. Their physiology is tightly regulated to control proliferation, cytotoxicity and cytokine production. In cancer, NK cells may be abnormal due to the cancer itself or possibly related to its therapy. The finding of class I recognizing inhibitory receptors may play a role in stem cell transplant rejection, immune surveillance and cancer immunotherapy. NK cells should no longer be thought of as direct cytotoxic killers alone, as they clearly play a critical role in cytokine production which may be important to control cancer and infection. Understanding NK cell function and homing may lead to novel therapeutic strategies for the treatment of human disease.

Sister cytotoxic CD8+ T cell clones differing in natural killer inhibitory receptor expression in human astrocytoma

Natural killer (NK) inhibitory receptors are thought to play a critical role in the regulation of cytotoxicity of NK cells and certain self-reactive T cells. In the present study, we investigated whether astrocytoma infiltrating T lymphocytes may be functionally compromised by NK receptors (NKRs). The NK inhibitory receptor CD94/NKG2A was found on a significant proportion of CD8+ astrocytoma infiltrating lymphocytes. The functional consequences of CD94/NKG2A expression were explored at the clonal level, using a T cell clone that exhibited substantial variation in the expression of this heterodimer. Triggering of CD94/NKG2A inhibited the killing properties of T cells with a high level of this receptor, but not those from T cells with a low level. Our data indicate that some astrocytoma infiltrating lymphocytes express functional inhibitory CD94/NKG2A, raising the possibility that they may represent silent T cells specific for self-antigens (Ags) expressed on tumor cells. Understanding the mechanisms of regulation of these receptors may bring new insights for optimizing an anti-tumor immune response.

The biology of natural killer cells in cancer, infection, and pregnancy

OBJECTIVE

NK cells are important cells of the immune system. They are ultimately derived from pluripotent hematopoietic stem cells. NK cell cytotoxicity and other functions are tightly regulated by numerous activating and inhibitory receptors including newly discovered receptors that selectively recognize major histocompatibility complex class I alleles. Based on their defining function of spontaneous cytotoxicity without prior immunization, NK cells have been thought to play a critical role in immune surveillance and cancer therapy. However, new insights into NK cell biology have suggested major roles for NK cells in infection control and uterine function. The purpose of this review is to provide an update on NK cell function, ontogeny, and biology in order to better understand the role of NK cells in health and disease.

DATA SOURCES

In the Medline database, the major subject heading "Natural Killer Cells" was introduced in 1983, identifying 16,848 citations as of December 31, 2000. Since 1986, there have been approximately 1000 citations per year under this subject heading. In this database, 68% of manuscripts are limited to human NK cells; 40% of citations cross with the major sub-heading of cytotoxicity, 40% with cytokines, 36% with neoplasm, 5% with antibody-dependent cellular cytotoxicity, 2.8% with pregnancy, and 1.3% with infection. Of references from the year 2000-2001, 46 were selected to combine with contributions from earlier literature.

CONCLUSIONS

NK cells should no longer be thought of as direct cytotoxic killers alone as they clearly serve a critical role in cytokine production which may be important to control cancer, infection, and fetal implantation. Understanding mechanisms of NK cell functions may lead to novel therapeutic strategies for the treatment of human disease.

The biology of natural killer cells and implications for therapy of human disease

Natural killer (NK) cells are unique lymphocytes capable of lysing target cells without prior immunization. NK cells activated with cytokines, like interleukin-2 (IL-2), have been used since the 1980s as adoptive immunotherapy against metastatic solid tumors, but their effectiveness has been limited. The mechanisms by which NK cells recognize their targets are complex, including newly identified receptors that recognize class I MHC molecules. Understanding these mechanisms may support the use of NK cells as clinical therapy against infectious diseases and cancer. We have been interested in the use of NK cells clinically for their potential to eradicate minimal residual disease and prevent relapses after autologous stem cell transplantation. Several strategies are discussed to increase the specificity and efficacy of NK cell therapy. One method is to increase the targeting of NK cells by the use of monoclonal antibodies. Another approach uses allogeneic NK cells to overcome the inhibitory receptor mechanisms that may block target cell lysis by recognition of class I molecules. These and other novel strategies may prove to be attractive and effective immunotherapeutic tools to manipulate NK cells to fight human disease.

Human natural killer cells with polyclonal lectin and immunoglobulinlike receptors develop from single hematopoietic stem cells with preferential expression of NKG2A and KIR2DL2/L3/S2

The stage of progenitor maturation and factors that determine the fate and clonal acquisition of human natural killer (NK) cell receptors during development are unknown. To study human NK cell receptor ontogeny, umbilical cord blood CD34(+)/Lin(-)/CD38(-) cells were cultured with a murine fetal liver line (AFT024) and defined cytokines. In the absence of lymphocyte-stimulating cytokines or when contact with AFT024 was prohibited, NK cell progeny were killer immunoglobulinlike receptor (KIR) and CD94 lectin receptor negative. In contrast, efficient NK cell differentiation and receptor acquisition was dependent on direct contact of progenitors with AFT024 and the addition of interleukin-15 (IL-15) or IL-2 but not IL-7. To address the question of whether receptor acquisition was determined at the stem cell level, single CD34(+)/Lin(-)/CD38(-) progenitors were studied. More than 400 single cell progeny were analyzed from cultures containing IL-15 or IL-2 and NK cells were always polyclonal, suggesting that receptor fate is determined beyond an uncommitted progenitor and that receptor-negative NK cells acquire class I-recognizing receptors after lineage commitment. KIR2DL2/L3/S2 was expressed more than KIR2DL1/S1 or KIR3DL1, and NKG2A was the dominant CD94 receptor, independent of whether the stem cell source contained the respective major histocompatibility complex class I ligand, suggesting a nonrandom sequence of receptor acquisition. The conclusion is that NK receptor fate is determined after NK cell commitment, does not require stromal presentation of human class I alleles, and is clonally stable after expression but dynamic because new receptors are acquired over time. (Blood. 2001;98:705-713)

Natural killer cells and the syndrome of chronic natural killer cell lymphocytosis

Natural killer (NK) cells provide anti-infectious, anti-neoplastic, and immunomodulatory function effected by both cytokine production and direct cellular cytotoxicity that is not major histocompatibility complex-restricted. NK cells lack truly specific cell surface determinants as well as antigen-specific receptors. Recent information suggests a variety of receptor-ligand interactions that underlie recognition and treatment of target cells by NK cells. Primary NK cell disorders in humans are currently classified into NK cell lymphomas and chronic NK cell lymphocytosis (CNKL). In this review, we summarize current understanding of the biology of NK cells and describe the clinical manifestations of CNKL.

Mouse CD94 participates in Qa-1-mediated self recognition by NK cells and delivers inhibitory signals independent of Ly-49

Inhibitory receptors expressed on NK cells recognize MHC class I molecules and transduce negative signals to prevent the lysis of healthy autologous cells. The lectin-like CD94/NKG2 heterodimer has been studied extensively as a human inhibitory receptor. In contrast, in mice, another lectin-like receptor, Ly-49, was the only known inhibitory receptor until the recent discovery of CD94/NKG2 homologues in mice. Here we describe the expression and function of mouse CD94 analyzed by a newly established mAb. CD94 was detected on essentially all NK and NK T cells as well as small fractions of T cells in all mouse strains tested. Two distinct populations were identified among NK and NK T cells, CD94(bright) and CD94(dull) cells, independent of Ly-49 expression. The anti-CD94 mAb completely abrogated the inhibition of target killing mediated by NK recognition of Qa-1/Qdm peptide on target cells. Importantly, CD94(bright) but not CD94(dull) cells were found to be functional in the Qa-1/Qdm-mediated inhibition. In the presence of the mAb, activated NK cells showed substantial cytotoxicity against autologous target cells as well as enhanced cytotoxicity against allogeneic and "missing self" target cells. These results suggest that mouse CD94 participates in the protection of self cells from NK cytotoxicity through the Qa-1 recognition, independent of inhibitory receptors for classical MHC class I such as Ly-49.

Identification of CCAAT displacement protein (CDP/cut) as a locus-specific repressor of major histocompatibility complex gene expression in human tumor cells

Human major histocompatibility (MHC) class I antigen expression is important in controlling the metastatic growth of malignant tumors. Locus-specific down-regulation of MHC class I gene expression is frequently observed in human tumors, leading to decreased susceptibility to cytotoxic T-cell-mediated lysis. The mechanism of this down-regulation is incompletely understood. Here, we describe the identification of human CCAAT displacement protein (CDP/cut) as a locus-specific repressor of HLA-B and C gene expression. Transient and stable transfections in HeLa and K562 cells demonstrated the presence of a repressor element 650 base pairs upstream of the first exon of HLA-B7. A specific binding complex with the HLA-B7 and Cw2 repressor elements was demonstrated by EMSA. Formation of the EMSA complex was inhibited specifically with polyclonal antiserum to human CDP/cut, demonstrating that CDP/cut binds the HLA-B7 repressor element. The corresponding region of the HLA-A2 promoter neither repressed HLA-A2 gene expression nor bound CDP/cut. Overexpression of CDP/cut in cell lines deficient in CDP/cut resulted in a nearly 4-fold repression of reporter constructs containing the HLA-B7 repressor element but not the corresponding region of the HLA-A2 promoter. Repression of HLA-B and C gene expression by CDP/cut does not involve displacement of NF-Y, nor is CDP/cut associated with the histone deacetylase HDAC1 when bound to the HLA-B7 repressor element. To our knowledge, these results identify CDP/cut as the first example of a locus-specific repressor of MHC class I gene transcription in human tumor cells.

Inhibition of natural killer cell-mediated cytotoxicity by Kaposi's sarcoma-associated herpesvirus K5 protein

Kaposi's sarcoma-associated herpesvirus (KSHV) K3 and K5 proteins dramatically downregulate MHC class I molecules. However, although MHC class I downregulation may protect KSHV-infected cells from cytotoxic T lymphocyte recognition, these cells become potential targets for natural killer (NK) cell-mediated lysis. We now show that K5 also downregulates ICAM-1 and B7-2, which are ligands for NK cell-mediated cytotoxicity receptors. As a consequence, K5 expression drastically inhibits NK cell-mediated cytotoxicity. Conversely, de novo expression of B7-2 and ICAM-1 resensitizes the K5-expressing cells to NK cell-mediated cytotoxicity. This is a novel viral immune evasion strategy where KSHV K5 achieves immune avoidance by downregulation of cellular ligands for NK cell-mediated cytotoxicity receptors.

Downregulation of major histocompatibility complex class I molecules by Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins

The T-cell-mediated immune response plays a central role in the defense against intracellular pathogens. To avoid this immune response, viruses have evolved elaborate mechanisms that target and modulate many different aspects of the host's immune system. A target common to many of these viruses is the major histocompatibility complex (MHC) class I molecules. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes K3 and K5 zinc finger membrane proteins which remove MHC class I molecules from the cell surface. K3 and K5 exhibit 40% amino acid identity to each other and localize primarily near the plasma membrane. While K3 and K5 dramatically downregulated class I molecules, they displayed different specificities in downregulation of HLA allotypes. K5 significantly downregulated HLA-A and -B and downregulated HLA-C only weakly, but not HLA-E, whereas K3 downregulated all four HLA allotypes. This selective downregulation of HLA allotypes by K5 was partly due to differences in amino acid sequences in their transmembrane regions. Biochemical analyses demonstrated that while K3 and K5 did not affect expression and intracellular transport of class I molecules, their expression induced rapid endocytosis of the molecules. These results demonstrate that KSHV has evolved a novel immune evasion mechanism by harboring similar but distinct genes, K3 and K5, which target MHC class I molecules in different ways.

The selective downregulation of class I major histocompatibility complex proteins by HIV-1 protects HIV-infected cells from NK cells

To avoid detection by CTL, HIV encodes mechanisms for removal of class I MHC proteins from the surface of infected cells. However, class I downregulation potentially exposes the virus-infected cell to attack by NK cells. Human lymphoid cells are protected from NK cell cytotoxicity primarily by HLA-C and HLA-E. We present evidence that HIV-1 selectively downregulates HLA-A and HLA-B but does not significantly affect HLA-C or HLA-E. We then identify the residues in HLA-C and HLA-E that protect them from HIV down-regulation. This selective downregulation allows HIV-infected cells to avoid NK cell-mediated lysis and may represent for HIV a balance between escape from CTL and maintenance of protection from NK cells. These results suggest that subpopulations of CTL and NK cells may be uniquely suited for combating HIV.

The natural killer gene complex genetic locus Chok encodes Ly-49D, a target recognition receptor that activates natural killing

Previously, we established that natural killer (NK) cells from C57BL/6 (B6), but not BALB/c, mice lysed Chinese hamster ovary (CHO) cells, and we mapped the locus that determines this differential CHO-killing capacity to the NK gene complex on chromosome 6. The localization of Chok in the NK gene complex suggested that it may encode either an activating or an inhibitory receptor. Here, results from a lectin-facilitated lysis assay predicted that Chok is an activating B6 NK receptor. Therefore, we immunized BALB/c mice with NK cells from BALB.B6-Cmv1(r) congenic mice and generated a mAb, designated 4E4, that blocked B6-mediated CHO lysis. mAb 4E4 also redirected lysis of Daudi targets, indicating its reactivity with an activating NK cell receptor. Furthermore, only the 4E4(+) B6 NK cell subset mediated CHO killing, and this lysis was abrogated by preincubation with mAb 4E4. Flow cytometric analysis indicated that mAb 4E4 specifically reacts with Ly-49D but not Ly-49A, B, C, E, G, H, or I transfectants. Finally, gene transfer of Ly-49DB6 into BALB/c NK cells conferred cytotoxic capacity against CHO cells, thus establishing that the Ly-49D receptor is sufficient to activate NK cells to lyse this target. Hence, Ly-49D is the Chok gene product and is a mouse NK cell receptor capable of directly triggering natural killing.

The functionality of HLA-G is emerging

In view of the recently published data, the HLA-G class Ib gene appears to be a functional locus. This is based on the following observations: 1) HLA-G is capable of presenting nonamer peptides and of exerting antigen-presenting functions; 2) HLA-G is a ligand for at least three natural killer (NK) and other cell inhibitory receptors of the immunoglobulin superfamily, namely leukocyte immunoglobulin-like receptor-1/immunoglobulin-like transcript (ILT)-2, ILT-4 and p49; 3) in addition to the extravillous cytotrophoblast cells, HLA-G proteins have been detected in endothelial cells of placental chorionic villi, as well as in amniotic fluid and in some medullary thymic epithelial cells; 4) major histocompatibility complex (MHC) class Ib genes that share the unique characteristics of HLA-G, including a high expression in placenta, have been reported in other mammalian species. In addition to the classical MHC class I roles (antigen presentation and ligation to NK receptors inducing inhibitory and/or activatory signals), HLA-G is likely to exert other, novel functions: first, HLA-G was shown to be involved in the control of HLA-E expression by furnishing the appropriate class I leader sequence nonamer peptide; second, we hypothesize that HLA-G could be a regulator of placental angiogenesis; third, soluble HLA-G isoforms may act as specific immunosuppressors during pregnancy. Such functional properties, although incompletely understood, are likely to be important in the outcome of human pregnancies but also in normal adult life.