Signal transduction during natural killer cell activation: inside the mind of a killer

Human leukocyte antigen-C in short- and long-term liver graft acceptance

In liver transplantation, rejection is still an important problem, and the role of human leukocyte antigens (HLA) has not been clearly established. At present, the possible involvement of HLA-C antigen in liver transplantation is still unexplored. The aim of this work was to analyze the influence of HLA-C polymorphism on the outcome of liver transplantation. For this purpose, genotyping of 100 orthotopic liver transplant recipient-donor pairs for HLA-C was performed with polymerase chain reaction-sequence-specific primers (PCR-SSPs). Liver recipients were stratified according to the occurrence of acute rejection. Patients without acute rejection were found to have a lower frequency of the HLA-Cw*06 allele compared with those with acute rejection or the control group. Moreover, when the role of HLA-C dimorphism was analyzed, natural killer (NK)1-alloantigens were found to be predominant in recipients without acute rejection. When the match of HLA-C single alleles and NK-alloantigens between donor and recipient was analyzed, it appeared that the frequency of acute rejection gradually decreased with decrease of the number of allele mismatches. Graft survival was increased when the number of mismatches in both HLA-C or NK-alloantigens was lower. In conclusion, the HLA-C locus may play a role in liver graft alloreactivity or allotolerance and, therefore, may be useful to avoid acute rejection and to achieve graft acceptance, resulting in a better final outcome in liver transplantation.

The role of Tec family kinases in T cell development and function

Three members of the Tec family kinases, Itk, Rlk and Tec, have been implicated in signaling downstream of the T cell receptor (TCR). The activity of these kinases in T cells has been shown to be important for the full activation of phospholipase C-gamma1 (PLC-gamma1). Disruption of Tec family signaling in Itk-/- and Rlk-/-Itk-/- mice has multiple effects on T cell development, cytokine production and T-helper cell differentiation. Furthermore, mice possessing mutations in signaling molecules upstream of PLC-gamma1, such as Src homology 2 (SH2) domain-containing phosphoprotein of 76 kDa (SLP-76), linker for activation of T cells (LAT) and Vav1, or in members of the nuclear factor for activated T cells (NFAT) family of transcription factors, which are downstream of PLC-gamma1, have been found to have similar phenotypes to Tec family-deficient mice, emphasizing the importance of this pathway in regulating T cell activation, differentiation and homeostasis.

Lysis of salmonella typhi intracellularly infected U937 cells by human natural killer cells: effect of protein kinase inhibitors

We examined the effect of Salmonella typhi (wild-type Ty2 and mutant strain TYT1231)-infected U937 cells on natural killer cell (NKC) cytotoxicity of peripheral blood mononuclear cells (PBMCs) and highly purified NKC (HPNKCs; CD16(+)/CD56(+) > 95%; the rest corresponding to CD3(+) T cells). We also analyzed the possible role of various protein kinases involved in natural cytotoxicity on these processes. PBMC cytotoxicity against S typhi-infected U937 cells was significantly higher (paired Student t test; P < 0.05) than its lytic effect against noninfected cells (control) at the various effector-to-target cell ratios used (30:1 [24.4 +/- 9.7, 25.1 +/- 11.8, and 17.5 +/- 8.6]; 50:1 [26.6 +/- 9.7, 26.7 +/- 12.8, and 21.2 +/- 7.5] and 70:1 [32.4 +/- 14.4, 30.1 +/- 12.4, and 23.1 +/- 7.2], respectively). PBMC NKC activity seemed to be dependent on such ratios and was similar against both Salmonella strains studied. Approximately half of the individual samples tested (n = 12; 8 male and 4 female subjects of comparable age) showed at least a 20% specific lysis increase against their own control; essentially no changes or smaller increases in NKC activity were observed in all other samples. Similar results were obtained using HPNKCs as effector cells (5:1 ratio [38.9 +/- 12.3, 43.3 +/- 11.2, and 27.5 +/- 4.9] and 10:1 ratio [51.3 +/- 9.1, 46.1 +/- 9.8, and 37.7 +/- 15.5, respectively]). In general, specimens significantly lysed after incubation with PBMCs responded in a similar manner to a challenge with HPNKCs. PBMC and HPNKC cytotoxicity against S typhi wild-type-infected U937 cells was significantly decreased in a dose-dependent manner by the addition of genistein (50-200 micromol) or GFX (0.5-2.0 micromol) to the cytotoxicity assay mixture. NKC activity was almost completely inhibited at the highest genistein and GFX concentrations. In similar experiments, wortmannin (100-500 nmol) failed to inhibit PBMC cytotoxicity and significantly decreased HPNKC activity only at the highest concentration tested. These results show that in the process of NKC recognition and lysis of S typhi-infected U937 cells, there is not a requisite for full bacterial intracellular survival capacity and that S typhi-infected U937 cells are a significantly better target than noninfected U937 cells. NKC signaling pathways activated during the S typhi-infected U937 cell recognition and lysis process are mainly protein tyrosine kinase and protein kinase-C, and they can be blocked by the same protein kinase inhibitors known to inhibit natural cytotoxicity.

Function of CMV-encoded MHC class I homologues

Homologues of MHC class I proteins have been identified in the genomes of human, murine and rat cytomegaloviruses (CMVs). Given the pivotal role of the MHC class I protein in cellular immunity, it has been postulated that the viral homologues subvert the normal antiviral immune response of the host, thus promoting virus replication and dissemination in an otherwise hostile environment. This review focuses on recent studies of the CMV MHC class I homologues at the molecular, cellular and whole animal level and presents current hypotheses for their roles in the CMV life cycle.

SH2-containing inositol phosphatase (SHIP-1) transiently translocates to raft domains and modulates CD16-mediated cytotoxicity in human NK cells

Membrane recruitment of the SH2-containing 5' inositol phosphatase 1 (SHIP-1) is responsible for the inhibitory signals that modulate phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathways. Here we have investigated the molecular mechanisms underlying SHIP-1 activation and its role in CD16-mediated cytotoxicity. We initially demonstrated that a substantial fraction of SHIP-1-mediated 5' inositol phosphatase activity associates with CD16 zeta chain after receptor cross-linking. Moreover, CD16 stimulation on human primary natural killer (NK) cells induces the rapid and transient translocation of SHIP-1 in the lipid-enriched plasma membrane microdomains, termed rafts, where it associates with tyrosine-phosphorylated zeta chain and shc adaptor protein. As evaluated by confocal microscopy, CD16 engagement by reverse antibody-dependent cellular cytotoxicity (ADCC) rapidly induces SHIP-1 redistribution toward the area of NK cell contact with target cells and its codistribution with aggregated rafts where CD16 receptor also colocalizes. The functional role of SHIP-1 in the modulation of CD16-induced cytotoxicity was explored in NK cells infected with recombinant vaccinia viruses encoding wild-type or catalytic domain-deleted mutant SHIP-1. We found a significant SHIP-1-mediated decrease of CD16-induced cytotoxicity that is strictly dependent on its catalytic activity. These data demonstrate that CD16 engagement on NK cells induces membrane targeting and activation of SHIP-1, which acts as negative regulator of ADCC function.

Natural killer cell activation in mice and men: different triggers for similar weapons?

The signaling pathways that regulate B and T lymphocytes are remarkably conserved between humans and mice. However, recent evidence suggests that the pathways regulating natural killer (NK) cell activation may actually differ between these two species. We discuss the controversies in the field and propose that this divergence could be deceptive: despite some clear differences between human and mouse NK cell receptors, the many ways of activating NK cells and their functions may well be conserved.

Native human blood dendritic cells as potent effectors in antibody-dependent cellular cytotoxicity

Functional studies on native human dendritic cells (DCs) are hampered by technical difficulties in preparing fresh DCs. Recently, with the help of the monoclonal antibody M-DC8, we succeeded in isolating a major subpopulation of human blood DCs by a one-step immunomagnetic separation procedure. These cells strongly express FcγRIII (CD16) and FcγRII (CD32) and are quite efficient in the antigen-specific activation of naive T cells. Because some Fcγ receptor-bearing cell types are known as effector cells in antibody-dependent cellular cytotoxicity (ADCC), we investigated whether M-DC8+ DCs are capable of effectuating ADCC. In this report we show that freshly prepared M-DC8+ DCs efficiently mediate tumor-directed ADCC and that both types of Fcγ receptors as well as tumor necrosis factor α essentially contribute to the cytotoxic activity. The results provide evidence that, in addition to their pivotal role in primary T-cell activation, a subset of blood DCs displays efficient cytotoxicity in ADCC.

Src-dependent Syk activation controls CD69-mediated signaling and function on human NK cells

CD69 C-type lectin receptor represents a functional triggering molecule on activated NK cells, capable of directing their natural killing function. The receptor-proximal signaling pathways activated by CD69 cross-linking and involved in CD69-mediated cytotoxic activity are still poorly understood. Here we show that CD69 engagement leads to the rapid and selective activation of the tyrosine kinase Syk, but not of the closely related member of the same family, ZAP70, in IL-2-activated human NK cells. Our results indicate the requirement for Src family kinases in the CD69-triggered activation of Syk and suggest a role for Lck in this event. We also demonstrate that Syk and Src family tyrosine kinases control the CD69-triggered tyrosine phosphorylation and activation of phospholipase Cgamma2 and the Rho family-specific exchange factor Vav1 and are responsible for CD69-triggered cytotoxicity of activated NK cells. The same CD69-activated signaling pathways are also observed in an RBL transfectant clone, constitutively expressing the receptor. These data demonstrate for the first time that the CD69 receptor functionally couples to the activation of Src family tyrosine kinases, which, by inducing Syk activation, initiate downstream signaling pathways and regulate CD69-triggered functions on human NK cells.

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.

Interleukin-2 and interleukin-15: immunotherapy for cancer

Interleukin (IL)-2 and IL-15 are two cytokine growth factors that regulate lymphocyte function and homeostasis. Early clinical interest in the use of IL-2 in the immunotherapy of renal cell carcinoma and malignant melanoma demonstrated the first efficacy for cytokine monotherapy in the treatment of neoplastic disease. Advances in our understanding of the cellular and molecular biology of IL-2 and its receptor complex have provided rationale to better utilize IL-2 to expand and activate immune effectors in patients with cancer. Exciting new developments in monoclonal antibodies recognizing tumor targets and tumor vaccines have provided new avenues to combine with IL-2 therapy in cancer patients. IL-15, initially thought to mediate similar biological effects as IL-2, has been shown to have unique properties in basic and pre-clinical studies that may be of benefit in the immunotherapy of cancer. This review first summarizes the differences between IL-2 and IL-15 and highlights that better understanding of normal physiology creates new ideas for the immunotherapy of cancer. The application of high, intermediate, and low/ultra low dose IL-2 therapy in clinical trials of cancer patients is discussed, along with new avenues for its use in neoplastic diseases. The growing basic and pre-clinical evidence demonstrating that IL-15 may be useful in immunotherapy approaches to cancer is also presented.

Differential expression of inhibitory or activating CD94/NKG2 subtypes on MART-1-reactive T cells in vitiligo versus melanoma: a case report

Selection and activation of T cells is tightly regulated by both antigen-specific receptors and co-receptors to ensure that responses to self antigens are largely avoided. By T cell receptor clonotypic mapping and staining with tetrameric HLA-peptide complexes, we demonstrate the presence of melanocyte differentiation antigen MART-1 specific T cells in the areas of destruction of both neoplastic and normal melanocytic cells in a case of a primary melanoma and its associated hypopigmentation. These self reactive T cells expressed CD94/NKG2 major histocompatibility complex class I specific C-type lectin-like receptors. This family of receptors includes both activating and inhibitory isoforms. Thus, we performed a detailed analysis that revealed the exclusive presence of inhibitory NKG2-A/B receptors in the vitiligo-like leukoderma, whereas both the inhibitory receptors and the activating NKG2-C/E isoforms were present within the tumor. Our data suggest the differential expression of killer inhibitory receptors as a possible mechanism to regulate T cell responses to self antigens.

The Pmed1 gene promoter of human Fc gamma RIIIA can function as a NK/T cell-specific restriction element, which involves binding of Sp1 transcription factor

The low-affinity receptor for IgG (human FcgammaRIIIA) is selectively expressed by a subset of T lymphocytes, NK cells, and macrophages. To understand the mechanisms underlying this pattern of cell type-specific expression, we initially identified alternative promoters, Pmed1/2 and Pprox, in the 5' end of the FcgammaRIIIA gene. In this study, we focused on the Pmed1 promoter and demonstrated this 93-bp region to be highly specific in governing restriction to NK/T cell lines. This property of Pmed1 is context independent and can extend to a disparate promoter. Deletion analysis defined a contribution of two separate elements located to the 5' 21-bp (-942/-922) and 3' 72-bp (-921/-850) regions of Pmed1 in conferring NK/T cell specificity. The 5' part of Pmed1 contains binding sites for Sp1 and NK element-recognizing factors and substitution mapping studies revealed a critical requirement of the Sp1-I site. The importance of Sp1 protein to regulate maximal Pmed1 promoter activity was further established by EMSAs and cotransfection experiments in Sp1-null Drosophila SL2 cells. Our data suggest that Sp1 can contribute, in part, to NK/T cell restriction and further indicate that the FcgammaRIIIA Pmed1 sequence might be useful to direct the NK/T cell-specific expression of heterologous genes.

Upregulation of natural killer cells functions underlies the efficacy of intratumorally injected dendritic cells engineered to produce interleukin-12

OBJECTIVE

Injection of dendritic cells (DC) engineered with recombinant adenoviral vectors to produce interleukin-12 (IL-12) inside experimental murine tumors frequently achieves complete regressions. In such a system the function of CD8(+) T cells has been shown to be an absolute requirement, in contrast to observations made upon depletion of CD4(+) T cells, which minimally affected the outcome. The aim of this work was to study the possible involvement of natural killer (NK) cells in this setting.

MATERIALS, METHODS, AND RESULTS

Depletions with anti-AsialoGM1 antiserum showed only a small decrease in the proportion of complete regressions obtained that correlated with induction of NK activities in lymphatic tissues into which DC migrate, whereas combined depletions of CD4(+) and NK cells completely eliminated the antitumor effects. Likewise in vivo neutralization of interferon-gamma (IFN-gamma) also eliminated those therapeutic effects. Trying to define the cellular role played by NK cells in vivo, it was observed that injection of cultured DC inside the spleen of T- and B-cell-deficient (Rag1(-/-)) mice induced upregulation of NK activity only if DC had been adenovirally engineered to produce IL-12. In addition, identically transfected fibroblasts also activated NK cells, indicating that IL-12 transfection was the unique requirement. Equivalent human DC only activated in vitro the cytolytic and cytokine-secreting functions of autologous NK cells if transfected to express human IL-12.

CONCLUSIONS

Overall, these results point out an important role played by NK cell activation in the potent immunotherapeutic effects elicited by intratumoral injection of IL-12--secreting DC and that NK activation under these conditions is mainly, if not only, dependent on IL-12.

Structure and function of major histocompatibility complex (MHC) class I specific receptors expressed on human natural killer (NK) cells

Natural killer (NK) cells express receptors that are specific for MHC class I molecules. These receptors play a crucial role in regulating the lytic and cytokine expression capabilities of NK cells. In humans, three distinct families of genes have been defined that encode for receptors of HLA class I molecules. The first family identified consists of type I transmembrane molecules belonging to the immunoglobulin (Ig) superfamily and are called killer cell Ig-like receptors (KIR). A second group of receptors belonging to the Ig superfamily, named ILT (for immunoglobulin like transcripts), has more recently been described. ILTs are expressed mainly on B, T and myeloid cells, but some members of this group are also expressed on NK cells. They are also referred to as LIRs (for leukocyte Ig-like receptor) and MIRs (for macrophage Ig-like receptor). The ligands for the KIR and some of the ILT receptors include classical (class Ia) HLA class I molecules, as well as the nonclassical (class Ib) HLA-G molecule. The third family of HLA class I receptors are C-type lectin family members and are composed of heterodimers of CD94 covalently associated with a member of the NKG2 family of molecules. The ligand for most members is the nonclassical class I molecule HLA-E. NKG2D, a member of the NKG2 family, is expressed as a homodimer, along with the adaptor molecule DAP10. The ligands of NKG2D include the human class I like molecules MICA and MICB, and the recently described ULBPs. Each of these three families of receptors has individual members that can recognize identical or similar ligands yet signal for activation or inhibition of cellular functions. This dichotomy correlates with particular structural features present in the transmembrane and intracytoplasmic portions of these molecules. In this review we will discuss the molecular structure, specificity, cellular expression patterns, and function of these HLA class I receptors, as well as the chromosomal location and genetic organization.

NK cell activation by dendritic cells is dependent on LFA-1-mediated induction of calcium-calmodulin kinase II: inhibition by HIV-1 Tat C-terminal domain

In this study, we show that binding to autologous dendritic cells (DC) induces a calcium influx in NK cells, followed by activation of the calcium-calmodulin kinase II (CAMKII), release of perforin and granzymes, and IFN-gamma secretion. CAMKII is induced via LFA-1: indeed, oligomerization of LFA-1 leads to CAMKII induction in NK cells. Moreover, release of lytic enzymes and cytotoxic activity is strongly reduced by masking LFA-1 or by adding CAMKII inhibitors such as KN62 and KN93, at variance with the inactive compound KN92. NK cell-mediated lysis of DC and IFN-gamma release by NK cells upon NK/DC contact are inhibited by exogenous HIV-1 Tat: the protein blocks calcium influx and impairs CAMKII activation elicited via LFA-1 in NK cells, eventually inhibiting degranulation. Experiments performed with synthetic, overlapping Tat-derived peptides showed that the C-terminal domain of the protein is responsible for inhibition. Finally, both KN62 and Tat reduced the extension of NK/DC contacts, possibly affecting NK cell granule polarization toward the target. These data provide evidence that exogenous Tat inhibits NK cell activation occurring upon contact with DC: this mechanism might contribute to the impairment of natural immunity in HIV-1 infection.

A role for a RhoA/ROCK/LIM-kinase pathway in the regulation of cytotoxic lymphocytes

Polarization of lipid rafts and granules to the site of target contact is required for the development of cell-mediated killing by cytotoxic lymphocytes. We have previously shown that these events require the activation of proximal protein tyrosine kinases. However, the downstream intracellular signaling molecules involved in the development of cell-mediated cytotoxicity remain poorly defined. We report here that a RhoA/ROCK/LIM-kinase axis couples the receptor-initiated protein tyrosine kinase activation to the reorganization of the actin cytoskeleton required for the polarization of lipid rafts and the subsequent generation of cell-mediated cytotoxicity. Pharmacologic and genetic interruption of any element of this RhoA/ROCK/LIM-kinase pathway inhibits both the accumulation of F-actin and lipid raft polarization to the site of target contact and the subsequent delivery of the lethal hit. These data define a specialized role for a RhoA-->ROCK-->LIM-kinase pathway in cytotoxic lymphocyte activation.

The biology of human natural killer-cell subsets

Human natural killer (NK) cells comprise approximately 15% of all circulating lymphocytes. Owing to their early production of cytokines and chemokines, and ability to lyse target cells without prior sensitization, NK cells are crucial components of the innate immune system. Human NK cells can be divided into two subsets based on their cell-surface density of CD56--CD56(bright) and CD56(dim)--each with distinct phenotypic properties. Now, there is ample evidence to suggest that these NK-cell subsets have unique functional attributes and, therefore, distinct roles in the human immune response. The CD56(dim) NK-cell subset is more naturally cytotoxic and expresses higher levels of Ig-like NK receptors and FCgamma receptor III (CD16) than the CD56(bright) NK-cell subset. By contrast, the CD56(bright) subset has the capacity to produce abundant cytokines following activation of monocytes, but has low natural cytotoxicity and is CD16(dim) or CD16(-). In addition, we will discuss other cell-surface receptors expressed differentially by human NK-cell subsets and the distinct functional properties of these subsets.

Interleukin-12- and gamma interferon-dependent innate immunity are essential and sufficient for long-term survival of passively immunized mice infected with herpes simplex virus type 1

Interferon (IFN) type I (alpha/beta IFN [IFN-alpha/beta]) is very important in directly controlling herpes simplex virus type I (HSV-1) replication as well as in guiding and upregulating specific immunity against this virus. By contrast, the roles of IFN type II (IFN-gamma) and antibodies in the defense against HSV-1 are not clear. Mice without a functional IFN system and no mature B and T cells (AGR mice) did not survive HSV-1 infection in the presence or absence of neutralizing antibodies to the virus. Mice without a functional IFN type I system and with no mature B and T cells (AR129 mice) were unable to control infection with as little as 10 PFU of HSV-1 strain F. By contrast, in the presence of passively administered neutralizing murine antibodies to HSV-1, some AR129 mice survived infection with up to 10(4) PFU of HSV-1. This acute immune response was dependent on the presence of interleukin-12 (IL-12) p75. Interestingly, some virus-infected mice stayed healthy for several months, at which time antibody to HSV-1 was no longer detectable. Treatment of these virus-exposed mice with dexamethasone led to death in approximately 40% of the mice. HSV-1 was found in brains of mice that did not survive dexamethasone treatment, whereas HSV-1 was absent in those that survived the treatment. We conclude that in the presence of passively administered HSV-1-specific antibodies, the IL-12-induced IFN-gamma-dependent innate immune response is able to control low doses of virus infection. Surprisingly, in a significant proportion of these mice, HSV-1 appears to persist in the absence of antibodies and specific immunity.

Cellular localization and functional role of phosphatidylcholine-specific phospholipase C in NK cells

Although several classes of phospholipases have been implicated in NK cell-mediated cytotoxicity, no evidence has been reported to date on involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) in NK activation by lymphokines and/or in lytic granule exocytosis. This study demonstrated the expression of two PC-PLC isoforms (M(r) 40 and 66 kDa) and their IL-2-dependent distribution between cytoplasm and ectoplasmic membrane surface in human NK cells. Following cell activation by IL-2, cytoplasmic PC-PLC translocated from the microtubule-organizing center toward cell periphery, essentially by kinesin-supported transport along microtubules, while PC-PLC exposed on the outer cell surface increased 2-fold. Preincubation of NK cells with a PC-PLC inhibitor, tricyclodecan-9-yl-xanthogenate, strongly reduced NK-mediated cytotoxicity. In IL-2-activated cells, this loss of cytotoxicity was associated with a decrease of PC-PLC exposed on the cell surface, and accumulation of cytoplasmic PC-PLC in the Golgi region. Massive colocalization of PC-PLC-rich particles with perforin-containing granules was found in the cytoplasm of NK-activated (but not NK-resting) cells; both organelles clustered at the intercellular contact region of effector-target cell conjugates. These newly detected mechanisms of PC-PLC translocation and function support an essential role of this enzyme in regulated granule exocytosis and NK-mediated cytotoxicity.

Ubiquitination and degradation of Syk and ZAP-70 protein tyrosine kinases in human NK cells upon CD16 engagement

Syk and ZAP-70 nonreceptor protein tyrosine kinases (PTKs) are essential elements in several cascades coupling immune receptors to intracellular responses. The critical role of these kinases in promoting the propagation of intracellular signaling requires a tight regulation of their activity, thus the existence of a negative feedback loop regulating their expression can be hypothesized. Herein, we have investigated whether ubiquitin-dependent proteolysis could be a mechanism responsible for controlling the fate of Syk and ZAP-70 after their immunoreceptor-induced activation. We found that both Syk and ZAP-70 become ubiquitinated in response to aggregation of the low affinity Fc receptor for IgG (CD16) on human natural killer cells. We confirmed the identity of the major in vivo ubiquitinated kinase species by performing an in vitro ubiquitination assay. In addition, we found that after CD16 stimulation, ubiquitinated forms of Syk and ZAP-70 associate with the receptor complex. After CD16 engagement, we also observed a decrease in the stability of Syk and ZAP-70 PTKs that is counteracted by pretreatment with either proteasome or lysosomal inhibitors. Moreover, in the presence of the proteasome inhibitor, epoxomicin, we observed an accumulation of ubiquitinated forms of both kinases. Our findings provide evidence of ligand-induced ubiquitination of nonreceptor PTKs belonging to the Syk family and propose the ubiquitin-dependent proteasome-mediated degradation pathway as a mechanism for attenuating the propagation of intracellular signaling initiated by immune receptor engagement.

Evolution of natural killer cell receptors: coexistence of functional Ly49 and KIR genes in baboons

Natural killer (NK) cells represent an important first line of defense against viruses and malignancy [1]. NK cells express a variety of inhibitory and activating receptors that interact with classical major histocompatibility complex (MHC) class I molecules on potential target cells and determine the NK cell response [2-4]. Mouse NK receptors are encoded by the C-type lectin multigene family Ly49. However, in humans, a completely different family of receptors, the immunoglobulin-like killer inhibitory receptors (KIRs), performs the same function [2-4]. One Ly49-like gene, Ly49L, exists in humans but is incorrectly spliced and assumed to be nonfunctional [5, 6]. Mouse KIR-like genes have not been found, and evidence suggests that the primate KIRs amplified after rodents and primates diverged [7, 8]. Thus, two structurally dissimilar families, Ly49 and KIR, have evolved to play similar roles in mouse and human NK cells. This apparent example of functional convergent evolution raises several questions. It is unknown, for example, when the Ly49L gene became nonfunctional and if this event affected the functional evolution of the KIRs. The distribution of these gene families in different mammals is unstudied, and it is not known if any species uses both types of receptors. Here, we demonstrate that the Ly49L gene shows evidence of conservation in other mammals and that the human gene likely became nonfunctional 6-10 million years ago. Furthermore, we show that baboon lymphocytes express both full-length Ly49L transcripts and multiple KIR genes.

Regulation of the natural killer cell receptor repertoire

Natural killer cells express inhibitory receptors specific for MHC class I proteins and stimulatory receptors with diverse specificities. The MHC-specific receptors discriminate among different MHC class I alleles and are expressed in a variegated, overlapping fashion, such that each NK cell expresses several inhibitory and stimulatory receptors. Evidence suggests that individual developing NK cells initiate expression of inhibitory receptor genes in a sequential, cumulative, and stochastic fashion. Superimposed on the receptor acquisition process are multiple education mechanisms, which act to coordinate the stimulatory and inhibitory specificities of developing NK cells. One process influences the complement of receptors expressed by individual NK cells. Other mechanisms may prevent NK cell autoaggression even when the developing NK cell fails to express self-MHC-specific inhibitory receptors. Together, these mechanisms ensure a self-tolerant and maximally discriminating NK cell population. Like NK cells, a fraction of memory phenotype CD8(+) T cells, as well as other T cell subsets, express inhibitory class I--specific receptors in a variegated, overlapping fashion. The characteristics of these cells suggest that inhibitory receptor expression may be a response to prior antigenic stimulation as well as to poorly defined additional signals. A unifying hypothesis is that both NK cells and certain T cell subsets initiate expression of inhibitory receptors in response to stimulation.

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 nitric oxide

Natural killer (NK) cells and nitric oxide (NO) are both important components of the natural or innate immune response. NK cells are large granular lymphocytes capable of destroying cells infected by virus or bacteria and susceptible tumor cells without prior sensitization and restriction by MHC antigens. They are abundant in blood, spleen, liver and lungs and are distinct from both T and B lymphocytes in their circulation patterns, profile of surface antigens, receptor repertoire and the way in which they discriminate between self and non-self. Uniquely, NK cells express receptors that can recognize and discriminate between normal and altered MHC class I determinants. NK cell cytotoxic activity is strongly induced by cytokines such as IL-2 and IL-12, and this activation is associated with synthesis of NO. Inhibitors of NO synthesis impair NK cell-mediated target cell killing, demonstrating a role for NO in NK cell function. Furthermore, NO itself can regulate NK cell activation. In this article, evidence that NO is a mediator of NK cell-mediated target cell killing, and that NO is a regulator of NK cell activation will be reviewed. Results of NO synthase gene deletion studies will be discussed, and rodent and human NK cells will be compared.

Molecular cloning of rat NK target structure--the possibility of CD44 involvement in NK cell-mediated lysis

The nature of target molecules of natural killer (NK) cell-mediated lysis remains to be elucidated. As we previously reported, mAb 109 recognizes one of the tumor-associated antigens, designated as 109 antigen (Ag), expressed on the cell surface of rat fibrosarcomas W31 and W14, which are transformants of WFB (rat fetal fibroblast cell line) with H-ras oncogene. 109Ag was thought to be a target structure of NK cells since mAb 109 inhibited NK cell-mediated lysis against W31 and W14. Here, we demonstrate by molecular cloning that 109Ag is identical to rat CD44. Immunoprecipitation and immunoblotting studies also showed that mAb 109 and anti-rat CD44 mAb OX-50 recognize the same protein of W31 cell lysates with an 86 kDa molecular size. CD44 was suggested to be a target structure of NK cell-mediated lysis; however, rat CD44 cDNA transfection alone into CD44 null cell lines did not result in up-regulation of target cell susceptibility to NK cell-mediated lysis. Our results therefore indicated that CD44 may play a crucial role as one of the target structures in our rat fibrosarcoma system though the cell surface expression of CD44 alone does not affect NK susceptibility of the target cells.

Functional dichotomy in natural killer cell signaling: Vav1-dependent and -independent mechanisms

The product of the protooncogene Vav1 participates in multiple signaling pathways and is a critical regulator of antigen-receptor signaling in B and T lymphocytes, but its role during in vivo natural killer (NK) cell differentiation is not known. Here we have studied NK cell development in Vav1-/- mice and found that, in contrast to T and NK-T cells, the absolute numbers of phenotypically mature NK cells were not reduced. Vav1-/- mice produced normal amounts of interferon (IFN)-gamma in response to Listeria monocytogenes and controlled early infection but showed reduced tumor clearance in vivo. In vitro stimulation of surface receptors in Vav1-/- NK cells resulted in normal IFN-gamma production but reduced tumor cell lysis. Vav1 was found to control activation of extracellular signal-regulated kinases and exocytosis of cytotoxic granules. In contrast, conjugate formation appeared to be only mildly affected, and calcium mobilization was normal in Vav1-/- NK cells. These results highlight fundamental differences between proximal signaling events in T and NK cells and suggest a functional dichotomy for Vav1 in NK cells: a role in cytotoxicity but not for IFN-gamma production.

Ig-binding receptors on human NK cells as effector and regulatory surface molecules

The receptors on human natural killer 9NK cells which can specifically bind the Fc portion of immunoglobulin molecules (Fc receptors) have been extensively studied. The best known and studied Fc receptor on human NK cells is FcgammaRIIIa. Interactions of NK cells with IgG antibodies via this receptor are well known to induce a signal transduction cascade and lead to antibody-dependent cell-mediated cytotoxicity (ADCC) as well as release of various cytokines. In addition, interactions with monomeric IgG and FcgammaRIIIa have been demonstrated, which result in negative regulation of NK activity and other immunomodulatory effects. Over the past several years, it has also become increasingly appreciated that human NK cells express a variety of other Fc receptors, including FcmuR, which also can mediate effector and immunoregulatory functions. Also, a novel form of FcgammaR has been demonstrated on human NK cells, termed FcgammaRIIc. Recent molecular studies have shown considerable polymorphism in the genes for FcgammaIIc and the functional consequences are being dissected. This appears to include cross-talk between FcgammaRIIIa and at least some forms of FcgammaRIIc, which may have important functional consequences.

Human natural killer cell receptors and signal transduction

Natural killer (NK) cells express numerous receptors, which continually engage with ligands on cell surfaces. Until 1995, only a handful of these receptors were characterized and the molecular basis of NK cell activation was obscure. Recently, considerable advances have been made in characterizing the receptor repertoire on human NK cells. Both activating and inhibitory receptors can transduce positive or negative signals to regulate NK cell cytotoxicity and cytokine release responses. The inhibitory receptors normally predominate in this balance of signals. Certain tumor cells and virally infected cells that lack major histocompatibility complex (MHC) class I molecules, however, can rapidly trigger NK cell activation. The basis of this activation is the loss of negative signals that are normally transmitted by MHC class I-binding inhibitory receptors, and the corresponding domination of activating receptor signals. While ligand specificity for a number of the recently described receptors is still a mystery, their signal transduction properties have begun to be defined. The dynamic crosstalk between these receptors ultimately governs the NK cell activation state. Although the complexities of NK cell signalling are only marginally understood, several overall themes have been defined by characterizing the roles of distinct pathways during NK cell responses.

Human natural killer cells: a unique innate immunoregulatory role for the CD56(bright) subset

During the innate immune response to infection, monocyte-derived cytokines (monokines), stimulate natural killer (NK) cells to produce immunoregulatory cytokines that are important to the host's early defense. Human NK cell subsets can be distinguished by CD56 surface density expression (ie, CD56(bright) and CD56(dim)). In this report, it is shown that CD56(bright) NK cells produce significantly greater levels of interferon-gamma, tumor necrosis factor-beta, granulocyte macrophage-colony-stimulating factor, IL-10, and IL-13 protein in response to monokine stimulation than do CD56(dim) NK cells, which produce negligible amounts of these cytokines. Further, qualitative differences in CD56(bright) NK-derived cytokines are shown to be dependent on the specific monokines present. For example, the monokine IL-15 appears to be required for type 2 cytokine production by CD56(bright) NK cells. It is proposed that human CD56(bright) NK cells have a unique functional role in the innate immune response as the primary source of NK cell-derived immunoregulatory cytokines, regulated in part by differential monokine production.

Activation of c-Jun N-terminal kinase in the absence of NFkappaB function prior to induction of NK cell death triggered by a combination of anti-class I and anti-CD16 antibodies

Addition of antibodies to major histocompatibility complex class I (MHC class I) and F(c) gamma RIII (CD16) antigens resulted in the synergistic augmentation of natural killer (NK) cell death, and the loss of NK cell cytotoxic function. The binding of anti-CD16 and anti-class I antibodies to the same population of NK cells is required for the synergistic augmentation of NK cell death. Moreover, the addition of antibodies to leukocyte function antigen-1 (LFA-1), which significantly inhibited the phorbol 12-myristate 13-acetate (PMA) and ionomycin mediated NK cell death, had no effect on NK cell death mediated by anti-CD16 and anti-class I antibodies. The increase in NK cell death was associated with an increase in tumor necrosis factor-alpha (TNF-alpha) secretion, and concomitant inhibition of nuclear factor kappa B (NFkappaB) activation and the induction of c-jun N-terminal kinase (JNK) activity in NK cells treated with the combination of anti-class I and anti-CD16 antibodies. Furthermore, the inhibition of NFkappaB activation in anti-CD16 and anti-class I antibody treated NK cells was paralleled with a significant increase in inhibitor of kappa B (IkappaB) protein expression. Overexpression of IkappaB super-repressor in YT, a NK cell line, caused significant up-regulation of TNF-alpha, PMA and ionomycin and Fusobacterium nucleatum mediated NK cell death. Overall, our studies suggest an important regulatory role for NFkappaB and JNK activities in MHC class I mediated NK cell death.

Negative regulation of expression and function of Fc gamma RIII by CD3 zeta in murine NK cells

Fc gamma RIII is involved in Ab-dependent cell-mediated cytotoxicity (ADCC) and cytokine production by NK cells. Signaling and expression of Fc gamma RIII are dependent on FcR gamma. Although NK cells express not only FcR gamma but also CD3 zeta, the role of CD3 zeta in NK cell function remains unclear. Here, we found that the expression of Fc gamma RIII on NK cells from CD3 zeta-deficient mice is unexpectedly up-regulated compared with that on cells from normal mice. Furthermore, ADCC and IFN-gamma production upon Fc gamma RIII-cross-linking by NK cells from CD3 zeta-deficient mice were also up-regulated. Up-regulation of the surface expression of Fc gamma RIII on CD3 zeta-deficient NK cells is not mediated by transcriptional augmentation of either Fc gamma RIII or FcR gamma gene because there was no significant difference in the expression of mRNA for Fc gamma RIII and FcR gamma. Transfection of CD3 zeta into a cell line expressing Fc gamma RIII and FcR gamma induced a decrease in the cell surface expression of Fc gamma RIII. These findings reveal a negative regulatory role of CD3 zeta in Fc gamma RIII-mediated function of murine NK cells.

Regulation of p38 mitogen-activated protein kinase during NK cell activation

The mitogen-activated protein kinase (MAPK) p38 modulates a variety of cellular functions, including proliferation, differentiation and cell death. However, we report here a novel function for p38, i.e. the regulation of cytotoxic lymphocyte-mediated cytotoxicity. Stimulation of NK cells by either cross-linking of their FcgammaRIII receptors or by binding to NK-sensitive target cells induces the phosphorylation and activation of p38, and also of its upstream regulators MKK3/MKK6. Pharmacologic analyses suggest that Src-family and Syk-family protein tyrosine kinases couple the NK cell surface receptors to p38 activation. The role of p38 in the cytotoxic function of NK cells was tested by treatment of NK cells with the cell-permeable, p38-specific inhibitor SB203580. Interestingly, exposure to the drug reduced both antibody-dependent cellular cytotoxicity and natural cytotoxicity, but maximal inhibitory concentrations resulted in only partial inhibition. Collectively, these results suggest that the p38 MAPK pathway is stimulated during the development of NK cell-mediated cytotoxicity and that efficient killing is influenced by both p38-dependent and p38-independent pathways. More broadly, this study identifies the regulation of cell-mediated killing as a novel role for p38 in cytotoxic lymphocytes.

Roles of the SHP-1 tyrosine phosphatase in the negative regulation of cell signalling

The critical role for the SH2 domain-containing SHP-1 tyrosine phosphatase in regulating haemopoietic cell behaviour was initially revealed by data linking SHP-1 deficiency to the systemic autoimmunity and severe inflammation exhibited by motheaten mice. This discovery laid the groundwork for the identification of SHP-1 as an inhibitor of activation-promoting signalling cascades and for the coincident demonstration that protein tyrosine phosphatases (PTPs) such as SHP-1 show considerable specificity with respect to the mechanisms whereby they modulate the biochemical and biological sequelae of extracellular simulation. As outlined in this review, SHP-1 has now been implicated in the regulation of a myriad of signalling cascades and cell functions. As a result, the cumulative data generated from studies of this PTP have elucidated not only the functional relevance of SHP-1, but also a number of novel paradigms as to the molecular mechanisms whereby signalling cascades are regulated so as to either augment or abrogate specific cell behaviours.

Expression of the CD80 and CD86 molecules enhances cytotoxicity by human natural killer cells

In contrast to the inhibitory pathway of NK cell regulation, much less is known about stimulatory or activation signals in NK cells. Both CD80 and CD86 function as costimulatory molecules in T-cell cytotoxicity. Several previous reports, most of them in the murine system, have indirectly or directly indicated the possible role of B7 molecules (CD80 and CD86) triggering NK cell-mediated cytotoxicity in vitro. Nevertheless, only little is known about the role of these molecules on human target cells. Therefore, anti-CD80 and anti-CD86 mAbs were used in blocking experiments and both were shown to inhibit lysis by human NK cells. The degree of inhibition observed was variable. 64% of these NK clones were strongly inhibited by both anti-CD80 and anti-CD86 (Type 1). A small number (19%) were only moderately inhibited by both of these antibodies (Type 2), and 17% of these NK clones were inhibited strongly by anti-CD86 but weakly or not at all by anti-CD80 (Type 3). To further examine the importance of these proteins, B7.1 (CD80) and B7.2 (CD86) genes were transfected into the mouse mastocytoma P815 cell line that could not be killed by the human NK cells. These transfectant cell lines were then tested in cytotoxicity assays using a number of human NK lines. Expression of the CD80 and CD86 molecules resulted in enhanced lysis of P815 by most of the NK lines tested. Thus, both CD80 and CD86 molecules are involved in triggering of human NK cells.

The tyrosine kinase PYK-2/RAFTK regulates natural killer (NK) cell cytotoxic response, and is translocated and activated upon specific target cell recognition and killing

The compartmentalization of plasma membrane proteins has a key role in regulation of lymphocyte activation and development of immunity. We found that the proline-rich tyrosine kinase-2 (PYK-2/RAFTK) colocalized with the microtubule-organizing center (MTOC) at the trailing edge of migrating natural killer (NK) cells. When polyclonal NK cells bound to K562 targets, PYK-2 translocated to the area of NK-target cell interaction. The specificity of this process was assessed with NK cell clones bearing activatory or inhibitory forms of CD94/NKG2. The translocation of PYK-2, MTOC, and paxillin to the area of NK-target cell contact was regulated upon specific recognition of target cells through NK cell receptors, controlling target cell killing. Furthermore, parallel in vitro kinase assays showed that PYK-2 was activated in response to signals that specifically triggered its translocation and NK cell mediated cytotoxicity. The overexpression of both the wt and a dominant-negative mutant of PYK-2, but not ZAP-70 wt, prevented the specific translocation of the MTOC and paxillin, and blocked the cytotoxic response of NK cells. Our data indicate that subcellular compartmentalization of PYK-2 correlates with effective signal transduction. Furthermore, they also suggest an important role for PYK-2 on the assembly of the signaling complexes that regulate the cytotoxic response.

Self-MHC class Ia (RT1-A(n)) protects cells co-expressing the activatory allogeneic MHC class Ib molecule (RT1-E(u)) from NK lysis

We have previously shown activation of NK cells via recognition of an allogeneic, non-classical MHC class I molecule, RT1-E(u). In this study we investigated whether a self-MHC class I molecule could protect the allogeneic targets from being recognized and killed by the alloreactive NK (allo NK) cells. NK cells from BN (RT1 n) rats, primed in vivo by immunization with RT1(u)-expressing cells, manifested cytolytic activity against RT1(u)- as well as RT1(u/lv1)-expressing targets, but not against RT1(u/n)-expressing targets. The absence of cytolytic activity against semiallogeneic targets, i.e. targets expressing self-allotypes, was also valid for allo NK cells from alloimmunized F344 (RT1 (lv1)) rats. To analyze the ability of a distinct MHC class I molecule to protect target cells from NK lysis, Rat2 cells transfected with the activating allogeneic MHC class Ib, RT1-E(u) molecule were also transfected with the self-MHC class Ia, RT1-A1(n) molecule. The allo NK cells from BN rats immunized with RT1(u)-expressing cells were cytolytic against Rat2 transfected with the RT1-E(u) molecule. However, the allo NK cells manifested no cytolytic activity against double-transfected Rat2 cells, expressing the RT1-E(u) as well as the RT1-A1(n) molecule. We conclude that expression of a self-MHC class Ia (RT1-A) molecule protects targets from allo NK killing. Furthermore, the NK inhibition via recognition of the self-MHC class Ia molecule dominates over the activation via recognition of the allogeneic MHC class Ib molecule, RT1-E.

NK cell recognition of non-classical HLA class I molecules

NK cells recognize several HLA class Ib molecules employing both immunoglobulin-like (Ig-like) and C-type lectin receptors. The CD94/NKG2 and NKG2D lectin-like molecules, respectively, interact with HLA-E and MICA; CD94/NKG2A functions as an inhibitory receptor, while CD94/NKG2C and NKG2D trigger NK cell activity. HLA-E predominantly presents nonamers from the leader sequences of other class I molecules; a peptide derived from HLA-G1 constitutes the highest affinity ligand for both CD94/NKG2 receptors. Members of the Ig-like transcript (ILT) or leucocyte Ig-like receptor (LIR) family (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leucocyte lineages, interact with a broad spectrum of HLA class Ia molecules and HLA-G1. Among Ig-like KIRs, the KIR2DL4 (p49) receptor has been shown to specifically recognize HLA-G1; this molecule displays an unusual hybrid structure, sharing features with inhibitory and triggering KIRs.

Signaling pathways engaged by NK cell receptors: double concerto for activating receptors, inhibitory receptors and NK cells

Despite the absence of antigen-specific receptors at their surface, NK cells can selectively eliminate virus-infected cells, tumor cells and allogenic cells. A dynamic and precisely coordinated balance between activating and inhibitory receptors governs NK cell activation programs. Multiple activating and inhibitory NK cell surface molecules have been described, a group of them acting as receptors for MHC class I molecules. In spite of their heterogeneity, activating NK cell receptors present remarkable structural and functional homologies with T cell- and B cell-antigen receptors. Inhibitory NK cell receptors operate at early stages of activating cascades by recruiting protein tyrosine phosphatases via intra- cytoplasmic motifs (ITIM), a strategy which is widely conserved in hematopoietic and non-hematopoietic cells.

Tyrosine kinase SYK: essential functions for immunoreceptor signalling

The tyrosine kinase SYK plays critical roles in signalling through immune receptors. Gene-targeting studies have identified the cell types that require SYK for development and function, and the receptors that use SYK as well as their downstream signalling effectors. There is also evidence of a role for SYK in non-immune cells and in the maintenance of vascular integrity.

Human inhibitory and activating Ig-like receptors which modulate the function of myeloid cells

There is increasing evidence that myeloid cells express several receptor families, which include both inhibitory and stimulatory isoforms. The expression of receptor isoforms with similar specificities but opposite functions on the same cell is intriguing. What might be the interplay between these receptors? Some clues to the answer to this question may come from recent studies on two myeloid receptor families: the ILT/LIR/MIR (immunoglobulin-like transcript/leukocyte Ig-like receptor/monocyte/macrophage Ig-like receptor) and the SIRP (signal-regulatory protein).

Cutting edge: functional role for proline-rich tyrosine kinase 2 in NK cell-mediated natural cytotoxicity

Protein tyrosine kinase activation is one of the first biochemical events in the signaling pathway leading to activation of NK cell cytolytic machinery. Here we investigated whether proline-rich tyrosine kinase 2 (Pyk2), the nonreceptor protein tyrosine kinase belonging to the focal adhesion kinase family, could play a role in NK cell-mediated cytotoxicity. Our results demonstrate that binding of NK cells to sensitive target cells or ligation of beta2 integrins results in a rapid induction of Pyk2 phosphorylation and activation. By contrast, no detectable Pyk2 tyrosine phosphorylation is found upon CD16 stimulation mediated by either mAb or interaction with Ab-coated P815 cells. A functional role for Pyk2 in natural but not Ab-mediated cytotoxicity was demonstrated by the use of recombinant vaccinia viruses encoding the kinase dead mutant of Pyk2. Finally, we provide evidence that Pyk2 is involved in the beta2 integrin-triggered extracellular signal-regulated kinase activation, supporting the hypothesis that Pyk2 plays a role in the natural cytotoxicity by controlling extracellular signal-regulated kinase activation.

CD27-mediated activation of murine NK cells

CD27, a member of the TNF receptor superfamily, has been implicated in T cell activation, T cell development, and T cell-dependent Ab production by B cells. In the present study we examined the expression and function of CD27 on murine NK cells. Murine NK cells constitutively expressed CD27 on their surface. Stimulation with immobilized anti-CD27 mAb or murine CD27 ligand (CD70) transfectans solely could induce proliferation and IFN-gamma production of freshly isolated NK cells and enhanced the proliferation and IFN-gamma production of anti-NK1.1-sutimulated NK cells. Although NK cell cytotoxicity was not triggered by anti-CD27 mAb or against CD70 transfectants, prestimulation via CD27 enhanced the cytotoxic activity of NK cells in an IFN-gamma-dependent manner. These results suggest that CD27-mediated activation may be involved in the NK cell-mediated innate immunity against virus-infected or transformed cells expressing CD70.

A balance between positive and negative signals in cytotoxic lymphocytes regulates the polarization of lipid rafts during the development of cell-mediated killing

Plasma membrane microdomains containing sphingolipids and cholesterol (lipid rafts) are enriched in signaling molecules. The cross-linking of certain types of cell surface receptors initiates the redistribution of these lipid rafts, resulting in the formation of signaling complexes. However, little is known about the regulation of the initial raft redistribution and whether negative regulatory signaling pathways target this phase of cellular activation. We used natural killer (NK) cells as a model to investigate the regulation of raft redistribution, as both positive and negative signals have been implicated in the development of their cellular function. Here we show that after NK cells form conjugates with sensitive tumor cells, rafts become polarized to the site of target recognition. This redistribution of lipid rafts requires the activation of both Src and Syk family protein tyrosine kinases. In contrast, engagement of major histocompatibility complex (MHC)-recognizing killer cell inhibitory receptors (KIRs) on NK cells by resistant, MHC-bearing tumor targets blocks raft redistribution. This inhibition is dependent on the catalytic activity of KIR-associated SHP-1, a Src homology 2 (SH2) domain containing tyrosine phosphatase. These results suggest that the influence of integrated positive and negative signals on raft redistribution critically influences the development of cell-mediated cytotoxicity.

Inhibition of human NK cell function by valinomycin, a toxin from Streptomyces griseus in indoor air

Streptomyces griseus strains isolated from indoor dust have been shown to synthesize valinomycin. In this report, we show that human peripheral blood lymphocytes treated with small doses (30 ng ml(-1)) of pure valinomycin or high-pressure liquid chromatography-pure valinomycin from S. griseus quickly show mitochondrial swelling and reduced NK cell activity. Larger doses (>100 ng/ml(-1)) induced NK cell apoptosis within 2 days. Within 2 h, the toxin at 100 ng ml(-1) dramatically inhibited interleukin-15 (IL-15)- and IL-18-induced granulocyte-macrophage colony-stimulating factor and gamma interferon (IFN-gamma) production by NK cells. However, IFN-gamma production induced by a combination of IL-15 and IL-18 was somewhat less sensitive to valinomycin, suggesting a protective effect of the cytokine combination against valinomycin. Thus, valinomycin in very small doses may profoundly alter the immune response by reducing NK cell cytotoxicity and cytokine production.