Regulation of susceptibility to natural killer cells' cytotoxicity and regulation of HLA synthesis: differing efficacies of alpha, beta, and gamma interferons

Natural killer cells in malignant hematology: A primer for the non-immunologist

Natural killer cells were first described over 40years ago, but the last 15years has shown tremendous progress in our understanding of their biology and our ability to manipulate them for clinical therapeutic effect. Despite the increased understanding by clinicians and scientists investigating these cells, their biology remains a confusing subject for many because of the wide array of receptors, complex interactions, multiple models of predicting function, and contradictory data in the literature. While they are microscopically indistinguishable from T cells and share many of the same effector functions, their mechanisms of target recognition are completely distinct from yet complimentary to T cells. In this review we provide a basic understanding of NK cell biology and HLA recognition as compared and contrasted to T cells using a metaphor of border patrol and passports. We conclude with a summary of the evidence for NK cell effects in hematologic malignancies and describe new advances in NK cell immunotherapy aimed at improving these effects.

Simultaneous knockdown of multiple ligands of innate receptor NKG2D prevents natural killer cell-mediated fulminant hepatitis in mice

NKG2D activation plays an important role in initiating and maintaining liver inflammation, and blockade of NKG2D recognition becomes a promising approach to alleviate liver inflammation. Treatment by silencing NKG2D ligands on hepatocytes, but not NKG2D on circulating immune cells, is more liver-specific, and simultaneous knockdown of multiple NKG2D ligands on hepatocytes will be more efficient in liver disease intervention. Here, we constructed a single vector that could simultaneously express multiple short hairpin RNAs (shRNAs) against all murine NKG2D ligands including Rae1, Mult1, and H60. After hydrodynamic injection of plasmid containing the three shRNA sequences (shRae1-shMult1-shH60), also called pRNAT-shRMH, we found the expression of all three NKG2D ligands on hepatocytes was downregulated both on messenger RNA and protein levels. Moreover, natural killer (NK) cell-mediated NKG2D-dependent fulminant hepatitis of the mice was alleviated, along with inactivation of hepatic NK cells, by pRNAT-shRMH if compared with its counterpart RNA interference vectors against single or double ligands. The therapeutic efficacy of pRNAT-shRMH was equivalent to that of injecting three monoclonal antibodies against Rae1, Mult1, and H60. For better in vivo application, we constructed a recombinant adenovirus containing pRNAT-shRMH (called Ad-RMH) with efficient hepatotropic infection capacity and observed that Ad-RMH intravenous injection exerted a similar therapeutic efficiency as plasmid pRNAT-shRMH hydrodynamic injection. Noticeably, simultaneous knockdown of multiple human NKG2D ligands (MICA/B, ULBP2, and ULBP3) also significantly attenuated NK cell cytolysis against human NKG2D ligand-positive hepatocyte L-02 cells, suggesting a possible translation into human settings.

CONCLUSION

Simultaneous knockdown of multiple ligands of NKG2D prevents NK cell-mediated fulminant hepatitis and is a potential therapeutic approach to treat liver diseases.

Interferon gamma enhances clonal expansion and survival of CD4+ T cells

Interferon-gamma (IFN-gamma) serves numerous functions in the regulation of the immune response. During the early phase of the immune response IFN-gamma is produced by natural killer and natural killer T cells. Although the effects of this cytokine on antigen presenting cells and other cell types are known, its direct role on CD4(+) T cells remains unclear. We demonstrate that CD4(+) T cells exposed to IFN-gamma proliferate more vigorously than the controls in response to signals through the antigen receptor. The increased proliferation of IFN-gamma-treated CD4(+) T cells is not due to enhanced signaling through the antigen receptor, but is accounted for by their increased survival. Our data suggest that enhanced survival of IFN-gamma-treated CD4(+)T cells is independent of signal transducer and activator of transcription 1 (STAT 1), a transcription factor that controls the expression of a variety of IFN-gamma-targeted genes. In addition, we demonstrate that independent of STAT 1, IFN-gamma treatment increases the expression of double-stranded RNA-dependent protein kinase, a kinase involved in regulating protein synthesis. Taken together, our findings suggest a direct role of IFN-gamma on unstimulated CD4(+) T cells that is likely to enhance the advent of adaptive immunity by augmenting their survival during the initiation of the immune response.

Interferon--a major regulator of natural killer cell-mediated cytotoxicity

Natural killer (NK) cells probably function as an early defense line against viruses because of their ability to kill virus-infected cells as well as a variety of tumor cells. In both cases, the killing is major histocompatibility complex (MHC)-unrestricted. NK cells exhibit spontaneous activity but they are positively regulated by interferons (IFNs) or indirectly by such IFN inducers as viruses, bacterial products, poly(rI):(rC), and mitogens. In addition to their "positive" regulation on NK activity, IFNs often act as "negative signals" for NK and lymphokine-activated killer (LAK) cell-mediated cytotoxicity. If NK susceptible target cells are exposed to IFN prior to NK cells, their sensitivity to NK activity is often markedly diminished. The mechanism by which IFNs (IFN-alpha, -beta, and -gamma) affect the sensitivity of target cells to NK activity remains unknown, but it is clear that this function is not shared by other cell-mediated killing processes. The protective effect induced by IFN against NK activity is dependent on new mRNA and protein synthesis and can be abolished when target cells are incubated with a combination of IFN and metabolic inhibitors or by chemotherapeutic purine or pyrimidine analogs. IFN treatment neither affects the conjugate formation between NK cell and target cell nor the susceptibility of target cells to NK cytotoxic factor (NKCF), released by effector cells. However, IFN reduces the capacity of target cells to induce activation of conjugated NK cells. Because IFN has the ability to induce or increase class I MHC antigen expression (on NK target cells), it has been suggested that class I MHC antigens act as "negative signals" or NK-mediated cytotoxicity. Although many studies support this hypothesis, others present evidence for a lack of involvement of class I MHC antigens in mediating sensitivity to NK activity. This review summarizes and discusses the dual effect of IFNs in the regulation of NK activity, the relationship between the expression of class I MHC antigens on target cell surface and sensitivity to NK activity following treatment with IFNs, and the possible clinical relevance of the dual effect of IFN.

Resistance to NK cell-mediated cytotoxicity (in K-562 cells) does not correlate with class I MHC antigen levels

Natural Killer (NK) cells probably function as an early line of defense against virus-infected cells and tumor cells. In all cases, the killing by NK cell-mediated cytotoxicity (NK-CMC) is not MHC-restricted and the factors which determine the sensitivity to NK-CMC have not yet been identified. A positive correlation between resistance to NK-CMC and the level of class I MHC antigen (MHC I) expression on target cells has been reported in many studies, and in some cases a functional linkage between the two has been claimed. Several other studies have shown that there is no such correlation. By employing several experimental systems, we demonstrate here a lack of correlation between the level of MHC I and the sensitivity of K-562 cells to NK-CMC. Transfer of MHC I to MHC I-negative cells via vesicles had no effect on their resistance to NK-CMC. In addition, a decrease in resistance to NK-CMC and increase of MHC I levels was observed following target-cell membrane modulation by both application of cholesterol and hydrostatic pressure. Finally, no correlation between sensitivity to NK-CMC and MHC I expression was found in three sublines of K-562 cells. Since NK-CMC is a multistage process, it is concluded that components other than class I MHC antigens have a more prominent role in modulating the sensitivity of target cells to NK-CMC.

Natural killer susceptibility is independent of HLA class I antigen expression on cell lines obtained from human solid tumors

The susceptibility to natural killer (NK) cell-mediated cytotoxicity of 20 cell lines obtained from human solid tumors and their class I histocompatibility antigen (HLA) levels were studied in an attempt to determine whether major histocompatibility complex (MHC) products expressed on cells derived from human solid tumors influence NK susceptibility. The effect of interferon-gamma (IFN-gamma) treatment on these elements was also analyzed. The MHC class I (HLA-ABC, HLA-A and HLA-B) antigen levels and degree of NK lysis were very heterogeneous and no correlation was found on comparison. After treatment with IFN-gamma a marked decrease in NK susceptibility was observed in all the cell lines, including the control line K-562. However, the level of HLA class I expression was not modified in any of the lines with the exception of the K-562, which increased. In some cell lines the expression of HLA class I-like antigens. CDla, b and c, was also measured before and after IFN treatment; however, no correlation was found between CD1 levels and NK susceptibility. Consequently, from our results it is possible to conclude that HLA class I antigens do not play a decisive role in NK susceptibility of cell lines derived from human solid tumors and to suggest that molecules which are not HLA class I antigens but IFN-gamma inducible may confer NK resistance to these lines.

Interleukin-1 alpha and tumor necrosis factor-alpha protect cells against natural killer cell-mediated cytotoxicity and natural killer cytotoxic factor

The protective effects of interferons (IFNs) against NK cell-mediated cytotoxicity (NK-CMC) is well established. We report here that both recombinant tumor necrosis factor-alpha (TNF-alpha) and recombinant interleukin-1 alpha (IL-1 alpha) can also protect some adherent target cells (e.g., the amniotic cells WISH and the cervical epithelial carcinoma cells HeLa-229) from NK-CMC in a dose-dependent manner. Like in the case of IFNs, the level of conjugate formation between target and effector cells (nonadherent peripheral blood lymphocytes) is not affected by pretreatment of the target cells with either TNF-alpha or IL-1 alpha. However, while the main effect of IFNs is to reduce the ability of target cells to stimulate the release of NK cytotoxic factor (NKCF) from effector cells, TNF-alpha and IL-1 alpha do not affect this process but rather reduce the target cell sensitivity to the lytic effect of NKCF. Therefore TNF-alpha and IL-1 alpha induce resistance to NK-CMC by a mechanism that differs from the one attributed to IFNs. The protective effect of TNF-alpha and IL-1 alpha is not mediated by the induction of IFN-beta 2/IL-6.

(2'-5')Oligoadenylate synthetase activity in intestinal mononuclear and epithelial cells of inflammatory bowel disease patients

The activity of (2'-5')oligoadenylate synthetase, an enzyme induced by and mediating the antiviral action of interferon, was measured in extracts of intestinal mononuclear and epithelial cells isolated from patients with Crohn's disease, ulcerative colitis, and a control group. No significant differences were detected among (2'-5')oligoadenylate synthetase activities of lamina propria mononuclear cells derived from inflammatory bowel disease-involved and histologically normal control mucosa. Similarly, epithelial cells from inflammatory bowel disease and control patients expressed comparable levels of the enzyme, but these were significantly higher (p less than 0.01) than those found in autologous mononuclear cells. These results indicate that interferon is locally produced along the human intestinal mucosa under normal and inflammatory conditions. While this study supports the contention that induction of an antiviral state does not play a significant role in the pathogenesis of inflammatory bowel disease, it does not exclude the activation of the interferon system for other immunologic functions.

(2'-5')An-dependent endoribonuclease: enzyme levels are regulated by IFN beta, IFN gamma, and cell culture conditions

The levels of a (2'-5')An-dependent endonuclease (RNase L) were determined in extracts prepared from murine L cells and Ehrlich ascites tumor (EAT) cells by measuring specific binding of protein to a labeled derivative of (2'-5')An, (2'-5')A3[32P]pCp. RNase L levels were found to depend both on interferon (IFN) treatment and on cell growth conditions. Treatment of murine L cells and EAT cells with 100-2,000 IRU IFN beta or IFN gamma resulted in a similar 2-4-fold increase in the levels of RNase L when cells were present at low density. The levels of RNase L were also shown to increase 2-3-fold as cells approached saturation density. Serum-starved cells also displayed relatively high levels of RNase L. RNase L levels in cells maintained at high cell density did not change appreciably following treatment with IFN beta or IFN gamma. Regulation of RNase L levels by cell growth conditions as well as by IFN beta or IFN gamma treatment suggests that RNase L may play an important role in regulating the levels of cellular mRNAs as well as acting to degrade viral RNAs.

Interferon-mediated protection of B16 melanoma cells from cytotoxicity by activated macrophages

Corynebacterium parvum-activated macrophages (M phi), purified by adherence, were cytotoxic for B16 melanoma cells maintained in vitro. Pretreatment of the melanoma cells for 18 hr with interferon-alpha/beta or -gamma (IFN-alpha/beta or -gamma) caused a reduced susceptibility of the B16 cells to M phi-mediated cytotoxicity. The IFN-induced protective effect of B16 cells from cytotoxic M phi was found to be dose dependent. In addition, IFN-gamma was more protective than IFN-alpha/beta. The protective effect observed with partially purified IFN was reproduced by using highly purified IFN-alpha/beta or recombinant IFN-gamma. Monoclonal antibodies to IFN-gamma neutralized the protective effect provided by IFN-gamma. These results show that the susceptibility of a tumor cell line to killing by activated M phi can be altered by IFN pretreatment.

The interferon receptors

Early studies on the mode of action of interferons have indicated that a receptor system on the cell surface is involved in its action. The first direct evidence to a high-affinity binding site was found only after pure interferon was available. Two different receptors, one specific for interferons-alpha and beta, and the other for interferon-gamma were recognized. A correlation between affinity to the receptor and specific activity was established. Cross-linked complexes of labeled interferons with their receptors were visualized on gel electrophoresis and even partially purified. Internalization of interferons after binding to the receptor was reported. The role of gangliosides as helpers of interferon binding was recently investigated. Fragments of interferons which still retained binding capacity were described and helped in elucidating the binding site on the interferon molecule.

Interferon-gamma is a strong modulator of NK susceptibility and expression of beta 2-microglobulin but not of transferrin receptors of K562 cells

The human cell line K562 was treated with human natural leukocyte interferon (IFN-alpha) and recombinant immune interferon (IFN-gamma). Cell cultures exposed to both types of IFNs displayed a reduced susceptibility to the cytotoxic activity of human PBL (NK activity). While this effect occurred preferentially at high doses of IFN-alpha, as little as 10 U/ml of IFN-gamma caused a marked decrease in susceptibility to NK-cell-mediated lysis. Using a monoclonal antibody against human beta2-microglobulin (beta2M) a low level of specific binding to K562 cells was detected. The binding increased after treatment with IFN-alpha (1.4-fold) and IFN-gamma (1.7-fold). The expression of transferrin receptors (TR) was not changed significantly. A hybrid cell line between K562 and a Burkitt's lymphoma-derived cell line displayed a similar pattern of response to IFN-alpha and IFN-gamma as did K562, when effects on NK susceptibility, beta2M expression, and TR expression were studied. The Burkitt's lymphoma line PUT showed no consistent changes in expression of beta2M and TR. These results demonstrate that IFN-gamma is highly efficient in modulating the NK susceptibility, and the expression of beta2M on K562. The presented data do not support a role for expression of TR as the only property that determines the degree of NK susceptibility, since there was no correlation between NK susceptibility and TR expression among the cell lines tested or when IFN-treated and untreated cells were compared.

Increase of vulnerability to lymphotoxin in cells infected by vesicular stomatitis virus and its further augmentation by interferon

The cytotoxic effect of lymphotoxin (LT) and its modulation by interferon (IFN) was quantitatively assessed in uninfected and vesicular stomatitis virus (VSV)-infected cultured cells. Preparations of human LT, which were depleted of IFN, had a significant cytotoxic effect on VSV-infected HeLa, SV-80, WISH, and Vero cells. IFN, most notably IFN-gamma, further potentiated destruction of the infected cells by these LT preparations, when applied on the cells at sub-antiviral IFN concentrations. In contrast, no cytotoxic effect could be observed in any of the examined cells, when applying LT, IFN, or their combination, in the absence of viral infection. Infected cells in which VSV replication was suppressed by treatment with antiviral concentrations of IFN also resisted destruction by LT. These findings indicate that LT cytotoxicity can be selectively directed against virus-infected cells and that IFN can augment this cell-killing mechanism when failing to exert an antiviral effect.

The HLA proteins and a related protein of 28 kDa are preferentially induced by interferon-gamma in human WISH cells

The HLA-A,B,C proteins are preferentially induced by interferon (IFN)-gamma. An increase in their synthesis and of their expression on the cell surface can be observed at concentrations of IFN-gamma which are lower than those inducing an antiviral effect. On the other hand, with IFN-alpha and beta, induction of these proteins can be observed only in the antiviral range of IFN concentrations. In human WISH cells, IFN also induces a protein with a molecular mass of 28 kDa (28K). The efficiency of IFN-beta and gamma in inducing this protein is correlated to the efficiency with which they induce the HLA-A,B,C proteins. The 28K protein can be immunoprecipitated with antibodies against beta 2-microglobulin, just as the HLA proteins; yet it can be clearly distinguished from the HLA proteins in several respects: (a) it is not a cell surface protein but rather an intracellular one, with a relatively short half-life, (b) partial peptide mapping suggests that it contains sequences distinct from those of which the HLA alpha chains or beta 2-microglobulin are comprised and (c) the extent of its induction by IFN is much larger than that observed for the HLA proteins.

Enhanced release of lymphotoxins by interferon-treated cells

Pretreatment of human peripheral blood lymphocytes with interferon significantly enhances the release of lymphotoxins (LTs) observed at subsequent incubation of the cells, for 3 hr, with phytohemagglutinin (PHA). Fractionation of the LTs by gel filtration shows that interferon (IFN) strongly increases the release of certain LTs produced by these cells, while it has little effect on the release of others. The release of LTs from the IFN-treated cells is dependent on stimulation by PHA, requires Ca2+ ions, and can be blocked by prostaglandin E1, but it is independent of protein synthesis.

Interferon-induced resistance to the killing by NK cells: a preferential effect of IFN-gamma

HeLa cells show a decrease of susceptibility to the killing by natural killer (NK) cells when treated with IFN-alpha, beta, or gamma. The concentrations at which preparations of IFN-alpha or beta induce the resistance to killing are those which also induce resistance of HeLa cells to infection by vesicular stomatitis virus (VSV). Stimulation of the killing activity of NK cells is also induced at that same range of concentrations of IFN-alpha and beta. In contrast with preparations of IFN-gamma, induction of the resistance to killing occurs at IFN concentrations which have only marginal stimulatory effect on the activity of NK cells and have no antiviral effect against VSV. IFN-gamma, produced with cloned IFN-gamma cDNA, is as effective as lymphocyte-produced IFN in inducing the resistance to natural killing. The potent effect of IFN-gamma on the target cells is, therefore, not due to the function of lymphokines which might contaminate lymphocyte-produced preparations of IFN-gamma, but a genuine property of the IFN itself.