Do natural killer cells play a role in virus infections?

Inhibition of transmissible gastroenteritis coronavirus (TGEV) multiplication in vitro by non-immune lymphocytes

In vitro studies were undertaken to examine the effects of non-immune porcine peripheral blood leukocytes (PBL) on a Coronavirus infection due to transmissible gastroenteritis virus (TGEV). The assay consisted of TGEV-infected epithelial cells expressing viral antigens on the cell surface and producing low amounts of interferon (IFN). Non-immune PBL were found to limit virus replication at an effector-to-target ratio of 100/1 even when effector cells were depleted of phagocytic cells. Neutralizing anti-IFN antibodies did not abrogate the effect. PBL from newborn animals were as effective as adult cells, whereas fibroepithelial cells, human and mouse lymphoid cells did not exert antiviral effects. Under similar conditions, PBL from adult animals could lyse TGEV-infected cells even in the presence of anti-IFN antibodies. However, newborn PBL were not cytotoxic. Moreover, depletion of NK cells by monoclonal antibodies plus complement did not alter the inhibitory effect. These latter observations suggest that virus multiplication-inhibition effects and cytotoxic (or NK) activities are unrelated.

Activity of the Keggin polyoxotungstate PM-19 against herpes simplex virus type 2 infection in immunosuppressed mice: role of peritoneal macrophage activation

The in vivo antiviral activity of the Keggin polyoxotungstate PM-19 [K7(PTi2W10O40).6H2O] against herpes simplex virus type 2 (HSV-2) was investigated in mice immunosuppressed by cyclophosphamide (CY). When PM-19 was administered intraperitoneally to immunosuppressed mice for 3 days (once daily) starting at the time of infection, it prevented death due to HSV-2 encephalitis in a dose-dependent manner (10-25 mg/kg). The in vivo anti-HSV-2 activity of PM-19 was superior to that of acyclovir. Intraperitoneal administration of PM-19 to the immunosuppressed mice significantly increased the number of peritoneal cells, especially macrophages. PM-19 did not stimulate interferon-inducing activity or natural killer cell activity, but markedly enhanced peritoneal macrophage functions: (1) phagocytic activity as assessed by measuring the amount of 51Cr-labeled sheep red blood cells taken into the macrophages, and (2) extrinsic antiviral activity as monitored by reduction in the numbers of plaque formed upon cocultivation of HSV-2-infected HEL cells with the macrophages. These results point to the role of peritoneal macrophage activation in the activity of PM-19 against HSV-2 infection in immunosuppressed mice.

Protective activity of lipid A analogue GLA-60 against murine cytomegalovirus infection in mice

A chemically synthesized lipid A subunit analogue, GLA-60 (2-deoxy-4-O-phosphono-2-[(3R)-3-hydroxytetradecanamido]-3-O-[(3R) - 3-tetradecanoyloxytetradecanoyl]-D-glucose), has many of the activities of endotoxins but little, if any, toxicity. We investigated the protective activity of GLA-60 against murine cytomegalovirus (MCMV) infection in NMRI mice. Intraperitoneal administration of GLA-60 at 1 day before MCMV infection at doses of 1, 10, or 100 micrograms per mouse significantly reduced mortality. GLA-60 stimulated peritoneal natural killer (NK) cell and macrophage activities, and these activities were abolished by in vitro treatment with anti-asialo GM1 antibody and anti-Mac1 antibody, respectively. GLA-60 proved also protective against MCMV infection in mice in which either NK cells or macrophages were depleted by in vivo treatment with anti-asialo GM1 or anti-Mac1 antibody. The anti-MCMV activity of GLA-60 can at least be partially attributed to activation of NK cells and macrophages.

An inducer of NKCF (NK cytotoxic factor) release: localization on target-cell membrane and initial characterization

Natural killer (NK) cells are probably involved in the elimination of virus-infected cells and of certain tumor cells. NK cell-mediated cytotoxicity (NK-CMC) was extensively studied and was found to consist of several steps. Following recognition and conjugation between the effector and the target cell, the latter one induces release of NK cytotoxic factor (NKCF) from the effector cells. The NKCF binds to the target cell which is subsequently killed. None of the molecules involved in these steps was completely characterized. In the present study it is demonstrated that isolated membranes of target cells can effectively induce the release of NKCF. Furthermore, the activity of such isolated membranes was found to be modulated by interferon (IFN) treatment of the cells prior to membrane isolation. It was therefore concluded that an NKCF-inducing structure (NKIS) is present on plasma membranes and is distinct from the NK-recognition structure. Similarly, the sensitivity to NK-CMC could be transferred from sensitive cells to IFN-gamma-treated (NK-resistant) cells by membrane fusion with the aid of Sendai virus envelope glycoproteins. It is proposed that transfer of NKIS is responsible for the acquired sensitivity to NK-CMC. In addition, it is shown that NKIS activity was recovered following membrane solubilization and reconstitution. Its level on cell surface was modulated by treatment of cells with tunicamycin, thus indicating that NKIS was probably a cell surface glycoprotein.

Thy 1+ asialo GM1+ dendritic epidermal cells in skin defense mechanisms of vaccinia virus-infected mice

To define different growths of vaccinia virus (VV) in the footpad, ear and tail skin of ddY and BALB c mice, we investigated the roles of Ia+ Langerhans cells (LC) and Ia- dendritic epidermal cells (DEC) in the skin defense mechanisms. With respect to LC obtained from the footpad, ear and tail skin of BALB/c mice, the level of lysosomal enzyme, phagocytic, antigen-presenting and antiviral activities were almost the same. Footpad DEC exhibited higher natural killer (NK)-like lysis activity against YAC-1 target cells than ear and tail DEC. The NK-like lysis activity of footpad DEC was also seen in athymic nude mice (BALB/c nu/nu) but not in NK cell-defective beige mice (C57BL/6 bg/bg). The footpad DEC activity was completely abolished by in vitro and in vivo treatment with anti-Thy 1.2 or anti-asialo GM1 antibody, and VV replicated in the footpad 10(3) fold more than in the untreated group, but not by treatment with anti-Iad antibody. When footpad Thy 1.2+ asialo GM1+ DEC were adoptively transferred to the tail skin of syngeneic mice, followed by intradermal challenge of the mice with VV, viral replication in the tail skin was markedly suppressed. These results indicated that the pathogenicity of VV in the skin is affected by Thy 1.2+ asialo GM1+ DEC rather than by Ia+ LC.

Natural killer cell suppression of Friend virus-induced preleukemic hemopoietic stem cells

To determine whether hemopoietic cells infected with Friend polycythemia-inducing spleen focus-forming virus (SFFVp) are conserved or suppressed via natural surveillance in leukemia-resistant adult mice, we engrafted C57BL/6 recipients with isologous transgenic (donor origin marker) or natural killer (NK) cell-deficient B6 beige marrow cells exposed to SFFVp in vitro. Both groups of primary recipients were viremic and nonleukemic. Spleen cells from primary SFFVp-infected chimeras were engrafted into irradiated leukemia-susceptible secondary recipients to reveal dormant leukemia and grew as tumors of donor origin in 8 of 38 (21%) and 33 of 47 (70%) instances, respectively. Treatment of marrow donors and recipients with anti-asialo GM1 serum resulted in the depression of NK cell activity and the rapid development of dormant leukemia. We conclude that NK cells are an effective surveillance mechanism able to suppress SFFVp-induced preleukemic stem cells.

Murine thymocytes mediate a natural killer-like activity against herpes virus-infected target cells but not YAC-1 target cells

In this report, we demonstrated a natural killer (NK)-like activity against HSV-1 infected cells mediated by CD-1 mouse thymocytes. This cytolytic activity is specific for HSV-1-infected MCN cells, since both uninfected MCN and YAC-1 target cells are not susceptible to thymocyte lysis. Antibody plus complement depletion experiments indicate that a portion of the activity is associated with the Lyt 2-/L3T4- thymocyte subpopulation. This NK-like activity cannot be enhanced by addition of interleukin 2 in vitro.

Identification of functional subpopulations of murine natural killer cells based on their cell surface asialo GM1 phenotype

HSV-1 infection renders a mouse fibroblast cell line (MCN) sensitive to murine splenic NK killing which is independent of interferon (IFN) induction during the assay. This NK (HSV-1) activity is distinctive from conventional NK (YAC-1) in that they cannot be aborted by anti-asialo GM1 (anti-ASGM1) antibody plus complement treatment as NK (YAC-1) does. Further characterization of these two subpopulations was carried out by fluorescence-activated cell sorting (FACS) technique based on their cell surface asialo GM1 (ASGM1) phenotype. While almost all NK (YAC-1) activity resides within FACS-positive population, both ASGM1 positive and negative cell populations can kill the virally infected MCN equally well. One interesting observation is that only the ASGM1 positive cells respond significantly to IL-2 NK boosting. Five different mouse strains (CD-1, C57BL/6J, C57BL/6J-BG, SM/J, and SJL) were compared on their FACS profile with anti-ASGM1 antibody as well as their NK function. The differences observed are discussed.

Natural killer cells--toward clinical application

Natural killer (NK) cells are non-B, non-T lymphoid cells of uncertain lineage that rapidly recognize and lyse a large variety of tumor or virus-infected cells, without the need for either prior sensitization or major histocompatibility complex (MHC)-dependent recognition. Though some essential problems in understanding NK cell function are still unsolved, considerable progress has been achieved in recent years following the identification of the characteristic large granular lymphocyte (LGL) morphology of NK cells and their purification, the study of their function at the single-cell level, and the cloning of mouse and human NK cell lines. Activated mainly by interferon (IFN), as well as important producers of IFN, NK cells appear to have a distinct role in immunoregulation in addition to their postulated major role in "immune surveillance," for which convincing in vivo data has accumulated. Future clinical applications may therefore include manipulations of the NK system through expansion and activation of patient's LGL or the use of cloned human NK cell lines.

Defective leukocyte interferon response in children with recurrent infections accompanied by arthralgia

Children with recurrent and/or unusually severe infections were investigated for possible defects in the interferon (IFN)-natural killer (NK) cell system. Two series, each of 13 children, were examined, one in 1982 and one in 1983. Healthy children, seven in 1982 and eight in 1983, served as controls. Peripheral blood mononuclear leukocytes were examined for IFN production induced by the IFN-alpha inducers Sendai virus and Escherichia coli and by the IFN-gamma inducers Concanavalin A and Lens culinaris lectin. None of these inducers discriminated patients from controls. However, the bacteria Staphylococcus aureus Cowan I (SACol), inducers of atypical IFN in null lymphocytes, yielded significantly lower IFN production in infection-prone children than in controls, particularly in children with recurrent infections accompanied by arthralgia. No differences in basal NK activity or in the in vitro enhancement of such activity by IFN-alpha were found between patients and controls.

Herpes simplex virus - induced interferon production and activation of natural killer cells in SM/J mice. Relation to antiviral resistance

We have studied the susceptibility of SM/J mice to intraperitoneal (i.p.) infection with herpes simplex virus type 1 (HSV) and have searched for correlations of susceptibility with the activation of Natural Killer (NK) cells and with local induction of interferon. SM/J were exceedingly susceptible to virus infection as they could be killed by less than 10 plaque forming units (PFU). The NK cell system of these mice, as measured by the activity of spleen cells against YAC-1 lymphoma cells, was hyperreactive, which is in agreement with previous findings of others. The peritoneal exudate cells (PEC) of uninfected mice had no activity. However, 18 h after i.p. injection of HSV NK cell activity was detected in the PEC population, which was at least as high as that in C57BL/6 mice that are resistant to HSV infection. Thus it appears as if the NK cell system does not play a major role in antiviral resistance in our experimental system. In contrast, from our previous work it would rather appear that the magnitude of the early local interferon response is important for resistance. The current data obtained in SM/J mice are in accordance with this, in that these highly susceptible mice are deficient in their early interferon response. Homozygous beige mice were found to be as resistant to infection with HSV as C57BL/6 mice. While the NK cell activity in their PEC population after injection of HSV was low, the titers of locally induced interferon were as high as those in the controls.

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.

Myxovirus- and coronavirus-induced in vitro stimulation of spontaneous cell-mediated cytotoxicity by porcine blood leukocytes

Spontaneous cell-mediated cytotoxicity (SCMC) by porcine blood leukocytes toward human myeloid tumour target cells (K₅₆₂) was shown by Koren and coworkers in 1978, using a 4-h ⁵¹Cr-release assay. In the present paper, SCMC was found to be enhanced following a 24-h incubation of leukocytes with swine influenza virus (SIV) of the H₃N₂ type, and with the transmissible gastroenteritis virus (TGEV), a porcine coronavirus.

In most of our experiments, both SIV and TGEV induced interferon (IFN) production by leukocytes (titres ranging from 30-7,500 units/ml). When porcine leukocytes were incubated with TGEV in the presence of anti-IFN antiserum, no SCMC stimulation occurred, suggesting that the virus-induced SCMC stimulation was mainly due to endogenously produced IFN. On the other hand, when either human αIFN or porcine virus-induced IFN were added to porcine leukocytes, a rapid and marked SCMC increase was observed.

Both the SIV-induced SCMC increase and IFN production were suppressed when SIV was UV-inactivated. On the contrary, UV-inactivated TGEV was still able to enhance SCMC and to induce IFN production by porcine leukocytes.

On the whole, these results suggest that myxovirus and coronavirus are able to activate porcine leukocyte SCMC via endogenously produced IFN.

Modulation by cyclosporin A of murine natural resistance against herpes simplex virus infection. II. Influence on the HSV-induced natural killer cell responses, macrophage activities and interferon levels

Cyclosporin A (CyA) interfered locally at the site of injection with several resistance functions which are of potential importance in experimental herpes simplex virus (HSV) infections of mice. HSV-induced stimulation of macrophage phagocytosis was reduced by CyA when the mice were infected 5 days before the assay. The in vitro replication of the virus in macrophages, however, was enhanced. Natural killer (NK) cell response were severely impaired. To some extent this could be attribute to the induction of suppressive macrophages by the drug treatment. Interferon levels induced by HSV were not diminished but rather enhanced in some experiments. Inhibitory effects ceased after termination of CyA treatment and could be prevented by presensitization of the mice with attenuated HSV type 2.