Requirement for HLA-DR+ accessory cells in natural killing of cytomegalovirus-infected fibroblasts

The leukocyte function-associated antigen-1 (LFA-1) is involved in the interferon-alpha response induced by herpes simplex virus in blood leukocytes

The role of the leukocyte function-associated antigen-1 (LFA-1) family of integrins (beta 2 integrins) in the interferon-alpha (IFN-alpha) response was examined, using human peripheral blood mononuclear cells (PBMCs) stimulated in vitro by glutaraldehyde-fixed Herpes simplex virus-infected WISH amnion cells. Monoclonal antibodies (mAbs) to the beta 2 chain (CD18) and to the alpha chain of LFA-1 (CD11a) reduced the number of IFN-alpha-producing cells (IPCs) by 30-50%, but mAbs to CD11b or c caused no inhibition. The IB4 mAb to CD18 was inhibitory when added during the first 2 h of the IFN-alpha response, but did not alter its kinetic. In contrast, the IB4 prevented the early enhancement of the IFN-alpha response caused by addition of interleukin-3 (IL-3) or granulocyte-macrophage colony-stimulating factor (GM-CSF). However, a delayed down-regulation of the IPC response occurred in such PBMC cultures, and a paradoxical increase in the total production of IFN-alpha. The results suggest that LFA-1 (CD11a/CD18) participates in the early phase of the IFN-alpha response and may be activated by cytokines such as IL-3 and GM-CSF.

Role for major histocompatibility complex class I in regulating natural killer cell-mediated killing of virus-infected cells

Target structures important for natural killer (NK) cell recognition of virally infected cells are not well defined. Since major histocompatibility complex (MHC) class I molecules bind viral peptides during acute infection, we evaluated whether an interaction between MHC and virus might influence the susceptibility of infected cells to NK cell-mediated lysis. To control for MHC class I expression on target cells, we used either HLA class I-deficient C1R cells or C1R sublines expressing transfected HLA class I gene products. Human NK cells were unable to preferentially lyse class I-deficient C1R cells after infection with herpes simplex virus (HSV). In contrast, HLA class I transfectants were significantly more susceptible to NK cell-mediated cytotoxicity after HSV infection. This occurred for HSV-infected C1R cells expressing any of the three HLA class I gene products tested (i.e., HLA-B27, HLA-A3, or HLA-Aw68), indicating that NK cell recognition in this system does not require "self" MHC and is not unique for a single haplotype. Productive HSV infection is required for the increased killing, since inoculation with UV-inactivated virus did not lead to increased lysis. In addition, since HSV infection of the transfectants did not significantly alter the level of class I expression, the change in susceptibility appears to be due to qualitative changes in the target structures on HSV-infected, HLA class I+ targets. These results demonstrate a role for MHC class I in regulating NK cell-mediated killing of virus-infected cells.

The induction of interferon-alpha and interferon-beta mRNA in human natural interferon-producing blood leukocytes requires de novo protein synthesis

The induction of interferon-alpha (IFN-alpha) and IFN-beta mRNA in natural IFN producing (NIP) cells in cultures of human peripheral blood mononuclear cells (PBMCs), stimulated by glutaraldehyde-fixed Herpes simplex virus type 1 (HSV)-infected WISH cells, was studied. The protein synthesis inhibitor cycloheximide (CHX) totally prevented the appearance of both IFN-alpha and IFN-beta mRNA, also in cultures supplemented with a conditioned medium (CM) assumed to contain soluble factors necessary for the IFN induction. However, when PBMCs were preincubated for 4 h in medium supplemented with fetal bovine serum (FBS) with or without addition of CM, the subsequent induction of IFN-alpha/beta mRNA became partially resistant to CHX. In serum-free medium containing interleukin-3 (IL-3) or granulocyte-macrophage colony-stimulating factor (GM-CSF), the early induction of IFN-alpha mRNA became resistant to CHX, and, in contrast to FBS and CM supplemented medium, this was observed also without a preincubation of the PBMCs. In contrast, IL-1, IL-2, IL-4, IL-6, tumor necrosis factor-alpha (TNF-alpha), IFN-alpha, or IFN-gamma had no such effects. Our results suggests that de novo synthesis of proteins normally is required for the induction of IFN-alpha/beta mRNA. Such proteins might be cytokines, possibly CSFs, which in turn also may require protein synthesis for their actions. In contrast, the actual triggering signal provided by the HSV-inducer is independent of protein synthesis.

Flow cytometric analysis of natural interferon-alpha producing cells

Herpes simplex virus-infected cells induce high interferon-alpha (IFN-alpha) production in infrequent cells among peripheral blood mononuclear cells (PBMC), designated natural IFN-alpha producing (NIP) cells. The properties of such NIP cells were compared with defined populations of leucocytes by means of flow cytometric analysis and sorting. The NIP cells are characterized as a discrete population of cells with high forward and low to intermediate orthogonal light scattering, similar to that of early progenitors of myeloid and lymphoid cells. However, they appear to lack the stem cell-associated molecule CD34. Furthermore, NIP cells cannot be localized to the myeloid line of cell differentiation, because they do not express the CD33, CD13, CD11b, CD15 or CD14 antigens. Neither do they express CD10 and CD19 antigens which are present in all stages of B-cell differentiation plasma cells excepted, nor CD7 antigens expressed on early T cells. In combination with previous results, our data support the view that the NIP cell is a unique and distinct cell type in peripheral blood, possibly with a physiological role in the defence against certain viral infections.

Rhinovirus induces natural killer-like cytotoxic cells and interferon alpha in mononuclear leukocytes

Natural killer-like cellular cytotoxicity was augmented by incubation of human rhinovirus serotype 2 with peripheral blood mononuclear leukocytes collected from healthy donors. The production of alpha interferon but not gamma interferon was identified in the same cell cultures. A specific interaction of conformationally intact rhinovirus with peripheral blood mononuclear leukocytes was required for induction of the response, since the response was extinguished at reduced quantities of infectious rhinovirus, and acid inactivated rhinovirus did not augment cellular cytotoxicity. Productive replication of rhinovirus was not observed in cultures of peripheral blood mononuclear leukocytes. The replicative failure was not related merely to interferon production, since the rate of disappearance of rhinovirus was similar to that observed in cell free medium. The findings suggest that natural killer cells should be considered as a potential component of the local nasopharyngeal pathophysiology of rhinovirus infection.

Potentiation of human natural killer cell cytotoxicity by Salmonella bacteria is an interferon- and interleukin-2-independent process that utilizes CD2 and CD18 structures in the effector phase

Incubation of large granular lymphocytes (LGL) with glutaraldehyde-fixed bacteria stimulated in the supernatant the production of interferon (IFN), which proved to be mainly IFN-gamma. Even though IFN-gamma was produced upon exposure of LGL to bacteria, anti-IFN-gamma antibodies failed to interfere with induction of cytotoxicity by bacterial contact. Anti-IFN-gamma receptor antibodies had no effect on the induction of activated killing by bacterial contact either. We also tested the effect of anti-IFN-alpha antibody, but it failed to interfere with induction of cytotoxicity by bacterial contact. No interleukin-2 (IL-2) was detected in the culture supernatant of bacterially activated LGL by the mouse HT2 cell assay, nor did we detect any IL-2 mRNA in bacterially activated LGL by Northern RNA blot assay. Neutralizing anti-IL-2 antiserum had no effect on the induction of activated killing by bacterial contact, and recombinant IL-4 did not interfere with the induction of activated killing. We then studied the membrane structures involved in bacterially activated killing. Anti-CD18 monoclonal antibody did not interfere with the induction phase of bacterially activated killing. However, both anti-CD18 and anti-CD2 antibodies inhibited the effector phase of bacterially activated killing. The effector pathways utilized by activated LGL depended on the mode of activation in that even though bacterially activated LGL were sometimes blocked by anti-CD2 monoclonal antibody, recombinant-IL-2-stimulated LGL were not. In conclusion, our present results suggest that there may be mediators other than exogenously secreted IFNs and IL-2 which are responsible for the induction of activated killing after bacterial contact. CD18 and CD2 structures were shown to be involved in the effector phase of bacterially activated killing.

Immediate-early gene expression is sufficient for induction of natural killer cell-mediated lysis of herpes simplex virus type 1-infected fibroblasts

Herpes simplex virus type 1 (HSV-1)-infected human fibroblast (HSV-FS) targets are susceptible to lysis by natural killer (NK) cells, whereas uninfected FS are resistant to lysis. Studies were undertaken to determine the mechanism of this preferential susceptibility. HSV-FS were not intrinsically less stable than FS, as determined by a 51Cr release assay under hypotonic shock in the presence of rat granule cytolysin and by sensitivity to anti-human leukocyte antigen class I antibody plus complement. Single-cell assays in agarose demonstrated that although similar numbers of large granular lymphocytes bound to the HSV-FS and FS targets, the conjugates with HSV-FS were lysed at a much higher frequency than those with FS. These results suggested that both targets are bound by the NK cells but only the HSV-FS were able to trigger lysis. The requirement for active virus expression was demonstrated by failure of emetine-treated HSV-FS targets or targets infected with UV-inactivated HSV to be lysed by NK effectors. To evaluate the role of viral glycoproteins in conferring susceptibility to lysis, Fab were prepared from HSV-1-seropositive sera; these Fab were unable to block lysis of the HSV-FS. Furthermore, incubation in phosphonoacetic acid failed to reduce NK(HSV-FS) activity despite sharp reductions in viral glycoprotein synthesis. Finally, targets infected with tsLB2 at the nonpermissive temperature were lysed as well as or better than targets infected with wild-type virus, indicating that HSV immediate-early gene product expression is sufficient for conferring susceptibility to lysis. We conclude that expression of nonstructural viral proteins or virally induced cellular gene products early in the course of infection rather than structural glycoproteins is required for NK lysis of HSV-FS targets.

In vitro infection of natural killer cells with different human immunodeficiency virus type 1 isolates

Natural killer (NK) cells are a discrete subset of leukocytes, distinct from T and B lymphocytes. NK cells mediate spontaneous non-MHC-restricted killing of a wide variety of target cells without prior sensitization and appear to be involved in initial protection against certain viral infections. Depressed NK cell-mediated cytotoxicity, one of the many immunological defects observed in AIDS patients, may contribute to secondary virus infections. Here we report that clonal and purified polyclonal populations of NK cells, which expressed neither surface CD4 nor CD4 mRNA, were susceptible to infection with various isolates of human immunodeficiency virus type 1 (HIV-1). Viral replication was demonstrated by detection of p24 antigen intracellularly and in culture supernatants, by the presence of HIV DNA within infected cells, and by the ability of supernatants derived from HIV-infected NK cells to infect peripheral blood mononuclear cells or CD4+ cell lines. Infection of NK cells was not blocked by anti-CD4 or anti-Fc gamma RIII monoclonal antibodies. NK cells from HIV-infected and uninfected cultures were similar in their ability to lyse three different target cells. Considerable numbers of cells died in HIV-infected NK cell cultures. These results suggest that loss of NK cells in AIDS patients is a direct effect of HIV infection but that reduced NK cell function involves another mechanism. The possibility that NK cells serve as a potential reservoir for HIV-1 must be considered.

Induction of interferon gamma production by natural killer cell stimulatory factor: characterization of the responder cells and synergy with other inducers

We previously reported that natural killer cell stimulatory factor (NKSF), a heterodimeric lymphokine purified from the conditioned medium of human B lymphoblastoid cell lines, induces interferon gamma (IFN-gamma) production from resting peripheral blood lymphocytes (PBL) and synergizes with interleukin 2 in this activity. In this study, we show that human NKSF induces IFN-gamma production from both resting and activated human PBL and from freshly isolated murine splenocytes. Human T and NK cells produce IFN-gamma in response to NKSF, but resting PBL require the presence of nonadherent human histocompatibility leukocyte antigens DR+ (HLA-DR+) accessory cells to respond to NKSF. The mechanism(s) by which NKSF induces IFN-gamma production results in accumulation of IFN-gamma mRNA, is insensitive to cyclosporin A, and synergizes with those mediated by phytohemagglutinin, phorbol diesters, anti-CD3 antibodies, and allogeneic antigens, but not by Ca2+ ionophores. The ability of NKSF to directly induce IFN-gamma production and to synergize with other physiological IFN-gamma inducers, joined with the previously described ability to enhance lymphocyte cytotoxicity and proliferation, indicates that this lymphokine is a powerful immunopotentiating agent.

Natural killer-mediated lysis of some but not all HSV-1- or VSV-infected targets requires the participation of HLA-DR-positive accessory cells

Natural killer (NK)-mediated lysis of herpes simplex virus type 1-infected fibroblasts (HSV-FS) has been previously shown to require the co-operation of CD16-positive NK cells and an HLA-DR-positive accessory cell population. In contrast, lysis of K562 tumour cells requires the presence of only the Leu-11-positive cells. In the current study, targets of different morphologies, both virally infected and non-infected, were tested in an attempt to dissect out which target characteristics determine the need for accessory cell participation for NK-mediated lysis. Effector populations were obtained through antibody plus complement (C) depletions of subpopulations of human peripheral blood mononuclear cells using anti-HLA-DR+C (accessory cell depleted) or anti-CD16+C (NK depleted). The subpopulations were tested both alone and mixed together for their ability to mediate target lysis. Although NK-mediated lysis of most HSV-infected targets required the presence of HLA-DR-positive accessory cells, there was one set of exceptions. Lysis of the non-adherent Epstein-Barr virus (EBV)-transformed lymphoblastoid lines HSV-Raji, HSV-ARH and HSV-CCRF demonstrated only partial accessory cell dependence. All infected adherent cell lines were accessory cell dependent. In contrast, none of the adherent or non-adherent non-infected targets tested required the presence of DR-positive accessory cells for killing. Therefore, the presence of virus was an indicator of accessory cell dependence for NK-mediated kill except in the cases where HSV-infected EBV-transformed targets were used. Assay times of 4 hr versus 14 hr were conducted to determine if the kinetics of kill of various targets correlated with the requirement for accessory cells. A substantial percentage of the total lysis seen at 14 hr occurred within 4 hr for accessory cell independent lysis of the non-infected targets. In contrast, accessory cell-dependent kill of infected targets usually required longer incubation time for substantial lysis to occur, and correlated with interferon (IFN) production. NK-mediated lysis of vesicular stomatitis virus-infected fibroblasts required the presence of both the CD16- and HLA-DR-positive subpopulations, extending the role of DR-positive cells in NK-mediated killing beyond herpes virally infected targets.

Sequential enrichment and immunocytochemical visualization of human interferon-alpha-producing cells

Human HLA-DR+ peripheral mononuclear cells (PBMC) produce interferon-alpha (IFN-alpha) in response to herpes simplex virus type 1 (HSV-1) or HSV-1-infected fibroblasts (HSV-FS). We have developed a protocol, based partly on a technique known to enrich for dendritic cells, that allows for a greater than 125-fold enrichment of these IFN-alpha-producing cells. Nylon wood nonadherent PBMC (NWNA) were fractionated on a 48% Percoll gradient into low-density (LD) and high-density (HD) populations. The LD cells were 10- to 30-fold enriched for the production of IFN-alpha compared to PBMC when stimulated with HSV-FS. LD cells were treated further to deplete any contaminating monocytes, CD3+ T cells and CD56+ natural killer (NK) cells. The resulting population (CD3/CD56-depleted) produced approximately 30,000 IU/ml IFN-alpha compared to 3,000-10,000 IU/ml for the corresponding LD cells and 30-300 IU/ml for PBMC. Immunocytochemistry to detect cytoplasmic IFN-alpha indicated that PBMC, NWNA, HD, LD, and CD3/CD56-depleted populations contained an average of less than 0.1%, 0.3%, less than 0.1%, 3%, and 12% IFN-alpha-producing cells, respectively. The cells responsible for IFN-alpha production in response to HSV-1 were of medium to large diameter and possessed eccentric nuclei that were often indented, with lightly staining perinuclear areas. The CD3/CD56-depleted populations were fivefold enriched for HLA-DR+ cells. This enrichment procedure partially overcomes the barrier of low frequency that has contributed to the elusive identification of these cells.

Natural killer cell-mediated lysis of T cell lines chronically infected with HIV-1

The susceptibility of HIV-1-infected CD4+ T cell lines to natural killer (NK) cell-mediated lysis was examined. Non-adherent peripheral blood mononuclear cells (PBMC) of healthy adults lysed HUT cells chronically infected with the IIIB or WMJ1 strains of HIV-1 to a significantly greater extent than uninfected HUT cells. In contrast, Sup-T1 cells chronically infected with these two strains of HIV-1 were not lysed to a greater extent than uninfected Sup-T1 cells. Clone A1.25-infected Sup-T1 (A1.25/Sup-T1), derived from IIIB-infected Sup-T1 cells (IIIB/Sup-T1), were susceptible to non-adherent PBMC-mediated lysis, as were A1.25-infected HUT cells (A1.25/HUT). When non-adherent PBMC were depleted of CD16 (Leu-11b)+ NK cells by treatment with anti-Leu-11b plus C, lysis of HIV-1-infected HUT or Sup-T1 cells was reduced to low levels, indicating that the lysis was mediated by NK cells. Expression of HIV antigens on these target cells did not correlate with their susceptibility to NK cell-mediated lysis. Depletion of interferon-alpha (IFN-alpha) producing HLA-DR+ cells from non-adherent PBMC had no effect on the magnitude of NK cell-mediated lysis of IIIB or WMJ1-infected HUT cells. In contrast, lysis of A1.25/Sup-T1 or A1.25/HUT cells required the presence of HLA-DR+ cells. IFN-alpha production appeared to be required for NK cell-mediated lysis of A1.25/Sup-T1 or A1.25/HUT cells, while lysis of HUT cells infected with the WMJ1 or IIIB strains of HIV-1 was IFN-alpha independent. These results indicate considerable variability in the susceptibility of different HIV-1 infected T cell lines to NK cell-mediated lysis and suggest the existence of alternative mechanisms of activation of NK cells for lysis of HIV-1-infected T cell lines.

Infrequent but efficient interferon-alpha-producing human mononuclear leukocytes induced by herpes simplex virus in vitro studied by immuno-plaque and limiting dilution assays

Human blood mononuclear leukocytes (PBMCs) producing interferon-alpha (IFN-alpha) after stimulation by herpes simplex virus type 1 (HSV) in vitro were identified by a filter immuno-plaque assay. Individual IFN-alpha-producing cells (IPCs) yielded between 0.5 and 2 units IFN-alpha, sufficient to protect cultures of MDBK cells against a viral challenge. Therefore, their frequency could be determined by a limiting dilution assay as well as by an immuno-plaque assay. Similar estimates of between 2 and 55 IPCs per 10(4) PBMCs at the peak of the IFN-alpha response were obtained by the two methods. IPCs were first detected 3 h after stimulation by HSV; their number peaked at 8 h and then declined. IPC frequencies were influenced by the concentrations of HSV and PBMCs during induction, but the quantity of IFN-alpha produced per IPC was relatively constant. The relation between the numbers of IPCs and PBMCs was linear at high PBMC concentrations, whereas at low PBMC concentrations fewer IPCs than expected were detected. The response could be fully restored by adding a combination of filler cells (Namalwa or U937 cells) and conditioned medium from 6-h HSV-induced PBMC cultures. Our results suggest that HSV induces an IFN-alpha response in a relatively rare population of efficient IPCs by complex mechanisms, which may involve cell cooperation and/or production of soluble factors.

Dendritic cells and IFN-alpha-producing cells are two functionally distinct non-B, non-monocytic HLA-DR+ cell subsets in human peripheral blood

At least two distinct HLA-DR+ cell subsets lacking surface markers specific for B cells, monocytes or other known lineages are present in human peripheral blood. One subset is non-adherent to plastic, produces interferon-alpha (IFN-alpha) when incubated with cytomegalovirus-infected target cells and provides an accessory function required for natural killer (NK) cell-mediated lysis of such cells. These non-adherent HLA-DR+ cells express the surface antigen recognized by antibody anti-D44 and do not stimulate mixed leucocyte reaction (MLR). The other HLA-DR+ cell subset is loosely adherent to plastic, produces only minimal levels of IFN-alpha when incubated with cytomegalovirus-infected target cells and does not provide the accessory function required for NK cell-mediated lysis of such cells. These HLA-DR+ cells stimulate a strong MLR, do not express D44 antigen and meet the criteria of dendritic cells (DC) morphologically and functionally.

Characterization of the blood mononuclear leucocytes producing alpha interferon after stimulation with herpes simplex virus in vitro, by means of combined immunohistochemical staining and in situ RNA-RNA hybridization

A procedure is described for combined immunohistochemical staining and in situ RNA-RNA hybridization of human peripheral blood mononuclear leucocytes (PBMC). These cells were first stimulated in vitro by herpes simplex virus (HSV)-infected fibroblasts and after 6 h fixed and stained with a panel of antibodies against differentiation antigens. Alpha interferon (IFN-alpha)-producing cells (IPC) were identified by in situ hybridization by means of a 35S-labelled IFN-alpha 2 cRNA probe. The IPC were infrequent, one in 200-5000 PBMC, but heavily labelled with the cRNA probe. They lacked antigens typical of T and B lymphocytes, and were also essentially negative for the Leu-M5 antigen, present on a majority of monocytes. However, 50% of IPC expressed OKM5 antigens, corresponding to the thrombospondin receptor. The IPC lacked the antigens present on null lymphocytes detected by OKT16, but most of them expressed HLA-DR, -DP and -DQ antigens. The IPC may represent a small subpopulation in the monocyte/macrophage lineage, resembling cells described as antigen presenting and stimulators of autologous mixed lymphocyte reactions. Alternatively, they constitute a subpopulation among the null lymphocytes.

Human immunodeficiency virus-specific cytotoxic responses of seropositive individuals: distinct types of effector cells mediate killing of targets expressing gag and env proteins

By using target cells that expressed isolated env, gag, p27nef, or p23vif molecules introduced by recombinant vaccinia viruses containing genes encoding these polypeptides, it was possible to identify env, gag, p27nef, and p23vif as cytolytic target antigens for freshly isolated blood cells from human immunodeficiency virus 1 (HIV-1) seropositive patients. Most of the patients tested (95%) manifested a specific cytotoxic activity against vaccinia virus-env-infected target cells. The env-specific cytotoxic activity was not restricted by the major histocompatibility complex and was not mediated by T lymphocytes, as shown by the absence of blocking effect with an anti-CD3 monoclonal antibody and by the inefficiency of CD3+, CD8+, or CD4+ and CD8+ depletion to reduce the cytotoxic activity against the env-expressing target cells. In the same conditions, the cytotoxic activity specific for gag was abrogated and gag major histocompatibility complex-restricted cytotoxic T lymphocytes were detected in 85% of the subjects tested. Therefore, in a HIV-1 seropositive subject, distinct types of effector cells mediate the lysis of target cells expressing gag and env proteins.

Sex-dependent, early cytokine production by NK-like spleen cells following infection with the D variant of encephalomyocarditis virus (EMCV-D)

Spleen cell cultures from diabetes-resistant ICR Swiss females exhibited an increase in expression of Ia antigens 24 hours post-infection (PI) with EMCV-D while comparable spleen cell cultures from diabetes-susceptible males of this strain did not exhibit this increase in Ia antigens expression. A monoclonal antibody specific for mouse interferon-gamma (IFN gamma) eliminated this increase in Ia antigens expression. Interferon-gamma (IFN gamma) and interleukin 2 (IL-2) production by EMCV-D-infected spleen cell cultures were monitored at 4-hour intervals for 24 hours. Female spleen cells produced IFN gamma earlier (less than 16 hours PI) and in greater amounts than did comparably treated male spleen cells. Addition of a monoclonal rat anti-mouse IL-2 to virus-infected cultures did not significantly affect the early (less than 16 hours PI) production of IFN gamma by spleen cells of females. Treatment of the spleen cell donors with rabbit anti-asialo GM1 (AAGM1) abolished early production of IFN gamma in virus-infected female spleen cell cultures and reduced the early IL-2 production by infected male and female cells. These results suggest that an NK-like cell is responsible for the early female IFN gamma production; this may be a factor in the resistance of female ICR Swiss mice to EMCV-D-induced diabetes.

Biology of natural killer cells

Studies of cytotoxicity by human lymphocytes revealed not only that both allogeneic and syngeneic tumor cells were lysed in a non-MHC-restricted fashion, but also that lymphocytes from normal donors were often cytotoxic. Lymphocytes from any healthy donor, as well as peripheral blood and spleen lymphocytes from several experimental animals, in the absence of known or deliberate sensitization, were found to be spontaneously cytotoxic in vitro for some normal fresh cells, most cultured cell lines, immature hematopoietic cells, and tumor cells. This type of nonadaptive, non-MHC-restricted cellmediated cytotoxicity was defined as “natural” cytotoxicity, and the effector cells mediating natural cytotoxicity were functionally defined as natural killer (NK) cells. The existence of NK cells has prompted a reinterpretation of both the studies of specific cytotoxicity against spontaneous human tumors and the theory of immune surveillance, at least in its most restrictive interpretation. Unlike cytotoxic T cells, NK cells cannot be demonstrated to have clonally distributed specificity, restriction for MHC products at the target cell surface, or immunological memory. NK cells cannot yet be formally assigned to a single lineage based on the definitive identification of a stem cell, a distinct anatomical location of maturation, or unique genotypic rearrangements.

Peripheral blood mononuclear cell-mediated cytolytic activity during cytomegalovirus (CMV) infection of guinea pigs

Inbred Strain-2 guinea pigs exhibited endogenous peripheral blood mononuclear cell (PBMC)-mediated cytolytic activity against xenogeneic MA104 targets and guinea pig cytomegalovirus (gpCMV)-infected syngeneic and allogeneic targets. This endogenous cytolysis was unaffected by monoclonal T-cell antibody depletion but was diminished by removal of plastic adherent cells. In nonadherent effector populations, cytolysis was mediated predominately by large granular lymphocytes (LGL). During gpCMV infection, cytolysis of both target types was augmented (MA 104 for 3 weeks and gpCMV targets for 10-14 weeks). Augmented cytolysis of gpCMV targets was MHC-unrestricted and was diminished by removal of plastic adherent cells or monoclonal antibody depletion of T-cells, being found largely in LGL enriched populations. A role for this augmented activity in limiting gpCMV infection in inbred guinea pigs is suggested by the temporal association of augmented cytolysis of gpCMV targets with the cessation of viremia and clinical recovery.

Natural killing of fibroblasts infected with low-passage clinical isolates of human cytomegalovirus

Fibroblasts infected with most low-passage clinical isolates of human cytomegalovirus (CMV) were as susceptible to lysis by human natural killer (NK) cells as high passage AD-169-infected fibroblasts. NK lysis occurred despite the absence of detectable CMV-specific late membrane antigen(s) on the majority of the target cells infected with most of the low passage strains. The magnitude of NK lysis of different CMV-infected target cells did not correlate with their ability to induce IFN-alpha. NK cell-mediated lysis of cells infected with low-passage clinical isolates of CMV required both NK cells and HLA-DR+ accessory cells, as previously shown for AD-169-infected target cells.

Natural killer cell activation and interferon production by peripheral blood lymphocytes after exposure to bacteria

We have previously shown that peripheral blood natural killer (NK) cells have significant levels of cytotoxic activity against Shigella flexneri-infected HeLa cells. In this report, we show that NK cell activity against K562 tumor cells and Shigella flexneri-infected HeLa cells can be greatly enhanced by preincubating peripheral blood lymphocytes (PBL) for 18 h with kanamycin-treated Shigella flexneri or Salmonella typhimurium. Cell-free supernatants obtained from PBL-bacteria cultures contained high levels of interferon (IFN) activity, which was characterized as a mixture of IFN-gamma and IFN-alpha. Cytotoxic activity associated with PBL precultured with shigellae was associated with predominantly CD16+ (Leu-11+) and CD2+ (OKT-11+) cells. Further, IFN production was dependent upon the presence of CD16+ and CD2+ cells at culture initiation. Enhancement of cytotoxic activity associated with PBL-bacteria cultures did not, however, appear to be dependent upon IFN production, since low numbers of bacteria which failed to stimulate IFN production induced high levels of NK cell activity. Lipopolysaccharide appeared not to be involved in bacteria-induced IFN production or enhanced NK cell activity, since Salmonella lipopolysaccharide failed to induce IFN production or enhance NK cell activity. These results suggest that IFN production by NK cells and the killing of bacteria-infected cells play an important role in host defense against facultative intracellular bacterial infections.

Generation of human cytomegalovirus-specific cytotoxic T-lymphocytes in a short-term culture

A method to generate human cytomegalovirus (HCMV)-specific CTL (cytotoxic T lymphocytes) from human peripheral blood mononuclear cells is described. This assay is unique in comparison with other methods reported to date, because it only requires a short-term (6 days) coculture of PBM and autologous infected fibroblasts without the addition of exogenous IL-2 (interleukin-2) and nevertheless is sensitive enough to determine HCMV-specific killing in a short (6 h) 51Cr-release assay using autologous HCMV-infected fibroblasts as targets. The virus-specific killing is mediated by CTL of the CD8 phenotype and it can be inhibited by a HLA class I monoclonal antibody. The sensitivity of the assay can be significantly enhanced by pretreating the targets with interferon-gamma (IFN-gamma) prior to infection with HCMV. HCMV-specific 51Cr-release is more than doubled when the IFN-gamma pretreated targets are used. This increase is mostly due to enhanced sensitivity of the fibroblasts to killing mediated by CD8-positive CTL, but some killing can be attributed to CTL of the CD4 phenotype.

Determination of herpes simplex virus-induced alpha interferon-secreting human blood leucocytes by a filter immuno-plaque assay

A filter immuno-plaque assay was developed which detects alpha interferon (INF-alpha)-secreting human peripheral blood leucocytes (PBL). Polyclonal anti-IFN-alpha antibodies were fixed to the nitrocellulose membrane bottoms of 96-well Millititer plates, which also contained monolayers of glutaraldehyde-fixed human WISH amnion cells infected by Herpes simplex virus type 1 (HSV). Such cells are potent IFN inducers and during a 16 h cocultivation with PBL, IFN-alpha was absorbed by the membrane-bound polyclonal antibodies around IFN-alpha-secreting cells. This IFN-alpha was detected with murine monoclonal antibodies against IFN-alpha and peroxidase-labelled antibodies against murine immunoglobulin, using diaminobenzidine as substrate. Distinctly stained plaques were seen, the frequency of which gave a minimal estimate of approximately 10 IFN-alpha-producing cells in 10(4) PBL (range in 12 blood donors 2.95-25.1). Fewer plaques than expected were seen at low PBL numbers per culture, one explanation being that cell interactions then limit the IFN-alpha response. The immuno-plaque assay should be useful in further studies of the cellular basis of the IFN-alpha response.

Regulation of hematopoiesis by T lymphocytes and natural killer cells

T lymphocytes and natural killer (NK) cells exert both stimulatory and suppressive effects that regulate growth and differentiation of hematopoietic cells. Activated T and NK cells have been demonstrated in different pathological states of bone marrow failure and are proposed to play a role in the pathogenesis of the disease. T and NK cells have also been shown to be responsible for bone marrow graft rejection in both allogeneic and syngeneic donor/recipient combinations. Lymphocytes can regulate hematopoietic cell growth by direct cellular contact or by releasing soluble factors, such as colony-stimulating factors, immune interferon, lymphotoxin, and tumor necrosis factor, active on hematopoietic precursor cells.