Lysis of uninfected and virus-infected cells in vivo: a rejection mechanism in addition to that mediated by natural killer cells

Rapid conversion of effector mechanisms from NK to T cells during virus-induced lysis of allogeneic implants in vivo

Viral infections can strongly stimulate both NK cell and allospecific CD8 T cell responses, and these same effector cells can lyse allogeneic cell lines in vitro. However, the impact of viral infections on the effector systems mediating rejection of allogeneic tissues in vivo has not been fully explored. Using in vivo cytotoxicity assays, we evaluated the effector systems mediating the rejection of CFSE-labeled allogeneic splenocytes after an infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus. Naive B6 mice predominantly used a NK cell-effector mechanism to reject allogeneic splenocytes because they rejected BALB/C (H2(d)) splenocytes but not CBA (H2(k)) splenocytes, and the rejection was prevented by immunodepletion of NK1.1(+) or Ly49D(+) NK cells. This rapid and efficient in vivo cytotoxicity assay recapitulated the specificity of NK cell-mediated rejection seen in longer duration in vivo assays. However, as early as 1 day after infection with lymphocytic choriomeningitis virus, a CD8 T cell-dependent mechanism participated in the rejection process and a broader range of tissue haplotypes (e.g., H2(k)) was susceptible. The CD8 T cell-mediated in vivo rejection process was vigorous at a time postinfection (day 3) when NK cell effector functions are peaking, indicating that the effector systems used in vivo differed from those observed with in vitro assays measuring the killing of allogeneic cells. This rapid generation of allospecific CTL activity during a viral infection preceded the peak of viral epitope-specific T cell responses, as detected by in vivo or in vitro cytotoxicity assays.

Role of an intact splenic microarchitecture in early lymphocytic choriomeningitis virus production

An acute infection with lymphocytic choriomeningitis virus (LCMV) is efficiently controlled by the cytotoxic-T-cell (CTL) response of the host, and LCMV titers in the spleen and peripheral solid organs usually fall sharply after day 4 to 6 postinfection. Surprisingly, infection of immunodeficient recombination-activating gene 2-deficient (RAG2-/-) mice with 5 x 10(2) PFU of LCMV-WE causes about 80-fold-lower LCMV titers in the spleen on day 4 postinfection compared with C57BL/6 control mice. This could not be attributed to NK cell activity, since common gamma-chain-deficient RAG2-/- mice lacking NK cells show low LCMV titers comparable to those for RAG2-/- mice. Furthermore, the reduced early LCMV production in spleens could not be explained by an enhanced gamma interferon production in RAG2-/- mice. Analysis of mutant mice exhibiting various defects in the splenic microarchitecture, including (i) tumor necrosis factor alpha-negative (TNF-alpha-/-), lymphotoxin alpha-negative (LTalpha-/-), B-cell-deficient muMT mice, (ii) immunoglobulin M-negative mice, and (iii) RAG2-/- mice reconstituted with wild-type versus TNF-alpha-/- LTalpha-/- B cells, revealed a clear correlation between an intact splenic marginal zone, rapid early replication of LCMV in the spleen, and efficient CTL induction. These results suggest that by the preferential infection of the highly organized splenic microarchitecture, LCMV seems to successfully exploit one of the key elements in the chain of the adaptive immune system. Not only does the early tropism of LCMV for the splenic marginal zone trigger a potent immune response, but at the same time the marginal zone may also become a target of early CTL-mediated immunopathology that impairs immune responsiveness.

Natural killer cells in antiviral defense: function and regulation by innate cytokines

Natural killer (NK) cells are populations of lymphocytes that can be activated to mediate significant levels of cytotoxic activity and produce high levels of certain cytokines and chemokines. NK cells respond to and are important in defense against a number of different infectious agents. The first indications for this function came from the observations that virus-induced interferons alpha/beta (IFN-alpha and -beta) are potent inducers of NK cell-mediated cytotoxicity, and that NK cells are important contributors to innate defense against viral infections. In addition to IFN-alpha/beta, a wide range of other innate cytokines can mediate biological functions regulating the NK cell responses of cytotoxicity, proliferation, and gamma interferon (IFN-gamma) production. Certain, but not all, viral infections induce interleukin 12 (IL-12) to elicit NK cell IFN-gamma production and antiviral mechanisms. However, high levels of IFN-alpha/beta appear to be unique and/or uniquely dominant in the context of viral infections and act to regulate other innate responses, including induction of NK cell proliferation in vivo and overall negative regulation of IL-12 production. A detailed picture is developing of particular innate cytokines activating NK cell responses and their consorted effects in providing unique endogenous milieus promoting downstream adaptive responses, most beneficial in defense against viral infections.

Mechanism of killing by virus-induced cytotoxic T lymphocytes elicited in vivo

The mechanism of lysis by in vivo-induced cytotoxic T lymphocytes (CTL) was examined with virus-specific CTL from mice infected with lymphocytic choriomeningitis virus (LCMV). LCMV-induced T cells were shown to have greater than 10 times the serine esterase activity of T cells from normal mice, and high levels of serine esterase were located in the LCMV-induced CD8+ cell population. Serine esterase was also induced in purified T-cell preparations isolated from mice infected with other viruses (mouse hepatitis, Pichinde, and vaccinia). In contrast, the interferon inducer poly(I.C) only marginally enhanced serine esterase in T cells. Serine esterase activity was released from the LCMV-induced T cells upon incubation with syngeneic but not allogeneic LCMV-infected target cells. Both cytotoxicity and the release of serine esterase were calcium dependent. Serine esterase released from disrupted LCMV-induced T cells was in the form of the fast-sedimenting particles, suggesting its inclusion in granules. Competitive substrates for serine esterase blocked killing by LCMV-specific CTL, but serine esterase-containing granules isolated from LCMV-induced CTL, in contrast to granules isolated from a rat natural killer cell tumor line, did not display detectable hemolytic activity. Fragmentation of target cell DNA was observed during the lytic process mediated by LCMV-specific CTL, and the release of the DNA label [125I]iododeoxyuridine from target cells and the accompanying fragmentation of DNA also were calcium dependent. These data support the hypothesis that the mechanism of killing by in vivo-induced T cells involves a calcium-dependent secretion of serine esterase-containing granules and a target cell death by a process involving nuclear degradation and DNA fragmentation.

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.

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.

Pathogenesis of murine cytomegalovirus infection in natural killer cell-depleted mice

The effect of natural killer (NK) cells on the course of acute and persistent murine cytomegalovirus (MCMV) infection was examined by selectively depleting NK cell activity by inoculation of mice with antibody to asialo GM1, a neutral glycosphingolipid present at high concentrations on NK cells. The dose of MCMV required to cause 50% mortality or morbidity in control C57BL/6 mice dropped 4- and greater than 11-fold, respectively, in mice first treated with anti-asialo GM1. NK cell-depleted mice had higher (up to 1,000-fold) virus titers in their lungs, spleens, and livers at days 3, 5, 7, and 9 postinfection. Spleens and livers of control mice were virus-free by day 7 postinfection, and their lungs showed no signs of active infection at any time. In contrast, MCMV had disseminated to the lungs of NK cell-depleted mice by day 5, and these mice still had moderate levels of virus in their lungs, spleens, and livers at day 9. Markedly severe pathological changes were noted in the livers and spleens of NK cell-depleted, MCMV-infected mice. These included ballooning degeneration of hepatocytes and spleen necrosis. MCMV-infected, NK cell-depleted mice had severe spleen leukopenia, and their spleen leukocytes exhibited a significantly lower (up to 13-fold) response to the T cell mitogen concanavalin A when compared with those of uninfected and MCMV-infected controls. It appeared that NK cells exerted their most potent antiviral effect early in the infection, in a pattern correlating with interferon production and NK cell activation; treatment with anti-asialo GM1 later in infection had no effect on virus titers. The relative effect of NK cell depletion on MCMV pathogenesis depended on the injection route of the virus. NK cell depletion greatly augmented MCMV synthesis and pathogenesis in mice inoculated either intravenously or intraperitoneally but had no effect on the course of disease after intranasal inoculation, at any time point examined. One month after intraperitoneal inoculation of virus, NK cell depletion resulted in a six- to eightfold increase in salivary gland virus titers in persistently infected mice, suggesting that NK cells may be important in controlling virus synthesis in the salivary gland during persistent infection. This treatment did not, however, induce dissemination of virus to other organs. These data support the hypothesis that NK cells limit the severity, extent, and duration of acute MCMV infection and that they may also be involved in regulating the persistent infection.