Mechanism of recovery from acute virus infection. III. Subclass of T lymphocytes mediating clearance of lymphocytic choriomeningitis virus from the spleens of mice

The hepatitis C virus non-structural NS5A protein impairs both the innate and adaptive hepatic immune response in vivo

The role of hepatitis C virus (HCV) protein non-structural (NS) 5A in HCV-associated pathogenesis is still enigmatic. To investigate the in vivo role of NS5A for viral persistence and virus-associated pathogenesis a transgenic (Tg) mouse model was established. Mice with liver-targeted NS5A transgene expression were generated using the albumin promoter. Alterations in the hepatic immune response were determined by Western blot, infection by lymphocytic choriomeningitis virus (LCMV), and using transient NS3/4A Tg mice generated by hydrodynamic injection. Cytotoxic T lymphocyte (CTL) activity was investigated by the Cr-release assay. The stable NS5A Tg mice did not reveal signs of spontaneous liver disease. The intrahepatic immunity was disrupted in the NS5A Tg mice as determined by clearance of LCMV infection or transiently NS3/4A Tg hepatocytes in vivo. This impaired immunity was explained by a reduced induction of interferon beta, 2',5'-OAS, and PKR after LCMV infection and an impairment of the CTL-mediated elimination of NS3-expressing hepatocytes. In conclusion, these data indicate that in the present transgenic mouse model, NS5A does not cause spontaneous liver disease. However, we discovered that NS5A could impair both the innate and the adaptive immune response to promote chronic HCV infection.

Transient control of a virus-induced immunopathology by genetic immunosuppression

The ability to control T cell reactivity using suicide genes opens new perspectives for the treatment of T cell-mediated diseases. The therapeutic effect is achieved by the selective killing of thymidine kinase gene-modified activated T cells by ganciclovir (GCV). This strategy has been shown to control T cell alloreactivity efficiently after bone marrow or solid organ transplantation. Here, we aimed to determine whether an immunopathological process induced by a viral infection could be controlled by GCV when T cells express a thymidine kinase transgene. When transgenic mice were infected with the lymphocytic choriomeningitis virus, administration of GCV resulted in an efficient, but only transient, control of the immunopathological immune response. Further analysis revealed the existence of a minute population of GCV-insensitive T cells. These cells expand in response to the virus despite the presence of GCV and cause immunopathology before viral elimination is finally obtained. Thus, when confronted with a replicative virus, the efficacy of this genetic immunosuppression strategy is highly dependent on the presence of even small numbers of GCV-insensitive cells. These results emphasize the need for sufficient preclinical investigations with regard to the pathology and the nature of the immune response if suicide gene transfer is envisioned for new therapeutic indications.

Intestinal intraepithelial lymphocytes exert potent protective cytotoxic activity during an acute virus infection

After systemic infection of mice with 104 PFU of lymphocytic choriomeningitis virus (LCMV), infected cells are detected simultaneously in various organs, including spleen and intestinal mucosa. Most notably, virus-infected cells are also present among CD11c+ dendritic cells in the subepithelial area of the small intestinal mucosa. Some of these virus-infected cells are in close spatial association with intestinal intraepithelial lymphocytes (IEL). Therefore, we compared virus-specific cytotoxic activity of CD8 splenocytes with that of IEL subsets. While ex vivo isolated TCRalphabeta+CD8alphaalpha+ IEL exert only minimal virus-specific cytotoxicity, maximum specific killing mediated by TCRalphabeta+CD8alphabeta+ IEL on day 8 postinfection exceeds maximum cytotoxic activity observed with CD8 splenocytes when assessed in vitro. Maximum cytotoxic activity of IEL is preceded by peak perforin and granzyme B mRNA expression in IEL around day 6 postinfection, suggesting a recent activation in situ. The antivirus cytotoxicity of in vivo primed IEL is further demonstrated by the protection from virus production in the spleen of mice infected with LCMV 10 h before adoptive cell transfer. These data indicate a potent priming of LCMV-specific IEL in situ after systemic LCMV infection and suggest that cytotoxic IEL markedly contribute to the elimination of virus-infected cells in the intestinal mucosa.

Mechanisms of viral clearance and persistence

Using examples predominantly drawn from study of the lymphocytic choriomeningitis virus (LCMV) model system, this review describes the mechanisms involved in control of virus infections by the cell-mediated immune response, and some of the different strategies viruses have evolved to evade such immune clearance so that they can persist in their hosts. The important role played by the CD8+ cytotoxic lymphocyte (CTL) response in clearance of many systemic virus infections is discussed; and it is emphasized that although CD8+ CTL are classically thought of as lymphocytes which mediate lysis of virus-infected target cells, the principal mechanism by which CD8+ T cells effect clearance of persistent and many acute virus infections via production of antiviral cytokines such as tumour necrosis factor-alpha and interferon-gamma, not via destruction of virus-producing cells. To avoid immune-mediated clearance, viruses frequently use a combination of several different strategies. These can be grouped into mechanisms for avoiding recognition by the immune response (such as establishing latent infections, replicating in immune-privileged sites, down-regulating the expression of immune recognition signals on the surface of infected cells, or undergoing antigenic variation); and mechanisms for suppressing the immune response. The latter include generalized immune suppression mechanisms, and strategies for more precisely disabling the specific immune response such as inducing tolerance or exhaustion of virus-specific CTL. The value of understanding both immune clearance mechanisms and viral evasion strategies in the rational design of immune-based therapies to combat persistent virus infections is discussed.

A functional and kinetic comparison of antiviral effector and memory cytotoxic T lymphocyte populations in vivo and in vitro

To analyze the critical parameters for effective antiviral cytotoxic T lymphocyte (CTL) activity in vivo, control of lymphocytic choriomeningitis virus (LCMV) infection in the spleen was studied after adoptive transfer of different spleen cell populations into preinfected recipients. The quantitative, qualitative and kinetic requirements for virus control were defined and related to in vitro assays to compare the antiviral protective function of CTL from naive, acutely infected and memory mice. Treatment of mice with an established but limited LCMV infection by adoptive transfer of spleen cells from acutely LCMV-infected mice led to complete virus elimination mainly mediated by donor-derived CD8+ T cell-mediated, perforin-dependent cytotoxicity. Since virus is continuously spreading and the number of infected target cells rapidly increases, the time until target cell lysis is achieved was critical: if release of viral progeny was not prevented early, additional time to perform effector function did not improve overall virus control. When the function of various cell populations was compared in this model, we found that CTL from naive and memory mice perform considerably less well than CTL from acutely infected mice. In vitro studies indicated that this is probably due to the fact that they can not fulfill the limiting time requirements for immediate antiviral protection: while CTL from acutely infected mice can perform lytic effector function immediately, memory CTL require a considerable reactivation time before they can lyse infected target cells. This reactivation does not necessarily involve cell division. These findings illustrate how critical time limitations are for CTL to mediate early control of a dynamic virus infection in vivo.

Different roles for cytotoxic T cells in the control of infections with cytopathic versus noncytopathic viruses

The assessment of the role of T-cell-mediated cytotoxicity in immunity to viral infections has been difficult to address directly and therefore has been controversial. Recent experiments with perforin-deficient mice have shown that cytotoxicity is crucial for the resolution of infection with lymphocytic choriomeningitis virus but not for the resolution of infection with vaccinia, vesicular stomatitis, Semliki Forest or influenza virus. These findings may reflect the general pattern that T-cell-mediated cytotoxicity is crucial only for the resolution of infections with noncytopathic viruses, whereas infections with cytopathic viruses are mainly controlled by soluble mediators such as antibodies and interferons.

Analysis of neutralizing antibody specificities of different strains of lymphocytic choriomeningitis virus with strain-specific immune sera

The present study was aimed at comparing specificities and cross-reactivities of immune mouse sera obtained from mice infected with a low (2 x 10(2) or 20 immunological infectious focus units (ifu) or high (2 x 10(6) ifu) dose of various strains of lymphocytic choriomeningitis virus (LCMV). Neutralization titres of the various antisera were determined by an infectious focus reduction assay. This assay was performed on MC57G and on Vero E6 cells using the commonly utilized laboratory strains of LCMV WE, Docile, Armstrong, Armstrong Clone 13, Pasteur, Traub and Aggressive. Experiments with immune sera revealed broad cross-neutralization, demonstrating a variable but close serological relationship between the various strains of LCMV.

Immune response against lymphocytic choriomeningitis virus infection in mice without CD8 expression

The immune response against lymphocytic choriomeningitis virus (LCMV) was studied in a mutant mouse strain that does not possess CD8+ T lymphocytes. Virus-specific cytotoxic T cell activity was generated in spleens of wild-type mice in an acute LCMV infection but was not measurable in mutant mice. Injection of replicating LCMV into footpads of wild-type mice induced a CD8+ T cell-mediated swelling that peaked on day 8, followed by a CD4+ T cell-mediated swelling that peaked on day 11, whereas mutant mice exhibited only the CD4+ T cell-mediated swelling. After intracerebral inoculation with LCMV-Armstrong, all wild-type mice died of classical CD8+ T cell-dependent choriomeningitis in 8-10 days. Mutant mice showed symptoms of general malaise but most of them survived. Mutant mice depleted of CD4+ T cells by monoclonal antibody treatment showed no clinical signs of sickness. On day 9 after intravenous infection with LCMV-WE, virus was detected at high titers in spleens and livers of mutant mice but not in those of wild-type mice. On day 70 after injection of LCMV-WE into footpads, virus was not detected in wild-type mice and in one of the three mutant mice tested, but was still measurable in kidneys of the other two mutant mice. These results confirm in a new animal model that CD8+ T cell-mediated immunity is crucial in LCMV clearance and in the immunopathological disease during LCMV infection. In addition, our results demonstrated a less severe form of choriomeningitis mediated by CD4+ T cells and slow clearance of LCMV by alternative pathways independent of CD8+ T cells.

Mechanism of recovery from acute virus infection. VIII. Treatment of lymphocytic choriomeningitis virus-infected mice with anti-interferon-gamma monoclonal antibody blocks generation of virus-specific cytotoxic T lymphocytes and virus elimination

In acutely infected mice the lymphocytic choriomeningitis (LCM) virus multiplies to high titers in essentially all tissues. Around day 6, virus clearance sets in, which has previously been shown to be mediated by CD8+ cytotoxic T lymphocytes (CTL), probably by releasing (or inducing other cells to release) anti-viral cytokines. To ascertain whether interferon-gamma plays a role, infected mice were injected once i.v. with monoclonal antibody known to neutralize this lymphokine, and the effect this had on both termination of the infection and development of LCM virus-specific CTL was determined. Administration 1 day after infection blocked virus elimination from spleen and liver and decreased the generation of CTL; also, limiting dilution analysis revealed absence of activation of CTL precursors. In contrast, when the antibody was given 3 days after or 1 day before the virus, neither clearance nor generation of CTL was measurably affected. Furthermore, the antiviral effect of immune spleen cells after their transfer into infected recipients was not altered by treatment of the latter with monoclonal antibody. We conclude that in the generation of LCM virus-specific CTL an early event is dependent on constitutively produced interferon-gamma; when its activity is blocked, CTL do not mature, resulting in the mouse's inability to terminate the infection.

Clearance of lymphocytic choriomeningitis virus in antibody- and B-cell-deprived mice

The role of antibody in immune recovery from infection with lymphocytic choriomeningitis virus (LCMV) strain WE was evaluated in B-cell-depleted mice. Mice were treated from birth with either affinity-purified rabbit anti-mouse immunoglobulin M (IgM), normal rabbit immunoglobulin, or, alternatively, an affinity-purified monoclonal rat anti-mouse IgM antibody (LO-MM-9); untreated mice served as controls. B-cell depletion was considered complete in specifically treated mice according to the following criteria: absence of a significant response to the B-cell mitogen lipopolysaccharide, absence of B cells expressing immunoglobulin on their surfaces, absence of detectable IgM or IgG in serum, and presence in the serum of free anti-IgM antibodies. In organs of mu-suppressed BALB/c mice, LCMV-WE replicated, dependent upon organ, at the same rate or more rapidly and, in general, to higher titers than in normal rabbit immunoglobulin-treated mice; untreated mice eliminated the virus most rapidly and showed lower virus titers. In addition, LCMV-primed control mice cleared a second LCMV challenge very rapidly and contained no virus by day 3, whereas mu-suppressed mice had virus in their blood and organs (except the spleen) up to days 3 to 6. The observed effects of anti-mu treatment may reflect the action of neutralizing antibodies (which so far have been difficult to demonstrate in vivo) or other antibody-dependent antiviral mechanisms which, together with T cells, efficiently control LCMV clearance.

The immune response of the mouse to lymphocytic choriomeningitis virus. V. High numbers of cytolytic T lymphocytes are generated in the spleen during acute infection

In the spleens of C57BL/6J (B6) and CBA/J (CBA) mice undergoing acute infection with lymphocytic choriomeningitis (LCM) virus, lymphocytes with the ability to develop in vitro into LCM virus-specific cytolytic clones were enumerated by use of the limiting dilution method. At intervals after virus inoculation, defined numbers of cells were cultivated with virus-infected syngeneic stimulator cells and T cell growth factor in multiple wells of microculture plates. After 7 days, individual cell cultures were tested for their ability to cause release of 51Cr from infected and uninfected syngeneic target cells. In cultures seeded with spleen cells from uninfected mice or from mice infected 3 days previously, no cytolytic activity was observed. On day 5, cells developing into LCM virus-specific cytolytic effector cells were detected. They rose in numbers, and on days 8 to 9 after infection, values of approximately 1/10 and 1/200 in B6 and CBA mice, respectively, were calculated. A low proportion of microcultures proved cytolytic also for noninfected syngeneic target cells, but the counts thus released were consistently much lower than the counts set free from infected targets, and no regular dose-response relationships existed between seeded cells and positive cultures. Determination of cell surface antigens of responder cells by negative and positive selection procedures disclosed that they were predominantly T lymphocytes and expressed Lyt-2 but not L3T4 surface markers. Lysis by the great majority of LCM virus-specific clones was restricted by products of the major histocompatibility gene complex (MHC), but a few lysed, in addition, allogeneic infected or uninfected targets; however, a consistent pattern of alloreactivity was not observed. Furthermore, cells of a proportion of the cultures also lysed uninfected YAC cells. Probably this natural killer-like activity was acquired by T lymphocytes during prolonged cultivation. We conclude that most spleen cells that during acute infection with LCM virus attained the ability to develop in vitro into LCM virus-specific cytolytic clones were derived from MHC-restricted Lyt-2+, L3T4- antigen-specific cytolytic T lymphocytes and their activated precursors.

Mechanism of recovery from acute virus infection: treatment of lymphocytic choriomeningitis virus-infected mice with monoclonal antibodies reveals that Lyt-2+ T lymphocytes mediate clearance of virus and regulate the antiviral antibody response

After intravenous infection of mice, lymphocytic choriomeningitis virus multiplied in spleens and livers, attaining highest concentrations on days 4 to 6. The subsequent clearance was as rapid, and 8 to 10 days after inoculation, infectivity was usually below detectability. During the effector phase of virus elimination, both cytotoxic T-cell (CTL) activity and the number of cells producing antiviral antibodies were high. Monoclonal antibodies directed against T lymphocytes and T-lymphocyte subsets were inoculated once intravenously 5, 6, or 7 days after infection of the animals, and the effects on antiviral immune responses, as well as on elimination of virus from the organs, were determined. Treatment with anti-Thy-1 and anti-Lyt-2 antibodies blocked elimination of the virus and profoundly diminished the activity of spleen CTLs but reduced the antibody response partially (anti-Thy-1) or increased it (anti-Lyt-2). In contrast, treatment with the anti-L3T4 antibody had essentially no effect on either virus elimination or CTL response but abolished antibody production. We conclude that Lyt-2+ (cytotoxic-suppressive) T lymphocytes are needed for elimination of the virus and also regulate the humoral response but that antiviral antibodies are not essential for control of the infection.

A specific serine proteinase is inducible in Lyt-2+,L3T4- and Lyt-2-,L3T4+ T cells in vitro but is mainly associated with Lyt-2+,L3T4- effector cells in vivo

Recently, we and others reported on the expression of a serine proteinase in long-term cultured murine T lymphocyte cell lines. In an attempt to explore the distribution and possible regulation of this enzyme in T lymphocyte subsets, we performed the presented detailed study. We found that the proteinase is not expressed by thymocytes and resting T cells but can be induced by lectin or antigen in combination with lymphokine sources in vitro in macrophage-depleted unselected T cells as well as in both T cell subsets (Lyt-2+,L3T4- and Lyt-2-,L3T4+) separated by flow cytofluorometry. Furthermore, it appears that cell-associated proteinase activity is increasing with prolonged culture period of sensitized T lymphocytes and that it is higher in antigen-activated as compared to lectin-activated T cells. When tested for substrate specificity the T cell-associated proteinase was shown to preferentially cleave model peptide substrates carrying L-arginine at position P1 in combination with nonpolar amino acids at position P2 and P3. As concluded from its sensitivity to proteinase inhibitors the enzyme can be classified as a serine proteinase and by molecular sieving at high ionic strength it was shown to have a mol. mass of approximately 50-60 kDa. Analysis of in vivo activated T cells revealed that this particular proteinase was expressed in flow cytofluorometry sorted lymphocytic choriomeningitis virus-specific Lyt-2+,L3T4- cytolytic T lymphocytes but not in Lyt-2-,L3T4+ T cells presensitized with either Listeria monocytogenes or I-A alloantigens. The data demonstrate that the two T cell subsets (Lyt-2+,L3T4-; Lyt-2-,L3T4+) have distinct in vitro induction requirements for the expression of proteinase and that after activation of T cells in vivo the enzyme is preferentially associated with Lyt-2+,L3T4- effector cells.