Studies on immunological tolerance to LCM virus. 9. Induction of immunological tolerance to the virus in the adult mouse

Differential tissue-specific regulation of antiviral CD8+ T-cell immune responses during chronic viral infection

The hallmarks of the immune response to viral infections are the expansion of antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) after they encounter antigen-presenting cells in the lymphoid tissues and their subsequent redistribution to nonlymphoid tissues to deal with the pathogen. Control mechanisms exist within CTL activation pathways to prevent inappropriate CTL responses against disseminating infections with a broad distribution of pathogen in host tissues. This is demonstrated during overwhelming infection with the noncytolytic murine lymphocytic choriomeningitis virus, in which clonal exhaustion (anergy and/or deletion) of CTLs prevents immune-mediated pathology but allows persistence of the virus. The mechanism by which the immune system determines whether or not to mount a full response to such infections is unknown. Here we present data showing that the initial encounter of specific CTLs with infected cells in lymphoid tissues is critical for this decision. Whether the course of the viral infection is acute or persistent for life primarily depends on the degree and kinetics of CTL exhaustion in infected lymphoid tissues. Virus-driven CTL expansion in lymphoid tissues resulted in the migration of large quantities of CTLs to nonlymphoid tissues, where they persisted at stable levels. Surprisingly, although virus-specific CTLs were rapidly clonally exhausted in lymphoid tissues under conditions of chronic infection, a substantial number of them migrated to nonlymphoid tissues, where they retained an effector phenotype for a long time. However, these cells were unable to control the infection and progressively lost their antiviral capacities (cytotoxicity and cytokine secretion) in a hierarchical manner before their eventual physical elimination. These results illustrate the differential tissue-specific regulation of antiviral T-cell responses during chronic infections and may help us to understand the dynamic relationship between antigen and T-cell populations in many persistent infections in humans.

Persistence of lymphocytic choriomeningitis virus at very low levels in immune mice

Lymphocytic choriomeningitis virus (LCMV), strain WE, is a non-cytopathic RNA virus that is highly adapted to its natural host, the mouse. Acute infection of adult mice leads to generalized virus spread, followed by cytotoxic T lymphocyte-mediated virus clearance below the detection levels of conventional assays within 2-3 weeks. Indirect evidence had suggested that virus or viral antigen might persist in the immune mouse. Here we demonstrate LCMV-WE persistence at low levels after infection with 10(2) or 10(6) plaque-forming units, shown as viral genome, viral antigen, and replicative virus using sensitive in vitro and in vivo assays. The finding that LCMV-WE persists in the face of apparently intact immune responses resembles the situation in some viral (hepatitis B and C, HIV) and bacterial (tuberculosis, leprosy) infections in humans; the results are relevant to the understanding not only of other murine and human persistent viral infections but also of protective immunological memory by "infection immunity."

Mechanism of recovery from acute virus infection. II. Effect of treatment of mice with cyclosporin A on their ability to eliminate the lymphocytic choriomeningitis virus

Illness and death of mice infected by intracerebral inoculation with lymphocytic choriomeningitis virus are pathologic immune phenomena, and mice thus infected were protected when immunosuppressed with cyclosporin A (CSA), although the virus multiplied extensively in all major organs. Concentrations remained high as long as the drug was administered but declined when the treatment was discontinued; also, cytotoxic T lymphocytes appeared. Virus elimination was slower and cytotoxic T cell activity lower than in mice not previously treated with CSA.

Dose-response relationships of lymphocytic choriomeningitis viruses in mice and L cell tube cultures

The dose—response relationships between two strains of lymphocytic choriomeningitis virus (WE3and Armstrong) and two hosts (mice and L cell tube cultures) were determined. The statistical analysis showed that, if infection was regarded as the response, and in the case of Armstrong virus in mice also death, the shapes of the empirical dose—response curves did not deviate from expectation which was based on the zero term of the Poisson distribution. Hence, the hypothesis that individual infectious units are capable of initiating infection and that co-operation is not required, was not contradicted. Furthermore, the units of assay were found to be equally susceptible under the experimental conditions applied.

By way of contrast, the relationship between WE3virus and mice dying after intracerebral inoculations was found to be more complex. In this case the empirical curve did not run the expected sigmoid course at all, but rather was bell-shaped with a maximum of mortality (92%) at approximately 50 ID 50.

The work was supported by a research grant from the Deutsche Forschungs-gemeinschaft.