Persistent virus infection despite chronic cytotoxic T-lymphocyte activation in gamma interferon-deficient mice infected with lymphocytic choriomeningitis virus

Abstract

The role of gamma interferon (IFN-gamma) in the permanent control of infection with a noncytopathic virus was studied by comparing immune responses in wild-type and IFN-gamma-deficient (IFN-gamma -/-) mice infected with a slowly invasive strain of lymphocytic choriomeningitis virus (LCMV Armstrong). While wild-type mice rapidly cleared the infection, IFN-gamma -/- mice became chronically infected. Virus persistence in the latter mice did not reflect failure to generate cytotoxic T-lymphocyte (CTL) effectors, as an unimpaired primary CTL response was observed. Furthermore, while ex vivo CTL activity gradually declined in wild-type mice, long-standing cytolytic activity was demonstrated in IFN-gamma -/- mice. The prolonged effector phase in infected IFN-gamma -/- mice was associated with elevated numbers of CD8(+) T cells. Moreover, a higher proportion of these cells retained an activated phenotype and was actively cycling. However, despite the increased CD8(+) T-cell turnover, which might have resulted in depletion of the memory CTL precursor pool, no evidence for exhaustion was observed. In fact, at 3 months postinfection we detected higher numbers of LCMV-specific CTL precursors in IFN-gamma -/- mice than in wild-type mice. These findings indicate that in the absence of IFN-gamma, CTLs cannot clear the infection and are kept permanently activated by the continuous presence of live virus, resulting in a delicate new balance between viral load and immunity. This interpretation of our findings is supported by mathematical modeling describing the effect of eliminating IFN-gamma-mediated antiviral activity on the dynamics between virus replication and CTL activity.

FIG. 1

Organ virus titers in IFN-γ −/− (●) and wild-type (○) B6 mice infected i.v. with 4,800 PFU of LCMV Armstrong. Points represent individual mice. (Parallel analysis of organ virus levels in mice deficient in both IFN-γ and perforin revealed titers roughly 3 log₁₀ higher [not shown].) LD₅₀, mean lethal dose.

FIG. 2

Time course of ex vivo T-cell-mediated cytotoxicity in IFN-γ −/− and wild-type (wt) B6 mice infected i.v. with 4,800 PFU of LCMV Armstrong. Cytotoxicity toward LCMV GP33-41 (gp)- and NP396-404 (np)-pulsed, histocompatible EL-4 cells was analyzed; unpulsed EL-4 cells served as control (con) targets. CTL activity in individual mice was assayed; medians of two to eight mice per time point are depicted. On day 60 p.i., effector/target ratios were 40:1, 20:1, 10:1, and 5:1; at all other time points, the ratios were 80:1, 40:1, 20:1, and 10:1.

FIG. 3

Phenotypic analysis of splenic CD8⁺ T cells in B6 and IFN-γ −/− mice infected i.v. with 4,800 PFU of LCMV Armstrong. (A) CD62L and BrdU expression on CD8⁺ T cells. Numbers in parentheses refer to mean sizes of the same subsets in uninfected controls. (B) Total number of splenic CD8⁺ T cells as a function of time after infection. Values shown are averages ± standard deviations from groups of four to five mice. (C) Phenotype of CD8⁺ BrdU⁺ splenocytes as a function of time after infection. Values shown are averages ± standard deviations from groups of four to five mice. Note that on day 240 p.i., only two IFN-γ −/− mice were analyzed.

FIG. 4

Effect of IFN-γ-mediated virus inhibition (q) on the outcome of LCMV infection, as predicted by the mathematical model. IFN-γ −/− mice are characterized by a value of q = 0. (a) Effect of IFN-γ-mediated virus inhibition on the context of CTL-induced pathology, described by the total number of target cells (uninfected plus infected) at equilibrium. Reducing IFN-γ activity results in increased levels of target cell depletion. However, the exact extent of CTL-mediated pathology strongly depends on the replication rate of the virus, β. The faster the replication kinetics of the virus, the stronger the degree of CTL-induced pathology, especially in IFN-γ −/− mice. If viral replication is slow, there is no significant pathology even in the absence of IFN-γ. (b) Effect of IFN-γ-mediated virus inhibition on the level of virus load and CTL activity at equilibrium. Decreasing the rate of IFN-γ activity results in an increase in both virus load and virus-specific CTL activity. Baseline parameters were chosen as follows: λ = 10; d = 0.1; β = 0.05; a = 0.1; p = 0.1; c = 0.2; b = 0.1.

FIG. 5

Time series of virus load and CTL activity in wild-type (q = 1) and IFN-γ −/− (q = 0) mice as predicted by the mathematical model. In IFN-γ −/− mice, virus load and CTL activity are higher than in wild-type mice, both during the acute phase of the infection and at equilibrium. Parameters were chosen as follows: λ = 10l; d = 0.1; β = 0.01; a = 0.1; p = 1; c = 0.5; b = 0.1.