Counting Antigen-Specific CD8 T Cells: A Reevaluation of Bystander Activation during Viral Infection

Interleukin-6 (IL-6) is a multifunctional cytokine that regulates various aspects of the immune response, acute-phase reaction and haematopoiesis (for reviews see refs 1, 2). In vitro, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and interleukin-11 display overlapping activities with IL-6. This functional redundancy may be explained by the interactions of specific binding receptors with a common signal-transducing receptor (gp130) (for reviews see refs 3, 4). To elucidate the unique function of IL-6 in vivo, we have disrupted the IL-6 gene by homologous recombination. IL-6-deficient mice develop normally. They fail to control efficiently vaccinia virus and infection with Listeria monocytogenes, a facultative intracellular bacterium. The T-cell-dependent antibody response against vesicular stomatitis virus is impaired. Further, the inflammatory acute-phase response after tissue damage or infection is severely compromised, whereas it is only moderately affected after challenge with lipopolysaccharide. We conclude that IL-6 production induced by injury or infection is an important in vivo SOS signal which coordinates activities of liver cells, macrophages and lymphocytes.

Impaired immune and acute-phase responses in interleukin-6-deficient mice

The immune system can remember, sometimes for a lifetime, the identity of a pathogen. Understanding how this is accomplished has fascinated immunologists and microbiologists for many years, but there is still considerable debate regarding the mechanisms by which long-term immunity is maintained. Some of the controversy stems from a failure to distinguish between effector and memory cells and to define their roles in conferring protection against disease. Here the current understanding of the cellular basis of immune memory is reviewed and the relative contributions made to protective immunity by memory and effector T and B cells are examined.

https://science.sciencemag.org/content/sci/272/5258/54.full.pdf

Immunological Memory and Protective Immunity: Understanding Their Relation

Viral clearance during hepatitis B virus (HBV) infection has been thought to reflect the destruction of infected hepatocytes by CD8 + T lymphocytes. However, in this study, HBV DNA was shown to largely disappear from the liver and the blood of acutely infected chimpanzees long before the peak of T cell infiltration and most of the liver disease. These results demonstrate that noncytopathic antiviral mechanisms contribute to viral clearance during acute viral hepatitis by purging HBV replicative intermediates from the cytoplasm and covalently closed circular viral DNA from the nucleus of infected cells.

https://science.sciencemag.org/content/sci/284/5415/825.full.pdf

Viral Clearance Without Destruction of Infected Cells During Acute HBV Infection

In this study, we have examined the relative contributions of CD4+ and CD8+ T cells in controlling an acute or chronic lymphocytic choriomeningitis virus (LCMV) infection. To study acute infection, we used the LCMV Armstrong strain, which is cleared by adult mice in 8 to 10 days, and to analyze chronic infection, we used a panel of lymphocyte-tropic and macrophage-tropic variants of LCMV that persist in adult mice for several months. We show that CD4+ T cells are not necessary for resolving an acute LCMV infection. CD4+ T-cell-depleted mice were capable of generating an LCMV-specific CD8+ cytotoxic T-lymphocyte (CTL) response and eliminated virus with kinetics similar to those for control mice. The CD8+ CTL response was critical for resolving this infection, since beta 2-microglobulin knockout (CD8-deficient) mice were unable to control the LCMV Armstrong infection and became persistently infected. In striking contrast to the acute infection, even a transient depletion of CD4+ T cells profoundly affected the outcome of infection with the macrophage- and lymphocyte-tropic LCMV variants. Adult mice given a single injection of anti-CD4 monoclonal antibody (GK1.5) at the time of virus challenge became lifelong carriers with high levels of virus in most tissues. Unmanipulated adult mice infected with the different LCMV variants contained virus for prolonged periods (> 3 months) but eventually eliminated infection from most tissues, and all of these mice had LCMV-specific CD8+ CTL responses. Although the level of CTL activity was quite low, it was consistently present in all of the chronically infected mice that eventually resolved the infection. These results clearly show that even in the presence of an overwhelming viral infection of the immune system, CD8+ CTL can remain active for long periods and eventually resolve and/or keep the virus infection in check. In contrast, LCMV-specific CTL responses were completely lost in chronically infected CD4-depleted mice. Taken together, these results show that CD4+ T cells are dispensable for short-term acute infection in which CD8+ CTL activity does not need to be sustained for more than 2 weeks. However, under conditions of chronic infection, in which CD8+ CTLs take several months or longer to clear the infection, CD4+ T-cell function is critical. Thus, CD4+ T cells play an important role in sustaining virus-specific CD8+ CTL during chronic LCMV infection. These findings have implications for chronic viral infections in general and may provide a possible explanation for the loss of human immunodeficiency virus-specific CD8+ CTL activity that is seen during the late stages of AIDS, when CD4+ T cells become limiting.

CD4+ T cells are required to sustain CD8+ cytotoxic T-cell responses during chronic viral infection.

Transgenic mice homozygous for a beta 2-microglobulin (beta 2-m) gene disruption and normal mice that had been treated with a CD8-specific mAb were infected intranasally with an H3N2 influenza A virus. Both groups of CD8T cell-deficient mice eliminated the virus from the infected respiratory tract. Potent CTL activity was detected in lung lavage populations taken from mice with intact CD8+ T cell function, with minimal levels of cytotoxicity being found for inflammatory cells obtained from the antibody-treated and beta 2-m mutant mice. We therefore conclude that cells infected with an influenza A virus can be cleared from the respiratory tract of mice lacking both functional class I major histocompatibility complex (MHC) glycoproteins and class I MHC-restricted, CD8+ effector T cells.

https://rupress.org/jem/article-pdf/174/4/875/1101898/875.pdf

Clearance of Influenza Virus Respiratory Infection in Mice Lacking Class I Major Histocompatibility Complex-restricted CD8 + T Cells

The cellularity of the mediastinal lymph nodes of mice infected intranasally with a high dose of an H3N2 influenza A virus increases massively within 5 days. All classes of lymphocytes are involved. A similar, but much smaller, expansion in cell numbers occurs after exposure to a comparable dilution of normal chick allantoic fluid. In the control group, this increase in lymph node size is totally prevented by the in vivo depletion of CD4+ T cells whereas there is only a 50% reduction in the virus-infected mice. The lymphocyte component of the cellular exudate in the lungs of infected mice is dominated by activated, CD8+ T cells, which are also prevalent in the mediastinal lymph nodes. Elimination of the CD4+ subset does not greatly diminish the severity of this inflammatory process. The CD4-depleted mice clear the virus from the lung, and there is little effect on the frequency of virus-specific, cytotoxic T lymphocyte precursors in either the lymph node or the lung. Substantial involvement of CD4+ T cells is not essential for the development of effective cell-mediated immunity in mice with influenza.

Cellular events in the lymph node and lung of mice with influenza. Consequences of depleting CD4+ T cells.

The current status of T-cell subset involvement in viral immunity is summarized for experimental studies in mice. The immunobiology of the normal host response is discussed, with particular reference to lymphocytic choriomeningitis (LCM) and influenza. The general impression is that CD8+ cytotoxic T lymphocytes, CD4+ TH1 cells, γ interferon, and IL-2 are of major importance, with these different components of the immune system interacting to promote an optimal response. However, experiments with a variety of virus systems indicate that there is considerable plasticity, at least in young, adult mice. Other mechanisms often compensate if a key lymphocyte subset is absent throughout the development of the immune response. Influenza-infected mice depleted of either CD4+ or CD8+ T cells clear virus and recover, though the latter may not be true for the elimination of LCM Virus. Emerging information on the involvement of γδ T cells in viral pneumonia is summarized, but there is as yet no understanding of the bi...

Roles of alpha beta and gamma delta T cell subsets in viral immunity.

The patterns of cytokine mRNA expression in mice with primary or secondary influenza pneumonia have been assessed by in situ hybridization analysis of cells from both the mediastinal lymph node (MLN) and the virus-infected lung. Evidence of substantial transcriptional activity was found in all lymphocyte subsets recovered from both anatomical sites. The kinetics of cytokine mRNA expression after primary infection with an H3N2 virus were in accord with the idea that the initial response occurs in regional lymphoid tissue, with the effector T cells later moving to the lung. This temporal separation was much less apparent for the more rapid secondary response resulting from challenge of H3N2-primed mice with an H1N1 virus. Among the T cell receptor alpha/beta+ subsets, transcripts for interferon (IFN) gamma and tumor necrosis factor beta were most commonly found in the CD8+ population whereas mRNA for interleukin (IL) 4 and IL-10 was much more prevalent in CD4+ T cells. The gamma/delta T cells expressed mRNA for all cytokines tested, with IL-2, IL-4, and IFN-gamma predominating among those recovered from the inflammatory exudate. At particular time points, especially early in the MLN and late in the infected lung, the frequency of mRNA+ lymphocytes was much higher than would be expected from current understanding of the prevalence of virus-specific precursors and effectors. If this response is typical, induction of cytokine gene expression for T cells that are not responding directly to the invading pathogen may be a prominent feature of acute virus infections.

Activation of cytokine genes in T cells during primary and secondary murine influenza pneumonia.

The distribution and clearance of viral RNA (vRNA) and mRNA has been analysed for the acute and recovery stages of the pneumonia induced by intranasal infection of C57BL/6J mice with H3N2 influenza A viruses. Amplification of viral genomic material by the polymerase chain reaction showed that the influenza haemagglutinin (HA) gene was eliminated from the lungs of immunologically intact mice by day 14 post-infection, whereas in vitro depletion of the CD4+ T cells delayed clearance by at most 4 days. Viral RNA encoding the HA gene was first demonstrated in the regional mediastinal lymph nodes at 48 h, and continued to be present until day 6 or day 10 after infection of the intact and CD4-depleted mice, respectively. Evidence for the presence of vRNA in the thymus, but not in the mesenteric lymph nodes or the spleen, was found in some situations. Otherwise, the distribution and clearance of vRNA was as would be predicted from earlier studies using virus isolation procedures to monitor localization patterns, and shows a lack of long-term persistence of the influenza virus genome.

William Allan

Papers

Clearance of Influenza Virus Respiratory Infection in Mice Lacking Class I Major Histocompatibility Complex-restricted CD8 + T Cells

Cellular events in the lymph node and lung of mice with influenza. Consequences of depleting CD4+ T cells.

Roles of alpha beta and gamma delta T cell subsets in viral immunity.

Activation of cytokine genes in T cells during primary and secondary murine influenza pneumonia.

Influenza virus RNA in the lung and lymphoid tissue of immunologically intact and CD4-depleted mice