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Keywords:

  • T cell;
  • Memory;
  • B cell;
  • LCMV

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 CD8+ T cells, CD4+ T cell help and antibody interact to control the levels of LCMV and other viruses
  5. 3 Implications for persistent virus infection in humans
  6. Acknowledgements

In animal models, lymphocytic choriomeningitis virus (LCMV) may be controlled after acute infection or may establish various levels of persistence. Cytotoxic responses mediated by CD8+ T cells are responsible for both initial control of LCMV and for immunopathology. As discussed in this article, there is emerging evidence that the levels of antigen to which the immune system is exposed over time are important in controlling CD8+ T cell activation, memory responses and exhaustion, and that these levels are affected by the efficiency of T cell help and the presence of antibody. To enable lasting control of LCMV infection, CD8+ T cells, CD4+ T cell help and B cells are all required. These findings have important implications for the preventionand treatment of infection by viruses such as hepatitis B and C viruses, cytomegalovirus and HIV.

Abbreviations:
HCV

Hepatitis C virus

LCMV:

Lymphocytic choriomeningitis virus

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 CD8+ T cells, CD4+ T cell help and antibody interact to control the levels of LCMV and other viruses
  5. 3 Implications for persistent virus infection in humans
  6. Acknowledgements

Most immunologists would agree that multiple pathways are involved in protective immune responses — particularly against tricky pathogens such as viruses. However, typically most analyses of cellular immune responses pay little attention to humoral immunity, in both human and animal studies. In this issue, Martin Bachmann and colleagues redress the balance with an investigation into the failure of "help deficient" mice to control lymphocytic choriomeningitis virus (LCMV) 1. LCMV is a model pathogen, which has been used for decades to analyse the immune responses against noncytopathic viruses 2. Cytotoxic T cell responses mediated by CD8+ T cells are responsible both for initial control of the virus and also for immunopathology inthis model. The extreme CD8+ T cell responses seen against LCMV, which are both very large and also possess strong lytic capacity, have been the focus of much recent attention in terms of the evolution of CD8+ T cell memory after acute infection 3. However, although CD8+ T cells do have a major role to play, they play it as part of a team, and it is this aspect that is the subject of the new work.

2 CD8+ T cells, CD4+ T cell help and antibody interact to control the levels of LCMV and other viruses

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 CD8+ T cells, CD4+ T cell help and antibody interact to control the levels of LCMV and other viruses
  5. 3 Implications for persistent virus infection in humans
  6. Acknowledgements

To understand the issue fully, it is important to recognize that LCMV — a variable RNA virus — is a persistent pathogen, although the degree of persistence varies from strain to strain and between different model systems. High-level persistence is well recognized, and long-term carrier states are readily established — a situation in which T cells are generally ultimately deleted 4. "Low level" persistence is much less well studied and probably more variable between models, but under some circumstances virus may exist for long periods in tissues, even when simple plaque assays may become negative 5. Although studies relying on detection of viral nucleic acids do not necessarily inform us of the true state of the virus, there are compellingexamples where virus is apparently cleared (especially from blood), only to re-emerge weeks later from some tissue source 6. The difficulties in defining the exact state of the virus in the "latent" situation should not come as a surprise, since it is also difficult even in examples such as cytomegaloviruses, which are known to establish long-term low-level persistence (see below) 7.

Earlier data from different groups studying the LCMV model have shown that although CD8+ T cells play a major part in host defense, it is also possible to identify important roles forB cells producing antibody, and for CD4+ T cells 6, 810. In many of these cases, although initial control over the virus was established, and normal CD8+ T cell responses emerged, mice failed to maintain long-term control, and recrudescence was seen after weeks or months. These initial studies did not define the exact mechanism for the loss of control. However, they did raise the profile of the antibody response, which, since it is typically relatively weak and delayed against LCMV, had been somewhat neglected. One could ask — given the obvious importance of CD4+ T cells and antibody — whether CD8+ T cells play any role in long-term virus control. Interestingly, although initial control over virus may be gained in the absence of CTL, in the long term the virus recrudesces again, through a combination of immune escape (from antibodies), and exhaustion/functional deletion of CD4+ T cells 1113. The lesson from these experiments — and others, where innate immunity is disrupted 14 — is that a full team is required. Failure of any one component can be temporarily compensated for by extra "work" from the other team members but this ultimately leads to loss of control over the virus through exhaustion/dysfunction or immune escape through viral mutation.

Previous studies in different settings have demonstrated an important role for CD40–CD40L interactions in establishing functional T cell memory (as discussed by Bachmann and colleagues 1). Since CD4+ T cell deficient mice have problems with control of LCMV and have dysfunctional CD8+ T cells, it might be assumed that failure of CD4+ T cells to provide help for CD8+ T cells through CD40–CD40L might be responsible for this phenotype. However, as Bachmann and colleagues show, failure to generate efficient B cell responses plays animportant part in allowing the CD8+ T cell response to fail. In the absence of effective help, the CD8+ T cell responses are ultimately lost and virus returns, but prior to that there are clues from the study that ongoing "low level" or tissue exposure is important. Populations of CD8+ T cells that do not see meaningful levels of antigen after acute infection typically regain expression of CD62L and revert to a "central" memory phenotype (CD62Lhigh). In the absence of help or of CD40–CD40L interactions, this does not occur, suggesting repeated antigen exposure despite the lack of detectable virus in blood. This effect is reversed by addition of anti-LCMV antibody, implying that additional control by humoral immunity can contain replication and prevent T cell activation.

A neat experiment using CFSE-labeled T cells to "measure" low-level antigen in one group of helpless (CD40L deficient) mice also suggests that the level of antigen presented to CD8+ Tcells is significantly higher in this setting. Thus, overall, the experiments do point to a potentially important role for antibody in "protecting" T cells from the effects of over-exposure to virus. Whether such overexposure results also in the emergence of escape mutations (as has been seen in other settings for LCMV), or simply all through dysfunction and deletion of T cells would be interesting to establish.

3 Implications for persistent virus infection in humans

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 CD8+ T cells, CD4+ T cell help and antibody interact to control the levels of LCMV and other viruses
  5. 3 Implications for persistent virus infection in humans
  6. Acknowledgements

The authors point out the potential implications for other viruses, particularly persistent human pathogens. One interesting comparison is with cytomegalovirus, which sets up low-level persistence in mouse and man, and where CD8+ T cells play an important role. In this setting, virus replication is never completely controlled, and T cells remain continuously exposed to some viral antigens throughout the "latent" period 7. In murine cytomegalovirus infection the virus-specific CD8+ T cells typically remain largely CD62Llow and do not revert to a central memory phenotype (CD62Lhigh). Nevertheless, unlike the LCMV helper deficient model described above, the responses in normal, immunocompetent mice do not decline over time, but rather increase, and retain functionality, with similar results seen in man 15, 16. This does suggest that it is not simply antigen persistence per se that leads to T cell dysfunction or exhaustion, but other aspects of the viral setting may play an important role in determining the function and phenotype of T cells 17.

Other important examples include the hepatitis B and C viruses (HBV and HCV), which can set up persistence accompanied by weak or absent T cell responses; this is particularly the case for HCV. In early HCV infection, immune responses may be quite strong, in both CD4+ and CD8+ T cell compartments, but antibody responses are often delayed and may fail to provide neutralizing activity (for example through envelope gene mutation) 18, 19. Interestingly, in some patients who go on to persistent infection, there is a transient period of viral control (virus becomes undetectable in blood by PCR), before recrudescence and loss of T cell responses 20. Failure of effective antibody responses to emerge in this setting could account for the re-emergence of virus, as in the murine LCMV experiments. Interestingly, recent experiments using "helpless" (in this case CD4+ T cell depleted) chimpanzees have shown that such re-emerging HCV strains have escaped from CD8+ T cells through epitope mutation, suggesting that here the CTL are functional, but fail on their own to control viral replication 21.

Finally, HIV plays multiple tricks on the immune system, and failure to neutralize this virus (for example as a result of a "glycan shield"), could readily contribute to virus persistence 22. CD8+ T cell responses to HIV are very strong and remain activated even after initial control over the virus is gained. It is possible that more effective antibody responses, or other immunological factors that lower the viral load, would protect the CD8+ T cell response against the dual threats of escape and exhaustion. Like the CFSE-labeled transferred cells in the experiments of Bachmann and colleagues, the endogenous anti-HIV responses tracked by tetramers may in some cases simply be marking the presence of virus, rather than effectively contributing to control, although the emerging weight of data on escape from CTL suggests that functional CD8+ T cells do provide an important selective force that shapes the evolution of the virus 23.

In the case of HBV, neutralizing antibodies still play an important role — hence the success of the vaccine — but for HCV and HIV, viral variation and other aspects of the biology of the envelope proteins may preclude really effective antibody generation in most cases. Does this mean that we cannot maintain effective CTL in the long term in these situations? This might depend on the ultimate role of the antibody. The simplest interpretation of the findings of Bachmann and colleagues is that further lowering of the viral load — already at low levels anyway — protects and maintains CTL, even in the absence of optimal help. If this is the case, other measures to lower viral load, through innate immunity, interferons, and drugs, might have similar effects (see model in Fig. 1). Since antigen drives CTL into a sustained "effector" state, and may ultimately drive them on into senescence or apoptosis, simply lowering this drive may maintain CTL and redress the balance that is lost when antibody is absent or ineffective. Transient lowering of the viral load in acute HIV through structured treatment interruptions may be one example of such re-alignment 24. The situation is less clear-cut in HCV, but marked increases in some aspects of T cell function can be seen during antiviral therapy with interferon-alpha and ribavirin 25.

There is still much to understand about the control of viruses by immune responses in low-level states in tissues. Better studies of the viral load and localization, and the distribution and fate of the T cells concerned are needed in this and other models. The specificities of the relevant antibodies involved and their role in vivo should be further dissected. Overall, however,those involved in the study of antiviral immunity should carefully consider the results reported in this issue. Unilateral action by individual players in the immune response, however powerful, seems ultimately doomed to failure. A multilateral approach against such complex pathogens, particularly those that can vary in vivo, is more likely to be successful. Like the rest of us, T cells — even CD4+ T cells — get by with a little help from their friends.

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Figure 1.  A model for the role of antibody in protection of memory CTL. (A) The normal situation. In the presence of antibody (and adequate help) CD8+ T cells are functional and can act to lower viral load, in concert with other mediators. Antigen drives central memory cells into effector cells (of various levels of maturity not depicted here — see reference 17 for a discussion), and may then drive them on to senescence and apoptotic cell death. Before they die, such cells may become dysfunctional. If antigen load is low, most cells can revert to a resting central memory state (thus here the central memory compartment is depicted larger than the effector memory compartment) and those effectors that are present are functional. (B) The "diseased" situation. If antibody is lacking, viral load may increase in tissues. Extra antigen pushes CD8+ T cells through effector memory and on towards death. T cell help may play a role in protecting CD8+ T cells from dysfunction/exhaustion or may also act directly to lower the viral load. Failure of other mediators such as Type I interferons may lead to similar consequences (see reference 14). It may be possible to repair situation B to situation A by addition of antibody or drug therapy if these lower the viral load.

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Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 CD8+ T cells, CD4+ T cell help and antibody interact to control the levels of LCMV and other viruses
  5. 3 Implications for persistent virus infection in humans
  6. Acknowledgements

The author is supported by the Wellcome Trust and the European Union. Thanks to Cheryl Day, Michaela Lucas and Sophie Sierro for helpful discussions on the subject of help and helplessness.

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