The impact of variation in the number of CD8(+) T-cell precursors on the outcome of virus infection

Modelling of Experimental Infections

This chapter aims to give a clear idea of how mathematical analysis for experimental systems could help in the process of data assimilation, parameter estimation and hypothesis testing.

Understanding Experimental LCMV Infection of Mice: The Role of Mathematical Models

Virus infections represent complex biological systems governed by multiple-level regulatory processes of virus replication and host immune responses. Understanding of the infection means an ability to predict the systems behaviour under various conditions. Such predictions can only rely upon quantitative mathematical models. The model formulations should be tightly linked to a fundamental step called “coordinatization” (Hermann Weyl), that is, the definition of observables, parameters, and structures that enable the link with a biological phenotype. In this review, we analyse the mathematical modelling approaches to LCMV infection in mice that resulted in quantification of some fundamental parameters of the CTL-mediated virus control including the rates of T cell turnover, infected target cell elimination, and precursor frequencies. We show how the modelling approaches can be implemented to address diverse aspects of immune system functioning under normal conditions and in response to LCMV and, importantly, make quantitative predictions of the outcomes of immune system perturbations. This may highlight the notion that data-driven applications of meaningful mathematical models in infection biology remain a challenge.

Clonal exhaustion as a mechanism to protect against severe immunopathology and death from an overwhelming CD8 T cell response

The balance between protective immunity and immunopathology often determines the fate of the virus-infected host. How rapidly virus is cleared is a function of initial viral load, viral replication rate, and efficiency of the immune response. Here, we demonstrate, with three different inocula of lymphocytic choriomeningitis virus (LCMV), how the race between virus replication and T cell responses can result in different disease outcomes. A low dose of LCMV generated efficient CD8 T effector cells, which cleared the virus with minimal lung and liver pathology. A high dose of LCMV resulted in clonal exhaustion of T cell responses, viral persistence, and little immunopathology. An intermediate dose only partially exhausted the T cell responses and resulted in significant mortality, and the surviving mice developed viral persistence and massive immunopathology, including necrosis of the lungs and liver. This suggests that for non-cytopathic viruses like LCMV, hepatitis C virus, and hepatitis B virus, clonal exhaustion may be a protective mechanism preventing severe immunopathology and death.

In situ evolution of virus-specific cytotoxic T cell responses in the lung

Cytotoxic T cells (CTL) play a critical role in the clearance of respiratory viral infections, but they also contribute to disease manifestations. In this study, we infected mice with a genetically modified pneumonia virus of mice (PVM) that allowed visualization of virus-specific CTL and infected cells in situ. The first virus-specific T cells entered the lung via blood vessels in the scattered foci of PVM-infected cells, which densely clustered around the bronchi at day 7 after infection. At this time, overall pulmonary virus load was maximal, but the mice showed no overt signs of disease. On days 8 to 9, T cells gained access to the infected bronchial epithelium and to the lung interstitium, which was associated with a reduction in the number of virus-infected cells within the initial clusters but could not prevent further virus spread throughout the lung tissue. Interestingly, recruitment of virus-specific CTL throughout the parenchyma was still ongoing on day 10, when the virus infection was already largely controlled. This also represented the peak of clinical disease. Thus, disease was associated with an exuberant T cell infiltration late in the course of the infection, which may be required to completely eliminate virus at residual foci of infection. PVM-induced immunopathology may thus result from the need to generate widespread T cell infiltrates to complete the elimination of virus-infected cells in a large organ like the lung. This experimental model provides the first insights into the spatiotemporal evolution of pulmonary antiviral T cell immunity in vivo.

Antigen amount dictates CD8+ T-cell exhaustion during chronic viral infection irrespective of the type of antigen presenting cell

Chronic viral infections lead to CD8(+) T-cell exhaustion, characterized by impaired cytokine secretion and loss of proliferative capacity. While viral load and T-cell dysfunction correlate, it is currently unclear whether the quality of a cell type presenting antigen determines the degree of T-cell exhaustion or if the overall amount of antigen recognized by T cells promotes exhaustion. We found that chronic lymphocytic chorio-meningitis virus infection led to decreased CD8(+) T-cell exhaustion in DC-MHC class I (MHCI) mice, in which CD8(+) T cells can only recognize antigen on DCs. However, this increase in CD8(+) T-cell function came at the expense of fatal immunopathology. Additional antigen recognition on nonhematopoietic cells in DC-MHCI mice promoted T-cell exhaustion and avoidance of immunopathology. Likewise, increased numbers of antigen-expressing hematopoietic cells, as well as a selective elevation of the number of DCs as the only cell type presenting antigen in DC-MHCI mice, resulted in compromised T-cell function. These results favor a scenario in which the overall amount of antigen exposure, rather than the type of cell engaging with virus-specific CD8(+) T cells, is responsible for their functional exhaustion. Furthermore, exhaustion of virus-specific CD8(+) T cells leads to avoidance of life-threatening immunopathology.

Subtle differences in CTL cytotoxicity determine susceptibility to hemophagocytic lymphohistiocytosis in mice and humans with Chediak-Higashi syndrome

Perforin-mediated cytotoxicity is important for controlling viral infections, but also for limiting immune reactions. Failure of this cytotoxic pathway leads to hemophagocytic lymphohistiocytosis (HLH), a life-threatening disorder of uncontrolled T-cell and macrophage activation. We studied susceptibility to HLH in 2 mouse strains (souris and beige(J)) and a cohort of patients with partial defects in perforin secretion resulting from different mutations in the LYST gene. Although both strains lacked NK-cell cytotoxicity, only souris mice developed all clinical and histopathologic signs of HLH after infection with lymphocytic choriomeningitis virus. The 2 strains showed subtle differences in CTL cytotoxicity in vitro that had a large impact on virus control in vivo. Whereas beige(J) CTLs eliminated lymphocytic choriomeningitis virus infection, souris CTLs failed to control the virus, which was associated with the development of HLH. In LYST-mutant patients with Chediak-Higashi syndrome, CTL cytotoxicity was reduced in patients with early-onset HLH, whereas it was retained in patients who later or never developed HLH. Thus, the risk of HLH development is set by a threshold that is determined by subtle differences in CTL cytotoxicity. Differences in the cytotoxic capacity of CTLs may be predictive for the risk of Chediak-Higashi syndrome patients to develop HLH.

Vaccination alters the balance between protective immunity, exhaustion, escape, and death in chronic infections

While T cell-based vaccines have the potential to provide protection against chronic virus infections, they also have the potential to generate immunopathology following subsequent virus infection. We develop a mathematical model to investigate the conditions under which T cells lead to protection versus adverse pathology. The model illustrates how the balance between virus clearance and immune exhaustion may be disrupted when vaccination generates intermediate numbers of specific CD8 T cells. Surprisingly, our model suggests that this adverse effect of vaccination is largely unaffected by the generation of mutant viruses that evade T cell recognition and cannot be avoided by simply increasing the quality (affinity) or diversity of the T cell response. These findings should be taken into account when developing vaccines against persistent infections.

Epitope specificity and relative clonal abundance do not affect CD8 differentiation patterns during lymphocytic choriomeningitis virus infection

To evaluate the impact of immunodominance on CD8 T-cell properties, we compared the functional properties of dominant and subdominant populations in the response to lymphocytic choriomeningitis virus (LCMV). To improve functional discrimination, in addition to the usual tests of phenotype and function, we used a sensitive technique that allows the screening of all CD8 effector genes simultaneously in single cells. Surprisingly, these methods failed to reveal a major impact of clonal dominance in CD8 properties throughout the response. Aiming to increase clonal dominance, we examined high-frequency transferred P14 T-cell receptor transgenic (TCR Tg) cells. Under these conditions LCMV is cleared faster, and accordingly we found an accelerated response. However, when Tg and endogenous cells were studied in the same mice, where they should be subjected to the same antigen load, they showed overlapping properties, and the presence of P14 cells did not modify endogenous responses to other LCMV epitopes or a perturbed immunodominance hierarchy in the memory phase. Using allotype-labeled Tg cells, we found that during acute infection up to 80% downregulated their TCR and were undetectable by tetramer binding, and that tetramer-negative and tetramer-positive cells had very different features. Since Tg cells are not available to evaluate immune responses in humans and, in many cases, are not available from the mouse, the tetramer-based evaluation of early immune responses in most situations of high viremia may be incomplete and biased.

Protection of hepatocytes from cytotoxic T cell mediated killing by interferon-alpha

BACKGROUND

Cellular immunity plays a key role in determining the outcome of hepatitis C virus (HCV) infection, although the majority of infections become persistent. The mechanisms behind persistence are still not clear; however, the primary site of infection, the liver, may be critical. We investigated the ability of CD8+ T-cells (CTL) to recognise and kill hepatocytes under cytokine stimulation.

METHODS/PRINCIPLE FINDINGS

Resting hepatocytes cell lines expressed low levels of MHC Class I, but remained susceptible to CTL cytotoxicity. IFN-alpha treatment, in vitro, markedly increased hepatocyte MHC Class I expression, however, reduced sensitivity to CTL cytotoxicity. IFN-alpha stimulated hepatocyte lines were still able to present antigen and induce IFN-gamma expression in interacting CTL. Resistance to killing was not due to the inhibition of the FASL/FAS- pathway, as stimulated hepatocytes were still susceptible to FAS-mediated apoptosis. In vitro stimulation with IFN-alpha, or the introduction of a subgenomic HCV replicon into the HepG2 line, upregulated the expression of the granzyme-B inhibitor-proteinase inhibitor 9 (PI-9). PI-9 expression was also observed in liver tissue biopsies from patients with chronic HCV infection.

CONCLUSION/SIGNIFICANCE

IFN-alpha induces resistance in hepatocytes to perforin/granzyme mediate CTL killing pathways. One possible mechanism could be through the expression of the PI-9. Hindrance of CTL cytotoxicity could contribute to the chronicity of hepatic viral infections.

A reduction in selective immune pressure during the course of chronic hepatitis C correlates with diminished biochemical evidence of hepatic inflammation

It is considered that selection pressure exerted by the host immune response during early HCV infection might influence the outcome of that infection particularly as it relates to persistence or clearance of the agent. However, it is unclear whether positive selection pressure plays a role in determining the severity of hepatitis C during the course of persistent HCV infection. To address the evolutionary mechanism by which HCV escapes from the host immune response and to assess the relationship between viral evolution and hepatic inflammation, we determined 57 sequences (3-5 serial samples per patient) from 5 individuals with persistent HCV infection of genotype 1a who were under long-term follow-up ranging from 15.6 to 21.6 years. We applied a novel method to estimate serial alternations of selective pressure against the HCV enveloped region and compared this to fluctuation in transaminase level over time. Positive selection pressure was reduced over time postinfection, as evidenced by a reduction in nonsynonymous substitutions in the later phase of infection. Furthermore, serum transaminase, as a measure of inflammatory necrosis of hepatocytes, was reduced in parallel with decreased positive selection pressure. These results suggest that during persistent HCV infection, the virus faces diminished immune pressure over time, either from mutation to an immune resistant sequence or from immunologic exhaustion, and that this diminished immune attack is reflected in diminished inflammatory activity. This observation may be applicable to other viruses characterized by a slow rate of disease progression.

Pathology during acute infections: contributions of intracellular pathogens and the CTL response

Previous work has shown how, in the case of cytotoxic T-lymphocyte (CTL) responses to persistent viral infections, pathology may arise as a consequence of cell destruction directly by the virus or indirectly due to the CTL response, leading to maximum pathology at intermediate efficacy of the immune response. We expand these studies to consider pathology arising during acute infections with intracellular pathogens controlled by the CTL response. We show that, in contrast to persistent infections, pathology during acute infections is minimized with increasing efficacy of the immune response. The implications of these results for vaccination are discussed.

A stochastic model of cytotoxic T cell responses

We have constructed a stochastic stage-structured model of the cytotoxic T lymphocyte (CTL) response to antigen and the maintenance of immunological memory. The model follows the dynamics of a viral infection and the stimulation, proliferation, and differentiation of naïve CD8(+) T cells into effector CTL, which can eliminate virally infected cells. The model is capable of following the dynamics of multiple T cell clones, each with a T cell receptor represented by a digit string. MHC-viral peptide complexes are also represented by strings and a string match rule is used to compute the affinity of a T cell receptor for a viral epitope. The avidities of interactions are also computed by taking into consideration the density of MHC-viral peptides on the surface of an infected cell. Lastly, the model allows the probability of T cell stimulation to depend on avidity but also incorporates the notion of an antigen-independent programmed proliferative response. We compare the model to experimental data on the cytotoxic T cell response to lymphocytic choriomeningitis virus infections.

High-resolution phylogenetic analysis of hepatitis C virus adaptation and its relationship to disease progression

Hepatitis C virus (HCV) persists in the majority of those infected despite host immune responses. Evidence has accrued that selectively fixed mutations in the envelope genes (E1 and E2) are associated with viral persistence, particularly those that occur within the first hypervariable region of E2 (HVR1). However, the individual amino acid residues under selection have not been identified, nor have their selection pressures been measured, despite the importance of this information for understanding disease pathogenesis and for vaccine design. We performed a high-resolution analysis of published gene sequence data from individuals undergoing acute HCV infection, employing two phylogenetic methods to determine site-specific selection pressures. Strikingly, we found a statistically significant association between the number of sites selected and disease outcome, with the fewest selected sites in fulminant HCV cases and the greatest number of selected sites in rapid progressors, reflecting the duration and intensity of the arms race between host and virus. Moreover, sites outside the HVR1 appear to play a major role in viral evolution and pathogenesis, although there was no association between viral persistence and specific mutations in E1 and E2. Our analysis therefore allows fine dissection of immune selection pressures, which may be more diverse than previously thought. Such analyses could play a similarly informative role in studies of other persistent virus infections, such as human immunodeficiency virus.

Underwhelming the immune response: effect of slow virus growth on CD8+-T-lymphocyte responses

The speed of virus replication has typically been seen as an advantage for a virus in overcoming the ability of the immune system to control its population growth. Under some circumstances, the converse may also be true: more slowly replicating viruses may evoke weaker cellular immune responses and therefore enhance their likelihood of persistence. Using the model of lymphocytic choriomeningitis virus (LCMV) infection in mice, we provide evidence that slowly replicating strains induce weaker cytotoxic-T-lymphocyte (CTL) responses than a more rapidly replicating strain. Conceptually, we show a "bell-shaped" relationship between the LCMV growth rate and the peak CTL response. Quantitative analysis of human hepatitis C virus infections suggests that a reduction in virus growth rate between patients during the incubation period is associated with a spectrum of disease outcomes, from fulminant hepatitis at the highest rate of viral replication through acute resolving to chronic persistence at the lowest rate. A mathematical model for virus-CTL population dynamics (analogous to predator [CTL]-prey [virus] interactions) is applied in the clinical data-driven analysis of acute hepatitis B virus infection. The speed of viral replication, through its stimulus of host CTL responses, represents an important factor influencing the pathogenesis and duration of virus persistence within the human host. Viruses with lower growth rates may persist in the host because they "sneak through" immune surveillance.

Modelling the dynamics of LCMV infection in mice: II. Compartmental structure and immunopathology

In this study, we develop a mathematical model for analysis of the compartmental aspects and immunopathology of lymphocytic choriomeningitis virus (LCMV) infection in mice. We used sets of original and published data on systemic (extrasplenic) virus distribution to estimate the parameters of virus growth and elimination for spleen and other anatomical compartments, such as the liver, kidney, thymus and lung as well as transfer rates between blood and the above organs. A mathematical model quantitatively integrating the virus distribution kinetics in the host, the specific cytotoxic T lymphocyte (CTL) response in spleen and the re-circulation of effector CTL between spleen, blood and liver is advanced to describe the CTL-mediated immunopathology (hepatitis) in mice infected with LCMV. For intravenous and "peripheral" routes of infection we examine the severity of the liver disease, as a function of the virus dose and the host's immune status characterized by the numbers of precursor and/or cytolytic effector CTL. The model is used to predict the efficacy of protection against virus persistence and disease in a localized viral infection as a function of the composition of CTL population. The modelling analysis suggests quantitative demands to CTL memory for maximal protection against a wide range of doses of infection with a primarily peripheral site of virus replication without the risk of favoring immunopathology. It specifies objectives for CTL vaccination to ensure virus elimination with minimal immunopathology vs. vaccination for disease.

Antigen-driven T-cell turnover

A mathematical model is developed to characterize the distribution of cell turnover rates within a population of T lymphocytes. Previous models of T-cell dynamics have assumed a constant uniform turnover rate; here we consider turnover in a cell pool subject to clonal proliferation in response to diverse and repeated antigenic stimulation. A basic framework is defined for T-cell proliferation in response to antigen, which explicitly describes the cell cycle during antigenic stimulation and subsequent cell division. The distribution of T-cell turnover rates is then calculated based on the history of random exposures to antigens. This distribution is found to be bimodal, with peaks in cell frequencies in the slow turnover (quiescent) and rapid turnover (activated) states. This distribution can be used to calculate the overall turnover for the cell pool, as well as individual contributions to turnover from quiescent and activated cells. The impact of heterogeneous turnover on the dynamics of CD4(+) T-cell infection by HIV is explored. We show that our model can resolve the paradox of high levels of viral replication occurring while only a small fraction of cells are infected.

High numbers of IL-2-producing CD8+ T cells during viral infection: correlation with stable memory development

Using infections with lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus in mice as model systems, we have investigated the ability of antigen-primed CD8+ T cells generated in the context of viral infections to produce IL-2. Our results indicate that acute immunizing infection normally leads to generation of high numbers of IL-2-producing antigen-specific CD8+ T cells. By costaining for IL-2 and IFN-gamma intracellularly, we found that IL-2-producing cells predominantly constitute a subset of cells also producing IFN-gamma. Comparison of the kinetics of generation revealed that IL-2-producing cells appear slightly delayed compared with the majority of IFN-gamma producing cells, and the relative frequency of the IL-2-producing subset increases with transition into the memory phase. In contrast to acute immunizing infection, few IL-2-producing cells are generated during chronic LCMV infection. Furthermore, in MHC class II-deficient mice, which only transiently control LCMV infection, IL-2-producing CD8+ T cells are initially generated, but by 4 weeks after infection this subset has nearly disappeared. Eventually the capacity to produce IFN-gamma also becomes impaired, while cell numbers are maintained at a level similar to those in wild-type mice controlling the infection. Taken together, these findings indicate that phenotyping of T cell populations based on capacity to produce cytokines, and especially IL-2, can provide important information as to the functional status of the analysed cell subset. Specifically, combined analysis of the capacity to produce IL-2 and IFN-gamma can be used as a predictor for loss of function within the CD8+ T cell compartment.

Effects of promyelocytic leukemia protein on virus-host balance

The cellular promyelocytic leukemia protein (PML) associates with the proteins of several viruses and in some cases reduces viral propagation in cell culture. To examine the role of PML in vivo, we compared immune responses and virus loads of PML-deficient and control mice infected with lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus (VSV). PML(-/-) mice exhibited accelerated primary footpad swelling reactions to very-low-dose LCMV, higher swelling peaks upon high-dose inoculation, and higher viral loads in the early phase of systemic LCMV infection. T-cell-mediated hepatitis and consequent mortality upon infection with a hepatotropic LCMV strain required 10- to 100-times-lower inocula despite normal cytotoxic T-lymphocyte reactivity in PML(-/-) mice. Furthermore, PML deficiency rendered mice 10 times more susceptible to lethal immunopathology upon intracerebral LCMV inoculation. Accordingly, 10-times-lower VSV inocula elicited specific neutralizing-antibody responses, a replication-based effect not observed with inactivated virus or after immunization with recombinant VSV glycoprotein. These in vivo observations corroborated our results showing more virus production in PML(-/-) fibroblasts. Thus, PML is a contributor to innate immunity, defining host susceptibility to viral infections and to immunopathology.

Estimating the precursor frequency of naive antigen-specific CD8 T cells

The constraint of fitting a diverse repertoire of antigen specificities in a limited total population of lymphocytes results in the frequency of naive cells specific for any given antigen (defined as the precursor frequency) being below the limit of detection by direct measurement. We have estimated this precursor frequency by titrating a known quantity of antigen-specific cells into naive recipients. Adoptive transfer of naive antigen-specific T cell receptor transgenic cells into syngeneic nontransgenic recipients, followed by stimulation with specific antigen, results in activation and expansion of both donor and endogenous antigen-specific cells in a dose-dependent manner. The precursor frequency is equal to the number of transferred cells when the transgenic and endogenous responses are of equal magnitude. Using this method we have estimated the precursor frequency of naive CD8 T cells specific for the H-2D(b)-restricted GP33-41 epitope of LCMV to be 1 in 2 x 10(5). Thus, in an uninfected mouse containing approximately 2-4 x 10(7) naive CD8 T cells we estimate there to be 100-200 epitope-specific cells. After LCMV infection these 100-200 GP33-specific naive CD8 T cells divide >14 times in 1 wk to reach a total of approximately 10(7) cells. Approximately 5% of these activated GP33-specific effector CD8 T cells survive to generate a memory pool consisting of approximately 5 x 10(5) cells. Thus, an acute LCMV infection results in a >1,000-fold increase in precursor frequency of D(b)GP33-specific CD8 T cells from 2 x 10(2) naive cells in uninfected mice to 5 x 10(5) memory cells in immunized mice.

Exhaustion of cytotoxic T cells during adoptive immunotherapy of virus carrier mice can be prevented by B cells or CD4+ T cells

Rapid disappearance of antiviral CTL after transfusion into persistently infected individuals is a serious limitation of adoptive immunotherapy protocols. In the mouse model of persistent infection with lymphocytic choriomeningitis virus (LCMV) naive or immune virus-specific donor CD8+ T cells are exhausted after transfusion into carrier recipients with similar kinetics. Here we show that cotransfusion of immune CD4+ T cells prevents exhaustion of immune CD8+ T cells. Interestingly, cotransfer of primed B cells also prevented CD8+ T cell exhaustion in carriers even in the absence of T helper cells. This effect required the presence of immune B cells as repetitive treatment with hyperimmune serum led to the generation of antibody escape mutants. A combination of primed CD4+ T cells and primed B cells enhanced antiviral effects and prevented exhaustion also of naive CD8+ T cells. One key factor for prevention of CD8+ T cell exhaustion was the antiviral effect of the cotransfused cells thus reducing the time that CD8+ T cells are confronted with a high systemic viral load. These findings have implications for improving adoptive immunotherapy for persistent human viral infections.

Immunity to hepatitis C virus: stunned but not defeated

Hepatitis C virus (HCV) readily causes a persistent infection, although some individuals spontaneously control infection. 'Successful' immune responses appear to be multi-specific and sustained-including a major role for CD4(+)T cells. Some antiviral CD8(+)T cells show reduced capacity to secrete antiviral cytokines either temporarily ('stunning') or in the long term ('stunting'). The co-ordination of multiple immune effector functions may be required to gain control of HCV.

Low level viral persistence after infection with LCMV: a quantitative insight through numerical bifurcation analysis

Many important viruses persist at very low levels in the body in the face of host immunity, and may influence the maintenance of this state of 'infection immunity'. To analyse low level viral persistence in quantitative terms, we use a mathematical model of antiviral cytotoxic T lymphocyte (CTL) response to lymphocytic choriomeningitis virus (LCMV). This model, described by a non-linear system of delay differential equations (DDEs), is studied using numerical bifurcation analysis techniques for DDEs. Domains where low level LCMV coexistence with CTL memory is possible, either as an equilibrium state or an oscillatory pattern, are identified in spaces of the model parameters characterising the interaction between virus and CTL populations. Our analysis suggests that the coexistence of replication competent virus below the conventional detection limit (of about 100 pfu per spleen) in the immune host as an equilibrium state requires the per day relative growth rate of the virus population to decrease at least 5-fold compared to the acute phase of infection. Oscillatory patterns in the dynamics of persisting LCMV and CTL memory, with virus population varying between 1 and 100 pfu per spleen, are possible within quite narrow intervals of the rates of virus growth and precursor CTL population death. Whereas the virus replication rate appears to determine the stability of the low level virus persistence, it does not affect the steady-state level of the viral population, except for very low values.

Depletion of CD4+ T cells precipitates immunopathology in immunodeficient mice infected with a noncytocidal virus

IFN-gamma-deficient (IFN-gamma(-/-)) mice inoculated with intermediate doses of a slowly replicating strain of lymphocytic choriomeningitis virus become chronically infected. In such mice a hypercompensated CTL response is observed that partially controls virus replication. Here we have investigated whether CD4(+) Th cells are required to establish and maintain this new equilibrium. The absence of IFN-gamma does not impair the generation of IL-2-producing CD4(+) cells, and depletion of these cells precipitates severe CD8(+) T cell-mediated immunopathology in IFN-gamma(-/-) mice, indicating an important role of CD4(+) T cells in preventing this syndrome. Analysis of organ virus levels revealed a further impairment of virus control in IFN-gamma(-/-) mice following CD4(+) cell depletion. Initially the antiviral CTL response did not require CD4(+) cells, but with time an impaired reactivity toward especially the glycoprotein 33--41 epitope was noted. Enumeration of epitope-specific (glycoprotein 33--41 and nucleoprotein 396--404) CD8(+) T cells by use of tetramers gave similar results. Finally, limiting dilution analysis of CTL precursors reveal an impaired capacity to sustain this population in CD4(+)-depleted mice, especially in mice also deficient in IFN-gamma. Thus, our findings disclose that T cell help is required to sustain the expanded CTL precursor pool required in IFN-gamma(-/-) mice. This interpretation is supported by mathematical modeling that predicts an increased requirement for help in IFN-gamma(-/-) hosts similar to what is found with fast replicating virus strains in normal hosts. Thus, the functional integrity of CD8(+) effector T cells is one important factor influencing the requirement for T cell help during viral infection.

Cytotoxic T-lymphocyte memory, virus clearance and antigenic heterogeneity

Cytotoxic T-lymphocyte (CTL) memory to viruses has traditionally been studied in an isolated setting. However, recent experiments have indicated that the presence of antigenically heterologous challenges can result in the attrition of CTL memory. Here we use mathematical models in order to explore the consequence of these dynamics for the ability of the immune system in controlling multiple infections. Mathematical models suggest that antigen-independent persistence of CTL memory is required in order to resolve and clear an infection. This ensures strong immunological pressure at low loads when the virus population declines towards extinction. If the number of antigenic stimuli exposed to the immune system crosses a threshold, we find that immunological pressure is significantly reduced at low loads and this can prevent virus clearance and reduces overall control of viral replication. Hence, exposure to many heterologous challenges reduces the ability of CTL memory to contribute to virus control. The higher the number of infections present in the host, the higher the overall virus load and the higher the total number of memory CTLs. Beyond a given threshold, addition of new viruses to the system results in accelerated loss of virus control which eventually leads to a reduction in the overall memory CTL population. These dynamics might contribute to the progressively weaker immunity observed as a result of ageing. In this context, antigenically variable pathogens expose the immune system to many heterologous challenges within a short period of time and this could result in accelerated ageing of the immune system. These results have important implications for vaccination and treatment strategies directed against viral infections.

High-avidity CTL exploit two complementary mechanisms to provide better protection against viral infection than low-avidity CTL

Previously, we observed that high-avidity CTL are much more effective in vivo than low-avidity CTL in elimination of infected cells, but the mechanisms behind their superior activity remained unclear. In this study, we identify two complementary mechanisms: 1) high-avidity CTL lyse infected cells earlier in the course of a viral infection by recognizing lower Ag densities than those distinguished by low-avidity CTL and 2) they initiate lysis of target cells more rapidly at any given Ag density. Alternative mechanisms were excluded, including: 1) the possibility that low-avidity CTL might control virus given more time (virus levels remained as high at 6 days following transfer as at 3 days) and 2) that differences in efficacy might be correlated with homing ability. Furthermore, adoptive transfer of high- and low-avidity CTL into SCID mice demonstrated that transfer of a 10-fold greater amount of low-avidity CTL could only partially compensate for their decreased ability to eliminate infected cells. Thus, we conclude that high-avidity CTL exploit two complementary mechanisms that combine to prevent the spread of virus within the animal: earlier recognition of infected cells when little viral protein has been made and more rapid lysis of infected cells.

Predicting the dynamics of antiviral cytotoxic T-cell memory in response to different stimuli: cell population structure and protective function

This paper examines the numerical and functional consequences of various stimuli on antiviral CD8+ T-cell memory using a mathematical model. The model is based upon biological evidence from the murine model of infection with lymphocytic choriomeningitis virus (LCMV) that the phenotype of immunological memory represents low-level responses driven by various stimuli, and the memory CTL population is partitioned between resting, cycling and effector cells. These subpopulations differ in their lifespan, their potential to mediate antiviral protection and in the stimuli needed for their maintenance. Three types of maintenance stimuli are examined: non-antigen-specific (bystander) stimulation, persisting antigen stimulation and reinfection-mediated stimulation. The modelling predicts that: (i) stable persistence of CTL memory requires the presence of either bystander or antigen-specific stimulation above a certain threshold depending on the sensitivity of memory CTL to stimulation and their life-span; (ii) a relatively low level of stimuli (approximately 10(4) fold less on a per CTL basis compared to acute infection) is needed to stabilize the expanded memory CTL population; (iii) the presence of CTL subsets in the memory pool of different activation states and lifespans ensures the robustness of memory persistence in the face of temporal variation in the low-level stimuli and; (iv) an 'optimal' population structure of the memory CTL pool, in terms of immediate protection, requires the presence of both activated cycling and effector CTL. For this, persisting antigen alone or synergistically with bystander signals provide the appropriate stimulation, so that the stimuli equivalent to approximately 30 p.f.u. of LCMV in the spleen are sufficient to maintain approximately 10(5)-10(6) specific CTL in the memory pool. These observations are relevant both to our understanding of natural protective immunity and to vaccine design.

Direct ex vivo kinetic and phenotypic analyses of CD8(+) T-cell responses induced by DNA immunization

CD8(+) T-cell responses can be induced by DNA immunization, but little is known about the kinetics of these responses in vivo in the absence of restimulation or how soon protective immunity is conferred by a DNA vaccine. It is also unclear if CD8(+) T cells primed by DNA vaccines express the vigorous effector functions characteristic of cells primed by natural infection or by immunization with a recombinant live virus vaccine. To address these issues, we have used the sensitive technique of intracellular cytokine staining to carry out direct ex vivo kinetic and phenotypic analyses of antigen-specific CD8(+) T cells present in the spleens of mice at various times after (i) a single intramuscular administration of a plasmid expressing the nucleoprotein (NP) gene from lymphocytic choriomeningitis virus (LCMV), (ii) infection by a recombinant vaccinia virus carrying the same protein (vvNP), or (iii) LCMV infection. In addition, we have evaluated the rapidity with which protective immunity against both lethal and sublethal LCMV infections is achieved following DNA vaccination. The CD8(+) T-cell response in DNA-vaccinated mice was slightly delayed compared to LCMV or vvNP vaccinees, peaking at 15 days postimmunization. Interestingly, the percentage of antigen-specific CD8(+) T cells present in the spleen at day 15 and later time points was similar to that observed following vvNP infection. T cells primed by DNA vaccination or by infection exhibited similar cytokine expression profiles and had similar avidities for an immunodominant cytotoxic T lymphocyte epitope peptide, implying that the responses induced by DNA vaccination differ quantitatively but not qualitatively from those induced by live virus infection. Surprisingly, protection from both lethal and sublethal LCMV infections was conferred within 1 week of DNA vaccination, well before the peak of the CD8(+) T-cell response.

Host factors influencing viral persistence

With the aim of characterizing the antiviral immune response to a non-cytocidal virus, we studied the outcome of lymphocytic choriomeningitis virus infection in a number of gene knockout mouse strains. Two virus strains differing markedly in their capacity to spread and replicate inside the murine host were used. Our results reveal that very different outcomes may be observed depending on virus strain and immunocompetence of the host. Thus while CD4+ cells are not critical during the initial phase of virus control, infectious virus reappear in mice lacking CD4+ cells, B cells or CD40 ligand. Reappearance of virus is associated with impaired long-term CD8+ T-cell mediated immune surveillance, and the time to virus resurgence is inversely correlated to the replication rate of the virus. Our studies also reveal that interferon-gamma is a central cytokine, and depending on the rate of virus replication, mice lacking the ability to produce interferon-gamma may develop either a severe, mostly fatal, T-cell mediated wasting syndrome or a chronic infection characterized by long-term coexistence of antiviral cytotoxic T lymphocytes and infectious virus. Mathematical modelling indicates that these different outcomes may be explained in relatively simple mathematical terms. This suggests that modelling may be used as a means to predict critical host and virus parameters. Therefore, combining mathematical modelling with precise, quantitative, in vivo analyses looks to be a promising approach in addressing central quantitative issues in immunobiology.

The role of antigen-independent persistence of memory cytotoxic T lymphocytes

We use mathematical models to analyze the role of a memory cytotoxic T lymphocyte (CTL) response in viral infections. The model predicts that antigen-independent persistence of an elevated number of precursor CTL (CTLp) does not protect the host from clinical symptoms upon re-infection. Instead, we find that antigen-independent long-term persistence of CTLp is required to clear virus infections. This mechanism also applies to infection in hosts that have never experienced the pathogen before. Requirement of antigen for the long-term maintenance of CTLp results in failure to clear the infection, even in hosts characterized by a high CTL responsiveness. We compare the CTL model to a B cell model. In keeping with experimentally established findings, B cells are efficient in protecting against re-infection, but are unlikely to clear viral infections unless the virus is cytopathic. We conclude that the role of 'memory CTLp' is different from the role of memory B cells in viral infections: antigen-independent long-term persistence of CTLp is a pre-requisite to ensure clearance of infection.

A clonal view of alphabeta T cell responses

Quantitative analyses of antigen (Ag)-specific alphabeta T cell populations have provided a large body of information on the natural course of T cell immune responses. New tools are now available to determine the clonal composition of Ag-specific pools in individual responders, an approach which offers direct insights into the generation of T cell immune responses and establishment of protective immunity. The present review discusses the parameters that determine the composition of Ag-specific T cell responses. Emphasis is placed on the role of the naive alphabeta T cell repertoire and on the dynamics of individual Ag-specific T cell clones during the successive phases of an immune response.