Role of virus and host variables in virus persistence or immunopathological disease caused by a non-cytolytic virus

Can the triumph of mRNA vaccines against COVID-19 be extended to other viral infections of humans and domesticated animals?

The unprecedented success of mRNA vaccines in managing the COVID-19 pandemic raises the prospect of applying the mRNA platform to other viral diseases of humans and domesticated animals, which may lead to more efficacious vaccines for some agents. We briefly discuss reasons why mRNA vaccines achieved such success against COVID-19 and indicate what other virus infections and disease conditions might also be ripe for control using mRNA vaccines. We also evaluate situations where mRNA could prove valuable to rebalance the status of immune responsiveness and achieve success as a therapeutic vaccine approach against infections that induce immunoinflammatory lesions.

Vectored antibody gene delivery restores host B and T cell control of persistent viral infection

Passive antibody therapy and vectored antibody gene delivery (VAGD) in particular offer an innovative approach to combat persistent viral diseases. Here, we exploit a small animal model to investigate synergies of VAGD with the host's endogenous immune defense for treating chronic viral infection. An adeno-associated virus (AAV) vector delivering the lymphocytic choriomeningitis virus (LCMV)-neutralizing antibody KL25 (AAV-KL25) establishes protective antibody titers for >200 days. When therapeutically administered to chronically infected immunocompetent wild-type mice, AAV-KL25 affords sustained viral load control. In contrast, viral mutational escape thwarts therapeutic AAV-KL25 effects when mice are unable to mount LCMV-specific antibody responses or lack CD8⁺ T cells. VAGD augments antiviral germinal center B cell and antibody-secreting cell responses and reduces inhibitory receptor expression on antiviral CD8⁺ T cells. These results indicate that VAGD fortifies host immune defense and synergizes with B cell and CD8 T cell responses to restore immune control of chronic viral infection.

Immunotherapeutic Blockade of CD47 Inhibitory Signaling Enhances Innate and Adaptive Immune Responses to Viral Infection

Paradoxically, early host responses to infection include the upregulation of the antiphagocytic molecule, CD47. This suggests that CD47 blockade could enhance antigen presentation and subsequent immune responses. Indeed, mice treated with anti-CD47 monoclonal antibody following lymphocytic choriomeningitis virus infections show increased activation of both macrophages and dendritic cells (DCs), enhancement of the kinetics and potency of CD8⁺ T cell responses, and significantly improved virus control. Treatment efficacy is critically dependent on both APCs and CD8⁺ T cells. In preliminary results from one of two cohorts of humanized mice infected with HIV-1 for 6 weeks, CD47 blockade reduces plasma p24 levels and restores CD4⁺ T cell counts. The results indicate that CD47 blockade not only enhances the function of innate immune cells but also links to adaptive immune responses through improved APC function. As such, immunotherapy by CD47 blockade may have broad applicability to treat a wide range of infectious diseases.

Cham et al. describe a way to enhance natural immune responses to infections by blocking interactions between two molecules (CD47 and SIRPα) that normally put brakes on the immune system. Since this therapy targets the immune system, it could have broad applicability against a wide range of infectious agents.

A network of immune and microbial modifications underlies viral persistence in the gastrointestinal tract

Many pathogens subvert intestinal immunity to persist within the gastrointestinal tract (GIT); yet, the underlying mechanisms that enable sanctuary specifically in this reservoir are unclear. Using mass cytometry and network analysis, we demonstrate that chronic LCMV infection of the GIT leads to dysregulated microbial composition, a cascade of metabolic alterations, increased susceptibility to GI disease, and a system-wide recalibration of immune composition that defines viral persistence. Chronic infection led to outgrowth of activated Tbet–expressing T reg cell populations unique to the GIT and the rapid erosion of pathogen-specific CD8 tissue-resident memory T cells. Mechanistically, T reg cells and coinhibitory receptors maintained long-term viral sanctuary within the GIT, and their targeting reactivated T cells and eliminated this viral reservoir. Thus, our data provide a high-dimensional definition of the mechanisms of immune regulation that chronic viruses implement to exploit the unique microenvironment of the GIT and identify T reg cells as key modulators of viral persistence in the intestinal tract.

Biomechanics of T Cell Dysfunctions in Chronic Diseases

Understanding the mechanisms behind T cell dysfunctions during chronic diseases is critical in developing effective immunotherapies. As demonstrated by several animal models and human studies, T cell dysfunctions are induced during chronic diseases, spanning from infections to cancer. Although factors governing the onset and the extent of the functional impairment of T cells can differ during infections and cancer, most dysfunctional phenotypes share common phenotypic traits in their immune receptor and biophysical landscape. Through the latest developments in biophysical techniques applied to explore cell membrane and receptor-ligand dynamics, we are able to dissect and gain further insights into the driving mechanisms behind T cell dysfunctions. These insights may prove useful in developing immunotherapies aimed at reinvigorating our immune system to fight off infections and malignancies more effectively. The recent success with checkpoint inhibitors in treating cancer opens new avenues to develop more effective, targeted immunotherapies. Here, we highlight the studies focused on the transformation of the biophysical landscape during infections and cancer, and how T cell biomechanics shaped the immunopathology associated with chronic diseases.

Dampening antiviral immunity can protect the host

Viral infections are very common, and in most cases, the virus is well controlled and eliminated by the immune system. Nevertheless, in some cases, damage of the host tissue inflicted by the virus itself or by the elicited immune response may result in severe disease courses. Thus, regulatory mechanisms are necessary to control virus‐induced and immune pathology. This ensures immune responses are elicited in a potent but controlled manner. In this review, we will outline how immune regulation may contribute to this process. We focus on regulatory T cells and co‐inhibitory receptors and outline how these two regulatory immune components allow for and may even promote potent but not pathologic immune responses. By enabling a balanced immune response, regulatory mechanisms can thus contribute to pathogen control as well as tissue and host protection.

Immune Exhaustion: Past Lessons and New Insights from Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a paradigm-forming experimental system with a remarkable track record of contributing to the discovery of many of the fundamental concepts of modern immunology. The ability of LCMV to establish a chronic infection in immunocompetent adult mice was instrumental for identifying T cell exhaustion and this system has been invaluable for uncovering the complexity, regulators, and consequences of this state. These findings have been directly relevant for understanding why ineffective T cell responses commonly arise during many chronic infections including HIV and HCV, as well as during tumor outgrowth. The principal feature of exhausted T cells is the inability to elaborate the array of effector functions necessary to contain the underlying infection or tumor. Using LCMV to determine how to prevent and reverse T cell exhaustion has highlighted the potential of checkpoint blockade therapies, most notably PD-1 inhibition strategies, for improving cellular immunity under conditions of antigen persistence. Here, we discuss the discovery, properties, and regulators of exhausted T cells and highlight how LCMV has been at the forefront of advancing our understanding of these ineffective responses.

T cell inhibitory mechanisms in a model of aggressive Non-Hodgkin's Lymphoma

A reduced immune surveillance due to immune deficiency or treatment with immunosuppressive drugs is associated with a higher risk to develop aggressive Non-Hodgkin's lymphoma (NHL). Nevertheless, NHL also develops in immunocompetent patients indicating an escape from the immune system. T cell function in advanced aggressive lymphoma is not well characterized and the molecular mechanisms how malignant B cells influence T cell function are ill-defined. We therefore studied T cell function in Eμ-myc transgenic mice that develop an aggressive B cell lymphoma with some similarities to human Burkitt-lymphoma (BL). In advanced lymphoma, the number of T cells was severely reduced and the remaining CD4⁺ and CD8⁺ T cells lost the capacity to produce effector cytokines and expand upon re-stimulation. T cells in lymphoma-bearing mice were characterized by the expression of the immune inhibitory molecules programmed death (PD)-1, 2B4 and lymphocyte activation protein (LAG)-3. The proto-oncogene c-Myc not only drives cell proliferation and disease progression but also induces apoptosis of the malignant cells. We found that apoptotic lymphoma cells release purine metabolites that inhibit T cell function. Taken together, our data document that the characteristic high cell turnover and apoptotic rate in aggressive NHL induce a severe T cell dysfunction mediated by several immune-inhibitory mechanisms including ligation of inhibitory ligands and purine metabolites. Blocking a single mechanism only partially restored T cell function and did not increase survival of lymphoma mice.

T cells responding to Trypanosoma cruzi detected by membrane TNF-α and CD154 in chagasic patients

Introduction

Chagas disease is a parasitic infection whose pathogenesis is related to parasite persistence and a dysfunctional cellular immune response. Variability in cytokine secretion among chronic Trypanosoma cruzi‐infected patients might preclude the identification of the pool of antigen specific T cells. The goal of this study was to determine the fraction of T cells responding to T. cruzi antigen measured by the expression of membrane TNF‐α and CD154.

Methods

A total of 21 chagasic patients, 11 healthy and 5 non‐chagasic cardiomyopathy controls were analyzed. PBMCs were short‐term cultured in the presence of anti‐CD28, anti‐CD49d, anti‐TNF‐α, and TACE (TNF‐α converting enzyme) inhibitor either under T. cruzi‐lysate or polyclonal stimuli. Cells were stained with anti‐CD3, anti‐CD4, anti‐CD8, and anti‐CD154, and analyzed with flow cytometry.

Results

CD4+ and CD8+ T cells in chagasic patients displayed higher percentages of membrane‐bound TNF‐α+ and CD154+ compared with controls after T. cruzi‐antigen stimulation. Both markers displayed a positive correlation in the T cell subpopulations analyzed. Symptomatic chagasic patients were differentiated from asymptomatic patients based on the expression of CD154 and membrane TNF‐α in TCD4+ and TCD8+ compartments, respectively.

Conclusions

These results show that both markers could be useful for assessing the pool of antigen‐specific T cells in chronic chagasic patients.

Evolutionary analysis of Old World arenaviruses reveals a major adaptive contribution of the viral polymerase

The Old World (OW) arenavirus complex includes several species of rodent-borne viruses, some of which (i.e., Lassa virus, LASV and Lymphocytic choriomeningitis virus, LCMV) cause human diseases. Most LCMV and LASV infections are caused by rodent-to-human transmissions. Thus, viral evolution is largely determined by events that occur in the wildlife reservoirs. We used a set of human- and rodent-derived viral sequences to investigate the evolutionary history underlying OW arenavirus speciation, as well as the more recent selective events that accompanied LASV spread in West Africa. We show that the viral RNA polymerase (L protein) was a major positive selection target in OW arenaviruses and during LASV out-of-Nigeria migration. No evidence of selection was observed for the glycoprotein, whereas positive selection acted on the nucleoprotein (NP) during LCMV speciation. Positively selected sites in L and NP are surrounded by highly conserved residues, and the bulk of the viral genome evolves under purifying selection. Several positively selected sites are likely to modulate viral replication/transcription. In both L and NP, structural features (solvent exposed surface area) are important determinants of site-wise evolutionary rate variation. By incorporating several rodent-derived sequences, we also performed an analysis of OW arenavirus codon adaptation to the human host. Results do not support a previously hypothesized role of codon adaptation in disease severity for non-Nigerian strains. In conclusion, L and NP represent the major selection targets and possible determinants of disease presentation; these results suggest that field surveys and experimental studies should primarily focus on these proteins.

Hybrid approach to model the spatial regulation of T cell responses

BACKGROUND

Moving from the molecular and cellular level to a multi-scale systems understanding of immune responses requires the development of novel approaches to integrate knowledge and data from different biological levels into mechanism-based integrative mathematical models. The aim of our study is to present a methodology for a hybrid modelling of immunological processes in their spatial context.

METHODS

A two-level hybrid mathematical model of immune cell migration and interaction integrating cellular and organ levels of regulation for a 2D spatial consideration of idealized secondary lymphoid organs is developed. It considers the population dynamics of antigen-presenting cells, CD4 + and CD8 + T lymphocytes in naive-, proliferation- and differentiated states. Cell division is assumed to be asymmetric and regulated by the extracellular concentration of interleukin-2 (IL-2) and type I interferon (IFN), together controlling the balance between proliferation and differentiation. The cytokine dynamics is described by reaction-diffusion PDEs whereas the intracellular regulation is modelled with a system of ODEs.

RESULTS

The mathematical model has been developed, calibrated and numerically implemented to study various scenarios in the regulation of T cell immune responses to infection, in particular the change in the diffusion coefficient of type I IFN as compared to IL-2. We have shown that a hybrid modelling approach provides an efficient tool to describe and analyze the interplay between spatio-temporal processes in the emergence of abnormal immune response dynamics.

DISCUSSION

Virus persistence in humans is often associated with an exhaustion of T lymphocytes. Many factors can contribute to the development of exhaustion. One of them is associated with a shift from a normal clonal expansion pathway to an altered one characterized by an early terminal differentiation of T cells. We propose that an altered T cell differentiation and proliferation sequence can naturally result from a spatial separation of the signaling events delivered via TCR, IL-2 and type I IFN receptors. Indeed, the spatial overlap of the concentration fields of extracellular IL-2 and IFN in lymph nodes changes dynamically due to different migration patterns of APCs and CD4 + T cells secreting them.

CONCLUSIONS

The proposed hybrid mathematical model of the immune response represents a novel analytical tool to examine challenging issues in the spatio-temporal regulation of cell growth and differentiation, in particular the effect of timing and location of activation signals.

Timing of CD8 T cell effector responses in viral infections

CD8 T cell or cytotoxic T lymphocyte (CTL) responses are an important branch of the immune system in the fight against viral infections. The dynamics of anti-viral CTL responses have been characterized in some detail, both experimentally and with mathematical models. An interesting experimental observation concerns the timing of CTL responses. A recent study reported that in pneumonia virus of mice the effector CTL tended to arrive in the lung only after maximal virus loads had been achieved, an observation that seems at first counterintuitive because prevention of pathology would require earlier CTL-mediated activity. A delay in CTL-mediated effector activity has also been quoted as a possible explanation for the difficulties associated with CTL-based vaccines. This paper uses mathematical models to show that in specific parameter regimes, delayed CTL effector activity can be advantageous for the host in the sense that it can increase the chances of virus clearance. The increased ability of the CTL to clear the infection, however, is predicted to come at the cost of acute pathology, giving rise to a trade-off, which is discussed in the light of evolutionary processes. This work provides a theoretical basis for understanding the described experimental observations.

Loss of the death receptor CD95 (Fas) expression by dendritic cells protects from a chronic viral infection

Chronic viral infections incapacitate adaptive immune responses by "exhausting" virus-specific T cells, inducing their deletion and reducing productive T-cell memory. Viral infection rapidly induces death receptor CD95 (Fas) expression by dendritic cells (DCs), making them susceptible to elimination by the immune response. Lymphocytic choriomeningitis virus (LCMV) clone 13, which normally establishes a chronic infection, is rapidly cleared in C57Black6/J mice with conditional deletion of Fas in DCs. The immune response to LCMV is characterized by an extended survival of virus-specific effector T cells. Moreover, transfer of Fas-negative DCs from noninfected mice to preinfected animals results in either complete clearance of the virus or a significant reduction of viral titers. Thus, DC-specific Fas expression plays a role in regulation of antiviral responses and suggests a strategy for stimulation of T cells in chronically infected animals and humans to achieve the clearance of persistent viruses.

The role of viral persistence in flavivirus biology

In nature, vector borne flaviviruses are persistently cycled between either the tick or mosquito vector and small mammals such as rodents, skunks, and swine. These viruses account for considerable human morbidity and mortality worldwide. Increasing and substantial evidence of viral persistence in humans, which includes the isolation of RNA by RT PCR and infectious virus by culture, continues to be reported. Viral persistence can also be established in vitro in various human, animal, arachnid, and insect cell lines in culture. Although some research has focused on the potential roles of defective virus particles, evasion of the immune response through the manipulation of autophagy and/or apoptosis, the precise mechanism of flavivirus persistence is still not well understood. We propose additional research for further understanding of how viral persistence is established in different systems. Avenues for additional studies include determining whether the multifunctional flavivirus protein NS5 has a role in viral persistence, the development of relevant animal models of viral persistence, and investigating the host responses that allow vector borne flavivirus replication without detrimental effects on infected cells. Such studies might shed more light on the viral–host relationships and could be used to unravel the mechanisms for establishment of persistence.

Persistent infections by vector borne flaviviruses are an important, but inadequately studied topic.

VSV-GP: a potent viral vaccine vector that boosts the immune response upon repeated applications

Antivector immunity limits the response to homologous boosting for viral vector vaccines. Here, we describe a new, potent vaccine vector based on replication-competent vesicular stomatitis virus pseudotyped with the glycoprotein of the lymphocytic choriomeningitis virus (VSV-GP), which we previously showed to be safe in mice. In mice, VSV and VSV-GP encoding ovalbumin (OVA) as a model antigen (VSV-OVA and VSV-GP-OVA) induced equal levels of OVA-specific humoral and cellular immune responses upon a single immunization. However, boosting with the same vector was possible only for VSV-GP-OVA as neutralizing antibodies to VSV limited the immunogenicity of the VSV-OVA boost. OVA-specific cytotoxic T-lymphocyte (CTL) responses induced by VSV-GP-OVA were at least as potent as those induced by an adenoviral state-of-the-art vaccine vector and completely protected mice in a Listeria monocytogenes challenge model. VSV-GP is so far the only replication-competent vaccine vector that does not lose efficacy upon repeated application.

IMPORTANCE

Although there has been great progress in treatment and prevention of infectious diseases in the past several years, effective vaccines against some of the most serious infections, e.g., AIDS, malaria, hepatitis C, or tuberculosis, are urgently needed. Here, several approaches based on viral vector vaccines are under development. However, for all viral vaccine vectors currently in clinical testing, repeated application is limited by neutralizing antibodies to the vector itself. Here, we have exploited the potential of vesicular stomatitis virus pseudotyped with the glycoprotein of the lymphocytic choriomeningitis virus (VSV-GP) as a vaccine platform. VSV-GP is the first replication-competent viral vector vaccine that does not induce vector-specific humoral immunity, i.e., neutralizing antibodies, and therefore can boost immune responses against a foreign antigen by repeated applications. The vector allows introduction of various antigens and therefore can serve as a platform technology for the development of novel vaccines against a broad spectrum of diseases.

Modeling inoculum dose dependent patterns of acute virus infections

Inoculum dose, i.e. the number of pathogens at the beginning of an infection, often affects key aspects of pathogen and immune response dynamics. These in turn determine clinically relevant outcomes, such as morbidity and mortality. Despite the general recognition that inoculum dose is an important component of infection outcomes, we currently do not understand its impact in much detail. This study is intended to start filling this knowledge gap by analyzing inoculum dependent patterns of viral load dynamics in acute infections. Using experimental data for adenovirus and infectious bronchitis virus infections as examples, we demonstrate inoculum dose dependent patterns of virus dynamics. We analyze the data with the help of mathematical models to investigate what mechanisms can reproduce the patterns observed in experimental data. We find that models including components of both the innate and adaptive immune response are needed to reproduce the patterns found in the data. We further analyze which types of innate or adaptive immune response models agree with observed data. One interesting finding is that only models for the adaptive immune response that contain growth terms partially independent of viral load can properly reproduce observed patterns. This agrees with the idea that an antigen-independent, programmed response is part of the adaptive response. Our analysis provides useful insights into the types of model structures that are required to properly reproduce observed virus dynamics for varying inoculum doses. We suggest that such models should be taken as basis for future models of acute viral infections.

Reversal of chronic to resolved infection by IL-10 blockade is LCMV strain dependent

Chronic viral infections lead to CD8(+) T-cell exhaustion, characterized by impaired cytokine secretion. The immune-regulatory cytokine IL-10 promotes chronicity of infection with lymphocytic choriomeningitis virus (LCMV) Clone 13, as absence of IL-10 or blocking of IL-10R during early LCMV Clone 13 infection results in viral clearance. Thus, treatment of humans suffering from chronic viral infections with IL-10 neutralizing or IL-10R blocking antibodies was proposed to boost virus-specific T-cell responses to enhance control or even clear the viral infection. Here we demonstrate that although CD4(+) and CD8(+) T cells can produce elevated levels of cytokines in IL-10(-/-) mice early after infection compared with WT mice, IL-10(-/-) mice cannot clear an infection with the quicker replicating LCMV strain Docile, eventually resulting in T-cell exhaustion. These data suggest that the success of IL-10 blockade to control chronic viral infections may critically depend on the virulence of the infecting strain.

HIV evolution and progression of the infection to AIDS

In this paper, we propose and discuss a possible mechanism, which, via continuous mutations and evolution, eventually enables HIV to break from immune control. In order to investigate this mechanism, we employ a simple mathematical model, which describes the relationship between evolving HIV and the specific CTL response and explicitly takes into consideration the role of CD4(+)T cells (helper T cells) in the activation of the CTL response. Based on the assumption that HIV evolves towards higher replication rates, we quantitatively analyze the dynamical properties of this model. The model exhibits the existence of two thresholds, defined as the immune activation threshold and the immunodeficiency threshold, which are critical for the activation and persistence of the specific cell-mediated immune response: the specific CTL response can be established and is able to effectively control an infection when the virus replication rate is between these two thresholds. If the replication rate is below the immune activation threshold, then the specific immune response cannot be reliably established due to the shortage of antigen-presenting cells. Besides, the specific immune response cannot be established when the virus replication rate is above the immunodeficiency threshold due to low levels of CD4(+)T cells. The latter case implies the collapse of the immune system and beginning of AIDS. The interval between these two thresholds roughly corresponds to the asymptomatic stage of HIV infection. The model shows that the duration of the asymptomatic stage and progression of the disease are very sensitive to variations in the model parameters. In particularly, the rate of production of the naive lymphocytes appears to be crucial.

Major histocompatibility complex-dependent cytotoxic T lymphocyte repertoire and functional avidity contribute to strain-specific disease susceptibility after murine respiratory syncytial virus infection

Susceptibility to respiratory syncytial virus (RSV) infection in mice is genetically determined. While RSV causes little pathology in C57BL/6 mice, pulmonary inflammation and weight loss occur in BALB/c mice. Using major histocompatibility complex (MHC)-congenic mice, we observed that the H-2(d) allele can partially transfer disease susceptibility to C57BL/6 mice. This was not explained by altered viral elimination or differences in the magnitude of the overall virus-specific cytotoxic T lymphocyte (CTL) response. However, H-2(d) mice showed a more focused response, with 70% of virus-specific CTL representing Vβ8.2(+) CTL directed against the immunodominant epitope M2-1 82, while in H-2(b) mice only 20% of antiviral CTL were Vβ9(+) CTL specific for the immunodominant epitope M187. The immunodominant H-2(d)-restricted CTL lysed target cells less efficiently than the immunodominant H-2(b) CTL, probably contributing to prolonged CTL stimulation and cytokine-mediated immunopathology. Accordingly, reduction of dominance of the M2-1 82-specific CTL population by introduction of an M187 response in the F1 generation of a C57BL/6N × C57BL/6-H-2(d) mating (C57BL/6-H-2(dxb) mice) attenuated disease. Moreover, disease in H-2(d) mice was less pronounced after infection with an RSV mutant failing to activate M2-1 82-specific CTL or after depletion of Vβ8.2(+) cells. These data illustrate how the MHC-determined diversity and functional avidity of CTL responses contribute to disease susceptibility after viral infection.

Expanded potential for recombinant trisegmented lymphocytic choriomeningitis viruses: protein production, antibody production, and in vivo assessment of biological function of genes of interest

The recombinant engineering of trisegmented lymphocytic choriomeningitis virus (LCMV) to express two genes of interest was recently reported. We used this technology to efficiently express green fluorescent protein (GFP) and the immunoregulatory gene product interleukin-10 (IL-10) in vitro, assess IL-10 function in vivo during viral meningitis, and generate specific, robust monoclonal antibody responses to IL-10. Tripartite viruses were attenuated in wild-type and TLR7(-/-) mice. However, IFNAR1(-/-) mice sustained systemic viral replication when 2 nucleotide substitutions from a persistent LCMV variant were present. These findings demonstrate the utility of tripartite LCMV in vitro and in vivo to study genes in the context of a well-defined model system.

A mechanism of virus-induced demyelination

Myelin forms an insulating sheath surrounding axons in the central and peripheral nervous systems and is essential for rapid propagation of neuronal action potentials. Demyelination is an acquired disorder in which normally formed myelin degenerates, exposing axons to the extracellular environment. The result is dysfunction of normal neuron-to-neuron communication and in many cases, varying degrees of axonal degeneration. Numerous central nervous system demyelinating disorders exist, including multiple sclerosis. Although demyelination is the major manifestation of most of the demyelinating diseases, recent studies have clearly documented concomitant axonal loss to varying degrees resulting in long-term disability. Axonal injury may occur secondary to myelin damage (outside-in model) or myelin damage may occur secondary to axonal injury (inside-out model). Viral induced demyelination models, has provided unique imminent into the cellular mechanisms of myelin destruction. They illustrate mechanisms of viral persistence, including latent infections, virus reactivation and viral-induced tissue damage. These studies have also provided excellent paradigms to study the interactions between the immune system and the central nervous system (CNS). In this review we will discuss potential cellular and molecular mechanism of central nervous system axonal loss and demyelination in a viral induced mouse model of multiple sclerosis.

Treatment with a sphingosine analog does not alter the outcome of a persistent virus infection

There is no known antiviral drug treatment that routinely terminates persistent virus infections. A recent provocative report indicated that low dosage of the sphingosine analog FTY720 caused lymphopenia in mice persistently infected with lymphocytic choriomeningitis virus (LCMV)-clone 13 (Cl 13) and induced viral clearance within 30 days post-treatment (Premenko-Lanier et al., 2008). However, we find that low dosage of FTY720 fails to purge LCMV-Cl 13 infection and does not induce lymphopenia in LCMV-Cl 13-infected mice. In fact, infection with non-persistent LCMV-Arm53b or with persistent LCMV-Cl 13 induces an equivalent lymphopenia, demonstrating that the quantity of circulating cells has little bearing on viral persistence. In addition, treatment with FTY720 or the sphingosine-1-phosphate receptor 1 (S1P1)-specific agonist, AUY954, does not alleviate T cell exhaustion and exacerbates disruption of the CD8(+) T cells response following LCMV-Cl 13 infection. Therefore, treatment with a sphingosine analog does not ameliorate persistent LCMV-Cl 13 infection.

The rhesus macaque pediatric SIV infection model - a valuable tool in understanding infant HIV-1 pathogenesis and for designing pediatric HIV-1 prevention strategies

Worldwide, the AIDS pandemic continues almost relentlessly. Women are now representing the fastest growing group of newly infected HIV-1 infected patients. The risk of mother-to-child-transmission (MTCT) of HIV-1 increases proportionally as many of these women are of childbearing age. The screening of pregnant women, the early diagnosis of HIV-1 infection, and the administration of antiretroviral therapy (ART) have helped to reduce MTCT significantly. However, this holds true only for developed countries. In many resource-poor countries, access to ART is limited, and breastfeeding, a major route of HIV-1 transmission, is essential to protect the infant from other infectious diseases preponderant in those geographic regions. HIV-1 infected children, in contrast to adult patients, have higher levels of virus replication that decline only slowly, and a subset progresses to AIDS within the first two years. Thus, it is imperative to understand pediatric HIV-1 pathogenesis to design effective prevention strategies and/or a successful pediatric HIV-1 vaccine. The review summarizes how MTCT of HIV-1 in humans can be modeled in the infant macaque model of SIV infection. Importantly, the infant macaque model of SIV infection provides the opportunity to study early virus-host interactions in multiple anatomic compartments. Furthermore, the review underlines the importance of evaluating SIV/HIV immune responses in the context of the normal developmental changes the immune system undergoes in the newborn. Thus, the pediatric SIV infection model provides a unique resource for preclinical studies of novel intervention therapies and vaccine strategies to stop MTCT of HIV-1.

Genomic and biological characterization of aggressive and docile strains of lymphocytic choriomeningitis virus rescued from a plasmid-based reverse-genetics system

Arenaviruses include several causative agents of haemorrhagic fever disease in humans. In addition, the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a superb model for the study of virus-host interactions, including the basis of viral persistence and associated diseases. There is little understanding about the molecular mechanisms concerning the regulation and specific role of viral proteins in modulating arenavirus-host cell interactions either associated with an acute or persistent infection, and associated disease. Here, we report the genomic and biological characterization of LCMV strains 'Docile' (persistent) and 'Aggressive' (not persistent) recovered from cloned cDNA via reverse genetics. Our results confirmed that the cloned viruses accurately recreated the in vivo phenotypes associated with the corresponding natural Docile and Aggressive viral isolates. In addition, we provide evidence that the ability of the Docile strain to persist is determined by the nature of both S and L RNA segments. Thus, our findings provide the foundation for studies aimed at gaining a detailed understanding of viral determinants of LCMV persistence in its natural host, which may aid in the development of vaccines to prevent or treat the diseases caused by arenaviruses in humans.

Demyelinating and nondemyelinating strains of mouse hepatitis virus differ in their neural cell tropism

Some strains of mouse hepatitis virus (MHV) can induce chronic inflammatory demyelination in mice that mimics certain pathological features of multiple sclerosis. We have examined neural cell tropism of demyelinating and nondemyelinating strains of MHV in order to determine whether central nervous system (CNS) cell tropism plays a role in demyelination. Previous studies demonstrated that recombinant MHV strains, isogenic other than for the spike gene, differ in the extent of neurovirulence and the ability to induce demyelination. Here we demonstrate that these strains also differ in their abilities to infect a particular cell type(s) in the brain. Furthermore, there is a correlation between the differential localization of viral antigen in spinal cord gray matter and that in white matter during acute infection and the ability to induce demyelination later on. Viral antigen from demyelinating strains is detected initially in both gray and white matter, with subsequent localization to white matter of the spinal cord, whereas viral antigen localization of nondemyelinating strains is restricted mainly to gray matter. This observation suggests that the localization of viral antigen to white matter during the acute stage of infection is essential for the induction of chronic demyelination. Overall, these observations suggest that isogenic demyelinating and nondemyelinating strains of MHV, differing in the spike protein expressed, infect neurons and glial cells in different proportions and that differential tropism to a particular CNS cell type may play a significant role in mediating the onset and mechanisms of demyelination.

Control of virus-specific CD8+ T-cell exhaustion and immune-mediated pathology by E3 ubiquitin ligase Cbl-b during chronic viral infection

A characteristic feature in the immune response to many persistent viral infections is the dysfunction or deletion of antigen-specific T cells (exhaustion). This down-regulation of virus-specific T-cell response represents a critical control mechanism that exists within T-cell activation pathways to prevent lethal disease by inappropriate responses against disseminating virus infections. However, the molecular mechanisms by which the immune system determines whether to mount a full response to such infections remain largely unexplored. Here, we have established that in the murine lymphocytic choriomeningitis virus (LCMV) model, induction of the T-cell receptor signaling inhibitor molecule E3 ligase Cbl-b is critically involved in this decision. In particular, our data revealed that Cbl-b controls the program responsible for T-cell tolerance (exhaustion) induction during a chronic viral infection. Thus, Cbl-b(-/-) mice infected with a low dose of LCMV Docile mount a strong CD8(+) T-cell response that rapidly clears the infection, and the animals remain healthy; in contrast, down-regulation of the epitope-specific CD8(+) T-cell population in persistently infected Cbl-b(-/-) mice, compared to that in chronically infected B6 mice, was significantly delayed, and this was associated with increased morbidity and eventual death in nearly 20% of the animals. Interestingly, infection of Cbl-b(-/-) mice with a moderate virus dose resulted in rapid death with 100% mortality by 7 to 8 days after infection, caused by a dysregulated antiviral T-cell response, whereas the infected B6 mice survived and remained healthy. In conclusion, our results suggest that Cbl-b is critically involved in T-cell exhaustion and prevention of lethal disease.

Evolution of parasite virulence when host responses cause disease

The trade-off hypothesis of virulence evolution rests on the assumption that infection-induced mortality is a consequence of host exploitation by parasites. This hypothesis lies at the heart of many empirical and theoretical studies of virulence evolution, despite growing evidence that infection-induced mortality is very often a by-product of host immune responses. We extend the theoretical framework of the trade-off hypothesis to incorporate such immunopathology and explore how this detrimental aspect of host defence mechanisms affects the evolution of pathogen exploitation and hence infection-induced mortality. We argue that there are qualitatively different ways in which immunopathology can arise and suggest ways in which empirical studies can tease apart these effects. We show that immunopathology can cause infection-induced mortality to increase or decrease as a result of pathogen evolution, depending on how it covaries with pathogen exploitation strategies and with parasite killing by hosts. Immunopathology is thus an important determinant of whether public and animal health programmes will drive evolution in a clinically beneficial or detrimental direction. Immunopathology complicates our understanding of disease evolution, but can nevertheless be readily accounted for within the framework of the trade-off hypothesis.

Regulatory T cell adjustment of quorum growth thresholds and the control of local immune responses

The consequences of regulatory T cell (Treg) inhibition of interleukine 2 secretion is examined by mathematical modelling. We demonstrate that cytokine dependent growth exhibits a quorum T cell population threshold that determines if immune responses develop on activation. Secretion inhibition manipulates the growth dynamics and effectively increases the quorum threshold, i.e. to develop immune responses a higher number of T cells need to be activated. Thus Treg induced secretion inhibition can provide a mechanism for tissue specific regulation of the balance between suppression (control) and immune responses, a balance that can be varied at the local tissue level through the regulation of the local active Treg population size. However, nonspecific inhibition is prone to escape of initially controlled autoimmune T cells through cross reactivity to pathogens and bystander proliferation on unrelated immune responses.

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.

Murine coronavirus spike glycoprotein mediates degree of viral spread, inflammation, and virus-induced immunopathology in the central nervous system

The mouse hepatitis virus (MHV) spike glycoprotein is a major determinant of neurovirulence. We investigated how alterations in spike affect neurovirulence using two isogenic recombinant viruses differing exclusively in spike. S(4)R, containing the MHV-4 spike gene, is dramatically more neurovirulent than S(A59)R, containing the MHV-A59 spike gene (J. J. Phillips, M. M. Chua, E. Lavi, and S. R. Weiss, 1999, J. Virol. 73, 7752-7760). We examined the contribution of differences in cellular tropism, viral spread, and the immune response to infection to the differential neurovirulence of S(4)R and S(A59)R. MHV-4 spike-mediated neurovirulence was associated with extensive viral spread in the brain in both neurons and astrocytes. Infection of primary hippocampal neuron cultures demonstrated that S(4)R spread more rapidly than S(A59)R and suggested that spread may occur between cells in close physical contact. In addition, S(4)R infection induced a massive influx of lymphocytes into the brain, a higher percentage of CD8(+) T cells, and a higher frequency of MHV-specific CD8(+) T cells relative S(A59)R infection. Despite this robust and viral-specific immune response to S(4)R infection, infection of RAG1-/- mice suggested that immune-mediated pathology also contributes to the high neurovirulence of S(4)R.

Dynamic regulation of T cell immunity by CD43

During a viral response, Ag-specific effector T cells show dramatically increased binding by the mAb 1B11 and the lectin peanut agglutinin (PNA). We investigated the contribution of CD43 expression to 1B11 and PNA binding as well as its role in generation and maintenance of a CD8 T cell response. Analysis of CD43(-/-) mice revealed no increased 1B11 binding and reduced PNA binding on virus-specific CD8 T cells from -/- mice compared with +/+ mice. Furthermore, we examined the role of CD43 in the kinetics of an immune response. We show that CD43 expression modestly effects generation of a primary virus-specific CD8 T cell response in vivo but plays a more significant role in trafficking of CD8 T cells to tissues such as the brain. More interestingly, CD43 plays a role in the contraction of the immune response, with CD43(-/-) mice showing increased numbers of Ag-specific CD8 T cells following initial expansion. Following the peak of expansion, Ag-specific CD8 T cells from -/- mice show similar proliferation but demonstrate increased Bcl-2 levels and decreased apoptosis of Ag-specific effector CD8 T cells in vitro. Consistent with a delay in the down-modulation of the immune response, following chronic viral infection CD43(-/-) mice show increased morbidity. These data suggest a dynamic role of CD43 during an immune response: a positive regulatory role in costimulation and trafficking of T cells to the CNS and a negative regulatory role in the down-modulation of an immune response.

Critical role for perforin-, Fas/FasL-, and TNFR1-mediated cytotoxic pathways in down-regulation of antigen-specific T cells during persistent viral infection

Viral persistence following infection with invasive strains of lymphocytic choriomeningitis virus (LCMV) can be achieved by selective down-regulation of virus-specific T lymphocytes. High viral burden in the onset of infection drives responding cells into functional unresponsiveness (anergy) that can be followed by their physical elimination. In this report, we studied down-regulation of the virus-specific CD8(+)-T-cell response during persistent infection of adult mice with LCMV, with emphasis on the role of perforin-, Fas/FasL-, or tumor necrosis factor receptor 1 (TNFR1)-mediated cytolysis in regulating T-cell homeostasis. The results reveal that the absence of perforin, Fas-ligand, or TNFR1 has no significant effect on the kinetics of proliferation and functional inactivation of virus-specific CD8(+) T cells in the onset of chronic LCMV infection. However, these molecules play a critical role in the homeostatic regulation of T cells, influencing the longevity of the virus-specific CD8(+)-T-cell population once it has become anergic. Thus, CD8(+) T cells specific to the dominant LCMV NP(396-404) epitope persist in an anergic state for at least 70 days in perforin-, FasL-, or TNFR1-deficient mice, but they were eliminated by day 30 in C57BL/6 controls. These effects were additive as shown by a deficit of apoptotic death of NP(396-404) peptide-specific CD8(+) T cells in mice lacking both perforin and TNFR1. This suggests a role for perforin-, FasL-, and TNFR1-mediated pathways in down-regulation of the antiviral T cell response during persistent viral infection by determining the fate of antigen-specific T cells. Moreover, virus-specific anergic CD8(+) T cells in persistently infected C57BL/6 mice contain higher levels of Bcl-2 and Bcl-XL than functionally intact T cells generated during acute LCMV infection. In the case of proapoptotic factors, Bax expression did not differ between T-cell populations and Bad was below the limit of detection in all samples. As expression of the Bcl-2 family members controls susceptibility to apoptosis, this finding may provide a molecular basis for the survival of anergic cells under conditions of prolonged antigen stimulation.

Critical role for alpha/beta and gamma interferons in persistence of lymphocytic choriomeningitis virus by clonal exhaustion of cytotoxic T cells

Under conditions of high antigenic load during infection with invasive lymphocytic choriomeningitis virus (LCMV) strains, virus can persist by selective clonal exhaustion of antigen-specific CD8(+) T cells. In this work we studied the down-regulation of the virus-specific CD8(+)-T-cell response during a persistent infection of adult mice, with particular emphasis on the contribution of the interferon response in promoting host defense. Studies were conducted by infecting mice deficient in receptors for type I (alpha/beta interferon [IFN-alpha/beta]), type II (IFN-gamma), and both type I and II IFNs with LCMV isolates that vary in their capacity to induce T-cell exhaustion. The main conclusions of this study are as follows. (i) IFNs play a critical role in LCMV infection by reducing viral loads in the initial stages of infection and thus modifying both the extent of CD8(+)-T-cell exhaustion and the course of infection. The importance of IFNs in this context varies with the biological properties of the LCMV strain. (ii) An inverse correlation exists between antigen persistence and responsiveness of virus-specific CD8(+) T cells. This results in distinct programs of activation or tolerance (functional unresponsiveness and/or physical elimination of antigen-specific cells) during acute and chronic virus infections, respectively. (iii) A successful immune response associated with definitive viral clearance requires an appropriate balance between cellular and humoral components of the immune system. We discuss the role of IFNs in influencing virus-specific T cells that determine the outcome of persistent infections.

CD4+ T-cell-epitope escape mutant virus selected in vivo

Mutations in viral genomes that affect T-cell-receptor recognition by CD8+ cytotoxic T lymphocytes have been shown to allow viral evasion from immune surveillance during persistent viral infections. Although CD4+ T-helper cells are crucially involved in the maintenance of effective cytotoxic T-lymphocyte and neutralizing-antibody responses, their role in viral clearance and therefore in imposing similar selective pressures on the virus is unclear. We show here that transgenic virus-specific CD4+ Tcells, transferred into mice persistently infected with lymphocytic choriomeningitis virus, select for T-helper epitope mutant viruses that are not recognized. Together with the observed antigenic variation of the same T-helper epitope during polyclonal CD4+ T-cell responses in infected pore-forming protein-deficient C57BL/6 mice, this finding indicates that viral escape from CD4+ T lymphocytes is a possible mechanism of virus persistence.

Hypercholesterolemia exacerbates virus-induced immunopathologic liver disease via suppression of antiviral cytotoxic T cell responses

The immune system has to be optimally balanced to be highly effective against infections with cytopathic microbial pathogens and must guarantee efficient destruction of cells infected with noncytopathic agents while leaving the integrity of noninfected cells largely unaltered. We describe here the effects of genetically induced hypercholesterolemia on cellular immunity in apolipoprotein E (ApoE(-/-)) and low density lipoprotein receptor-deficient (LDLR(-/-)) mice during infection with the hepatotropic lymphocytic choriomeningitis virus WE strain. In both ApoE(-/-) and LDLR(-/-) mice hypercholesterolemia aggravated virus-induced immunopathologic liver disease. ApoE(-/-) mice exhibited a higher susceptibility to virus-induced immunopathology than LDLR(-/-) mice and usually succumbed to immunopathologic disease when infected with high doses of virus. Initial virus spread was not influenced by the hypercholesterolemia, whereas clearance of the virus from spleen and nonlymphoid organs, including liver, was delayed. Activation of antiviral CTL, measured by ex vivo cytotoxicity and IFN-gamma production, and recruitment of specific CTL into blood and liver were impaired in hypercholesterolemic mice, indicating that hypercholesterolemia had a significant suppressive effect on cellular immunity. Taken together, these data provide evidence that hypercholesterolemia suppresses antiviral immune responses, thereby changing the host-virus balance, and can increase susceptibility to acute or chronic and potentially lethal virus-induced immunopathologic disease. These findings impinge on our understanding of hypercholesterolemia as a disease parameter and may explain aspects of the frequent association of persistent pathogens with hypercholesterolemia-induced diseases, such as atherosclerosis.

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.

Impairment of CD4(+) T cell responses during chronic virus infection prevents neutralizing antibody responses against virus escape mutants

We have shown previously that neutralizing antibodies (nAbs) are important contributors to the long-term immune control of lymphocytic choriomeningitis virus infection, particularly if cytotoxic T cell responses are low or absent. Nevertheless, virus escape from the nAb response due to mutations within the surface glycoprotein gene may subsequently allow the virus to persist. Here we show that most of the antibody-escape viral mutants retain their immunogenicity. We present evidence that the failure of the infected host to mount effective humoral responses against emerging neutralization-escape mutants correlates with the rapid loss of CD4(+) T cell responsiveness during the establishment of viral persistence. Similar mechanisms may contribute to the persistence of some human pathogens such as hepatitis B and C viruses, and human immunodeficiency virus.

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

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.

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.

Defining CTL-induced pathology: implications for HIV

The relationship between virus and host cells is multifactorial and nonlinear. This indicates that the effect of an immune response on infection can lead to several different outcomes. These include severe immunopathology. We seek to define properties of CTL-induced pathology in viral infections and examine the implications for HIV disease progression. We find that CTL-induced pathology is observed if the rate of viral replication is fast relative to the CTL responsiveness of the host. Theoretical predictions are consistent with empirical data on LCMV infection. These conditions are also sufficient to induce pathology in HIV infection. However, the absence of HIV-specific CTL can result in an equivalent depletion of the CD4 T cell pool as a consequence of the short life span of activated T cells. A mathematical model describing the evolution of HIV coreceptor usage in the context of lytic and nonlytic CD8 cell responses might account for the relatively long time span required to result in disease. Viral evolution toward parameter ranges allowing CTL-induced pathology is difficult to achieve. It requires the emergence of fast viral replication together with escape from nonlytic CTL responses. However, according to the model, fast viral replication can result in the evolution of virus strains that are susceptible to chemokine-mediated inhibition of viral replication.

Role of CD40 ligand and CD28 in induction and maintenance of antiviral CD8+ effector T cell responses

The primary aim of this report was to evaluate the immune responses of CD40 ligand-deficient (CD40L-/-) mice infected with two viruses known to differ markedly in their capacity to replicate in the host. Lymphocytic choriomeningitis virus (LCMV) is a natural mouse pathogen that replicates widely and extensively, whereas vesicular stomatitis virus (VSV) spreads poorly. We found that the primary response of CD40L-/- mice toward VSV is significantly impaired; proliferation of both CD4+ and CD8+ cells is reduced 2- to 3-fold, few CD8+ cells acquire an activated phenotype, and little functional activity is induced. Very similar results were obtained in VSV-infected, CD28-deficient mice. In contrast, neither CD40L nor CD28 was required for induction of a primary CD8+ response toward LCMV. Surprisingly, lack of CD4+ T cells had no impact on the primary immune response toward any of the viruses, even though the CD40 ligand dependence demonstrated for VSV would be expected to be associated with CD4 dependence. Upon coinfection of VSV-infected mice with LCMV, the requirement for CD40 ligand (but not CD28) could be partially bypassed, as evidenced by a 3-fold increase in the frequency of VSV-specific CD8+ T cells on day 6 postinfection. Finally, despite the fact that the primary LCMV-specific CD8+ response is virtually unimpaired in CD40L-/- mice, their capacity to maintain CD8+ effector activity and to permanently control the infection is significantly reduced. Thus, our results demonstrate that the importance of CD40/CD40L interaction for activation of CD8+ T cells varies between viruses and over time.

Viral persistence in vivo through selection of neutralizing antibody-escape variants

Despite initial virus control by CD8(+) cytotoxic T lymphocytes (CTLs), noncytopathic or variably cytopathic viruses (e.g., hepatitis B and C viruses, HIV) are able to establish persistent infections. The role of neutralizing antibodies (nAbs) in controlling disease progression is unclear. Therefore, the phenomenon of viral evasion from the nAb response and its implications for virus persistence remain controversial. Here we demonstrate nAb-mediated viral clearance in CTL-deficient mice infected with the prototypic noncytopathic lymphocytic choriomeningitis virus (strain WE). During prolonged CTL absence, neutralization-resistant virus mutants were selected in individual mice within 70-90 days. In naive animals infected with these virus variants only low nAb responses were induced, resulting in an increased tendency of virus to persist.

Abundant Tax protein expression in CD4+ T cells infected with human T-cell lymphotropic virus type I (HTLV-I) is prevented by cytotoxic T lymphocytes

The role of the cellular immune response in human T-cell leukemia virus type I (HTLV-I) infection is not fully understood. A persistently activated cytotoxic T lymphocyte (CTL) response to HTLV-I is found in the majority of infected individuals. However, it remains unclear whether this CTL response is protective or causes tissue damage. In addition, several observations paradoxically suggest that HTLV-I is transcriptionally silent in most infected cells and, therefore, not detectable by virus-specific CTLs. With the use of a new flow cytometric procedure, we show here that a high proportion of naturally infected CD4+ peripheral blood mononuclear cells (PBMC) (between 10% and 80%) are capable of expressing Tax, the immunodominant target antigen recognized by virus-specific CTLs. Furthermore, we provide direct evidence that autologous CD8+ T cells rapidly kill CD4+ cells naturally infected with HTLV-I and expressing Tax in vitro by a perforin-dependent mechanism. Consistent with these observations, we observed a significant negative correlation between the frequency of Tax11-19-specific CD8+ T cells and the percentage of CD4+ T cells in peripheral blood of patients infected with HTLV-I. Those results are in accordance with the view that virus-specific CTLs participate in a highly efficient immune surveillance mechanism that persistently destroys Tax-expressing HTLV-I-infected CD4+ T cells in vivo.

Antibody Prevents Virus Reactivation Within the Central Nervous System

The neurotropic JHM strain of mouse hepatitis virus (JHMV) produces an acute CNS infection characterized by encephalomyelitis and demyelination. The immune response cannot completely eliminate virus, resulting in persistence associated with chronic ongoing CNS demyelination. The contribution of humoral immunity to viral clearance and persistent infection was investigated in mice homozygous for disruption of the Ig μ gene (IgM−/−). Acute disease developed with equal kinetics and severity in IgM−/− and syngeneic C57BL/6 (wt) mice. However, clinical disease progressed in IgM−/− mice, while wt mice recovered. Viral clearance during acute infection was similar in both groups, supporting a primary role of cell-mediated immunity in viral clearance. In contrast to wt mice, in which infectious virus was reduced to below detection following acute infection, increasing infectious virus was recovered from the CNS of the IgM−/− mice following initial clearance. No evidence was obtained for selection of variant viruses nor was there an apparent loss of cell-mediated immunity in the absence of Ab. Passive transfer of anti-JHMV Ab following initial clearance prevented reactivation of infectious virus within the CNS of IgM−/− mice. These data demonstrate the clearance of infectious virus during acute disease by cell-mediated immunity. However, immunologic control is not maintained in the absence of anti-viral Ab, resulting in recrudescence of infectious virus. These data suggest that humoral immunity plays no role in controlling virus during acute infection, but plays an important role in establishing and maintaining CNS viral persistence.