Detection of measles virus nucleoprotein mRNA in autopsied brain tissues

Rubella virus chronic inflammatory disease and other unusual viral phenotypes in inborn errors of immunity

Infectious susceptibility is a component of many inborn errors of immunity. Nevertheless, antibiotic use is often used as a surrogate in history taking for infectious susceptibility, thereby disadvantaging patients who present with viral infections as their phenotype. Further complicating clinical evaluations are unusual manifestations of viral infections which may be less familiar that the typical respiratory viral infections. This review covers several unusual viral phenotypes arising in patients with inborn errors of immunity and other settings of immune compromise. In some cases, chronic infections lead to oncogenesis or tumor-like growths and the conditions and mechanisms of viral-induced oncogenesis will be described. This review covers enterovirus, rubella, measles, papillomavirus, and parvovirus B19. It does not cover EBV and hemophagocytic lymphohistiocytosis nor lymphomagenesis related to EBV. EBV susceptibility has been recently reviewed. Our goal is to increase awareness of the unusual manifestations of viral infections in patients with IEI and to describe treatment modalities utilized in this setting. Coincidentally, each of the discussed viral infections can have a cutaneous component and figures will serve as a reminder of the physical features of these viruses. Given the high morbidity and mortality, early recognition can only improve outcomes.

Comparable Infection Level and Tropism of Measles Virus and Canine Distemper Virus in Organotypic Brain Slice Cultures Obtained from Natural Host Species

Measles virus (MV) and canine distemper virus (CDV) are closely related members of the family Paramyxoviridae, genus Morbillivirus. MV infection of humans and non-human primates (NHPs) results in a self-limiting disease, which rarely involves central nervous system (CNS) complications. In contrast, infection of carnivores with CDV usually results in severe disease, in which CNS complications are common and the case-fatality rate is high. To compare the neurovirulence and neurotropism of MV and CDV, we established a short-term organotypic brain slice culture system of the olfactory bulb, hippocampus, or cortex obtained from NHPs, dogs, and ferrets. Slices were inoculated ex vivo with wild-type-based recombinant CDV or MV expressing a fluorescent reporter protein. The infection level of both morbilliviruses was determined at different times post-infection. We observed equivalent infection levels and identified microglia as main target cells in CDV-inoculated carnivore and MV-inoculated NHP brain tissue slices. Neurons were also susceptible to MV infection in NHP brain slice cultures. Our findings suggest that MV and CDV have comparable neurotropism and intrinsic capacity to infect CNS-resident cells of their natural host species.

Mechanisms of viral persistence in the brain and therapeutic approaches

There is growing recognition of the diversity of viruses that can infect the cells of the central nervous system (CNS). While the majority of CNS infections are successfully cleared by the immune response, some viral infections persist in the CNS. As opposed to resolved infections, persistent viruses can contribute to ongoing tissue damage and neuroinflammatory processes. In this manuscript, we provide an overview of the current understanding of factors that lead to viral persistence in the CNS including how viruses enter the brain, how these pathogens evade antiviral immune system responses, and how viruses survive and transmit within the CNS. Further, as the CNS may serve as a unique viral reservoir, we examine the ways in which persistent viruses in the CNS are being targeted therapeutically.

Should we expect neurological symptoms in the SARS-CoV-2 epidemic?

INTRODUCTION

There is growing evidence that SARS-CoV-2 can gain access to the central nervous system (CNS). We revise the literature on coronavirus infection of the CNS associated with neurological diseases.

DEVELOPMENT

Neurological symptoms were rarely reported in the SARS-CoV and MERS-CoV epidemics, although isolated cases were described. There are also reports of cases of neurological symptoms associated with CoV-OC43 and CoV-229E infection. The presence of neurological lesions, especially demyelinating lesions in the mouse hepatitis virus model, may explain the mechanisms by which coronaviruses enter the CNS, particularly those related with the immune response. This may explain the presence of coronavirus in patients with multiple sclerosis. We review the specific characteristics of SARS-CoV-2 and address the question of whether the high number of cases may be associated with greater CNS involvement.

CONCLUSION

Although neurological symptoms are not frequent in coronavirus epidemics, the high number of patients with SARS-CoV-2 infection may explain the presence of the virus in the CNS and increase the likelihood of early- or delayed-onset neurological symptoms. Follow-up of patients affected by the SARS-CoV-2 epidemic should include careful assessment of the CNS.

Should we expect neurological symptoms in the SARS-CoV-2 epidemic?

Introduction

There is growing evidence that SARS-CoV-2 can gain access to the central nervous system (CNS). We revise the literature on coronavirus infection of the CNS associated with neurological diseases.

Development

Neurological symptoms were rarely reported in the SARS-CoV and MERS-CoV epidemics, although isolated cases were described. There are also reports of cases of neurological symptoms associated with CoV-OC43 and CoV-229E infection. The presence of neurological lesions, especially demyelinating lesions in the mouse hepatitis virus model, may explain the mechanisms by which coronaviruses enter the CNS, particularly those related with the immune response. This may explain the presence of coronavirus in patients with multiple sclerosis. We review the specific characteristics of SARS-CoV-2 and address the question of whether the high number of cases may be associated with greater CNS involvement.

Conclusion

Although neurological symptoms are not frequent in coronavirus epidemics, the high number of patients with SARS-CoV-2 infection may explain the presence of the virus in the CNS and increase the likelihood of early- or delayed-onset neurological symptoms. Follow-up of patients affected by the SARS-CoV-2 epidemic should include careful assessment of the CNS.

Immune-Mediated Control of a Dormant Neurotropic RNA Virus Infection

Genomic material from many neurotropic RNA viruses (e.g., measles virus [MV], West Nile virus [WNV], Sindbis virus [SV], rabies virus [RV], and influenza A virus [IAV]) remains detectable in the mouse brain parenchyma long after resolution of the acute infection. The presence of these RNAs in the absence of overt central nervous system (CNS) disease has led to the suggestion that they are viral remnants, with little or no potential to reactivate. Here we show that MV RNA remains detectable in permissive mouse neurons long after challenge with MV and, moreover, that immunosuppression can cause RNA and protein synthesis to rebound, triggering neuropathogenesis months after acute viral control. Robust recrudescence of viral transcription and protein synthesis occurs after experimental depletion of cells of the adaptive immune response and is associated with a loss of T resident memory (T_rm) lymphocytes within the brain. The disease associated with loss of immune control is distinct from that seen during the acute infection: immune cell-depleted, long-term-infected mice display severe gait and motor problems, in contrast to the wasting and lethal disease that occur during acute infection of immunodeficient hosts. These results illuminate the potential consequences of noncytolytic, immune-mediated viral control in the CNS and demonstrate that what were once considered "resolved" RNA viral infections may, in fact, induce diseases later in life that are distinct from those caused by acute infection.IMPORTANCE Viral infections of neurons are often not cytopathic; thus, once-infected neurons survive, and viral RNAs can be detected long after apparent viral control. These RNAs are generally considered viral fossils, unlikely to contribute to central nervous system (CNS) disease. Using a mouse model of measles virus (MV) neuronal infection, we show that MV RNA is maintained in the CNS of infected mice long after acute control and in the absence of overt disease. Viral replication is suppressed by the adaptive immune response; when these immune cells are depleted, viral protein synthesis recurs, inducing a CNS disease that is distinct from that observed during acute infection. The studies presented here provide the basis for understanding how persistent RNA infections in the CNS are controlled by the host immune response, as well as the pathogenic consequences of noncytolytic viral control.

What if protective immunity is antigen-driven and not due to so-called "memory" B and T cells?

Vaccines or early childhood exposure to infection mediate immunity, that is, improved resistance against disease and death caused by a second infection with the same agent. This has been explained by and equaled to immunological memory, that is, an "altered immune system behavior" that is maintained in a presumably antigen-independent fashion. This review summarizes epidemiological and experimental data, that largely falsify this idea and that show that periodic re-exposure to antigen either, artificially as vaccines or naturally as low-level persisting antigens or infections, or immune complexes on follicular dendritic cells or endemic re-exposure is necessary for protection. Both, the huge success of vaccines in controlling childhood infections, the reduction in clinical disease and the chance of endemically re-exposure, have gradually reduced periodical re-exposure to infections and thereby endangered protective herd immunity. In parallel, vaccine deniers have created susceptibility islands even in an otherwise well vaccinated population, thereby creating a very new situation when compared to the later parts of the 20th century. If protective Immunity is-as emphasized here-antigen driven, then increasingly frequent revaccinations will be necessary (even more so with too much attenuated vaccines) to maintain both herd immunity and individual resistance to acute infections. Of course, this rule also applies to tumor vaccines.

Subacute sclerosing panencephalitis - current perspectives

Subacute sclerosing panencephalitis is a progressive neurodegenerative disease. It usually occurs 7–10 years after measles infection. The clinical course is characterized by progressive cognitive decline and behavior changes followed by focal or generalized seizures as well as myoclonus, ataxia, visual disturbance, and later vegetative state, eventually leading to death. It is diagnosed on the basis of Dyken’s criteria. There is no known cure for subacute sclerosing panencephalitis to date, but it is preventable by ensuring that an effective vaccine program for measles is made compulsory for all children younger than 5 years in endemic countries.

MiRNA-124 is a link between measles virus persistent infection and cell division of human neuroblastoma cells

Measles virus (MV) infects a variety of lymphoid and non-lymphoid peripheral organs. However, in rare cases, the virus can persistently infect cells within the central nervous system. Although some of the factors that allow MV to persist are known, the contribution of host cell-encoded microRNAs (miRNA) have not been described. MiRNAs are a class of noncoding RNAs transcribed from genomes of all multicellular organisms and some viruses, which regulate gene expression in a sequence-specific manner. We have studied the contribution of host cell-encoded miRNAs to the establishment of MV persistent infection in human neuroblastoma cells. Persistent MV infection was accompanied by differences in the expression profile and levels of several host cell-encoded microRNAs as compared to uninfected cells. MV persistence infection of a human neuroblastoma cell line (UKF-NB-MV), exhibit high miRNA-124 expression, and reduced expression of cyclin dependent kinase 6 (CDK6), a known target of miRNA-124, resulting in slower cell division but not cell death. By contrast, acute MV infection of UKF-NB cells did not result in increased miRNA-124 levels or CDK6 reduction. Ectopic overexpression of miRNA-124 affected cell viability only in UKF-NB-MV cells, causing cell death; implying that miRNA-124 over expression can sensitize cells to death only in the presence of MV persistent infection. To determine if miRNA-124 directly contributes to the establishment of MV persistence, UKF-NB cells overexpressing miRNA-124 were acutely infected, resulting in establishment of persistently infected colonies. We propose that miRNA-124 triggers a CDK6-dependent decrease in cell proliferation, which facilitates the establishment of MV persistence in neuroblastoma cells. To our knowledge, this is the first report to describe the role of a specific miRNA in MV persistence.

Keeping it in check: chronic viral infection and antiviral immunity in the brain

It is becoming clear that the manner by which the immune response resolves or contains infection by a pathogen varies according to the tissue that is affected. Unlike many peripheral cell types, CNS neurons are generally non-renewable. Thus, the cytolytic and inflammatory strategies that are effective in controlling infections in the periphery could be damaging if deployed in the CNS. Perhaps for this reason, the immune response to some CNS viral infections favours maintenance of neuronal integrity and non-neurolytic viral control. This modified immune response - when combined with the unique anatomy and physiology of the CNS - provides an ideal environment for the maintenance of viral genomes, including those of RNA viruses. Therefore, it is possible that such viruses can reactivate long after initial viral exposure, contributing to CNS disease.

Acute measles encephalitis in partially vaccinated adults

BACKGROUND

The pathogenesis of acute measles encephalitis (AME) is poorly understood. Treatment with immune-modulators is based on theories that post-infectious autoimmune responses cause demyelination. The clinical course and immunological parameters of AME were examined during an outbreak in Vietnam.

METHODS AND FINDINGS

Fifteen measles IgM-positive patients with confusion or Glasgow Coma Scale (GCS) score below 13, and thirteen with uncomplicated measles were enrolled from 2008-2010. Standardized clinical exams were performed and blood collected for lymphocyte and measles- and auto-antibody analysis. The median age of AME patients was 21 years, similar to controls. Eleven reported receiving measles vaccination when aged one year. Confusion developed a median of 4 days after rash. Six patients had GCS <8 and four required mechanical ventilation. CSF showed pleocytosis (64%) and proteinorrhachia (71%) but measles virus RNA was not detected. MRI revealed bilateral lesions in the cerebellum and brain stem in some patients. Most received dexamethasone +/- IVIG within 4 days of admission but symptoms persisted for ≥3 weeks in five. The concentration of voltage gated calcium channel-complex-reactive antibodies was 900 pM in one patient, and declined to 609 pM ∼ 3 months later. Measles-reactive IgG antibody avidity was high in AME patients born after vaccine coverage exceeded 50% (∼ 25 years earlier). AME patients had low CD4 (218/µl, p = 0.029) and CD8 (200/µl, p = 0.012) T-cell counts compared to controls.

CONCLUSION

Young adults presenting with AME in Vietnam reported a history of one prior measles immunization, and those aged <25 years had high measles-reactive IgG avidity indicative of prior vaccination. This suggests that one-dose measles immunization is not sufficient to prevent AME in young adults and reinforces the importance of maintaining high coverage with a two-dose measles immunization schedule. Treatment with corticosteroids and IVIG is common practice, and should be assessed in randomized clinical trials.

Immunological memory ≠ protective immunity

So-called 'immunological memory' is, in my view, a typical example where a field of enquiry, i.e. to understand long-term protection to survive reexposure to infection, has been overtaken by 'l'art pour l'art' of 'basic immunology'. The aim of this critical review is to point out some key differences between academic text book-defined immunological memory and protective immunity as viewed from a co-evolutionary point of view, both from the host and the infectious agents. A key conclusion is that 'immunological memory' of course exists, but only in particular experimental laboratory models measuring 'quicker and better' responses after an earlier immunization. These often do correlate with, but are not the key mechanisms of, protection. Protection depends on pre-existing neutralizing antibodies or pre-activated T cells at the time of infection-as documented by the importance of maternal antibodies around birth for survival of the offspring. Importantly, both high levels of antibodies and of activated T cells are antigen driven. This conclusion has serious implications for our thinking about vaccines and maintaining a level of protection in the population to deal with old and new infectious diseases.

Measles virus, immune control, and persistence

Measles remains one of the most important causes of child morbidity and mortality worldwide with the greatest burden in the youngest children. Most acute measles deaths are owing to secondary infections that result from a poorly understood measles-induced suppression of immune responses. Young children are also vulnerable to late development of subacute sclerosing panencephalitis, a progressive, uniformly fatal neurologic disease caused by persistent measles virus (MeV) infection. During acute infection, the rash marks the appearance of the adaptive immune response and CD8(+) T cell-mediated clearance of infectious virus. However, after clearance of infectious virus, MeV RNA persists and can be detected in blood, respiratory secretions, urine, and lymphoid tissue for many weeks to months. This prolonged period of virus clearance may help to explain measles immunosuppression and the development of lifelong immunity to re-infection, as well as occasional infection of the nervous system. Once MeV infects neurons, the virus can spread trans-synaptically and the envelope proteins needed to form infectious virus are unnecessary, accumulate mutations, and can establish persistent infection. Identification of the immune mechanisms required for the clearance of MeV RNA from multiple sites will enlighten our understanding of the development of disease owing to persistent infection.

T cell-, interleukin-12-, and gamma interferon-driven viral clearance in measles virus-infected brain tissue

Genetic studies with immunocompetent mice show the importance of both T cells and gamma interferon (IFN-γ) for survival of a measles virus (MV) challenge; however, the direct role of T cells and IFN-γ within the MV-infected brain has not been addressed. Organotypic brain explants represent a successful ex vivo system to define central nervous system (CNS)-specific mechanisms of leukocyte migration, activation, and MV clearance. Within the heterogeneous, brain-derived, primed leukocyte population which reduced MV RNA levels in brain explants by 60%, CD3 T cells are the active antiviral cells, as purified CD3-positive cells are highly antiviral and CD3-negative leukocytes are unable to reduce the viral load. Neutralization of CCL5 and CXCL10 decreases leukocyte migration to areas of infection by 70%. However, despite chemokines directing the migration of T cells to infected neurons, chemokine neutralization revealed that migration is not required for viral clearance, suggesting a cytokine-mediated antiviral mechanism. In accordance with our hypothesis, the ability of leukocytes to clear the virus is abrogated when explants are treated with anti-IFN-γ neutralizing antibodies. IFN-γ applied to infected slices in the absence of primed leukocytes reduces the viral load by more than 80%; therefore, in brain tissue, IFN-γ is both necessary and sufficient to clear MV. Secretion of IFN-γ is stimulated by interleukin-12 (IL-12) in the brain, as neutralization of IL-12 results in loss of antiviral activity and stimulation of leukocytes with IL-12/IL-18 enhances their immune effector function of viral clearance. MV-primed leukocytes can reduce both West Nile and mouse hepatitis viral RNAs, indicating that cytokine-mediated viral clearance occurs in an antigen-independent manner. The IFN-γ signal is transduced within the brain explant by the Jak/STAT signaling pathway, as inhibition of Jak kinases results in a loss of antiviral activity driven by either brain-derived leukocytes or recombinant IFN-γ. These results reveal that primed T cells directly act to clear MV infection of the brain by using a noncytolytic IL-12- and IFN-γ-dependent mechanism in the CNS and that this mechanism relies upon Jak/STAT signaling.

Measles virus neurovirulence and host immunity

Measles virus is highly neuroinvasive, yet host immune responses are highly effective at limiting neurovirulence in humans. We know that neurons are an important target of infection and that both IFN-γ and -β expression are observed in the measles virus-infected human brain. Rodent models can be used to understand how this response is orchestrated. Constitutive expression of the major inducible 70-kDa heat-shock protein is a feature of primate tissues that is lacking in mice. This article examines the importance of addressing this difference when modeling outcomes of brain infection in mice, particularly in terms of understanding how infected neurons may activate uninfected brain macrophages to produce IFN-β and support T-cell production of IFN-γ, a mediator of noncytolytic viral clearance. New and historical data suggest that the virus heat-shock protein 70 relationship is key to a protective host immune response and has potential broad relevance.

Blue moon neurovirology: the merits of studying rare CNS diseases of viral origin

While measles virus (MV) continues to have a significant impact on human health, causing 150,000-200,000 deaths worldwide each year, the number of fatalities that can be attributed to MV-triggered central nervous system (CNS) diseases are on the order of a few hundred individuals annually (World Health Organization 2009). Despite this modest impact, substantial effort has been expended to understand the basis of measles-triggered neuropathogenesis. What can be gained by studying such a rare condition? Simply stated, the wealth of studies in this field have revealed core principles that are relevant to multiple neurotropic pathogens, and that inform the broader field of viral pathogenesis. In recent years, the emergence of powerful in vitro systems, novel animal models, and reverse genetics has enabled insights into the basis of MV persistence, the complexity of MV interactions with neurons and the immune system, and the role of immune and CNS development in virus-triggered disease. In this review, we highlight some key advances, link relevant measles-based studies to the broader disciplines of neurovirology and viral pathogenesis, and propose future areas of study for the field of measles-mediated neurological disease.

Major histocompatibility complex haplotype determines hsp70-dependent protection against measles virus neurovirulence

In vitro studies show that hsp70 promotes gene expression for multiple viral families, although there are few reports on the in vivo significance of virus-hsp70 interaction. Previously we showed that hsp70-dependent stimulation of Edmonston measles virus (Ed MeV) transcription caused an increased cytopathic effect and mortality in transgenic hsp70-overexpressing C57BL/6 mice (H-2(b)). The response to MeV infection is influenced by the major histocompatibility complex haplotype; H-2(d) mice are resistant to brain infection due to robust antiviral immune responses, whereas H-2(b) mice are susceptible due to deficiencies in this response. We therefore tested the hypothesis that the outcome of MeV-hsp70 interaction may be dependent upon the host H-2 haplotype. The impact of selective neuronal hsp70 overexpression on Ed MeV brain infection was tested with congenic C57BL/10 H-2(d) neonatal mice. In this context, hsp70 overexpression conferred complete protection against virus-induced mortality, compared to >30% mortality in nontransgenic mice. Selective depletion of T-cell populations showed that transgenic mice exhibit a diminished reliance on T cells for protection. Brain transcript analysis indicated enhanced innate immune activation and signaling through Toll-like receptors 2 and 4 at early times postinfection for transgenic infected mice relative to those for nontransgenic infected mice. Collectively, results suggest that hsp70 can enhance innate antiviral immunity through Toll-like receptor signaling, supporting a protective role for physiological responses that enhance tissue levels of hsp70 (e.g., fever), and that the H-2 haplotype determines the effectiveness of this response.

Making it to the synapse: measles virus spread in and among neurons

Measles virus (MV) is one of the most transmissible microorganisms known, continuing to result in extensive morbidity and mortality worldwide. While rare, MV can infect the human central nervous system, triggering fatal CNS diseases weeks to years after exposure. The advent of crucial laboratory tools to dissect MV neuropathogenesis, including permissive transgenic mouse models, the capacity to manipulate the viral genome using reverse genetics, and cell biology advances in understanding the processes that govern intracellular trafficking of viral components, have substantially clarified how MV infects, spreads, and persists in this unique cell population. This review highlights some of these technical advances, followed by a discussion of our present understanding of MV neuronal infection and transport. Because some of these processes may be shared among diverse viruses, comparisons are made to parallel studies with other neurotropic viruses. While a crystallized view of how the unique environment of the neuron affects MV replication, spread, and, ultimately, neuropathogenesis is not fully realized, the tools and ideas are in place for exciting advances in the coming years.

Protective 'immunity' by pre-existent neutralizing antibody titers and preactivated T cells but not by so-called 'immunological memory'

The idea of immunological memory originally arose from the observation that survivors of infections were subsequently resistant to disease caused by the same infection. While most immunologists accept a special 'remembering' memory quality, we have argued previously and document here that increased resistance against re-infection, i.e. immunity, reflects low-level antigen-driven T- and B-cell responses, resulting in elevated serum or mucosal titers of protective antibodies or of activated T cells, respectively. Periodic antigen re-exposure is from within, by persisting infection (long-term) or by immune complexes (short-term), or from without, by low-level re-infections. This simple concept is supported by clinical evidence and model experiments but is often ignored, although this concept, but not so-called 'immunological memory', as defined in textbooks (i.e. earlier and better responses of a primed host), is compatible with evolutionary maternal antibody transfer of protection as well as immunity against existing infections. The concept of 'immunity without immunological remembering memory' explains why it is easy to generate vaccines against acute cytopathic infections, particularly those of early childhood, where neutralizing antibodies are the key to protection, because it has been validated by adoptive transfer of maternal antibodies. It also explains why we have not succeeded (yet?) to generate truly protective vaccines against persisting infections, because we cannot imitate 'infection immunity' that is long-lasting, generating protective T- and B-cell stimulation against variable infections without causing disease by either immunopathology or tolerance.

hsp72, a host determinant of measles virus neurovirulence

Transient hyperthermia such as that experienced during febrile episodes increases expression of the major inducible 70-kDa heat shock protein (hsp72). Despite the relevance of febrile episodes to viral pathogenesis and the multiple in vitro roles of heat shock proteins in viral replication and gene expression, the in vivo significance of virus-heat shock protein interactions is unknown. The present work determined the in vivo relationship between hsp72 levels and neurovirulence of an hsp72-responsive virus using the mouse model of measles virus (MV) encephalitis. Transgenic C57BL/6 mice were created to constitutively overexpress hsp72 in neurons, and these mice were inoculated intracranially with Edmonston MV (Ed MV) at 42 h of age. The mean viral RNA burden in brain was approximately 2 orders of magnitude higher in transgenic animals than in nontransgenic animals 2 to 4 weeks postinfection, and this increased burden was associated with a fivefold increase in mortality. Mice were also challenged with an Ed MV variant exhibiting an attenuated in vitro response to hsp72-dependent stimulation of viral transcription (Ed N-522D). This virus exhibited an attenuated neuropathogenicity in transgenic mice, where mortality and viral RNA burdens were not significantly different from nontransgenic mice infected with either Ed N-522D or parent Ed MV. Collectively, these results indicate that hsp72 levels can serve as a host determinant of viral neurovirulence in C57BL/6 mice, reflecting the direct influence of hsp72 on viral gene expression.

Maintenance of long-term tumour-specific T-cell memory by residual dormant tumour cells

LacZ (Gal)-reactive immune cells were transferred into athymic nu/nu mice inoculated with Gal-expressing syngeneic tumour cells (ESbL-Gal) in order to study tumour-protective T-cell memory. This transfer prevented tumour outgrowth in recipients and resulted in the persistence of a high frequency of Gal-specific CD8(+) T cells in the bone marrow and spleen. In contrast, such Ag-specific memory CD8(+) T cells were not detectable by peptide-major histocompatibility complex (MHC) multimer staining in animals that had not previously received an antigenic challenge. Even though CD44(hi) memory T cells from the bone marrow showed a significantly higher turnover rate, as judged by bromodeoxyuridine (BrdU) incorporation, than respective cells from spleen or lymph nodes, as well as in comparison to CD44(lo) naïve T cells, these findings suggest that tumour-associated antigen (TAA) from residual dormant tumour cells are implicated in maintaining high frequencies of long-term surviving Gal-specific memory CD8(+) T cells. Memory T cells could be recruited to the peritoneal cavity by tumour vaccination of immunoprotected nu/nu mice and exhibited ex vivo antitumour reactivity. Long-term immune memory and tumour protection could be maintained over four successive transfers between tumour-inoculated recipients, which involved periodic antigenic restimulation in vivo prior to reisolating the cells for adoptive transfer. Using a cell line (ESbL-Gal-BM) that was established from dormant tumour cells isolated from the bone marrow of immunoprotected animals, it could be demonstrated that the tumour cells had up-regulated the expression of MHC class I molecules and down-regulated the expression of several adhesion molecules during the in vivo passage. Our results suggest that the bone marrow microenvironment has special features that are of importance for the maintenance of tumour dormancy and immunological T-cell memory, and that a low level of persisting antigen favours the maintenance of Ag-specific memory T cells over irrelevant memory T cells.

On 'reactivity' versus 'tolerance'

In Burnet's review on 'The impact of ideas on immunology' he considers himself an observer of nature using biochemical and molecular analysis for more detailed understanding, a description that applies also to me. I use three examples--repertoire selection of T cells, rules of immune reactivity versus non-reactivity and immunological memory--to illustrate the difficulties we all have in probing nature's immunological secrets and in critically testing immunologists' ideas. At one end of the spectrum of biological research one may argue everything is possible and therefore all results are correct, if correctly measured. But perhaps it is more important to always ask again and again what is frequent and enhances survival versus what is rare and an exception. At the same time one must keep in mind that special situations and special tricks may well be applied for medical benefits, although they may have little impact on physiology and species survival. I will attempt to use disease in virus-infected mice to obtain some answers to what I consider to be important immunological questions with the hope of improving the ratio of answers that are right for the right experimental reasons versus those that are right for the wrong reasons. Some of these experiments falsify hypotheses, previous experiments and interpretations and therefore are particularly important in correcting misleading concepts. They should help to find out which half of immunological ideas and truths in immunological text books written today are likely to be wrong. Ideas are important in immunology, but are often rather demagogically handled and therefore may cost us very dearly indeed. Evaluating immunity to infections and tumours in vivo should help prevent us from getting lost in immunology.

Search for morbillivirus proteins in multiple sclerosis brain tissue

We investigated brain samples of patients with multiple sclerosis (MS) and controls with immunohistochemistry using monoclonal antibodies (MoAbs) against canine distemper virus (CDV) and measles virus (MV) proteins. All stained negative except for MoAb F3-5, which recognises a conserved epitope on the fusion protein of morbilliviruses. F3-5 immunostaining was found in 8/9 MS plaques and 2/5 herpes simplex virus encephalitis brain samples, but not in six controls or four patients with ischaemic stroke. Using RT-PCR we found no evidence for the presence of MV in MS plaques. The F3-5 epitope may represent a protein that is upregulated during inflammation or point to a yet unrecognised morbillivirus in the human central nervous system that might be implicated in MS pathogenesis.

On natural and artificial vaccinations

This review summarizes the general parameters of cell- and antibody-mediated immune protection and the basic mechanisms responsible for what we call immunological memory. From this basis, the various successes and difficulties of vaccines are evaluated with respect to the role of antigen in maintaining protective immunity. Based on the fact that in humans during the first 12-48 months maternal antibodies from milk and serum protect against classical acute childhood and other infections, the concept is developed that maternal antibodies attenuate most infections of babies and infants and turn them into effective vaccines. If this "natural vaccination" under passive protective conditions does not occur, acute childhood diseases may be severe, unless infants are actively vaccinated with conventional vaccines early enough, i.e., in synchronization with the immune system's maturation. Although vaccines are available against the classical childhood diseases, they are not available for many seemingly milder childhood infections such as gastrointestinal and respiratory infections; these may eventually trigger immunopathological diseases. These changing balances between humans and infections caused by changes in nursing habits but also in hygiene levels may well be involved in changing disease patterns including increased frequencies of certain autoimmune and degenerative diseases.

The effectiveness and limitations of immune memory: understanding protective immune responses

Immune memory is the foundation of the practise of vaccination. Research on the molecular and cellular events leading to generation and development of memory T and B lymphocytes explain why there are heightened secondary immune responses after an initial encounter with antigen. In this review, we discuss how clonal expansion, targeted tissue localisation, more efficient antigen recognition and more proficient effector functions contribute to the improved effectiveness of memory cells. Despite the enhanced efficacy of memory cells and the recall immune response, there are numerous experimental and empirical examples in which protection provided by vaccines are short-lived, particularly against pathogens that replicate and cause pathology at their site of entry. In the absence of active immune effector activities, the ability of memory cells to respond quickly enough to control this type of infection is limited. The protective efficacy of bovine herpes virus-1 vaccines in experimental and field challenge conditions are used to illustrate the concept that full protection from disease conferred by vaccination requires the presence of active immune effector mechanisms. Thus, regardless of the many successful technological advances in vaccine design and better understanding of mechanisms underlining induction of memory responses by vaccination, we should recognise that vaccine immunoprophylaxis has limitations. Expectations for vaccines should be realistic and linked to the understanding of host immune responses and knowledge regarding the pathogen and disease pathogenesis.

Memory T cells and vaccines

T lymphocytes play a central role in the generation of a protective immune response in many microbial infections. After immunization, dendritic cells take up microbial antigens and traffic to draining lymph nodes where they present processed antigens to naïve T cells. These naïve T cells are stimulated to proliferate and differentiate into effector and memory T cells. Activated, effector and memory T cells provide B cell help in the lymph nodes and traffic to sites of infection where they secrete anti-microbial cytokines and kill infected cells. At least two types of memory cells have been defined in humans based on their functional and migratory properties. T central-memory (T(CM)) cells are found predominantly in lymphoid organs and can not be immediately activated, whereas T effector-memory (T(EM)) cells are found predominantly in peripheral tissue and sites of inflammation and exhibit rapid effector function. Most currently licensed vaccines induce antibody responses capable of mediating long-term protection against lytic viruses such as influenza and small pox. In contrast, vaccines against chronic pathogens that require cell-mediated immune responses to control, such as malaria, Mycobacterium tuberculosis (TB), human immunodeficiency virus (HIV) and hepatitis C virus (HCV), are currently not available or are ineffective. Understanding the mechanisms by which long-lived cellular immune responses are generated following vaccination should facilitate the development of safe and effective vaccines against these emerging diseases. Here, we review the current literature with respect to memory T cells and their implications to vaccine development.

Neuronal survival strategies in the face of RNA viral infection

Neurons of the mammalian central nervous system (CNS) are an essential and largely nonrenewable cell population. Thus, viral infections that result in neuronal depletion, either by viral lysis or by induction of the cytolytic immune response, would likely lead to profound neurologic impairment. However, many viral infections that result in tissue destruction elsewhere in the host produce few overt symptoms in the CNS, despite readily detectable virus expression. This observation has lead to the speculation that neurons possess strategies to limit the replication and spread of otherwise cytopathic viruses. These strategies either favor the clearance of virus in the absence of appreciable neuronal loss or promote the establishment of noncytolytic persistent infections. This review discusses some of these strategies, with an emphasis on how such survival techniques lessen the potential for CNS neuropathology.

Detection of measles virus genome in bone-marrow aspirates from adults

We investigated the presence of the measles virus genome in order to identify asymptomatic infections in the adult population. Bone-marrow aspirates were obtained from 179 patients, 20-96 years of age, for the diagnosis of malignant diseases (29 with malignant lymphoma, 28 with acute leukaemia, 21 with myelodysplastic syndrome, five with multiple myeloma and 96 with other diseases). The measles virus genome was detected in 17 (9.5%) of 179 individuals by RT-PCR and 28 (15.6%) through hybridization. The genomes detected in bone marrow were all in the same cluster, D5, the strain circulating during the study period, and no evidence of persistent infection was obtained. We conclude that asymptomatic infections of measles virus are common in adults and the presence of the measles virus genome would not be related to the pathogenesis of illness.

On differences between immunity and immunological memory

The evolutionary benefits of immunological memory are important: whereas antibodies can be transmitted to offspring by their mother and thereby benefit the species, T cell memory may function to help the individual combat persistent infection in peripheral tissues. Although experimental immunological memory is largely maintained antigen-independently, protective immunity is antigen-dependent.

Viral studies in the cerebrospinal fluid in subacute sclerosing panencephalitis

OBJECTIVES

The pathogenesis of subacute sclerosing panencephalitis (SSPE), and particularly, the cause of measles virus (MV) reactivation following a latent period after primary measles infection is unknown. The hypothesis of other viruses contributing to the pathogenesis of SSPE by affecting the in vivo state of MV was investigated.

METHODS

We examined the cerebrospinal fluid of SSPE patients (n=43) for DNA or RNA and antibodies against HSV type 1 and 2, EBV, CMV, VZV, Hepatitis B, Hepatitis C, JC virus, human herpesvirus (HHV)-6, HHV-7, HHV-8, HTLV-1, and HTLV-2. We compared the findings with those of patients with other neurological disorders (n=39).

RESULTS

CMV DNA and HSV type 1 IgG were found more frequently in SSPE patients. Other positive results were at similar incidence in SSPE and control groups. The clinical features of SSPE cases with and without positive viral tests did not differ from each other.

CONCLUSION

These data do not support a specific role for these agents in SSPE, but imply that the passage of some viruses to the CNS and local antibody synthesis may be facilitated by inflammation. The persistence or reactivation of MV in SSPE may be related to other factors pertaining to the host or environment.

Detection of measles virus genome in lymphocytes from asymptomatic healthy children

A total of 342 samples of peripheral blood mononuclear cells (PBMC) were obtained from 145 healthy individuals, which we examined for the presence of measles virus genome RNA by reverse transcription-polymerase chain reaction (RT-PCR), to identify whether asymptomatic infection of measles virus has occurred in healthy children. Measles virus genome was detected in 11 (23.4%) of 47 nonimmunized individuals; all positives for RT-PCR were infants who experienced measles exposure. No genome was detected in those without measles exposure. In 83 individuals immunized with measles vaccine, the vaccine strain genome was detected in 10 (71.4%) of 14 recipients whose PBMC were obtained within 2 months of vaccination. Measles wild-type genome was detected in 36 (46.2%) of 78 individuals, 40 (25.2%) of 159 samples, who had been immunized more than 2 months before. The wild-type measles genome was also detected in 6 (46.2%) of 13 individuals who had been infected with measles in the distant past. The measles PCR-positive rate was not related to the period since immunization or natural infection. Sequence analysis of PCR products demonstrated they were all in the same cluster of D5 lineage, which was the circulating strain during the study period. We obtained 13 samples of nasopharyngeal secretion (NPS) simultaneously from individuals whose PBMC were positive for measles PCR but did not detect virus genome. Measles genome was, however, detected from NPS in cases of acute infection. We conclude that asymptomatic measles infection is common but would rarely become a source of transmission because of negative PCR in NPS.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

Neuroinvasion by human respiratory coronaviruses

Human coronaviruses (HCoV) cause common colds but can also infect neural cell cultures. To provide definitive experimental evidence for the neurotropism and neuroinvasion of HCoV and its possible association with multiple sclerosis (MS), we have performed an extensive search and characterization of HCoV RNA in a large panel of human brain autopsy samples. Very stringent reverse transcription-PCR with two primer pairs for both viral strains (229E and OC43), combined with Southern hybridization, was performed on samples from 90 coded donors with various neurological diseases (39 with MS and 26 with other neurological diseases) or normal controls (25 patients). We report that 44% (40 of 90) of donors were positive for 229E and that 23% (21 of 90) were positive for OC43. A statistically significant higher prevalence of OC43 in MS patients (35.9%; 14 of 39) than in controls (13.7%; 7 of 51) was observed. Sequencing of nucleocapsid protein (N) gene amplicons revealed point mutations in OC43, some consistently found in three MS patient brains and one normal control but never observed in laboratory viruses. In situ hybridization confirmed the presence of viral RNA in brain parenchyma, outside blood vessels. The presence of HCoV in human brains is consistent with neuroinvasion by these respiratory pathogens. Further studies are needed to distinguish between opportunistic and disease-associated viral presence in human brains.

On immunological memory

Immunological memory may not represent a special characteristic of lymphocytes but simply reflect low-level responses driven by antigen that is re-encountered or persists within the host. T-cell memory is important to control persistent infections within the individual host and cannot be transmitted to offspring because of MHC polymorphism and MHC-restricted T-cell recognition. In contrast, antibody memory is transmissible from mother to offspring and may function essentially to protect offspring during the phase of physiological immuno-incompetence before, at and shortly after birth. This physiological immuno-incompetence is a result of MHC polymorphism and the dangers of the graft-versus-host and host-versus-graft reaction between mother and embryo, which necessitate immunosuppression of the mother and immuno-incompetence of the offspring. One may argue therefore that immunological memory of transmissible immunological experience is the basis on which MHC-restricted T-cell recognition could develop or coevolve.

Detection of canine distemper virus nucleoprotein RNA by reverse transcription-PCR using serum, whole blood, and cerebrospinal fluid from dogs with distemper

Reverse transcription-PCR (RT-PCR) was used to detect canine distemper virus (CDV) nucleoprotein (NP) RNA in serum, whole blood, and cerebrospinal fluid (CSF) samples from 38 dogs with clinically suspected distemper. Results were correlated to clinical findings, anti-CDV neutralizing antibody titers, postmortem findings, and demonstration of CDV NP antigen by immunohistochemistry. The specificity of the RT-PCR was ensured by amplification of RNA from various laboratory CDV strains, restriction enzyme digestion, and Southern blot hybridization. In 29 of 38 dogs, CDV infection was confirmed by postmortem examination and immunohistochemistry. The animals displayed the catarrhal, systemic, and nervous forms of distemper. Seventeen samples (serum, whole blood, or CSF) from dogs with distemper were tested with three sets of primers targeted to different regions of the NP gene of the CDV Onderstepoort strain. Expected amplicons were observed in 82, 53, and 41% of the 17 samples, depending upon the primer pair used. With the most sensitive primer pair (primer pair I), CDV NP RNA was detected in 25 of 29 (86%) serum samples and 14 of 16 (88%) whole blood and CSF samples from dogs with distemper but not in body fluids from immunohistochemically negative dogs. Nucleotide sequence analysis of five RT-PCR amplicons from isolates from the field revealed few silent point mutations. These isolates exhibited greater homology to the Rockborn (97 to 99%) than to the Onderstepoort (95 to 96%) CDV strain. In summary, although the sensitivity of the RT-PCR for detection of CDV is strongly influenced by the location of the selected primers, this nucleic acid detection system represents a highly specific and sensitive method for the antemortem diagnosis of distemper in dogs, regardless of the form of distemper, humoral immune response, and viral antigen distribution.

Immune response-mediated protection of adult but not neonatal mice from neuron-restricted measles virus infection and central nervous system disease

In many cases of neurological disease associated with viral infection, such as measles virus (MV)-induced subacute sclerosing panencephalitis in children, it is unclear whether the virus or the antiviral immune response within the brain is the cause of disease. MV inoculation of transgenic mice expressing the human MV receptor, CD46, exclusively in neurons resulted in neuronal infection and fatal encephalitis within 2 weeks in neonates, while mice older than 3 weeks of age were resistant to both infection and disease. At all ages, T lymphocytes infiltrated the brain in response to inoculation. To determine the role of lymphocytes in disease progression, CD46(+) mice were back-crossed to T- and B-cell-deficient RAG-2 knockout mice. The lymphocyte deficiency did not affect the outcome of disease in neonates, but adult CD46(+) RAG-2(-) mice were much more susceptible to both neuronal infection and central nervous system disease than their immunocompetent littermates. These results indicate that CD46-dependent MV infection of neurons, rather than the antiviral immune response in the brain, produces neurological disease in this model system and that immunocompetent adult mice, but not immunologically compromised or immature mice, are protected from infection.

The matrix gene expression of subacute sclerosing panencephalitis (SSPE) virus (Osaka-1 strain): a comparison of two sibling viruses isolated from different lobes of an SSPE brain

The Fr/V and Oc/V sibling viruses of the Osaka-1 strain of the subacute sclerosing panencephalitis (SSPE) virus were defective in cell-free virus production. By radioimmunoprecipitation assay, the matrix (M) protein was not detected in cells persistently infected with the Osaka-1 strain. This undetectable expression was consistent with the selective reduction of antibody response to the M protein in the patient from whom the Osaka-1 strain was isolated. The sequence of the M gene, however, predicted that the protein could be synthesized because the translational start and stop codons for the protein were not altered. Northern blot hybridization demonstrated the selective defect of the monocistronic mRNAs for the M protein and the phosphoprotein (P) together with the dominant presence of the P-M bicistronic mRNA. This absence of the M mRNA was further confirmed by primer extension analysis. Therefore, the undetectable expression of the M protein in the infected cells was proved to be caused by a transcriptional defect. The two sibling viruses, isolated from remote portions of an SSPE brain, were indistinguishable in their viral characters, including the M gene sequences, which indicates the possibility of clonal expansion of the strain in the brain.

A matrix-less measles virus is infectious and elicits extensive cell fusion: consequences for propagation in the brain

Measles viruses (MV) can be isolated from the brains of deceased subacute sclerosing panencephalitis patients only in a cell-associated form. These viruses are often defective in the matrix (M) protein and always seem to have an altered fusion protein cytoplasmic tail. We reconstituted a cell-free, infectious M-less MV (MV-DeltaM) from cDNA. In comparison with standard MV, MV-DeltaM was considerably more efficient at inducing cell-to-cell fusion but virus titres were reduced approximately 250-fold. In MV-DeltaM-induced syncytia the ribonucleocapsids and glycoproteins largely lost co-localization, confirming the role of M protein as the virus assembly organizer. Genetically modified mice were inoculated with MV-DeltaM or with another highly fusogenic virus bearing glycoproteins with shortened cytoplasmic tails (MV-Delta(tails)). MV-DeltaM and MV-Delta(tails) lost acute pathogenicity but penetrated more deeply into the brain parenchyma than standard MV. We suggest that enhanced cell fusion may also favour the propagation of mutated, assembly-defective MV in human brains.

Chimeric measles viruses with a foreign envelope

Measles virus (MV) and vesicular stomatitis virus (VSV) are both members of the Mononegavirales but are only distantly related. We generated two genetically stable chimeric viruses. In MGV, the reading frames of the MV envelope glycoproteins H and F were substituted by a single reading frame encoding the VSV G glycoprotein; MG/FV is similar but encodes a G/F hybrid in which the VSV G cytoplasmic tail was replaced by that of MV F. In contrast to MG/FV, MGV virions do not contain the MV matrix (M) protein. This demonstrates that virus assembly is possible in the absence of M; conversely, the cytoplasmic domain of F allows incorporation of M and enhances assembly. The formation of chimeric viruses was substantially delayed and the titers obtained were reduced about 50-fold in comparison to standard MV. In the novel chimeras, transcription and replication are mediated by the MV ribonucleoproteins but the envelope glycoproteins dictate the host range. Mice immunized with the chimeric viruses were protected against lethal doses of wild-type VSV. These findings suggest that it is feasible to construct MV variants bearing a variety of different envelopes for use as vaccines or for gene therapeutic purposes.

Nucleotide sequences of the matrix protein gene of subacute sclerosing panencephalitis viruses compared with local contemporary isolates from patients with acute measles

Measles viruses isolated from brain cells of patients with subacute sclerosing panencephalitis (SSPE) have numerous mutations, especially in the matrix protein (M) gene. To find whether the M genes of these SSPE viruses were mutated randomly or in a pattern, we sequenced this gene from three strains of defective measles virus isolated in Osaka, Japan. We could deduce the sequence of the possible progenitor measles virus for each patient by comparison of the isolate with measles viruses prevailing at roughly the same time and place as the primary infection. Biased hypermutation affected the M genes of all three SSPE viruses, although the molecular mechanisms for the mutations might be various. Replacements of U with C in the plus strand accounted for 76% of all mutations in two of the strains, but in the other strain, replacements of A with G accounted for 52% of the mutations, and the U residues were unchanged.

Detection of measles virus mRNA from autopsied human tissues

By reverse transcription-PCR, measles virus (MV) mRNA was detected in the brain, kidney, spleen, liver, and lung tissues obtained from 23 (45.1%) of 51 autopsy subjects, with the detection rates of each tissue ranging from 8 to 20%. Sequence analysis revealed frequent mutations in the corresponding viral protein. These results suggest that MV mutants commonly persist in apparently healthy individuals.

Molecular epidemiology and changing distribution of genotypes of measles virus field strains in Japan

Based on phylogenetic and restriction fragment length polymorphism analyses of the hemagglutinin and nucleoprotein gene sequences, measles virus strains obtained in western Japan were divided into two types. Type 1 isolates have largely replaced type 2 isolates during the last 10 years in the area surveyed.

A transgenic mouse model for measles virus infection of the brain

In addition to the rash, fever, and upper respiratory tract congestion that are the hallmarks of acute measles virus (MV) infection, invasion of the central nervous system (CNS) can occur, establishing a persistent infection primarily in neurons. The recent identification of the human membrane glycoprotein, CD46, as the MV receptor allowed for the establishment of transgenic mice in which the CD46 gene was transcriptionally regulated by a neuron-specific promoter. Expression of the measles receptor rendered primary CD46-positive neurons permissive to infection with MV-Edmonston. Notably, viral transmission within these cultures occurred in the absence of extracellular virus, presumably via neuronal processes. No infection was seen in nontransgenic mice inoculated intracerebrally with MV-Edmonston. In contrast, scattered neurons were infected following inoculation of transgenic adults, and an impressive widespread neuronal infection was established in transgenic neonates. The neonatal infection resulted in severe CNS disease by 3-4 weeks after infection. Illness was characterized initially by awkward gait and a lack of mobility, and in later stages seizures leading to death. These results show that expression of the MV receptor on specific murine cells (neurons) in vivo is absolutely essential to confer both susceptibility to infection and neurologic disease by this human virus. The disparity in clinical findings between neonatal and adult transgenic mice indicates that differences exist between the developing and mature CNS with respect to MV infection and pathogenesis.

Detection and comparative analysis of persistent measles virus infection in Crohn's disease by immunogold electron microscopy

AIMS

To determine the specificity of persistent measles virus infection in intestinal samples from Crohn's disease patients using quantitative immunogold electron microscopy. To compare the results with samples from ulcerative colitis, a granulomatous inflammatory control (tuberculous lymphadenitis), and a positive control.

METHODS

Formalin fixed, paraffin embedded intestinal tissue from patients with Crohn's disease was reprocessed and stained with antimeasles nucleocaspid protein primary antibody followed by 10 nm gold conjugated secondary antibody. Tissue samples were taken from granulomatous and non-granulomatous areas of the intestine. Intestinal samples from patients with ulcerative colitis, tuberculous lymphadenitis, or acute mesenteric ischaemia were similarly processed. Brain tissue from a patient with subacute sclerosing panencephalitis (SSPE) was used as the positive control. Duplicate sections of all tissues were processed without the primary antibody. Stained specimens were examined by electron microscopy.

RESULTS

In Crohn's disease patients, 8/9 foci of granulomatous inflammation and 0/4 foci of non-specific inflammation were positive for measles virus. Of controls, 0/5 non-inflamed intestinal tissues, 1/8 tuberculous tissues, 1/5 ulcerative colitis tissues, and 1/1 SSPE tissues were positive. Gold grain counts per nuclear field-of-view in both Crohn's disease granulomas (43.29) and SSPE (36.94) were significantly higher than in tissues from patients with ulcerative colitis (13.52) or tuberculous lymphadenitis (15.875), and nongranulomatous areas of Crohn's disease (4.89) (p < 0.001, p < 0.001, p = 0.0006, respectively), with no significant difference between Crohn's disease and SSPE (p > 0.1). In both SSPE and Crohn's disease staining was confined to a small population of cells exhibiting characteristic cytopathology.

CONCLUSION

These data support a role for measles virus in the aetiology of Crohn's disease.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.