T-cell tolerance: exposure to virus in utero does not cause a permanent deletion of specific T cells

T cell responses and clinical symptoms among infants with congenital cytomegalovirus infection

BACKGROUNDCongenital cytomegalovirus (cCMV) infection can cause developmental impairment and sensorineural hearing loss (SNHL). To determine the relationship between immune responses to cCMV infection and neurologic sequelae, T cell responses were compared for their connection to clinical symptoms at birth and neurodevelopmental outcomes.METHODSThirty cCMV-infected and 15 uninfected infants were enrolled in a single-center prospective observational case-control study. T cell pp65-specific cytokine responses; CD57, CD28, and PD-1 expression; and memory subsets were compared.RESULTSInfected neonates (73% symptomatic at birth) lacked pp65-specific cytokine-secreting T cells, with elevated frequencies of CD57+, CD28-, and PD-1+CD8+ T cells and effector memory subsets. Though frequencies overlapped between cCMV symptom groups, asymptomatic infants had higher frequencies of CD57+PD-1+CD8+ T cells. Neonates with subsequent developmental delay lacked detectable CMV-specific T cell responses, with patterns resembling those of uninfected infants. Two children with progressive SNHL had high frequencies of PD-1+CD8+ T cells over the first year compared with children without progressive SNHL.CONCLUSIONSimilar to published reports, neonatal viral antigen-specific cytokine-secreting T cell responses were not detected, but overall patterns indicate that globally differentiated memory CD8+ T cell populations were induced by cCMV infection, with higher frequencies of terminally differentiated PD-1+CD8+ T cells potentially associated with asymptomatic infection. In this cohort, a lack of in utero T cell differentiation was associated with developmental delay, and high frequencies of PD-1+CD8+ T cells persisted only in children with progressive SNHL. Further work is needed to define the specificity of these T cells and their mechanistic connection to these outcomes.FUNDINGThis study was funded through an intramural research award at Nationwide Children's Hospital, the Pediatric Infectious Disease Society Fellowship Award funded by Stanley and Susan Plotkin and Sanofi Pasteur, the Abigail Wexner Research Institute at Nationwide Children's Hospital, and the Pichichero Family Foundation Vaccines for Children Initiative Research Award from the Pediatric Infectious Diseases Society Foundation.

The potential role of the thymus in immunotherapies for acute myeloid leukemia

Understanding the factors which shape T-lymphocyte immunity is critical for the development and application of future immunotherapeutic strategies in treating hematological malignancies. The thymus, a specialized central lymphoid organ, plays important roles in generating a diverse T lymphocyte repertoire during the infantile and juvenile stages of humans. However, age-associated thymic involution and diseases or treatment associated injury result in a decline in its continuous role in the maintenance of T cell-mediated anti-tumor/virus immunity. Acute myeloid leukemia (AML) is an aggressive hematologic malignancy that mainly affects older adults, and the disease's progression is known to consist of an impaired immune surveillance including a reduction in naïve T cell output, a restriction in T cell receptor repertoire, and an increase in frequencies of regulatory T cells. As one of the most successful immunotherapies thus far developed for malignancy, T-cell-based adoptive cell therapies could be essential for the development of a durable effective treatment to eliminate residue leukemic cells (blasts) and prevent AML relapse. Thus, a detailed cellular and molecular landscape of how the adult thymus functions within the context of the AML microenvironment will provide new insights into both the immune-related pathogenesis and the regeneration of a functional immune system against leukemia in AML patients. Herein, we review the available evidence supporting the potential correlation between thymic dysfunction and T-lymphocyte impairment with the ontogeny of AML (II-VI). We then discuss how the thymus could impact current and future therapeutic approaches in AML (VII). Finally, we review various strategies to rejuvenate thymic function to improve the precision and efficacy of cancer immunotherapy (VIII).

The foundations of immune checkpoint blockade and the ipilimumab approval decennial

Cancer immunity, and the potential for cancer immunotherapy, have been topics of scientific discussion and experimentation for over a hundred years. Several successful cancer immunotherapies - such as IL-2 and interferon-α (IFNα) - have appeared over the past 30 years. However, it is only in the past decade that immunotherapy has made a broad impact on patient survival in multiple high-incidence cancer indications. The emergence of immunotherapy as a new pillar of cancer treatment (adding to surgery, radiation, chemotherapy and targeted therapies) is due to the success of immune checkpoint blockade (ICB) drugs, the first of which - ipilimumab - was approved in 2011. ICB drugs block receptors and ligands involved in pathways that attenuate T cell activation - such as cytotoxic T lymphocyte antigen 4 (CTLA4), programmed cell death 1 (PD1) and its ligand, PDL1 - and prevent, or reverse, acquired peripheral tolerance to tumour antigens. In this Review we mark the tenth anniversary of the approval of ipilimumab and discuss the foundational scientific history of ICB, together with the history of the discovery, development and elucidation of the mechanism of action of the first generation of drugs targeting the CTLA4 and PD1 pathways.

The Thymus in Chagas Disease: Molecular Interactions Involved in Abnormal T-Cell Migration and Differentiation

Chagas disease, caused by the protozoan parasite T. cruzi, is a prevalent parasitic disease in Latin America. Presently, it is spreading around the world by human migration, thus representing a new global health issue. Chronically infected individuals reveal a dissimilar disease progression: while nearly 60% remain without apparent disease for life, 30% develop life-threatening pathologies, such as chronic chagasic cardiomyopathy (CCC) or megaviscerae. Inflammation driven by parasite persistence seems to be involved in the pathophysiology of the disease. However, there is also evidence of the occurrence of autoimmune events, mainly caused by molecular mimicry and bystander activation. In experimental models of disease, is well-established that T. cruzi infects the thymus and causes locally profound structural and functional alterations. The hallmark is a massive loss of CD4⁺CD8⁺ double positive (DP) thymocytes, mainly triggered by increased levels of glucocorticoids, although other mechanisms seem to act simultaneously. Thymic epithelial cells (TEC) exhibited an increase in extracellular matrix deposition, which are related to thymocyte migratory alterations. Moreover, medullary TEC showed a decreased expression of AIRE and altered expression of microRNAs, which might be linked to a disrupted negative selection of the T-cell repertoire. Also, almost all stages of thymocyte development are altered, including an abnormal output of CD4⁻CD8⁻ double negative (DN) and DP immature and mature cells, many of them carrying prohibited TCR-Vβ segments. Evidence has shown that DN and DP cells with an activated phenotype can be tracked in the blood of humans with chronic Chagas disease and also in the secondary lymphoid organs and heart of infected mice, raising new questions about the relevance of these populations in the pathogenesis of Chagas disease and their possible link with thymic alterations and an immunoendocrine imbalance. Here, we discuss diverse molecular mechanisms underlying thymic abnormalities occurring during T. cruzi infection and their link with CCC, which may contribute to the design of innovative strategies to control Chagas disease pathology.

Insights into Thymus Development and Viral Thymic Infections

T-cell development in the thymus is a complex and highly regulated process, involving a wide variety of cells and molecules which orchestrate thymocyte maturation into either CD4⁺ or CD8⁺ single-positive (SP) T cells. Here, we briefly review the process regulating T-cell differentiation, which includes the latest advances in this field. In particular, we highlight how, starting from a pool of hematopoietic stem cells in the bone marrow, the sequential action of transcriptional factors and cytokines dictates the proliferation, restriction of lineage potential, T-cell antigen receptors (TCR) gene rearrangements, and selection events on the T-cell progenitors, ultimately leading to the generation of mature T cells. Moreover, this review discusses paradigmatic examples of viral infections affecting the thymus that, by inducing functional changes within this lymphoid gland, consequently influence the behavior of peripheral mature T-lymphocytes.

Silica exposure and chronic virus infection synergistically promote lupus-like systemic autoimmunity in mice with low genetic predisposition

Considerable evidence indicates that autoimmune disease expression depends on both genetic and environmental factors. Among potential environmental triggers, occupational airway exposure to crystalline silica and virus infections have been linked to lupus and other autoimmune diseases in both humans and mouse models. Here, we hypothesized that combined silica and virus exposures synergize and induce autoimmune manifestations more effectively than single exposure to either of these factors, particularly in individuals with low genetic predisposition. Accordingly, infection with the model murine pathogen lymphocytic choriomenigitis virus (LCMV) in early life, followed by airway exposure to crystalline silica in adult life, induced lupus-like autoantibodies to several nuclear self-antigens including chromatin, RNP and Sm, concurrent with kidney lesions, in non-autoimmune C57BL/6 (B6) mice. In contrast, given individually, LCMV or silica were largely ineffectual in this strain. These results support a multihit model of autoimmunity, where exposure to different environmental factors acting on distinct immunostimulatory pathways complements limited genetic predisposition and increases the risk of autoimmunity above a critical threshold.

Perturbation of Thymocyte Development Underlies the PRRS Pandemic: A Testable Hypothesis

Porcine reproductive and respiratory syndrome virus (PRRSV) causes immune dysregulation during the Critical Window of Immunological Development. We hypothesize that thymocyte development is altered by infected thymic antigen presenting cells (TAPCs) in the fetal/neonatal thymus that interact with double-positive thymocytes causing an acute deficiency of T cells that produces "holes" in the T cell repertoire allowing for poor recognition of PRRSV and other neonatal pathogens. The deficiency may be the result of random elimination of PRRSV-specific T cells or the generation of T cells that accept PRRSV epitopes as self-antigens. Loss of helper T cells for virus neutralizing (VN) epitopes can result in the failure of selection for B cells in lymph node germinal centers capable of producing high affinity VN antibodies. Generation of cytotoxic and regulatory T cells may also be impaired. Similar to infections with LDV, LCMV, MCMV, HIV-1 and trypanosomes, the host responds to the deficiency of pathogen-specific T cells and perhaps regulatory T cells, by "last ditch" polyclonal B cell activation. In colostrum-deprived PRRSV-infected isolator piglets, this results in hypergammaglobulinemia, which we believe to be a "red herring" that detracts attention from the thymic atrophy story, but leads to our second independent hypothesis. Since hypergammaglobulinemia has not been reported in PRRSV-infected conventionally-reared piglets, we hypothesize that this is due to the down-regulatory effect of passive maternal IgG and cytokines in porcine colostrum, especially TGFβ which stimulates development of regulatory T cells (Tregs).

Lupus acceleration by a MAVS-activating RNA virus requires endosomal TLR signaling and host genetic predisposition

Viruses have long been implicated in the pathogenesis of autoimmunity, yet their contribution remains circumstantial partly due to the lack of well-documented information on infections prior to autoimmune disease onset. Here, we used the lymphocytic choriomeningitis virus (LCMV) as a model to mechanistically dissect the impact of viral infection on lupus-like autoimmunity. Virus persistence strongly enhanced disease in mice with otherwise weak genetic predisposition but not in highly predisposed or non-autoimmune mice, indicating a synergistic interplay between genetic susceptibility and virus infection. Moreover, endosomal Toll-like receptors (TLRs) and plasmacytoid dendritic cells (pDCs) were both strictly required for disease acceleration, even though LCMV also induces strong TLR-independent type I interferon (IFN-I) production via RNA helicases and MAVS in conventional DCs. These results suggest that LCMV enhances systemic autoimmunity primarily by providing stimulatory nucleic acids for endosomal TLR engagement, whereas overstimulation of the MAVS-dependent cytosolic pathway in the absence of endosomal TLR signaling is insufficient for disease induction.

The human thymus perivascular space is a functional niche for viral-specific plasma cells

The human thymus is susceptible to viral infections that can severely alter thymopoiesis and compromise the mechanisms of acquired tolerance to self-antigens. In humans, plasma cells residing primarily in the bone marrow confer long-lasting protection to common viruses by secreting antigen-specific antibodies. Since the thymus also houses B cells, we examined the phenotypic complexity of these thymic resident cells and their possible protective role against viral infections. Using tissue specimens collected from subjects ranging in age from 5 days to 71 years, we found that starting during the first year of life, CD138⁺ plasma cells (PC) begin accumulating in the thymic perivascular space (PVS) where they constitutively produce IgG without the need for additional stimulation. These, thymic PC secrete almost exclusively IgG1 and IgG3, the two main complement-fixing effector IgG subclasses. Moreover, using antigen-specific ELISpot assays, we demonstrated that thymic PC include a high frequency of cells reactive to common viral proteins. Our study reveals an unrecognized role of the PVS as a functional niche for viral-specific PCs. The PVS is located between the thymic epithelial areas and the circulation. PCs located in this compartment may therefore provide internal protection against pathogen infections and preserve the integrity and function of the organ.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

Development of operational immunologic tolerance with neonatal gene transfer in nonhuman primates: preliminary studies

Achieving persistent expression is a prerequisite for effective genetic therapies for inherited disorders. These proof-of-concept studies focused on adeno-associated virus (AAV) administration to newborn monkeys. Serotype rh10 AAV expressing ovalbumin and green fluorescent protein (GFP) was administered intravenously at birth and compared with vehicle controls. At 4 months postnatal age, a second injection was administered intramuscularly, followed by vaccination at 1 year of age with ovalbumin and GFP. Ovalbumin was highest 2 weeks post administration in the treated monkey, which declined but remained detectable thereafter; controls demonstrated no expression. Long-term AAV genome copies were present in myocytes. At 4 weeks, neutralizing antibodies to rh10 were present in the experimental animal only. With AAV9 administration at 4 months, controls showed transient ovalbumin expression that disappeared with the development of strong anti-ovalbumin and anti-GFP antibodies. In contrast, increased and maintained ovalbumin expression was noted in the monkey administered AAV at birth, without antibody development. After vaccination, the experimental monkey maintained levels of ovalbumin without antibodies, whereas controls demonstrated high levels of antibodies. These preliminary studies suggest that newborn AAV administration expressing secreted and intracellular xenogenic proteins may result in persistent expression in muscle, and subsequent vector administration can result in augmented expression without humoral immune responses.

Viremia during pregnancy and risk of childhood leukemia and lymphomas in the offspring: Nested case-control study

A possible role for infections of the pregnant mother in the development of childhood acute leukemias and lymphomas has been suggested. However, no specific infectious agent has been identified. Offspring of 74,000 mothers who had serum samples taken during pregnancy and stored in a large-scale biobank were followed up to the age of 15 years (750,000 person years) through over-generation linkages between the biobank files, the Swedish national population and cancer registers to identify incident leukemia/lymphoma cases in the offspring. First-trimester sera from mothers of 47 cases and 47 matched controls were retrieved and analyzed using next generation sequencing. Anelloviruses were the most common viruses detected, found in 37/47 cases and in 40/47 controls, respectively (OR: 0.6, 95% CI: 0.2-1.9). None of the detected viruses was associated with leukemia/lymphoma in the offspring. Viremia during pregnancy was common, but no association with leukemia/lymphoma risk in the offspring was found.

Molecular mimicry and clonal deletion: A fresh look

In this article, I trace the historic background of clonal deletion and molecular mimicry, two major pillars underlying our present understanding of autoimmunity and autoimmune disease. Clonal deletion originated as a critical element of the clonal selection theory of antibody formation in order to explain tolerance of self. If we did have complete clonal deletion, there would be major voids, the infamous "black holes", in our immune repertoire. For comprehensive, protective adaptive immunity, full deletion is necessarily a rare event. Molecular mimicry, the sharing of epitopes among self and non-self antigens, is extraordinary common and provides the evidence that complete deletion of self-reactive clones is rare. If molecular mimicry were not common, protective adaptive immunity could not be all-encompassing. By taking a fresh look at these two processes together we can envision their evolutionary basis and understand the need for regulatory devices to prevent molecular mimicry from progressing to autoimmune disease.

Congenitally acquired persistent lymphocytic choriomeningitis viral infection reduces neuronal progenitor pools in the adult hippocampus and subventricular zone

Lymphocytic choriomeningitis virus (LCMV) can be transmitted through congenital infection, leading to persistent infection of numerous organ systems including the central nervous system (CNS). Adult mice persistently infected with LCMV (LCMV-cgPi mice) exhibit learning deficits, such as poor performance in spatial discrimination tests. Given that deficits in spatial learning have been linked to defects in adult neurogenesis, we investigated the impact of congenital LCMV infection on generation of neuroblasts from neural progenitor cells within neurogenic zones of adult mice. In LCMV-cgPi mice, QPCR and immunohistochemistry detected presence of LCMV glycoprotein-coding RNA and nucleoprotein in the hippocampal dentate gyrus and subventricular zone (SVZ), sites of neurogenesis that harbor populations of neuroblasts. Numbers of neuroblasts were reduced in LCMV-cgPi mice, as determined by IHC quantification, and analysis of BrdU incorporation by flow cytometry revealed lower numbers of BrdU-labeled neuroblasts. Additionally, TUNEL assays performed in situ showed increased numbers of apoptotic cells in the two neurogenic regions. Next, neurosphere cultures were infected in vitro with LCMV and differentiated to create a population of cells that consisted of both transit amplifying cells and neuroblasts. Immunocytochemical and TUNEL assays revealed increased numbers of TUNEL-positive cells that express nestin, suggesting that the drop in numbers of neuroblasts was due to a combination of impaired proliferation and apoptosis of progenitor cells. LCMV-cgPi mice exhibited transcriptional up-regulation several cytokines and chemokines, including gamma-interferon inducible chemokines CXCL9 and CXCL10. Chronic up-regulation of these chemokines can facilitate a pro-inflammatory niche that may contribute to defects in neurogenesis.

The great balancing act: regulation and fate of antiviral T-cell interactions

The fate of T lymphocytes revolves around a continuous stream of interactions between the T-cell receptor (TCR) and peptide-major histocompatibility complex (MHC) molecules. Beginning in the thymus and continuing into the periphery, these interactions, refined by accessory molecules, direct the expansion, differentiation, and function of T-cell subsets. The cellular context of T-cell engagement with antigen-presenting cells, either in lymphoid or non-lymphoid tissues, plays an important role in determining how these cells respond to antigen encounters. CD8(+) T cells are essential for clearance of a lymphocytic choriomeningitis virus (LCMV) infection, but the virus can present a number of unique challenges that antiviral T cells must overcome. Peripheral LCMV infection can lead to rapid cytolytic clearance or chronic viral persistence; central nervous system infection can result in T-cell-dependent fatal meningitis or an asymptomatic carrier state amenable to immunotherapeutic clearance. These diverse outcomes all depend on interactions that require TCR engagement of cognate peptide-MHC complexes. In this review, we explore the diversity in antiviral T-cell behaviors resulting from TCR engagement, beginning with an overview of the immunological synapse and progressing to regulators of TCR signaling that shape the delicate balance between immunopathology and viral clearance.

Tolerance has its limits: how the thymus copes with infection

The thymus is required for T cell differentiation; a process that depends on which antigens are encountered by thymocytes, the environment surrounding the differentiating cells, and the thymic architecture. These features are altered by local infection of the thymus and by the inflammatory mediators that accompany systemic infection. Although once believed to be an immune privileged site, it is now known that antimicrobial responses are recruited to the thymus. Resolving infection in the thymus is important because chronic persistence of microbes impairs the differentiation of pathogen-specific T cells and diminishes resistance to infection. Understanding how these mechanisms contribute to disease susceptibility, particularly in infants with developing T cell repertoires, requires further investigation.

Lessons learned and concepts formed from study of the pathogenesis of the two negative-strand viruses lymphocytic choriomeningitis and influenza

Viruses have unique lifestyles. To describe the pathogenesis and significance of viral infection in terms of host responses, resultant injury, and therapy, we focused on two RNA viruses: lymphocytic choriomeningitis (LCMV) and influenza (Flu). Many of the currently established concepts and consequences about viruses and immunologic tolerance, virus-induced immunosuppression, virus-induced autoimmunity, immune complex disease, and virus-lymphocyte and virus-dendritic cell interactions evolved through studies of LCMV in its natural murine host. Similarly, the mechanisms, aftermath, and treatment of persistent RNA viruses emerged, in large part, from research on LCMV. Analysis of acute influenza virus infections uncovered the prominent direct role that cytokine storm plays in the pathogenesis, morbidity, and mortality from this disease. Cytokine storm of influenza virus infection is initiated via a pulmonary endothelial cell amplification loop involving IFN-producing cells and virus-infected pulmonary epithelial cells. Importantly, the cytokine storm is chemically treatable with specific agonist therapy directed to the sphingosphine 1 phosphate receptor 1, which is located on pulmonary endothelial cells, pointing to the endothelial cells as the gatekeepers of this hyperaggressive host immune response.

AAV-based neonatal gene therapy for hemophilia A: long-term correction and avoidance of immune responses in mice

Hemophilia A gene therapy has been hampered by immune responses to vector-associated antigens and by neutralizing antibodies or inhibitors to the factor VIII (FVIII) protein; these ‘inhibitors’ more commonly effect hemophilia A patients than those with hemophilia B. A gene replacement strategy beginning in the neonatal period may avoid the development of these immune responses and lead to prolonged expression with correction of phenotype thereby avoiding long-term consequences. Serotype rh10 AAV was developed splitting the FVIII coding sequence into heavy and light chains with the chicken β-actin promoter/CMV enhancer for dual recombinant AAV vector delivery. Coinjection of virions of each FVIII chain intravenously to mice on the second day of life was performed. Mice express sustained FVIII antigen levels of ≥5% to 22 months of life without the development of antibodies to FVIII. Phenotypic correction was manifest in all AAV-FVIII-treated mice as demonstrated by functional assay and reduction in bleeding time. This study demonstrates the use of AAV in a gene replacement strategy in neonatal mice that establishes both long-term phenotypic correction of hemophilia A and lack of antibody development to FVIII in this disease model where AAV is administered shortly after birth. These studies support consideration of gene replacement therapy for diseases that are diagnosed in utero or in the early neonatal period.

A novel virus carrier state to evaluate immunotherapeutic regimens: regulatory T cells modulate the pathogenicity of antiviral memory cells

Restrictions in the diversity of an adaptive immune repertoire can facilitate viral persistence. Because a host afflicted with an immune deficiency is not likely to purge a persistent infection using endogenous mechanisms, it is important to explore adoptive therapies to supplement the host with a functional immune defense. In this study, we describe a virus carrier state that results from introducing lymphocytic choriomeningitis virus (LCMV) into adult mice possessing a restricted T cell repertoire. On infection of these mice, LCMV establishes systemic persistence, and within the CNS the virus infects astrocytes (and later oligodendrocytes) rather than its traditional parenchymal target neurons. To determine whether LCMV could be purged from a novel target selection in the absence of an endogenous immune repertoire, we adoptively transferred virus-specific memory cells into adult carrier mice. The memory cells purged virus from the periphery as well as the CNS, but they induced fatalities not typically associated with adoptive immunotherapy. When the repertoire of the recipient mice was examined, a deficiency in natural regulatory T cells was noted. We therefore supplemented carrier mice with regulatory T cells and simultaneously performed adoptive immunotherapy. Cotransfer of regulatory T cells significantly reduced mortality while still permitting the antiviral memory cells to purge the persistent infection. These data indicate that regulatory T cells can be used therapeutically to lessen the pathogenicity of virus-specific immune cells in an immunodeficient host. We also propose that the novel carrier state described herein will facilitate the study of immunotherapeutic regimens.

Lymphocytic choriomeningitis infection of the central nervous system

Viral infection of the central nervous system (CNS) can result in a multitude of responses including pathology, persistence or immune clearance. Lymphocytic choriomeningitis virus (LCMV) is a powerful model system to explore these potential outcomes of CNS infection due to the diversity of responses that can be achieved after viral inoculation. Several factors including tropism, timing, dose and variant of LCMV in combination with the development or suppression of the corresponding immune response dictates whether lethal meningitis, chronic infection or clearance of LCMV in the CNS will occur. Importantly, the functionality and positioning of the LCMV-specific CD8+ T cell response are critical in directing the subsequent outcome of CNS LCMV infection. Although a basic understanding of LCMV and immune interactions in the brain exists, the molecular machinery that shapes the balance between pathogenesis and clearance in the LCMV-infected CNS remains to be elucidated. This review covers the various outcomes of LCMV infection in the CNS and what is currently known about the impact of the virus itself versus the immune response in the development of disease or clearance.

Generation and maintenance of Listeria-specific CD8+ T cell responses in perforin-deficient mice chronically infected with LCMV

Disruption of the perforin gene results in primary immunodeficiency and an increased susceptibility to opportunistic pathogens. Perforin-deficient (PKO) mice fail to clear primary lymphocytic choriomeningitis virus (LCMV) Armstrong, resulting in persistent infection and functional exhaustion of virus-specific CD8+ T cells. CD8+ T cell responses to Listeria monocytogenes (LM) challenge within the first week after LCMV infection were diminished in both WT and PKO mice, and correlated with enhanced bacterial clearance. However, bacterial challenge at later time points generated similar CD8 T cell responses in both groups of mice. The phenotype and function of pre-existing LM-specific memory CD8+ T cells were maintained in persistently infected PKO mice. Thus persistent LCMV infection, as a result of perforin deficiency, results in dysfunction of the virus-specific CD8+ T cell response but does not compromise the host's ability to maintain pre-existing memory CD8+ T cells or to generate new memory CD8+ T cell responses against other pathogens.

Evasion of host immune responses by tumours and viruses

Viruses and tumours use various mechanisms to avoid immune surveillance. Oncogenic viruses have achieved a balance with the immune system through evolutionary time to ensure long-term persistence. Mutations that promote escape mechanisms favouring tumour growth to the detriment of host survival through reproductive age offer no selective advantage and will not generally be maintained in the viral genome that persists in nature. Conventional (non-oncogenic) and tumour viruses interact with various immune mediators and T cells in different ways. Oncogenic viruses cannot operate solely in the context of a lytic cycle, though this may be characteristic of the initial phase of infection that is limited by the acute immune response. Some oncogenic viruses interact with normal cellular growth control and signalling mechanisms. Synthesis of key viral proteins may be tightly controlled in replicating cells that are subject to T cell surveillance, such as basal epithelia, while productive infection occurs in non-proliferating progeny that are lost under normal physiological conditions, such as desquamating epithelia. Tumorigenesis may be an aberrant consequence of the molecular mechanisms needed to maintain this pattern of viral growth regulation in the context of the cell cycle. Vaccines designed to limit the acute phase of infection with cell-free oncogenic viruses should be as effective as those for conventional viruses.

Viral persistence: parameters, mechanisms and future predictions

For a virus to persist, it must actively curtail the host's antiviral immune response. Here, we review the conceptual basis by which this can occur and discuss the subsequent fate of differentiated cells infected over long periods of time. We also consider how the compromised antiviral immune response can be revigorated or replaced with a potent response that purges the virus and thereby terminates persistent infection.

Neonate-primed CD8+ memory cells rival adult-primed memory cells in antigen-driven expansion and anti-viral protection

Immunizations early in life, when the host is most susceptible to infection, allow protective immunological memory to develop. Decreasing the dose of Cas-Br-E murine leukemia virus when priming neonatal mice results in adult-like, Type 1 protective responses, but the resulting memory cell populations are smaller than after adult priming. After secondary challenge, virus-specific CD8+ memory cell populations expand twice as much in neonate-primed mice as in adult-primed mice. We found that when equivalent numbers of virus-specific cells were transferred into virus-susceptible mice, protection from disease was similar whether donor, immune mice were primed as neonates or adults, and IL-4 did not alter in vivo virus-specific CD8+ memory cell effector function. Hence, neonate-primed CD8+ cells develop into memory cells that rival adult-primed cells in proliferation and effector function.

Enhanced type 1 immunity after secondary viral challenge in mice primed as neonates

The goal of infant immunization against viral infection is to develop protective long term memory responses. Priming neonatal mice with a low dose of Cas-Br-E murine leukemia virus (Cas) results in adult-like, type 1 protective responses. However, other studies suggest that Ag priming of neonates leads to an increase in type 2 secondary responses even when primary responses were type 1. We assessed whether type 1 CD8+ T cell-mediated responses developed in murine neonates are maintained after secondary challenge with Cas in adulthood. Despite the induction of significant anti-viral CD8+-mediated cytotoxic T lymphocyte and IFN-gamma responses, initial neonatal priming led to a lower frequency of virus-specific T cells compared with adult priming. Adult frequencies were reached in mice primed as neonates only after secondary challenge in adulthood. A nonspecific and transient CD4+-mediated IL-4 response was present in all groups after secondary challenge with Cas or medium, indicating that this rise in type 2 cytokine production was not unique to mice that had been primed as neonates. Rather, type 1 anti-viral memory CD8+ T cell responses developed in neonatal mice are stable, protective, and enhanced after secondary challenge.

Pathogenesis of maternal-fetal syphilis revisited

Although congenital syphilis has been recognized for several centuries and an efficient treatment with penicillin became available more than a half-century ago, the disease is still with us. Inability to culture in vitro the causative agent, Treponema pallidum, and the lack of an adequate animal model have prevented exploration of the various immunopathological events affecting the natural course of congenital infection. The purpose of this review is to analyze the disease in the context of recent knowledge acquired from human and experimental animals, particularly from the guinea pig model of congenital and neonatal syphilis, and to describe how the infection interacts with the maternal-fetal unit and how it is further modulated by the conceptus' ontogenic development. We also attempt to elucidate several old immunologic concepts and misconceptions that have remained unchallenged for too long.

Immune escape by hepatitis B viruses

Hepatitis B viruses are DNA viruses characterized by their very small genome size and their unique replication via reverse transcription. The circular genome has been efficiently exploited, thereby limiting genome variation, and leaves no space for genes in addition to those essentially needed during the viral live cycle. Hepatitis B viruses are prototype non-cytopathic viruses causing persistent infection. Human hepatitis B virus (HBV), as well as the closely related animal viruses, most frequently are transmitted vertically from mothers to their offspring. Because infection usually persists for many years, if not lifelong, hepatitis B viruses need efficient mechanisms to hide from the immune response of the host. To escape the immune response, they exploit different strategies. Firstly, they use their structural and non-structural proteins multiplely. One of the purposes is to alter the immune response. Secondly, they replicate by establishing a pool of stable extrachromosomal transcription templates, which allow the virus to react sensitively to changes in its microenvironment by up- or downregulating gene expression. Thirdly, hepatitis B viruses replicate in the liver which is an immunopriviledged site.

Persistence of physiological self antigen is required for the regulation of self tolerance

Endogenous Ag requirement for induction and maintenance of T cell tolerance has been extensively investigated in mice that express a transgenic Ag and/or its cognate transgenic TCR. In contrast, studies on tolerance for physiologically expressed self Ag and normal T cells are limited. Herein, we showed that the murine ovarian-specific ZP3 Ag is detectable from birth. Tolerance to ZP3 is detected in female relative to male mice. In comparison to males, 100-fold more ovarian peptide (pZP3) is required to elicit a comparable pathogenic response in females. Female tolerance to pZP3 was dependent on the presence of endogenous ovarian Ag, because neonatal ovariectomy converted the female response to that of males. Moreover, in female mice that were ovariectomized from the ages of 1-6 wk, the pZP3 responses were enhanced to the male level if ovaries were removed up to 7 days, but not 3 days, before adult challenge with pZP3. Thus, the physiologically expressed ZP3 Ag induces tolerance to pZP3, and the maintenance of tolerance is critically dependent on the continuous presence of the endogenous ovarian Ag. In contrast, exposure to endogenous ovarian Ag confined to the neonatal period is insufficient for the induction and maintenance of tolerance to ZP3.

Bystander sensitization to activation-induced cell death as a mechanism of virus-induced immune suppression

Viral infections which induce strong T-cell responses are often characterized by a period of transient immunodeficiency associated with the failure of host T cells to proliferate in response to mitogens or to mount memory recall responses to other antigens. During acute infections, most of the activated, proliferating virus-specific T cells are sensitized to undergo apoptosis on strong T-cell receptor (TCR) stimulation, but it has not been known why memory T cells not specific for the virus fail to proliferate on exposure to their cognate antigen. Using a lymphocytic choriomeningitis virus (LCMV) infection model in which LCMV-immune Thy 1.1(+) splenocytes are adoptively transferred into Thy 1.2(+) LCMV carrier mice, we demonstrate here that T cells clearly defined as not specific for the virus are sensitized to undergo activation-induced cell death on TCR stimulation in vitro. This bystander sensitization was in part dependent on the expression of Fas ligand (FasL) on the activated virus-specific cells and gamma interferon (IFN-gamma) receptor expression on the bystander T cells. We propose that FasL from highly activated antiviral T cells may sensitize IFN-gamma-conditioned T cells not specific for the virus to undergo apoptosis rather than to proliferate on encountering antigen. This may in part explain the failure of memory T cells to respond to recall antigens during acute and persistent viral infections.

Immune escape by hepatitis B viruses

Hepatitis B viruses are DNA viruses characterized by their very small genome size and their unique replication via reverse transcription. The circular genome has been efficiently exploited, thereby limiting genome variation, and leaves no space for genes in addition to those essentially needed during the viral live cycle. Hepatitis B viruses are prototype non-cytopathic viruses causing persistent infection. Human hepatitis B virus (HBV), as well as the closely related animal viruses, most frequently are transmitted vertically from mothers to their offspring. Because infection usually persists for many years, if not lifelong, hepatitis B viruses need efficient mechanisms to hide from the immune response of the host. To escape the immune response, they exploit different strategies. Firstly, they use their structural and non-structural proteins multiplely. One of the purposes is to alter the immune response. Secondly, they replicate by establishing a pool of stable extrachromosomal transcription templates, which allow the virus to react sensitively to changes in its microenvironment by up- or downregulating gene expression. Thirdly, hepatitis B viruses replicate in the liver which is an immunopriviledged site.

How viruses escape from cytotoxic T lymphocytes: molecular parameters and players

Viruses that persist in infected hosts must evolve successful strategies to avoid recognition by the immune system. The primary player in antiviral immune surveillance is the CD8+ cytotoxic T lymphocyte (CTL), and the battle drawn between the CTLs and viruses is the focus of this review. In this struggle, viruses can follow multiple distinct pathways. For example, DNA viruses often adopt the strategy of encoding proteins that interfere with the immune response along routes of antigen presentation. Such interference prevents the viral peptide from binding to the major histocompatibility complex (MHC) class I glycoprotein; therefore, no virus-MHC complex forms for recognition by antiviral CTLs. RNA viruses, having fewer genes, generate swarms of quasispecies that can contain mutated viral proteins. When such mutants occur in viral peptides presented to the MHC protein or the residue recognized by the CTL receptor, CTL recognition and activation fail. If, instead, the mutation occurs in the viral peptide flanking sequence, the infected cell may not process the viral peptide from the cytosol to the endoplasmic reticulum. Viruses can also directly or indirectly attack dendritic cells and CD4+ or CD8+ T lymphocytes, other routes that interfere with immune functions. Dendritic cells are the primary professional antigen-presenting cells and are critical for the activation of CTL responses. CD4+ T lymphocytes provide help for long-term CD8+ CTL activity and are necessary for its maintenance. Consequently, interference with either dendritic or CD4+ cell types constitutes yet another way that viruses can disable the immune response and persistently infect their host.

Induction of protective CTL responses in newborn mice by a murine retrovirus

The susceptibility of neonates to virus-induced disease is thought to reflect, in part, the immaturity of their immune systems. However, inoculation of newborn mice with low doses of Cas-Br-M murine leukemia virus induced a protective cytotoxic T lymphocyte (CTL) response. The inability of neonates to develop a CTL response to high doses of virus was not the result of immunological immaturity but correlated with the induction of a nonprotective type 2 cytokine response. Thus, the initial viral dose is critical in the development of protective immunity in newborns.

DNA vaccination against persistent viral infection

This study shows that DNA vaccination can confer protection against a persistent viral infection by priming CD8+ cytotoxic T lymphocytes (CTL). Adult BALB/c (H-2d) mice were injected intramuscularly with a plasmid expressing the nucleoprotein (NP) gene of lymphocytic choriomeningitis virus (LCMV) under the control of the cytomegalovirus promoter. The LCMV NP contains the immunodominant CTL epitope (amino acids 118 to 126) recognized by mice of the H-2d haplotype. After three injections with 200 micrograms of NP DNA, the vaccinated mice were challenged with LCMV variants (clones 13 and 28b) that establish persistent infection in naive adult mice. Fifty percent of the DNA-vaccinated mice were protected, as evidenced by decreased levels of infectious virus in the blood and tissues, eventual clearance of viral antigen from all organs tested, the presence of an enhanced LCMV-specific CD8+ CTL response, and maintenance of memory CTL after clearance of virus infection. However, it should be noted that protection was seen in only half of the vaccinated mice, and we were unable to directly measure virus-specific immune responses in any of the DNA-vaccinated mice prior to LCMV challenge. Thus, at least in the system that we have used, gene immunization was a suboptimal method of inducing protective immunity and was several orders of magnitude less efficient than vaccination with live virus. In conclusion, our results show that DNA immunization works against a persistent viral infection but that efforts should be directed towards improving this novel method of vaccination.

Tissue-mediated selection of viral variants: correlation between glycoprotein mutation and growth in neuronal cells

Viral variants with different biological properties predominate in the central nervous system (CNS) and lymphoid tissues of carrier mice infected at birth with the Armstrong strain of lymphocytic choriomeningitis virus. The CNS isolates have the same phenotype as the parental strain and cause acute infections in adult mice, while the spleen-derived isolates cause chronic infections associated with suppressed T-cell responses and susceptibility to opportunistic infections. Our previous studies have identified a single amino acid change in the viral glycoprotein, a phenylalanine-to-leucine (F-->L) mutation at residue 260, that correlates with the tissue-specific selection and the persistent and immunosuppressive phenotype of the spleen isolates (R. Ahmed, C.S. Hahn, T. Somasundaram, L. Villarete, M. Matloubian, and J. H. Strauss, J. Virol. 65:4242-4247, 1991). In this study, we screened viral isolates obtained from the spleen, liver, kidney, and brain of carrier mice for the presence of this mutation and determined the temporal selection of variants as they appear in these organs. We found that this F-->L amino acid change is common to > 90% of the spleen and liver isolates and is selected for rapidly by day 32 postinfection (p.i.). Although the kinetics observed in the kidney are relatively slower than in the spleen and liver, this F-->L mutation predominates in the kidney-derived isolates by 250 days p.i. In contrast, the majority of the CNS isolates retain the parental sequence up to 250 days p.i. In addition, most of the brain isolates replicated efficiently in a neuronal cell line, and this enhanced growth phenotype in neurons correlated with the parental F genotype. This linkage with neurotropism, along with our earlier finding that the F-->L mutation is necessary for enhanced infection of macrophages (M. Matloubian, S. R. Kolhekar, T. Somasundaram, and R. Ahmed, J. Virol. 67:7340-7349, 1993), provides a cellular basis for the molecular changes associated with tissue-specific selection. Taken together, these results suggest that tropism for macrophages is a critical determinant in selection of variants with the F-->L mutation in tissues such as spleen and liver, and tropism for neurons is important in retention of the F genotype in the CNS.

Cytotoxic T lymphocyte response to a wild type hepatitis B virus epitope in patients chronically infected by variant viruses carrying substitutions within the epitope

Mutations that abrogate recognition of a viral epitope by class I-restricted cytotoxic T lymphocyte (CTL) can lead to viral escape if the CTL response against that epitope is crucial for viral clearance. The likelihood of this type of event is low when the CTL response is simultaneously directed against multiple viral epitopes, as has been recently reported for patients with acute self-limited hepatitis B virus (HBV) infection. The CTL response to HBV is usually quite weak, however, during chronic HBV infection, and it is generally acknowledged that this is a major determinant of viral persistence in this disease. If such individuals were to produce a mono- or oligospecific CTL response, however, negative selection of the corresponding mutant viruses might occur. We have recently studied two HLA-A2-positive patients with chronic hepatitis B who, atypically, developed a strong HLA-A2-restricted CTL response against an epitope (FLPSDFFPSV) that contains an HLA-A2-binding motif located between residues 18-27 of the viral nucleocapsid protein, hepatitis B core antigen (HBcAg). These patients failed, however, to respond to any of other HLA-A2-restricted HBV-derived peptides that are generally immunogenic in acutely infected patients who successfully clear the virus. Interestingly, DNA sequence analysis of HBV isolates from these two patients demonstrated alternative residues at position 27 (V --> A and V --> I) and position 21 (S --> N, S --> A, and S --> V) that reduced the HLA and T cell receptor-binding capacities of the variant sequences, respectively. Synthetic peptides containing these alternative sequences were poorly immunogenic compared to the prototype HBc18-27 sequence, and they could not be recognized by CTL clones specific for the prototype peptide. While we do not know if the two patients were originally infected by these variant viruses or if the variants emerged subsequent to infection because of immune selection, the results are most consistent with the latter hypothesis. If this is correct, the data suggest that negative selection of mutant viral genomes might contribute to viral persistence in a subset of patients with chronic HBV infection who express a narrow repertoire of anti-HBV CTL responses.

Resistance to Toxoplasma gondii in mice infected as neonates or exposed in utero

Mice were exposed to the protozoan parasite Toxoplasma gondii in utero or were infected as neonates in order to identify and characterize resistance mechanisms that function protectively during the first weeks after birth. About one-half of the mice born of mothers fed T. gondii cysts at 11 days of gestation survived to weaning age or beyond. No effect of major histocompatibility complex (MHC) haplotype on early survival was observed in a group of backcross progeny; however, long-term survival was strongly dependent on MHC haplotype. The ability of mice infected as neonates to survive until weaning was found to depend on gamma interferon and on Thy-1+ cells but not on CD4+ or CD8+ cells. Mice that survived to maturity after infection as neonates were slightly more resistant to challenge with virulent T. gondii parasites than were sham-infected controls but were less resistant than were mice infected as adults. Together the results indicate the following. (i) Mice congenitally infected with T. gondii have a gamma interferon-dependent mechanism of early resistance that involves Thy-1+ cells but not CD4+ or CD8+ cells. (ii) This mechanism is not under MHC-linked genetic control. (iii) Mice that exhibit long-term survival after congenital infection acquire a modest degree of protection against reinfection with virulent organisms. (iv) The extent of long-term survival of congenitally infected neonates, like that in mice infected as adults, is influenced by MHC genes, presumably via MHC-restricted CD4+ and/or CD8+ cells.

Viral infection of the thymus

We have examined infection of the thymus during congenitally acquired chronic lymphocytic choriomeningitis virus (LCMV) infection of mice, a classic model of antigen-specific T-cell tolerance. Our results show that (i) infection starts at the fetal stage and is maintained throughout adulthood, and (ii) this chronic infection of the thymus can be eliminated by transfer of virus-specific cytotoxic T lymphocytes (CTL) that infiltrate the thymus and clear all viral products from both medullary and cortical regions. Elimination of virus from the thymus results in abrogation of tolerance. During the fetal stage, the predominant cell type infected is the earliest precursor of T cells with a surface phenotype of Thy1+ CD4- CD8- J11d+. In the adult thymus, infection is confined primarily to the cortisone-resistant thymocytes present in the medullary region. The infected cells are CD4+ and J11d+. The presence of J11d, a marker usually associated with immature thymocytes, on infected single positive CD4+ "mature" thymocytes is intriguing and suggests that infection by this noncytolytic virus may affect development of T cells. There is minimal infection of the CD8+ medullary thymocytes or of the double positive (CD4+ CD8+) cells present in the cortex. Infection within the cortex is confined to the stromal cells. Interestingly, there is infection of the double negative (CD4- CD8-) thymocytes in the adult thymus, showing that even during adulthood the newly developing T cells are susceptible to infection by LCMV. Virus can be eliminated from the thymuses of these carrier mice by adoptive transfer of medullary region first and then from the thymic cortex. This result clearly shows the need to reevaluate the widely held notion that mature T cells are unable to reenter the thymus. In fact, in our experiments the donor T cells made up to 20 to 30% of the total cells in the thymus at 5 to 7 days after the transfer. The number of donor T cells declined as virus was eliminated from the thymus, and at 1 month posttransfer, the donor T cells were hardly detectable. The results of this study examining the dynamics of viral infection and clearance from the thymus, the primary site of T-cell development, have implications for understanding tolerance induction in chronic viral infections.

Borna disease: association with a maturation defect in the cellular immune response

Borna disease virus (BDV) is a negative-strand RNA virus which produces persistent infection in a variety of experimental animals. In the rat, the presence or absence of clinical signs of Borna disease, a characteristic, biphasic neurobehavioral illness, depends on host-related factors. A window of opportunity exists after birth wherein inoculation with BDV produces a persistently infected rat without signs of Borna disease or encephalitis (persistent, tolerant infection-newborn [PTI-NB] rat). Although immunopathological destruction of the nervous system does not occur in the PTI-NB rat, significant alterations in the development of the nervous system were noted, including site-specific lysis of neurons. Unlike the case with other pharmacologically produced, persistent, tolerant BDV infections, adoptive transfer of spleen cells from BDV-infected rats did not produce disease in the PTI-NB rats. PTI-NB rats developed Borna disease after being connected by parabiosis to rats with Borna disease. Bone marrow transplantation experiments revealed that bone marrow cells from PTI-NB rats produced Borna disease in lethally irradiated, BDV-infected recipient rats. Bone marrow from PTI-NB rats contained a complement of inflammatory cells capable of inducing Borna disease. Thus, the loss of BDV-specific cellular immunity appeared to occur after the release of cells from the bone marrow.

Monoclonal antibody against a tumor-associated sialoglycoprotein of superficial papillary bladder tumors and cervical condylomas

A mouse IgG1 monoclonal antibody (MAb), 19A211, defining a tumor-associated cell-surface antigen of superficial papillary bladder tumors, was generated by immunizing with fresh bladder tumor cells mice neonatally injected with normal human urothelial cells. The reactivity of the antibody with cell lines was examined by indirect immunofluorescence staining and was restricted to 3/14 bladder cancer lines and 3/31 cancer cell lines of non-bladder origin, including HeLa cervical cancer. No normal fibroblast, kidney cells, EBV-lymphocytes, erythrocytes or leukocytes expressed the antigen. Reactivity of MAb 19A211 was well preserved on tissue paraffin sections. Immunoperoxidase staining of normal adult or fetal tissues showed no reactivity except for a patchy or uniform staining of umbrella cells in 6/23 adult and 1/4 fetal urothelium samples. Positive and often heterogeneous staining was observed on 24/38 papillary superficial tumors (Ta) and 4/5 carcinoma in situ bladder lesions but on only 4/20 infiltrating tumors. It was also observed on 5/6 cervical condylomas and one bladder condyloma, but none of 6 penile or vulvar condylomas. All other tumors tested were negative. The antigenic determinant is present on a heterogeneous group of proteins with molecular weights ranging from 90 to 200 kDa. It is sensitive to periodate treatment and to neuraminidase but only partially sensitive to proteases. MAb 19A211 is different from other reported MAbs with similar reactivity to superficial bladder tumors and umbrella cells of normal urothelium. When tested in competition assays, several of these MAbs, but not 19A211, were found to react with Lewis X blood group determinant. Our results suggest that 19A211 may be useful for detection and stratification of bladder tumors.

Neonatal exposure to thymotropic gross murine leukemia virus induces virus-specific immunologic nonresponsiveness

Neonatal exposure to Gross murine leukemia virus results in a profound inhibition of the virus-specific T and B cell responses of adult animals. Animals exposed to virus as neonates exhibit a marked depression in virus-specific T cell function as measured by the virtual absence of in vivo delayed type hypersensitivity responses and in vitro proliferative responses to virally infected stimulator cells. Further, serum obtained from neonatally treated mice failed to either immunoprecipitate viral proteins or neutralize virus in an in vitro plaque assay, suggesting the concurrent induction of a state of B cell hyporesponsiveness. The specificity of this effect at the levels of both T and B cells was demonstrated by the ability of neonatally treated mice to respond normally after adult challenge with either irrelevant reovirus or influenza virus. The replication of Gross virus within both stromal and lymphocytic compartments of the neonatal thymus suggests that thymic education plays a key role in the induction of immunologic nonresponsiveness to viruses.

Viral persistence and immune dysfunction

Viruses can persist for years without provoking an effective host immune response or otherwise causing the cell destruction characteristic of an acute viral infection. Clinically relevant principles, generalizable to many persistent viruses, are exemplified by measles and lymphocyte choriomeningitis viruses. The LCMV model indicates that viral persistence and the anergic state are reversible.

T cell memory. Long-term persistence of virus-specific cytotoxic T cells

This study documents that virus-specific CTL can persist indefinitely in vivo. This was accomplished by transferring Thy-1.1 T cells into Thy-1.2 recipient mice to specifically identify the donor T cell population and to characterize its antigenic specificity and function by using a virus-specific CTL assay. Thy-1.1+ T cells from mice previously immunized with lymphocytic choriomeningitis virus (LCMV) were transferred into Thy-1.2 mice persistently infected with LCMV. The transferred LCMV-specific CTL (Thy-1.1+ CD8+) eliminate virus from the chronically infected carriers and persist in the recipient mice in small numbers, comprising only a minor fraction of the total T cells. Upon re-exposure to virus, these long-lived "resting" CD8+ T cells proliferate in vivo to become the predominant cell population. These donor CD8+ T cells can be recovered up to a year post-transfer and still retain antigenic specificity and biological function. They kill LCMV infected H-2-matched cells in vitro and can eliminate virus upon transfer into a second infected host. In addition, these long-lived CD8+ T cells appear not to be dependent on help from CD4+ T cells, since depletion of CD4+ T cells has minimal or no effect on their biological properties (proliferation, CTL response, viral clearance). These donor CTL also exhibit an immunodominance over the host-derived LCMV-specific CTL response. When both host and donor T cells are present, the donor CTL response is dominant over the potential CTL response of the cured carrier host. Taken together, these results suggest that virus-specific CTL can persist for the life span of the host as memory cells.

Recognition of idiotypic structures in the thymus

During the passage through the thymus, T cells are selected which recognize self major histocompatibility complex (MHC) antigens with low avidity. Whether the T-cell repertoire for recognition of altered self is also built up intrathymically or in the periphery, and whether it is determined exclusively by external antigens or is shaped by the internal environment is still a matter of debate. This question was addressed by analysing the responsiveness of thymocytes during post-natal development towards a nominal antigen [trinitrophenyl (TNP)] and an anti-TNP monoclonal antibody (Sp6), which carries a recurrent idiotype. During the first weeks of life, in vitro cultures of thymocytes proliferated strongly in the absence of nominal antigen. Proliferation rates were not increased by the addition of nominal antigen [TNP-ovalbumin (OA)], but a significant increase was noted in the presence of Sp6, thymocytes recognizing the processed immunoglobulin. After in vivo stimulation with TNP conjugates, 'antigen-specific' clones could also be detected in the thymus, the frequency of clones proliferating in response to Sp6 being further augmented. With increasing age, the proliferative capacity of thymocytes from unstimulated and antigenically stimulated mice decreased significantly. Responsiveness of spleen cells (SC) differed in some respects. The response towards Sp6 decreased with age, while antigen-specific clones were detected at increasing frequencies during post-natal development. Furthermore, after antigenic stimulation, the frequency solely of antigen-specific, but not of Sp6-specific clones was increased. Thus, it appears that the T-cell repertoire is shaped already during the intrathymic passage, being influenced primarily by the B-cell repertoire and modulated further by external antigen.

Virus-lymphocyte interactions. III. Biologic parameters of a virus variant that fails to generate CTL and establishes persistent infection in immunocompetent hosts

Viruses that cause in vivo persistent infections avoid the host's immunologic surveillance machinery. A major component of that armamentarium is virus-specific MHC-restricted cytotoxic T lymphocyte (CTL) response of the host. Studies with lymphocytic choriomeningitis virus (LCMV) have uncovered a parental virus (CTL+) that in immuno-competent adults induces CTL and terminates acute infection and a variant (CTL-) that fails to elicit CTL responses and establishes a persistent state (R. Ahmed et al. (1984) J. Exp. Med. 160, 521-540). The biologic properties, similarities, and differences between CTL+ and CTL- viruses as regards their interactions with lymphocytes of newborn and adult mice is recorded here. CTL+ and CTL- viruses persist in lymphocytes of newborn inoculated mice, primarily within the T helper subset. Approximately 2% of lymphocytes express viral nucleic acid sequences while only 0.04% score as infectious centers suggesting incomplete viral replication. These levels were maintained over the course of infectious. In contrast, CTL- virus but not CTL+ persists in lymphocytes of mice inoculated when adults. Lymphocytes easily scored as infecting centers but rarely displayed nucleic acid sequences suggesting a different balance of incomplete to complete virion replication. Further, infectious centers decreased by 10-fold from the 3rd to 68th day of infection and the total numbers of T lymphocytes in the circulation decreased suggesting CTL- may replicate in and destroy lymphocytes of adult mice. In the following paper the primary nucleotide structure of the LCMV small RNA segment, the segment responsible for generation of CTL and encoding the proteins recognized by CTL, for CTL+ and CTL- viruses is reported.