Lymphocytic choriomeningitis virus. VI. Isolation of a glycoprotein mediating neutralization

Strategies of rational and structure-driven vaccine design for Arenaviruses

The COVID-19 outbreak has highlighted the importance of pandemic preparedness for the prevention of future health crises. One virus family with high pandemic potential are Arenaviruses, which have been detected almost worldwide, particularly in Africa and the Americas. These viruses are highly understudied and many questions regarding their structure, replication and tropism remain unanswered, making the design of an efficacious and molecularly-defined vaccine challenging. We propose that structure-driven computational vaccine design will contribute to overcome these challenges. Computational methods for stabilization of viral glycoproteins or epitope focusing have made progress during the last decades and particularly during the COVID-19 pandemic, and have proven useful for rational vaccine design and the establishment of novel diagnostic tools. In this review, we summarize gaps in our understanding of Arenavirus molecular biology, highlight challenges in vaccine design and discuss how structure-driven and computationally informed strategies will aid in overcoming these obstacles.

Glycoprotein-Specific Polyclonal Antibodies Targeting Machupo Virus Protect Guinea Pigs against Lethal Infection

Convalescent plasma has been shown to be effective at protecting humans against severe diseases caused by New World (NW) arenaviruses, including Junin virus (JUNV) and Machupo virus (MACV). This plasma contains antibodies against the full complement of structural proteins including the nucleocapsid and envelope glycoproteins (GPcs) consisting of GP1 and GP2. To gain insights into the protective and cross-protective properties of anti-GPc-specific polyclonal antibodies, we evaluated the ability of a DNA vaccine-produced anti-GPc rabbit antisera targeting MACV strain Carvallo to provide heterologous protection against another MACV strain termed Chicava in the Hartley guinea pig model. The neutralizing activity of the rabbit antisera against the heterologous MACV strains Chicava and Mallale was found to be 54-fold and 23-fold lower, respectively, compared to the titer against the homologous MACV strain Carvallo in the PRNT50 assay. Despite lower neutralizing activity against the strain Chicava, the rabbit antisera protected 100% of the guinea pigs from this strain when administered up to four days post-infection, whereas all the control animals succumbed to the disease. Using vesicular stomatitis virus (VSV) particles pseudotyped with MACV GPc, we identified a single amino acid difference at position 122 between the strains Chicava and Carvallo GPc that significantly influenced the neutralization activity of the rabbit antisera. These findings indicate that polyclonal antibodies targeting the MACV glycoproteins can protect against lethal infection in a post-challenge setting. These data will help guide future antibody-based therapeutics development against NW arenaviruses.

Central tolerance shapes the neutralizing B cell repertoire against a persisting virus in its natural host

Viral mimicry of host cell structures has been postulated to curtail the B cell receptor (BCR) repertoire against persisting viruses through tolerance mechanisms. This concept awaits, however, experimental testing in a setting of natural virus-host relationship. We engineered mouse models expressing a monoclonal BCR specific for the envelope glycoprotein of lymphocytic choriomeningitis virus (LCMV), a naturally persisting mouse pathogen. When the heavy chain of the LCMV-neutralizing antibody KL25 was paired with its unmutated ancestor light chain, most B cells underwent receptor editing, a behavior reminiscent of autoreactive clones. In contrast, monoclonal B cells expressing the same heavy chain in conjunction with the hypermutated KL25 light chain did not undergo receptor editing but exhibited low levels of surface IgM, suggesting that light chain hypermutation had lessened KL25 autoreactivity. Upon viral challenge, these IgMlow cells were not anergic but up-regulated IgM, participated in germinal center reactions, produced antiviral antibodies, and underwent immunoglobulin class switch as well as further affinity maturation. These studies on a persisting virus in its natural host species suggest that central tolerance mechanisms prune the protective antiviral B cell repertoire.

Molecular Engineering of a Mammarenavirus with Unbreachable Attenuation

Several mammarenaviruses cause severe hemorrhagic fever (HF) disease in humans and pose important public health problems in their regions of endemicity. There are no United States (US) Food and Drug Administration (FDA)-approved mammarenavirus vaccines, and current anti-mammarenavirus therapy is limited to an off-label use of ribavirin that has limited efficacy. Mammarenaviruses are enveloped viruses with a bi-segmented negative-strand RNA genome. Each genome segment contains two open reading frames (ORF) separated by a noncoding intergenic region (IGR). The large (L) segment encodes the RNA dependent RNA polymerase, L protein, and the Z matrix protein, whereas the small (S) segment encodes the surface glycoprotein precursor (GPC) and nucleoprotein (NP). In the present study, we document the generation of a recombinant form of the prototypic mammarenavirus lymphocytic choriomeningitis virus (LCMV) expressing a codon deoptimized (CD) GPC and containing the IGR of the S segment in both the S and L segments (rLCMV/IGR-CD). We show that rLCMV/IGR-CD is fully attenuated in C57BL/6 (B6) mice but able to provide complete protection upon a single administration against a lethal challenge with LCMV. Importantly, rLCMV/IGR-CD exhibited an unbreachable attenuation for its safe implementation as a live-attenuated vaccine (LAV). IMPORTANCE Several mammarenaviruses cause severe disease in humans and pose important public health problems in their regions of endemicity. Currently, no FDA-licensed mammarenavirus vaccines are available, and anti-mammarenaviral therapy is limited to an off-label use of ribavirin whose efficacy is controversial. Here, we describe the generation of recombinant version of the prototypic mammarenavirus lymphocytic choriomeningitis virus (rLCMV) combining the features of a codon deoptimized (CD) GPC and the noncoding intergenic region (IGR) of the S segment in both S and L genome segments, called rLCMV/IGR-CD. We present evidence that rLCMV/IGR-CD has excellent safety and protective efficacy features as live-attenuated vaccine (LAV). Importantly, rLCMV/IGR-CD prevents, in coinfected mice, the generation of LCMV reassortants with increased virulence. Our findings document a well-defined molecular strategy for the generation of mammarenavirus LAV candidates able to trigger long-term protective immunity, upon a single immunization, while exhibiting unique enhanced safety features, including unbreachable attenuation.

Antibody bivalency improves antiviral efficacy by inhibiting virion release independently of Fc gamma receptors

Across the animal kingdom, multivalency discriminates antibodies from all other immunoglobulin superfamily members. The evolutionary forces conserving multivalency above other structural hallmarks of antibodies remain, however, incompletely defined. Here, we engineer monovalent either Fc-competent or -deficient antibody formats to investigate mechanisms of protection of neutralizing antibodies (nAbs) and non-neutralizing antibodies (nnAbs) in virus-infected mice. Antibody bivalency enables the tethering of virions to the infected cell surface, inhibits the release of virions in cell culture, and suppresses viral loads in vivo independently of Fc gamma receptor (FcγR) interactions. In return, monovalent antibody formats either do not inhibit virion release and fail to protect in vivo or their protective efficacy is largely FcγR dependent. Protection in mice correlates with virus-release-inhibiting activity of nAb and nnAb rather than with their neutralizing capacity. These observations provide mechanistic insights into the evolutionary conservation of antibody bivalency and help refining correlates of nnAb protection for vaccine development.

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

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

Viral nucleoprotein antibodies activate TRIM21 and induce T cell immunity

Nucleoprotein (N) is an immunodominant antigen in many enveloped virus infections. While the diagnostic value of anti‐N antibodies is clear, their role in immunity is not. This is because while they are non‐neutralising, they somehow clear infection by coronavirus, influenza and LCMV in vivo. Here, we show that anti‐N immune protection is mediated by the cytosolic Fc receptor and E3 ubiquitin ligase TRIM21. Exploiting LCMV as a model system, we demonstrate that TRIM21 uses anti‐N antibodies to target N for cytosolic degradation and generate cytotoxic T cells (CTLs) against N peptide. These CTLs rapidly eliminate N‐peptide‐displaying cells and drive efficient viral clearance. These results reveal a new mechanism of immune synergy between antibodies and T cells and highlights N as an important vaccine target.

Chronic Viral Infection Promotes Efficient Germinal Center B Cell Responses

Persistent viral infections subvert key elements of adaptive immunity. To compare germinal center (GC) B cell responses in chronic and acute lymphocytic choriomeningitis virus infection, we exploit activation-induced deaminase (AID) fate-reporter mice and perform adoptive B cell transfer experiments. Chronic infection yields GC B cell responses of higher cellularity than acute infections do, higher memory B cell and antibody secreting cell output for longer periods of time, a better representation of the late B cell repertoire in serum immunoglobulin, and higher titers of protective neutralizing antibodies. GC B cells of chronically infected mice are similarly hypermutated as those emerging from acute infection. They efficiently adapt to viral escape variants and even in hypermutation-impaired AID mutant mice, chronic infection selects for GC B cells with hypermutated B cell receptors (BCRs) and neutralizing antibody formation. These findings demonstrate that, unlike for CD8⁺ T cells, chronic viral infection drives a functional, productive, and protective GC B cell response.

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Chronic viral infection elicits potent and sustained germinal center (GC) responses

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Chronic infection triggers prolonged plasma cell and memory B cell output from GCs

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GC B cells hypermutate efficiently and are potently selected in chronic infection

Enhancing immunity prevents virus-induced T-cell-mediated immunopathology in B cell-deficient mice

Hyper-activated or deviated immune responses can result in immunopathological diseases. Paradoxically, immunodeficiency represents a frequent cause of such immune-mediated pathologies. Immunopathological manifestations are commonly treated by immunosuppression, but in situations in which immunodeficiency is the basis of disease development, enhancing immunity may represent an alternative treatment option. Here, we tested this counterintuitive concept in a preclinical model using infection of mice with lymphocytic choriomeningitis virus (LCMV). Firstly, we demonstrate that infection of B-cell-deficient (B^-/- ) but not of wild-type (WT) mice with the LCMV strain Docile induced a rapid and fatal CD8⁺ T-cell-mediated immunopathological disease. Similar to WT mice, LCMV-infected B^-/- mice generated a potent, functional LCMV-specific CD8⁺ T-cell response but exhibited prolonged viral antigen presentation and increased vascular leakage in liver and lungs. Secondly, we were able to prevent this virus-induced immunopathology in B^-/- mice by active or passive T-cell immunizations or by treatment with LCMV-specific virus neutralizing or non-neutralizing monoclonal antibodies (mAb). Thus, boosting antiviral immunity did not aggravate immunopathology in this model, but prevented it by decreasing the formation of target structures for damage-causing CD8⁺ T cells.

Replication-incompetent rabies virus vector harboring glycoprotein gene of lymphocytic choriomeningitis virus (LCMV) protects mice from LCMV challenge

BACKGROUND

Lymphocytic choriomeningitis virus (LCMV) causes a variety of diseases, including asymptomatic infections, meningitis, and congenital infections in the fetus of infected mother. The development of a safe and effective vaccine against LCMV is imperative. This study aims to develop a new candidate vaccine against LCMV using a recombinant replication-incompetent rabies virus (RV) vector.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we have generated a recombinant deficient RV expressing the LCMV glycoprotein precursor (GPC) (RVΔP-LCMV/GPC) which is lacking the RV-P gene. RVΔP-LCMV/GPC is able to propagate only in cells expressing the RV-P protein. In contrast, the LCMV-GPC can be expressed in general cells, which do not express RV-P protein. The ability of RVΔP-LCMV/GPC to protect mice from LCMV infection and induce cellular immunity was assessed. Mice inoculated intraperitoneally with RVΔP-LCMV/GPC showed higher survival rates (88.2%) than those inoculated with the parental recombinant RV-P gene-deficient RV (RVΔP) (7.7%) following a LCMV challenge. Neutralizing antibody (NAb) against LCMV was not induced, even in the sera of surviving mice. CD8+ T-cell depletion significantly reduced the survival rates of RVΔP-LCMV/GPC-inoculated mice after the LCMV challenge. These results suggest that CD8+ T cells play a major role in the observed protection against LCMV. In contrast, NAbs against RV were strongly induced in sera of mice inoculated with either RVΔP-LCMV/GPC or RVΔP. In safety tests, suckling mice inoculated intracerebrally with RVΔP-LCMV/GPC showed no symptoms.

CONCLUSIONS/SIGNIFICANCE

These results show RVΔP-LCMV/GPC might be a promising candidate vaccine with dual efficacy, protecting against both RV and LCMV.

Immunological tolerance to LCMV antigens differently affects control of acute and chronic virus infection in mice

Cytotoxic T lymphocytes (CTLs) play a key role in the control of lymphocytic choriomeningitis virus (LCMV) infection. In C57BL/6 mice (H-2^b ), the CTL response is mainly directed against epitopes from the LCMV glycoprotein (GP) and the nucleoprotein (NP) which represent the two major viral proteins. The role of GP- versus NP-derived epitopes for viral clearance was examined using transgenic (tg) mice ubiquitously expressing LCMV GP and NP, respectively. These mice lack GP- or NP-specific CTLs and show decreased levels of GP- or NP-specific antibodies as a result of tolerance induction. During acute LCMV infection, CTLs specific for GP- and NP-derived epitopes are generated with similar frequencies. Nonetheless, we found that lack of GP- but not of NP-specific CTLs abolished control of acute LCMV infection. In contrast, after high-dose or chronic LCMV infection, virus elimination was delayed to a similar extent in GP- and NP-tg mice. Thus, immunological tolerance to LCMV antigens differently affects virus clearance in acute and chronic infection settings. In addition, our data reveal that immunodominance of H-2^b -restricted LCMV-specific CTL epitopes and their antiviral activity do not strictly correlate.

Differences in Glycoprotein Complex Receptor Binding Site Accessibility Prompt Poor Cross-Reactivity of Neutralizing Antibodies between Closely Related Arenaviruses

The glycoprotein complex (GPC) of arenaviruses, composed of stable signal peptide, GP1, and GP2, is the only antigen correlated with antibody-mediated neutralization. However, despite strong cross-reactivity of convalescent antisera between related arenavirus species, weak or no cross-neutralization occurs. Two closely related clade B viruses, Machupo virus (MACV) and Junín virus (JUNV), have nearly identical overall GPC architecture and share a host receptor, transferrin receptor 1 (TfR1). Given structural and functional similarities of the GP1 receptor binding site (RBS) of these viruses and the recent demonstration that the RBS is an important target for neutralizing antibodies, it is not clear how these viruses avoid cross-neutralization. To address this, MACV/JUNV chimeric GPCs were assessed for interaction with a group of α-JUNV GPC monoclonal antibodies (MAbs) and mouse antisera against JUNV or MACV GPC. All six MAbs targeted GP1, with those that neutralized JUNV GPC-pseudovirions competing with each other for RBS binding. However, these MAbs were unable to bind to a chimeric GPC composed of JUNV GP1 containing a small disulfide bonded loop (loop 10) unique to MACV GPC, suggesting that this loop may block MAbs interaction with the GP1 RBS. Consistent with this loop causing interference, mouse anti-JUNV GPC antisera that solely neutralized pseudovirions bearing autologous GP1 provided enhanced neutralization of MACV GPC when this loop was removed. Our studies provide evidence that loop 10, which is unique to MACV GP1, is an important impediment to binding of neutralizing antibodies and contributes to the poor cross-neutralization of α-JUNV antisera against MACV.IMPORTANCE Multiple New World arenaviruses can cause severe disease in humans, and some geographic overlap exists among these viruses. A vaccine that protects against a broad range of New World arenaviruses is desirable for purposes of simplicity, cost, and broad protection against multiple National Institute of Allergy and Infectious Disease-assigned category A priority pathogens. In this study, we sought to better understand how closely related arenaviruses elude cross-species neutralization by investigating the structural bases of antibody binding and avoidance. In our studies, we found that neutralizing antibodies against two New World arenaviruses, Machupo virus (MACV) and Junín virus (JUNV), bound to the envelope glycoprotein 1 (GP1) with JUNV monoclonal antibodies targeting the receptor binding site (RBS). We further show that altered structures surrounding the RBS pocket in MACV GP1 impede access of JUNV-elicited antibodies.

Most neutralizing human monoclonal antibodies target novel epitopes requiring both Lassa virus glycoprotein subunits

Lassa fever is a severe multisystem disease that often has haemorrhagic manifestations. The epitopes of the Lassa virus (LASV) surface glycoproteins recognized by naturally infected human hosts have not been identified or characterized. Here we have cloned 113 human monoclonal antibodies (mAbs) specific for LASV glycoproteins from memory B cells of Lassa fever survivors from West Africa. One-half bind the GP2 fusion subunit, one-fourth recognize the GP1 receptor-binding subunit and the remaining fourth are specific for the assembled glycoprotein complex, requiring both GP1 and GP2 subunits for recognition. Notably, of the 16 mAbs that neutralize LASV, 13 require the assembled glycoprotein complex for binding, while the remaining 3 require GP1 only. Compared with non-neutralizing mAbs, neutralizing mAbs have higher binding affinities and greater divergence from germline progenitors. Some mAbs potently neutralize all four LASV lineages. These insights from LASV human mAb characterization will guide strategies for immunotherapeutic development and vaccine design.

Crystal structure of the prefusion surface glycoprotein of the prototypic arenavirus LCMV

Arenaviruses exist worldwide and can cause hemorrhagic fever and neurologic disease. A single glycoprotein expressed on the viral surface mediates entry into target cells. This glycoprotein, termed GPC, contains a membrane-associated signal peptide, a receptor-binding subunit termed GP1 and a fusion-mediating subunit termed GP2. Although GPC is a critical target of antibodies and vaccines, the structure of the metastable GP1-GP2 prefusion complex has remained elusive for all arenaviruses. Here we describe the crystal structure of the fully glycosylated prefusion GP1-GP2 complex of the prototypic arenavirus LCMV at 3.5 Å. This structure reveals the conformational changes that the arenavirus glycoprotein must undergo to cause fusion and illustrates the fusion regions and potential oligomeric states.

Glycoprotein-Specific Antibodies Produced by DNA Vaccination Protect Guinea Pigs from Lethal Argentine and Venezuelan Hemorrhagic Fever

Several members of the Arenaviridae can cause acute febrile diseases in humans, often resulting in lethality. The use of convalescent-phase human plasma is an effective treatment in humans infected with arenaviruses, particularly species found in South America. Despite this, little work has focused on developing potent and defined immunotherapeutics against arenaviruses. In the present study, we produced arenavirus neutralizing antibodies by DNA vaccination of rabbits with plasmids encoding the full-length glycoprotein precursors of Junín virus (JUNV), Machupo virus (MACV), and Guanarito virus (GTOV). Geometric mean neutralizing antibody titers, as measured by the 50% plaque reduction neutralization test (PRNT(50)), exceeded 5,000 against homologous viruses. Antisera against each targeted virus exhibited limited cross-species binding and, to a lesser extent, cross-neutralization. Anti-JUNV glycoprotein rabbit antiserum protected Hartley guinea pigs from lethal intraperitoneal infection with JUNV strain Romero when the antiserum was administered 2 days after challenge and provided some protection (∼30%) when administered 4 days after challenge. Treatment starting on day 6 did not protect animals. We further formulated an IgG antibody cocktail by combining anti-JUNV, -MACV, and -GTOV antibodies produced in DNA-vaccinated rabbits. This cocktail protected 100% of guinea pigs against JUNV and GTOV lethal disease. We then expanded on this cocktail approach by simultaneously vaccinating rabbits with a combination of plasmids encoding glycoproteins from JUNV, MACV, GTOV, and Sabia virus (SABV). Sera collected from rabbits vaccinated with the combination vaccine neutralized all four targets. These findings support the concept of using a DNA vaccine approach to generate a potent pan-arenavirus immunotherapeutic.

IMPORTANCE

Arenaviruses are an important family of emerging viruses. In infected humans, convalescent-phase plasma containing neutralizing antibodies can mitigate the severity of disease caused by arenaviruses, particularly species found in South America. Because of variations in potency of the human-derived product, limited availability, and safety concerns, this treatment option has essentially been abandoned. Accordingly, despite this approach being an effective postinfection treatment option, research on novel approaches to produce potent polyclonal antibody-based therapies have been deficient. Here we show that DNA-based vaccine technology can be used to make potently neutralizing antibodies in rabbits that exclusively target the glycoproteins of several human-pathogenic arenaviruses found in South America, including JUNV, MACV, GTOV, and SABV. These antibodies protected guinea pigs from lethal disease when given post-virus challenge. We also generated a purified antibody cocktail with antibodies targeting three arenaviruses and demonstrated protective efficacy against all three targets. Our findings demonstrate that use of the DNA vaccine technology could be used to produce candidate antiarenavirus neutralizing antibody-based products.

Arenavirus Glycan Shield Promotes Neutralizing Antibody Evasion and Protracted Infection

Arenaviruses such as Lassa virus (LASV) can cause severe hemorrhagic fever in humans. As a major impediment to vaccine development, delayed and weak neutralizing antibody (nAb) responses represent a unifying characteristic of both natural infection and all vaccine candidates tested to date. To investigate the mechanisms underlying arenavirus nAb evasion we engineered several arenavirus envelope-chimeric viruses and glycan-deficient variants thereof. We performed neutralization tests with sera from experimentally infected mice and from LASV-convalescent human patients. NAb response kinetics in mice correlated inversely with the N-linked glycan density in the arenavirus envelope protein’s globular head. Additionally and most intriguingly, infection with fully glycosylated viruses elicited antibodies, which neutralized predominantly their glycan-deficient variants, both in mice and humans. Binding studies with monoclonal antibodies indicated that envelope glycans reduced nAb on-rate, occupancy and thereby counteracted virus neutralization. In infected mice, the envelope glycan shield promoted protracted viral infection by preventing its timely elimination by the ensuing antibody response. Thus, arenavirus envelope glycosylation impairs the protective efficacy rather than the induction of nAbs, and thereby prevents efficient antibody-mediated virus control. This immune evasion mechanism imposes limitations on antibody-based vaccination and convalescent serum therapy.

Neutralizing antibodies (nAbs) represent a key principle of antiviral immunity. Protective vaccines aim at inducing nAbs to prevent viral infection, and infusion of nAbs in convalescent patient serum can offer a potent antiviral therapy. Certain viruses, however, have found ways to evade nAb control. Amongst them are high-risk pathogens of the arenavirus family such as Lassa virus (LASV), which is a frequent cause of hemorrhagic fever in West Africa. Here we unveil the molecular strategy by which arenaviruses escape antibody neutralization and avoid efficient immune control. We show that their surface is decorated with sugar moieties, serving to shield the virus against the neutralizing effect of the host’s antibodies. This immune evasion strategy differs from those described for other viruses, in which sugars impair primarily the induction of antibodies or allow for viral mutational escape. The arenavirus sugar coat renders the host nAb response inefficient and as a consequence thereof, the host fails to promptly control the infection. Our results offer a compelling explanation for the long history of failures in trying to make a nAb-based vaccine against LASV or in using convalescent serum for therapy. These mechanistic insights will support vaccine development efforts against arenaviruses such as LASV.

Generation of cellular immune memory and B-cell immunity is impaired by natural killer cells

The goal of most vaccines is the induction of long-lived memory T and B cells capable of protecting the host from infection by cytotoxic mechanisms, cytokines and high-affinity antibodies. However, efforts to develop vaccines against major human pathogens like HIV and HCV have not been successful, thereby highlighting the need for novel approaches to circumvent immunoregulatory mechanisms that limit induction of protective immunity. Here we show that mouse natural killer (NK) cells inhibit generation of long-lived virus-specific memory T- and B-cells as well as virus-specific antibody production after acute infection. Mechanistically, NK cells suppressed CD4 T cells and follicular helper T cells (T_FH) in a perforin-dependent manner during the first few days of infection, resulting in a weaker germinal center (GC) response and diminished immune memory. We anticipate that innovative strategies to relieve NK cell-mediated suppression of immunity should facilitate development of efficacious new vaccines targeting difficult-to-prevent infections.

Vaccine-elicited CD4 T cells induce immunopathology after chronic LCMV infection

CD4 T cells promote innate and adaptive immune responses, but how vaccine-elicited CD4 T cells contribute to immune protection remains unclear. We evaluated whether induction of virus-specific CD4 T cells by vaccination would protect mice against infection with chronic lymphocytic choriomeningitis virus (LCMV). Immunization with vaccines that selectively induced CD4 T cell responses resulted in catastrophic inflammation and mortality after challenge with a persistent strain of LCMV. Immunopathology required antigen-specific CD4 T cells and was associated with a cytokine storm, generalized inflammation, and multi-organ system failure. Virus-specific CD8 T cells or antibodies abrogated the pathology. These data demonstrate that vaccine-elicited CD4 T cells in the absence of effective antiviral immune responses can trigger lethal immunopathology.

Arenavirus stable signal peptide is the keystone subunit for glycoprotein complex organization

The rodent arenavirus glycoprotein complex encodes a stable signal peptide (SSP) that is an essential structural component of mature virions. The SSP, GP1, and GP2 subunits of the trimeric glycoprotein complex noncovalently interact to stud the surface of virions and initiate arenavirus infectivity. Nascent glycoprotein production undergoes two proteolytic cleavage events: first within the endoplasmic reticulum (ER) to cleave SSP from the remaining precursor GP1/2 (glycoprotein complex [GPC]) glycoprotein and second within the Golgi stacks by the cellular SKI-1/S1P for GP1/2 processing to yield GP1 and GP2 subunits. Cleaved SSP is not degraded but retained as an essential glycoprotein subunit. Here, we defined functions of the 58-amino-acid lymphocytic choriomeningitis virus (LCMV) SSP in regard to glycoprotein complex processing and maturation. Using molecular biology techniques, confocal microscopy, and flow cytometry, we detected SSP at the plasma membrane of transfected cells. Further, we identified a sorting signal (FLLL) near the carboxyl terminus of SSP that is required for glycoprotein maturation and trafficking. In the absence of SSP, the glycoprotein accumulated within the ER and was unable to undergo processing by SKI-1/S1P. Mutation of this highly conserved FLLL motif showed impaired glycoprotein processing and secretory pathway trafficking, as well as defective surface expression and pH-dependent membrane fusion. Immunoprecipitation of SSP confirmed an interaction between the signal peptide and the GP2 subunit; however, mutations within this FLLL motif disrupted the association of the GP1 subunit with the remaining glycoprotein complex.

Several members of the Arenaviridae family are neglected human pathogens capable of causing illness ranging from a nondescript flu-like syndrome to fulminant hemorrhagic fever. Infections by arenaviruses are mediated by attachment of the virus glycoprotein to receptors on host cells and virion internalization by fusion within an acidified endosome. SSP plays a critical role in the fusion of the virus with the host cell membrane. Within infected cells, the retained glycoprotein SSP plays a neglected yet essential role in glycoprotein biosynthesis. Without this 6-kDa polypeptide, the glycoprotein precursor is retained within the endoplasmic reticulum, and trafficking to the plasma membrane where SSP, GP1, and GP2 localize for glycoprotein assembly into infectious virions is inhibited. To investigate SSP contributions to glycoprotein maturation and function, we created an SSP-tagged glycoprotein to directly detect and manipulate this subunit. This resource will aid future studies to identify host factors that mediate glycoprotein maturation.

Nucleoprotein-specific nonneutralizing antibodies speed up LCMV elimination independently of complement and FcγR

CD8(+) T cells have an essential role in controlling lymphocytic choriomeningitis virus (LCMV) infection in mice. Here, we examined the contribution of humoral immunity, including nonneutralizing antibodies (Abs), in this infection induced by low virus inoculation doses. Mice with impaired humoral immunity readily terminated infection with the slowly replicating LCMV strain Armstrong but showed delayed virus elimination after inoculation with the faster replicating LCMV strain WE and failed to clear the rapidly replicating LCMV strain Docile, which is in contrast to the results obtained with wild-type mice. Thus, the requirement for adaptive humoral immunity to control the infection was dependent on the replication speed of the LCMV strains used. Ab transfers further showed that LCMV-specific IgG Abs isolated from LCMV immune serum accelerated virus elimination. These Abs were mainly directed against the viral nucleoprotein (NP) and completely lacked virus neutralizing activity. Moreover, mAbs specific for the LCMV NP were also able to decrease viral titers after transfer into infected hosts. Intriguingly, neither C3 nor Fcγ receptors were required for the antiviral activity of the transferred Abs. In conclusion, our study suggests that rapidly generated nonneutralizing Abs specific for the viral NP speed up virus elimination and thereby may counteract T-cell exhaustion.

A multivalent vaccination strategy for the prevention of Old World arenavirus infection in humans

Arenaviruses cause severe human disease ranging from aseptic meningitis following lymphocytic choriomeningitis virus (LCMV) infection to hemorrhagic fever syndromes following infection with Guanarito virus (GTOV), Junin virus (JUNV), Lassa virus (LASV), Machupo virus (MACV), Sabia virus (SABV), or Whitewater Arroyo virus (WWAV). Cellular immunity, chiefly the CD8(+) T-cell response, plays a critical role in providing protective immunity following infection with the Old World arenaviruses LASV and LCMV. In the current study, we evaluated whether HLA class I-restricted epitopes that are cross-reactive among pathogenic arenaviruses could be identified for the purpose of developing an epitope-based vaccination approach that would cross-protect against multiple arenaviruses. We were able to identify a panel of HLA-A*0201-restricted peptides derived from the same region of the glycoprotein precursor (GPC) of LASV (GPC spanning residues 441 to 449 [GPC(441-449)]), LCMV (GPC(447-455)), JUNV (GPC(429-437)), MACV (GPC(444-452)), GTOV (GPC(427-435)), and WWAV (GPC(428-436)) that displayed high-affinity binding to HLA-A*0201 and were recognized by CD8(+) T cells in a cross-reactive manner following LCMV infection or peptide immunization of HLA-A*0201 transgenic mice. Immunization of HLA-A*0201 mice with the Old World peptide LASV GPC(441-449) or LCMV GPC(447-455) induced high-avidity CD8(+) T-cell responses that were able to kill syngeneic target cells pulsed with either LASV GPC(441-449) or LCMV GPC(447-455) in vivo and provided significant protection against viral challenge with LCMV. Through this study, we have demonstrated that HLA class I-restricted, cross-reactive epitopes exist among diverse arenaviruses and that individual epitopes can be utilized as effective vaccine determinants for multiple pathogenic arenaviruses.

Neutralization of IFNgamma defeats haemophagocytosis in LCMV-infected perforin- and Rab27a-deficient mice

Hereditary haemophagocytic lymphohistiocytosis (HLH) is a fatal inflammatory disease and treatments currently may lead to serious side effects. There is a pressing need for effective, less toxic treatments for this disease. Previous reports have suggested that interferon γ (IFNγ) has a role in the pathogenesis of HLH. Here, we report that blocking IFNγ had a therapeutic effect in two different murine models of human hereditary HLH (perforin-deficient and Rab27a-deficient mice, both infected with lymphocytic choriomeningitis virus). Therapeutic administration of an anti-IFNγ antibody induced recovery from haemophagocytosis in both genetic models, as evidenced by increased survival in perforin-deficient mice and correction of blood cytopenia, moderation of body temperature changes, decreased cytokinaemia, restoration of splenic architecture and reduced haemophagocytosis in the liver of both murine models. Involvement of the central nervous system in Rab27a-deficient mice was prevented by anti-IFNγ therapy. Hepatic T-cell infiltrates and virus persisted, with no detectable harm during the time course of these studies. These data strongly suggest that neutralization of IFNγ could be used in humans to safely alleviate the clinical manifestations of haemophagocytosis.

Impaired antibody response causes persistence of prototypic T cell-contained virus

CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor gamma chain or Fc gamma receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell-controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

A Griscelli syndrome type 2 murine model of hemophagocytic lymphohistiocytosis (HLH)

Griscelli syndrome type 2 is caused by mutations in the RAB27A gene and is a rare and potentially fatal immune disorder associated with hemophagocytic lymphohistiocytosis (HLH). Animal models could provide assistance for better understanding the mechanisms and finding new treatments. Rab27a-deficient (ashen) mice do not spontaneously develop HLH. When injected with lymphocytic choriomeningitis virus (LCMV) strain WE, Rab27a-deficient C57BL/6 mice developed wasting disease, hypothermia, splenomegaly, cytopenia (anemia, neutropenia and thrombocytopenia), hypertriglyceridemia and increased levels of IFN-gamma, TNF-alpha, GM-CSF, IL-12, CCL5 and IL-10. Activated macrophages with hemophagocytosis were found in liver sections of these mice. Compared with perforin-deficient mice, LCMV-infected Rab27a-deficient mice showed a substantially better survival rate and slightly higher viral doses were needed to trigger HLH in Rab27a-deficient mice. This study demonstrates that LCMV-infected Rab27a-deficient C57BL/6 mice develop features consistent with HLH and, therefore, represent a murine model of HLH in human Griscelli syndrome type 2.

Absence of CTL responses to early viral antigens facilitates viral persistence

CD8+ T cells are crucial for the control of intracellular pathogens such as viruses and some bacteria. Using lymphocytic choriomeningitis virus (LCMV) infection of mice--the prototypic arenavirus evolutionarily closely related to human Lassa fever and South American hemorrhagic fever viruses, we have shown previously that the kinetics of Ag presentation determine immunodominance of the LCMV-specific CTL response due to progressive exhaustion of LCMV nucleoprotein (NP)-specific CTL upon increasing viral load. In this study, we provide evidence that CTL against early LCMV NP-derived epitopes are more important in virus control than those against late glycoprotein-derived epitopes. We show that mice that are tolerant to all NP-derived T cell epitopes are severely compromised in their ability to control larger inocula of LCMV, supporting our hypothesis that CD8+ T cells specific for early viral Ags play a major role in acute virus control. Thus, the kinetics with which virus-derived T cell epitopes are presented has a strong impact on the efficacy of the antiviral immunity. This aspect should be taken into consideration for the development of vaccines.

Signal peptide requirements for lymphocytic choriomeningitis virus glycoprotein C maturation and virus infectivity

Insertion of the lymphocytic choriomeningitis virus (LCMV) precursor glycoprotein C (GP-C) into the membrane of the endoplasmic reticulum is mediated by an unusual signal peptide (SP(GP-C)). It is comprised of 58 amino acid residues and contains an extended hydrophilic N-terminal region, two hydrophobic regions, and a short C-terminal region. After cleavage by signal peptidase, SP(GP-C) accumulates in cells and virus particles. In the present study, we identified the LCMV SP(GP-C) as being an essential component of the GP complex and show that the different regions of SP(GP-C) are required for distinct steps in GP maturation and virus infectivity. More specifically, we show that one hydrophobic region of SP(GP-C) is sufficient for the membrane insertion of GP-C, while both hydrophobic regions are required for the processing and cell surface expression of the GPs. The N-terminal region of SP(GP-C), on the other hand, is essential for pseudoviral infection of target cells. Furthermore, we show that unmyristoylated SP(GP-C) exposes its N-terminal region to the exoplasmic side. This SP(GP-C) can promote GP-C maturation but is defective in pseudoviral infection. Myristoylation and topology of SP(GP-C) in the membrane may thus hold the key to an understanding of the role of SP(GP-C) in GP-C complex maturation and LCMV infectivity.

Adaptive immunity to Lymphocytic choriomeningitis virus: new insights into antigenic determinants

Lymphocytic choriomeningitis virus (LCMV) is one of the most studied infectious disease models in mice. Human infection with LCMV can result in severe disease, ranging from aseptic meningitis in immunocompetent individuals, hydrocephalus, chorioretinitis or microcephaly in fetal infection, or to a highly lethal outcome in immunosuppressed individuals. This review examines recent advances in our understanding of the adaptive immune response to LCMV and how the cell-mediated and humoral immune responses contribute to protective immunity. New insights into the antigenicity of the LCMV proteome and the complexity of the cell-mediated immune response are addressed. We also discuss state-of-the-art approaches for T-cell epitope discovery in murine and human backgrounds and their recent application to LCMV. New findings regarding CD4+ T-cell dysregulation during chronic LCMV infection, and potential avenues for the treatment of chronic viral infection through modulation of the programmed cell death-1 receptor and/or IL-10 signaling pathways, are also evaluated.

Early antibodies specific for the neutralizing epitope on the receptor binding subunit of the lymphocytic choriomeningitis virus glycoprotein fail to neutralize the virus

Lymphocytic choriomeningitis virus (LCMV) is a murine arenavirus whose glycoprotein consists of a transmembrane subunit (GP-2) and a receptor-binding subunit (GP-1). LCMV-neutralizing antibodies (nAbs) are directed against a single site on GP-1 and occur 1 month after the infection of cytotoxic-T-lymphocyte (CTL) deficient mice. In wild-type mice, however, CTLs control early infection, and weak nAb titers emerge very late (after 70 to 150 days) if at all. Production of recombinant GP-1 in native conformation enabled us to study the emergence of GP-1-binding antibodies directed against the neutralizing epitope. By combining binding and neutralization assays, we correlated the development of binding antibodies versus nAbs in wild-type and CTL-deficient mice after infection with different LCMV doses. We found that wild-type mice developed GP-1-specific antibodies already by day 8 after exposure to high but not low doses, demonstrating that naive GP-1-specific B cells were infrequent. Furthermore, the induced antibodies bound to the neutralizing GP-1 epitope but failed to neutralize the virus and therefore were of low affinity. In CTL-deficient mice, where massive viremia quickly levels initial differences in viral load, low and high doses induced low-affinity non-neutralizing GP-1-binding antibodies with kinetics similar to high-dose-infected wild-type mice. Only in CTL-deficient mice, however, the GP-1-specific antibodies developed into nAbs within 1 month. We conclude that LCMV uses a dual strategy to evade nAb responses in wild-type mice. First, LCMV exploits a "hole" in the murine B-cell repertoire, which provides only a small and narrow initial pool of low-affinity GP-1-specific B cells. Second, affinity maturation of the available low-affinity non-neutralizing antibodies is impaired.

A lymphocytic choriomeningitis virus glycoprotein variant that is retained in the endoplasmic reticulum efficiently cross-primes CD8(+) T cell responses

Recent studies indicate that T cell cross-priming preferentially occurs against long-lived, stable proteins. We have studied cross-priming by using the glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV), a protein that normally is not MHC class I cross-presented. This study shows that a C-terminally truncated, noncleavable variant of LCMV-GP led to the accumulation of stable, soluble GP trimers in the endoplasmic reticulum (ER) of the antigen donor cell, and thereby converted LCMV-GP into a potent immunogen for cytotoxic T lymphocyte cross-priming. Immunization of mice with tumor cells expressing an ER-retained LCMV-GP variant cross-primed protective antiviral cytotoxic T lymphocyte responses in vivo at least 10,000-fold better than immunization with cells expressing the cross-presentation-"resistant" wild-type LCMV-GP. Thus the ER is a cellular compartment that can provide antigen for cross-presentation, and modifications affecting stability and subcellular localization of the antigen significantly increase its availability for MHC class I cross-presentation. These findings impinge on vaccine strategies.

A retroviral packaging cell line for pseudotype vectors based on glioma-infiltrating progenitor cells

BACKGROUND

Early clinical trials for gene therapy of human gliomas with retroviral packaging cells (PC) have been hampered by low transduction efficacy and lack of dissemination of PC within the tumor. In the current approach, these issues have been addressed by creating a stable packaging cell line for retroviral vectors pseudotyped with glycoproteins of lymphocytic choriomeningitis virus (LCMV) based on tumor-infiltrating progenitor cells.

METHODS

Tumor-infiltrating progenitor cells, which had been isolated from adult rat bone marrow (BM-TIC), were modified to stably express Gag-Pol proteins of moloney murine leukemia virus (Mo-MLV) and glycoproteins of LCMV. Packaging of a retroviral vector was measured by titration experiments on human fibroblast cells as well as on mouse and human glioma cell lines. Additionally, gene transfer was tested in a rat glioma model in vivo.

RESULTS

The BM-TIC-derived packaging cell line (BM-TIPC) produced retroviral vectors with titers between 2-8 x 10(3) transducing units (TU)/ml. Extended culturing of BM-TIPC over several weeks and freezing/thawing of cells did not affect vector titers. No replication-competent retrovirus was released from BM-TIPC. In a rat glioma model, BM-TIPC infiltrated the tumors extensively and with high specificity. Moreover, BM-TIPC mediated transduction of glioma cells in vivo.

CONCLUSION

This proof-of-principle study shows that primary adult progenitor cells with tumor-infiltrating capacity can be genetically modified to stably produce retroviral LCMV pseudotype vectors. These BM-TIPC may be a useful tool to enhance specificity and efficacy of gene transfer to gliomas in patients.

No essential role for tripeptidyl peptidase II for the processing of LCMV-derived T cell epitopes

The proteasome is critically involved in the production of MHC class I-restricted T cell epitopes. Approximately 20% of all peptides generated by the proteasome are too large for direct presentation by MHC class I molecules. Reits et al. (Immunity 2004. 20: 495-506) suggested that a major portion of proteasomal products are larger than 15 amino acids and require further degradation by the tripeptidyl peptidase II (TPPII) before becoming ligands of MHC class I molecules. Using the well-characterized lymphocytic choriomeningitis virus (LCMV) model, the role of TPPII in the processing of several LCMV-derived T cell epitopes was investigated. In contrast to Reits' proposal, TPPII inhibition and TPPII overexpression experiments revealed that five out of six LCMV-derived CD8(+) T cell epitopes were not affected by inhibition of TPPII, while one epitope (GP276) was slightly reduced upon TPPII overexpression. Additionally, we demonstrated that the processing of two epitopes derived from ovalbumin and murine cytomegalovirus were not altered by TPPII inhibition. We propose that TPPII is not generally required for the production of MHC class I peptides, but the presentation of some peptides can be negatively affected by TPPII.

Viral hemorrhagic fevers

A taxonomically diverse set of single-stranded ribonucleic acid(ssRNA) viruses from four diverse viral families Arenaviridae,Bunyaviridae, Filoviridae, and Flaviviridae cause an acute systemic febrile syndrome called viral hemorrhagic fever (VHF). The syndrome produces combinations of prostration, malaise, increased vascular permeability, and coagulation maladies. In severe illness,VHF may include generalized bleeding but the bleeding does not typically constitute a life-threatening loss of blood volume. To a certain extent, it is a sign of damage to the vascular endothelium and is an indicator of disease severity in specific target organs. Although the viruses that cause hemorrhagic fever (HF) can productively replicate in endothelial cells, much of the disease pathology including impairment to the vascular system is thought to result primarily from the release of a variety of mediators from virus-infected cells, such as monocytes and macrophages that subsequently alter vascular function and trigger the coagulation disorders that epitomize these infections. While significant progress has been made over the last several years in dissecting out the molecular biology and pathogenesis of the HF viruses, there are currently no vaccines or drugs licensed available for most of the VHFs.

A-to-G hypermutation in the genome of lymphocytic choriomeningitis virus

The interferon-inducible adenosine deaminase that acts on double-stranded RNA (ADAR1-L) has been proposed to be one of the antiviral effector proteins within the complex innate immune response. Here, the potential role of ADAR1-L in the innate immune response to lymphocytic choriomeningitis virus (LCMV), a widely used virus model, was studied. Infection with LCMV clearly upregulated ADAR1-L expression and activity. The editing activity of ADAR1-L on an RNA substrate was not inhibited by LCMV replication. Accordingly, an adenosine-to-guanosine (A-to-G) and uracil-to-cytidine (U-to-C) hypermutation pattern was found in the LCMV genomic RNA in infected cell lines and in mice. In addition, two hypermutated clones with a high level of A-to-G or U-to-C mutations within a short stretch of the viral genome were isolated. Analysis of the functionality of viral glycoprotein revealed that A-to-G- and U-to-C-mutated LCMV genomes coded for nonfunctional glycoprotein at a surprisingly high frequency. Approximately half the GP clones with an amino acid mutation lacked functionality. These results suggest that ADAR1-L-induced mutations in the viral RNA lead to a loss of viral protein function and reduced viral infectivity. This study therefore provides strong support for the contribution of ADAR1-L to the innate antiviral immune response.

Antiviral antibody responses: the two extremes of a wide spectrum

Viruses elicit a diverse spectrum of antiviral antibody responses. In this review, we discuss two widely used experimental model systems for viral infections - non-cytopathic lymphocytic choriomeningitis virus (LCMV) and acutely cytopathic vesicular stomatitis virus (VSV) - to analyse two fundamentally different types of antiviral antibody response. The basic principles found in these model infections are discussed in the context of other viral infections, and with regard to protective neutralizing versus non-protective enzyme-linked immunosorbent assay (ELISA)-detected antibody responses. Issues of antibody specificity, affinity and avidity, maturation and escape are discussed in the context of co-evolution of the host and viruses.

O Mannosylation of alpha-dystroglycan is essential for lymphocytic choriomeningitis virus receptor function

Alpha-dystroglycan (alpha-DG) was identified as a common receptor for lymphocytic choriomeningitis virus (LCMV) and several other arenaviruses including the human pathogenic Lassa fever virus. Initial work postulated that interactions between arenavirus glycoproteins and alpha-DG are based on protein-protein interactions. We found, however, that susceptibility toward LCMV infection differed in various cell lines despite them expressing comparable levels of DG, suggesting that posttranslational modifications of alpha-DG would be involved in viral receptor function. Here, we demonstrate that glycosylation of alpha-DG, and in particular, O mannosylation, which is a rare type of O-linked glycosylation in mammals, is essential for LCMV receptor function. Cells that are defective in components of the O-mannosylation pathway showed strikingly reduced LCMV infectibility. As defective O mannosylation is associated with severe clinical symptoms in mammals such as congenital muscular dystrophies, it is likely that LCMV and potentially other arenaviruses may have selected this conserved and crucial posttranslational modification as the primary target structure for cell entry and infection.

Immunoproteasomes Down-Regulate Presentation of a Subdominant T Cell Epitope from Lymphocytic Choriomeningitis Virus

The cytotoxic T cell response to pathogens is usually directed against a few immunodominant epitopes, while other potential epitopes are either subdominant or not used at all. In C57BL/6 mice, the acute cytotoxic T cell response against lymphocytic choriomeningitis virus is directed against immunodominant epitopes derived from the glycoprotein (gp33-41) and the nucleoprotein (NP396-404), while the gp276-286 epitope remains subdominant. Despite extensive investigations, the reason for this hierarchy between epitopes is not clear. In this study, we show that the treatment of cells with IFN-gamma enhanced the presentation of gp33-41, whereas presentation of the gp276-286 epitope from the same glycoprotein was markedly reduced. Because proteasomes are crucially involved in epitope generation and because IFN-gamma treatment in vitro and lymphocytic choriomeningitis virus infection in vivo lead to a gradual replacement of constitutive proteasomes by immunoproteasomes, we investigated the role of proteasome composition on epitope hierarchy. Overexpression of the active site subunits of immunoproteasomes LMP2, LMP7, and MECL-1 as well as overexpression of LMP2 alone suppressed the presentation of the gp276-286 epitope. The ability to generate gp276-286-specific CTLs was enhanced in LMP2- and LMP7-deficient mice, and macrophages from these mice showed an elevated presentation of this epitope. In vitro digests demonstrated that fragmentation by immunoproteasomes, but not constitutive proteasomes led to a preferential destruction of the gp276 epitope. Taken together, we show that LMP2 and LMP7 can at least in part determine subdominance and shape the epitope hierarchy of CTL responses in vivo.

Deliberate removal of T cell help improves virus-neutralizing antibody production

The B cell response to lymphocytic choriomeningitis virus is characterized by a CD4(+) T cell-dependent polyclonal hypergammaglobulinemia and delayed formation of virus-specific neutralizing antibodies. Here we provide evidence that, paradoxically, because of polyclonal B cell activation, virus-specific T cell help impairs the induction of neutralizing antibody responses. Experimental reduction in CD4(+) T cell help in vivo resulted in potent neutralizing antibody responses without impairment of CD8(+) T cell activity. These unexpected consequences of polyclonal B cell activation may affect vaccine strategies and the treatment of clinically relevant chronic bacterial, parasitic and viral infections in which hypergammaglobulinemia is regularly found.

Antiviral immune responses in gene-targeted mice expressing the immunoglobulin heavy chain of virus-neutralizing antibodies

Two gene-targeted immunoglobulin heavy chain transgenic mouse strains, TgH(KL25) and TgH(VI10), expressing neutralizing specificities for lymphocytic choriomeningitis virus and vesicular stomatitis virus, respectively, have been generated. Three days after lymphocytic choriomeningitis virus infection, TgH(KL25) mice showed a thymus-independent neutralizing IgM response followed by thymus-dependent (TD) IgG. In contrast, WT mice mounted only a TD IgG response around day 80. These observations indicated that not only structural properties of the virus but also immunological parameters such as the frequency of B cells were indicative for the induction of thymus-independent versus TD Ig responses. Naïve vesicular stomatitis virusspecific Ig heavy chain transgenic mice displayed greatly elevated natural antibody titers. However, despite these high naïve titers, de novo activation of naïve CD4+ T and B cells was not blocked. Therefore, B cells giving rise to natural antibodies do not participate in virus-induced antibody responses.

Long-lived signal peptide of lymphocytic choriomeningitis virus glycoprotein pGP-C

Signal peptides (SPs) direct nascent secretory and membrane proteins to the membrane of the endoplasmic reticulum. They are usually cleaved from the nascent polypeptide by signal peptidase and then further proteolytically processed. The SP of the pre-glycoprotein (pGP-C) of the lymphocytic choriomeningitis virus SPGP-C (signal peptide of pGP-C) shows different properties: 1) The SPGP-C is unusually long (58 amino acid residues) and contains two hydrophobic segments interrupted by a lysine residue. 2) The SPGP-C is cleaved only from a subset of pGP-C proteins. A substantial portion of pGP-C accumulates that still contains the SPGP-C.3)The cleaved SPGP-C is rather long-lived (t(1/2) of more than 6 h). 4) The cleaved SPGP-C resides in the membrane and is resistant to digestion with proteinase K even in the presence of detergents, suggesting a very compact structure. 5) SPGP-C accumulates in virus particles. These unusual features of the cleaved SPGP-C suggest that SPGP-C not only targets the nascent pGP-C to the endoplasmic reticulum membrane but also has additional functions in lymphocytic choriomeningitis virus life cycle.

Endoproteolytic processing of the lymphocytic choriomeningitis virus glycoprotein by the subtilase SKI-1/S1P

The envelope glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV) is posttranslationally cleaved into two subunits. We show here that this endoproteolytic processing is not required for transport to the cell surface but is essential for LCMV GP to mediate infectivity of pseudotyped retroviral vectors. By systematic mutational analysis of the LCMV GP cleavage site, we determined that the consensus motif R-(R/K/H)-L-(A/L/S/T/F)(265) is essential for the endoproteolytic processing. In agreement with the identified consensus motif, we show that the cellular subtilase SKI-1/S1P cleaves LCMV GP.

Oncoretrovirus and lentivirus vectors pseudotyped with lymphocytic choriomeningitis virus glycoprotein: generation, concentration, and broad host range

Lymphocytic choriomeningitis virus (LCMV) is a noncytopathic arenavirus shown to infect a broad range of different cell types. Here, we combined the beneficial characteristics of the LCMV glycoprotein (LCMV-GP) and those of retroviral vectors to generate a new, safe, and efficient gene transfer system. These LCMV-GP pseudotypes were systematically compared with vectors containing the widely used amphotropic murine leukemia virus envelope (A-MLVenv) or the vesicular stomatitis virus G protein (VSV-G). Production of LCMV-GP-pseudotyped oncoretroviral and lentiviral vectors by transient transfection resulted in vector titers similar to those with A-MLVenv or VSV-G. In contrast to A-MLVenv particles, LCMV-GP pseudotypes could be efficiently concentrated by ultracentrifugation without loss of vector titer. Unlike the cell-toxic VSV-G, a stable retroviral packaging cell line constitutively expressing LCMV-GP could be established. Vectors pseudotyped with LCMV-GP efficiently transduced many cell lines from different species and tissues relevant for gene therapy. Transduction of human glioma cells was studied in detail. These cells are a major target for cancer gene therapy and were transduced more efficiently with LCMV-GP-pseudotyped vectors than with the generally used A-MLVenv particles. The high stability, low toxicity, and broad host range make LCMV-GP-pseudotyped vectors attractive for gene transfer applications. The recombinant LCMV-GP-pseudotyped vectors will also allow functional characterization of naturally occurring and recombinant LCMV-GP variants.

Immunoproteasomes largely replace constitutive proteasomes during an antiviral and antibacterial immune response in the liver

The proteasome is critically involved in the production of MHC class I-restricted T cell epitopes. Proteasome activity and epitope production are altered by IFN-gamma treatment, which leads to a gradual replacement of constitutive proteasomes by immunoproteasomes in vitro. However, a quantitative analysis of changes in the steady state subunit composition of proteasomes during an immune response against viruses or bacteria in vivo has not been reported. Here we show that the infection of mice with lymphocytic choriomeningitis virus or Listeria monocytogenes leads to an almost complete replacement of constitutive proteasomes by immunoproteasomes in the liver within 7 days. Proteasome replacements were markedly reduced in IFN-gamma(-/-) mice, but were only slightly affected in IFN-alphaR(-/-) and perforin(-/-) mice. The proteasome regulator PA28alpha/beta was up-regulated, whereas PA28gamma was reduced in the liver of lymphocytic choriomeningitis virus-infected mice. Proteasome replacements in the liver strongly altered proteasome activity and were unexpected to this extent, since an in vivo half-life of 12 days had been previously assigned to constitutive proteasomes in the liver. Our results suggest that during the peak phase of viral and bacterial elimination the antiviral cytotoxic T lymphocyte response is directed mainly to immunoproteasome-dependent T cell epitopes, which would be a novel parameter for the design of vaccines.

Recombinant expression of lymphocytic choriomeningitis virus strain WE glycoproteins: a single amino acid makes the difference

Cytoplasmic vector systems are generally used for expression of lymphocytic choriomeningitis virus (LCMV) proteins. However, we achieved high levels of cell surface glycoproteins using a standard nuclear expression plasmid. Expression was independent of other LCMV proteins but was blocked by a missense mutation within the original LCMV(WE) glycoprotein cDNA.

Overexpression of the proteasome subunits LMP2, LMP7, and MECL-1, but not PA28 alpha/beta, enhances the presentation of an immunodominant lymphocytic choriomeningitis virus T cell epitope

The proteasome is a large protease complex that generates most of the peptide ligands of MHC class I molecules either in their final form or in the form of N-terminally extended precursors. Upon the stimulation of cells with IFN-gamma, three constitutively expressed subunits of the 20S proteasome are replaced by the inducible subunits LMP2 (low-molecular mass polypeptide 2), LMP7, and MECL-1 (multicatalytic endopeptidase complex-like-1) to form so-called immunoproteasomes. We show in this study that overexpression of these three subunits in triple transfectants led to a marked enhancement in the H-2Ld-restricted presentation of the immunodominant nonameric epitope NP118, which is derived from the nucleoprotein (NP) of lymphocytic choriomeningitis virus. Overexpression of the alpha and beta subunits of the IFN-gamma-inducible proteasome regulator PA28, in contrast, did not have a comparable effect. In vitro, immunoproteasomes as compared with constitutive proteasomes generated higher amounts of 11- and 12-mer fragments containing the NP118 epitope. These are likely to be cytosolic precursors of NP118, as a proline anchor residue in the second position of NP118 may interfere with TAP-mediated transport of the nonameric epitope itself. In conclusion, we provide evidence that up-regulation of the three inducible subunits, LMP2, LMP7, and MECL-1, can result in a marked improvement of Ag presentation and that, depending on the epitope, PA28 and immunoproteasomes may differentially affect Ag processing.

Additive effect of neutralizing antibody and antiviral drug treatment in preventing virus escape and persistence

Poorly cytopathic or noncytopathic viruses can escape immune surveillance and establish a chronic infection. Here we exploited the strategy of combining antiviral drug treatment with the induction of a neutralizing antibody response to avoid the appearance of neutralization-resistant virus variants. Despite the fact that H25 immunoglobulin transgenic mice infected with lymphocytic choriomeningitis virus mounted an early neutralizing antibody response, the virus escaped from neutralization and persisted. After ribavirin treatment of H25 transgenic mice, the appearance of neutralization-resistant virus was prevented and virus was cleared. Thus, the combination of virus-neutralizing antibodies and chemotherapy efficiently controlled the infection, whereas each defense line alone did not. Similar additive effects may be unexpectedly efficient and beneficial in humans after infections with persistent viruses such as hepatitis C virus and hepatitis B virus and possibly human immunodeficiency virus.

The selective proteasome inhibitors lactacystin and epoxomicin can be used to either up- or down-regulate antigen presentation at nontoxic doses

The complete inhibition of proteasome activities interferes with the production of most MHC class I peptide ligands as well as with cellular proliferation and survival. In this study we have investigated how partial and selective inhibition of the chymotrypsin-like activity of the proteasome by the proteasome inhibitors lactacystin or epoxomicin would affect Ag presentation. At 0.5-1 microM lactacystin, the presentation of the lymphocytic choriomeningitis virus-derived epitopes NP118 and GP33 and the mouse CMV epitope pp89-168 were reduced and were further diminished in a dose-dependent manner with increasing concentrations. Presentation of the lymphocytic choriomeningitis virus-derived epitope GP276, in contrast, was markedly enhanced at low, but abrogated at higher, concentrations of either lactacystin or epoxomicin. The inhibitor-mediated effects were thus epitope specific and did not correlate with the degradation rates of the involved viral proteins. Although neither apoptosis induction nor interference with cellular proliferation was observed at 0.5-1 microM lactacystin in vivo, this concentration was sufficient to alter the fragmentation of polypeptides by the 20S proteasome in vitro. Our results indicate that partial and selective inhibition of proteasome activity in vivo is a valid approach to modulate Ag presentation, with potential applications for the treatment of autoimmune diseases and the prevention of transplant rejection.

Retroviral vectors pseudotyped with lymphocytic choriomeningitis virus

Pseudotyping can improve retroviral vector stability and transduction efficiency. Here, we describe a novel pseudotype of murine leukemia virus packaged with lymphocytic choriomeningitis virus (LCMV). This pseudotype was stable during ultracentrifugation and infected several cell lines from different species. Moreover, LCMV glycoproteins were not cell toxic.

Secondary rearrangements and hypermutation generate sufficient B cell diversity to mount protective antiviral immunoglobulin responses

Variable (V) region gene replacement was recently implicated in B cell repertoire diversification, but the contribution of this mechanism to antibody responses is still unknown. To investigate the role of V gene replacements in the generation of antigen-specific antibodies, we analyzed antiviral immunoglobulin responses of "quasimonoclonal" (QM) mice. The B cells of QM mice are genetically committed to exclusively express the anti-(4-hydroxy-3-nitrophenyl) acetyl specificity. However, approximately 20% of the peripheral B cells of QM mice undergo secondary rearrangements and thereby potentially acquire new specificities. QM mice infected with vesicular stomatitis virus (VSV), lymphocytic choriomeningitis virus, or poliovirus mounted virus-specific neutralizing antibody responses. In general, kinetics of the antiviral immunoglobulin responses were delayed in QM mice; however, titers similar to control animals were eventually produced that were sufficient to protect against VSV-induced lethal disease. VSV neutralizing single-chain Fv fragments isolated from phage display libraries constructed from QM mice showed VH gene replacements and extensive hypermutation. Thus, our data demonstrate that secondary rearrangements and hypermutation can generate sufficient B cell diversity in QM mice to mount protective antiviral antibody responses, suggesting that these mechanisms might also contribute to the diversification of the B cell repertoire of normal mice.

In Vivo Selection of Neutralization-Resistant Virus Variants But No Evidence of B Cell Tolerance in Lymphocytic Choriomeningitis Virus Carrier Mice Expressing a Transgenic Virus-Neutralizing Antibody

B cell tolerance is maintained by active deletion and functional anergy of self-reactive B cells depending on the time, amount, and site of the self-antigen expression. To study B cell tolerance toward a transplacentally transmitted viral Ag, we crossed transgenic mice expressing the μ heavy and the κ light chain of the lymphocytic choriomeningitis virus (LCMV)-neutralizing mAb KL25 (HL25-transgenic mice) with persistently infected LCMV carrier mice. Although HL25-transgenic LCMV carrier mice exhibited the same high virus titers as nontransgenic LCMV carrier mice, no evidence for B cell tolerance was found. In contrast, enhanced LCMV-neutralizing Ab titers were measured that, however, did not clear the virus. Instead, LCMV isolates from different tissues turned out to be neutralization resistant Ab escape variants expressing different substitutions of amino acid Asn119 of the LCMV-glycoprotein 1 that displays the neutralizing B cell epitope. Virus variants with the same mutations were also selected in vitro in the presence of the transgenic mAb KL25 confirming that substitutions of Asn119 have been selected by LCMV-neutralizing Abs. Thus, despite abundant expression of viral neo-self-antigen in HL25-transgenic LCMV carrier mice, transgenic B cells expressing LCMV-neutralizing Abs were rather stimulated than tolerized and neutralization resistant Ab escape variants were selected in vivo.