Viral pathogenesis and resistance to defective interfering particles

Virus-like Particles: Measures and Biological Functions

Virus-like particles resemble infectious virus particles in size, shape, and molecular composition; however, they fail to productively infect host cells. Historically, the presence of virus-like particles has been inferred from total particle counts by microscopy, and infectious particle counts or plaque-forming-units (PFUs) by plaque assay; the resulting ratio of particles-to-PFUs is often greater than one, easily 10 or 100, indicating that most particles are non-infectious. Despite their inability to hijack cells for their reproduction, virus-like particles and the defective genomes they carry can exhibit a broad range of behaviors: interference with normal virus growth during co-infections, cell killing, and activation or inhibition of innate immune signaling. In addition, some virus-like particles become productive as their multiplicities of infection increase, a sign of cooperation between particles. Here, we review established and emerging methods to count virus-like particles and characterize their biological functions. We take a critical look at evidence for defective interfering virus genomes in natural and clinical isolates, and we review their potential as antiviral therapeutics. In short, we highlight an urgent need to better understand how virus-like genomes and particles interact with intact functional viruses during co-infection of their hosts, and their impacts on the transmission, severity, and persistence of virus-associated diseases.

Distinct Host-Dependent Pathogenic Mechanisms Leading to a Similar Clinical Anemia After Infection with Lymphocytic Choriomeningitis Virus

The Docile strain of lymphocytic choriomeningitis virus (LCMV) induces anemia in a number of inbred strains of mice, including C3HeB/FeJ and CBA/Ht animals. A difference in the kinetics of anemia and in compensatory reticulocytosis suggested that impaired erythropoiesis was the major pathogenic mechanism Involved in CBA/Ht mice, but not in C3HeB/FeJ mice. In both mouse strains an antierythrocyte autoantibody production that depended on the presence of functional CD4+ T lymphocytes was observed. Although depletion of T helper lymphocytes prevented anemia in C3HeB/FeJ mice, this treatment largely failed to inhibit the development of the disease in CBA/Ht animals. This observation indicated that the antierythrocyte autoimmune response induced by the infection was at least Partly responsible for the anemia of C3HeB/FeJ mice, but not of CBA/Ht mice. Erythrophagocytosis was enhanced in both mouse strains after LCMV infection, but did not appear to be a major cause of anemia. These data clearly indicate that similar disease profiles induced by the same virus in two different host strains can be the result of distinctly different mechanisms.

The Lymphocytic Choriomeningitis Virus Matrix Protein PPXY Late Domain Drives the Production of Defective Interfering Particles

Arenaviruses cause severe diseases in humans but establish asymptomatic, lifelong infections in rodent reservoirs. Persistently-infected rodents harbor high levels of defective interfering (DI) particles, which are thought to be important for establishing persistence and mitigating virus-induced cytopathic effect. Little is known about what drives the production of DI particles. We show that neither the PPXY late domain encoded within the lymphocytic choriomeningitis virus (LCMV) matrix protein nor a functional endosomal sorting complex transport (ESCRT) pathway is absolutely required for the generation of standard infectious virus particles. In contrast, DI particle release critically requires the PPXY late domain and is ESCRT-dependent. Additionally, the terminal tyrosine in the PPXY motif is reversibly phosphorylated and our findings indicate that this posttranslational modification may regulate DI particle formation. Thus we have uncovered a new role for the PPXY late domain and a possible mechanism for its regulation.

Arenaviruses cause severe and often fatal diseases in humans yet typically establish lifelong, asymptomatic infections in their rodent reservoirs. Several families of enveloped RNA viruses, including the arenaviruses, encode short amino acid motifs, called late domains, to hijack host proteins in the endosomal sorting complex required for transport (ESCRT) to drive the release of virus particles from the host cell’s outer membrane. Many late domain-containing viruses produce defective interfering (DI) particles in addition to standard, infectious virus. DI particles cannot self-replicate but interfere with the production of infectious virus and mitigate virus-induced cytopathic effect. Arenaviruses such as lymphocytic choriomeningitis virus (LCMV) generate high levels of DI particles, yet, the mechanism that drives their formation is not known. We show that LCMV’s only encoded late domain, PPXY, and a functional ESCRT pathway are critical for DI particle production, but in contrast, are not absolutely required for infectious virus production. We also demonstrate that the LCMV PPXY late domain is phosphorylated and that this modification may regulate DI particle production. In summary, we have discovered a new and unexpected role for a viral late domain in selectively driving the production of DI particles independently of standard infectious virus particles.

Original antigenic sin impairs cytotoxic T lymphocyte responses to viruses bearing variant epitopes

Some viruses, including human immunodeficiency virus (HIV) and hepatitis B virus (HBV) in humans, and lymphocytic choriomeningitis virus (LCMV) in mice, are initially controlled by cytotoxic T lymphocytes (CTLs), but may subsequently escape through mutation of the relevant T-cell epitope. Some of these mutations preserve the normal binding to major histocompatibility complex class I molecules, but present an altered surface to the T-cell antigen receptor. The exact role of these so-called altered peptide ligands in vivo is not clear. Here we report that mice primed with LCMV-WE strain respond to a subsequent infection by WE-derived CTL epitope variants with a CTL response directed against the initial epitope rather than against the new variant epitope. This phenomenon of 'original antigenic sin' was initially described in influenza and is an asymmetric pattern of protective antibody crossreactivity determined by exposure to previously existing strains, which may therefore extend to some CTL responses. Original antigenic sin by CTL leads to impaired clearance of variant viruses infecting the same individual and so may enhance the immune escape of mutant viruses evolving in an individual host.

Analysis of neutralizing antibody specificities of different strains of lymphocytic choriomeningitis virus with strain-specific immune sera

The present study was aimed at comparing specificities and cross-reactivities of immune mouse sera obtained from mice infected with a low (2 x 10(2) or 20 immunological infectious focus units (ifu) or high (2 x 10(6) ifu) dose of various strains of lymphocytic choriomeningitis virus (LCMV). Neutralization titres of the various antisera were determined by an infectious focus reduction assay. This assay was performed on MC57G and on Vero E6 cells using the commonly utilized laboratory strains of LCMV WE, Docile, Armstrong, Armstrong Clone 13, Pasteur, Traub and Aggressive. Experiments with immune sera revealed broad cross-neutralization, demonstrating a variable but close serological relationship between the various strains of LCMV.

Defective interfering particles and virus evolution

Almost all viruses produce replication-defective mutants that have complex effects on the growth and evolution of the virus in culture. These effects can be explained qualitatively by a simple mathematical model. However, the model shows that the quantitative effects of these mutants are intrinsically unpredictable.

Virally induced immunosuppression

A mouse model of virus-triggered, T-cell mediated acquired immunosuppression is analyzed. Lymphocytic choriomeningitis virus initially infects mostly macrophages and antigen-presenting cells; these are destroyed by lymphocytic choriomeningitis virus specific cytotoxic T cells resulting in immunosuppression. Similar immunopathological mechanisms may play a role in acquired immune deficiency syndrome.

Evidence for polyclonal B cell activation as the mechanism for LCMV-induced autoimmune hemolytic anemia

A docile substrain of lymphocytic choriomeningitis virus (LCMV) causes a persistent infection in adult C3HeB mice and induces a severe autoimmune hemolytic anemia (AIHA) which is maximal around three weeks post infection (PI). Evaluations of serum immunoglobulin levels of these mice demonstrated grossly elevated IgG2a levels along with increased IgG1 and IgG2b levels, suggesting that these animals also develop polyclonal B cell activation (PBA). Interestingly, LCMV-infected B10.BR mice did not demonstrate a marked hypogammaglobulinemia nor did they experience a severe hemolytic anemia. Although evaluations of the hematocrits indicated that these animals endure a mild anemia 21 days PI, a below normal reticulocyte count until day 18 PI suggests that there was a prolonged suppression in hematopoiesis. It is clear from RBC survival studies that there is not an accelerated rate of RBC elimination, as seen in infected C3H mice, demonstrating that the anemia in B10.BR mice is not due to a hemolytic process. These results imply a correlation between the development of PBA and AIHA, suggesting a cause and effect relationship.

Vaccination with a synthetic peptide modulates lymphocytic choriomeningitis virus-mediated immunopathology

Vaccination with a nucleopeptide (NP 118; amino acids 118 to 132) representing a cytotoxic T-cell epitope of lymphocytic choriomeningitis virus (LCMV) can modulate immunopathology. Immunization with NP 118 protected H-2d mice against intracerebral infection with the LCMV-ARMSTRONG isolate. However, when NP 118-primed H-2d mice were challenged intracerebrally with an intermediate dose (5 x 10(4) PFU) of the LCMV-DOCILE strain, all mice primed with NP 118 emulsified in incomplete Freund's adjuvant died, whereas unprimed mice survived. Correspondingly, peptide vaccination enhanced specifically the cytotoxic T-cell response, influencing the critical balance between T-cell response and virus spread.

Quantification of lymphocytic choriomeningitis virus with an immunological focus assay in 24- or 96-well plates

Titers of lymphocytic choriomeningitis virus (LCMV) were determined on adherent fibroblast cell lines in 24- or 96-well plates. After absorption of virus by cells and 48 h incubation under a methylcellulose overlay, cell monolayers were fixed with 4% formaldehyde in phosphate-buffered saline, permeabilized by incubation in 0.5% Triton X-100 in balanced salt solution and then stained with a monoclonal rat anti-LCMV and a peroxidase-labeled second stage antibody. The sensitivity of the assay is within a factor of 2-4 of conventional plaquing methods. The method also detects poorly or non-plaquing LCMV isolates, and therefore drastically reduces the need for titration of LCMV in mice. The method is quicker (2-3 days), as compared to conventional methods (4-6 days) and less expensive in terms of work and materials.

The presence of an anti-erythrocyte autoantibody in C3HeB/FeJ mice after lymphocytic choriomeningitis virus infection

Lymphocytic Choriomeningitis (LCM) virus, substrain Docile, causes a chronic infection in adult C3HeB/FeJ mice. The virus also induces a severe anemia which, unlike the viremia, eventually resolves. Initially, there is frank bone marrow deficit, but the anemia persists well beyond a strong erythroid compensatory response. An immune-mediated basis for the hemolytic anemia was suggested by its abrogation in cyclophosphamide-treated mice, as well as an abnormal number of spherocytes in the circulation. We now show by ELISA assay, using either anti-mouse Ig or RBC membrane ghosts as catching antigen, that unusually high quantities of antibodies can be eluted from the RBCs of virus-infected mice. Furthermore, the high transient antibody concentration correlates with the severity of the anemia. With no evidence for complement playing a role in the anemia, these data indicate that erythrophagocytosis (via macrophage FcRs) may be the mechanism for RBC elimination. The possibility of molecular mimicry (antibody cross-reactivity between LCM and RBC membrane epitopes) was considered but appeared unlikely since the RBC antibody eluates gave no signal in an LCM-specific ELISA (which showed an ever increasing serum titer of virus-specific antibody). Isotype determination of the RBC eluates revealed the following: IgG2a much greater than IgG1 greater than IgG2b greater than IgG3 greater than IgM. The precise role, if any, of LCM-virus induced polyclonal activation (most strikingly in the IgG2a subclass) has yet to be determined.

Evidence for immune-mediated destruction as mechanism for LCMV-induced anemia in persistently infected mice

A docile substrain of lymphocytic choriomeningitis virus (LCMV) causes a persistent infection in adult C3HeB mice and induces a severe anemia, which, unlike the viremia, eventually resolves. Measurements of red blood cell (RBC) survival rates demonstrated an increased rate of RBC clearance in these animals, indicating a hemolytic process for the anemia. Normal clearance rates of RBCs from infected mice transfused into control mice suggested that there was not an intrinsic defect in these cells. It also appeared that RBC destruction was immune-mediated, as cyclophosphamide treatments prevented the onset of anemia in infected mice, whereas adoptive transfer (AT) of immune splenocytes into immunocompromised mice reestablished the condition. The AT experiments also demonstrated that the onset of anemia correlated with the functional state of the immune cells. In addition, opsonization of RBCs was demonstrated by macrophage phagocytosis, and the appearance of opsonized RBCs corresponded with the course of the anemia. These findings support a hypothesis of RBC opsonization and subsequent phagocytosis by macrophages of the reticuloendothelial system as the mechanism for RBC destruction in LCMV-induced hemolytic anemia.

Effects of defective interfering viruses on virus replication and pathogenesis in vitro and in vivo

DI viruses and defective viruses generally are widespread in nature. Laboratory studies show that they can sometimes exert powerful disease-modulating effects (either attenuation or intensification of symptoms). Their role in nature remains largely unexplored, despite recent suggestive evidence for their importance in a number of systems.

Major histocompatibility complex-linked susceptibility or resistance to disease caused by a noncytopathic virus varies with the disease parameter evaluated

The influence of major transplantation antigens on susceptibility to T cell-mediated disease caused by infection with the noncytopathic virus lymphocytic choriomeningitis virus (LCMV) was evaluated in B10 H-2-congenic mice. Susceptibility to early T cell-mediated liver cell destruction (day 7-9) and early mortality (before day 12) was H-2Dq linked and correlated directly with early (day 6-8) and high cytotoxic T cell activity. In contrast, susceptibility to become an LCMV carrier, inability to rapidly clear virus, or tendency to develop late hepatitis (day 14-17) was linked to Dk and correlated with absence of early cytotoxic T cell activity. Thus, H-2D-regulated T cell-immune responses controlling both virus spread and immunopathology may directly determine the type and severity of disease. The results illustrate that susceptibility to disease caused by one virus may be linked to distinct MHC alleles dependent upon the disease parameter studied.

Enhanced virus replication and inhibition of lymphocytic choriomeningitis virus disease in anti-gamma interferon-treated mice

The role of gamma interferon (IFN-gamma) induced during a viral infection in the ability of the host to acquire antiviral immunity was studied in mice. They were injected subcutaneously daily with an ammonium sulfate-precipitated sheep anti-IFN-gamma antibody preparation able to neutralize 10(4) U of IFN-gamma. Specificity of the anti-IFN-gamma antiserum was demonstrated by absence of detectable activity against natural IFN-alpha and -beta. Controls were treated with a similarly prepared normal sheep serum. Treatment with the IFN-gamma-specific antibody preparation had no influence on the ability of mice to generate anti-vaccinia virus- or anti-vesicular stomatitis virus (VSV)-specific cytotoxic T-cell (CTL) responses or T helper-dependent immunoglobulin G responses to VSV. In contrast, treatment of mice with sheep anti-IFN-gamma impaired CTL responses against lymphocytic choriomeningitis (LCM) virus (LCMV, Aggressive isolate); in addition, under the experimental conditions used, it prevented lethal LCM. Cytotoxic T-cell activity measured in the spleens of anti-IFN-gamma-treated mice was comparable to that found in mice initially infected with a 100-fold-larger dose of LCMV. Evaluation of the effects of treatment on the kinetics of virus replication revealed that in both euthymic and athymic nude C57BL/6 mice, anti-IFN-gamma treatment led to an increase of virus titers up to 100-fold compared with control mice. Therefore, IFN-gamma may play a role in controlling viruses with tropism for lymphocytes and monocytes/macrophages, such as LCMV.

The combination of major histocompatibility complex (MHC) and non-MHC genes influences murine lymphocytic choriomeningitis virus pathogenesis

Resistance to the acute lethal disease caused by the docile strain of lymphocytic choriomeningitis (LCM) virus varies widely between different mouse strains. In order to study the inheritance of host influence on susceptibility to this strain of LCM virus, we crossed the F1 to the parent with the recessive disease phenotype. In all cases, susceptibility was dominant. In backcross progeny obtained from matings of parental strains differing in both major histocompatibility complex (MHC) and non-MHC (SWR; C3H), 90% of the challenged mice died, indicating that at least three loci controlled susceptibility to the disease. When the parental strains carried similar MHC haplotypes but dissimilar background genes (B10.BR; CBA), 78% of the backcross mice succumbed, indicating that at least two non-MHC loci influenced disease susceptibility. It is unlikely, however, that the same two non-MHC loci are critical in all genetic combinations, since F1 produced from two H-2 identical, resistant strains (B10.BR; C3H) were found to be fully susceptible. When congenic mice, differing only in the D-end of the MHC region, were analysed, 50% of the backcross animals died, indicating that one gene in the MHC region was important; segregation analysis comparing MHC serotype and disease outcome indicated the H-2D locus itself as the determining factor.

Altered replicase specificity is responsible for resistance to defective interfering particle interference of an Sdi- mutant of vesicular stomatitis virus

The in vitro resistance of an Sdi- mutant of vesicular stomatitis virus to interference by wild-type defective interfering (DI) particles was expressed quantitatively in a cell-free replication system derived from mutant-infected cells. Added wild-type DI particle templates were replicated very poorly by extracts of Sdi- mutant-infected cells. However, the addition of purified viral polymerase (a complex of L and NS proteins) from wild-type vesicular stomatitis virus allowed efficient replication of wild-type DI particle genomes in these cell extracts. Added wild-type NS protein alone did not complement DI particle genome replication in these cell extracts, but it did complement a defect in the in vitro transcriptional activity of Sdi- mutant virus. These results clearly implicate the vesicular stomatitis virus polymerase complex in the inability of Sdi- mutants to replicate DI particles and in the quantitative escape from DI particle interference in evolving virus populations.

Host cell-dependent homologous interference in lymphocytic choriomeningitis virus infection

The generation of virus progeny as well as transcription, translation, and replication of the viral small RNA (S-RNA), which codes for the nucleoprotein (NP) and the glycoprotein precursor (GPC), was followed in L and MDCK cells after infection with multiplicities (m.o.i.) ranging from 0.01 to 100. In L cells, the yields of both plaque-forming units and interfering particles varied inversely with the m.o.i. Northern blot analysis revealed that early after infection with high multiplicity NP-mRNA was present, but later few or no signals of any specificity were registered. After low m.o.i. the results were negative at 8 hr, but large quantities of mRNAs for NP and GPC as well as viral genomic S-RNA and genomic-sized complementary S-RNA had been synthesized at 48 hr. In MDCK cells, throughout the range of m.o.i. both entities attained lower levels and most were generated at m.o.i. one. The degree of hybridization correlated roughly with the quantity of infectious virus to which the cells had been exposed. In the cells of both lines the NP-mRNA corresponded to the synthesis of its translation product, but once produced, most of it appeared to be retained in the phosphorylated form. We assume that the homologous interference seen in L cells after infection with high m.o.i. results from a host-dependent inhibition of viral transcription and replication mediated by NP.

Molecular analysis of viral RNAs in mice persistently infected with lymphocytic choriomeningitis virus

Infection of newborn mice with lymphocytic choriomeningitis virus (LCMV) results in a lifelong persistent infection. Persistently infected animals continuously produce low levels of infectious virus and accumulate large amounts of intracellular viral nucleic acid (P. J. Southern, P. Blount, and M. B. A. Oldstone, Nature [London] 312:555-558, 1984). We have used gel electrophoresis and hybridization techniques to analyze viral RNAs that appear during the establishment and maintenance of a persistent LCMV infection in vivo to identify any role for defective and/or defective interfering RNAs. We have found a complex, heterogeneously sized population of viral RNAs in multiple independent tissues that is uniquely associated with persistent infections in vivo, but we have not yet established whether these RNAs have a causal or a consequential association with persistent infection by LCMV. Within the complex virus RNA population, full-length genomic L and S RNAs were readily detectable and represented the most abundant individual viral RNA species. RNAs apparently corresponding in size to the viral nucleoprotein and glycoprotein mRNAs could also be detected in these tissue RNA samples. The presence of glycoprotein mRNA indicates a potential mechanism of posttranscriptional regulation to account for the previously documented restriction in viral glycoprotein expression in persistently infected mice (M. B. A. Oldstone and M. J. Buchmeier, Nature (London) 300:360-362, 1982).

Meningeal macrophages reflect lymphocytic choriomeningitis virus pathogenic phenotypes

Intracerebral (i.c.) infection of adult mice with lymphocytic choriomeningitis (LCM) virus can result in acute lethal central nervous system (CNS) disease which is the result of the host's thymus-derived lymphocyte (T cell) response against the virus. Whether the specific effector function of the T cell is that of a cytotoxic cell (Tc) or a delayed-type hypersensitivity cell (Td) is still under debate. We assumed that if Td cells were important in pathogenesis then accessory cells in the brain (specifically, glass-adherent macrophages) might vary with the outcome of i.c. infection. We found that accumulation of macrophages in the brain (washed from meninges and skull cap) appeared to be independent of the severity of the infection (controlled by the mouse strain as well as the strain and dose of virus used). However, differentiation of macrophages was clearly linked to whether or not the infection caused rapid death. In mice that were destined to survive, macrophages became large, extensively vacuolated, and phagocytically active. In lethally-infected mice macrophages were small and had poor phagocytic abilities. At present this dichotomy could be viewed as either a cause or a consequence of disease outcome. However, the data are not inconsistent with the hypothesis that Td lymphocytes may be of primary importance in pathogenesis.

Characterization of virus-specific cytotoxic T cell clones from allogeneic bone marrow chimeras

We established several H-2-restricted lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T cell clones from spleens of virus-primed C57BL/6 or C57BL/10 (H-2b) and B10.BR (H-2k) mice and from allogeneic C57BL/10----B10.BR and B10.BR----C57BL/10 bone marrow chimeras. Two T cell clones of H-2b origin and restricted to H-2b, 3 of H-2k origin and restricted to H-2k were compared with two clones each derived from the two types of chimeras. Their surface phenotype was found to be Lyt-2+, L3/T4- and KJ16-133+ (2 of 9). Clones from chimeras expressed bone marrow donor H-2 and are restricted to the recipient H-2. H-2k-restricted clones were all specific for Kk whereas all H-2b-restricted clones were specific for Db. These restriction specificities could be further defined by the blocking activity of various monoclonal anti-H-2 antibodies. Interestingly the anti-H-2Db antibodies blocked the restricted virus-specific killing activity of the clones derived B10.BR----C57BL/10 chimeras much more effectively than the activity of the clones derived from conventional H-2b mice. The various clones differed with respect to their fine specificity for LCMV strains. The 3 clones of conventional B10.BR origin only recognized LCMV-WE but not LCMV-Armstrong, Aggressive or Docile; H-2b-restricted conventional clones recognized target cells infected with all LCMV strains except LCMV-UBC-Docile; the T cell clones from the bone marrow chimeras recognized with one exception all LCMV strains tested.

Continuing coevolution of virus and defective interfering particles and of viral genome sequences during undiluted passages: virus mutants exhibiting nearly complete resistance to formerly dominant defective interfering particles

We quantitatively analyzed the interference interactions between defective interfering (DI) particles and mutants of cloned vesicular stomatitis virus passaged undiluted hundreds of times in BHK-21 cells. DI particles which predominated at different times in these serial passages always interfered most strongly (and very efficiently) with virus isolated a number of passages before the isolation of the DI particles. Virus isolated at the same passage level as the predominant DI particles usually exhibited severalfold resistance to these DI particles. Virus mutants (Sdi- mutants) isolated during subsequent passages always showed increasing resistance to these DI particles, followed by decreasing resistance as new DI particles arose to predominate and exert their own selective pressures on the virus mutant population. It appears that such coevolution of virus and DI particle populations proceeds indefinitely through multiple cycles of selection of virus mutants resistant to a certain DI particle (or DI particle class), followed by mutants resistant to a newly predominant DI particle, etc. At the peak of resistance, virus mutants were isolated which were essentially completely resistant to a particular DI particle; i.e., they were several hundred thousand-fold resistant, and they formed plaques of normal size and numbers in the presence of extremely high multiplicities of the DI particle. However, they were sensitive to interference by other DI particles. Recurring population interactions of this kind can promote rapid virus evolution. Complete sequencing of the N (nucleocapsid) and NS (polymerase associated) genes of numerous Sdi- mutants collected at passage intervals showed very few changes in the NS protein, but the N gene gradually accumulated a series of stable nucleotide and amino acid substitutions, some of which correlated with extensive changes in the Sdi- phenotype. Likewise, the 5' termini (and their complementary plus-strand 3' termini) continued to accumulate extensive base substitutions which were strikingly confined to the first 47 nucleotides. We also observed addition and deletion mutations in noncoding regions of the viral genome at a level suggesting that they probably occur at a high frequency throughout the genome, but usually with lethal or debilitating consequences when they occur in coding regions.

Immunity to lymphocytic choriomeningitis virus in B cell-depleted mice: evidence for B cell and antibody-independent protection by memory T cells

Immunity against lymphocytic choriomeningitis virus (LCMV) in anti-IgM-treated B cell-depleted mice was evaluated. We found that the following immune phenomena were independent of antibodies: the generation of virus-specific cytotoxic T cells; the footpad swelling response against locally injected LCMV; natural killer cell activity basic levels or after LCMV or poly(I) X poly(C) stimulation; immunopathologically mediated LCM after primary intracerebral inoculation; immunological memory in LCMV-immune mice assessed by immune protection against LCM after intracerebrally injected virus or as resistance against the local footpad swelling response to LCMV. This study demonstrates that humoral immunity plays no crucial role in immune protection and immunopathology in murine LCMV infection and suggests that protective memory T cell function is B cell and antibody independent.

Molecular basis for interference of defective interfering particles of pseudorabies virus with replication of standard virus

Serial passage of pseudorabies virus (PrV) at high multiplicity yields defective interfering particles (DIPs), but the sharp cyclical increases and decreases in titer of infectious virus that are observed upon continued passage at high multiplicity of most DIPs of other viruses are not observed with DIPs of PrV (T. Ben-Porat and A. S. Kaplan, Virology 72:471-479). We have studied the dynamics of the interactions of the virions present in a population of DIPs to assess the cis functions for which the genomes of the DIPs are enriched. The defective genomes present in one population of DIPs, [PrV(1)42], replicate preferentially over the nondefective genomes present in that virion population at early stages of infection, indicating that the DIP DNA is enriched for sequences that can serve as origins of replication at early stages of infection. This replicative advantage of the DIP DNA is transient and disappears at later stages of infection. The defective DNA does not appear to be encapsidated preferentially over the nondefective DNA present in this virion population, which might indicate that it is not enriched for cleavage-encapsidation sites. However, the nondefective DNA in the DIP virion population has become modified and has acquired reiterations of sequences originating from the end of the unique long (UL) region of the genome. Furthermore, both the infectious and defective genomes present in the DIP population compete for encapsidation more effectively than do the genomes of standard PrV. These results indicate that the defective genomes in the population of virions studied are enriched not only for an origin of replication but probably also for sequences necessary for efficient cleavage-encapsidation. Furthermore, the nondefective genomes present in this population of DIPs have also been modified and have acquired the ability to compete with the defective genomes for cleavage-encapsidation.

Induction or prevention of immunopathological disease by cloned cytotoxic T cell lines specific for lymphocytic choriomeningitis virus

Cloned lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T lymphocyte (CTL) lines were prepared from spleens of 129/J (H-2b) mice immunized 7-9 months earlier with LCMV (UBC strain), or of C57BL/10J (H-2b) mice immunized 4 to 5 weeks earlier with LCMV (Armstrong strain). One uncloned and 3 cloned cytotoxic T cell lines were assessed for their respective abilities to produce, or protect against, fatal disease upon transfer to appropriate recipients or to induce specific footpad-swelling reaction. The effects of all lines were essentially identical. In recipient mice acutely infected with LCMV and immunosuppressed either by irradiation (750-990 rds) or treatment with cyclophosphamide, cloned T cells administered intracerebrally (i.c.) caused a convulsive disease and death within 1-4 days. No disease was produced when the same CTL were transferred to uninfected recipients or when they had been frozen and thawed prior to transfer to infected recipients. When admixed with 500 plaque-forming units of LCMV and transferred i.c. to immunocompetent H-2b mice, the T cell clones prevented overt disease. Allogeneic (H-2k) recipients of this same admixture all developed typical LCM disease as did H-2b recipients of the admixture after T cells had been frozen and thawed. Inoculation of cloned CTL into preinfected footpads induced a specific footpad-swelling reaction, which reached maximum levels after about 36 h. Irradiated and infected recipients of cloned LCMV-specific T cells showed the footpad-swelling reaction only when they had been reconstituted with bone marrow cells. In contrast, cloned T cells induced LCM disease in i.c. infected and irradiated mice independent of bone marrow reconstitution. These findings indicate that both fatal LCMV-induced neurologic disease and protection against it are mediated directly by virus-specific CTL.

Susceptibility to lymphocytic choriomeningitis virus isolates correlates directly with early and high cytotoxic T cell activity, as well as with footpad swelling reaction, and all three are regulated by H-2D

The lymphocytic choriomeningitis virus (LCMV) isolates Docile (D) and Aggressive (A) of Pfau et al. were studied in various strains of mice. Disease susceptibility, assessed as mortality and time to death to LCMV-D or -A varied greatly amongst mouse strains, and all four possible susceptibility patterns were observed: susceptibility to both (e.g. SWR/J), resistance to both (e.g. DBA/2), susceptibility to A but resistance to D (C57BL/6), or vice versa (CBA/J). Irrespective of the virus isolate or the mouse strain tested, susceptibility correlated with both early and high cytotoxic T cell activity found in spleens or leptomeningeal infiltrates, and with early and high primary footpad swelling reaction after local infection. C57BL/6 mice infected with A or SWR/J infected with A or with D showed, in both test systems, early and high activities; in contrast, DBA/2 mice infected with either D or A, and C57BL/6 infected with D showed no or only slow and low responses in both tests. Early and high LCMV-specific cytotoxic T cell activity, and the rapidity and extent of the primary footpad reaction directly correlated with susceptibility to LCM and all were dominantly regulated by H-2D.

Vesicular stomatitis virus mutants resistant to defective-interfering particles accumulate stable 5'-terminal and fewer 3'-terminal mutations in a stepwise manner

We have studied the evolution of sequences which include the RNA polymerase binding sites at the 5' and 3' termini of vesicular stomatitis virus mutants (Sdi-) resistant to defective-interfering particles. We observed a striking stepwise accumulation of stable base substitutions within the area of replication initiation at the 5'-terminal 54 nucleotides of Sdi- mutants isolated at intervals from persistent infections and undiluted lytic passage series. Fewer mutations accumulated in the region of transcription initiation at the 3' end and in those portions of the N and L protein coding cistrons examined. The termini changes are not strictly required to obtain the Sdi- phenotype. However, it is possible that they represent stepwise compensatory changes to accommodate Sdi- mutations affecting viral replication or encapsidation gene products or both. These results have important implications for RNA virus genome evolution.

Congenital retinitis in the rat following maternal exposure to lymphocytic choriomeningitis virus

A combined clinical and histopathological study of the eyes of the offspring of females inoculated with LCMV shows that about two-thirds of the pups develop some degree of retinal inflammation. This may range from a mild, subclinical reaction to an overt retinitis characterized clinically by demonstrable inflammatory and degenerative changes. Histopathologically, the latter condition presents the picture of an inflammatory reaction with extensive loss of photoreceptors and retinal neurons in general, macrophagic invasion, mild microcystoid degeneration and total or subtotal retinal detachment. This vertically-transmitted disease does not show the relentless progression and hemorrhagic tendency characteristic of the retinitis which occurs after direct viral inoculation. However, in the most severe cases the final outcome is the same, namely severe retinal impairment subsequent to widespread loss of photoreceptors. In the presence of consistently negative virological data, the hypothesis is proposed that this retinitis could be the result of a vertically-transmitted autoimmune disease.

Defective interfering particles of mouse hepatitis virus

After six to eight serial undiluted passages of mouse hepatitis virus (JHM strain) in DBT cell culture, a decrease in the yield of infectious virus occurred, and with further passages fluctuating yields of infectious virus were observed. The serially passaged virus interfered with the multiplication of the standard JHM virus, but not with vesicular stomatitis virus. After sucrose equilibrium centrifugation of high passage virus, a single peak contained both infectious virus and interfering activity. This virus population resembled the original JHM virus in its structural proteins, but it contained an increased proportion of a protein with a molecular weight of 65 X 10(3). Genomic RNA from standard JHM virus contained a single species of RNA with a molecular weight of 5.4 X 10(6). After five undiluted passages, however, the virion population contained two RNA species with molecular weights of 5.4 X 10(6) and 5.2 X 10(6). RNase T1 resistant oligonucleotide finger-printing of these RNAs showed that the lower molecular weight RNA had lost several oligonucleotide spots that were present in the genomic RNA of the standard JHM virus. After several serial diluted passages of passage 10 virus, a single virus population was obtained which again had only standard virus RNA with a molecular weight of 5.4 X 10(6) and lacked interfering activity. These results indicated that defective interfering particles were generated by serial undiluted passages of JHM virus.

A replication-efficient mutant of West Nile virus is insensitive to DI particle interference

A previous report described the isolation of a mutant of West Nile virus (WNV) from culture fluid obtained from persistently infected genetically resistant C3H/RV mouse cells that replicates significantly more efficiently in cultures of C3H/RV cells than does the parental virus. This replication-efficient mutant, designated RE-WNV, has now been found to be insensitive to interference by WNV defective interfering (DI) particles. This characteristic was demonstrated by several means. The RE-WNV mutant was able to superinfect persistently infected cultures that were no longer producing detectable parental virus, while the parental virus was not. Good yields of the mutant virus were produced during six serial undiluted passages of RE-WNV in both resistant C3H/RV and congenic susceptible C3H/HE cells. In contrast, during passage of parental virus in C3H/RV cells, progeny virus could not be detected after the third passage, due to an enhanced interference by WNV DI particles with standard virus replication in these cells. The RE-WNV was also insensitive to interference by a pool of parental virus enriched for DI particles. Analysis of the mutant genome by oligonucleotide fingerprinting indicated that the genome RNA of the mutant differs by two unique spots from the parental RNA. The relevance of this mutant to the eventual understanding of the mechanism by which C3H/RV and C3H/HE cells manifest their flavivirus-specific difference in the efficiency of progeny virus production is discussed.

Lymphocytic choriomeningitis virus killer T cells are lethal only in weakly disseminated murine infections

Two types of lymphocytic choriomeningitis (LCM) viruses were studied which, upon intracerebral injection into adult C3H mice, provoked either (a) acute fatal central nervous system (CNS) disease or (b) life-long persistent infection. Both virus types, (a) aggressive and (b) docile, had been found to induce LCM-specific lymphocytes with comparable in vitro lytic activity (11). Because the requirement for T cells in the development of adult LCM disease has been extensively documented, we sought other reasons for the lack of acute disease in mice infected with docile virus. A striking correlation was found between the outcome of the infection and spread of virus to visceral organs. Adoptive transfer experiments showed that a 300-plaque forming unit inoculum of docile virus induced a population of T cells in donor mice fully capable of causing CNS disease in identically infected recipients. This disease causing ability was lost if the interaction was delayed beyond 3 d after infection of the recipients, but could be preserved by lowering the size of the viral inoculum in the recipients. Furthermore, without adoptive transfer, very low intracerebral doses of docile virus (which mimicked the normally slow spread of aggressive virus) were lethal. On the other hand, very high doses of aggressive virus, which mimicked the normally rapid spread of docile virus, did not induce fatal CNS disease. The results suggest that rapid dissemination of the LCM infection creates multiple target organs which divert the focused lethal T cell attack on the brain.

Cytotoxic T cells are induced in mice infected with lymphocytic choriomeningitis virus strains of markedly different pathogenicities

The ability of two lymphocytic choriomeningitis virus substrains to induce cytotoxic T-lymphocyte (CTL) responses in intracerebrally infected mice was examined. One strain, designated A (aggressive), provoked a convulsive type of death in 100% of the mice within 8 to 9 days, whereas the other strain, designated D (docile), killed less than 10% of the mice during 28-day observation periods. CTL activity was assessed by the capacity of partially purified splenocytes to lyse 51Cr-labeled L-cell targets infected with either type of lymphocytic choriomeningitis substrain. The CTL population was identified by its sensitivity to anti-Thy-1 serum and its inability to lyse uninfected target cells or infected target cells with which it differed at the level of antigens controlled by the major histocompatibility gene complex. A strong CTL response developed in mice infected with either lymphocytic choriomeningitis substrain, although the activity provoked by substrain D was somewhat less than that seen after substrain A infection. Peak CTL activities induced by both strains occurred at about the same time. Even though docile virus replicated more extensively in the brain than did aggressive virus and fluorescent antibody staining revealed similar distributions of viral antigen, no inflammatory response was noted in the brains of mice infected with docile virus. These results are discussed with regard to the role of CTLs in mediating classic central nervous system pathology.

Rapid evolution of RNA genomes

RNA viruses show high mutation frequencies partly because of a lack of the proofreading enzymes that assure fidelity of DNA replication. This high mutation frequency is coupled with high rates of replication reflected in rates of RNA genome evolution which can be more than a millionfold greater than the rates of the DNA chromosome evolution of their hosts. There are some disease implications for the DNA-based biosphere of this rapidly evolving RNA biosphere.

Defective interfering particles of Sindbis virus do not interfere with the homologous virus obtained from persistently infected BHK cells but do interfere with Semliki Forest virus

Defective interfering particles derived from wild-type Sindbis virus no longer interfere with the infectious virus cloned from BHK cells persistently infected with Sindbis virus for 16 months. These particles do interfere with the replication of Semliki Forest virus.