Identification of lactate dehydrogenase-elevating virus as the etiological agent of genetically restricted, age-dependent polioencephalomyelitis of mice

Ectopic expression of murine CD163 enables cell-culture isolation of lactate dehydrogenase-elevating virus 63 years after its discovery

IMPORTANCE

Mouse models of viral infection play an especially large role in virology. In 1960, a mouse virus, lactate dehydrogenase-elevating virus (LDV), was discovered and found to have the peculiar ability to evade clearance by the immune system, enabling it to persistently infect an individual mouse for its entire lifespan without causing overt disease. However, researchers were unable to grow LDV in culture, ultimately resulting in the demise of this system as a model of failed immunity. We solve this problem by identifying the cell-surface molecule CD163 as the critical missing component in cell-culture systems, enabling the growth of LDV in immortalized cell lines for the first time. This advance creates abundant opportunities for further characterizing LDV in order to study both failed immunity and the family of viruses to which LDV belongs, Arteriviridae (aka, arteriviruses).

Proceedings of the 2022 National Toxicology Program Satellite Symposium

The 2022 annual National Toxicology Program Satellite Symposium, entitled "Pathology Potpourri," was held in Austin, Texas at the Society of Toxicologic Pathology's 40th annual meeting during a half-day session on Sunday, June 19. The goal of this symposium was to present and discuss challenging diagnostic pathology and/or nomenclature issues. This article presents summaries of the speakers' talks along with select images that were used by the audience for voting and discussion. Various lesions and topics covered during the symposium included induced and spontaneous neoplastic and nonneoplastic lesions in the mouse lung, spontaneous lesions in the reproductive tract of a female cynomolgus macaque, induced vascular lesions in a mouse asthma model and interesting case studies in a rhesus macaque, dog and genetically engineered mouse model.

Degenerative Myelopathy and Neuropathy in NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) Mice Caused by Lactate Dehydrogenase-Elevating Virus (LDV)

Following implantation of patient-derived xenograft (PDX) breast carcinomas from three separate individuals, 33/51 female NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ (NSG) mice presented with progressive, unilateral to bilateral, ascending hindlimb paresis to paralysis. Mice were mildly dehydrated, in thin to poor body condition, with reduced to absent hindlimb withdrawal reflex and deep pain sensation. Microscopically, there was variable axonal swelling, vacuolation, and dilation of myelin sheaths within the ventral spinal cord and spinal nerve roots of the thoracolumbar and sacral spinal cord, as well as within corresponding sciatic nerves. Results of PCR screening of PDX samples obtained at necropsy and pooled environmental swabs from the racks housing affected animals were positive for lactate dehydrogenase-elevating virus (LDV). LDV is transmitted through animal-animal contact or commonly as a contaminant of biologic materials of mouse origin. Infection is associated with progressive degenerative myelopathy and neuropathy in strains of mice harboring endogenous retrovirus (AKR, C58), or in immunosuppressed strains (NOD-SCID, Foxn1^nu), and can interfere with normal immune responses and alter engraftment and growth of xenograft tumors in immunosuppressed mice. This is the first reported series of LDV-induced poliomyelitis in NSG mice and should be recognized as a potentially significant confounder to biomedical studies utilizing immunodeficient xenograft models.

Poliomyelitis in MuLV-infected ICR-SCID mice after injection of basement membrane matrix contaminated with lactate dehydrogenase-elevating virus

The arterivirus lactate dehydrogenase-elevating virus (LDV) causes life-long viremia in mice. Although LDV infection generally does not cause disease, infected mice that are homozygous for the Fv1(n) allele are prone to develop poliomyelitis when immunosuppressed, a condition known as age-dependent poliomyelitis. The development of age-dependent poliomyelitis requires coinfection with endogenous murine leukemia virus. Even though LDV is a common contaminant of transplantable tumors, clinical signs of poliomyelitis after inadvertent exposure to LDV have not been described in recent literature. In addition, LDV-induced poliomyelitis has not been reported in SCID or ICR mice. Here we describe the occurrence of poliomyelitis in ICR-SCID mice resulting from injection of LDV-contaminated basement membrane matrix. After exposure to LDV, a subset of mice presented with clinical signs including paresis, which was associated with atrophy of the hindlimb musculature, and tachypnea; in addition, some mice died suddenly with or without premonitory signs. Mice presenting within the first 6 mo after infection had regions of spongiosis, neuronal necrosis and astrocytosis of the ventral spinal cord, and less commonly, brainstem. Axonal degeneration of ventral roots prevailed in more chronically infected mice. LDV was identified by RT-PCR in 12 of 15 mice with typical neuropathology; positive antiLDV immunolabeling was identified in all PCR-positive animals (n = 7) tested. Three of 8 mice with neuropathology but no clinical signs were LDV negative by RT-PCR. RT-PCR yielded murine leukemia virus in spinal cords of all mice tested, regardless of clinical presentation or neuropathology.

The lactate dehydrogenase-elevating virus capsid protein is a nuclear-cytoplasmic protein

Arteriviruses replicate in the cytoplasm and do not require the nucleus function for virus multiplication in vitro. However, nucleocapsid (N) protein of two arteriviruses, porcine reproductive respiratory syndrome virus and equine arteritis virus, has been observed to localize in the nucleus and nucleolus of virus-infected and N-gene-transfected cells in addition to their normal cytoplasmic distribution. In the present study, the N protein of lactate dehydrogenase-elevating virus (LDV) of mice was examined for nuclear localization. The subcellular localization of LDV-N was determined by tagging N with enhanced green fluorescence protein (EGFP) at the N- and C-terminus. Both N-EGFP and EGFP-N fusion proteins localized to the nucleus and nucleolus of gene-transfected cells. Labeled N also accumulated in the perinuclear region, the site of virus replication. The LDV-N sequence contains a putative ‘pat4’-type nuclear localization signal (NLS) consisting of 38-KKKK. To determine its functional significance, a series of deletion constructs of N were generated and individually expressed in cells. The results showed that the ‘pat4’ NLS was essential for nuclear translocation. In addition, the LDV-N interacted with the importin-α and -β proteins, suggesting that the LDV-N nuclear localization may occur via the importin-mediated nuclear transport pathway. These results provide further evidence for the nuclear localization of N as a common feature within the arteriviruses.

Increased efficacy of the immunoglobulin G2a subclass in antibody-mediated protection against lactate dehydrogenase-elevating virus-induced polioencephalomyelitis revealed with switch mutants

Immunoglobulin G1 (IgG1), IgG2a, and IgG2b switch variants were derived from an IgG3 monoclonal antibody directed against the VP3 envelope glycoprotein of lactate dehydrogenase-elevating virus (LDV). Among the four antibodies, IgG2a delayed the onset and progression of LDV-induced polioencephalomyelitis more than did the other subclasses. This suggests that the IgG2a predominance observed in many IgG antibody responses elicited by live viruses could, at least under some circumstances, correspond to the selection of the best protection for the infected host.

Age-dependent poliomyelitis in mice is associated with respiratory failure and viral replication in the central nervous system and lung

Age-dependent poliomyelitis (ADPM) is a virally induced neuroparalytic disease of mice and a model for amyotrophic lateral sclerosis (ALS). ADPM is triggered in genetically susceptible mice by immunosuppression and infection with lactate dehydrogenase-elevating virus (LDV). Both ADPM and ALS are characterized by progressive degeneration of anterior horn motor neurons, and death in ALS is usually associated with respiratory failure. To assess respiratory function in ADPM, we investigated ventilation in conscious control and LDV-infected C58/J mice breathing air and then 6.5% CO(2) in O(2). Three days after LDV infection, ventilation in response to CO(2) was half of that compared to the uninfected state, but become normalized by 10 days. Administration of cyclophosphamide alone (200 mg/kg, ip), an immunosuppressant, had no effect on ventilation. Induction of ADPM by concomitant administration of LDV to cyclophosphamide-treated mice resulted in altered gait, hindlimb paralysis, wasting, decreased metabolism, and decreased body temperature by 4 degrees C relative to controls. Compared to baseline values, mice with ADPM had decreased tidal volume and ventilation while breathing air, and while exposed to the CO(2) challenge they were unable to increase tidal volume, frequency of breathing, or ventilation. Using in situ hybridization, LDV replication was noted within the spinal cord, brain, and lung, but not in the diaphragm. Thus, respiratory failure is a contributory mechanism leading to death in ADPM and is associated with LDV replication in the CNS and lung. This animal model may be useful to investigate physiological and molecular mechanisms associated with the development of respiratory failure in neurodegenerative diseases.

The neutralization epitope of lactate dehydrogenase-elevating virus is located on the short ectodomain of the primary envelope glycoprotein

We have measured by indirect ELISA the binding of neutralizing and non-neutralizing anti-lactate dehydrogenase-elevating virus (LDV) polyclonal and monoclonal antibodies to synthetic peptides representing unmodified hydrophilic segments of LDV proteins. Using this method a single neutralization epitope has been shown to be located in the very short (about 30 amino acid long) ectodomain of the primary envelope glycoprotein, VP-3P, encoded by ORF 5. Although the neutralization epitopes of neuropathogenic and non-neuropathogenic LDVs differ slightly in amino acid sequences, the neutralizing antibodies bind strongly to the epitopes of both groups of viruses. However, the neutralization epitopes of neuropathogenic and non-neuropathogenic LDVs are associated with different numbers of polylactosaminoglycan chains (1 and 3, respectively) which may affect the binding of neutralizing antibodies to the virions of these LDVs. The ELISA using synthetic peptides containing the neutralization epitope provides a novel, rapid, sensitive, and inexpensive method for quantitating LDV neutralizing antibodies in infected mice.

Detection of lactate dehydrogenase-elevating virus in transplantable mouse tumors by biological assay and RT-PCR assays and its removal from the tumor cell

It is known that lactate dehydrogenase-elevating virus (LDV) of mice is a common contaminant of transplantable tumors of both murine and human origin. It is imperative that tumors that are maintained by transplantation in mice are examined for LDV and freed of the virus, when present, before use in experimental studies, because an LDV infection of mice exerts considerable effects on lymphoid cell populations and cytokine production and other effects. Methods for LDV detection are described using a biological assay and reverse transcription (RT)-polymerase chain reaction (PCR) technology and their application is illustrated. A differential RT-PCR method that distinguishes between three quasispecies of LDV is also described and applied to an examination of LDVs isolated from a number of different tumors. Each of the LDV isolates was found to contain at least two different quasispecies, generally in different concentrations.

Coexistence in lactate dehydrogenase-elevating virus pools of variants that differ in neuropathogenicity and ability to establish a persistent infection

Neuropathogenic isolates of lactate dehydrogenase-elevating virus (LDV) differ from nonneuropathogenic isolates in their unique ability to infect anterior horn neurons of immunosuppressed C58 and AKR mice and cause paralytic disease (age-dependent poliomyelitis [ADPM]). However, we and others have found that neuropathogenic LDVs fail to retain their neuropathogenicity during persistent infections of both ADPM-susceptible and nonsusceptible mice. On the basis of a segment in open reading frame 2 that differs about 60% between the neuropathogenic LDV-C and the nonneuropathogenic LDV-P, we have developed a reverse transcription-PCR assay that distinguishes between the genomes of the two LDVs and detects as little as 10 50% infectious doses (ID50) of LDV. With this assay, we found that LDV-P and LDV-C coexist in most available pools of LDV-C and LDV-P. For example, various plasma pools of 10(9.5) ID50 of LDV-C/ml contained about 10(5) ID50 of LDV-P/ml. Injection of such an LDV-C pool into mice of various strains resulted in the rapid displacement in the circulation of LDV-C by LDV-P as the predominant LDV, but LDV-C also persisted in the mice at a low level along with LDV-P. We have freed LDV-C of LDV-P by endpoint dilution (LDV-C-EPD). LDV-C-EPD infected mice as efficiently as did LDV-P, but its level of viremia during the persistent phase was only 1/10,000 that observed for LDV-P. LDV-permissive macrophages accumulated and supported the efficient replication of superinfecting LDV-P. Therefore, although neuropathogenic LDVs possess the unique ability to infect anterior horn neurons of ADPM-susceptible mice, they exhibit a reduced ability to establish a persistent infection in peripheral tissues of mice regardless of the strain. The specific suppression of LDV-C replication in persistently infected mice is probably due in part to a more efficient neutralization of LDV-C than LDV-P by antibodies to the primary envelope glycoprotein, VP-3P. Both neuropathogenicity and the higher sensitivity to antibody neutralization correlated with the absence of two of three N-linked polylactosaminoglycan chains on the ca. 30-amino-acid ectodomain of VP-3P, which seems to carry the neutralization epitope(s) and forms part of the virus receptor attachment site.

Differential glycosylation of the ectodomain of the primary envelope glycoprotein of two strains of lactate dehydrogenase-elevating virus that differ in neuropathogenicity

ORF 5 encoding the primary envelope glycoprotein, VP-3P, of a highly neuropathogenic isolate of lactate dehydrogenase-elevating virus (LDV-v) has been sequenced. It exhibits 92% nucleotide identity with the ORF 5 of an LDV isolate that lacks neuropathogenicity, LDV-P, and the amino acid identities of the predicted VP-3Ps of the two strains is 90%. Most striking, however, is the absence in the ectodomain of LDV-v VP-3P of two out of three potential N-glycosylation sites present in the ectodomain of VP-3P of LDV-P. The ectodomain of VP-3P has been implicated to play an important role in host receptor interaction. VP-3P of another neuropathogenic LDV strain, LDV-C, lacks the same two N-glycosylation sites (Godeny et al., 1993). In vitro transcription/translation of the ORFs 5 of LDV-P and LDV-v indicated that all three N-glycosylation sites in the ectodomain of LDV-P VP-3P became glycosylated when synthesized in the presence of microsomal membranes, whereas the glycosylation of the ORF 5 proteins of LDV-v and LDV-C was consistent with glycosylation at a single site. No other biological differences between the neuropathogenic and non-neuropathogenic strains have been detected. They replicate with equal efficiency in mice and in primary macrophage cultures.

Cytokine regulation of lactate dehydrogenase-elevating virus: inhibition of viral replication by interferon-gamma

The mechanisms which regulate the replication of lactate dehydrogenase-elevating virus (LDV), a persistent murine model virus which infects macrophages, are unclear. For this study, the effects of murine recombinant interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on LDV replication were examined. LDV permissiveness was reduced in macrophages obtained from uninfected mice treated with IFN-gamma prior to cell harvest and in vitro LDV infection. Virus inhibition by IFN-gamma was also observed when neonatal LDV-infected mice were injected with this cytokine prior to macrophage harvest and analysis of LDV replication-positive cells. Persistently LDV-infected mice demonstrated an increase in viremia levels following treatment with TNF-alpha. Neither IFN-gamma nor TNF-alpha had any direct in vitro effect on LDV replication in cultured macrophages, suggesting that the actions of these cytokines required secondary or accessory in vivo events. These results provide evidence for cytokine-mediated regulation of LDV infection and support a role for the immune system in the LDV-host relationship.

Comparison of the ability of lactate dehydrogenase-elevating virus and its virion RNA to infect murine leukemia virus-infected or -uninfected cell lines

Lactate dehydrogenase-elevating virus (LDV) has a strict species specificity. Cells or cell lines other than a particular subset of mouse primary macrophages which can support LDV replication in vitro have not been identified. LDV induces neurological disorders in old C58 or AKR strains, in which the involvement of multiple copies of the endogenous N-tropic murine leukemia virus (MuLV) genome and the Fv-1 locus of the mouse has been implicated. Our previous studies have demonstrated that LDV could infect and replicate in cell lines of the mouse or other species in vitro when they were infected with MuLV. The significance of and the precise mechanism underlying this phenomenon, however, remain unclear. We demonstrated in this study the efficient infection and replication of the virus in vitro by inoculation of its RNA mixed with liposome. No significant difference either in the efficiency of RNA transfection or in the ability to support its replication was observed among the various species' cell lines examined. In addition, by RNA transfection the virus replicated with equal efficiency in MuLV-infected and -uninfected cells or in macrophages derived from mice irrespective of their age. In contrast, the pattern of the infection by virus particles was quite different; LDV replication was observed only in macrophages (particularly from newborn mice) and MuLV-infected cells. By using various LDV isolates, it was demonstrated that the capability of replication between neurovirulent, LDV type C, and the other avirulent strains was almost the same in mouse cell lines when their RNA was introduced into the cells. Higher infectivity of LDV-C to MuLV-infected cells may be due to its efficient incorporation of the particles into MuLV-infected cells.

Lactate dehydrogenase-elevating virus, equine arteritis virus, and simian hemorrhagic fever virus: a new group of positive-strand RNA viruses

The last comprehensive reviews of nonarbotogaviruses included discussions on pestiviruses, rubella virus, lactate dehydrogenase-elevating virus (LDV), equine arteritis virus (EAV), simian hemorrhagic fever virus (SHFV), cell fusion agent, and nonarboflaviviruses. The inclusion of all these viruses in the family Togaviridae was largely based on the similarities in morphological and physical–chemical properties of these viruses, and in the sizes and polarities of their genomes. In the intervening years, considerable new information on the replication strategies of these viruses and the structure and organization of their genomes has become available that has led to the reclassification or suggestions for reclassification of some of them. The replication strategy of EAV resembles that of the coronaviruses, involving a 3'-coterminal nested set of mRNAs. Therefore, EAV has been suggested to be included in a virus superfamily, along with coronaviruses and toroviruses. Recent evidence indicates that LDV not only resembles EAV in morphology, virion and genome size, and number and size of their structural proteins, but also in genome organization and replication via a 3'-coterminal set of mRNAs. SHFV, although not fully characterized, exhibits properties resembling those of LDV and EAV, and the recent evidence suggest that it may possess the same genome organization as these viruses. The three viruses may, therefore, represent a new family of positive-strand RNA viruses and are reviewed together in this chapter. In this chapter, emphasis is on the recent information concerning their molecular properties and pathogenesis in vitro and in vivo and on the host immune responses to infections by these viruses.

Map location of lactate dehydrogenase-elevating virus (LDV) capsid protein (Vp1) gene

Lactate dehydrogenase-elevating virus (LDV) is currently classified within the Togaviridae family. In an effort to obtain further information on the characteristics of this virus, we have begun to sequence the viral RNA genome and to map the virion structural protein genes. A sequence of 1064 nucleotides, which represents the 3' terminal end of the genome, was obtained from LDV cDNA clones. A 3' noncoding region of 80 nucleotides followed by two complete open reading frames (ORFs) were found within this sequence. The two ORFs were in different reading frames and overlapped each other by 11 nucleotides. One ORF encoded a protein of 170 amino acids and the other ORF, located adjacent to the 3' noncoding region of the viral genome, encoded a 114 amino acid protein. Thirty-three N-terminal residues were sequenced directly from purified LDV capsid protein, Vp1, and this amino acid sequence mapped to the ORF adjacent to the 3' noncoding region. The presence of overlapping ORFs and the 3' terminal map position of Vp1 indicate that LDV differs significantly from the prototype alpha togaviruses.

Animal models of amyotrophic lateral sclerosis and the spinal muscular atrophies

The causes of human amyotrophic lateral sclerosis (ALS) and the spinal muscular atrophies (SMA) are, almost without exception, unknown. This ignorance has stimulated the search for animal models to obtain insight into the etiology, pathogenesis and biochemical mechanisms underlying the human disorders. None of the 38 animal models, described in this review, provides an exact animal copy of a specific human motor neuron disease. Most of the models reproduce certain structural or physiological aspects of their human counterparts. The various experimental models can be classified according to the pathogenetic mechanism involved and according to the structural changes observed. Models based on experimentally induced disease, include heavy metals and trace elements (lead intoxication in guinea pigs, rabbits, rats, cats and primates; mercury intoxication in rats; aluminium intoxication in rabbits; swayback in goat kids; calcium and magnesium deficient rabbits and primates and calcium deficient cynomolgus monkeys), toxins (IDPN, vincristine, vinblastine, podophyllotoxin, colchicine, maytansine, maytanprine, L-BMAA, lectins, adriamycin), nutritional factors (ascorbic acid deficient guinea pigs), virus infection (spongiform polioencephalomyelitis, attenuated poliovirus, lactate dehydrogenase-elevating virus), and immunological factors (immunization with motor neurons). Hereditary models comprise hereditary canine spinal muscular atrophy, hereditary neurogenic amyotrophy in the pointer dog, Stockard paralysis, Swedish Lapland dog paralysis, "wobbler" mouse, "shaker" calf, and hereditary spinal muscular atrophy in zebra foals, crossbred rabbits,

Autoimmunity induced by lactate dehydrogenase-elevating virus: monoclonal autoantibodies against Golgi antigens and other subcellular elements

Immunocompetent mice infected with LDVAGIA developed autoantibodies against Golgi antigen as early as 6-7 days postinfection preceding anti-viral antibodies for nearly a week. The anti-Golgi antibody titres peaked between two and three weeks postinfection independent of the applied virus dose. Already one week postinfection anti-Golgi autoantibodies were prominent in IgG subclasses IgG2a and b. Spleen cells from these mice were fused with myeloma cells and the culture fluids were screened by indirect immunofluorescence for antibodies reactive with the Golgi antigen of normal cultured cells. A panel of cloned stable antibody-producing hybridomas has been obtained. Some antibodies showed broad cross-species reactivity, recognizing similar antigenic determinants in the Golgi region of mouse, rat and other mammalian cells and also in avian and piscine cells, whereas others recognized determinants in this cell compartment only in mammalian cells and one recognized Golgi antigen only in particular murine tumor cells. From 19 Golgi antibody producing hybridomas 3 secreted IgM-antibodies, 10 synthesized autoantibodies of the subclass IgG2a and 6 of IgG2b. Applied in immunoelectron microscopy mABs decorated the Golgi organelle. A considerable amount of LDVAGIA-induced hybridomas produced antibodies against conserved antigens associated with the cytoskeleton, mitochondria, and the nucleus, LDV-induced monoclonal anti-Golgi antibodies decorated the Golgi area in LDV- and mock-infected macrophages. However, cytoplasmic fluorescence characteristic for LDV-infected cells was not observed indicating that the anti-Golgi autoantibodies do not cross-react with the virus.

Susceptibility of C58 mice to paralytic disease induced by lactate dehydrogenase-elevating virus correlates with increased expression of endogenous retrovirus in motor neurons

The induction of poliomyelitis by lactate dehydrogenase-elevating virus (LDV) in C58 mice is dependent upon several host factors including old age, loss of immune competence and genetic predisposition. Two genetic components segregate with susceptibility to this neurological disease: the presence of multiple proviral copies of N-tropic endogenous murine leukemia viruses (MuLV) and homozygosity of the permissive allele for N-tropic viral replication (Fv-1n/n). We have quantified the levels of RNA for several endogenous retroviruses, using virus specific oligonucleotide probes, in various tissues of C58 mice in relation to age and immunosuppression. A tissue specific increase in expression of 3.0 kb AKR MuLV RNA in the spinal cords of mice occurred with increasing age of the mice and was enhanced several-fold by immunosuppression in old mice. Susceptibility to LDV-induced poliomyelitis occurs in the same age dependent manner as AKR MuLV expression and is also enhanced by immunosuppression. In contrast, the mink cell focus forming virus (MCF) RNA levels in the spinal cord remained constant despite apparent variations in MCF RNA expression in other tissues, and no xenotropic retrovirus RNA was detectable in spinal cords or brains of the C58 mice. The increased AKR MuLV RNA in the spinal cord was shown by in situ hybridization to be mainly located in the same motor neurons that become infected with LDV in these mice and are destroyed as paralysis develops. These results support a novel dual virus virus hypothesis for LDV-induced poliomyelitis in which increased endogenous retroviral expression in motor neurons renders these cells susceptible to cytocidal replication of LDV and hence to the development of LDV-induced poliomyelitis.

Formalin inactivation of the lactate dehydrogenase-elevating virus reveals a major neutralizing epitope not recognized during natural infection

Five hybridomas that secrete monoclonal antibodies which neutralize the infectivity of lactate dehydrogenase-elevating virus (LDV) were isolated from BALB/c mice primed with Formalin-inactivated LDV. Competition analyses indicated that all five neutralizing monoclonal antibodies recognize contiguous, if not identical, epitopes on the envelope glycoprotein of LDV (VP-3) which are not recognized by nonneutralizing VP-3-specific monoclonal antibodies isolated from the same fusion. Despite the presence of neutralizing activity, polyclonal anti-LDV antibodies obtained from persistently infected mice did not compete for binding to LDV with four of the five neutralizing monoclonal antibodies tested. The results indicate that the envelope glycoprotein of LDV possesses a major neutralizing epitope which is poorly recognized, if at all, by mice during a natural infection but is rendered immunogenic by Formalin inactivation of the virus. The epitope was also not immunogenic in a rabbit, since its polyclonal LDV-neutralizing antibodies did not inhibit binding of the mouse monoclonal antibodies to LDV. Passive immunization with the neutralizing monoclonal antibodies did not protect mice from LDV infection and did not alter the course of infection. Neutralizing monoclonal antibodies have been used to select a neutralization escape variant by a novel combination of in vitro and in vivo isolation.

Potential detrimental effects of rodent viral infections on long-term experiments

Healthy animals are of paramount importance in obtaining meaningful, reliable scientific results. Viral infections of rodents often have a significant impact on various types of biomedical research. Laboratory animal specialists and researchers must be aware of the possible consequences associated with the use of infected animals. The objective of the paper is a discussion of the frequently encountered viral infections that can complicate or invalidate the interpretation of results by altering the host's response.

Protection of C58 mice from lactate dehydrogenase-elevating virus-induced motor neuron disease by non-neutralizing antiviral antibodies without interference with virus replication

The paralytic poliomyelitis induced in old, immunosuppressed C58 mice by a primary infection with the lactate dehydrogenase-elevating virus (LDV) is prevented by the presence of anti-LDV antibodies in the virus inoculum or by passive transfer of LDV-free plasma from chronically LDV-infected mice one day before infection. Non-neutralizing antibodies were protective and specifically directed to the lowest molecular weight form of the envelope glycoprotein of LDV (VP-3), which seems to exist in virions in at least ten molecular forms ranging from 24 to 44 kDa. The antibodies did not prevent the productive infection of the subpopulation of macrophages that represents the primary permissive cell type in the mouse as evidenced by normal plasma LDV levels nor the spread of LDV to the central nervous system. Many non-neuronal cells containing LDV RNA were detected by in situ hybridization in the spinal cords of mice that had been infected with LDV in the presence of protective antibodies. However, no LDV RNA-positive neurons were detected, which are normally found coincidental with the development of paralytic symptoms in LDV-infected C58 mice. We propose that an early event after infection is critical for the infection of neurons and is inhibited by the presence of non-neutralizing antibodies to the LDV glycoprotein.

Molecular pathogenesis of virus infections

Although a very wide range of viral diseases exists in vertebrates, certain generalizations can be made regarding pathogenetic pathways on the molecular level. The presentation will focus on interactions of virions and their components with target cells. Using coronaviruses as examples the changes in virulence have been traced back to single mutational events; recombination, however, is likely to be an alternative mechanism by which virus-host interactions (e.g. the cell-, organ- or animal species-spectrum) can dramatically change. Receptor molecules are essential for the early interactions during infection and some of these have been identified. Events in the target cell and the host organism are discussed, and wherever possible, aspects of virus evolution and cooperation between infectious agents are highlighted.

Detection of viral-specific nucleic acid and intracellular virions in ventral horn neurons of lactate dehydrogenase-elevating virus infected C58 mice

C58 mice which have been immunosuppressed by treatment with cyclophosphamide (200 mg/kg) one day prior to infection with the C strain of lactate dehydrogenase-elevating virus (LDV-C) develop poliomyelitis. Using in situ hybridisation, we found that some ventral horn neurons in these mice contain cytoplasmic viral-specific nucleic acid. Viral-specific nucleic acid was also found within a few small cells located near inflammatory foci. In addition, mature virus particles were observed by electron microscopy in some ventral horn neurons, indicating that these cells are productively infected in C58 mice. Neither viral nucleic acid nor virions were found in the ventral horn neurons of poliomyelitis-resistant mouse strains or C58 mice that were not immunosuppressed prior to infection. Ventral horn neurons which contained viral nucleic acid or virions within cytoplasmic vesicles generally were normal in appearance and were not located within poliomyelitis inflammatory foci. Our data are consistent with the hypothesis that infected neurons first replicate virus and subsequently are attacked and cleared by inflammatory cells.

Age-related loss of Lyt-1,2 cells in C58 mice results in susceptibility to lactic dehydrogenase virus-induced polioencephalomyelitis

C58 mice (aged greater than or equal to 5 months) are susceptible to age-dependent polioencephalomyelitis, a paralytic central nervous system disease induced by lactic dehydrogenase virus. Susceptibility results from the loss of protective T cells. Data are presented showing a positive correlation between the age-related loss of Lyt-1,2 cells and the development of susceptibility to neuroparalytic lactic dehydrogenase virus infection.

Failure to demonstrate a role for line Ib tumor-associated surface antigen in the etiology of age-dependent polioencephalomyelitis

In the preceding paper [Morris et al. (1982), Molec. Immun. 19, 973-982] we demonstrate an associative interaction between the line Ib tumor-associated surface antigen (Ib-TASA) and the Dk/Kk regions of the major histocompatibility complex, i.e. 'altered-self' antigen. We originally hypothesized that age-dependent polioencephalomyelitis (ADPE) occurred as the result of immune recognition of a 'self'-determinant on the 'altered-self' antigen. In this report we used the non-ionic detergent, NP-40, to solubilize Ib cell surface antigens. Although immunization of immunocompetent C58 mice with the soluble NP-40 Ib cell extract afforded protection to lethal tumor challenge, the extract failed to induce ADPE in immunosuppressed mice. Data presented here demonstrate that Ib-TASA is not involved in the etiology of ADPE. The evidence suggests that lactic dehydrogenase virus, which is a silent virus passaged with line Ib leukemia, is the causative agent of the paralytic disease.

Co-infection by lactic dehydrogenase virus and C-type retrovirus elicits neurological disease

Intraperitoneal injection of neuropathogenic strains of lactic dehydrogenase virus (LDV) causes a histologically distinctive fatal paralytic disease characterized by an inflammatory destruction of motor neurones in the brain stem and cord in C58 mice aged over 9 months. To elicit the disease in the naturally susceptible C58 strain requires an age-associated or X-ray induced loss of immunological competence, LDV infection and genetic susceptibility. Genetic studies of the common inbred mouse strains showed that susceptibility to the disease was not linked to the major histocompatibility complex but correlated with the FV-1n allele, susceptibility to spontaneous leukaemia, and infection by neuropathogenic strains of LDV. These observations suggested that neuropathogenic strains of LDV elicit the disease only in those strains of mice that carry multiple copies of N-tropic C-type retroviruses in their genomes and that are permissive for retrovirus replication. Presumably the expression of these viral genomes (high titres of virus in tissues correlating with age) is the important factor. Here we present genetic evidence to support this hypothesis and briefly discuss the possible implications.