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

Implications of infectious agents on results of animal experiments

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

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

B lymphocyte activation by coinfection prevents immune control of friend virus infection

Although the adaptive immune response almost invariably fails to completely eliminate retroviral infections, it can exert significant protection from disease and long-term control of viral replication. Friend virus (FV), a mouse retrovirus, causes persistent infection in all strains of mice and erythroleukaemia in susceptible strains, the course of which can be strongly influenced by both genetic and extrinsic factors. In this study we examine the impact of coinfection on the requirements for immune control of FV infection. We show that congenic C57BL/6 mice, in which the introduction of an allele of the Friend virus susceptibility 2 gene provides the potential for FV-induced leukemia development, effectively resist FV infection, and both T cell- and Ab-dependent mechanisms contribute to their resistance. However, we further demonstrate that coinfection with lactate dehydrogenase-elevating virus (LDV) renders these otherwise immunocompetent mice highly susceptible to FV infection and subsequent disease. The presence of LDV delays induction of FV-specific neutralizing Abs and counteracts the protective contribution of adaptive immunity. Importantly, the disease-enhancing effect of LDV coinfection requires the presence of a polyclonal B cell repertoire and is reproduced by direct polyclonal B cell activation. Thus, immune activation by coinfecting pathogens or their products can contribute to the pathogenicity of retroviral infection.

Two cellular proteins that interact with a stem loop in the simian hemorrhagic fever virus 3'(+)NCR RNA

Both full-length and subgenomic negative-strand RNAs are initiated at the 3′ terminus of the positive-strand genomic RNA of the arterivirus, simian hemorrhagic fever virus (SHFV). The SHFV 3′(+) non-coding region (NCR) is 76 nts in length and forms a stem loop (SL) structure that was confirmed by ribonuclease structure probing. Two cell proteins, p56 and p42, bound specifically to a probe consisting of the SHFV 3′(+)NCR RNA. The 3′(+)NCR RNAs of two additional members of the arterivirus genus specifically interacted with two cell proteins of the same size. p56 was identified as polypyrimidine tract-binding protein (PTB) and p42 was identified as fructose bisphosphate aldolase A. PTB binding sites were mapped to a terminal loop and to a bulged region of the SHFV 3′SL structure. Deletion of either of the PTB binding sites in the viral RNA significantly reduced PTB binding activity, suggesting that both sites are required for efficient binding of this protein. Changes in the top portion of the SHFV 3′SL structure eliminated aldolase binding, suggesting that the binding site for this protein is located near the top of the SL. These cell proteins may play roles in regulating the functions of the genomic 3′ NCR.

Characterization of lactate dehydrogenase-elevating virus ORF6 protein expressed by recombinant baculoviruses

Lactate dehydrogenase-elevating virus (LDV) has a strict species-specificity and can replicate only in a subset of mouse primary macrophages in vitro. Because it is difficult to grow and purify sufficient quantities of LDV virions from the primary macrophages, it has been difficult to further characterize LDV envelope proteins. A few expression systems have been reported for structural analysis of the nonglycosylated envelope protein M/VP-2, however, very few studies of the antigenicity of M/VP-2 have been reported. We cloned and expressed the ORF6 gene, which encodes the M/VP-2, as a fusion protein with a polyhistidine metal-binding tag (6 x His-tag) in Autographa californica nuclear polyhedrosis virus (baculovirus) under the control of the polyhedrin promoter. In Western blotting analysis, the expressed protein was similar in size to the native M/VP-2 plus 6 x His-tag. The usefulness of the baculovirus-expressed LDV ORF6 protein for analysis of the immunogenicity of LDV M/VP-2 was discussed.

Analysis of protein expression by mammalian cell lines stably expressing lactate dehydrogenase-elevating virus ORF 5 and ORF 6 proteins

Lactate dehydrogenase-elevating virus (LDV) has a strict species-specificity. Because only a subset of mouse primary macrophages have been identified that can support LDV replication in vitro, the precise molecular mechanism of viral entry and replication remains unclear. To analyze the LDV envelope proteins, which probably mediate viral attachment to the host cell, we developed a mammalian system for stable co-expression of LDV open reading frame (ORF) 5- and ORF 6-encoded proteins (ORF 5 and ORF 6 proteins), which correspond to envelope VP-3 and M/VP-2, respectively, and compared these expressed proteins to the native ones. Western blotting analysis combined with N-glycanase digestion revealed that ORF 5 and ORF 6 proteins were similar in size to native VP-3 and M/VP-2, and that ORF 5 protein was N-glycosylated, like the native VP-3. Immunofluorescence microscopy revealed that both ORF 5 and ORF 6 proteins were distributed throughout the cytoplasm and were colocalized in most cells. Moreover, ORF 5 protein was localized both in the perinuclear region and the Golgi complex and transported to the cell surface. This mammalian expression system in which the exogenously expressed proteins closely resemble the native proteins will provide the experimental basis for further studies of the interactions between LDV envelope proteins and host cells.

Production of virus-specific antiserum corresponding to sequences in the lactate dehydrogenase-elevating virus (LDV) ORF6 protein

The elucidation of the antigenic structure of the envelope proteins of Arteriviridae which includes lactate dehydrogenase-elevating virus (LDV) will provide further understanding of a mechanism of strict host cell specificity. To analyze the linkage between LDV envelope proteins, M/VP-2 and VP-3, which may play an important role in viral infectivity, we generated specific antibody against M/VP-2 that has not been reported in previous studies. A synthetic polypeptide corresponding to the C-terminal region of LDV strain C (LDV-C) ORF6, which encodes M/VP-2, was chemically synthesized and coupled to keyhole limpet hemocyanin (KLH). The peptide was immunogenic in rabbits and induced antibody specific for viral protein. Western blotting and immunofluorescence analysis of virion M/VP-2 in infected macrophages showed that the antibody was able to react specifically with authentic virion protein. The immunoreactive antibody against LDV M/VP-2 described in this study will be useful for further studies of the specific roles of the envelope proteins in arterivirus assembly and infectivity.

Involvement of sialoadhesin in entry of porcine reproductive and respiratory syndrome virus into porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) shows a very restricted tropism for cells of the monocyte/macrophage lineage. It enters cells via receptor-mediated endocytosis. A monoclonal antibody (MAb) that is able to block PRRSV infection of porcine alveolar macrophages (PAM) and that recognizes a 210-kDa protein (p210) was described previously (MAb41D3) (X. Duan, H. Nauwynck, H. Favoreel, and M. Pensaert, J. Virol. 72:4520-4523, 1998). In the present study, the p210 protein was purified from PAM by immunoaffinity using MAb41D3 and was subjected to internal peptide sequencing after tryptic digestion. Amino acid sequence identities ranging from 56 to 91% with mouse sialoadhesin, a macrophage-restricted receptor, were obtained with four p210 peptides. Using these peptide data, the full p210 cDNA sequence (5,193 bp) was subsequently determined. It shared 69 and 78% amino acid identity, respectively, with mouse and human sialoadhesins. Swine (PK-15) cells resistant to viral entry were transfected with the cloned p210 cDNA and inoculated with European or American PRRSV strains. Internalized virus particles were detected only in PK-15 cells expressing the recombinant sialoadhesin, demonstrating that this glycoprotein mediated uptake of both types of strains. However, nucleocapsid disintegration, like that observed in infected Marc-145 cells as a result of virus uncoating after fusion of the virus with the endocytic vesicle membrane, was not observed, suggesting a block in the fusion process. The ability of porcine sialoadhesin to mediate endocytosis was demonstrated by specific internalization of MAb41D3 into PAM. Altogether, these results show that sialoadhesin is involved in the entry process of PRRSV in PAM.

Construction of chimeric arteriviruses reveals that the ectodomain of the major glycoprotein is not the main determinant of equine arteritis virus tropism in cell culture

The recent development of arterivirus full-length cDNA clones makes possible the construction of chimeric arteriviruses for fundamental and applied studies. Using an equine arteritis virus (EAV) infectious cDNA clone, we have engineered chimeras in which the ectodomains of the two major envelope proteins, the glycoprotein GP(5) and the membrane protein M, were replaced by sequences from envelope proteins of related and unrelated RNA viruses. Using immunofluorescence microscopy, we monitored the transport of the hybrid GP(5) and M proteins to the Golgi complex, which depends on their heterodimerization and is a prerequisite for virus assembly. The only viable chimeras were those containing the GP(5) ectodomain from the porcine (PRRSV) or mouse (LDV) arteriviruses, which are both considerably smaller than the corresponding sequence of EAV. Although the two viable GP(5) chimeras were attenuated, they were still able to infect baby hamster kidney (BHK-21) and rabbit kidney (RK-13) cells. These cells can be infected by EAV, but not by either PRRSV or LDV. This implies that the ectodomain of the major glycoprotein GP(5), which has been postulated to be involved in receptor recognition, is not the main determinant of EAV tropism in cell culture.

An infectious cDNA clone of porcine reproductive and respiratory syndrome virus

A plasmid containing a full-length cDNA copy of the Lelystad virus isolate (LV) of porcine reproductive and respiratory syndrome virus was constructed. When RNA that was transcribed in vitro from this full-length cDNA clone was transfected to BHK-21 cells, infectious LV was produced and secreted. The virus was rescued by passage to porcine alveolar lung macrophages or CL2621 cells. When infectious transcripts were transfected to porcine alveolar lung macrophages or CL2621 cells no infectious virus was produced due to the poor transfection efficiency of these cells. The growth properties of the viruses produced by BHK-21 cells transfected with infectious transcripts of LV cDNA resembled the growth properties of the parental virus from which the cDNA was derived. The infectious clone of LV enables us to mutagenize the viral genome at specific sites and thus will be useful for detailed molecular characterization of the virus, as well as for the development of a safe and effective live vaccine for use in pigs.

Infectious transcripts from cloned genome-length cDNA of porcine reproductive and respiratory syndrome virus

The 5'-terminal end of the genomic RNA of the Lelystad virus isolate (LV) of porcine reproductive and respiratory syndrome virus was determined. To construct full-length cDNA clones, the 5'-terminal sequence was ligated to cDNA clones covering the complete genome of LV. When RNA that was transcribed in vitro from these full-length cDNA clones was transfected into BHK-21 cells, infectious LV was produced and secreted. The virus was rescued by passage to porcine alveolar lung macrophages or CL2621 cells. When infectious transcripts were transfected to porcine alveolar lung macrophages or CL2621 cells, no infectious virus was produced due to the poor transfection efficiency of these cells. The growth properties of the viruses produced by BHK-21 cells transfected with infectious transcripts of LV cDNA resembled the growth properties of the parental virus from which the cDNA was derived. Two nucleotide changes leading to a unique PacI restriction site directly downstream of the ORF7 gene were introduced in the genome-length cDNA clone. The virus recovered from this mutated cDNA clone retained the PacI site, which confirmed the de novo generation of infectious LV from cloned cDNA. These results indicate that the infectious clone of LV enables us to mutagenize the viral genome at specific sites and that it will therefore be useful for detailed molecular characterization of the virus, as well as for the development of a safe and effective live vaccine for use in pigs.

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.

Pseudotype virions formed between mouse hepatitis virus and lactate dehydrogenase-elevating virus (LDV) mediate LDV replication in cells resistant to infection by LDV virions

Infection of cultures of peritoneal macrophages with both lactate dehydrogenase-elevating virus (LDV) and mouse hepatitis virus (MHV) resulted in the formation of pseudotype virions containing LDV RNA which productively infected cells that are resistant to infection by intact LDV virions but not to infection by MHV. These cells were mouse L-2 and 3T3-17Cl-1 cells as well as residual peritoneal macrophages from persistently LDV-infected mice. Productive LDV infection of these cells via pseudotype virions was inhibited by antibodies to the MHV spike protein or to the MHV receptor, indicating that LDV RNA entered the cells via particles containing the MHV envelope. Simultaneous exposure of L-2 cells to both LDV and MHV resulted in infection by MHV but not by LDV. The results indicate that an internal block to LDV replication is not the cause of the LDV nonpermissiveness of many cell types, including the majority of the macrophages in an adult mouse. Instead, LDV permissiveness is restricted to a subpopulation of mouse macrophages because only these cells possess a surface component that acts as an LDV receptor.

Lactate dehydrogenase-elevating virus: an ideal persistent virus?

LDV contradicts all commonly held views about mechanisms of virus persistence, namely that persistence is primarily associated with noncytopathic viruses, or the selection of immune escape variants or other mutants, or a decrease in expression of certain viral proteins by infected cells, or replication in “immune-privileged sites”, or a general suppression of the host immune system, etc. [1, 2, 5, 54, 77, 78]. LDV is a highly cytocidal virus that invariably establishes a life-long, viremic, persistence in mice, in spite of normal anti-viral immune responses.

One secret of LDV's success in persistence is its specificity for a renewable, nonessential population of cells that is continuously regenerated, namely a subpopulation of macrophages. Since the continuous destruction of these cells is not associated with any obvious health effects, this macrophage population seems nonessential to the well-being of its host. The only function identified for this subpopulation of macrophages is clearance of the muscle type of LDH and some other enzymes [59, 67, 68]. Furthermore, the effects of LDV infection on the host immune system, namely the polyclonal activation of B cells and its associated production of autoantibodies, and the slight impairment of primary and secondary antibody responses also do not seem to be severe enough to cause any clinical consequences.

But how does LDV replication in macrophages escape all host defenses? Persistence is not dependent on the seletion of immune escape variants or other mutants ([58] and Palmer, Even and Plagemann, unpublished results). Also, LDV replication is not restricted to immune-privileged sites [5]. LDV replication persists in the liver, lymphoidal tissues and testis [66]. Only the latter could be considered a site not readily accessible to immune surveillance.

Most likely, resistance of LDV replication to antiviral immune responses is related to the unique structure of its envelope proteins and the production of large quantities of viral antigens. High titers of anti-LDV antibodies are generated in infected mice but they neutralize LDV infectivity only very inefficiently and, even though the antiviral antibodies are mainly of the IgG2a and IgG2b isotypes, they do not mediate complement lyses of virions [31]. Interaction of the antibodies and complement with the VP-3/VP-2 heterodimers in the viral envelope may be impeded by the exposure of only very short peptide segments of these proteins at the envelope surface and the presence of large oligosaccharide side chains. Furthermore, since LDV maturation is restricted to intracytoplasmic cisternae [59, 71], the question arises of whether any of the viral proteins are available on the surface of infected cells for ADCC.

CTLs also fail to control LDV replication. Altough CTLs specific for N/VP-1 are rapidly generated, these have disappeared by 30 days p.i. [26]. The reasons for this loss are unknown, but high-dose clonal exhaustion [41, 51, 77, 78] is a reasonable possibility since, regardless of the infectious dose, large amounts of LDV proteins are present in all the lymphoidal tissues at the time of the induction of the CTL response. Furthermore, after exhaustion of CTLs in the periphery, continuous replication of LDV in the thymus [65] assures that the mice become permanently immunologically tolerant with respect to LDV antigen-specific CTLs as a result of negative selection in the thymus. LDV might be a primary example for the effectiveness of a permanent clonal CTL deletion in adult animals under natural conditions of infection.

The presumed modes of transmission of LDV in nature and the events associated with its infection of mice are strikingly similar to those observed during the acute and asymptomatic phases of infection with human immunodeficiency virus (HIV) [24, 29, 74, 78]. These include: (1) primary inefficient transmission via sexual and transplacental routes but effective transmission via blood; (2) primary replication in renewable populations of lymphoidal cells with production of large amounts of virus after the initial infection of the host followed by persistent low level of viremia in spite of antiviral immune responses; (3) persistence, reflecting continuous rounds of productive, cytocidal infection of permissive cells [59, 74] and the rate of generation of permissive cells which may be the main factor in determining the level of virus production (in the case of HIV, the rate of activation of CD4⁺ T cells to support a productive HIV replication might be the factor determining the rate of virus production and the progression of the disease); (4) rapid antibody formation but delayed production of neutralizing antibodies with limited neutralizing capacity; (5) rapid but transient generation of virus-specific CTLs; and (6) accumulation of large amounts of virus in newly formed germinal centers in the spleen and lymph nodes concomitant with an initiation of a permanent polyclonal activation of B cells resulting in an elevation of plasma IgG2a.

The events described under points 2–6 might be generally associated with natural viremic persistent virus infections. Such persistent viruses, by necessity, have evolved properties that allow them to escape all host defenses and control of their infection by immunological processes is, therefore, difficult, if not impossible. Prevention of infection and chemotherapy may be the only approaches available to combat such virus infections.