Focus Assay on RD114 Virus in QN10S Cells

Establishment of CRFK cells for vaccine production by inactivating endogenous retrovirus with TALEN technology

Endogenous retroviruses (ERVs) are retroviral sequences present in the host genomes. Although most ERVs are inactivated, some are produced as replication-competent viruses from host cells. We previously reported that several live-attenuated vaccines for companion animals prepared using the Crandell-Rees feline kidney (CRFK) cell line were contaminated with a replication-competent feline ERV termed RD-114 virus. We also found that the infectious RD-114 virus can be generated by recombination between multiple RD-114 virus-related proviruses (RDRSs) in CRFK cells. In this study, we knocked out RDRS env genes using the genome-editing tool TAL Effector Nuclease (TALEN) to reduce the risk of contamination by infectious ERVs in vaccine products. As a result, we succeeded in establishing RDRS knockout CRFK cells (RDKO_CRFK cells) that do not produce infectious RD-114 virus. The growth kinetics of feline herpesvirus type 1, calicivirus, and panleukopenia virus in RDKO_CRFK cells differed from those in parental cells, but all of them showed high titers exceeding 10⁷ TCID₅₀/mL. Infectious RD-114 virus was undetectable in the viral stocks propagated in RDKO_CRFK cells. This study suggested that RDRS env gene-knockout CRFK cells will be useful as a cell line for the manufacture of live-attenuated vaccines or biological substances with no risk of contamination with infectious ERV.

Barriers to Infection of Human Cells by Feline Leukemia Virus: Insights into Resistance to Zoonosis

The human genome displays a rich fossil record of past gammaretrovirus infections, yet no current epidemic is evident, despite environmental exposure to viruses that infect human cells in vitro. Feline leukemia viruses (FeLVs) rank high on this list, but neither domestic nor workplace exposure has been associated with detectable serological responses. Nonspecific inactivation of gammaretroviruses by serum factors appears insufficient to explain these observations. To investigate further, we explored the susceptibilities of primary and established human cell lines to FeLV-B, the most likely zoonotic variant. Fully permissive infection was common in cancer-derived cell lines but was also a feature of nontransformed keratinocytes and lung fibroblasts. Cells of hematopoietic origin were generally less permissive and formed discrete groups on the basis of high or low intracellular protein expression and virion release. Potent repression was observed in primary human blood mononuclear cells and a subset of leukemia cell lines. However, the early steps of reverse transcription and integration appear to be unimpaired in nonpermissive cells. FeLV-B was subject to G→A hypermutation with a predominant APOBEC3G signature in partially permissive cells but was not mutated in permissive cells or in nonpermissive cells that block secondary viral spread. Distinct cellular barriers that protect primary human blood cells are likely to be important in protection against zoonotic infection with FeLV.

IMPORTANCE Domestic exposure to gammaretroviruses such as feline leukemia viruses (FeLVs) occurs worldwide, but the basis of human resistance to infection remains incompletely understood. The potential threat is evident from the human genome sequence, which reveals many past epidemics of gammaretrovirus infection, and from recent cross-species jumps of gammaretroviruses from rodents to primates and marsupials. This study examined resistance to infection at the cellular level with the most prevalent human cell-tropic FeLV variant, FeLV-B. We found that blood cells are uniquely resistant to infection with FeLV-B due to the activity of cellular enzymes that mutate the viral genome. A second block, which appears to suppress viral gene expression after the viral genome has integrated into the host cell genome, was identified. Since cells derived from other normal human cell types are fully supportive of FeLV replication, innate resistance of blood cells could be critical in protecting against cross-species infection.

Comparison between S+L- assay and LacZ marker rescue assay for detecting replication-competent gammaretroviruses

To avoid contamination of adventitious gammaretroviruses in biological products such as vaccines, it is necessary to check the master seed cells for manufacturing. There are several assays to detect infectious gammaretroviruses. Among these, sarcoma-positive, leukemia-negative (S+L-) assay is a classical infectivity assay, which is often recommended in governmental guidelines. The S+L- cells used in S+L- assay generate unique focus upon the infection of replication-competent gammaretroviruses. Although S+L- assay is well recognized for the detection, their applicability is questionable in some cases. On the other hand, LacZ marker rescue (LMR) assay detects infectious gammaretroviruses by transducing LacZ marker gene to the target cells, which shows lacZ-positive foci if the infectious virus is present. In this study, we compared LMR and S+L- assays for detection of a variety of endogenous and exogenous gammaretroviruses. As results, LMR assay could detect all gammaretroviruses examined. On the other hand, S+L- assay using feline S+L- cells, termed QN10S, could not detect porcine endogenous retrovirus (PERV) subgroups A/B. Further, S+L- mink cells could not detect feline leukemia virus subgroups B in addition to PERV-A/B. These data indicate that LMR assay is better suited to detect wider range of gammaretroviruses.

Suppression of production of baboon endogenous virus by dominant negative mutants of cellular factors involved in multivesicular body sorting pathway

Baboon endogenous virus (BaEV) is an infectious endogenous gammaretrovirus isolated from a baboon placenta. BaEV-related sequences have been identified in both Old World monkeys and African apes, but not in humans or Asian apes. Recently, it was reported that BaEV-like particles were produced from Vero cells derived from African green monkeys by chemical induction, and thus BaEV-like particles may contaminate biological products manufactured using Vero cells. In this study, we constructed an infectious molecular clone of BaEV strain M7. We found two putative L-domain motifs, PPPY and PSAP, in the pp15 region of Gag. To examine the function of the L-domain motifs, we conducted virus budding assay using L-domain motif mutants. We revealed that the PPPY motif, but not the PSAP motif, plays a major role as the L-domain in BaEV budding. We also demonstrated that Vps4A/B are involved in BaEV budding. These data suggest that BaEV Gag recruits the cellular endosomal sorting complex required for transport (ESCRT) machinery through the interaction of the PPPY L-domain with cellular factors. These data will be useful for controlling contamination of BaEV-like particles in biological products in the future.

Up-regulation of MDP and tuftsin gene expression in Th1 and Th17 cells as an adjuvant for an oral Lactobacillus casei vaccine against anti-transmissible gastroenteritis virus

The role of muramyl dipeptide (MDP) and tuftsin in oral immune adjustment remains unclear, particularly in a Lactobacillus casei (L. casei) vaccine. To address this, we investigated the effects of different repetitive peptides expressed by L. casei, specifically the MDP and tuftsin fusion protein (MT) repeated 20 and 40 times (20MT and 40MT), in mice also expressing the D antigenic site of the spike (S) protein of transmissible gastroenteritis virus (TGEV) on intestinal and systemic immune responses and confirmed the immunoregulation of these peptides. Treatment of mice with a different vaccine consisting of L. casei expressing MDP and tuftsin stimulated humoral and cellular immune responses. Both 20MT and 40MT induced an increase in IgG and IgA levels against TGEV, as determined using enzyme-linked immunosorbent assay. Increased IgG and IgA resulted in the activation of TGEV-neutralising antibody activity in vitro. In addition, 20MT and 40MT stimulated the differentiation of innate immune cells, including T helper cell subclasses and regulatory T (Treg) cells, which induced robust T helper type 1 and T helper type 17 (Th17) responses and reduced Treg T cell immune responses in the 20MT and 40MT groups, respectively. Notably, treatment of mice with L. casei expressing 20MT and 40MT enhanced the anti-TGEV antibody immune responses of both the humoral and mucosal immune systems. These findings suggest that L. casei expressing MDP and tuftsin possesses substantial immunopotentiating properties, as it can induce humoral and T cell-mediated immune responses upon oral administration, and it may be useful in oral vaccines against TGEV challenge.

Detection of RD-114 virus by a reverse transcriptase assay based on product enhancement

RD-114 virus is a feline endogenous retrovirus that exists in the genome of all cats. It can be assumed that feline and canine live vaccines manufactured by culturing cells of feline origin are contaminated with the virus. The current study is attempted to develop a product enhanced reverse transcriptase (PERT) assay to detect replication-competent RD-114 virus. Since culture supernatants of Crandell-Rees feline kidney (CRFK) cells do not have detectable reverse transcriptase activity in case of do not passage on other cell lines, these results raise the possibility that RD-114 virus grow efficiently in other retrovirus producing cell lines. For the PERT assay, RD-114 virus isolated from CRFK cells may need to be passaged more than four times on 293T cells to be expressed at appreciable levels. The PERT assay described here provides an accurate method for determining the presence of RD-114 in culture supernatants and will help monitor live vaccines produced in endogenous virus producing cell lines.

Sequence comparison of three infectious molecular clones of RD-114 virus

RD-114 virus is a replication-competent feline endogenous retrovirus. RD-114 virus contaminates several feline and canine live attenuated vaccines and the issue of contamination of RD-114 virus in vaccines should be solved. To date, three infectious molecular clones (pSc3c, pCRT1, and pRD-UCL) have been reported. In this study, we sequenced the entire nucleotide sequence of pRD-UCL and compared the nucleotide sequences of the three infectious molecular clones. As a result, these three infectious clones were nearly identical with each other in gag-pol and env coding regions. These data support the notion that the active locus of infectious RD-114 virus is single in the feline genome. The length of long terminal repeat (LTR) of pCRT1 was 47 bp shorter than those of pSc3c and pRD-UCL. The 47-bp sequence named direct repeat A (DR-A) was duplicated in the U3 region in pSc3c and pRD-UCL. Although several potential enhancer binding sites are present in the DR-A, there was no significant difference in promoter activities between the LTRs of pRD-UCL and pCRT1 in two human cell lines. We also analyzed the splicing pattern of the RD-114 virus by reverse transcription-polymerase chain reaction and confirmed that RD-114 virus is a simple retrovirus. The data presented here will provide basic information about RD-114 virus to solve the contamination issue in live attenuated vaccines.

Viral and cellular requirements for the budding of feline endogenous retrovirus RD-114

BACKGROUND

RD-114 virus is a feline endogenous retrovirus and produced as infectious viruses in some feline cell lines. Recently, we reported the contamination of an infectious RD-114 virus in a proportion of live attenuated vaccines for dogs and cats. It is very difficult to completely knock out the RD-114 proviruses from cells, as endogenous retroviruses are usually integrated multiply into the host genome. However, it may be possible to reduce the risk of contamination of RD-114 virus by regulating the viral release from cells.

RESULTS

In this study, to understand the molecular mechanism of RD-114 virus budding, we attempted to identify the viral and cellular requirements for RD-114 virus budding. Analyses of RD-114 L-domain mutants showed that the PPPY sequence in the pp15 region of Gag plays a critical role in RD-114 virus release as viral L-domain. Furthermore, we investigated the cellular factors required for RD-114 virus budding. We demonstrated that RD-114 virus release was inhibited by overexpression of dominant negative mutants of Vps4A, Vps4B, and WWP2.

CONCLUSIONS

These results strongly suggest that RD-114 budding utilizes the cellular multivesicular body sorting pathway similar to many other retroviruses.

Canine ASCT1 and ASCT2 are functional receptors for RD-114 virus in dogs

All domestic cats carry an infectious endogenous retrovirus termed RD-114 virus. Recently, we and others found that several live-attenuated vaccines for dogs were contaminated with infectious RD-114 virus. In this study, we confirmed that the RD-114 virus efficiently infected and proliferated well in canine primary kidney cells, as well as three tested canine cell lines. Further, we identified canine ASCT1 and ASCT2, sodium-dependent neutral amino acid transporters, as RD-114 virus receptors. Canine ASCT2 also acts as a functional receptor for simian retrovirus 2, a pathogenic retrovirus that induces immunodeficiency in rhesus macaques. Identification of the canine receptor for RD-114 virus will help in evaluating the risk from vaccines contaminated by the virus.

Development of a real-time reverse-transcription-PCR method for detection of RD114 virus in canine vaccines

It is known that certain feline cell lines, such as the Crandell-Rees feline kidney cell, produce an RD-114-like virus. As a feline endogenous retrovirus, RD114 virus, exists in the genome of all cats, it can be assumed that contamination with the virus in feline and canine live vaccines manufactured by culturing cells of feline origin occurs.

To detect an infectious RD114 virus in vitro, a LacZ marker rescue assay has recently been established. In feline and canine live vaccines approved in Japan, feline cell lines are widely used to produce vaccines, especially those containing canine parvovirus components. The LacZ marker rescue assay detects infectious viral particles, but the real-time reverse-transcription-PCR detects both infectious and defective viruses. The canine live vaccines manufactured in cells of feline origin showed positive results for the env gene by the real-time reverse-transcription-PCR, including all of the 8 vaccines produced in feline cell lines that were negative in the LacZ marker rescue assay. In conclusion, the present investigation suggests that the newly developed method has the advantages of shorter time requirements and can be applied as a valuable screening method to detect RD114 viral RNA in vaccines.

Experimental infection of dogs with a feline endogenous retrovirus RD-114

Background

The feline endogenous retrovirus RD114 is contained in the genome of cats. The virus may contaminate live canine vaccines based on cultured feline cells. The in vivo infectivity, acute and subacute pathogenicity, and viral proliferation of the RD114 virus were evaluated by experimental infection of dogs.

Methods

Nine specific pathogen free dogs were divided into three groups, with each group consisting of one female and two male dogs. Dogs were subcutaneously inoculated in the neck with either 1 ml RD114 stock virus (group A), inactivated RD114 virus suspension (group B), or cell culture medium (group C) as a negative control. To assess blood cell counts and biochemical properties, blood samples from each group were collected 5 days before inoculation, just prior to inoculation, and 1, 3, 7 and 10 days post-inoculation.

Result

During the experimental period of 51 days, none of the dogs inoculated with RD114 virus showed any clinical signs, significant increases in rectal temperature or abnormal blood biochemical characteristics including C-reactive protein when compared with the negative controls. We were not able to re-isolate the RD114 virus from buffy coat cells of group A dogs. Additionally, we could not detect RD114 provirus in the genomic DNA isolated from peripheral blood leukocytes, lymph node, spleen and sternal bone marrow cells.

Conclusions

Signs of RD114 virus proliferation were not found after subcutaneous infection of dogs. Although the potential risk caused by infection with RD114 virus in dogs could not be assessed in this study, we suspect that RD114 virus has little or no virulence in dogs.

Focus assay on FeLIX-dependent feline leukemia virus

T-lymphotropic feline leukemia virus (FeLV-T) induces immunodeficiency in cats. FeLV-T is fusion-defective and requires a cofactor, termed FeLIX, for infection. FeLIX is a truncated envelope glycoprotein of an endogenous FeLV and mediates infection by binding a phosphate transporter Pit-1. In this study, we established a feline sarcoma-positive leukemia-negative cell line expressing FeLIX, named QN/FeLIX cells. Upon infection, FeLV-T induced prominent foci with syncytia in QN/FeLIX cells and could be titrated by the focus assay. In addition, we established a FeLIX-expressing feline fibroblast cell line, named AH/FeLIX cells. FeLV-T productively infected AH/FeLIX cells and induced severe CPE with syncytia. QN/FeLIX and AH/FeLIX cells will be useful for the study of FeLIX-dependent mutants in FeLV-infected cats.