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Short Communication |
Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University, 9820 Merelbeke, Belgium
Correspondence
Hans J. Nauwynck
hans.nauwynck{at}ugent.be
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ABSTRACT |
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A figure showing co-localization of biotinylated particles and FIPV virions is available with the online version of this paper.
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MAIN TEXT |
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TOPABSTRACTMAIN TEXTREFERENCES |
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; Cowley et al., 2000; Gorbalenya et al., 2006). Based on comparative sequence analysis, human coronavirus 229E (HCoV 229E), porcine transmissible gastroenteritis virus (TGEV), porcine epidemic diarrhea virus, canine coronavirus (CCoV) and feline coronaviruses (FCoV) are grouped in phylogenetic group 1 (González et al., 2003; Spaan et al., 2005; Gorbalenya et al., 2006). Feline coronaviruses are divided into two serotypes: serotype I and the less prevalent serotype II (Pedersen et al., 1984; Hohdatsu et al., 1992). Type II strains are related closely to CCoV and TGEV (Pedersen et al., 1983) and are thought to be recombinants of type I strains and CCoV (Vennema et al., 1995). Unlike type I strains, type II strains grow easily in cell culture, such as Crandell feline kidney (CrFK) cells, and are therefore used frequently in FIPV studies.
Aminopeptidase N (APN) serves as a receptor for several group 1 coronaviruses (Tresnan et al., 1996), including HCoV 229E (Yeager et al., 1992), porcine respiratory coronavirus and TGEV (Delmas et al., 1992, 1993, 1994). For both serotype I and II FCoVs, stably expressed APN cDNA isolated from the Felis catus whole fetus (fcwf) cell line induced susceptibility in FCoV-resistant cells. Although strains of both serotypes were able to infect these cells, the efficiency of infection was different (Tresnan et al., 1996). Hohdatsu et al. (1998) showed that only serotype II strains of FCoV use feline APN (fAPN) as a receptor for internalization into feline cell lines. Infection of bone marrow-derived macrophages with FIPV II (79-1146) was restricted from approximately 33 to 3 % of cells by a mAb against fAPN (Rottier et al., 2005). Whether fAPN is the receptor for FIPV II on in vivo target cells, feline blood monocytes, is not known. The receptor for FIPV I has not been identified.
Viruses may enter cells via two pathways. Viral envelopes may fuse with the plasma membrane or with endosomal membranes after entering cells via endocytosis. Several group 1 coronaviruses enter cells via endocytosis: HCoV 229E (Nomura et al., 2004), TGEV (Hansen et al., 1998) and CCoV (Savarino et al., 2003). Mouse hepatitis virus, a group 2 coronavirus, enters cells by both endosomal and non-endosomal pathways. The pathway of internalization was shown to depend upon the virus strain and the nature of the cell being infected (Kooi et al., 1991; Nash & Buchmeier, 1997). Internalization of FIPV has not been studied in either primary cells or cell lines.
In this study, kinetics of attachment and internalization were studied for FIPV in feline blood monocytes, the primary target cells, and compared with those in CrFK cells, used frequently as a model cell line for FCoV–cell interactions. Further, it was investigated whether FIPV enters monocytes and CrFK cells via fusion with the plasma membrane or via endocytosis.
Monocytes were isolated from blood collected from two FCoV-, feline leukemia virus- and feline immunodeficiency virus-negative cats as described previously (Dewerchin et al., 2005) and seeded on glass coverslips. The adherent cells consisted of 86±7 % monocytes [assessed with monocyte marker DH59B; Veterinary Medical Research and Development (VMRD)]. Experiments were performed at 36 h post-seeding, using third passages of type I FIPV strain Black (Black, 1980) on fcwf cells and third passages of type II FIPV strain 79-1146 (McKeirnan et al., 1981) on CrFK cells. Both strains were kindly provided by Dr H. F. Egberink (Utrecht University, The Netherlands). Virus purification was performed as described by Delputte et al. (2002) and Delputte & Nauwynck (2004). Purified virus was biotinylated with Sulfo-NHS-LC-Biotin (Pierce) according to the manufacturer's instructions. Purified viral-particle suspension (200 µg) was applied to 104.4 monocytes or 106 CrFK cells throughout the experiments. Application of diluted samples of biotinylated virus suspension to streptavidin-coated coverslips (Xenopore) and staining with fluorescein isothiocyanate (FITC)-labelled streptavidin (Molecular Probes) showed that this amount of virus contains 7.9x109 FIPV I (Black) particles and 5.0x1010 FIPV II (79-1146) particles. Double staining with polyclonal anti-FIPV–FITC (VMRD) and streptavidin–Texas red (Molecular Probes) confirmed the concurrence of biotin and FIPV antigens (see Supplementary Fig. S1, available in JGV Online). Further, it was shown that the biotinylation procedure did not influence the obtained kinetics (data not shown). Cell vitality was checked at the end of the experiments. Triplicate assays were performed and kinetics were compared by using the Wilcoxon signed ranks (WSR) test from the SPSS software package (version 12.0; SPSS Inc.).
To determine kinetics of attachment, cells were chilled on ice for 15 min and inoculated with biotinylated FIPV I (Black) or II (79-1146) at 4 °C. At different time points, cells were washed with PBS and fixed with 1 % formaldehyde. Attached particles were visualized by using FITC-labelled streptavidin and counted manually for 50 randomly chosen cells by means of a DM IRB inverted microscope (Leica). Fig. 1(a) illustrates the origin of the graphs shown in Fig. 1(b). FIPV I (Black) bound to almost all monocytes (98.8 % of the monocytes for cat 1 and 99.5 % for cat 2). For cat 1, the number of attached particles per cell reached 14±1 after 15 min; for cat 2, it was 18±1 after 1 h. Thereafter, no further increase was observed. Attachment to CrFK cells was restricted to 15.4 % of the cells and, after 2 h, only 1±1 particle was bound per cell (all cells considered). FIPV II (79-1146) bound to only 66.2 % of the monocytes for cat 1 and to 95.0 % for cat 2. For cat 1, the number of attached particles per cell increased slowly to 5±3 during the first hour. For cat 2, a strong increase to 19±10 particles per cell was observed during the first 30 min. Afterwards, attachment remained at the same level. Clearly, cat 2 reached a higher level of attached FIPV II (79-1146) particles than cat 1. In contrast to the results for FIPV I (Black), almost all CrFK cells (99.8 %) bound FIPV II (79-1146) particles. Also, more particles were attached, although the kinetics were slow. During 4 h incubation, the number of bound particles per cell increased gradually to 49±19. Longer incubation times increased the amount of bound virus slightly. Based on the numbers of inoculated and attached virus particles, it is clear that virus was present in excess in all experiments. So, for both FIPV I (Black) and II (79-1146), all possible binding sites were taken.
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). Cells were washed and plates were transferred from 4 to 37 °C (time point zero for the kinetics of internalization). Then, cells were fixed at different time points. Attached particles were visualized by using streptavidin–Texas red. After permeabilizing cells with 0.1 % Triton X-100, internalized particles were stained with streptavidin–FITC. Kinetics of internalization are shown in Fig. 2. The percentages given were calculated by quantifying particles inside the cytosol and on the surface of 10 cells with attached and/or internalized particles. The kinetics of internalization for FIPV I (Black) in monocytes were similar for both cats. Almost all monocytes that bound FIPV I (Black) virions internalized virions (98.9 % of the monocytes for cat 1 and 98.5 % for cat 2). Remarkably, immobilization of the cells at 4 °C could not inhibit internalization completely, as there were already particles internalized at time point zero. Within the first minute, 89±6 % of the bound particles were internalized per cell. This level of internalization was roughly maintained, although a slight, not significant, decrease to 85±13 % was observed after 1 h. However, unlike for monocytes, uptake was restricted in CrFK cells to 20.8 % of the cells that had bound virus or only 3.2 % of all cells. No internalization was observed after the first minute, but during the first 2 h, kinetics gradually increased to a maximum, with 52 % of the bound particles internalized per cell. The percentages of internalized particles per cell varied between 0 and 100 %, as often only one particle was bound, so only one particle could be internalized. As for FIPV I (Black), internalization of FIPV II (79-1146) was mediated by nearly all monocytes that had bound virus particles (92.5 % of the analysed monocytes from cat 1 and all analysed monocytes from cat 2) and kinetics were similar for both cats. After 1 min, a mean of 60±9 % of the bound virions were internalized per cell. Internalization further increased gradually up to 90±7 % after 1 h. All CrFK cells were capable of internalizing FIPV II (79-1146) virions. Within the first minute, only 3±2 % of the bound particles were internalized. However, the percentage of internalization per cell increased gradually to 67±4 % after 2 h. Longer incubation times increased the percentage of internalization per cell only slightly.
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shows that all FIPV material at the plasma membrane co-localized with nucleocapsid proteins. Thus, the biotinylated material consisted of intact virions. This implies that, under the present experimental conditions, both FIPV strains enter monocytes exclusively via endocytosis, and FIPV II (79-1146) also does so in CrFK cells.
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). As only two cats were considered, these data cannot lead to general conclusions. Further research will focus on the receptors of FIPV in monocytes, which will allow interpretation of the present findings.
Internalization in monocytes was very efficient for both serotypes and was mediated exclusively by endocytosis. The efficiency of internalization of TGEV and HCoV 229E, also group 1 coronaviruses, and of porcine reproductive and respiratory syndrome virus (PRRSV), another member of the order Nidovirales, in macrophages is comparable to that of FIPV in monocytes (Hansen et al., 1998; Nauwynck et al., 1999; Nomura et al., 2004). Other mutual observations between TGEV, PRRSV and FIPV are the accumulation of particles in endosomes (already occurring 15 min after the start of virus uptake for FIPV) and the decreasing number of particles inside the cell, due to particle disassembly, at later time points in the kinetics (within 1 h for FIPV). The decreasing number of particles explains the apparent drop in internalization described for FIPV I (Black), where the percentages of uptake per cell were higher after 1 min than after 1 h.
Replication of FIPV is limited to a small fraction (<1 % for FIPV 79-1146) of monocytes (Dewerchin et al., 2005) and peritoneal macrophages (Morahan et al., 1985; Stoddart & Scott, 1989). However, virus binding and uptake took place in almost all monocytes. Thus, the mechanism behind the resistance of most monocytes/macrophages must lie in inhibition of genome release and/or translation. Similar observations were obtained for PRRSV replication in alveolar macrophages (Duan et al., 1997; Nauwynck et al., 1999).
Attachment and internalization of FIPV I (Black) in CrFK cells were very inefficient. It is possible that no specific mechanism is involved. Particles may accidentally co-internalize with other compounds. The ability of FIPV I (Black) to induce infection of CrFK cells was restricted to only 0.003 % of the cells at 12 h post-inoculation. Infectivity of FIPV I (Black) in CrFK cells is the subject of contradictory literature. Black (1980) reported susceptibility of CrFK cells, whereas Hohdatsu et al. (1998) failed to induce infection. It is possible that susceptibility of CrFK cells may differ due to genetic changes during long-term subpassaging. CrFK cells are often used as a model cell line for FIPV serotype II infection studies. However, internalization of FIPV II (79-1146) was slower and less efficient than in monocytes. Thus, FIPV seems to interact differently with CrFK cells and with monocytes. This confirms earlier observations (Dewerchin et al., 2005). It is concluded that the CrFK cell line is not suitable for studying FIPV entry into target cells.
The findings concerning attachment and internalization in monocytes are useful for further characterization of the FIPV entry process. Unravelling this mechanism may lead to new targets for drug development.
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ACKNOWLEDGEMENTS |
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Received 9 March 2007;
accepted 21 May 2007.
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-FIPV–FITC and nucleocapsid protein with mAb E22-2 and goat