key: cord-0835257-qoi00ydx
authors: Florek, Dominik; Burmistrz, Michal; Potempa, Jan; Pyrc, Krzysztof
title: Stability of infectious human coronavirus NL63
date: 2014-09-01
journal: J Virol Methods
DOI: 10.1016/j.jviromet.2014.04.001
sha: a68006d01847e1b81aad9a4f33db967ec99f0756
doc_id: 835257
cord_uid: qoi00ydx

The human coronavirus NL63 was identified in 2004 and subsequent studies showed its worldwide distribution. Infection with this pathogen is associated with upper and lower respiratory tract diseases of mild to moderate severity. Furthermore, HCoV-NL63 is the main cause of croup in children. Within this study an optimal protocol for freeze-drying that allows safe and effective preservation of HCoV-NL63 infectious material was developed. Lyophilized virus preparations can be stored either at ambient temperature or at +4 °C. In the latter case samples may be stored for at least two months. Surprisingly, conducted analysis showed that HCoV-NL63 virions are exquisitely stable in liquid media and can be stored also without preservatives at ambient temperature for up to 14 days.

East respiratory syndrome coronavirus (MERS-CoV) -crossed the species border, proving that the emergence of SARS-CoV was not an isolated case (Centers for Disease and Prevention, 2013; Zaki et al., 2012) .

Intensified research on coronaviruses and development of a novel research tool based on a coronaviral backbone (e.g., vectors, vaccines) requires a method for safe storage and shipment of infectious material. Coronavirus particles are relatively stable, but previous reports show that this stability is insufficient for storage and transport of coronavirus-based vaccines, vectors and for the material exchange between laboratories (Lamarre and Talbot, 1989; Muller et al., 2008) . Lyophilization is the method of choice for cost-effective stabilization of infectious material. It is a process, where a sample is frozen and subsequently the solvent is removed by sublimation (primary drying) and then by desorption (secondary drying) to the level that inhibits biological growth and chemical reactions (Jennings, 1999) . To improve the stability of a sample during lyophilization and storage stabilizing substances, including lyoprotectants (Townsend and DeLuca, 1988 ) and bulking agents (Jennings, 1999) , are added. There are numerous reports describing the freeze-drying of viruses, mainly for vaccine production (Amorij et al., 2008; Audouy et al., 2011; Bieganski et al., 1998; Burger et al., 2008; Croyle et al., 1998; de Jonge et al., 2007; Furuya et al., 2010; Geeraedts et al., 2010; Gupta et al., 2010; Kang et al., 2010; Lang et al., 2009; Levy and Fieldsteel, 1982; Yannarell et al., 2002) . These methods differ in terms of process parameters and buffer content and stability of preparations ranging from 12 weeks http://dx.doi.org/10.1016/j.jviromet.2014.04.001 0166-0934/© 2014 Elsevier B.V. All rights reserved.

at +4 • C (de Jonge et al., 2007) to one year at ambient temperature (Geeraedts et al., 2010; Levy and Fieldsteel, 1982) . Precausta et al. developed a protocol that allowed storage of the Infectious Bronchitis Virus (IBV) for 12 months at +6 • C using 40 mg/ml mannitol as the lyoprotectant (Precausta et al., 1980) . Roseto et al. lyophilized Bovine Enteric Coronavirus without addition of any stabilizing formulation for structural studies using electron microscopy (Roseto et al., 1982) . Virus stability during storage was not tested. No studies on freeze-drying preservation of other coronaviruses, including human coronaviruses were previously reported.

Human coronavirus NL63 belongs to the genus alphacoronavirus (International Committee on Taxonomy of Viruses and King, 2012; Pyrc et al., 2006 Pyrc et al., , 2007 van der Hoek et al., 2004) . This pathogen is spread worldwide and is responsible for 1-10% of cases of viral respiratory tracts diseases (Bastien et al., 2005; Cabeca et al., 2012; Gaunt et al., 2010; Kon et al., 2012; Moes et al., 2005) . Symptoms of HCoV-NL63 upper respiratory tract infection include rhinitis, cough, fever and sore throat, while infection of the lower respiratory tract manifests in bronchitis, bronchiolitis and pneumonia (Pyrc et al., 2007) . Furthermore, HCoV-NL63 was proved to be the main cause of croup in children (Sung et al., 2010; van der Hoek et al., 2005) .

The aim of this study was the development of a safe method for freeze-drying preservation of infectious HCoV-NL63. To this end LLC-MK2 cells (ATCC: CCL-7; Macaca mulatta kidney epithelial cell line) were used for in vitro culture of HCoV-NL63 (Schildgen et al., 2006) . The cells were cultured on T25 flasks (TPP, Germany) at +37 • C with 5% CO 2 in minimal essential medium (MEM), containing 2 parts of Hank's MEM and 1 part of Earle's MEM (Life Technologies, Poland) supplemented with 3% heat-inactivated fetal bovine serum (Life Technologies, Poland), penicillin (100 U/ml), and streptomycin (100 g/ml) (H/E medium). HCoV-NL63 (Amsterdam I strain) stock was generated by infecting LLC-MK2 cells. Infected cells were lysed 6 days post-infection by two freeze-thaw cycles. The virus-containing fluid was cleared by centrifugation, aliquoted and stored at −80 • C. A control from mock infected cells was prepared in the same manner as the virus stocks. Virus yield was assessed by virus titration on fully confluent LLC-MK2 cells, according to Reed and Muench formula (Reed and Muench, 1938) . All formulations used during lyophilization are presented in Table 1 and were prepared as 2× concentrated solutions in distilled water. Five times concentrated HBS buffer was prepared in distilled water. All samples were sterilized by filtration. Samples were prepared as follows: formulations 1-7: 80 l 5× HBS, 20 l sterile water, 200 l 2× concentrated stabilizing formulation, 100 l HCoV-NL63 stock (TCID 50 = 1.4 million) in medium H/E 3% P/S; formulations 8-13: 100 l sterile water, 200 l of 2× concentrated stabilizing formulation, 100 l HCoV-NL63 stock (TCID 50 = 1.4 million) in medium H/E 3% P/S; HBS control: 80 l 5× HBS, 220 l sterile water, 100 l HCoV-NL63 stock (TCID 50 = 1.4 million) in medium H/E 3% P/S; PBS control: 300 l 1× PBS (NaCl 8.0 g/l, KCl 0.2 g/l, Na 2 HPO 4 1.44 g/l, KH 2 PO 4 0.24 g/l, pH 7.4), 100 l HCoV-NL63 stock (TCID 50 = 1,4 million) in medium H/E 3% P/S:-H/E 3% P/S. The solutions were prepared in 2 ml tubes (Sarstdedt, Germany), snap frozen in liquid nitrogen and positioned within the sterile, pre-cooled lyophilization container with open caps.

The drying process was carried out in the shelf lyophilizer -TMFreeZone Triad 7400030 (Labconco) with attached vacuum pump 195 (Labconco). To ensure safety, the sealed box was prepared from poly(methyl methacrylate) with a rubber seal. The container was designed for a standard probe stand. For sterilization, the box was disinfected with 70% ethanol and exposed to ultraviolet radiation. The gas exchange between the container and the lyophilizer was possible via the vent protected with a 0.1 m air filter (Sartorius stedim, Germany). Before the lyophilization the container was pre-cooled at −20 • C. Process parameters were chosen based on literature data. In order to maintain stable temperature, the apparatus was stabilized for 3-4 h before the process (shelf temperature −30 • C, collector temperature −80 • C, vacuum off). The first drying step was carried out for 40 h at −30 • C and the pressure of 0.22 mBar. After this time the temperature was increased to −10 • C and the process was carried out for another 24 h. Subsequently, samples in the container were transferred to the laminar flow hood and capped. All samples were stored at −20 • C, +4 • C or at room temperature for stability testing.

The freeze-dried virus was dissolved in 400 l of sterile water and incubated at +37 • C with mixing for 15 min to facilitate the dissolving of the cake. Obtained solution was diluted 5 times in H/E medium. LLC-MK2 cells were seeded on the 96-well culture plate and incubated for 48 h at +37 • C with 5% CO 2 to obtain fully confluent cell monolayer. Medium was removed from the cells and fresh medium containing serially diluted virus or control samples reconstituted from lyophilized material were added. Two hours post-inoculation medium was removed and fresh H/E medium was applied. Six days post infection the cytopathic effect on the LLC-MK2 cells was assessed. Obtained data were re-calculated to TCID 50 according to Reed and Muench formula (Reed and Muench, 1938) .

The primary screening of different buffer composition revealed that already during the freeze-drying process some formulations do not provide sufficient protection. First, 1× HBS buffer was selected over the 1× PBS buffer, as PBS exhibits significant pH instability during the freezing process (Amorij et al., 2008) . Second, the requirement for a bulking agent was confirmed, in line 50Mi2Leu a Myo-inositol 50 g/l 2 g/l Leucine 13. 30Mi15Gel a Myo-inositol 30 g/l 15 g/L Gelatin a Samples supplemented with gelatin (25 g/l; with the exception of formulation 13), L-arginine (16 g/l), alanine (1 g/l), and histidine (2.1 g/l). Presented data is shown as % of TCID50 in the relation to PBS control. All formulation names are described in Table 1. with observations made by Jennings (1999) . Lack of a bulking agent (e.g., in sample Sucr0.5) resulted in excessive foaming during freeze-drying process, what rendered the storage of the sample impossible. Conversely, samples buffered with Sucr74.6, containing similar concentration of sucrose as samples buffered with Sucr0.5, produce during the freeze-drying a cake of good structural properties. This can be explained by the presence of gelatin.

Formulations that contained high concentration of sugars (Treh100, Sucr0.5, and Sucr74.6); sorbitol (35Mi15Sor, Sor10) or inulin (In25, In40) turned out to be toxic for the cells during the incubation with the virus. To prevent the cytotoxic effect of these formulations, the virus/mock medium was applied on cells for 2 h; following this time cells were washed and fresh medium was applied. Samples lyophilized with Sor2, Sor10 and Treh100 (Table 1) buffers showed lower TCID 50 values, compared to reference samples, i.e., 1× PBS and 1× HBS (Fig. 1) . The conducted experiments proved that mannitol is a preferable bulking agent compared to sorbitol, as cell toxicity was observed with the latter carbohydrate but not the former one. This analysis showed also that myo-inositol is optimal as a lyoprotectant for HCoV-NL63 virions.

Based on obtained results three formulations yielding most promising results were selected. Selected samples contained myo-inositol as lyoprotectant and mannitol as bulking agent (25Mi25Mann, 35Mi15Mann and 50Mi). All these buffers were supplemented with gelatin, l-arginine (Merck, Germany), alanine and histidine (Sigma-Aldrich, Poland).

To test the stability of freeze-dried virus in different conditions, lyophilized samples were stored at −20 • C, +4 • C or at room temperature (RT). Preparations of the lyophilized virus were solubilized at days 0, 7, 14, 28 and 56 post-lyophilization and titrated. A set of original, not lyophilized samples was stored in identical conditions as lyophilized samples in H/E 3% P/S medium and used as a control (Fig. 2) .

Conducted analysis revealed that two formulations containing myo-inositol (50 g/l and 35 g/l with 15 g/l mannitol) supplemented with gelatin (25 g/l), l-arginine (16 g/l), alanine (1 g/l) and histidine (2.1 g/l) in 20 mM HEPES provided optimal stability of HCoV-NL63 virions. Sample storage at −20 • C or +4 • C for 56 days did not affect virion infectivity. When the preparation was stored at ambient temperature, the considerable virus titer decrease was observed during first weeks. These most likely resulted from hydrolysis reactions caused by residual water present in samples (Jennings, 1999) . TCID50 as a measure of infectivity of the lyophilized virus was assessed 6 days post infection. All assays were performed in duplicate in at least three independent experiments and average values with standard errors (error bars) are presented. Time point 0 represents samples titrated immediately after lyophilization. All formulations are described in Table 1 . Graphs represent samples stored at −20 • C (A), +4 • C (B) or at ambient temperature (C).

Further, a slight increase in the virus titer after 56 days of storage at +4 • C for both storage buffers was observed. Similar phenomenon was previously observed and it was suggested that virions aggregate during freeze-drying process what lowers their infectivity. Prolonged storage results in aggregate dissociation and restoration of the infectivity (Cowdery et al., 1976) .

Coronavirus stability in suspension was researched by several groups. Muller et al. showed that HCoV-NL63 was still infective after 7 days of incubation at ambient temperature in PBS medium (Muller et al., 2008) . Suspension of 229E infectivity was lost after 14 days of incubation at 22, 33 or 37 degrees (Lamarre and Talbot, 1989) . Here we showed that HCoV-NL63 virions were still detectable after 14 days of storage in cell culture medium at room temperature. Further, virions retained infectivity at +4 • C for 56 days, showing similar decrease in virus TCID 50 as freeze dried samples.

Summarizing, the optimal protocol for freeze-drying that allows safe and effective preservation of HCoV-NL63 infectious material was developed. Safety of the process is provided by usage of the inner container, sealed with the air filtration unit. Considering the need for distribution of known and novel coronaviral pathogens to different laboratories and as a reference material, the developed method may facilitate the shipment process and significantly lower the transport cost. Surprisingly, it appeared that HCoV-NL63 virions are highly stable and can be stored at ambient temperature for up to 14 days. Obtained preparations can be shipped either at ambient temperature or at +4 • C. In the latter case samples may be stored for at least two months.

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This work was in supported by the LIDER grant from the National Centre for Research and Development (Lider/27/55/L-2/10/2011) (KP). The Faculty of Biochemistry, Biophysics and Biotechnology of the Jagiellonian University is a beneficiary of the structural funds from the European Union (grant no: POIG.02.01.00-12-064/08 -"Molecular Biotechnology for Health").