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Cluj Vet J 2021, 26, 3 http://clujveterinaryjournal.ro 

Communication 

Assessing two strategies for production of murine ascites with 
anti-SARS-CoV-2 monoclonal antibodies 
 
Joel Javier Pérez-Paz1, Reinaldo Blanco1, Dayamí Dorta1, Andy Domínguez1, Maylin Pérez-Bernal1,*, Celia Tamayo1, 
Carlos Hernández1, Ricardo Pina1, Javier Díaz1, Shaylí Pérez1, Ivis Pasarón1 and Enrique Pérez1 

1 Center for Genetic Engineering and Biotechnology of Sancti Spiritus, Circunvalante Norte, Olivos 3, Sancti 
Spiritus, Cuba 

* Correspondence: maylin.perez@cigb.edu.cu 

Abstract: Studies were conducted to improve the production of murine ascites with monoclonal antibodies that recognize 
SARS-CoV-2 proteins. BALB/c mice were primed with 0.5 mL of mineral oil into the abdominal cavity. Seven days after priming, 
mice were divided in two groups: the group 1 was inoculated intraperitoneally with 2x106 cells/mL of MAb-secreting hybridomas 
against the nucleocapsid and spike proteins of SARS-CoV-2; the group 2 was injected simultaneously with the same inoculum of 
hybridoma cells and mineral oil, 18 days after priming. No disturbances or suffering signals were observed in mice from both 
groups, suggesting that double administration of mineral oil did not produce significant distress with respect to the single dose 
used for priming, and that none of the hybridoma cell lines were particularly aggressive for the inoculated mice. Ascites was col-
lected in 90.48% and 97.68% of mice from groups 1 and 2, respectively. Ascites was not collected in 7.42% of all mice. The main 
cause was they never developed ascites tumors but no solid tumors were observed either. The volume of ascitic fluid per mouse 
was increased significantly in mice from group 2, and there were no significant differences between groups in terms of the con-
centration of IgG in clarified ascites. According to these results, to obtain higher amounts of MAb the strategy applied in group 2 
should be used, since it showed the best results in the development of ascites tumors and it significantly increased the volume of 
ascites fluid per mouse. This could allow the use of fewer animals for ascites production, which is an ethical and economic benefit. 

Keywords: ascites; hybridoma; mice; mineral oil 
 

1. Introduction 
At the end of 2019, a new coronavirus began to spread in China to become the 

pandemic that has cost the most human lives: the SARS-CoV-2. All efforts of healthcare 
personnel and scientists have had to focus on its rapid diagnosis and treatment. In the 
strategies to reduce the viral transmission, monoclonal antibodies (MAbs) have become 
reliable tools, especially for immunoassay techniques used for diagnosis, which are 
cost-effective, sensitive, rapid and selective [1].  
     There are basically two main phases in the production of MAbs: the selection of 
MAb-producing hybridoma cells, generated by the fusion of antibody-producing lym-
phoid cells from an immunized mouse and murine myeloma cells, and the propagation 
of selected hybridoma clones, in vivo or in vitro. The in vivo production of MAbs has been 
carried out by injecting the hybridoma cells into the mice abdominal cavity. The propa-
gation of these cells in the ascitic fluid offers a rapid and economical route to the pro-
duction of MAbs [2]. In vitro production techniques have been developed over the years 
as alternatives to in vivo production of MAbs, but many of these techniques have not 
been practical due to their high cost, requirements for specialized equipment or their 
propensity to become contaminated [3]. MAb production in transgenic plants is also a 
promising technology, since they are considered inexpensive and facile production 
platforms for recombinant MAbs [4], but it still not solves the great demand of these 
molecules. Consequently, the total replacement of the ascites method is not yet possible, 
much less in the current pandemic context, when the rapid and effective production of 
MAbs is needed for the diagnosis and control of viral transmission.   

Received: 20.10.2021 

Accepted: 26.11.2021 

Published: 31.12.2021 

DOI: 10.52331/cvj.v26i3.31 

 

 

Copyright: © 2021 by the authors. 

Submitted for possible open access 

publication under the terms and 

conditions of the Creative Commons 

Attribution (CC BY) license 

(http://creativecommons.org/licenses

/by/4.0/). 



Cluj Vet J 2021, 26, 3  
 

3 

Under these circumstances, medium or large-scale productions of MAbs are required. Medium-scale produc-
tion is demanded to make 0.1-1.0 g quantities for diagnostic or developing assays [3]. Large-scale production re-
quires an extensive inoculation program with a large number of mice, mostly when a single mouse is inoculated 
with a single antigen. Hence, from an ethical and economic perception, it would be necessary to acquire efficient 
and high throughput strategies to maximize the MAb production and to reduce the number of animals used [5]. 

The Center for Genetic Engineering and Biotechnology of Sancti Spiritus (CIGBSS), Cuba, has been in charge of 
generating several MAb-secreting hybridomas against the nucleocapsid and spike proteins of SARS-CoV-2 virus. 
These antibodies must be produced to satisfy the increasing demands of the enzyme-linked immunosorbent assay 
(ELISA) applied currently for detecting SARS-CoV-2 antigen. The present work aimed to improve the production of 
murine ascites containing anti-SARS-CoV-2 MAbs, by assessing two strategies for the administration of mineral oil 
for BALB/c mice: a single injection, as priming agent, 7 days before the inoculation of hybridoma cells and the sim-
ultaneous injection of mineral oil and hybridoma cells to mice previously primed with mineral oil. The capability of 
mice to develop ascitic tumors, the volume of ascites per mouse and the IgG concentration in clarified ascites were 
monitored to select the best strategy for production of ascites with anti-SARS-CoV-2 MAbs.  

 
2. Materials and Methods 

2.1 Hybridoma cell lines  
For the production of anti-SARS-CoV-2 MAbs in murine ascites, there were used six hybridoma cell lines 

generated by CIGBSS, Cuba. Three of them secrete MAbs that recognize the SARS-CoV-2 nucleocapsid protein 
(CBSSNCOV.2, CBSSNCOV.3 and CBSSNCOV.10) and the others recognize the receptor binding domain (RBD) of 
the SARS-CoV-2 spike protein (CBSSRBD-S.1, CBSSRBD-S.2 and CBSSRBD-S.3). 

 

2.2 Mouse priming and inoculation  
BALB/c male and female mice of 22 ± 1 and 24 ± 1 g of weight, respectively, were used for ascitic fluid pro-

duction. They were maintained in Eurostandard type II cages (268 mm x 215 mm x 141 mm) at (22-25) °C and 
50-65% relative humidity. All animals were primed with 0.5 mL of mineral oil (Zahori, Mexico) in the abdominal 
cavity. Each hybridoma inoculum was prepared at a dilution of 2 x 106 cells per milliliter of RPMI-1640 medium 
(Sigma, Hybri-Max). Before the inoculation of cells, the mice were divided into two groups: the group 1 was in-
oculated intraperitoneally with 1 mL of cell suspension seven days after priming, and the group 2 was inoculated 
analogously to group 1 but 18 days after priming and simultaneously with 0.5 mL of mineral oil. 

The mice were observed at least twice daily, including weekends, by personnel familiar with the clinical signs 
related to the production of ascites, to assess the degree of abdominal distention and to monitor health and 
well-being. 

All the experimental procedures were approved by the Ethical Committee on Animal Experimentation of the 
Center for Genetic Engineering and Biotechnology (CIGB, Havana, Cuba). 

 

2.3 Ascites harvest 
The extraction of ascites was carried out in two times: 10 and 12 days after the inoculation of the hybridoma 

cells. Before the ascites extraction, the abdomen of each mouse was cleaned with 70% ethanol. The abdominal 
paracentesis was performed in the right side of the inguinal-abdominal region with an 18-gauge needle, inserted 
at a 35-45° angle. The ascites fluid was harvested in 50 mL Corning tubes with 150 µL of 8% EDTA. The ascites was 
clarified by centrifugation at 1125 x g for 30 min at (22-25) °C and filtered through 0.45 µm glass wool.  

 

2.4 Quantification of IgG in clarified ascites 
The quantification of murine IgG in clarified ascites was performed by a sandwich ELISA. Polystyrene 

96-well microtiter plates (Costar 3590 High Binding) were coated with 10 µg/mL goat anti-mouse IgG polyclonal 
antibody in 100µL/well coating buffer (10mM carbonate-bicarbonate buffer, pH 9.6) and incubated 2 h at 37°C. 
Wells were washed twice with 380µL/well washing buffer (0.05% Tween-20) and blocked with 200µL/well 
blocking buffer (phosphate-buffered saline, pH 7.2, and 1% nonfat milk). This and the two subsequent steps were 
carried out 1 h at 37°C. The plate was washed once and 100 µL of ascites diluted 1:20 0000 with blocking buffer 
were added to wells. The calibration curve was prepared in the range of 150 ng/mL to 2.34 ng/mL using the 
CBSSPSA.4, supplied by CIGBSS as standard MAb. Plates were washed three times and 100µL of peroxidase 



Cluj Vet J 2021, 26, 3  
 

4 

(HRP)-conjugated goat anti-mouse IgG diluted 1:8 000 with blocking buffer were added per well and incubated. 
After four washes, 100µL/well 5.5mg/mL o-phenylenediamine dihydrochloride with 0.015% hydrogen peroxide 
in 0.1M citrate-phosphate buffer, pH 5.0, were added and incubated 20 min at (22-25) °C in the dark. The reaction 
was stopped with 100µL/well of 2M sulfuric acid, and the absorbance was measured at 492 nm in a microplate 
reader (Labsystems Multiskan® Plus, Finland). 

 
2.5 Statistical analysis 
The data of ascites volume per mouse and IgG concentration in clarified ascites, obtained from each group of 

mice, were analyzed by means of independent samples t-test (ɑ=0.05) using the Statistical Package for Social Sci-
ence, version 15.0. 

 
3. Results 

Mice developed the gradual swelling of the abdomen that accompanies the accumulation of ascites fluid over a 
period of 7 days after the injection of hybridoma cells. The daily observation did not detect evidence of distress in 
the animals of both groups under study: their coats were clean and smooth, they maintained food and water con-
sumption and the activity into the cages was normal in all of them.  

The first extraction of ascites was performed 10 days after the inoculation of the cells. At that time, the ab-
dominal distention was moderate since ascites fluid volumes did not exceed 20% of the baseline body weight of 
mice. Each mouse was tapped two times. After the second and last tap, the following clinical irregularities were 
perceived in some animals of both groups: hunched posture, piloerection and decreased activity.  

Table 1 shows the number of mice used in both ascites producing groups and the number of mice from which it 
was possible to collect ascites in two taps. There were included 889 mice in the study, 29.13% of them received the 
simultaneous injection of mineral oil and hybridoma cells after being primed with mineral oil. In this group, 97.68% 
of the mice developed ascitic tumors and it was possible to collect ascites from them, while from the group that re-
ceived a single dose of mineral oil the ascites was collected in a smaller number of mice (90.47%). Ascites was not 
collected in 7.42% of all mice; the main cause was they never developed ascites tumors but no solid tumors were 
observed either. Within this percentage, the minority fraction, 9.09%, corresponded to the group 2. The observations 
performed by personnel familiar with the clinical signs associated with the production of ascites, did not report 
differences in the behavior and appearance of the mice, related to the hybridoma cell lines that were inoculated, 
which means that none of these lines was particularly aggressive to the well-being of mice. 

 

Table 1.  Distribution of the mice inoculated with hybridoma cell lines and the mice that pro-
duced ascites in both groups under study.  

Hybridoma cell 
lines 

No. mice injected No. mice collected 
Group 1 Group 2 Group 1 Group 2 

CBSSNCOV.2 130 10 124 10 
CBSSNCOV.3 100 130 93 126 
CBSSNCOV.10 120 50 117 50 
CBSSRBD-S.1 150 25 120 23 
CBSSRBD-S.2 50 24 39 24 
CBSSRBD-S.3 80 20 77  20 

Group 1: Mice were inoculated intraperitoneally with 2x106 cells of each hybridoma sus-
pended in 1 mL of RPMI-1640 medium, 7 days after priming with 0.5 mL of mineral oil.  
Group 2: Mice were inoculated simultaneously with the same inoculum of hybridoma 
cells and 0.5 mL of mineral oil, 18 days after priming with 0.5 mL of mineral oil. 

The volume of ascites per mouse was calculated by dividing the total volume of ascites harvested by the 
number of mice collected in each group. The values ranged from 1.38 to 4.90 mL per animal. The independent 
samples t-test demonstrated that both groups under study were significant different in terms of the volume of as-



Cluj Vet J 2021, 26, 3  
 

5 

cites per mouse. Simultaneous injection of mice with mineral oil and hybridoma cells increased these volumes in all 
cases with respect to the mice injected with a single dose of mineral oil (Figure 1). The mean ascites volume per 
mouse in this group was 1.75-fold higher than the estimated mean for group 1. 

Fluctuations in the IgG concentration quantified in clarified ascites, both within and between groups, were 
observed (Table 2). Nevertheless, the analysis of the data from both groups following an independent samples 
t-test, showed no significant differences in the mean IgG concentration in clarified ascites between both groups 
(t=1.115; p=0.291). In addition, an approximation of the MAb mass yield per mouse was also similar, 17.64 mg for 
group 1 and 16.48 mg for group 2.  

 

              

Figure 1.  Volume of ascites produced per mouse in both groups under study. All mice were 
primed with 0.5 mL of mineral oil into the abdominal cavity. Group 1: Seven days after priming, 
mice were inoculated intraperitoneally with 2x106 cells of each hybridoma suspended in 1 mL of 
RPMI-1640 medium. Group 2: Eighteen days after priming, mice were inoculated simultaneously 
with the same inoculum of hybridoma cells and 0.5 mL of mineral oil. The independent samples 
t-test showed that the volume of ascites per mouse was significant different between the groups 
(t= -3.747; p= 0.004). 

 

Table 2. Volume of clarified ascites and resultant quantity of IgG from each hybridoma cell line 
inoculated in both groups of mice.  

 
Hybridoma cell 

lines 

Clarified ascites 
volume (mL) 

IgG in ascites 
(mg/mL) 

Group 1 Group 2 Group 1 Group 2 

CBSSNCOV.2 215 25 9.06 8.56 
CBSSNCOV.3 187 280 7.19 9.05 
CBSSNCOV.10 300 82 9.95 6.28 
CBSSRBD-S.1 300 72 2.49 5.81 
CBSSRBD-S.2 103 72 13.04 3.64 
CBSSRBD-S.3 265 98 6.83 3.67 

Group 1: Mice were inoculated intraperitoneally with 2x106 cells of each hybridoma sus-
pended in 1 mL of RPMI-1640 medium, 7 days after priming with 0.5 mL of mineral oil.  
Group 2: Mice were inoculated simultaneously with the same inoculum of hybridoma 
cells and 0.5 mL of mineral oil, 18 days after priming with 0.5 mL of mineral oil. 

 

 

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

CBSSNCOV.2 CBSSNCOV.3 CBSSNCOV.10 CBSSRBDS-1 CBSSRBDS-2 CBSSRBDS-3

A
sc

ite
s 

pe
r m

ou
se

 (m
L/

m
ou

se
)

Hybridoma cell lines

Group 1 Group 2



Cluj Vet J 2021, 26, 3  
 

6 

4. Discussion 

In this work we assayed two strategies for production of murine ascites containing anti-SARS-CoV-2 MAbs 
using BALB/c mice. Since sensitive techniques have not been developed to measure signs that might indicate the 
presence of pain or distress [6], one of the most important tasks in the present study was the daily observation of the 
mice involved, including the examination and palpation of the injection site, the evaluation of abdominal distention 
and the detection of all possible side effects of the injected mixture. Some hybridoma cell lines can produce clinical 
signs in mice indicating distress, such as anorexia, hunched posture, hypothermia, rapid breathing and decreased 
activity [6]. But none of the six hybridoma cell lines used in this work caused worrisome adverse effects as a sign of 
an aggressive cell line, and this was an important ethical advantage. 

Regarding the timing of priming agent administration in relation to the inoculation of the hybridoma cells, it is 
common practice to perform priming and several days later to inject the hybridoma cell-suspension into the peri-
toneal cavity of the mouse. This leads to the development of a tumor, the accumulation of ascitic fluid and the ab-
dominal distention [2, 6]. However, it is not usual the simultaneous administration of the hybridoma cells with an 
additional dose of the priming agent. In the present study, the objective of the second administration of the mineral 
oil was to increase the throughput in the production of ascites. Mineral oil is thought to act by inducing granulom-
atous inflammation and interfering with peritoneal lymphatic drainage, thus increasing the volume of ascites 
produced [7]. We consider that the purpose was achieved because it was possible almost to double the volume of 
ascites per mouse in the group that received the second injection of mineral oil. This procedure could cause animal 
distress, but it did not occur in the mice included in this work. Some authors have evaluated the effects of priming 
agent injection on mouse well-being and ascites production, using parameters related with the mouse activity and 
food and water consumption [8]. In our study, the observation of mice at least twice a day by qualified personnel, 
did not inform significant evidences of distress or pain in the animals, since they maintained their normal activities 
and external appearance until the second tap. The volume of ascitic fluid did not cause gross abdominal distention 
even when a double volume of the priming agent was administered; moreover, the abdominal tap was done before 
fluid accumulation became excessive and distressful, following the recommendations that it not exceed 20% of the 
baseline body weight of the mouse [6]. 

Ascites yields can be improved also by performing several harvests; nevertheless, the well-being of mice must 
be strictly observed. The number of taps should be restricted according to animal welfare and characteristics of the 
hybridoma being used. Some hybridomas seem to cause little distress and various taps could be permissible [9], but 
the prolongation of tapping time increases the pathological abnormalities in the mice due to solid tumor growth 
within the peritoneal cavity and the accumulation of ascites [10]. Various guidelines and reports have required that 
the abdomen may be tapped no more than twice before the mouse is euthanized for final harvest of ascites [2, 8, 11]. 
The present study was careful with this aspect, since ascites pressure was relieved before abdominal distention was 
great enough to cause discomfort or disturb the normal activity of the animals, and only two taps were performed 
to obtain ascites, despite the fact that the hybridomas inoculated did not cause adverse effects in mice. 

A crucial finding was that there were no significant differences in the concentration of IgG in ascites and in the 
mass yield of MAb per mouse between the groups analyzed. According to these results, in order to obtain a greater 
mass of MAb, the strategy applied in the group of mice that significantly increased the volume of ascites fluid per 
mouse must be taken into account: the simultaneous injection of hybridoma cells and mineral oil eighteen days after 
priming. Furthermore, the lowest percentage of mice that never developed ascites tumors was found in this group. 
Consequently, this strategy could allow the usage of fewer animals for the production of ascites. It is clear that the 
number of mice to be used will be determined by the total amount of MAbs needed [10]; but, certainly, the reduc-
tion of the amount of animals required for the MAbs production offers important ethical and economic advantages. 

Medium-scale production of MAbs is demanded to make 0.1-1.0 g quantities for diagnostic or developing as-
says [3]. Applying the strategy used in group 2 it was possible to produce more quantities than 1.0 g of each MAb. 
For example, mice included in this group produced approximately 4.0 g of CBSSNCOV.3 and more than 1.0 g of 
CBSSNCOV.10. Both MAbs are being used in the ELISA developed in our country for the diagnosis of SARS-CoV-2 
by detecting viral antigen, and they are constantly demanded for immediate diagnosis as urgent necessity of cur-
rent epidemiological scenario. Therefore, with the simultaneous injection of hybridoma cells and mineral oil in 
previously primed mice, it is possible to guarantee the production of required quantities of both MAbs with a 
minimum number of animals involved. 



Cluj Vet J 2021, 26, 3  
 

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The MAbs against the RBD of the SARS-CoV-2 spike protein (CBSSRBD-S.1, CBSSRBD-S.2 and CBSSRBD-S.3) 
could play an important role in subunit vaccines development. For this reason, it is necessary to optimize all the 
productive stages to ensure the availability of these MAbs for future assays, and this work also contributes to this.  
5. Conclusions 

The double administration of mineral oil, for priming and for the inoculation of hybridoma cells, does not 
produce additional stress in the mice. None of the six anti-SARS-CoV-2 MAb-secreting hybridoma cell lines, which 
were injected to mice, are harmful to the animal well-being. The simultaneous inoculation of the hybridoma cells 
with the mineral oil to the primed mice improves the efficiency of ascites tumor formation and significantly in-
creases the volume of ascites per mouse. This inoculation strategy will optimize the production of ascites with an-
ti-SARS-CoV-2 MAbs, involving fewer animals, which translates into ethical and economic benefits. 

 

Author Contributions: Conceptualization, J.J.P.P. and M.P.B; methodology, J.J.P.P., R.B., D.D., A.D., C.T., R.P., J.D. and 
I.P.; validation, S.P.; formal analysis, C.H.; writing—original draft preparation, M.P.B. and J.J.P.P.; writing—review and 
editing, R.B., A.D., D.D. and M.P.B.; supervision, R.B. and E.P. All authors have read and agreed to the published ver-
sion of the manuscript.  

Funding: This research received no external funding.  

Conflicts of Interest: The authors declare no conflict of interest. 

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