viruses
Article
SARS–CoV–2 Spike Impairs DNA Damage Repair and Inhibits
V(D)J Recombination In Vitro
Hui Jiang 1,2,* and Ya-Fang Mei 2,*
Citation: Jiang, H.; Mei, Y.-F.
SARS–CoV–2 Spike Impairs DNA
Damage Repair and Inhibits V(D)J
Recombination In Vitro. Viruses 2021,
13, 2056. https://doi.org/10.3390/
v13102056
Academic Editor: Oliver Schildgen
Received: 20 August 2021
Accepted: 8 October 2021
Published: 13 October 2021
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1 Department of Molecular Biosciences, The Wenner–Gren Institute, Stockholm University,
SE-10691 Stockholm, Sweden
2 Department of Clinical Microbiology, Virology, Umeå University, SE-90185 Umeå, Sweden
* Correspondence: hui.jiang@su.se (H.J.); ya-fang.mei@umu.se (Y.-F.M.)
Abstract: Severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) has led to the coronavirus
disease 2019 (COVID–19) pandemic, severely affecting public health and the global economy.
Adaptive immunity plays a crucial role in fighting against SARS–CoV–2 infection and directly influences
the clinical outcomes of patients. Clinical studies have indicated that patients with severe
COVID–19 exhibit delayed and weak adaptive immune responses; however, the mechanism by
which SARS–CoV–2 impedes adaptive immunity remains unclear. Here, by using an in vitro cell line,
we report that the SARS–CoV–2 spike protein significantly inhibits DNA damage repair, which is
required for effective V(D)J recombination in adaptive immunity. Mechanistically, we found that
the spike protein localizes in the nucleus and inhibits DNA damage repair by impeding key DNA
repair protein BRCA1 and 53BP1 recruitment to the damage site. Our findings reveal a potential
molecular mechanism by which the spike protein might impede adaptive immunity and underscore
the potential side effects of full-length spike-based vaccines.
Keywords: SARS–CoV–2; spike; DNA damage repair; V(D)J recombination; vaccine
1. Introduction
Severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) is responsible for
the ongoing coronavirus disease 2019 (COVID–19) pandemic that has resulted in more
than 2.3 million deaths. SARS–CoV–2 is an enveloped single positive–sense RNA virus
that consists of structural and non–structural proteins [1]. After infection, these viral
proteins hijack and dysregulate the host cellular machinery to replicate, assemble, and
spread progeny viruses [2]. Recent clinical studies have shown that SARS–CoV–2 infection
extraordinarily affects lymphocyte number and function [3–6]. Compared with mild and
moderate survivors, patients with severe COVID–19 manifest a significantly lower number
of total T cells, helper T cells, and suppressor T cells [3,4]. Additionally, COVID–19 delays
IgG and IgM levels after symptom onset [5,6]. Collectively, these clinical observations
suggest that SARS–CoV–2 affects the adaptive immune system. However, the mechanism
by which SARS–CoV–2 suppresses adaptive immunity remains unclear.
As two critical host surveillance systems, the immune and DNA repair systems are
the primary systems that higher organisms rely on for defense against diverse threats and
tissue homeostasis. Emerging evidence indicates that these two systems are interdependent,
especially during lymphocyte development and maturation [7]. As one of the major doublestrand
DNA break (DSB) repair pathways, non-homologous end joining (NHEJ) repair
plays a critical role in lymphocyte–specific recombination–activating gene endonuclease
(RAG) –mediated V(D)J recombination, which results in a highly diverse repertoire of
antibodies in B cell and T cell receptors (TCRs) in T cells [8]. For example, loss of function
of key DNA repair proteins such as ATM, DNA–PKcs, 53BP1, et al., leads to defects
in the NHEJ repair which inhibit the production of functional B and T cells, leading to
immunodeficiency [7,9–11]. In contrast, viral infection usually induces DNA damage via
Viruses 2021, 13, 2056. https://doi.org/10.3390/v13102056 https://www.mdpi.com/journal/viruses
Viruses 2021, 13, 2056 2 of 10
different mechanisms, such as inducing reactive oxygen species (ROS) production and
host cell replication stress [12–14]. If DNA damage cannot be properly repaired, it will
contribute to the amplification of viral infection-induced pathology. Therefore, we aimed to
investigate whether SARS–CoV–2 proteins hijack the DNA damage repair system, thereby
affecting adaptive immunity in vitro.
2. Materials and Methods
2.1. Antibodies and Reagents
DAPI (Cat #MBD0015), doxorubicin (Cat #D1515), H2O2 (Cat #H1009), and β-tubulin
antibodies (Cat #T4026) were purchased from Sigma-Aldrich. Antibodies against His tag
(Cat #12698), H2A (Cat #12349), H2A.X (Cat #7631), γ–H2A.X (Cat #2577), Ku80 (Cat # 2753),
and Rad51(Cat #8875) were purchased from Cell Signaling Technology (Danvers, MA, USA).
53BP1(Cat #NB100-304) and RNF168 (Cat #H00165918–M01) antibodies were obtained from
Novus Biologicals (Novus Biologicals, Littleton, CO, USA). Lamin B (Cat #sc–374015), ATM
(Cat #sc–135663), DNA–PK (Cat #sc–5282), and BRCA1(Cat #sc–28383) antibodies were purchased
from Santa Cruz Biotechnology (Santa Cruz, CA, USA). XRCC4 (Cat #PA5–82264)
antibody was purchased from Thermo Fisher Scientific (Waltham, MA, USA).
2.2. Plasmids
pHPRT–DRGFP and pCBASceI were kindly gifted by Maria Jasin (Addgene plasmids
#26476 and #26477) [15]. pimEJ5GFP was a gift from Jeremy Stark (Addgene plasmid
#44026) [16]. The NSP1, NSP9, NSP13, NSP14, NSP16, spike, and nucleocapsid
proteins were first synthesized with codon optimization and then cloned into a mammalian
expression vector pUC57 with a C–terminal 6xHis tag. A 12–spacer RSS–GFP
inverted complementary sequence–a 23–spacer RSS was synthesized for the V(D)J reporter
vector. Then, the sequence was cloned into the pBabe–IRES–mRFP vector to
generate the pBabe–12RSS–GFPi–23RSS–IRES–mRFP reporter vector. 12–spacer RSS sequence:
5 –CACAGTGCTACAGACTGGAACAAAAACC–3 . 23–spacer RSS sequence:
5 –CACAGTGGTAGTACTCCACTGTCTGGCTGTACAAAAACC–3 . RAG1 and RAG2
expression constructs were generously gifted by Martin Gellert (Addgene plasmid #13328
and #13329) [17].
2.3. Cells and Cell Culture
HEK293T and HEK293 cells obtained from the American Type Culture Collection
(ATCC) were cultured under 5% CO2 at 37 ◦C in Dulbecco’s modified Eagle’s medium
(DMEM, high glucose, GlutaMAX) (Life Technologies, Carlsbad, CA, USA) containing 10%
(v/v) fetal calf serum (FCS, Gibco), 1% (v/v) penicillin (100 IU/mL), and streptomycin
(100 µg/mL). HEK293T–DR–GFP and HEK293T–EJ5–GFP reporter cells were generated as
previously described and cultured under 5% CO2 at 37 ◦C in the above-mentioned culture
medium.
2.4. HR and NHEJ Reporter Assays
HR and NHEJ repair in HEK293T cells were measured as described previously
using DR–GFP and EJ5–GFP stable cells. Briefly, 0.5 × 106 HEK293T stable reporter
cells were seeded in 6–well plates and transfected with 2 µg I–SceI expression plasmid
(pCBASceI) together with SARS–CoV–2 proteins expression plasmids. Forty–eight hours
post–transfection and aspirin treatment, cells were harvested and analyzed by flow cytometry
analysis for GFP expression. The means were obtained from three independent
experiments.
2.5. Cellular Fractionation and Immunoblotting
For the cellular fraction assay, the Subcellular Protein Fractionation Kit (Thermo Fisher)
was used according to the manufacturer’s instructions. Protein lysates were quantified using
the BCA reagent (Thermo Fisher Scientific, Rockford, IL, USA). Proteins were resolved
Viruses 2021, 13, 2056 3 of 10
by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE), transferred to
nitrocellulose membranes (Amersham protran, 0.45 µm NC), and immunoblotted with specific
primary antibodies followed by HRP–conjugated secondary antibodies. Protein bands
were detected using SuperSignal West Pico or Femto Chemiluminescence kit (Thermo
Fisher Scientific).
2.6. Comet Assay
Cells were treated with different DNA damage reagents and then harvested at the
indicated time points for analysis. Cells (1 × 105 cells/mL in cold phosphate-buffered
saline [PBS]) were resuspended in 1% low–melting agarose at 40 ◦C at a ratio of 1:3 vol/vol
and pipetted onto a CometSlide. Slides were then immersed in prechilled lysis buffer
(1.2 M NaCl, 100 mM EDTA, 0.1% sodium lauryl sarcosinate, 0.26 M NaOH pH > 13) for
overnight (18–20 h) lysis at 4 ◦C in the dark. Slides were then carefully removed and
submerged in rinse buffer (0.03 M NaOH and 2 mM EDTA, pH > 12) at room temperature
(RT) for 20 min in the dark. This washing step was repeated twice. The slides were
transferred to a horizontal electrophoresis chamber containing rinse buffer and separated
for 25 min at a voltage of 0.6 V/cm. Finally, the slides were washed with distilled water,
stained with 10 µg/mL propidium iodide, and analyzed by fluorescence microscopy.
Twenty fields with approximately 100 cells in each sample were evaluated and quantified
using the Fiji software to determine the tail length (tail moment).
2.7. Immunofluorescence
Cells were seeded on glass coverslips in a 12–well plate and transfected with the
indicated plasmid for 24 h. Then, the cells were treated with or without DNA damage
reagents according to the experimental setup. The cells were fixed in 4% paraformaldehyde
(PFA) in PBS for 20 min at RT and then permeabilized in 0.5% Triton X–100 for 10 min.
Slides were blocked in 5% normal goat serum (NGS) and incubated with primary antibodies
diluted in 1% NGS overnight at 4 ◦C. Samples were then incubated with the indicated
secondary antibodies labeled with Alexa Fluor 488 or 555 (Invitrogen) diluted in 1% NGS
at RT for 1 h. Thereafter, they were stained with DAPI for 15 min at RT. Coverslips
were mounted using Dako Fluorescence Mounting Medium (Agilent) and imaged using a
Nikon confocal microscope (Eclipse C1 Plus). All scoring was performed under blinded
conditions.
2.8. Analysis of V(D)J Recombination
Briefly, V(D)J reporter plasmid contains inverted-GFP and IRES driving continuously
expressed RFP. Continuously expressed RFP is the internal transfection control. After
Recombination activation gene1/2 (RAG1/2) co–transfected into the cells, RAG1/2 will
cut the RSS and mediated induction of DSBs, if V(D)J recombination occurs, the inverted
GFPs are ligated in positive order by NHEJ repair. Then the cell will express functional
GFP. So, the GFP and RFP double positive cells are the readout of the V(D)J reporter
assay [18]. 293T cells at 70% confluency were transfected with the V(D)J GFP reporter alone
(background) or in combination with RAG1 and RAG2 expression constructs, at a ratio of
1 µg V(D)J GFP reporter: 0.5 µg RAG1: 0.5 µg RAG2. The following day, the medium was
changed, and after an additional 48 h, cells were harvested and analyzed by flow cytometry
for GFP and RFP expression.
2.9. Statistical Analysis
All experiments were repeated at least three times using independently collected or
prepared samples. Data were analyzed by Student’s t test or ANOVA followed by Tukey’s
multiple-comparison tests using GraphPad 8.
Viruses 2021, 13, 2056 4 of 10
3. Results
3.1. Effect of Nuclear–Localized SARS–CoV–2 Viral Proteins on DNA Damage Repair
DNA damage repair occurs mainly in the nucleus to ensure genome stability. Although
SARS–CoV–2 proteins are synthesized in the cytosol [1], some viral proteins are
also detectable in the nucleus, including Nsp1, Nsp5, Nsp9, Nsp13, Nsp14, and Nsp16 [19].
We investigated whether these nuclear-localized SARS–CoV–2 proteins affect the host
cell DNA damage repair system. For this, we constructed these viral protein expression
plasmids together with spike and nucleoprotein expression plasmids, which are generally
considered cytosol–localized proteins. We confirmed their expression and localization
by immunoblotting and immunofluorescence (Figures 1A and S1A). Our results were
consistent with those from previous studies [19]; Nsp1, Nsp5, Nsp9, Nsp13, Nsp14, and
Nsp16 proteins are indeed localized in the nucleus, and nucleoproteins are mainly localized
in the cytosol. Surprisingly, we found the abundance of the spike protein in the nucleus
(Figure 1A). NHEJ repair and homologous recombination (HR) repair are two major DNA
repair pathways that not only continuously monitor and ensure genome integrity but are
also vital for adaptive immune cell functions [9]. To evaluate whether these viral proteins
impede the DSB repair pathway, we examined the repair of a site-specific DSB induced by
the I–SceI endonuclease using the direct repeat–green fluorescence protein (DR–GFP) and
the total-NHEJ-GFP (EJ5–GFP) reporter systems for HR and NHEJ, respectively [15,16].
Overexpression of Nsp1, Nsp5, Nsp13, Nsp14, and spike proteins diminished the efficiencies
of both HR and NHEJ repair (Figures 1B–E and S2A,B). Moreover, we also found
that Nsp1, Nsp5, Nsp13, and Nsp14 overexpression dramatically suppressed proliferation
compared with other studied proteins (Figure S3A,B). Therefore, the inhibitory effect of
Nsp1, Nsp5, Nsp13, and Nsp14 on DNA damage repair may be due to secondary effects,
such as growth arrest and cell death. Interestingly, overexpressed spike protein did not
affect cell morphology or proliferation but significantly suppressed both HR and NHEJ
repair (Figures 1B–E, S2A,B and S3A,B).
3.2. SARS–CoV–2 Spike Protein Inhibits DNA Damage Repair
Because spike proteins are critical for mediating viral entry into host cells and are
the focus of most vaccine strategies [20,21], we further investigated the role of spike
proteins in DNA damage repair and its associated V(D)J recombination. Spike proteins
are usually thought to be synthesized on the rough endoplasmic reticulum (ER) [1]. After
posttranslational modifications such as glycosylation, spike proteins traffic via the cellular
membrane apparatus together with other viral proteins to form the mature virion [1]. Spike
protein contains two major subunits, S1 and S2, as well as several functional domains or
repeats [22] (Figure 2A). In the native state, spike proteins exist as inactive full–length
proteins. During viral infection, host cell proteases such as furin protease activate the S
protein by cleaving it into S1 and S2 subunits, which is necessary for viral entry into the
target cell [23]. We further explored different subunits of the spike protein to elucidate
the functional features required for DNA repair inhibition. Only the full–length spike
protein strongly inhibited both NHEJ and HR repair (Figures 2B–E and S4A,B). Next, we
sought to determine whether the spike protein directly contributes to genomic instability
by inhibiting DSB repair. We monitored the levels of DSBs using comet assays. Following
different DNA damage treatments, such as γ–irradiation, doxorubicin treatment, and H2O2
treatment, there is less repair in the presence of the spike protein (Figure 2F,G). Together,
these data demonstrate that the spike protein directly affects DNA repair in the nucleus.
Viruses 2021, 13, 2056 5 of 10
5 of 10
Figure 1. Effect of severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) nuclear-localized
proteins on DNA damage repair. (A) Subcellular distribution of the SARS–CoV–2 proteins.
Immunofluorescence was performed at 24 h after transfection of the plasmid expressing the viral
proteins into HEK293T cells. Scale bar: 10 μm. (B) Schematic of the EJ5-GFP reporter used to monitor
non-homologous end joining (NHEJ). (C) Effect of empty vector (E.V) and SARS–CoV–2 proteins on
NHEJ DNA repair. The values represent the mean ± standard deviation (SD) from three independent
experiments (see representative FACS plots in Figure S2A). (D) Schematic of the DR-GFP reporter used
to monitor homologous recombination (HR). (E) Effect of E.V and SARS–CoV–2 proteins on HR DNA
repair. The values represent the mean ± SD from three independent experiments (see representative
FACS plots in Figure S2B). The values represent the mean ± SD, n = 3. Statistical significance was determined
using one-way analysis of variance (ANOVA) in (C) and (E). ** p < 0.01, *** p < 0.001, **** p <  0.0001.
3.2. SARS–CoV–2 Spike Protein Inhibits DNA Damage Repair
Because spike proteins are critical for mediating viral entry into host cells and are the
Nsp1 Nsp5 Nsp9 Nsp13 Nsp14 Nsp16 Spike Nucleocapsid
DAPI
His-tag
Merge
A
E.V
Nsp1
Nsp5
Nsp9Nsp13Nsp14Nsp16Spike
Nucleocapsid
***
***
***
****
0
50
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HRefficiency(%)
E.V
Nsp1
Nsp5
Nsp9Nsp13Nsp14Nsp16Spike
Nucleocapsid
****
**
****
****
0
50
100
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NHEJefficiency(%)
****
B
C
D
E
Figure 1. Effect of severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) nuclear-localized proteins on DNA
damage repair. (A) Subcellular distribution of the SARS–CoV–2 proteins. Immunofluorescence was performed at 24 h after
transfection of the plasmid expressing the viral proteins into HEK293T cells. Scale bar: 10 µm. (B) Schematic of the EJ5-GFP
reporter used to monitor non-homologous end joining (NHEJ). (C) Effect of empty vector (E.V) and SARS–CoV–2 proteins
on NHEJ DNA repair. The values represent the mean ± standard deviation (SD) from three independent experiments (see
representative FACS plots in Figure S2A). (D) Schematic of the DR-GFP reporter used to monitor homologous recombination
(HR). (E) Effect of E.V and SARS–CoV–2 proteins on HR DNA repair. The values represent the mean ± SD from three
independent experiments (see representative FACS plots in Figure S2B). The values represent the mean ± SD, n = 3.
Statistical significance was determined using one-way analysis of variance (ANOVA) in (C,E). ** p < 0.01, *** p < 0.001,
**** p < 0.0001.
Viruses 2021, 13, 2056 6 of 10
functional features required for DNA repair inhibition. Only the full–length spike protein
strongly inhibited both NHEJ and HR repair (Figures 2B–E and S4A,B). Next, we sought
to determine whether the spike protein directly contributes to genomic instability by inhibiting
DSB repair. We monitored the levels of DSBs using comet assays. Following different
DNA damage treatments, such as γ–irradiation, doxorubicin treatment, and H2O2
treatment, there is less repair in the presence of the spike protein (Figure 2F,G). Together,
these data demonstrate that the spike protein directly affects DNA repair in the nucleus.
Figure 2. Severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) spike protein inhibits DNA
damage repair. (A) Schematic of the primary structure of the SARS–CoV–2 spike protein. The S1 subunit
includes an N–terminal domain (NTD, 14–305 residues) and a receptor–binding domain (RBD, 319–541
residues). The S2 subunit consists of the fusion peptide (FP, 788–806 residues), heptapeptide repeat sequence
1 (HR1, 912–984 residues), HR2 (1163–1213 residues), TM domain (TM, 1213–1237 residues), and
cytoplasm domain (CT,1237–1273 residues). (B,C) Effect of titrated expression of the spike protein on
DNA repair inHEK–293Tcells. (D,E) Only full-length spike protein inhibits non-homologous end joining
(NHEJ) and homologous recombination (HR) DNA repair. The values represent the mean ± SD from
three independent experiments (see representative FACS plots in Figure S4A,B). (F) Full–length spike (S–
FL) protein–transfected HEK293T cells exhibited more DNA damage than empty vector-, S1–, and S2–
transfected cells under different DNA damage conditions. For doxorubicin: 4 μg/mL, 2 h. For γ–irradiation:
10 Gy, 30 min. For H2O2: 100 μM, 1 h. Scale bar: 50 μm. (G) Corresponding quantification of the
comet tail moments from 20 different fields with n > 200 comets of three independent experiments. Statistical
significance was assessed using a two-way analysis of variance (ANOVA). NS (Not Significant): *
p > 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001.
Figure 2. Severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) spike protein inhibits
DNA damage repair. (A) Schematic of the primary structure of the SARS–CoV–2 spike protein.
The S1 subunit includes an N–terminal domain (NTD, 14–305 residues) and a receptor–binding
domain (RBD, 319–541 residues). The S2 subunit consists of the fusion peptide (FP, 788–806 residues),
heptapeptide repeat sequence 1 (HR1, 912–984 residues), HR2 (1163–1213 residues), TM domain
(TM, 1213–1237 residues), and cytoplasm domain (CT,1237–1273 residues). (B,C) Effect of titrated
expression of the spike protein on DNA repair in HEK–293T cells. (D,E) Only full-length spike protein
inhibits non-homologous end joining (NHEJ) and homologous recombination (HR) DNA repair.
The values represent the mean ± SD from three independent experiments (see representative FACS
plots in Figure S4A,B). (F) Full–length spike (S–FL) protein–transfected HEK293T cells exhibited
more DNA damage than empty vector-, S1–, and S2–transfected cells under different DNA damage
conditions. For doxorubicin: 4 µg/mL, 2 h. For γ–irradiation: 10 Gy, 30 min. For H2O2: 100 µM, 1 h.
Scale bar: 50 µm. (G) Corresponding quantification of the comet tail moments from 20 different fields
with n > 200 comets of three independent experiments. Statistical significance was assessed using
a two-way analysis of variance (ANOVA). NS (Not Significant): * p > 0.05, ** p < 0.01, *** p < 0.001,
**** p < 0.0001.
3.3. Spike Proteins Impede the Recruitment of DNA Damage Repair Checkpoint Proteins
To confirm the existence of spike protein in the nucleus, we performed subcellular
fraction analysis and found that spike proteins are not only enriched in the cellular membrane
fraction but are also abundant in the nuclear fraction, with detectable expression
even in the chromatin–bound fraction (Figure 3A). We also observed that the spike has
three different forms, the higher band is a highly glycosylated spike, the middle one is a
full–length spike, and the lower one is a cleaved spike subunit. Consistent with the comet
assay, we also found the upregulation of the DNA damage marker, γ–H2A.X, in spike
Viruses 2021, 13, 2056 7 of 10
protein–overexpressed cells under DNA damage conditions (Figure 3B). A recent study
suggested that spike proteins induce ER stress and ER–associated protein degradation [24].
To exclude the possibility that the spike protein inhibits DNA repair by promoting DNA
repair protein degradation, we checked the expression of some essential DNA repair proteins
in NHEJ and HR repair pathways and found that these DNA repair proteins were
stable after spike protein overexpression (Figure 3C). To determine how the spike protein
inhibits both NHEJ and HR repair pathways, we analyzed the recruitment of BRCA1 and
53BP1, which are the key checkpoint proteins for HR and NHEJ repair, respectively. We
found that the spike protein markedly inhibited both BRCA1 and 53BP1 foci formation
(Figure 3D–G). Together, these data show that the SARS–CoV–2 full–length spike protein
inhibits DNA damage repair by hindering DNA repair protein recruitment.
3.3. Spike Proteins Impede the Recruitment of DNA Damage Repair Checkpoint Proteins
To confirm the existence of spike protein in the nucleus, we performed subcellular
fraction analysis and found that spike proteins are not only enriched in the cellular membrane
fraction but are also abundant in the nuclear fraction, with detectable expression
even in the chromatin–bound fraction (Figure 3A). We also observed that the spike has
three different forms, the higher band is a highly glycosylated spike, the middle one is a
full–length spike, and the lower one is a cleaved spike subunit. Consistent with the comet
assay, we also found the upregulation of the DNA damage marker, γ–H2A.X, in spike
protein–overexpressed cells under DNA damage conditions (Figure 3B). A recent study
suggested that spike proteins induce ER stress and ER–associated protein degradation
[24]. To exclude the possibility that the spike protein inhibits DNA repair by promoting
DNA repair protein degradation, we checked the expression of some essential DNA repair
proteins in NHEJ and HR repair pathways and found that these DNA repair proteins were
stable after spike protein overexpression (Figure 3C). To determine how the spike protein
inhibits both NHEJ and HR repair pathways, we analyzed the recruitment of BRCA1 and
53BP1, which are the key checkpoint proteins for HR and NHEJ repair, respectively. We
found that the spike protein markedly inhibited both BRCA1 and 53BP1 foci formation
(Figure 3D–G). Together, these data show that the SARS–CoV–2 full–length spike protein
inhibits DNA damage repair by hindering DNA repair protein recruitment.
Figure 3. Severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) spike protein impedes the
recruitment of DNA damage repair checkpoint proteins. (A) Membrane fraction (MF), cytosolic fraction
(CF), soluble nuclearfraction (SNF), and chromatin-bound fraction(CBF) fromHEK293Tcells transfected
with SARS–CoV–2 spike protein were immunoblotted for His-tag spike and indicated proteins. (B) Left:
Immunoblots of DNA damage marker γH2AX in empty vector (E.V)– and spike protein–expressing
HEK293T cells after 10 Gy γ-irradiation. Right: corresponding quantification of immunoblots in left. The
values represent the mean ± SD (n = 3). Statistical significance was determined using Student’s t–test. ****
DAPI His-tag 53BP1 Merge
E.V
Spike
E.V
Spike
DAPI His-tag BRCA1 Merge
E.V S-FL
0
10
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50 ****
Average53BP1foci/Cell
E.V S-FL
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AverageBRCA1foci/Cell
MF CF SNFCBF
Spike
(His tag)
H2A
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(His tag)
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IRRelativeγH2A.Xintensity
(γH2A.X/H2A.X)
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DNAPK
Rad51
53BP1
RNF168
ATM
His tag
(Spike)
0 1 2
Spike overexpression
(μg)
XRCC4
Ku80
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15
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70
170
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95
15
15
170
170
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43
55
70
95
170
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kDa
kDa
kDa
Figure 3. Severe acute respiratory syndrome coronavirus 2 (SARS–CoV–2) spike protein impedes
the recruitment of DNA damage repair checkpoint proteins. (A) Membrane fraction (MF), cytosolic
fraction (CF), soluble nuclear fraction (SNF), and chromatin-bound fraction (CBF) from HEK293T cells
transfected with SARS–CoV–2 spike protein were immunoblotted for His-tag spike and indicated
proteins. (B) Left: Immunoblots of DNA damage marker γH2AX in empty vector (E.V)– and spike
protein–expressing HEK293T cells after 10 Gy γ-irradiation. Right: corresponding quantification of
immunoblots in left. The values represent the mean ± SD (n = 3). Statistical significance was determined
using Student’s t-test. **** p < 0.0001. (C) Immunoblots of DNA damage repair related proteins
in spike protein–expressing HEK293T cells. (D) Representative images of 53BP1 foci formation in
E.V– and spike protein-expressing HEK293 cells exposed to 10 Gy γ–irradiation. Scale bar: 10 µm.
(E) Quantitative analysis of 53BP1 foci per nucleus. The values represent the mean ± SEM, n = 50.
(F) BRCA1 foci formation in empty vector- and spike protein-expressing HEK293 cells exposed to
10 Gy γ–irradiation. Scale bar: 10 µm. (G). Quantitative analysis of BRCA1 foci per nucleus. The
values represent the mean ± SEM, n = 50. Statistical significance was determined using Student’s
t-test. **** p < 0.0001.
Viruses 2021, 13, 2056 8 of 10
3.4. Spike Protein Impairs V(D)J Recombination In vitro
DNA damage repair, especially NHEJ repair, is essential for V(D)J recombination,
which lies at the core of B and T cell immunity [9]. To date, many approved SARS–
CoV–2 vaccines, such as mRNA vaccines and adenovirus–COVID–19 vaccines, have been
developed based on the full–length spike protein [25]. Although it is debatable whether
SARS–CoV–2 directly infects lymphocyte precursors [26,27], some reports have shown
that infected cells secrete exosomes that can deliver SARS–CoV–2 RNA or protein to
target cells [28,29]. We further tested whether the spike protein reduced NHEJ–mediated
V(D)J recombination. For this, we designed an in vitro V(D)J recombination reporter
system according to a previous study [18] (Figure S5). Compared with the empty vector,
spike protein overexpression inhibited RAG–mediated V(D)J recombination in this in vitro
reporter system (Figure 4).
was determined using Student’s t–test. **** p < 0.0001.
3.4. Spike Protein Impairs V(D)J Recombination In vitro
DNA damage repair, especially NHEJ repair, is essential for V(D)J recombination,
which lies at the core of B and T cell immunity [9]. To date, many approved SARS–CoV–
2 vaccines, such as mRNA vaccines and adenovirus–COVID–19 vaccines, have been developed
based on the full–length spike protein [25]. Although it is debatable whether
SARS–CoV–2 directly infects lymphocyte precursors [26,27], some reports have shown
that infected cells secrete exosomes that can deliver SARS–CoV–2 RNA or protein to target
cells [28,29]. We further tested whether the spike protein reduced NHEJ–mediated V(D)J
recombination. For this, we designed an in vitro V(D)J recombination reporter system according
to a previous study [18] (Figure S5). Compared with the empty vector, spike protein
overexpression inhibited RAG–mediated V(D)J recombination in this in vitro reporter
system (Figure 4).
Figure 4. Spike protein impairs V(D)J recombination in vitro. (A) Schematic of the V(D)J reporter
system. (B) Representative plots of flow cytometry show that the SARS–CoV–2 spike protein impedes
V(D)J recombination in vitro. (C) Quantitative analysis of relative V(D)J recombination. The values represent
the mean ± SD, n = 3. Statistical significance was determined using Student’s t–test. **** p < 0.0001.
4. Discussion
Our findings provide evidence of the spike protein hijacking the DNA damage repair
machinery and adaptive immune machinery in vitro. We propose a potential mechanism
by which spike proteins may impair adaptive immunity by inhibiting DNA damage repair.
Although no evidence has been published that SARS–CoV–2 can infect thymocytes
or bone marrow lymphoid cells, our in vitro V(D)J reporter assay shows that the spike
protein intensely impeded V(D)J recombination. Consistent with our results, clinical observations
also show that the risk of severe illness or death with COVID–19 increases with
age, especially older adults who are at the highest risk [22]. This may be because SARS–
CoV–2 spike proteins can weaken the DNA repair system of older people and consequently
impede V(D)J recombination and adaptive immunity. In contrast, our data provide
valuable details on the involvement of spike protein subunits in DNA damage repair,
indicating that full–length spike–based vaccines may inhibit the recombination of V(D)J
in B cells, which is also consistent with a recent study that a full–length spike–based vaccine
induced lower antibody titers compared to the RBD–based vaccine [28]. This suggests
E.V Spike
0
50
100
150
****
RelativeV(D)Jrecombination
efficiency(%)
GFPi, +RAG1/2
E.V
Spike
GFP
RFP
B C
GFP
IRES
RFP
12RSS 23RSS
RAG1/RAG2
GFP
IRES
RFP
12/23RSS
A
Figure 4. Spike protein impairs V(D)J recombination in vitro. (A) Schematic of the V(D)J reporter system. (B) Representative
plots of flow cytometry show that the SARS–CoV–2 spike protein impedes V(D)J recombination in vitro. (C) Quantitative
analysis of relative V(D)J recombination. The values represent the mean ± SD, n = 3. Statistical significance was determined
using Student’s t-test. **** p < 0.0001.
4. Discussion
Our findings provide evidence of the spike protein hijacking the DNA damage repair
machinery and adaptive immune machinery in vitro. We propose a potential mechanism
by which spike proteins may impair adaptive immunity by inhibiting DNA damage
repair. Although no evidence has been published that SARS–CoV–2 can infect thymocytes
or bone marrow lymphoid cells, our in vitro V(D)J reporter assay shows that the spike
protein intensely impeded V(D)J recombination. Consistent with our results, clinical
observations also show that the risk of severe illness or death with COVID–19 increases
with age, especially older adults who are at the highest risk [22]. This may be because
SARS–CoV–2 spike proteins can weaken the DNA repair system of older people and
consequently impede V(D)J recombination and adaptive immunity. In contrast, our data
provide valuable details on the involvement of spike protein subunits in DNA damage
repair, indicating that full–length spike–based vaccines may inhibit the recombination of
V(D)J in B cells, which is also consistent with a recent study that a full–length spike–based
vaccine induced lower antibody titers compared to the RBD–based vaccine [28]. This
suggests that the use of antigenic epitopes of the spike as a SARS–CoV–2 vaccine might be
safer and more efficacious than the full–length spike. Taken together, we identified one of
the potentially important mechanisms of SARS–CoV–2 suppression of the host adaptive
immune machinery. Furthermore, our findings also imply a potential side effect of the
Viruses 2021, 13, 2056 9 of 10
full–length spike–based vaccine. This work will improve the understanding of COVID–19
pathogenesis and provide new strategies for designing more efficient and safer vaccines.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/
10.3390/v13102056/s1, Figure S1: Expression of nuclear–localized SARS–CoV–2 proteins in human
cells, Figure S2: Effect of nuclear SARS–CoV–2 proteins on NHEJ– and HR–DNA repair pathway,
Figure S3: Nsp1, Nsp5, Nsp13, Nsp14 but not spike inhibit cell proliferation, Figure S4: Effect of
SARS–CoV–2 spike mutants on NHEJ– and HR– DNA repair pathway, Figure S5: In vitro V(D)J
recombination assay.
Author Contributions: H.J. conceived and designed the study. H.J. and Y.-F.M. supervised the
study, performed experiments, and interpreted the data. Writing—original draft preparation, H.J.;
Writing—review and editing, H.J. and Y.-F.M.; funding acquisition, Y.-F.M. All authors have read and
agreed to the published version of the manuscript.
Funding: This work was supported by Umeå University, Medical Faculty’s Planning grants for
COVID–19 (research project number: 3453 16032 to Y.F.M.); the Lion’s Cancer Research Foundation
at Umeå University (grants: LP 17–2153, AMP 19–982, and LP 20–2256 to Y.F.M.), and the base unit’s
ALF funds for research at academic healthcare units and university healthcare units in the northern
healthcare region (ALF–Basenheten: 2019, 2020, 2021 to Y.F.M.).
Institutional Review Board Statement: Not applicable, because of this study not involving humans
or animals.
Informed Consent Statement: Not applicable, because of this study not involving humans.
Data Availability Statement: The data presented in this study are available in the main text and
Supplementary Materials.
Conflicts of Interest: The authors have declared that no competing interests exist. The funders
had no role in study design, data collection and analysis, decision to publish, or preparation of
the manuscript.
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