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Appl. Sci. 2020, 10, 1710; doi:10.3390/app10051710 www.mdpi.com/journal/applsci
Article
Cancer Stem Cell Target Labeling and Efficient Growth Inhibition of CD133 and PD‐L1 Monoclonal Antibodies Double Conjugated with Luminescent Rare‐Earth Tb3+ Nanorods
Thi Thao Do 1,2,3,*, Nhat Minh Le 4, Trong Nhan Vo 4, Thi Nga Nguyen 4, Thu Huong Tran 3,5
and Thi Kim Hue Phung 2,4
1 Institute of Biotechnology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road,
Hanoi 10000, Vietnam 2 Institute of Health Research and Educational Development in Central Highlands, Gia Lai 61000, Vietnam;
[email protected] 3 Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang
Quoc Viet Road, Hanoi 10000, Vietnam; [email protected] 4 Hung Vuong Gifted High School, Gia Lai 61000, Vietnam; [email protected] (N.M.L.);
[email protected] (T.N.V.); [email protected] (T.N.N.) 5 Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road,
Hanoi 10000, Vietnam
* Correspondence: [email protected]; Tel.: +84‐24‐38361774
Received: 22 January 2020; Accepted: 26 February 2020; Published: 2 March 2020
Abstract: Rare‐earth nanomaterials are being widely applied in medicine as cytotoxicity agents, in
radiation and photodynamic therapy, as drug carriers, and in biosensing and bioimaging
technology. Terbium (Tb), a rare‐earth element belonging to the lanthanides, has a long luminescent
lifetime, large stock displacement, narrow spectral width, and biofriendly probes. In cancer therapy,
cancer stem cell (CSC)‐targeted treatment is receiving considerable attention due to these cells’
harmful characteristics. However, CSCs remain barely understood. Therefore, to effectively label
and inhibit the growth of CSCs, we produced a nanocomplex in which TbPO4∙H2O nanorods were
double conjugated with CD133 and PD‐L1 monoclonal antibodies. The Tb3+ nanomaterials were
created in the presence of a soft template (polyethylene glycol 2000). The obtained nanomaterial
TbPO4∙H2O was hexagonal crystal and nanorod in shape, 40–80 nm in diameter, and 300–800 nm in
length. The nanorods were further surfaced through tetraethyl orthosilicate hydrolysis and
functionalized with amino silane. Finally, the glutaraldehyde‐activated Tb3+ nanorods were
conjugated with CD133 monoclonal antibody and PD‐L1 monoclonal antibody on the surface to
obtain the nanocomplex TbPO4∙H2O@silica‐NH2+mAb^CD133+mAb^PD‐L1 (TMC). The formed
nanocomplex was able to efficiently and specifically label NTERA‐2 cells, a highly expressed CD133
and PD‐L1 CSC cell line. The conjugate also demonstrated promising anti‐CSC activity by
significant inhibition (58.50%) of the growth of 3D tumor spheres of NTERA‐2 cells (p < 0.05).
Keywords: cancer stem cells; CD133 mAb; ion Tb3+; nanorod; NTERA‐2; PD‐L1; TMC
1. Introduction
In recent years, cancer studies have involved the consideration of cancer stem cell (CSC) theory.
CSCs are a subpopulation of cells in tumors that have self‐renewal, differentiation, and
tumorigenicity abilities [1]. These cells are related to therapy drug resistance, metastasis, and
recurrent cancer [2]. The identification of CSCs is based on typical cellular surface markers, such as
Cluster of Differentiation 133 (CD133), CD44, CD24, and Aldehyde dehydrogenases (ALDH), of
Appl. Sci. 2020, 10, 1710 2 of 11
which CD133 appears in various types of cancer cells in solid tumors. This glycoprotein is among the
most popular markers for isolation of CSCs [3]. CD133, also known as prominin‐1, is a cross‐
membrane glycoprotein. Evidence has shown that CD133 might be related to metastasis,
tumorigenesis, and drug resistance. Therefore, CD133 is used not only as a specific surface antigen
to detect and isolate CSCs, but also in therapeutic strategies [4].
Another typical feature of CSCs is immunosurveillance resistance [5]. Programmed death‐ligand
1 (PD‐L1) is also reported as a CSC surface marker which blocks PD‐1 on the surface of T cells. Thus,
PD‐L1 limits the response of T cells, helping CSCs to escape the immune system for their growth and
metastasis [6]. Since 2014, PD‐L1 monoclonal antibody has been clinically approved for anticancer
immunotherapy worldwide.
CD133 and PD‐L1 antibodies are reported to have the ability to detect and treat cancers,
especially when combined with nanomaterials. Nanomaterials have improved the therapeutic index
of clinical drugs by enhancing circulation time and increasing permeability and retention [7].
Nanomaterials help with probing, tracking, homing, and studying CSCs’ behavior. In this area, rare‐
earth nanomaterials such as Terbium (Tb), a lanthanide, have attracted considerable attention. The
advantages of lanthanide compounds include long luminescence lifetime, large stock displacement,
and narrow spectral width, which are useful for fluorescent marking, probes, and sensors for use in
tests and human body imaging [8]. Nanoscale lanthanides are highly stable, and it is easy to fabricate
and functionalize their surfaces using biological substances such as antigens, monoclonal antibodies,
enzymes, and aptamers. These molecules can be used to improve therapeutic efficacy or for locating
nanoparticles in vivo. Therefore, in this study, Tb3+ was used to produce nanomaterials to double
conjugate with the monoclonal antibodies against CD133 and PD‐L1 for the purpose of biolabeling
and growth inhibition of cancer stem cells, which were NTERA‐2 pluripotent human embryonic
carcinoma cells.
2. Materials and Methods
2.1. Materials
Cultured Dulbecco’s Modified Eagle Medium (DMEM), fetal bovine serum (FBS), Trypsin‐
EDTA, and antibiotics (antibiotics/antimycotics) were received from Invitrogen (Carlsbad, CA, USA).
Human CD133 monoclonal antibody, PD‐L1 monoclonal antibody, and CD133 antibody conjugated
with FITC (CD133‐FITC) were sourced from Thermo Fisher (Invitrogen; Carlsbad, CA, USA). Other
chemicals were provided by Sigma Aldrich (St. Louis, MO, USA).
2.2. Preparation of TbPO4.H2O@silica‐NH2 Nanomaterials
Terbium orthophosphate monohydrate (TbPO4∙H2O): Tb(NO3)3∙5H2O (Sigma, 99.9 %) was added
to NH4H2PO4 solution (Merck) in the presence of polyethylene glycol 2000 (PEG‐2000) and stirred for
3–12 h. The pH of the obtained solution was adjusted in the range of 4–12 by adding 10% NaOH
solution before incubating at 200 °C for 24 h. The product (TbPO4∙H2O) was centrifuged at 5900 rpm
and washed with ddH2O before drying at 60 °C for 5–10 h. The nanomaterial was then coated with
silica through a hydrolysis reaction with tetra ethyl orthosilicate (TEOS) (Aldrich, 99.99%). Briefly,
TbPO4∙H2O was added to a mixture solution containing TEOS, ethanol, acetic acid, and water and
stirred for 15 min (TbPO4/TEOS molar ratio of 1:0.2). The solution was then centrifuged and washed
three times with 33% ethanol solution. Glycerol solution (0.5 mL) was added to a hydrous mixture of
ethanol containing TbPO4∙H2O coated silica (TbPO4∙H2O@silica) and stirred for 30 min. 3‐
aminopropyl trimethoxy silane (APTMS) was dispersed in ethanol before being mixed with
TbPO4∙H2O@silica solution (TbPO4∙H2O@silica/APTMS molar ratio of 1:0.2) and stirred for 15 min to
functionalize the surface with ‐NH2. The TbPO4∙H2O@silica‐NH2 (TM) materials were washed two
times with ethanol, two times with ddH2O, and finally dispersed in phosphate buffer saline (PBS)
(1X, pH 7).
Appl. Sci. 2020, 10, 1710 3 of 11
2.3. Conjugation of TbPO4∙H2O@silica‐NH2 Nanomaterials with CD133 Monoclonal Antibody and PD‐L1
Monoclonal Antibody (mAb)
The TbPO4∙H2O@silica‐NH2 nanomaterial in sodium phosphate solution was gently vortexed
before adding 0.5% glutaraldehyde solution in a ratio of 1:0.5 (v/v) and mixed for 1 h at room
temperature (RT) to disperse completely. The mixture was centrifuged and washed three times with
PBS solution to remove glutaraldehyde. Then, 40 μg of CD133 antibodies (Thermo Fisher, Invitrogen,
Carlsbad, CA, USA) was added into the 400 μL glutaraldehyde pre‐activated TbPO4∙H2O@silica‐NH2
and incubated at 37 °C for 30 min. After incubation, the suspension was centrifuged at 6000 rpm for
5 min at 4 °C; the supernatant was retained to determine the amount of unconjugated antibodies in
the combined efficiency study. The TbPO4∙H2O@silica‐NH2‐mAb^CD133 residue after rinsing with
PBS three times was continuously conjugated with mAb^PD‐L1 by adding 40 μg of this PD‐L1 mAb
at 37 °C for a further 30 min. After a centrifuge step at 6000 rpm for 5 min at 4 °C, the supernatant
solution was retained to determine the conjugation efficiency. The PBS washing residue of
TbPO4∙H2O@silica‐NH2‐mAb^CD133‐mAb^PD‐L1 (TMC) nanocomplex was reconstituted in PBS
and stored at 4 °C before being used for further experiments.
Conjugation efficiency was measured through the indirect detemination of unbound IgG in the
supernatant after combining mAb with nanomaterials using a NANOPHOTOMETER P300 system
(IMPLEN.INC., USA). The conjugated efficiency was calculated using the following fomula:
CE% = 100% −
× 100. (1)
2.4. Characterization of the Obtained Nanocomplex
The morphology of nanomaterials was observed by field emission scanning electron microscopy
(FESEM, Hitachi). The structure of the material was determined using an X‐ray diffraction measuring
system (Siemens D5000 with = 1.5406 Å, diffraction angle in the range of 15° ≤ 2 ≤ 75°). Infrared spectra of the samples were measured on a NICOLET impact 410 Fourier transform infrared
spectrometer (FTIR). The fluorescence spectrum of the product was measured at a wavelength of 355
nm by using the Horiba Jobin Yvon IHR 320 (USA) system at Hanoi Polytechnic University, and some
samples were measured on the Horiba Jobin Yvon IHR 550 system (USA) at the Institute of Materials
Science, Vietnam Academy of Science and Technology (VAST).
2.5. Cell Culture
In this study, the NTERA‐2 cell line, which is a pluripotent human embryonic carcinoma cell
line, served as CSCs and CCD‐18Co cells (the human colon normal) were used as healthy cells. These
cell lines were kindly provided by Dr. P. Wongtrakoongate, Mahidol University, Thailand and Prof.
Chi‐Ying Huang, National Yang‐Ming University, Taiwan. Cells were maintained in DMEM medium
supplement with 10% fetal bovine serum and 1% antibiotics (antibiotics/antimycotics solution,
Invitrogen, Carlsbad, CA, USA) in incubator at 37 °C, 5% CO2, and 100% humidity.
2.6. Observing and Imaging TMC‐Nanocomplex‐Labeled Cells
Cells at log phase were seeded into 96‐well plates with a concentration of 10,000 cells/well and
incubated at 37 °C, 5% CO2 for 24 h. The culture medium was removed, then cells were fixed with
10% formaldehyde for 10 min at RT. TMC nanocomplex (10 μL) was dilluted in 190 μL of PBS before
it was added into each well and incubated at 4 °C for 1 h. The unbound TMC were removed and
washed with PBS three times. At the end of the process, PBS was added to the wells before the cells
were observed under an Olympus Scan ^R fluorescence microscope (Olympus Europa SE & Co.KG,
Hamburg, DE).
Appl. Sci. 2020, 10, 1710 4 of 11
2.7. Detecting the TMC‐Nanocomplex‐Labeled Cells by Flow Cytometry
NTERA‐2 cells and CCD‐18Co cells at log phase were harvested with trypsin‐EDTA and
collected into a Falcon tube. Cells were re‐suspended with DMEM medium containing 2% FBS and
separated into several tubes, then TMC nanocomplex was added to the cells and incubated at 4 °C
for at least 15 min and protected from light. After that, cells were washed three time with PBS to
remove the unbound TMC. The cells incubated with CD133 mAb conjugated FITC (Thermo Fisher,
Invitrogen; Carlsbad, CA, USA) served as a reference control. The numbers of labeled and
luminescent cells in 10.000–12.000 counting cells were measured and analyzed using the Novocyte
flow cytometry system and NovoExpress software (ACEA Bioscience Inc.).
2.8. TMC Cytotoxicity Determination
MTT ((3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide ) assay was used for
measurement of the cytotoxic activity of the TMC nanocomplex. This method is based on the
formation of formazan by MTT relating to the effectiveness of enzymatic activities in viable cells [9].
Briefly, cells were seeded in 96‐well plates (10,000 cells/well) and treated with TMC at various
concentrations of corresponding 0.08, 0.4, 2, or 10 μg/mL of PD‐L1 mAb amounts, for 72 h at 37 °C,
5% CO2. The experiments were performed in triplicate to ensure accuracy. Then, 10 μL fresh MTT (5
mg/mL) was added to the each well of the experimental plate and incubated at 37 °C. After 4 h, all
medium was discarded and the formazan crystal formations were dissolved by adding 50 μL/well
DMSO 100%. The OD values were measured at 540 nm using a spectrophotometer (BioTek, ELx800).
The number of surviving cells was calculated by the formula:
% survival =
100. (2)
2.9. Effective of TMC on the Growth of Tumor Spheroids Co‐Cultured with Macrophages
Macrophages were isolated from the peritoneum of healthy BALB/c mice using a Macrophage
mouse Isolation Kit (Peritoneum) (Miltenyi Biotech., Bergisch Gladbach, Germany). The isolation
cells were cultured in DMEM containing 10% FBS, 1% antibiotics and incubated at 37 °C and 5% CO2.
In order to form 3D tumor spheroids, the hanging drop method was used. NTERA‐2 cells (1500
cells) in 20 μL medium were dropped onto the underside of the lid of a 60 mm tissue culture dish.
The lids were then inverted onto 5 mL medium‐filled bottom dishes and incubated at 37 °C, 5% CO2,
95% humidity. After 3 days of incubation, cell aggregates were formed.
The obtained spheroids were then co‐cultured with macrophages in 96‐well plates. Wells were
covered by 1% agarose before spheroids were transferred to the wells. The macrophage cells were
then co‐cultured with the spheroids in the wells. The TMC treatment was executed by directly adding
TMC into the co‐culture wells and further incubating for 3 days. The growth of spheroids was
observed under microscopy. The images were analyzed using ImageJ software to determine the
growth area of the spheroids and to compare with the negative control.
2.10. Statistical Analysis
The data are reported as mean ± standard deviation (SD), which were analyzed using GraphPad
Prism 7 software and unpaired t‐tests. p < 0.05 was considered to indicate statistical significance.
3. Results
3.1. Characteristics of the Synthesiszed Nanomaterials
The morphologies of nano TbPO4∙H2O and TMC are presented in Figure 1. From the FESEM
images, TbPO4∙H2O formed nanorods with diameter 30–40 nm and length 300–800 nm. After coating
with silica, surfacing with –NH2, and conjugating with mAb, the diameter of the complex slightly
increased to 40–80 nm, but the length remained at 300–800 nm.
Appl. Sci. 2020, 10, 1710 5 of 11
(a)
(b)
(c)
Figure 1. FESEM images of the produced nanomaterials: (a) TbPO4∙H2O nanorods, (b)
TbPO4∙H2O@silica‐NH2; (c) TbPO4∙H2O@silica‐NH2‐mAb^CD133‐mAb^PD‐L1 nanocomplex (TMC).
The designed TbPO4∙H2O nanorods were also typical hexagonal crystals, as proven by the X‐ray
diagram (Figure 2).
10 20 30 40 50 60
PDF 20 - 1244
Inte
nsit
y (a
.u.)
2- Theta (degree)
PDF 20 - 1244
TbPO4.H
2O
Figure 2. The X‐ray diagram of Terbium phosphate monohydrate hexagonal crystals determined
using an X‐ray diffraction measuring system (Siemens D5000 with = 1.5406 Å, diffraction angle in the range of 15° ≤ 2 ≤ 75°).
To be suitable for biological labeling, created nanomaterials must be strongly luminescent after
design. Therefore, the fluorescence of the obtained nano TbPO4∙H2O and TMC was measured. The
results in Figure 3 show that strong fluorescence levels of TbPO4∙H2O and TMC were excited at 355
nm and emitted at 545 nm. They also exhibit that the main emission peaks of the TbPO4∙H2O product
were at 488, 545, 586, and 620 nm, which correspond to the 5D4−7Fj (J = 6, 5, 4, 3) transitions of Tb3+
ions. These results are consistent with the data reported by Lien et al. [10].
Appl. Sci. 2020, 10, 1710 6 of 11
450 500 550 600 650
(2)
5D4-
7F3
5D4-
7F4
5D4-
7F5
5D4-
7F6
Inte
nsity (a
.u.)
Wavelength (nm)
(1) TbPO4.H
2O
(2) TbPO4.H
2O@silica-NH
2
(1)
Figure 3. Fluorescence spectra of excitation at 355 nm of TbPO4∙H2O incubated at 200 °C for 24 h (1)
and TbPO4∙H2O@silica‐NH2 coated with a silica layer attached to NH2 groups (2).
The FTIR spectra of TbPO4∙H2O, TbPO4∙H2O@silica‐NH2, TbPO4∙H2O@silica‐NH2‐mAb are
shown in Figure 4. Curve (a) is the FTIR spectrum of TbPO4∙H2O. Curves (b) and (c) (corresponding
to the products TbPO4∙H2O@silica‐NH2 and TbPO4∙H2O@silica‐NH2‐mAb, respectively) have the
same profile as curve (a), which indicates strong absorption in the region 770–600 cm−1. The two peaks
at 660 and 600 cm−1 are typically attributed to the Tb‐O and PO43‐ vibrations, respectively. In Fig. 4,
curve (a), we can also observe oscillations of the O‐H bond at around 1600 cm−1 and near 3600 cm−1.
In curves (b) and (c), the unique absorption peaks from internal vibration of the amino bands (1642
cm−1) and the strong absorption band (3443 cm−1) from symmetric and asymmetric N–H stretching
vibration can be observed, which demonstrate the appearance of APTMS and mAb on the obtained
TbPO4∙H2O@silica‐NH2 and TbPO4∙H2O@silica‐NH2‐mAb. The strong absorption band in the region
1000–950 cm−1 (two peaks at 1008 and 950 cm−1) arises from Si‐O‐Si asymmetric vibration. Thus, it can
be suggested that conjugation (linkage) between luminescent nanorods and mAb was formed in the
TbPO4∙H2O@silica‐NH2‐mAb nanocomplex.
3000 2000 100040
60
80
100
Tra
nsm
itta
nce
(%
)
Wavenumber (cm-1)
TbPO4.H2O
(a)
Appl. Sci. 2020, 10, 1710 7 of 11
3000 2000 100040
60
80
100TbPO4.H2O@silica-NH2
Tra
nsm
itta
nce
(%
)
Wavenumber (cm-1)
(b)
3000 2000 100040
60
80
100TbPO4.H2O@silica - NH2 - mAb
Tra
nsm
itta
nce
(%)
Wavenumber (cm-1)
(c)
Figure 4. The FTIR spectra of TbPO4∙H2O (a), TbPO4∙H2O@silica‐NH2 (b), and TbPO4∙H2O@silica‐NH2‐
mAb (c) obtained using a NICOLET impact 410 Fourier transform infrared spectrometer.
3.2. Probing NTERA‐2 and CCD‐18Co Cells with TMC Nanocomplex
As presented in Table 1, the CE index, which reports the conjugation efficiency of
TbPO4∙H2O@silica‐NH2 nanorods with mAb against CD133 and PD‐L1, was high, ranging from 60%
to 100%.
Table 1. The conjugated efficiency of TbPO4∙H2O@silica‐NH2 nanorods with mAb against CD133
and PD‐L1.
No. Samples Input
mAb
Free mAb in
Supernatant CE
1 TbPO4∙H2O@silica‐NH2 ‐CD133 mAb 40 μg 0 100%
2 TbPO4∙H2O@silica‐NH2‐mAb^CD133‐mAb^PD‐L1 40 μg 8–16 μg 60%–80%
To evaluate the binding ability of TMC to cells, cells were observed under a fluorescence
microscope. According to Feng et al., NTERA‐2 highly express CD133, so this cell line was chosen for
this experiment [11]. As shown in Figure 5, NTERA‐2 cells labeled with luminescent nanocomplex
TMC expressed strong luminescence under fluorescence microscopy compared with the negative
control (Figure 5). Although TbPO4∙H2O@silica‐NH2 could bond with the cells, the bonds were weak;
therefore, the fluorescence image was not bright.
Appl. Sci. 2020, 10, 1710 8 of 11
(a)
(b)
(c)
(d)
Figure 5. NTERA‐2 cells after 1 h of incubation with (a) TbPO4∙H2O@silica‐NH2; (b) TMC
(TbPO4∙H2O@silica‐NH2‐mAb^CD133‐mAb^PD‐L1); (c) CD133‐FITC; and (d) negative control;
observed using an Olympus Scan^R 100X fluorescent microscope system.
The flow cytometry results also provided the percentage of probed cells using the TMC
nanocomplex. CD133‐FITC (Invitrogen; Carlsbad, CA, USA) served as a reference control. As shown
in Figure 6, CD133 expression was found in about 95.83% ± 7.31% of NTERA‐2 cells when stained
with CD133‐FITC. A similar result (97.77% ± 5.69%) was found in NTERA‐2 cells that were incubated
with TMC (p > 0.05). The percentage of CD133‐positive cells was only 1.11% ± 0.06% for NTERA‐2
cells incubated with unconjugated TbPO4∙H2O nanorods.
(a) (b) (c) (d)
Figure 6. Flow cytometry analysis of labeled NTERA‐2 cells incubated with (a) TbPO4∙H2O@silica‐
NH2; (b) TMC, (c) CD133‐FITC; and (d) negative control using a Novocyte flow cytometry system
and NovoExpress software (ACEA Bioscience Inc.).
CCD‐18Co cells were also incubated with the nanorods conjugated with mAb under the same
conditions. However, luminescent expression of CCD‐18Co cells bound with nanomaterials was not
noticed under the fluorescent microscopic observation. The results from flow cytometry analysis also
showed a very low percentage of CD133‐positive cells in the CCD‐18Co cell population (0.85% ±
0.07%) which had been incubated with TMC (Figure 7). Lodi reported that the expression of CD133
in CCD‐18Co cells is hardly noticeable [12]. Thus, this cell line served as the negative control for
CD133 markers. These results prove that the TMC nanocomplex could specifically bind to CSCs.
(a) (b) (c) (d)
Figure 7. Flow cytometry analysis of CCD‐18Co cells incubated with (a) TbPO4∙H2O@silica‐NH2; (b)
TMC, (c) CD133‐FITC; and (d) negative control using a Novocyte flow cytometry system and
NovoExpress software (ACEA Bioscience Inc.).
Appl. Sci. 2020, 10, 1710 9 of 11
3.3. Effect of TMC on the Proliferation of NTERA‐2 and CCD‐18Co Cells
The proliferation of NTERA‐2 and CCD‐18Co cells treated with TMC was assessed using the
MTT assay. TMC showed the ability to inhibit the growth of NTERA‐2 cells by up to 14.12% at the
highest concentration of 10 μg/mL (Table 2). The antiproliferative activity of TMC on CCD‐18Co cells
was slightly lower than that on NTERA‐2 cells (P > 0.05). TbPO4∙H2O@silica‐NH2 did not show any
cytotoxicity on either NTERA‐2 or CCD‐18Co cells.
Table 2. The effects of TMC on the proliferation of NTERA‐2 and CCD‐18Co cells
Samples % Proliferation
NTERA‐2 CCD‐18Co
TM (TbPO4∙H2O @silica‐NH2) 92.23 ± 3.68 93.41 ± 2.19
TMC (TbPO4∙H2O @silica ‐NH2 ‐mAb^CD133‐mAb^PD‐L1) 85.88 ± 5.76 89.67 ± 4.36
CD133‐FITC (ThermoFisher) 87.63 ± 7.08 90.33 ± 2.41
Negative control 100 100
3.4. Effect of TMC on NTERA‐2 Spheroids
One of the key features of CSCs in tumors is adaptive immune resistance. This unique
characteristic helps CSCs escape destruction by immune cells such as lympho T and NK cells or
macrophages, resulting in tumor progression and metastasis. This phenomenon is thought to be
related to programmed cell death ligand 1 (PD‐L1). PD‐L1 is expressed on tumors and binds to
programmed cell death 1 (PD1) on immune cells, leading to the inhibition of tumor‐infiltrating
lymphocytes (TILs) [13]. Therefore, PD‐L1 blocking decreases the growth of tumors. In this study, we
measured tumor growth inhibition due to the activity of TMC using the research model, which was
3D NTERA‐2 spheroids co‐cultured with macrophages. TMC showed the ability to inhibit the growth
of 3D tumor spheroids (Figure 8). As a result of TMC application, the areas of the 3D spheroids were
reduced by 58.50% ± 1.60%, which is a significant reduction in comparison with the negative control
after three days of treatment (p < 0.05).
(a) (b) (c)
Figure 8. 3D tumor spheroid images at Day 3 under treatments of TbPO4∙H2O@silica‐NH2 (a), TMC
(b), and negative control (c) using an Olympus Scan^R 100X fluorescence microscope system for
observation.
4. Discussion
Lanthanides have now been widely applied in medicine. Their applications include therapy and
imaging. Unlike other metals, lanthanides are luminescent, stable, and biosafe [14]. Among the
lanthanides, Tb is a typical lanthanide with strong green fluorescence and has potential for
biomedical labeling or imaging. This material has also been studied for use as a carrier for drugs such
as a measles virus antibody [15] or cobra venom antigens [10]. Due to the advantages of Tb, we chose
this material to produce a nanocomplex, TMC, which is Tb3+ nanorods double conjugated with CD133
and PD‐L1 mAb for the purpose of CSC labeling and therapeutic solution. TbPO4∙H2O formed
nanorods 30–50 nm in diameter and 300–800 nm in length. After surface functionalization, the
nanorods were successfully double conjugated with CD133 and PD‐L1 mAb with high efficiency
Appl. Sci. 2020, 10, 1710 10 of 11
(60%–100%). The labeling ability of TMC to detect CSCs is equivalent to that of the reference CD133‐
FITC. However, CD133 is also expressed in stemlike cells throughout the body. Thus, mAb against
other CSC‐specific markers such as EpCAM, CD44, CD24, etc., will be double conjugated with our
TbPO4∙H2O@silica‐NH2‐mAb^CD133 (instead of PD‐L1 mAb) in order to improve the specific
targeting activity of the nanocomplex for fundamental CSC research or for future clinical
applications.
Together with CSC probing, TMC with PD‐L1 mAb was produced in a structure selected for the
purpose of cancer treatment. PD‐L1 is a ligand of PD‐1, and the interaction of PD‐L1 and PD‐1 in
immune cells may cause inactivation of these cells [16]. However, PD‐L1 acts as an antiapoptotic
receptor in response to Fas ligation. Therefore, PD‐L1 is closely related to cancer stem cell
proliferation [17]. PD‐L1 antibodies are commercially available to clinically treat several types of
cancer [18]. Herein, although TMC only slightly inhibited the growth of NTERA‐2 cells in vitro, the
nanocomplex strongly inhibited the growth of these 3D NTERA‐2 spheroids when co‐cultured with
macrophages. According to Genevieve, PD‐L1 monoclonal antibodies enhance the ability of
macrophages to proliferate and activate, leading to increased numbers of TAM (tumor‐associated
macrophages) and thereby inhibiting the growth of tumor tissues [17].
5. Conclusions
Tb3+ nanomaterials were produced using polyethylene glycol 2000 (PEG‐2000) as a soft template.
The obtained TbPO4∙H2O nanomaterial was a hexagonal crystal and nanorod in shape, 40–80 nm in
diameter, and 300–800 nm in length. These Tb3+ nanorods were further silica surfaced using tetraethyl
orthosilicate (TEOS) hydrolysis and functionalized with amino silane to obtain TbPO4∙H2O@silica‐
NH2 (TM) materials. The glutaraldehyde‐activated TM was double conjugated with CD133 and PL‐
D1 monoclonal antibodies to produce the nanocomplex TbPO4∙H2O@silica‐
NH2+mAb^CD133+mAb^PD‐L1 (TMC). The formed nanocomplex presented highly efficient and
specific labeling of NTERA‐2 cells, a CSC cell line strongly expressing CD133 and PD‐L1. The
nanoconjugate also exhibited promising anti‐CSC properties, including 58.50% inhibition of the
growth of 3D tumor spheres of NTERA‐2 cells, and its anti‐tumor properties should be further tested
in in vivo experiments.
Author Contributions: Conceptualization: N.M.L., T.N.V., T.T.D. Methodology: T.T.D., T.K.H.P., T.H.T.
Software, validation, formal analysis, investigation, resources, data curation: T.N.N., T.H.T., N.M.L., T.N.V.,
Writing—original draft preparation: T.N.N. Writing—review and editing: T.T.D. All authors have read and
agreed to the published version of the manuscript.
Acknowledgments: The authors thank The Institute of Natural Product Chemistry, Vietnam Academy of
Science and Technology (VAST) for the Olympus Scan^R fluorescent microscope (Olympus Europa SE & Co.KG,
Hamburg, DE) and the Vietnam Ministry of Education and Training for their partly support under grant number
B2019‐MDA‐04.
Conflicts of Interest: The authors declare no conflict of interest.
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