Peptide-free Synthetic Nicotine Vaccine Candidates with α-Galactosylceramide as Adjuvant

Article abstract of DOI:10.1021/acs.molpharmaceut.8b01095

Peptides are generally needed as T-helper epitopes in nicotine vaccines to induce effective antibody responses, but the highly polymorphic property of major histocompatibility complex (MHC) molecules may limit opportunities of B cell to receive CD4⁺ T-cell help. Invariant natural killer T (iNKT) cells recognize lipid antigens presented by the nonpolymorphic CD1d molecule that is conserved in mammals to a great extent. iNKT cells also display some similar functions to conventional CD4⁺ T-helper cells, especially they license dendritic cells stimulate antibody isotype switching by B cells. Herein, α-galactosylceramide (αGalCer), a classical iNKT cell agonist, serves as an adjuvant in synthetic nicotine vaccine candidates absent of peptide or protein. Our study reveals that αGalCer displays better adjuvant activity than Pam₃CSK₄ (a commonly used lipopeptide TLR agonist). Remarkably, the covalent linker between the nicotine hapten and αGalCer is not critical. Self-assembly of the lipid-tailed nicotine and αGalCer into the liposome represents a structurally simple but immunologically effective way to develop nicotine vaccines. This is the first time to introduce the iNKT cell agonist as an adjuvant to an antidrug vaccine. This discovery may contribute to improving the efficacy of clinical candidate nicotine vaccines in the future.

Full text of DOI:10.1021/acs.molpharmaceut.8b01095

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Article
Peptide-free synthetic nicotine vaccine
candidates with #-Galactosylceramide as adjuvant
Xiang-Zhao Chen, Ru-Yan Zhang, Xi-Feng Wang, Xu-Guang Yin, Jian
Wang, Ya-Cong Wang, Xiu Liu, Jing-Jing Du, Zheng Liu, and Jun Guo
Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/acs.molpharmaceut.8b01095 • Publication Date (Web): 01 Mar 2019
Downloaded from http://pubs.acs.org on March 1, 2019
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Peptide-free Synthetic Nicotine Vaccine Candidates
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with -Galactosylceramide as Adjuvant
Xiang-Zhao Chen,‡ Ru-Yan Zhang,‡ Xi-Feng Wang, Xu-Guang Yin, Jian Wang, Ya-Cong Wang,
Xiu Liu, Jing-Jing Du, Zheng Liu* and Jun Guo*
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Hubei International
Scientific and Technological Cooperation Base of Pesticide and Green Synthesis, International
Joint Research Center for Intelligent Biosensing Technology and Health, College of Chemistry,
Central China Normal University, Wuhan, Hubei 430079, P. R. China
Corresponding author: liuz1118@mail.ccnu.edu.cn (Z. L.); jguo@mail.ccnu.edu.cn (J. G.)
Graphical abstract
lipidated nicotine
N
(1)
N
formation of
peptide-free
liposome
Me
OH
GalCer
(iNKT-cell agonist)
O

HO
HO
O
( )
14
NH OH
OH
HO
O
(2)
immunization
( )
5
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ABSTRACT: Peptides are generally needed as T-helper epitopes in nicotine vaccines to induce
effective antibody responses, but the highly polymorphic property of MHC molecules may limit
opportunities of B cell to receive CD4+ T-cell help. iNKT cells recognize lipid antigens presented
by the nonpolymorphic CD1d molecule that is conserved in mammals to a great extent. iNKT cells
also display some similar functions to conventional CD4+ T-helper cells, especially they license
dendritic cells stimulate antibody isotype switching by B cells. Herein, -galactosylceramide
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(GalCer), a classical iNKT cell agonist, serves as an adjuvant in synthetic nicotine vaccine
candidates absent peptide or protein. Our study reveals that GalCer displays better adjuvant
activity than Pam3CSK4 (a commonly used lipopeptide TLR agonist). Remarkably, the covalent
linker between the nicotine hapten and GalCer is not critical. Self-assembly of the lipid-tailed
nicotine and GalCer into the liposome represents a structurally simple but immunologically
effective way to develop nicotine vaccines. This is the first time to introduce the iNKT cell agonist
as an adjuvant to an anti-drug vaccine. This discovery may contribute to improving the efficacy of
clinical candidate nicotine vaccines in the future.
KEYWORDS: iNKT cell, α-galactosylceramide, liposomal formulation, covalent conjugation,
nicotine vaccine
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INTRODUCTION
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Tobacco addiction is a main cause of cardiovascular risk and tobacco-related cancer, and
nicotine is the main ingredient leading to addiction in tobacco.1 Approximately 6 million
mortalities and immeasurable economic loss are caused by nicotine consumption every year
globally. Current therapies to promote smoking cessation mainly focus on nicotine replacement
therapies, receptor antagonists, or behavioural intervention, but the overall cessation rate is highly
limited, and these therapies might also have potential severe side effects.2,3 An alternative
therapeutic strategy is known as immunopharmacotherapy, which depends on the induction of
nicotine specific antibodies that could bind nicotine in the blood circulation, and prevent it from
entering the brain, thus reducing nicotine addiction.4,5
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One of bottlenecks for the development of nicotine vaccine is to trigger the production of
high titers of anti-nicotine IgG antibodies with strong affinity.6 Because of its poor
immunogenicity, nicotine need to be covalently attached to macromolecular carriers (such as BSA
and KLH proteins) to induce antibody response. Importantly, the proteins are also digested by
APCs to produce shorter peptides, which are presented by MHC class II molecules, serving as the
crucial Th epitopes to promote antibody class-switching from IgM to IgG. Therefore, peptides (or
proteins) are an indispensable component in nicotine vaccines. However, the fact that human MHC
class II molecules are highly polymorphic would narrow the population that can respond to a
related peptide.7 Although this problem can be solved by introducing multiple Th epitopes, the
syntheses are highly challenging, and can only be managed by experienced organic chemists.
There are several small populations of T lymphocytes that can recognize nonpeptide
antigens without the participation of MHC molecules.8 These populations do not obey the rule that
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T cells can only detect peptides binding with MHC. Invariant natural killer T (iNKT) cell is one
of the most studied subset among these unconventional T cells.9 iNKT cells share many functions
with conventional CD4+ T helper cells, especially they license DCs stimulate antibody isotype
switching and affinity maturation by B cells.10-12 Unlike CD4+ T helper cells, the receptors of iNKT
cells can recognize lipid antigens presented by CD1d molecules, instead of peptides presented by
MHC molecules. Given the non-polymorphic nature of CD1d, iNKT cell can function as helper in
a more universal way compared to the majority of CD4+ T cells.13 The first glycolipid ligand of
iNKT cells was -galactosylceramide (1, abbreviated as GalCer or KRN7000, Scheme 1a),
which was originally extracted from a marine sponge and optimized through SAR studies.14-16 The
adjuvanticity of GalCer have been revealed in many animal models in which the activated iNKT
cells act as “universal helpers” to augment both humoral and cellular responses to the co-
administered antigens.17
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Adjuvants have been considered a crucial component in vaccines.18 Pam3CSK4 (3, Scheme
1a) is a popular lipopeptide adjuvant that resemble the N-terminus of bacterial lipoproteins.19
Pam3CSK4 play an adjuvant role through the cooperation between TLR2 and TLR1, resulting in
NF-B activation. Because lipopeptides tend to be incorporated into liposomes, Pam3CSK4 has
been widely used in liposomal synthetic vaccines as good inducers of antibody responses,
providing appropriate “danger signals”.20 To elicit effective immune responses adjuvanted by
Pam3CSK4, several approaches of potential benefit have been or can be applied. First, Pam3CSK4
is preferably used in a self-adjuvanting manner.21 The covalent conjugation of Pam3CSK4 with
antigens is able to induce superior antibody response to a simple mixture of TLR-agonist and free
antigen. Indeed, this covalent concept has been extended to the use of other lipid adjuvants, such
as monophosphoryl lipid A,22 muramyl dipeptide,23 and αGalCer24 as well. Second, for T-
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independent antigens, Th epitope peptides generally has to be incorporated. For example, the
liposomal construct lacking the Th epitope was less effective than the corresponding tricomponent
construct that consists of Pam3CSK4, Th epitope, and B epitope.25 Third, combination of a TLR
agonist and another adjuvant of immunologically distinct mode of action could have synergistic
effects.26
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As part of our ongoing investigations of structurally simple but immunologically effective
synthetic vaccines,27 we report herein the nicotine vaccine candidates with αGalCer as adjuvants.
The immune responses induced in mice by the vaccine candidates were characterized by nicotine-
specific serum antibody titers, concentrations of cytokines (IFN- and IL-4), and antibody
affinities. The efficacies of all vaccine candidates were further evaluated by acute pharmacological
effects (nicotine-induced antinociception and hypothermia) in mice. We found that the iNKT cell
agonist αGalCer more potently induces the anti-nicotine antibody immune responses in mice than
the TLR agonist Pam3CSK4. Our results also showed that no synergy of the co-adminstrated
adjuvants (Pam3CSK4 and αGalCer) was observed relative to either Pam3CSK4 or αGalCer alone.
Next, whether the covalent linker between the nicotine hapten and the glycolipid αGalCer
(covalent modality) can further improve vaccine efficacy was investigated. The results suggest
that self-assembly of αGalCer with Nic-βGalCer by the noncovalent modality represents a
practical means for use of iNKT cell agonists in nicotine vaccines. To the best of our knowledge,
αGalCer has been employed for the first time as an adjuvant in the vaccine targeting drugs of
abuse.
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(a)
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OH
HO
6''
O
C25H51
O
C25H51
OH
HO
O
HO
NH OH
NH OH

O
HO
O

HO
O
C14H29
C14H29
HO
OH
OH
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GalCer ( )
2
GalCer ( )
(immunologically inactive)
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(iNKT cell agonist)
HO
O
H
O
O
N
C15H31
O
S
N
H
O
OH
4
O
HN
O
O
3
Pam3CSK4 ( )
TLR2/1 ligand
C15H31
C15H31
O
NH2
(b)
O
O
( )4
N
H
N
O
O
O
C25H51
NH OH
C14H29
H
NH
HO
O
O

HO
N
Me
()
HO
N
OH
4
Nic-GalCer ( )
nicotine hapten
(c)
GalCer
O
O
O
( )4
N
O
N
O
H
H
NH
HO
O
C25H51
O
HO
N
Me
NH OH

HO
O
()
N
C14H29
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Nic-GalCer ( )
OH
GalCer
nicotine hapten
(d)
hapten/adjuvant
noncovalent
or
hapten/adjuvant
covalent modality
modality
adjuvant
Nic-GalCer
DSPC
Nic-GalCer
DSPC
cholesterol
cholesterol
A
B
C
D
: GalCer
: Pam3CSK4
: Freund's adjuvant
: No adjuvant
GalCer & Pam3CSK4
liposome
F1
F2
F3
0.6
3.0
g
 )
g
 )
: Nic-GalCer (
: Nic-GalCer (
: Nic-GalCer (  )
15
g
E:
Scheme 1. (a) The structure of αGalCer (1), βGalCer (2), and Pam3CSK4 (3); (b) and (c) The
structure of Nic-βGalCer (4) and Nic-αGalCer (5); (d) liposomal formulations of vaccine
candidates A–F (The adjuvants in each formulation are shown in bold).
EXPERIMENTAL SECTION
Chemistry
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Se-phenyl 2-((tert-butoxycarbonyl)amino)ethaneselenoate (7). To Ph2Se2 (780 mg, 2.5
mmol) and N-Boc-glycine (441 mg, 2.5 mmol) in a solution of DMF (10 mL) was added 2.5 mL
of PBu3 under Ar atmosphere. The reaction mixture was stirred until N-Boc-glycine was
completely consumed on the TLC. The solvents were removed in vacuo. Purification of the
resultant residue through flash column chromatography on silica gel (petroleum ether/DCM 9:1 to
1:1) provide 7 as a pale yellow solid (672 mg, 85%). 1H NMR (400 MHz, CDCl3) δ 7.49 (d, J =
6.7 Hz, 2H), 7.37 (d, J = 6.1 Hz, 3H), 5.45 (s, 1H), 4.04 (d, J = 6.0 Hz, 2H), 1.53 (s, 9H); 13C NMR
(100 MHz, CDCl3) δ 200.96, 155.37, 135.77, 129.12, 128.75, 125.16, 80.57, 53.17, 28.43, 28.29;
ESI-MS calcd. for C13H17NNaO3Se+ [M + Na]+: 338.03, found: 337.95.
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3,4-Di-O-benzyl-1-O-(2,3-di-O-benzyl-4-hydroxyl-6-(2-((tert-butoxycarbonyl)amino)
ethylamino)--D-galactopyranosyl)-2-hexacosylaminooctadecane-1,3,4-triol (8). To the
solution of 626 (50 mg, 0.04 mmol) in wet solvents of MeOH/THF (9:1, 10 mL) was added PMe3
(0.4 mL, 0.4 mmol). The reaction mixture was stirred at rt for 4 h. After the solvents was removed
in vacuo. the residue was placed under high vacuum for 24 h to further remove trimethylphosphine
oxide. The resultant amine was applied directly without column purification. To the solution of
amine and DIPEA (15 L) in DCM (3 mL) was treated with the solution of 7 (19 mg, 0.06 mmol)
in DCM (3 mL) at rt. After stirred at rt for 2 h, the reaction mixture was concentrated under reduce
pressure to offer the residue which was purified by flash column chromatography on silica gel
(DCM/MeOH 20:1) to give 8 as a white solid (90.3 mg, 96% yield over two steps). 1H NMR (400
MHz, CDCl3) δ 7.37 – 7.23 (m, 20H), 7.04 – 6.98 (m, 1H), 6.06 (d, J = 8.7 Hz, 1H), 5.59 (s, 1H),
4.78 (d, J = 11.2 Hz, 1H), 4.72 – 4.64 (m, 4H), 4.60 (d, J = 11.8 Hz, 1H), 4.55 (d, J = 3.4 Hz, 2H),
4.29 (d, J = 7.7 Hz, 1H), 3.88 (d, J = 3.5 Hz, 1H), 3.85 – 3.74 (m, 5H), 3.70 (dd, J = 17.0, 5.5 Hz,
2H), 3.64 – 3.56 (m, 2H), 3.43 (ddd, J = 16.0, 9.1, 3.6 Hz, 2H), 3.25 (ddd, J = 13.1, 8.6, 3.9 Hz,
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1H), 2.59 (s, 1H), 1.81 (q, J = 7.0 Hz, 2H), 1.71 – 1.61 (m, 2H), 1.42 (s, 9H), 1.23 (d, J = 16.1 Hz,
70H), 0.88 (t, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 172.90, 170.16, 155.71, 138.22,
138.16, 137.42, 128.33, 128.29, 128.23, 128.18, 127.92, 127.83, 127.70, 127.60, 127.52, 104.36,
81.10, 80.33, 79.61, 78.78, 75.13, 72.90, 72.40, 72.00, 69.03, 67.15, 50.59, 44.12, 40.22, 36.69,
32.04, 30.60, 29.83, 29.49, 29.15, 28.47, 27.78, 25.96, 25.71, 22.84, 14.32. ESI-MS calcd. for
C57H111N3NaO11+ [M + Na]+: 1397.00, found: 1397.30.
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Nic-βGalcer (4). To a degassed solution of 8 (53 mg, 0.039 mmol) in DCM/MeOH (4 mL,
2:1) was treated with a catalytic amount of Pd(OH)2/C (25 mg, 20% Pd on carbon, normally
contain 50% water). The suspension was stirred under H2 atmosphere until 8 was consumed. After
restoring Ar atmosphere, the mixture was filtered by a Celite pad which was prewashed by MeOH
followed by DCM. Upon the removal of solvent in vacuo, the residue was subjected to the
purification of flash column chromatography on silica gel (DCM/MeOH 10:1) to give S2 (see the
1
structure in the Supporting Information) as a white solid (32.3 mg, 82%). H NMR (400 MHz,
CDCl3/CD3OD 1:1) δ 4.24 (s, 1H), 4.19 (d, J = 7.2 Hz, 1H), 4.07 (d, J = 5.6 Hz, 1H), 3.77 – 3.66
(m, 4H), 3.63 – 3.42 (m, 7H), 2.21 (t, J = 7.6 Hz, 2H), 1.62 (m, 2H), 1.46 (s, 9H), 1.27 (s, 70H),
0.89 (t, J = 6.6 Hz, 6H). ESI-MS calcd. for [M + H]+: 1014.82, found: 1015.06. A solution of S2
in DCM (4 mL) was treated with anhydrous trifluoroacetic acid (1 mL, 0.013 mmol) at 0 °C. The
resultant mixture was stirred at 0 °C for 15 min followed by at rt for 30 min. The solvents were
removed in vacuo to afford amine S3 as a crude product. To the solution of amine S3 in
DCM/MeOH (4 mL, 1:1) was added the solutions of 9 in DCM (1 mL) and Et3N (50 L) at rt.
After stirred at rt for 5 h, the reaction mixture was concentrated under reduced pressure and then
purified on a LH-20 column using DCM/MeOH (1:1) as the eluent to offer Nic-βGalCer (4) after
1
lyophiliztion as a white solid (6.9 mg, 53% yield over two steps). H NMR (400 MHz,
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CDCl3/CD3OD 1:1) δ 8.55 (dd, J = 14.4, 3.5 Hz, 1H), 7.95 (d, J = 8.0 Hz, 1H), 7.47 (dd, J = 7.9,
4.7 Hz, 1H), 4.24 (s, 1H), 4.21 (d, J = 6.9 Hz, 1H), 4.11 (dd, J = 10.3, 4.6 Hz, 1H), 3.86 (d, J = 6.3
Hz, 1H), 3.77 (d, J = 2.9 Hz, 1H), 3.64 (q, J = 7.1 Hz, 4H), 3.60 – 3.45 (m, 11H), 3.38 (s, 6H),
2.44 – 2.11 (m, 10H), 1.65 (s, 9H), 1.27 (s, 70H), 0.89 (t, J = 6.7 Hz, 6H). 13C NMR (100 MHz,
CDCl3) δ 174.31, 174.03, 173.95, 170.33, 148.94, 148.35, 135.94, 123.80, 103.29, 73.70, 72.65,
72.41, 71.70, 71.47, 70.63, 69.75, 69.39, 68.97, 68.48, 56.75, 55.16, 49.78, 48.60, 48.38, 48.17,
47.96, 47.74, 47.53, 47.32, 47.11, 46.08, 42.06, 39.23, 38.76, 38.58, 35.75, 35.01, 34.73, 31.39,
31.28, 29.14, 29.05, 29.00, 28.92, 28.81, 28.70, 28.68, 25.48, 25.29, 24.55, 24.38, 21.98, 16.90,
13.07. ESI-MS calcd. for [M + H]+: 1304.01, found: 1304.30.
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Nic-αGalcer (5). The preparation of Nic-αGalCer (5) from 10 was similar to that of Nic-
βGalCer from 6. The crude product was purified through LH-20 chromatography column using
DCM/MeOH (1:1) as the eluent to give Nic-αGalCer (5) after lyophiliztion as a white solid (11.7
mg, 61% yield over two steps). 1H NMR (400 MHz, CDCl3/CD3OD 1:1) δ 8.58 – 8.40 (m, 2H),
7.92 (d, J = 7.9 Hz, 1H), 7.41 (dd, J = 7.9, 4.9 Hz, 1H), 4.76 (d, J = 3.5 Hz, 1H), 4.09 (d, J = 5.2
Hz, 1H), 3.77 – 3.60 (m, 7H), 3.56 – 3.35 (m, 13H), 3.25 (dd, J = 12.6, 3.8 Hz, 4H), 2.39 (s, 2H),
2.21 (d, J = 6.7 Hz, 3H), 2.12 (dd, J = 9.3, 5.8 Hz, 5H), 1.55 (d, J = 7.0 Hz, 9H), 1.17 (s, 70H),
13
0.79 (t, J = 6.6 Hz, 6H). C NMR (100 MHz, CDCl3/CD3OD 1:1) δ 174.25, 174.04, 173.90,
170.10, 149.08, 148.88, 136.04, 123.91, 98.95, 73.76, 71.44, 71.25, 70.53, 69.75, 69.41, 69.34,
69.15, 68.91, 68.30, 68.21, 66.21, 56.71, 55.02, 49.91, 48.34, 48.12, 47.91, 47.70, 47.49, 47.27,
47.06, 41.96, 39.33, 38.53, 38.28, 35.72, 34.99, 34.71, 31.63, 31.24, 29.14, 29.05, 29.02, 28.96,
28.79, 28.71, 28.67, 28.64, 25.31, 25.22, 25.06, 24.55, 24.34, 21.95, 16.86, 13.04. ESI-MS calcd.
for C73H134N6NaO13+ [M + Na]+: 1325.99, found: 1326.19.
Immunological test
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Immunization of mice. Female BALB/c mice (6-8 weeks age) in this study were
purchased from the Center for Animal Experiments of Wuhan University. All mice were used
according to the animal ethics guidelines. The mice were divided into 8 groups (5 mice per group)
and bred in the Center for Animal Experiments of Wuhan University. The Mice were immunized
by intraperitoneal injection on the 1st, 15th, and 29th days. The mice were bled (day 0) before
initial immunization and on 14th, 28th, and 42nd days after boost immunizations. Th sera collected
at 2 and 24 h after 1st injection was analysed for the cytokine profiles (IFN- and IL-4). Mouse
blood samples were clotted to obtain antisera that were stored at -80 °C before use. The vaccine
efficacy upon nicotine challenge was evaluated on day 56 by nicotine-induced antinociception (hot
plate procedure) and hypothermia.
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ELISA procedure. A 96-well microtiter plate (Costar type 3590, Corning Inc.) was first
coated with Nic-BSA (18 nicotine hapten per BSA protein), which had been dissolved in 0.1 M
NaHCO3 buffer (pH = 9.6). Each microwell contains 0.1 µg of Nic-BSA in 100 µL of NaHCO3
buffer solution. Next, the plate was kept at 4 °C overnight and washed 3 times with PBST (0.05%
Tween-20 in PBS). The solution of 1.5% (w/v) BSA in PBS was added to each well and incubated
at 37 °C for 1 h. After washed again with PBST, the plate was treated with serially diluted sera
(100 μL/well), incubated for 1 h at 37 °C and washed (3 times) with PBST. A diluted solution
(1:5000) of horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG (-chain specific) or
IgM (-chain specific), IgG1, IgG2a, IgG2b, IgG3, IgA or IgE in 0.1% BSA/PBS (100 µL per
well) was added to each microwell respectively. After incubation (1 h at 37 °C) and a final wash
(3 times), substrate solutions (freshly prepared with 9.5 mL critic buffer at pH 5.0, 0.5 mL 2 mg/mL
tetramathyl benzidine (TMB), and 32 L 3% (w/v) urea hydrogen peroxide) were added to the
wells (100 L per well). The plate was placed in the dark for 15 min for the color development
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and terminated by a solution of 2 M H2SO4. The optical density (OD) value was then recorded at
450 nm. The antibody titer was described as the highest dilution indicating an 0.1 absorbance, after
subtracting the background. Pre-immunization sera (day 0) have been used as negative control.
Antibody affinity by competition ELISA. The relative affinity of anti-nicotine antibody
elicited by the vaccine candidates was measured by a competition ELISA method.28 The sera were
diluted in 0.1% BSA in PBS until the absorbance values reach approximately 1.0 at 450 nm
following the IgG-HRP antibody development assay. The nicotine tartrate was serially diluted
from 100 mM to 2 nM using 0.1% BSA in PBS. Equal volumes (65 µL) of diluted sera and nicotine
were added to a non-absorbent 96-well plate and incubated for 1 h at 37 °C, allowing equilibrium
be achieved. The mixture of serum/nicotine were then added (100 L/well) to the ELISA plates
coated with Nic-BSA (The antigen-coated ELISA plates was prepared identically as “ELISA
procedure”). The plates were incubated for 30 min at 37 °C and then washed 3 times with PBST.
Next, detection of IgG antibody binding was carried out as described in “ELISA procedure”. The
OD readings at 450 nm of diluted serum without nicotine was B0, and B/B0 were plotted against
the molar concentration of nicotine and the 50% inhibition (IC50) was the concentration of
competitors where the binding rate was 50%.
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In vivo cytokine assay. The cytokine levels in sera were evaluated using ELISA kits (IFN-
 and IL-4, Biolegend) according to the manufacturer’s protocol.
Hot plate assessment. Two weeks following the last bleeding, the antinociception effect
against nicotine (0.5 mg/kg, injected subcutaneously) was evaluated by the hot plate test following
the reported procedure with slightly modification.29 A mouse was placed in a glass beaker (11 cm
diameter × 16 cm) whose bottom was heated to 55 °C. The timing latency to exhibit one of the
following nociceptive responses was recorded: licking of paw, shaking/withdrawal of paw, or
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jumping. The typical baseline latency was between 8-15 s, and a 35 s cutoff was imposed to protect
the mice from damage. After response, mice were taken out from the beaker. The antinociceptive
response for the hot-plate test is calculated as percentage of maximum possible effect (%MPE),
where %MPE = (test-control) / [(35 s for the hot plate) – control)] × 100.
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Hypothermia evaluation. Rectal temperature was determined by inserting a 1-cm
thermistor probe into the rectum and temperature read from the digital thermometer. The
measurements were carried out at 5, 15, and 30 min after injection of nicotine. Baseline values
were recorded just before injection.
Evaluation of the vaccine safety through weight change. The safety of all vaccine
candidates was preliminarily evaluated by checking the weight changes of the mice,30 and the mice
were weighed at 0, 28th, 42nd and 56th days before bleedings.
Statistical analyzes. Comparisons were carried out via one-way ANOVA followed by a
Turkey’s posthoc test. Differences were judged to be insignificant when P-values are greater than
0.05. Data were analyzed using GraphPad Prism (GraphPad Software, San Diego, CA).
RESULTS
Design rationale for noncovalent vaccine modality
We began by comparing the adjuvant activity of αGalCer with the commonly used
adjuvants. The simple liposomal formulation developed by Savage and co-workers was
employed.31,32 Self-assembly of the lipid-tailed hydrophilic antigen is able to provide the crucial
multivalency for cross-linking of B cell receptors, eliminating the need for covalent attachment to
carrier proteins. Due to the amphipathic property, the glycolipid adjuvant can be inserted to the
liposomal formulation. In this model, there is no covalent linker between adjuvant and antigen.
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There are two key issues need to be addressed with regard to the design of such vaccine
formulation: choosing a suitable lipid anchor that is immunologically inactive and a conjugation
site on nicotine hapten. As illustrated in Scheme 1b, Nic-βGalCer (4) was designed to present a
nicotine hapten. The choice of βGalCer (2) as lipid anchor is according to our previous research
on the anti-STn cancer vaccine.27 The covalent conjugation of βGalCer with STn antigen only
induces weak anti-STn IgG antibody response, but allows the formation of liposomes for
multivalent antigen presentation. A 3’-substituted nicotine analogue was chosen as the hapten,
because this regiochemical modification around nicotine molecule has been adopted in the nicotine
vaccines (NicVAX33 and NicQβ34) that entered to clinical trials.
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Scheme 1d exhibits the vaccine formulations investigated in our present work. The
formulations A, B, and C contained αGalCer, Pam3CSK4, and the Freund’s adjuvant, respectively.
The complete Freund’s adjuvant contains several TLR agonists originated from inactivated
Mycobacteria, which stimulate extremely strong immune responses in a murine model, but its use
is prohibited in humans due to severe adverse reactions. The construct D lacking adjuvants was
used as a negative control. In order to investigate whether there is co-operation between Pam3CSK4
and αGalCer, the formulation E was prepared. In each formulation, DSPC and cholesterol are
added. Both of them are non-immunomodulatory, but the addition of them in liposomal vaccines
has been shown to facilitate the induction of strong antibody responses in small animals.35
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(a) Synthesis of Nic- GalCer (4)

NHBoc
SePh
O
C25H51
N3
HO
O
7
O
O
NH OBn
O
O
( )4
SePh
BnO
N
O
O
C14H29
H
HO
OBn
a) PMe3
6
N
Me
()
9
N
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7
b)
NHBoc
NH
O
C25H51
NH OBn
C14H29
O
c) H2, Pd(OH)2/C
d) TFA
HO
4
Nic-GalCer ( )
O
BnO
8
O
BnO
9
e)
(43%, three steps)
OBn
(96%, two steps)
(b) Synthesis of Nic- GalCer (5)

similar synthetic
sequence to
N3
HO
O
C25H51
Nic-GalCer (4)
O
BnO
5
Nic-GalCer ( )
NH OBn
BnO
(41%, five steps)
O
C14H29
OBn
10
(c) Synthesis of Nic-protein conjugates Nic-BSA (11)
O
O
( )4
BSA
N
N
O
O
H
H
BSA
PBS (pH 7.5)
f)
9
N
Me
N
()
18
11
Nic-BSA (
)
Scheme 2. (a) and (b) Synthesis of 4 and 5; (c) Preparation of protein conjugate Nic-BSA (11).
Synthesis of Nic-βGalCer and protein conjugates Nic-BSA
As shown in Scheme 2a, the preparation of 4 began with the amide formation between the
glycine selenoester 7 and the 6’-NH2 derivative of βGalCer prepared by the reduction of azide 6.
The attempt to treat 6 via simultaneous reduction of N3 and deprotection of Bn group led to an
unidentified mixture. The hydrogenlysis of 8 and subsequent deprotection of Boc group provided
the amine that reacts with the nicotine selenoester 9 to afford 4. The purity of 4 was confirmed by
HPLC. The protein conjugate Nic-BSA (11) were prepared by coupling 9 with BSA.36 According
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to our previous study on protein conjugation, the selenoester displays better combination of
stability and reactivity than the commonly used active esters (N-hydroxysuccinimide, p-
nitrophenyl, and polyfluorophenyl esters).36, 37 The incorporation number per BSA molecule is 18,
which was determined by the MALDI-TOF mass spectrometry. 11 was applied as the coating
antigen for ELISA of anti-nicotine antibodies (Scheme 2c).
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Vaccine formulation and immunization
GalCer (1 µg) and Nic-βGalCer (3 µg, contains 0.38 µg of the nicotine hapten) was
administered in the liposomal formulation (A) prepared with cholesterol and DSPC in a molar ratio
39,40
of 0.5:1:5:4 by the reported method.22,38 An equimolar amount of Pam3CSK4
(0.0011 µmol,
1.7 µg) instead of αGalCer was employed in B. No adjuvant was used in D. In the construct E,
both αGalCer (1 µg) and Pam3CSK4 (1.7 µg) were introduced. After hydration of a lipid film that
contained all comonents in a HEPES buffer, the liposomes A, B, D, and E were prepared by 20-
minute sonication.21,41 The dynamic light scattering (DLS) confirmed that the liposomes are
uniformly sized with a diameter of approximately 600 nm (Figure S1 in Supporting Information).
In the formulation C, the complete/incomplete Freund’s adjuvant was employed based on the
manufacturer’s instructions (Table S1 in Supporting Information).
Female BALB/c mice (6−8 weeks old) were divided to 8 groups (5 mice per group), and
immunized with the vaccine candidates A-E via intraperitoneal injections. Scheme 3 demonstrates
the vaccine schedule. During the immunization study, no mouse showed signs of significant weight
loss (Figure S15 in Supporting Information).
antinociception and
immunizations
hypothermia assessment
1 d
2 h
15 d
14 d
29 d
28 d
56 d
Day
24 h
after 1st injection
42 d
0
bleedings
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Scheme 3. Study design for nicotine vaccine candidates in mice. Mice were immunized on the 1st,
15th, and 29th days, and the blood was taken before the initial immunization (day 0) as well as on
14th, 28th, and 42nd days after the boost immunizations. The sera collected at 2 and 24 h after 1st
injection were analyzed for cytokine profile (IFN- and IL-4). Nicotine-induced antinociception
(the hot plate procedure) and hypothermia were evaluated on the 56th day.
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Figure 1. The titers of anti-nicotine antibodies (IgG, IgM, IgG1, and IgG3) elicited by the vaccine
candidates (solid bars for noncovalent modality A-E, hatched bars for covalent modality F1, F2,
F3) after three immunizations (day 42). The data are indicated as the average of two independent
experiments ± SEM. Significantly different compared to D group: *, P < 0.05; **, P < 0.01; ***,
P < 0.001; ****, P < 0.0001. Significantly different: , P < 0.01; , P < 0.05; ns, no significant
difference.
##
#
Figure 2. IFN- (a) and IL-4 (b) production induced by the vaccine candidates (solid bars for
noncovalent modality A-E, hatched bars for covalent modality F1, F2, F3). The serum
concentrations of cytokines were evaluated by ELISA at the indicated time points after 1st
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injection. The data are indicated as the average of two independent experiments ± SEM.
Significantly different compared to D group: *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P <
0.0001. Significantly different: ##, P < 0.01; #, P < 0.05; ns, no significant difference.
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Figure 3. (a) Relative affinities of nicotine specific antibody measured by competition ELISA.
The serum collected on the 42nd day was subjected to a competitive ELISA using serially diluted
(-)-nicotine as the competitor. The data are shown as the average of two independent experiments
± SEM. Difference between individual bleeds is insignificant (P > 0.05). (b) Antinociception
effects to nicotine measured by responses to hot plates. Mice were challenged at day 56 with
nicotine (0.5 mg/kg) by the subcutaneous route. The mice were placed on a surface with a steady
temperature of 55 °C and the time between placement and hind paw licking or jumping (whichever
occurred first) was recorded as nociceptive latencies. A 35-s cut-off was used to prevent tissue
damages. “% MPE”: percent maximal possible effect. “Unvaccinated & no nicotine”: the mice
that were unvaccinated and were not injected with nicotine as well. The data are shown as the
average ± SEM. Significantly different compared to “unvaccinated” group: *, P < 0.05; **, P <
0.01; ***, P < 0.001; ****, P < 0.0001.
αGalCer is more effective than Pam3CSK4 in inducing anti-nicotine responses
Figure 1 gave the ELISA results of the antisera obtained on the 42nd day from the mice
immunized with A-E. Compared to the nonadjuvant-loaded D, the immunization of A or B resulted
in significant enhancements of anti-nicotine IgG antibodies; the IgG titer initiated by A is about
twice as high as that of B. To our surprise, given the extremely powerful adjuvant activity of the
Freund's adjuvant, no significant IgG production was observed in C. This result parallels the
previous reports on anticancer synthetic liposomal vaccines which the emulsification process
required by the use of Freund’s adjuvant may destroy the structure of liposomes.42,43
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The pattern of IgG subclasses was also checked. The sera from the mice immunized with
A stimulated the almost equal levels of IgG1 (Th2) and IgG3 (Th1) subclasses, and no significant
titers of IgG2a or IgG2b were observed, indicating a Th1/Th2 mixed type antibody response,
consistent with the polarization of IFN- (Th1)/IL-4 (Th2) cytokine profile (Figure 2).44 In
contrast, B triggered the similar level of IgG3 but the lower level of IgG1 relative to A, displaying
a skewed Th1 antibody response, which is also reflective in cytokine production: the high level of
IFN- but almost no production of IL-4.45,46 Because the murine IgG3, instead of IgG1, can be
generated without T cell helps,32,47 the lack of Th epitopes in B may explain the relatively low titer
of the IgG1 antibody.
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In addition to antibody titers, the affinities of these antibodies were also measured by the
competition ELISA (Figure 3a) according to the reported methods.48 Compared to B, the
antibodies stimulated by A exhibits higher affinity (lower IC50), although the difference is
statistically insignificant. To evaluate antibody function, antinociception and hypothermia caused
by nicotine were determined.29 As shown in Figure 1h, the αGalCer-adjuvanting cohorts (A)
exhibited less antinociception effect than B. Consistent to the results of the hot plate assay, A also
more effectively attenuate nicotine-induced hypothermia than B. The hypothermia effect was
measured by the rectal body temperature (Figure S14 in Supporting Information). Therefore,
αGalCer-adjuvanting cohorts (A) exhibited more significantly blunted responses to nicotine than
B.
No synergy between αGalCer and Pam3CSK4 in stimulating antibody responses
Because Pam3CSK4 and αGalCer initiate immune response through two immunologically
distinct modes of action, these two adjuvants could have synergistic effects to enhance the humoral
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response.26,49 Therefore, we compared the antibody responses elicited by the immunizations using
liposomal formulations containing αGalCer alone (A), Pam3CSK4 alone (B), or the mixture of
these two (E) to find whether the cooperation between two adjuvants could be observed. Although
the production of IL-4 induced by the immunization of E did enhance relative to the use of A or B
(Figure 2), the augmentation of the IgG titer with individual adjuvants was not observed (Figure
1), nor did the improvement of the antibody affinity and the antinociception effect be detected
(Figure 3), indicating that there is no cooperation between αGalCer and Pam3CSK4 in stimulating
antibody responses. The results are consistent with the previous reports that αGalCer and the TLR
agonists do not have synergistic effect on the amplification of antibody responses.50,51
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Noncovalent modality stimulates antibody response as effectively as covalent modality
It has been shown that the direct conjugation of iNKT cell agonist glycolipids to antigens
is crucial for excellent immune responses.38,52-57 Therefore, Nic-αGalCer (5) was prepared via the
similar synthetic sequence to 4 (Scheme 2b). The procedures of vaccine formulation and
immunization are similar to those of the noncovalent modality. As indicated in Figure 1a and 1b,
the immunization with the covalent modality candidates (F1, F2, and F3) induced both IgG and
IgM anti-nicotine antibodies in a dose-dependent manner. However, the titer of IgG antibody
triggered by F3 that containing the most Nic-αGalCer is still less than half of that induced by A.
The patterns of IgG subclasses of F1, F2, and F3 are similar to that of A (Figure 1c and 1d), except
that their titers are lower than those of A, and demonstrate a Th1/Th2 mixed-type response. In term
of the cytokine profile (Figure 2), A exhibited comparable potency to F1. Given the content of
αGalCer in F1 is approximately equal to that of A, the modification at the 6’’-position of αGalCer
caused by conjugation probably does not have a detrimental effect on the stimulation of cytokines.
The cytokine production stimulated by F1, F2, and F3 also exhibited a dose-dependent manner.
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Impressively, the administration of F1 induced mouse lymphocytes to produce the Th1 cytokines
selectively, which is in agreement to the study by Mori group that the low concentration of
glycolipids results in more selective cytokine generation.58
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The affinities of the antibodies stimulated by the covalent modality candidates do not
appear to be dose-dependent, and the antibodies initiated by F2 and F3 display the comparable
affinities (Figure 3a). It is worth noting that the enhancement of the antibody affinities by means
of the covalent modality was not achieved relative to the noncovalent modality. The other
remarkable observation is that the antibody affinity is not closely correlated with the secretion of
IFN- and IL-4 cytokine, which however is an important indicator for the immunological
evaluation of iNKT cell agonists. For the effects of antinociception and hypothermia, slightly but
insignificant difference was observed between A and F candidates (Figure 3b). Taken together,
the covalent linking of the nicotine hapten to αGalCer does not give rise to a greater vaccine
efficacy compared to the noncovalent modality.
DISCUSSION
The major challenge for nicotine vaccines is to induce high titers of specific IgG antibodies
that bind the free nicotine in the blood and hinder them from crossing the blood brain barrier.5 The
rational use of adjuvants plays a critical role in addressing this challenge.59 αGalCer has not been
investigated in vaccines against drugs of abuse and addiction. To the best of our knowledge, this
is the first vaccine candidate that targets addiction drugs employing the iNKT cell agonist as an
adjuvant. The key findings from our study are as follows.
First, αGalCer displays better adjuvant activity than Pam3CSK4, Pam3CSK4 has also been
widely used in synthetic vaccines, preferably in a manner of self-adjuvanting.21 However, without
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Th epitope peptides, the vaccines only stimulate inadequate antibody class-switching from IgM to
IgG.60 Because iNKT cells can provide help to B cells without the participation of CD4+ T cells,
Th epitope is not indispensable. As a result, αGalCer is able to play multiple roles including
liposomal carrier, stimulation of APCs, and Th epitope-like function. Generally, combination of
several adjuvant properties to a single molecule provide the potential to enhance the vaccine
efficacy.18,29 Notably, it is known that a single administration of αGalCer tends to cause iNKT cell
anergy which resistant to further aGalCer activation within at least one month.61 However, iNKT
cell anergy might not be a serious problem in our liposomal vaccines. The antibody titers after the
2nd and 3rd injections do not decrease but increase relative to those after 1st injection (Figure S12
in Supporting Information). Our data of antibody response agree with the report on the synthetic
vaccine against Tn, a typical tumor-associated carbohydrate antigen.38 Anergy has frequently been
observed when αGalCer is administrated in solution as a soluble form, probably the liposomal
formulation can more selectively deliver αGalCer to professional APCs, thus preventing or
reducing this phenomenon.62
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Second, there is no synergy of αGalCer and Pam3CSK4 in stimulating antibody responses.
Cerundolo and coworkers reported the cooperative actions between iNKT cell and TLR agonists
contribute to DC maturation and cytotoxic T cell stimulation.49 More relevantly, it was reported
that in M. tuberculosis infection model, co-administration of Pam3CSK4 with IFN- was able to
up-regulate CD1d on macrophages.63 However, the combination of αGalCer and Pam3CSK4 did
not elevate anti-nicotine IgG titers compared to the treatment of each adjuvant alone, in spite of
the distinct mechanisms through which two adjuvants function. These results are in agreement
with the reports about the lack of cooperation between αGalCer and TLR ligands for humoral
responses.50,51 Given some examples of synergistic TLR and iNKT cell collaborations in cellular
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immunity,64-68 perhaps humoral immune responses are separable from DC-dependent cytotoxic T
cell responses.50
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Third, the covalent attachment of nicotine hapten to αGalCer may not be key to induce an
effective immune response; in contrast, the noncovalent modality involving an admixture of the
lipid-tailed antigen and the glycolipid adjuvant represents a more practical means for vaccine
construction. Generally, conjugation of an adjuvant to an antigen has exhibited higher efficacy
than unconjugated mixtures.69,70 Because adjuvants and antigens usually have different
pharmacokinetic properties, simply mixing them without using covalent linker cannot ensure that
they are recognized concurrently by APC. However, the covalent modality is not the only
approach to guarantee antigen and adjuvant arrive at APC simultaneously. The noncovalent
modality is an alternative approach in which the antigen is appended a "hydrophobic foot" and
then combines with an adjuvant. One vaccine against pneumococcal bacteria (PBS150/PBS57)
prepared by this noncovalent design is able to trigger better responses than Prevnar, a conjugate
vaccine approved by FDA in 2010.31 Particularly, the direct comparison between covalent and
noncovalent modalities in which a model antigen, 4-hydroxy-3-nitrophenyl (NP) moiety was used,
demonstrates that the immunization of the noncovalent modality stimulates higher anti-NP IgG
antibody titers than that of the covalent one.71 From the perspective of synthesis, the noncovalent
modality is more practical. The lipid anchor need not to be βGalCer, and a structurally simple lipid
would also meet the requirement.72 Another advantage of this noncovalent modality is that no
modification of adjuvants is required, thus avoiding decrease or even disappearance of adjuvant
activity caused by covalent conjugation.38 Additionally, the ratio of hapten to adjuvant in covalent
modality is fixed at 1: 1, which in contrast can be altered in noncovalent modality. Given the fact
that iNKT cell anergy is closely correlated to high dose of αGalCer, 61 it is likely that the optimal
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ratio of hapten to adjuvant is much greater than one, which can be regarded as the third advantage
of noncovalent modality. Taken together, our present study suggests that the covalent linker
between the nicotine hapten and αGalCer might not be indispensable for an effective nicotine
vaccine.
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Finally, the cytokine profiles (IFN- and IL-4) of the αGalCer-adjuvanting vaccine
candidates, which are commonly used criteria for the iNKT cell agonists, do not correlate closely
with the vaccine efficacies in our system. Further study on the use of iNKT cell agonists
(particularly Th1 and Th2-skewing glycolipids) in nicotine synthetic vaccines is required for the
optimization of a vaccine construct.
CONCLUSIONS
Our study highlights the use of iNKT cell agonist αGalCer by means of noncovalent and
peptide-free vaccine modality in the liposomal synthetic nicotine vaccine, which is synthetically
simple but capable of triggering effective anti-nicotine antibody responses. In view of the
nonpolymorph of CD1d, these αGalCer-adjuvanting vaccine candidates has the potential to be
universally effective in all people. Although these findings are based on nicotine vaccines, they
should be also applicable for the development of vaccines against other addictive drugs.
ASSOCIATED CONTENT
Supporting Information
Synthetic procedures, compound characterization spectra (1H NMR, 13C NMR, MS
spectra), vaccine formulation, characterization of liposomes by dynamic light scattering, biological
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assays (antibody titers, antibody affinity, studies with antinociception and hypothermia). This
material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: liuz1118@mail.ccnu.edu.cn (Z. L.)
*E-mail: jguo@mail.ccnu.edu.cn (J. G.)
Author Contributions
‡X.-Z. C. and R.-Y. Z. contributed equally.
Notes
The authors declare no competing financial interests.
Funding Sources
This work was supported by the National Key Research and Development Program of
China (No. 2017YFA0505200), the National Natural Science Foundation of China (Nos.
21402058 and 21772056), the Natural Science Foundation of Hubei Province (No. 2017CFB482),
Wuhan Bureau of Science and Technology (2018060401011323), the self-determined research
funds of CCNU from the colleges’ basic research and operation of MOE (Nos. CCNU18TS011
and CCNU19QN068), and Program of Introducing Talents of Discipline to Universities of China
(111 program, B17019).
ACKNOWLEDGMENT
We thank Professor Richard W. Franck (CUNY) for help with preparation of this
manuscript.
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ABBREVIATIONS
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αGalCer, α-galactosylceramide; APC, antigen-presenting cells; BSA, bovine serum
albumin; DCs, dendritic cells; DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine; ELISA,
enzyme-linked immunosorbent assay; IFN-, interferon-; IL-4, interleukin-4; iNKT, invariant
natural killer T; MHC, major histocompatibility complex; MPE, maximum possible effect; MPLA,
monophosphoryl lipid A; Nic, nicotine; NP, 4-hydroxy-3-nitrophenyl; OVA, ovalbumin; Th, T
helper; TLR, toll-like receptor.
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