Angewandte
Research Articles
Chemie
[
17]
based vaccines have reached late stage of clinical develop-
ment and are entering the market. For these vaccines, pre-
clinical data in animal models and phase 3 clinical trials
tissues, which is currently a strong limitation for applying
imidazoquinoline TLR7/8 agonists in mass immunization
campaigns. We and others have reported on strategies to alter
the bio-distribution of imidazoquinolines through chemical
conjugation to a synthetic carrier that limits systemic circu-
lation but confers robust translocation to immune-inducing
[
6,7]
also support the hypothesis that these vaccines can effectively
prevent severe illness upon viral infection. However, little is
known about whether recombinant protein vaccines are
capable of conferring protective immunity. In contrast to
the aforementioned mRNA and viral vector-based vaccines,
recombinant protein vaccines are simpler as they consist of
a single entity antigen and do not require antigen expression
in the vaccinees. mRNA vaccines show promising results.
However, the need for formulation into lipid nanoparticles,
required to overcome the barrier of the endosomal membrane
before the mRNA reaches its molecular target the cytoplasm,
poses considerable challenges in the context of manufacturing
[
17–23]
sites in sentinel lymph nodes.
In the present work, we report on a novel amphiphilic
carrier for imidazoquinoline (IMDQ) TLR7/8 agonists with
high translational potential, based on conjugation of a single
imidazoquinoline to the chain end of a cholesteryl-polyeth-
ylene glycol macromolecular amphiphile (IMDQ-PEG-
CHOL; Figure 1A). This design mediates binding to serum
[
17,21–23]
proteins such as albumin
pure lipidation, the conjugate is well water-soluble.
(Figure 1B) and in contrast to
[
8]
and storage. Hence, exploring the viability of a recombinant
protein COVID-19 vaccine might be of considerable rele-
vance.
We demonstrate that IMDQ-PEG-CHOL is a potent
adjuvant which enhances vaccine efficiency and induces
robust Th1 skewed antibody responses in mice when deliv-
ered as a single shot with either admixed S protein (for SARS-
CoV-2) or seasonal quadrivalent inactivated influenza virus
vaccine (QIV, for influenza). Moreover, IMDQ-PEG-CHOL
was able to infer protection in SARS-CoV-2 or influenza virus
(H1N1)-infected mice. In this context, it is noteworthy that in
contrast to human ACE-2, murine ACE-2 is not targeted by
wild type SARS-CoV-2 virus because of species-specific
variations in ACE-2 receptors between mouse and human.
Here we made use of a mouse model where hACE-2 was
introduced through an adenoviral vector, which allows for
subsequent replication of SARS-CoV-2 upon infection in the
SARS-CoV-2 encodes four major structural proteins,
spike (S), membrane (M), nucleocapsid (N), and envelope
(
E). The spike protein comprises a homotrimeric structure
which is present on the surface of the virus and facilitates the
viral attachment and entry into the host via human angio-
tensin-converting enzyme 2 (hACE-2) receptors via its
[9–11]
receptor-binding domain (RBD).
Owing to its involvement in viral entry, the S protein is
a major target for current vaccine development against
[5]
SARS-CoV-2. Therefore, in this study we explored the
recombinant SARS-CoV-2 S protein as a potential vaccine
candidate. As recombinant protein antigens are poorly
immunogenic and are incapable of mounting antigen-specific
immunity of sufficient quality, amplitude and duration, co-
administration of adjuvants that shape B cell and T cell
responses are indispensable. Adjuvants like alum and oil-in-
water emulsions can act through a multitude of mechanisms.
More defined small molecule adjuvants that potently activate
innate immune cells by triggering specific innate immune
receptors might be more relevant for anti-viral vaccine
design. The Toll-like receptors 7 and 8 (TLR7/8) are widely
distributed amongst innate immune cell subsets over a broad
[
24]
airways of transduced mice.
[12]
range of species.
Akin to be an endosomal pattern
recognition receptor for viral RNA, triggering of these
receptors provokes robust type I interferon production that
can skew a Th1-type adaptive immune response against co-
Figure 1. A) Molecular structure of (A1) IMDQ-PEG-CHOL and (A2)
IMDQ-PEG. Conjugation was performed by amide bond formation
between, respectively cholesterylamine and PEG and PEG and IMDQ.
B) Representation of albumin hitchhiking-mediated lymphatic trans-
portation.
[13]
administered antigen.
The latter are characterized by
robust antibody titers capable of inducing viral neutralization
through a variety of mechanisms, including Fc-mediated
innate immune killing as well as inducing CD4 + and CD8 +
T-cell based immunological memory. Moreover, vaccines Results and Discussion
adjuvanted with TLR7/8 ligands have been shown to confer
enhanced protective immunity in both mouse and non-human
The imidazoquinoline 1-(4-(aminomethyl)benzyl)-2-bu-
[
14,15]
[25]
primate models.
Being well-defined small molecules, imidazoquinolines
tyl-1H-imidazo[4,5-c]quinolin-4-amine (IMDQ)
was con-
jugated to cholesteryl-poly(ethylene glycol) (PEG-CHOL),
yielding IMDQ-PEG-CHOL. As a control, non-amphiphilic
IMDQ-PEG was synthesized. (Figure 1A) PEG with a mo-
lecular weight of 3 kDa was chosen as an optimal compromise
between water solubility and drug load. Characterization of
the conjugate was performed by matrix assisted laser
desorption/ionization–time of flight (MALDI-ToF) (Support-
[16]
are a class of TLR7/8 agonists
that hold a massive
technological advantage in terms of production and phys-
icochemical stability. However, their pharmacokinetic profile
is characterized by rapid systemic dissemination upon local
(
subcutaneous or intramuscular) administration, thereby
causing unwanted innate immune activation at multiple distal
9
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 9467 – 9473