Even as several safe and effective COVID-19 vaccines are being administered to people worldwide, scientists are still hard at work developing different vaccine strategies that could provide even stronger or longer-lasting immunity against SARS-CoV-2 and its variants.
Now, researchers reporting in ACS Central Science have immunized mice with nanoparticles that mimic SARS-CoV-2 by displaying multiple copies of the receptor binding domain (RBD) antigen, showing that the vaccine triggers robust antibody and T cell responses.
Although the first vaccines to receive Emergency Use Authorization by the U.S. Food and Drug Administration were based on mRNA, more conventional protein-based vaccines have also shown promise in clinical trials. Most train the immune system to recognize the RBD, a peptide that is the portion of the SARS-CoV-2 spike protein that binds to the ACE-2 receptor on host cell surfaces.
However, not all of these vaccines elicit both antibody and T cell responses, both of which are thought to be important for longer-lasting immunity. Melody Swartz, Jeffrey Hubbell and colleagues had previously developed a vaccine delivery tool called polymersomes – self-assembling, spherical nanoparticles that can encapsulate antigens and adjuvants (helper molecules that boost the immune response) and then release them inside immune cells.
Polymersomes trigger robust T cell immunity, and the researchers wondered if they could further improve the antibody response by engineering the nanoparticles to mimic viruses by displaying multiple copies of the RBD on their surfaces.
So the team made polymersomes that were similar in size to SARS-CoV-2 and decorated them with many RBDs. After characterizing the nanoparticles in vitro, they injected them into mice, along with separate polymersomes containing an adjuvant, in two doses that were three weeks apart.
For comparison, they immunized another group of mice with polymersomes that encapsulated the RBD, along with the nanoparticles containing the adjuvant.
Although both groups of mice produced high levels of RBD-specific antibodies, only the surface-decorated polymersomes generated neutralizing antibodies that prevented SARS-CoV-2 infection in cells.
Both the surface-decorated and encapsulated RBDs triggered robust T cell responses.
Although the new vaccine still needs to be tested for safety and efficacy in humans, it could have advantages over mRNA vaccines with regard to widespread distribution in resource-limited areas, the researchers say.
That’s because the surface-decorated polymersomes are stable and active for at least six months with refrigeration, in contrast to mRNA vaccines that require subzero temperature storage.
Future directions based on polymersomes
Lacking a universal vaccine along with the increase in the number of cases opens the door for virus inhibitors to be recognized as powerful tools to suppress virus infection. On the other hand, in addition to a long time and high costs that are still required to reach the targeted safe and effective vaccine against COVID-19, the recently emerged reinfection dilemma has threatened the efforts and hopes for the ongoing COVID-19 vaccine trials.
Since the nanomedicine field has shown a variety of promising therapeutic applications against COVID-19, and previously against various viral infections and diseases, it is worth to emphasize that learning from the past can be an effective route towards therapeutics against COVID-19.
Here we propose a novel approach based on using polymersomes (polymer-like liposomes) as potential nano-objects with a significant imprint in the field of nanomedicine. Despite their immense potential, they have not been employed in the fight against COVID-19 so far.
Polymersomes are some of the most efficient nanomaterials for use as drug delivery systems with a special surface functionalization (Discher et al., 1999; Tuguntaev et al., 2016). They are artificial vesicles composed of amphiphilic block or grafted copolymers, and they emerged thanks to their high colloidal stability, strong membrane properties, as well as easy ligand conjugation with high biocompatibility (Ferji et al., 2015, 2018; Guan et al., 2015). Fig. 5 shows common amphiphilic block copolymers that are used to formulate polymersomes (Barnier Quer et al., 2011; Chun et al., 2018; Galan-Navarro et al., 2017; Scott et al., 2012).
Polymersomes were designed to mimic the cell structure with an aqueous cavity, and they showed a high capacity for drug loading, especially as a co-delivery system upon loading hydrophobic and hydrophilic drugs in their exterior layers and cores, respectively (Kim et al., 2013; Li et al., 2016).
Polymersomes have recently been exploited not only as vehicles for the delivery of various therapeutic compounds (Chun et al., 2018), but also based on their potential to regulate ROS (Kim et al., 2017). Owing to their immunogenic properties (Webster et al., 2013), polymersomes could play a vital role in improving subunit vaccines and therapeutics delivery against COVID-19 infection.
In a previous study, for example, polymersomes were loaded with influenza hemagglutinin (HA) antigens and then used as an immune adjuvant (Barnier Quer et al., 2011). Notably, a superior increase of serum IgG and hemagglutination inhibition titers were reported upon immunization with polymersome-loaded HA relative to free HA, without causing any cellular toxicity (Barnier Quer et al., 2011).
Therefore, polymersomes successfully enhanced the immunogenicity of HA, which indicated their potential not only as a delivery system, but also as an adjuvant for subunit vaccines. Furthermore, researchers have shown that loading specific protein antigens into the polymersome core can boost the antigen presentation by DCs in-vitro (Scott et al., 2012).
While polymersomes enhanced strong T cell immunity to protein antigens and induce the activation of antigen-specific CD4+ T cells (Stano et al., 2013), it has also been reported that polymersomes can regulate intracellular ROS levels when used as a delivery system for antiviral therapeutics against H1N1 infection in-vitro (Kim et al., 2017).
Their ability to reduce the ROS generation, which is normally increased during viral infection, could be one of the promising approaches in inhibiting viral replication, cell death, production of pro-inflammatory cytokines, and activation ISGs in the host (Drew et al., 2012; Hung et al., 2016; Lin et al., 2016; Reshi et al., 2014; Svegliati et al., 2005; Ting et al., 2018; Vlahos et al., 2012; Wong et al., 2016).
As a result, polymersomes can play a vital role as ROS regulators that can assist in the suppression of SARS-CoV-2 propagation and disease severity, as well as increase the cell survival rate.
In a study on Lassa virus (LASV) infected mice, recombinant LASV E protein was encapsulated inside oxidation-sensitive polymersomes as nanocarriers that induced intracellular drug transfer (Galan-Navarro et al., 2017). The results showed that immunization with polymersome-loaded LASV E protein, compared to the treatment with free LASV E protein, preferentially activated the humoral immune response.
LASV E protein loaded polymersome immunization elevated the antibody production with a higher binding affinity to the E protein of LASV virion, and also increased the production of IgG-secreting B cells and antiviral CD4+ T cells (Galan-Navarro et al., 2017). Another study used polymersomes to encapsulate two antivirals (favipiravir in the exterior layer and mir-323a in the core) for use in-vitro against H1N1 infection (Chun et al., 2018).
The surface density of polymersomes was controlled by functionalization via specific copolymers to maximize cellular uptake (Chun et al., 2018). This study showed promising synergistic effects upon using these functional polymersomes against H1N1 infection. Together, these studies indicate the potential efficiency of polymersome-based delivery systems in improving the transfection of antiviral therapeutics and vaccine substances against COVID-19, which has not been studied yet nor proposed.
We recently proposed a novel therapeutic approach for cancer based on nano-objects that have the capacity to target specific immune checkpoints along with the inhibition of DNA demethylation (Al-Hatamleh et al., 2019b). Here we hypothesize that there could be benefits arising from the readjustment of this approach involving the use of polymersomes as promising nanocarrier-based systems against COVID-19.
Based on the unique characteristics of polymersomes, it is possible to functionalize them and turn them into effective delivery systems for therapeutic substances or antibodies that block the pro-inflammatory cytokines or their cellular receptors. Owing to their potential for co-delivery of both hydrophobic and hydrophilic drugs, polymersomes are able to be loaded with DNA demethylation inhibitors along with cytokines blockers to cause a stronger blockage.
Using specific DNA demethylation inhibitors such as histone deacetylase (HDAC) inhibitors, histone methyltransferase (HMT) inhibitors, and dimethyltryptamine (DMT) inhibitors, might lead to epigenetic alteration and result in a decreased expression of genes encoding cytokines (e.g., IL-6, TNF, IL-10) and their respective receptors (i.e., IL-6 receptor, TNF receptors 1 and 2, and IL-10 receptor), and thus downregulate those cytokines. Thus, the synergistic effects of cytokine blockers and DNA demethylation inhibitors loaded into polymersomes would be a promising approach in fighting COVID-19 by suppression of the cytokine storm in patients.
More specifically, this approach can be tested first against IL-6, the most important member in the cytokine storm (Zhang et al., 2020a), but also against other cytokines in the advanced stages of the research. In the early days of the COVID-19 pandemic, researchers from Wuhan, China noted that levels of IL-6 were higher in critical cases than in severe and mild cases (Chen et al., 2020a). This report was confirmed later by another similar study showing significantly higher levels of IL-6 among severe cases compared to mild cases (Gao et al., 2020).
Interestingly, a retrospective study on data related to COVID-19 cases (68 mortality and 82 recovered cases) showed that IL-6 levels were significantly higher in died cases compared to the survivors (Ruan et al., 2020). Therefore, employing IL-6 inhibitors in the treatment of COVID-19 is considered as a promising immunotherapeutic approach to control the infection.
Some clinical trials are being conducted to repurpose the existing IL-6 inhibitors including anti-IL-6 antibodies (e.g., clazakizumab and siltuximab) and anti-IL-6 receptor antibodies (e.g., tocilizumab and sarilumab) against COVID-19 (Atal and Fatima, 2020). Overall, based on the above literature survey, we hypothesize that loading IL-6 receptor blockers along with DNA demethylation inhibitors into functionalized polymersomes might be a promising approach in fighting COVID-19 (Fig. 6 ).
Polymersomes could have specific advantages over other nanomaterial-based delivery systems (e.g., liposomes) for development of therapeutics and vaccines against COVID-19. A variety of highly reproducible and scalable production methods are used to produce polymersomes with low polydispersity, and the process became achievable within about 1 h (Poschenrieder et al., 2017).
The ability of polymersomes to encapsulate hydrophobic, hydrophilic and amphiphilic molecules makes them more suited for in-vivo studies compared to many other nanomaterials (Zhang and Zhang, 2017). Despite their similar amphiphilic nature, the bilayer thickness of polymersomes (5–50 nm) is greater compared with the bilayer of liposomes (3–5 nm), which causes more robust and impermeable wall (Rideau et al., 2018).
Thus, polymersomes have considerably higher membrane stability than liposomes (Poschenrieder et al., 2017), which widely used nowadays in development of COVID-19 vaccines. The higher stability and versatility of polymersomes gives them advantages towards more sustained and controlled release, and the improved metabolic stability of the loaded therapeutic agent (Zhang and Zhang, 2017; Gurunathan et al., 2020).
Furthermore, the immunogenicity of polymersomes can be reduced (stealthiness) if a dense PEG brush is used on the surface with relatively long PEG polymers, meanwhile their biological stability would be increased (Zhang and Zhang, 2017). Therefore, the use of a proper polymersome-based delivery system can help in reducing therapeutic doses, along with maintaining a constant concentration of drug in the targeted site or circulation for longer time. These factors support polymersomes to be applicable and universal carrier-systems for medical applications, more specifically in the fighting against COVID-19.
In addition to the potential polymersome-based system which is hypothesized above, polymersomes could have promising roles with other repurposed drugs that have regulatory effects on the immunity of COVID-19 patients, especially for severe cases.
Among these drugs, anticoagulant treatments (e.g., heparin and nafamostat), that also could inhibit the cytokine storm and increase the percentage of lymphocytes (Shi et al., 2020; Tang et al., 2020; Yamamoto et al., 2020), as well as some other immune-based therapies (e.g., interferon alfa-2B) which also expected to have similar effects, but are still awaiting experimental evaluation (Khan et al., 2020).
Also, other types of drugs are repurposed and currently being studied, such as antihypertensive drugs and non-steroidal anti-inflammatory drugs, but no scientific evidence proving the effectiveness of any drug or therapeutic compound against COVID-19 has been demonstrated so far.
Moreover, the potential roles of polymersome-based delivery systems are not limited to boosting immunity and suppressing cytokine storm in COVID-19 patients. Polymersomes can be functionalized to deliver several types of repurposed drugs that showed potential antiviral effects against SARS-CoV-2, including antimalarial drugs (e.g., chloroquine), antimalarial and antibiotic combinations (e.g., hydroxychloroquine and azithromycin), antiviral drugs (e.g., camostat, bromhexine, favipiravir, remdesivir and lopinavir), and antihelmintics/antiparasitic agents (e.g., nitazoxanide and ivermectin) (Khan et al., 2020; Rajoli et al., 2020; Santos et al., 2020).
However, the clinical effectiveness of these drugs has not yet been fully evaluated, while several clinical trials are still underway (Singh et al., 2020). Future studies may also investigate potential polymersome-formulations for combination therapy (using repurposed drugs) to COVID-19 infection.
REFERENCE LINK : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7857087/
More information: Lisa R. Volpatti et al, Polymersomes Decorated with the SARS-CoV-2 Spike Protein Receptor-Binding Domain Elicit Robust Humoral and Cellular Immunity, ACS Central Science (2021). DOI: 10.1021/acscentsci.1c00596