Nanomaterial-Based Vaccine Development and Immunomodulation
Following the publication of the genetic sequence of SARS-CoV-2 on January 11, 2020, intense research efforts have been devoted to developing a vaccine against COVID-19. With unprecedented speed, this extraordinary scientific mobilization led the first vaccine candidate to enter the Phase I human clinical trial on March 16, 2020, and other novel candidates are rapidly following.59 Up to May 22, 2020, there are 10 COVID-19 candidate vaccines in clinical evaluations and 114 in preclinical development.60
Concerning vaccine and immunization research, nanomaterials can assist in multiple ways to boost the upregulation required by the immune system and to direct the immune response specifically against antigens. Immune-targeted nanotherapeutics can be developed through their rational manufacture at the nanoscale level by designing nanomaterials that are able to amplify host’s immune response, for instance as adjuvants in the context of vaccination.
The development of a vaccine will rely either on the direct administration of viral antigens (e.g., in the form of recombinant proteins, vectored vaccines, or whole inactivated or attenuated virus) or RNA- or DNA-encoding viral antigens.27 Candidate antigens for immunization are surface proteins such as the immunogenic spike protein (S1), which is already targeted by antibodies of convalescent patients.27 Because the S1 protein is also essential for cellular uptake, many researchers are using this protein as the primary target for a vaccine.
There are many issues related to the delivery of a drug, protein, or RNA into the patient as the cargo is often degraded, not bioavailable, or is swiftly cleared. Nanotechnology provides multiple solutions to these challenges though, as nanocarriers can overcome some of these limitations.
Biocompatible polymeric-, lipid-based, or inorganic NPs can be tuned with respect to their physicochemical properties to encapsulate cargo proteins with high loading efficiency, improving protein delivery and pharmacokinetics over conventional approaches.38 Intranasal delivery of polymer-encapsulated antigen triggers a strong immune response, and the success of vaccination depends on the appropriate type of polymer in combination with the antigen.61,62 Similarly, researchers have developed lipid and lipid-based NPs as delivery platforms for mRNAs or siRNAs to enable the synthesis of key viral proteins for vaccination or to inactivate critical viral target genes, respectively.39 The field of therapeutic mRNAs to support vaccination has gained momentum, as well,63 at least in part due to nanotechnology-enabled strategies for cargo delivery. Moreover, the versatility of nanoplatforms as antigen-presenting systems offers great opportunities. Considering the possibility of random viral mutations that may alter the antigen shape, functionalizing nanomaterials with a wide number of molecules at the same time to target the virus, or motifs that are specific for pathogens, will increase the efficiency of vaccines and their ability to prevent viral infection.
Several companies are working on mRNA vaccines encoding SARS-CoV-2 proteins such as the spike protein, encapsulated in nanoliposomes with specific physicochemical properties that are potentially akin to those documented for immunization against certain tumor antigens.64 The design of such nanocarriers, which will need to escape recognition by scavenger cells and to be nontoxic and nonimmunogenic, is a challenge that will require substantial time prior to clinical availability.
On March 16, 2020, Moderna, through a partnership with the Vaccine Research Center at the U.S. National Institutes of Health, enrolled the first participants into a Phase I clinical trial testing an mRNA vaccine (mRNA-1273) encapsulated in lipid NPs—a record time of just 63 days following sequence selection (NCT04283461).65 The enrollment of the first cohort of participants (18 to 55-year-old healthy subjects) concluded on April 16, 2020.66 CureVac and BioNTech (in partnership with Pfizer) are currently working on similar vaccines; Pfizer/BioNTech, in particular, have recently started the recruitment in Phase I/II trials (NCT04368728, NCT04380701). A DNA plasmid vaccine by Inovio Pharmaceuticals (INO-4800) has showed promising results in mice and guinea pigs according to a recent article published in Nature Communications(67) and has entered Phase I testing in humans (NCT04336410).
Another candidate COVID-19 vaccine for Phase I clinical trial is from the University of Oxford and AstraZeneca (NCT04324606).68 Around 1110 people will take part in the trial, which started recruitment at the end of April 2020. The vaccine is based on a chimpanzee adenovirus vaccine vector (ChAdOx1) and the SARS-CoV-2 spike protein. Chimpanzee adenoviral vectors against different pathogens have been already tested in thousands of subjects with demonstrated safety. To date, ChAdOx1 has been administered to six rhesus macaques exposed to high doses of SARS-CoV-2. The vaccine was unable to prevent infections, although it reduced the severity of the disease: no signs of virus replication were observed in the lungs, with significantly lower levels of respiratory disease and no lung damage compared to control animals, according to a recent article deposited in bioRxiv.69 Another adenoviral vector vaccine developed by CanSino Biological Inc. and Beijing Institute of Biotechnology using a genetically engineered replication-defective adenovirus type 5 vector to express the SARS-CoV-2 spike protein (Ad5-nCoV) is currently being tested in Phase I/II trials (NCT04398147, NCT04341389, NCT0431312).
Although vaccines based on novel DNA and mRNA technologies might be promising, there are no such kinds of vaccines on the market, and it is unknown whether they can be effective in humans. Moderna, for instance, has generated preliminary safety data on different mRNA-based vaccines targeting other respiratory viruses, but their more advanced program (on a cytomegalovirus vaccine) is still in Phase II clinical testing. However, on May 18, Moderna announced that mRNA-1273 elicited antibody titers above the levels observed in convalescent individuals (and therefore considered potentially protective) in all eight initial participants across the 25 and 100 μg dose cohorts of the Phase I trial (NCT04283461).70 The company, after having received a fast-track approval from the FDA, is now moving on Phase II trials.
Conversely, strong evidence exists regarding the efficacy of protein-based vaccines such as recombinant-protein, viral-vector, attenuated, or inactivated vaccines across different infectious diseases, with licensed vaccines already existing for all of these platforms.27 All of the aforementioned approaches are currently being explored in the context of SARS-CoV-2.27 An inactivated vaccine developed by the China National Pharmaceutical Group (Sinopharm), in collaboration with the Wuhan Institute of Biological Products, is currently tested in a Phase I/II trial (ChiCTR2000031809), and a second inactivated vaccine (in collaboration with the Beijing Institute of Biological Products) has been currently approved for clinical testing (ChiCTR2000032459). A third aluminum salt (alum) adjuvanted inactivated vaccine, developed by Beijing-based Sinovac Biotech’s, was able to provide partial or complete protection in rhesus macaques, according to a recent article published in Science,71 and is currently being tested in Phase I/II trials (NCT04383574, NCT04352608).
Live-attenuated vaccines are intrinsically immunogenic (e.g., due to the presence of viral DNA), but extensive safety tests are required due to the rare possibility of reversion to a pathogenic form able to cause infection. Vaccine candidates based in this application have previously been designed for SARS-CoV with high stability.72,73 Recombinant-protein vaccines and inactivated vaccines are safer but might require adjuvants to increase their immunogenicity. In the context of SARS-CoV-2, adjuvants are important for two reasons. First, adjuvants might increase the efficacy of the vaccine, especially in subjects with impaired immunological function, such as the elderly, or in subjects with comorbidities resulting in immune dysfunctions; in these patient cohorts, SARS-CoV-2 has a high lethality rate. Second, adjuvants can reduce the amount of vaccine protein(s) required per dose, which could facilitate scaling-up vaccine production in a reduced time frame.
Beyond alum, which is in fact a nanoscale material74 that was developed in the 1920s for the tetanus and diphtheria toxoids,75 approval for a new adjuvant did not occur until 1997, with the introduction of the oil-in-water emulsion of squalene oil and polysorbate 80 and sorbitan trioleate surfactants (MF59) in the seasonal influenza vaccine for the elderly.76 The use of MF59 was further expanded to pandemic and avian influenza vaccines.77 Other adjuvants in licensed vaccines have been approved since 2000, as well: (a) AS03 (used for pandemic and avian influenza vaccines), similar to MF59, but including α-tocopherol as an additional immune stimulant; (b) AF 03 (used for pandemic influenza vaccines), an alternative squalene emulsion containing polyoxyethylene, cetostearyl ether, mannitol, and orbitan oleate;78 (c) AS 01 (used for herpes zoster vaccine), a liposome-based vaccine adjuvant system containing two immunostimulants, 3-O-desacyl-4′-monophosphoryl lipid A (MPL, a Toll-like receptor 4 agonist) and saponin QS-21, which activates the ACT-NLRP3 inflammasome pathway;79 and (d) AS04 (used for hepatitis B and human papilloma virus vaccines), which is a combination of MPL and aluminum hydroxide.76,80
In this context, the concept of “nanoimmunity by design” relies on the rational design of distinct physicochemical properties and specific functionalization of nanomaterials intended for fine-tuning their potential effects on the immune system.81 Nanomaterials have emerged as promising tools for immune modulation, either stimulating or suppressing the immune response. In fighting SARS-CoV-2, these properties may find applications for both prevention and therapy and in the context of vaccine development.
Mounting evidence indicates that nanomaterials such as graphene,82 nanodiamonds,83 carbon nanotubes,84 and polystyrene particles85 bear an intrinsic capacity to activate the immune system, depending on their functionalization.86 For instance, graphene oxide functionalized with amino groups (GO-NH2) induces activation of STAT1/IRF1 interferon signaling in monocytes and T cells, resulting in the production of T cell chemoattractants, and macrophage 1 (M1) 1/T-helper 1 (Th1) polarization of the immune response, with negligible toxicity.82 Remarkably, the ability of licensed adjuvants such as AS01 and AS03 to enhance adaptive immunity has been linked to their capacity to boost STAT1/IRF1 interferon signaling.87 In addition, recent SARS-CoV and MERS-CoV studies suggest that the development of a Th1-type response is central for controlling infection, which also may be true for SARS-CoV-2.88
Several groups and consortia have been screening and characterizing nanomaterials according to their immunomodulatory properties and absence of cytotoxicity.56,57,82−84,86 The development of an adjuvant for clinical use is a lengthy process that requires extensive Phase III randomized trials in large and diverse cohorts of subjects, and the process generally requires several years.76 Pharmaceutical giants such as GlaxoSmithKline (GSK), which owns the ASs mentioned above and other adjuvant platforms, are engaged in several partnerships to embed their adjuvant systems with SARS-CoV-2-protein-based vaccines. A full-length recombinant SARS-CoV-2 glycoprotein nanoparticle vaccine adjuvanted with the saponin-based Matrix M developed by Novavax is in Phase I clinical testing (NCT04368988).89
Although it is unlikely that novel adjuvants would be used in the context of the current pandemic, the SARS-CoV-2 pandemic offers an opportunity to reflect on the potential of nanotechnology for vaccine adjuvant development. In this context, it is critical to stream coherent pipelines covering in vitro and in vivo experiments specifically to select candidate materials that might be tested for clinical implementation as vaccine adjuvants.90
In this view, the status of nanomaterial-based vaccine adjuvants has been reviewed.91,92 In particular, immunomodulatory effects induced on the innate immune signaling have been demonstrated.91 For example, nanomaterials such as GO can elicit an inflammasome sensor (NLRP3)-dependent expression of IL-1β in macrophages.93 Notably, alum, the most commonly used adjuvant in human vaccines, induces the release of this cytokine in macrophages through the same NRLP-induced mechanism. These results suggest that nanomaterials such as GO and alum may be useful for medical applications.94
Considering that new vaccine development typically requires years to be approved and applied to the general population, there is also an urgent need to study how to improve treatment approaches. According to the first Phase III clinical studies, the antiviral drug Remdesivir, which was recently approved by the FDA for COVID-19 treatment in the United States, seems to be a promising treatment for adults diagnosed with COVID-19. Nano-based strategies have already been applied to enhance the effectiveness of Remdesivir in the context of other emerging viral infections (Nipah virus),95 suggesting the suitability of nanotechnology to assist with similar strategies for the treatment of COVID-19 as well as other possible pandemics in the future.96