Nano-Self-Assemblies Based on Synthetic Analogues of Mycobacterial Monomycoloyl Glycerol and DDA: Supramolecular Structure and Adjuvant Efficacy

Recent Advances in the Development of Mincle-Targeting Vaccine Adjuvants

The Macrophage-inducible C-type lectin (Mincle) is a pattern-recognition receptor (PRR), which has shown much promise as a molecular target for the development of TH1/TH17-skewing vaccine adjuvants. In 2009, the first non-proteinaceous Mincle ligands, trehalose dimycolate (TDM) and trehalose dibehenate (TDB), were identified. This prompted a search for other Mincle agonists and the exploration of Mincle agonists as vaccine adjuvants for both preventative and therapeutic (anti-cancer) vaccines. In this review, we discuss those classes of Mincle agonists that have been explored for their adjuvant potential. These Mincle agonists have been used as stand-alone adjuvants or in combination with other pathogen-associated molecular patterns (PAMPs) or immunomodulatory agents. We will also highlight recently identified Mincle ligands with hitherto unknown adjuvanticity. Conjugate vaccines that contain covalently linked adjuvants and/or adjuvant-antigen combinations are also presented, as well as the different formulations (e.g., oil-in-water emulsions, liposomes, and particulate delivery systems) that have been used for the codelivery of antigens and adjuvants. Insofar the reader is presented with a thorough review of the potential of Mincle-mediated vaccine adjuvants, including historical context, present-day research and clinical trials, and outstanding research questions, such as the role of ligand presentation and Mincle clustering, which, if better understood, will aid in the development of the much-needed TH1/TH17-skewing vaccine adjuvants.

A Model of Type II Collagen-Induced Spondylitis and Arthritis in F1 Hybrid Male Mice

In this study, we tested a new model of ankylosing spondylitis in order to determine its histological and radiological features needed to investigate peripheral arthritis, spondylitis, and formation of the new bone tissues. F1 hybrid male mice (BALB/c×DBA/1), a progeny of spondylitis-susceptible BALB/c male mice and rheumatoid arthritis-susceptible DBA/1 female mice, were immunized intraperitoneally with bovine type II collagen (CII) mixed with adjuvant dimethyldioctadecylammonium bromide. Radiological and histological studies were performed at the peak of swelling, redness, and stiffness. The incidence of peripheral arthritis and spondylitis induced by CII in F1 hybrid mice were 66 and 62%, respectively. X-ray examination revealed bone erosion and spondylitis in the peripheral joints, as well as the formation of new bone tissues in the coccygeal vertebrae and between LIII and LIV vertebrae. The histological study showed lymphocyte and plasma cell infiltration, capillary dilation, congestion, and endochondral ossification of the lumbar vertebrae. This novel model of CII-induced spondylitis in F1 hybrid mice provoked axial and peripheral arthritides inducing chronic inflammation. In this model, the formation of new bone tissue in the stiff spine is characterized by endochondral ossification. The advanced model is an additional and valuable tool for investigation of the autoimmune reactions in spondylitis.

Pulmonary Administration of the Liposome-Based Adjuvant CAF01: Effect of Surface Charge on Mucosal Adjuvant Function

Mucosal surfaces of the lungs represent a major site of entry for airborne pathogens, and pulmonary administration of vaccines is an attractive strategy to induce protective mucosal immunity in the airways. Recently, we demonstrated the potential of pulmonary vaccination with the tuberculosis subunit antigen H56 adjuvanted with the cationic liposomal adjuvant formulation CAF01, which consists of the cationic lipid dimethyldioctadecylammonium (DDA) bromide and the synthetic cord factor trehalose-6,6'-dibehenate. However, the cationic charge of DDA represents a major safety challenge. Hence, replacing DDA with a safer zwitterionic or anionic phospholipid is an attractive approach to improve vaccine safety, but the effect of liposomal surface charge on the induction of mucosal immunity after airway immunization is poorly understood. Here, we investigated the effect of surface charge by replacing the cationic DDA component of CAF01 with zwitterionic dipalmitoylphosphatidylcholine (DPPC) or anionic dipalmitoylphosphatidylglycerol (DPPG), and we show that charge modification enhances antigen-specific pulmonary T-cell responses against co-formulated H56. We systematically replaced DDA with either DPPC or DPPG and found that these modifications resulted in colloidally stable liposomes that have similar size and morphology to unmodified CAF01. DPPC- or DPPG-modified CAF01 displayed surface charge-dependent protein adsorption and induced slightly higher follicular helper T cells and germinal center B cells in the lung-draining lymph nodes than unmodified CAF01. In addition, modified CAF01 induced significantly higher levels of H56-specific Th17 cells and polyfunctional CD4⁺ T cells in the lungs, as compared to unmodified CAF01. However, the strong H56-specific humoral responses induced by CAF01 in the lungs and spleen were not influenced by surface charge. Hence, these results provide insights into the importance of surface charge for liposomal adjuvant function and can also guide the design of safe pulmonary subunit vaccines against other mucosal pathogens.

Liposomes as Adjuvants and Vaccine Delivery Systems

The review considers liposomes as systems of substantial interest as adjuvant carriers in vaccinology due to their versatility and maximal biocompatibility. Research and development on the use of liposomes and lipid nanoparticles to create subunit vaccines for the prevention and treatment of infectious diseases has been going on for several decades. In recent years, the area has seen serious progress due to the improvement of the technology of industrial production of various high-grade lipids suitable for parenteral administration and the emergence of new technologies and equipment for the production of liposomal preparations. When developing vaccines, it is necessary to take into account how the body’s immune system (innate and adaptive immunity) functions. The review briefly describes some of the fundamental mechanisms underlying the mobilization of immunity when encountering an antigen, as well as the influence of liposome carriers on the processes of internalization of antigens by immunocompetent cells and ways of immune response induction. The results of the studies on the interactions of liposomes with antigen-presenting cells in function of the liposome size, charge, and phase state of the bilayer, which depends on the lipid composition, are often contradictory and should be verified in each specific case. The introduction of immunostimulant components into the composition of liposomal vaccine complexes—ligands of the pathogen-associated molecular pattern receptors—permits modulation of the strength and type of the immune response. The review briefly discusses liposome-based vaccines approved for use in the clinic for the treatment and prevention of infectious diseases, including mRNA-loaded lipid nanoparticles. Examples of liposomal vaccines that undergo various stages of clinical trials are presented.

Rational Design and In Vivo Characterization of Vaccine Adjuvants

Many different adjuvants are currently being developed for subunit vaccines against a number of pathogens and diseases. Rational design is increasingly used to develop novel vaccine adjuvants, which requires extensive knowledge of, for example, the desired immune responses, target antigen-presenting cell subsets, their localization, and expression of relevant pattern-recognition receptors. The adjuvant mechanism of action and efficacy are usually evaluated in animal models, where mice are by far the most used. In this review, we present methods for assessing adjuvant efficacy and function in animal models: (1) whole-body biodistribution evaluated by using fluorescently and radioactively labeled vaccine components; (2) association and activation of immune cell subsets at the injection site, in the draining lymph node, and the spleen; (4) adaptive immune responses, such as cytotoxic T-lymphocytes, various T-helper cell subsets, and antibody responses, which may be quantitatively evaluated using ELISA, ELISPOT, and immunoplex assays and qualitatively evaluated using flow cytometric and single cell sequencing assays; and (5) effector responses, for example, antigen-specific cytotoxic potential of CD8+ T cells and antibody neutralization assays. While the vaccine-induced immune responses in mice often correlate with the responses induced in humans, there are instances where immune responses detected in mice are not translated to the human situation. We discuss some examples of correlation and discrepancy between mouse and human immune responses and how to understand them.

Immune responses induced by nano-self-assembled lipid adjuvants based on a monomycoloyl glycerol analogue after vaccination with the Chlamydia trachomatis major outer membrane protein

Nanocarriers based on inverse hexagonal liquid crystalline phases (hexosomes) show promising potential as vaccine delivery systems. Their unique internal structure, composed of both lipophilic domains and water-containing channels, renders them capable of accommodating immunopotentiating compounds and antigens. However, their adjuvant properties are poorly understood. We hypothesized that the supramolecular structure of the lyotropic liquid crystalline phase influences the immunostimulatory activity of lipid-based nanocarriers. To test this, hexosomes were designed containing the lipid phytantriol (Phy) and the immunopotentiator monomycoloyl glycerol-1 (MMG-1). Self-assembly of Phy and MMG-1 into nanocarriers featuring an internal hexagonal phase was confirmed by small-angle X-ray scattering and cryogenic transmission electron microscopy. The effect of the nanostructure on the adjuvant activity was studied by comparing the immunogenicity of Phy/MMG-1 hexosomes with MMG-1-containing lamellar liquid crystalline nanoparticles (liposomes, CAF04). The quality and magnitude of the elicited immune responses were determined after vaccination of CB6/F1 mice using the Chlamydia trachomatis major outer membrane protein (MOMP) as antigen. MMG-1-based hexosomes potentiated significantly stronger MOMP-specific humoral responses than CAF04 liposomes. The liposome-based vaccine formulation induced a much stronger MOMP-specific cell-mediated immune response compared to hexosome-adjuvanted MOMP, which elicited minimal MOMP-specific T-cell stimulation after vaccination. Hence, our data demonstrates that hexosomal and liposomal adjuvants activate the immune system via different mechanisms. Our work provides valuable insights into the adjuvant potential of hexosomes and emphasizes that engineering of the supramolecular structure can be used to design adjuvants with customized immunological properties.

Liposomal vaccine formulations as prophylactic agents: design considerations for modern vaccines

Vaccinology is one of the most important cornerstones in modern medicine, providing better quality of life. The human immune system is composed of innate and adaptive immune processes that interplay when infection occurs. Innate immunity relies on pathogen-associated molecular patterns which are recognized by pathogen recognition receptors localized in antigen presenting cells. After antigen processing and presentation, CD4+ T cell polarization occurs, further leading to B cell and CD8+ activation and humoral and cell-mediated adaptive immune responses. Liposomes are being employed as vaccine technologies and their design is of importance to ensure proper immune responses. Physicochemical parameters like liposome size, charge, lamellarity and bilayer fluidity must be completely understood to ensure optimal vaccine stability and efficacy. Liposomal vaccines can be developed to target specific immune cell types for the induction of certain immune responses. In this review, we will present promising liposomal vaccine approaches for the treatment of important viral, bacterial, fungal and parasitic infections (including tuberculosis, TB). Cationic liposomes are the most studied liposome types due to their enhanced interaction with the negatively charged immune cells. Thus, a special section on the cationic lipid dimethyldioctadecylammonium and TB is also presented.

Advances in structural design of lipid-based nanoparticle carriers for delivery of macromolecular drugs, phytochemicals and anti-tumor agents

The present work highlights recent achievements in development of nanostructured dispersions and biocolloids for drug delivery applications. We emphasize the key role of biological small-angle X-ray scattering (BioSAXS) investigations for the nanomedicine design. A focus is given on controlled encapsulation of small molecular weight phytochemical drugs in lipid-based nanocarriers as well as on encapsulation of macromolecular siRNA, plasmid DNA, peptide and protein pharmaceuticals in nanostructured nanoparticles that may provide efficient intracellular delivery and triggered drug release. Selected examples of utilisation of the BioSAXS method for characterization of various types of liquid crystalline nanoorganizations (liposome, spongosome, cubosome, hexosome, and nanostructured lipid carriers) are discussed in view of the successful encapsulation and protection of phytochemicals and therapeutic biomolecules in the hydrophobic or the hydrophilic compartments of the nanocarriers. We conclude that the structural design of the nanoparticulate carriers is of crucial importance for the therapeutic outcome and the triggered drug release from biocolloids.

Systematic Investigation of the Role of Surfactant Composition and Choice of oil: Design of a Nanoemulsion-Based Adjuvant Inducing Concomitant Humoral and CD4+ T-Cell Responses

PURPOSE

Induction of cell-mediated immune (CMI) responses is crucial for vaccine-mediated protection against difficult vaccine targets, e.g., Chlamydia trachomatis (Ct). Adjuvants are included in subunit vaccines to potentiate immune responses, but many marketed adjuvants stimulate predominantly humoral immune responses. Therefore, there is an unmet medical need for new adjuvants, which potentiate humoral and CMI responses. The purpose was to design an oil-in-water nanoemulsion adjuvant containing a synthetic CMI-inducing mycobacterial monomycoloyl glycerol (MMG) analogue to concomitantly induce humoral and CMI responses.

METHODS

The influence of emulsion composition was analyzed using a systematic approach. Three factors were varied: i) saturation of the oil phase, ii) type and saturation of the applied surfactant mixture, and iii) surfactant mixture net charge.

RESULTS

The emulsions were colloidally stable with a droplet diameter of 150-250 nm, and the zeta-potential correlated closely with the net charge of the surfactant mixture. Only cationic emulsions containing the unsaturated surfactant mixture induced concomitant humoral and CMI responses upon immunization of mice with a Ct antigen, and the responses were enhanced when squalene was applied as the oil phase. In contrast, emulsions with neutral and net negative zeta-potentials did not induce CMI responses. The saturation degree of the oil phase did not influence the adjuvanticity.

CONCLUSION

Cationic, MMG analogue-containing nanoemulsions are potential adjuvants for vaccines against pathogens for which both humoral and CMI responses are needed.

Adjuvants Based on Synthetic Mycobacterial Cord Factor Analogues: Biophysical Properties of Neat Glycolipids and Nanoself-Assemblies with DDA

Synthetic mycobacterial cord factor analogues, e.g., trehalose 6,6'-dibehenate (TDB), are highly promising adjuvants due to their strong immunopotentiating capabilities, but their biophysical properties have remained poorly characterized. Here, we report the synthesis of an array of synthetic TDB analogues varying in acyl chain length, degree of acylation, and headgroup display, which was subjected to biophysical characterization of neat nondispersed self-assembled nanostructures in excess buffer and as aqueous dispersions with cationic dimethyldioctadecylammonium (DDA) bromide. The array comprised trehalose mono- (TMX) and diester (TDX) analogues with symmetrically shortened acyl chains [denoted by X: arachidate (A), stearate (S), palmitate (P), myristate (Myr), and laurate (L)] and an analogue with a short hydrophilic polyethylene glycol (PEG) linker inserted between the trehalose headgroup of TDS and the acyl chains (PEG-TDS). All dispersions were liposomes, but in contrast to the colloidally stable and highly cationic TDX-containing liposomes, the zeta-potential was significantly reduced for DDA/TMX and DDA/PEG-TDS liposomes, suggesting a charge-shielding effect, which compromises the colloidal stability. An increased d-spacing was observed for the lamellar phase of neat TDB analogues in excess buffer (TDS < TMS < PEG-TDS), confirming that the charge shielding is caused by an extended molecular configuration of the more flexible headgroup. Differential scanning calorimetry showed highly cooperative phase transitions for all tested dispersions albeit the monoesters destabilized the lipid bilayers. Langmuir experiments demonstrated that incorporation of TDXs and PEG-TDS stabilized DDA monolayers due to improved hydrogen bonding and reduced intermolecular repulsions. In conclusion, data suggest that the DDA/TDS dispersions exhibit favorable physicochemical properties rendering these DDA/TDS liposomes an attractive vaccine adjuvant, and they emphasize that not only the receptor binding and immune activation but also the biophysical properties of immunopotentiator formulations should be collectively considered when designing adjuvants with optimal safety, efficacy, and storage stability.

Mucosal Vaccine Development Based on Liposome Technology

Immune protection against infectious diseases is most effective if located at the portal of entry of the pathogen. Hence, there is an increasing demand for vaccine formulations that can induce strong protective immunity following oral, respiratory, or genital tract administration. At present, only few mucosal vaccines are found on the market, but recent technological advancements and a better understanding of the principles that govern priming of mucosal immune responses have contributed to a more optimistic view on the future of mucosal vaccines. Compared to live attenuated vaccines, subcomponent vaccines, most often protein-based, are considered safer, more stable, and less complicated to manufacture, but they require the addition of nontoxic and clinically safe adjuvants to be effective. In addition, another limiting factor is the large antigen dose that usually is required for mucosal vaccines. Therefore, the combination of mucosal adjuvants with the recent progress in nanoparticle technology provides an attractive solution to these problems. In particular, the liposome technology is ideal for combining protein antigen and adjuvant into an effective mucosal vaccine. Here, we describe and discuss recent progress in nanoparticle formulations using various types of liposomes that convey strong promise for the successful development of the next generation of mucosal vaccines.

Conserved Molecular Superlattices in a Series of Homologous Synthetic Mycobacterial Cell-Wall Lipids Forming Interdigitated Bilayers

Synthetic analogues of the cell-wall lipid monomycoloyl glycerol (MMG) are promising as next-generation vaccine adjuvants. In the present study, the thermotropic phase behavior of an array of synthetic MMG analogues was examined by using simultaneous small- and wide-angle X-ray scattering under excess water conditions. The MMG analogues differed in the alkyl chain lengths and in the stereochemistry of the polar glycerol headgroup or of the lipid tails (native-like versus alternative compounds). All MMG analogues formed poorly hydrated lamellar phases at low temperatures and inverse hexagonal (H₂) phases at higher temperatures prior to melting. MMG analogues with a native-like lipid acid configuration self-assembled into noninterdigitated bilayers whereas the analogues displaying an alternative lipid acid configuration formed interdigitated bilayers in a subgel (L_c') state. This is in contrast to previously described interdigitated phases for other lipids, which are usually in a gel (L_β) state. All investigated MMG analogues displayed an abrupt direct temperature-induced phase transition from L_c' to H₂. This transition is ultimately driven by the lipid chain melting and the accompanying change in molecular shape. No intermediate structures were found, but the entire array of MMG analogues displayed phase coexistence during the lamellar to H₂ transition. The structural data also showed that the headgroups of the MMG analogues adopting the alternative lipid acid configuration were ordered and formed a two-dimensional molecular superlattice, which was conserved regardless of the lipid tail length. To our knowledge, the MMG analogues with an alternative lipid acid configuration represent the first example of a lipid system showing both interdigitation and superlattice formation, and as such could serve as an interesting model system for future studies. The MMG analogues are also relevant from a subunit vaccine perspective because they are well-tolerated and display promising immunopotentiating activity. The structural characterization described here will serve as a prerequisite for the rational design of nanoparticulate adjuvants with specific and tailored structural features.