The cationic lipid, diC14 amidine, extends the adjuvant properties of aluminum salts through a TLR-4- and caspase-1-independent mechanism

Emerging Cationic Nanovaccines

Cationic vaccines of nanometric sizes can directly perform the delivery of antigen(s) and immunomodulator(s) to dendritic cells in the lymph nodes. The positively charged nanovaccines are taken up by antigen-presenting cells (APCs) of the lymphatic system often originating the cellular immunological defense required to fight intracellular microbial infections and the proliferation of cancers. Cationic molecules imparting the positive charges to nanovaccines exhibit a dose-dependent toxicity which needs to be systematically addressed. Against the coronavirus, mRNA cationic nanovaccines evolved rapidly. Nowadays cationic nanovaccines have been formulated against several infections with the advantage of cationic compounds granting protection of nucleic acids in vivo against biodegradation by nucleases. Up to the threshold concentration of cationic molecules for nanovaccine delivery, cationic nanovaccines perform well eliciting the desired Th 1 improved immune response in the absence of cytotoxicity. A second strategy in the literature involves dilution of cationic components in biocompatible polymeric matrixes. Polymeric nanoparticles incorporating cationic molecules at reduced concentrations for the cationic component often result in an absence of toxic effects. The progress in vaccinology against cancer involves in situ designs for cationic nanovaccines. The lysis of transformed cancer cells releases several tumoral antigens, which in the presence of cationic nanoadjuvants can be systemically presented for the prevention of metastatic cancer. In addition, these local cationic nanovaccines allow immunotherapeutic tumor treatment.

Lipid-protein interactions regulating the canonical and the non-canonical NLRP3 inflammasome

The inflammatory response is a complex regulated effector mechanism of the innate immune system that is initiated after tissue injury or infection. The NLRP3 inflammasome is an important initiator of inflammation by regulating the activation of caspase-1, the maturation of pro-inflammatory cytokines and the induction of pyroptotic cell death. Numerous studies demonstrate that the NLRP3 inflammasome could be modulated by lipids, existing a relation between lipids and the activation of different inflammatory processes. In this review we will summarize how the mechanism of NLRP3 inflammasome activation is regulated by different lipids and how these lipids control specific cellular localization of NLRP3 during activation. Although being a cytosolic protein, NLRP3 interacts with lipids accessible in neighbor membranes. Also, the modulation of NLRP3 by endogenous lipids has been found causative of different metabolic diseases and bacterial-pathogenic lipids lead to NLRP3 activation during infection. The understanding of the modulation of the NLRP3 inflammasome by lipids has resulted not only in a better knowledge about the mechanism of NLRP3 activation and its implication in disease, but also opens a new avenue for the development of novel therapeutics and vaccines, as NLRP3 could be modulated by synthetic lipids used as adjuvants.

Harnessing biomaterials for therapeutic strategies against COVID-19

With the emergence of the coronavirus disease 2019 (COVID-19), the world is experiencing a profound human health crisis. The number of infections and deaths due to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to increase every minute, pinpointing major shortcomings in our ability to prevent viral outbreaks. Although several COVID-19 vaccines have been recently approved for emergency use, therapeutic options remain limited, and their long-term potency has yet to be validated. Biomaterials science has a pivotal role to play in pushing the boundaries of emerging technologies for antiviral research and treatment. In this perspective, we discuss how biomaterials can be harnessed to develop accurate COVID-19 infection models, enhance antiviral drug delivery, foster new antiviral strategies, and boost vaccine efficacy. These efforts will not only contribute to stop or mitigate the current pandemic but will also provide unorthodox platforms to understand, prevent, and protect us from future viral outbreaks.

Lipidic Aminoglycoside Derivatives: A New Class of Immunomodulators Inducing a Potent Innate Immune Stimulation

Development of simple and fully characterized immunomodulatory molecules is an active area of research to enhance current immunotherapies. Monophosphoryl lipid A (MPL), a nontoxic lipidic derivative from bacteria, is the first and currently only adjuvant approved in humans. However, its capacity to induce a potent response against weak immunogenic tumoral-associated antigens remains limited. Herein, a new generation of lipidic immunomodulators to conduct a structure-activity relationship study to determine the minimal structural elements conferring immunomodulatory properties is introduced. Two lead molecules characterized by a short succinyl linker between two oleyl chains and a polar headgroup consisting of either naturally occurring tobramycin (DOST) or kanamycin (DOSK) are identified. These two lipoaminoglycosides self-assemble in very small vesicles. In a wide variety of cells including 3D human cell culture, DOST and DOSK induce the upregulation of proinflammatory cytokines and interferon-inducible proteins in a dose and time-dependent manner via a caveolae-dependent proinflammatory mechanism and phosphatidylinositol phospholipase C activation. Furthermore, after intratumoral administration, these lipoaminoglycosides induce an efficient immune response leading to significant antitumor activity in a mouse breast cancer model. Altogether, these findings indicate that DOST and DOSK are two groundbreaking synthetic lipid immunostimulators that can be used as adjuvants to enhance current immunotherapeutic treatments.

Antigen Priming with Enantiospecific Cationic Lipid Nanoparticles Induces Potent Antitumor CTL Responses through Novel Induction of a Type I IFN Response

Certain types of cationic lipids have shown promise in cancer immunotherapy, but their mechanism of action is poorly understood. In this study, we describe the properties of an immunotherapeutic consisting of the pure cationic lipid enantiomer R-1,2-dioleoyl-3-trimethyl-ammonium-propane (R-DOTAP) formulated with modified viral or self-peptide Ags. R-DOTAP formulations with peptide Ags stimulate strong cross-presentation and potent CD8 T cell responses associated with a high frequency of polyfunctional CD8 T cells. In a human papillomavirus tumor model system, a single s.c. injection of tumor-bearing mice with R-DOTAP plus human papillomavirus Ags induces complete regression of large tumors associated with an influx of Ag-specific CD8 T cells and a reduction of the ratio of regulatory/Ag-specific CD8 T cells. R-DOTAP also synergizes with an anti-PD1 checkpoint inhibitor, resulting in a significant inhibition of B16 melanoma tumor growth. We found that R-DOTAP stimulates type I IFN production by dendritic cells in vivo and in vitro. s.c. injection of R-DOTAP results in an IFN-dependent increase in draining lymph node size and a concomitant increase in CD69 expression. Using knockout mice, we show that type I IFN is required for the induction of CD8 T cell activity following administration of R-DOTAP plus Ag. This response requires Myd88 but not TRIF or STING. We also show that R-DOTAP stimulates both TLR7 and 9. Collectively, these studies reveal that R-DOTAP stimulates endosomal TLRs, resulting in a Myd88-dependent production of type I IFN. When administered with Ag, this results in potent Ag-specific CD8 T cell responses and antitumor activity.

Cationic lipids as one-component vaccine adjuvants: A promising alternative to alum

Effective vaccine formulations consist of several components: an antigen carrier, the antigen, a stimulator of cellular immunity such as a Toll-like Receptors (TLRs) ligand, and a stimulator of humoral response such as an inflammasome activator. Here, we investigated the immunostimulatory and adjuvant properties of lipopolyamines, cationic lipids used as gene carriers. We identified new lipopolyamines able to activate both TLR2 and TLR4 and showed that lipopolyamines interact with TLRs via a mechanism different from the one used by bacterial ligands, activating a strong type-I IFN response, pro-inflammatory cytokines and IL-1β secretion. The TLR and inflammasome stimulations, together with the antigen carrier properties of lipopolyamines, resulted in both humoral and cellular immunity in mice vaccinated against OVA and make lipopolyamines promising one-component vaccine adjuvants.

An optimized whole blood assay measuring expression and activity of NLRP3, NLRC4 and AIM2 inflammasomes

The proinflammatory protease caspase-1 plays pivotal roles in central pathways of innate immunity, thereby contributing to pathogen clearance. Beside its physiological role, dysregulated activity of caspase-1 is known to contribute to an increasing number of diseases. In this study, we optimized and validated a low-volume human whole blood assay facilitating the measurement of caspase-1 activation and inflammasome-related gene expression upon stimulation of the NLRP3, NLRC4 or AIM2 inflammasome. Using the NLRP3 inflammasome specific inhibitor MCC950, we were able to measure the activity of canonical or alternative NLRP3 pathways, AIM2 and NLRC4 inflammasomes in whole blood. Based on our data we assume a superposition of NLRP3 and NLRC4 inflammasome activities in human whole blood following stimulation with S. typhimurium. The optimized whole blood assay may be suitable for diagnostic and research purposes for pediatric patients who can only donate small amounts of blood.

A Tetravalent Sub-unit Dengue Vaccine Formulated with Ionizable Cationic Lipid Nanoparticle induces Significant Immune Responses in Rodents and Non-Human Primates

Dengue virus has emerged as an important arboviral infection worldwide. As a complex pathogen, with four distinct serotypes, the development of a successful Dengue virus vaccine has proven to be challenging. Here, we describe a novel Dengue vaccine candidate that contains truncated, recombinant, Dengue virus envelope protein from all four Dengue virus serotypes (DEN-80E) formulated with ionizable cationic lipid nanoparticles (LNPs). Immunization studies in mice, Guinea pigs, and in Rhesus macaques, revealed that LNPs induced high titers of Dengue virus neutralizing antibodies, with or without co-administration or encapsulation of a Toll-Like Receptor 9 agonist. Importantly, LNPs were also able to boost DEN-80E specific CD4+ and CD8+ T cell responses. Cytokine and chemokine profiling revealed that LNPs induced strong chemokine responses without significant induction of inflammatory cytokines. In addition to being highly efficacious, the vaccine formulation proved to be well-tolerated, demonstrating no elevation in any of the safety parameters evaluated. Notably, reduction in cationic lipid content of the nanoparticle dramatically reduced the LNP's ability to boost DEN-80E specific immune responses, highlighting the crucial role for the charge of the LNP. Overall, our novel studies, across multiple species, reveal a promising tetravalent Dengue virus sub-unit vaccine candidate.

The synthetic cationic lipid diC14 activates a sector of the Arabidopsis defence network requiring endogenous signalling components

Natural and synthetic elicitors have contributed significantly to the study of plant immunity. Pathogen-derived proteins and carbohydrates that bind to immune receptors, allow the fine dissection of certain defence pathways. Lipids of a different nature that act as defence elicitors, have also been studied, but their specific effects have been less well characterized, and their receptors have not been identified. In animal cells, nanoliposomes of the synthetic cationic lipid 3-tetradecylamino-tert-butyl-N-tetradecylpropionamidine (diC14) activate the TLR4-dependent immune cascade. Here, we have investigated whether this lipid induces Arabidopsis defence responses. At the local level, diC14 activated early and late defence gene markers (FRK1, WRKY29, ICS1 and PR1), acting in a dose-dependent manner. This lipid induced the salicylic acid (SA)-dependent, but not jasmonic acid (JA)-dependent, pathway and protected plants against Pseudomonas syringae pv. tomato (Pst), but not Botrytis cinerea. diC14 was not toxic to plant or pathogen, and potentiated pathogen-induced callose deposition. At the systemic level, diC14 induced PR1 expression and conferred resistance against Pst. diC14-induced defence responses required the signalling protein EDS1, but not NDR1. Curiously, the lipid-induced defence gene expression was lower in the fls2/efr/cerk1 triple mutant, but still unchanged in the single mutants. The amidine headgroup and chain length were important for its activity. Given the robustness of the responses triggered by diC14, its specific action on a defence pathway and the requirement for well-known defence components, this synthetic lipid is emerging as a useful tool to investigate the initial events involved in plant innate immunity.

Quantitative measurement of Toll-like receptor 4 agonists adsorbed to Alhydrogel(®) by Fourier transform infrared-attenuated total reflectance spectroscopy

Aluminum salts have a long history as safe and effective vaccine adjuvants. In addition, aluminum salts have high adsorptive capacities for vaccine antigens and adjuvant molecules, for example, Toll-like receptor 4 (TLR4) agonists. However, the physicochemical properties of aluminum salts make direct quantitation of adsorbed molecules challenging. Typical methods for quantifying adsorbed molecules require advanced instrumentation, extreme sample processing, often destroy the sample, or rely on an indirect measurement. A simple, direct, and quantitative method for analysis of adsorbed adjuvant molecules is needed. This report presents a method utilizing Fourier transform infrared spectroscopy with a ZnSe-attenuated total reflectance attachment to directly measure low levels (<30 μg/mL) of TLR4 agonists adsorbed on aluminum salts with minimal sample preparation.

Activation of the NLRP3 inflammasome is not a feature of all particulate vaccine adjuvants

Particulate vaccine formulations, designed to improve the delivery of antigens to antigen-presenting cells (APCs) and to stimulate an immune response, have been shown to activate the NLRP3 inflammasome. This leads to the processing and secretion of interleukin (IL)-1β, which supports the recruitment of pro-inflammatory immune cells into the tissue and can therefore be beneficial for vaccine potency. Recent work suggested that this may be a common mechanism of action for all particulate formulations. The aim of this study was to investigate whether the activation of the NLRP3 inflammasome was common to many delivery systems. We prepared polymer-based chitosan nanoparticles (CNPs), lipid-based cubosomes, a water in oil emulsion of incomplete Freund's adjuvant (IFA) and alum formulations and examined inflammasome activation in vitro using murine bone-marrow-derived dendritic cells and human peripheral blood mononuclear cells and in vivo in mice. The formulations differed in their morphology, size and zeta-potential. Only the positively charged particles (CNPs and alum) were able to activate the inflammasome and increase the secretion of IL-1β. A decrease in the activation of the inflammasome with these particulates was observed when cathepsin B-mediated effects were blocked, implying a role of lysosomal rupture in the activation process. These findings demonstrate a role for the surface charge of particulates in the activation of the NLRP3 inflammasome, which should be considered when designing a novel vaccine formulation.

Oligonucleotide adsorption affects phase transition but not interdigitation of diC14-amidine bilayers

In this work, we investigate the effect of a small single-stranded oligonucleotide (ODN) on the colloid stability and structure of cationic diC14-amidine liposomes. Dynamic light scattering (DLS) shows that small, stable, anionic assemblies are formed in presence of excess ODN negative charge. This charge overcompensation condition was further characterized. A less cooperative bilayer phase transition is observed by differential scanning calorimetry (DSC). Electron spin resonance (ESR) spectra of probes at different bilayer depths show that ODN electrostatic adsorption increases the rigidity of both interdigitated gel and lamellar fluid phases. The increase in gel phase rigidity could be explained by the transformation of an adjacent to an interpenetrated interdigitation. Interdigitated fusogenic bilayers may find interesting applications in delivery of therapeutic oligonucleotides.

Cationic lipids activate intracellular signaling pathways

Cationic liposomes are commonly used as a transfection reagent for DNA, RNA or proteins and as a co-adjuvant of antigens for vaccination trials. A high density of positive charges close to cell surface is likely to be recognized as a signal of danger by cells or contribute to trigger cascades that are classically activated by endogenous cationic compounds. The present review provides evidence that cationic liposomes activate several cellular pathways like pro-apoptotic and pro-inflammatory cascades. An improved knowledge of the relationship between the cationic lipid properties (nature of the lipid hydrophilic moieties, hydrocarbon tail, mode of organization) and the activation of these pathways opens the way to the use and design of cationic tailored for a specific application (e.g. for gene transport or as adjuvants).

Disparate adjuvant properties among three formulations of "alum"

Aluminum adjuvants, commonly referred to as "alum," are the most widespread immunostimulants in human vaccines. Although the mechanisms that promote humoral responses to alum-adsorbed antigens are still enigmatic, alum is thought to form antigen depots and induce inflammatory signals that, in turn, promote antibody production. It was recently noted that Imject(®) alum, a commercial aluminum-containing adjuvant commonly used in animal studies, is not the physicochemical equivalent of aluminum adjuvant present in human vaccines. This difference raises concerns about the use of Imject(®) alum in animal research as a model for approved aluminum adjuvants. Here, we compared the capacity of Imject(®) alum, Alhydrogel(®), and a traditional alum-antigen precipitate to induce humoral responses in mice to the hapten-carrier antigen, NP-CGG [(4-hydroxy-3-nitrophenyl)acetyl-chicken γ-globulin]. The magnitude of humoral responses elicited by Alhydrogel(®) and precipitated alum was significantly greater than that induced by Imject(®) alum. The strength of the humoral responses elicited by different alum formulations was correlated with the quantity of pro-inflammatory cytokines induced and the numbers of inflammatory cells at the site of immunization. Moreover, Imject(®) exhibited a severely reduced capacity to adsorb protein antigens compared to Alhydrogel(®) and precipitated alum. These findings reveal substantial differences in the immunostimulatory properties of distinct alum preparations, an important point of consideration for the evaluation of novel adjuvants, the assessment of new alum-based vaccines, and in mechanistic studies of adjuvanticity.

Temperature-dependence of cationic lipid bilayer intermixing: possible role of interdigitation

In this work, we investigated the properties of a fusogenic cationic lipid, diC14-amidine, and show that this lipid possesses per se the capacity to adopt either an interdigitated structure (below and around its transition temperature) or a lamellar structure (above the transition temperature). To provide experimental evidence of this lipid bilayer organization, phospholipids spin-labeled at different positions of the hydrocarbon chain were incorporated into the membrane and their electron spin resonance (ESR) spectra were recorded at different temperatures. For comparison, similar experiments were performed with dimyristoyl phosphatidylcholine, a zwitterionic lipid (DMPC) which adopts a bilayer organization over a broad temperature range. Lipid mixing between diC14-amidine and asolectin liposomes was more efficient below (10-15 °C) than above the transition temperature (above 25 °C). This temperature-dependent "fusogenic" activity of diC14-amidine liposomes is opposite to what has been observed so far for peptides or virus-induced fusion. Altogether, our data suggest that interdigitation is a highly fusogenic state and that interdigitation-mediated fusion occurs via an unusual temperature-dependent mechanism that remains to be deciphered.