New adjuvants for vaccines containing purified protein antigens

Use of a quartz crystal microbalance platform to study protein adsorption on aluminum hydroxide vaccine adjuvants: Focus on phosphate-hydroxide ligand exchanges

Aluminum hydroxide (AH) salts are widely used as vaccine adjuvants and controlling antigen-AH interactions is a key challenge in vaccine formulation. In a previous work, we have developed a quartz crystal microbalance (QCM) platform, based on stable AH-coated sensors, to explore the mechanisms of model antigen adsorption. The QCM study of bovine serum albumin (BSA) adsorption at different pH and ionic strength (I) values showed that protein adsorption on AH adjuvant at physiological pH cannot be explained mainly by electrostatic interactions, in contrast with previous reports. Here, we exploit further the developed QCM platform to investigate the role of phosphate-hydroxyl ligand exchanges in the adsorption mechanism of BSA, human serum albumin (HSA) and ovalbumin (OVA) on two commercial AH adjuvants. BSA adsorption decreased on immobilized AH particles previously treated with KH₂PO₄, highlighting the role of exchangeable sites on AH particles in the adsorption process. BSA and OVA were dephosphorylated by treatment with an acid phosphatase to decrease their phosphate content by about 80% and 25%, respectively. Compared to native BSA, adsorption of dephosphorylated BSA decreased significantly on one AH adjuvant at pH 7. Adsorption of dephosphorylated OVA was comparable to the one of native OVA. Further QCM assays showed that phospho-amino acids (PO₄-serine and PO₄-threonine) displaced previously adsorbed BSA and OVA from AH particles in conditions that were depending on the protein and the AH. Taken together, these observations suggest that phosphate-hydroxyl ligand exchange is an important adsorption mechanism of proteins on AH. These results moreover confirm that the developed AH-coated QCM sensors offer a new platform for the study of antigen adsorption, to the benefit of vaccine formulation.

Immobilization of Aluminum Hydroxide Particles on Quartz Crystal Microbalance Sensors to Elucidate Antigen-Adjuvant Interaction Mechanisms in Vaccines

Aluminum hydroxide (AH) salts are the most widely used adjuvants in vaccine formulation. They trigger immunogenicity from antigenic subunits that would otherwise suffer from a lack of efficiency. Previous studies focusing on antigen-AH interaction mechanisms, performed with model proteins, suggested that electrostatic interactions and phosphate-hydroxyl ligand exchanges drive protein adsorption on AH. We however recently evidenced that NaCl, used in vaccine formulation, provokes AH particle aggregation. This must be taken into account to interpret data related to protein adsorption on AH. Here, we report on the successful development and use of a stable AH-coated surface to explore the mechanisms of protein adsorption by means of ultrasensitive surface analysis tools. Bovine serum albumin (BSA) adsorption was studied at different pHs and ionic strengths (I) using quartz crystal microbalance. The results show that protein adsorption on the AH adjuvant cannot be explained solely by electrostatic interactions and ligand exchanges. Hence, a higher adsorption was observed at pH 3 compared to pH 7, although AH and BSA respectively undergo repulsive and attractive electrostatic interactions at these pH values. Almost no effect of I on adsorption was moreover noted at pH 7. These new developments and observations not only suggest that other mechanisms govern protein adsorption on AH but also offer a new platform for the study of antigen adsorption in the context of vaccine formulation. Immobilizing particles on QCM sensors also enriches the range of applications for which QCM can be exploited, especially in colloid science.

Nonpyrogenic Molecular Adjuvants Based on norAbu-Muramyldipeptide and norAbu-Glucosaminyl Muramyldipeptide: Synthesis, Molecular Mechanisms of Action, and Biological Activities in Vitro and in Vivo

Fatty acyl analogues of muramyldipeptide (MDP) (abbreviated N-L18 norAbuGMDP, N-B30 norAbuGMDP, norAbuMDP-Lys(L18), norAbuMDP-Lys(B30), norAbuGMDP-Lys(L18), norAbuGMDP-Lys(B30), B30 norAbuMDP, L18 norAbuMDP) are designed and synthesized comprising the normuramyl-l-α-aminobutanoyl (norAbu) structural moiety. All new analogues show depressed pyrogenicity in both free (micellar) state and in liposomal formulations when tested in rabbits in vivo (sc and iv application). New analogues are also shown to be selective activators of NOD2 and NLRP3 (inflammasome) in vitro but not NOD1. Potencies of NOD2 and NLRP3 stimulation are found comparable with free MDP and other positive controls. Analogues are also demonstrated to be effective in stimulating cellular proliferation when the sera from mice are injected sc with individual liposome-loaded analogues, causing proliferation of bone marrow-derived GM-progenitors cells. Importantly, vaccination nanoparticles prepared from metallochelation liposomes, His-tagged antigen rOspA from Borrelia burgdorferi, and lipophilic analogue norAbuMDP-Lys(B30) as adjuvant, are shown to provoke OspA-specific antibody responses with a strong Th1-bias (dominance of IgG2a response). In contrast, the adjuvant effects of Alum or parent MDP show a strong Th2-bias (dominance of IgG1 response).

NaCl strongly modifies the physicochemical properties of aluminum hydroxide vaccine adjuvants

The immunostimulation capacity of most vaccines is enhanced through antigen adsorption on aluminum hydroxide (AH) adjuvants. Varying the adsorption conditions, i.e. pH and ionic strength (I), changes the antigen adsorbed amount and therefore the ability of the vaccine to stimulate the immune system. Vaccine formulations are thus resulting from an empirical screening of the adsorption conditions. This work aims at studying the physicochemical effects of adjusting the ionic strength of commercial AH adjuvant particles suspensions with sodium chloride (NaCl). X-ray photoelectron spectroscopy data show that AH particles surface chemical composition is neither altered by I adjustment with NaCl nor by deposition on gold surfaces. The latter result provides the opportunity to use AH-coated gold surfaces as a platform for advanced surface analysis of adjuvant particles, e.g. by atomic force microscopy (AFM). The morphology of adjuvant particles recovered from native and NaCl-treated AH suspensions, as studied by scanning electron microscopy and AFM, reveals that AH particles aggregation state is significantly altered by NaCl addition. This is further confirmed by nitrogen adsorption experiments: I adjustment to 150mM with NaCl strongly promotes AH particles aggregation leading to a strong decrease of the developed specific surface area. This work thus evidences the effect of NaCl on AH adjuvant structure, which may lead to alteration of formulated vaccines and to misinterpretation of data related to antigen adsorption on adjuvant particles.

Unleashing the therapeutic potential of NOD-like receptors

Nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are a family of intracellular sensors that have key roles in innate immunity and inflammation. Whereas some NLRs - including NOD1, NOD2, NAIP (NLR family, apoptosis inhibitory protein) and NLRC4 - detect conserved bacterial molecular signatures within the host cytosol, other members of this family sense 'danger signals', that is, xenocompounds or molecules that when recognized alert the immune system of hazardous environments, perhaps independently of a microbial trigger. In the past few years, remarkable progress has been made towards deciphering the role and the biology of NLRs, which has shown that these innate immune sensors have pivotal roles in providing immunity to infection, adjuvanticity and inflammation. Furthermore, several inflammatory disorders have been associated with mutations in human NLRgenes. Here, we discuss the effect that research on NLRs will have on vaccination, treatment of chronic inflammatory disorders and acute bacterial infections.

Adjuvancy enhancement of muramyl dipeptide by modulating its release from a physicochemically modified matrix of ovalbumin microspheres. II. In vivo investigation

In the present study, sustaining the release of adjuvants was investigated using microspheres as a means to increase the immune response (i.e. efficacy) and, ultimately, to reduce adverse effects to vaccine components. To date, most attempts have focused on sustaining the release of antigens. The utility of currently used vaccine adjuvants may be improved by sustaining their release. The development, modification and characterization of a two-component microsphere vaccine delivery system was demonstrated in our previous report [Puri et al., J. Control. Release (2000) in press]. Briefly, ovalbumin (OVA) was utilized as the model antigen (Ag) and delivery matrix and MDP or threonyl-MDP served as the model adjuvants. The release pattern of MDP was modulated from a physicochemically modified matrix of OVA microspheres (OVA-MSs). The purpose of the present study was to evaluate the adjuvancy of MDP in mice by modulating its release from OVA-MSs. Mice were immunized intradermally (i.d.) with various preparations of OVA-MSs, using a single-shot-immunization technique. Positive and negative control preparations were evaluated as well. An inverse relationship was observed between the in vitro release rate of MDP and the in vivo OVA-specific IgG antibody (Ab) immune response in mice. These results demonstrated that modulating the release pattern of MDP or threonyl-MDP enhanced their adjuvant effect. In conclusion, the current results demonstrate that the sustained and controlled release of adjuvants is extremely important for inducing a high level and prolonged period of immunostimulation while potentially minimizing therapy-limiting adverse effects.

Isolation and evaluation of immunological adjuvant activities of saponins from Polygala senega L

We have identified saponins in the root of Polygala senega L., a plant indigenous to the Canadian prairies, which display immunopotentiation activity to protein and viral antigens. By two-step extraction and hemolytic activity-guided fractionation by silica flush chromatography six saponin fractions were generated and their HPLC profiles determined. Two dominant fractions, designated as PS-1 and PS-2, were tested for adjuvant activity in mice immunized with ovalbumin, and hens immunized with rotavirus. The resulting adjuvant activity was compared with that of Quil A saponin. The P. senega saponins increased specific antibody levels to the antigens, in both mice and hens. In mice, there was a preferential increase of the IgG2a subclass, and upon in vitro secondary antigen stimulation, high IL-2 and IFN-gamma levels were observed in spleen cell cultures from P. senega saponins-immunized animals. The saponins were tested for their toxicity by lethality in mice and were found to be less toxic at the same dose than their counterpart Quil A. The results of this study indicated the potential of P. senega saponins as vaccine adjuvants to increase specific immune responses.

Production of interleukin 1 from macrophages incubated with poly (DL-lactic acid) granules containing ovalbumin

The production profile of interleukin 1 (IL-1) from mouse peritoneal macrophages (M phi) was determined following their incubation with poly(DL-lactic acid) (PDLLA) granules containing ovalbumin (OVA). Upon incubation, M phi produced IL-1 at a significantly high rate compared with those incubated with OVA in the free form or OVA-free granules. A simple mixture of empty granules and free OVA exhibited the same level of IL-1 production as induced by free OVA alone. IL-1 production by the granules with a fixed OVA loading increased with an increase in their amount added to M phi. When incubated with a fixed amount of granules containing OVA of different loadings, M phi produced more IL-1 with an increase in the total OVA amount, but the IL-1 production decreased at OVA loadings higher than 10%. The presence of free OVA enhanced IL-1 production with the increased addition of empty granules, but the level induced by OVA loaded in granules was higher than that by mixtures of free OVA and empty granules, when compared at a similar OVA dose, irrespective of the absolute amount of PDLLA added. These findings indicate that the sustained release of OVA from the granules is critical to enhance the OVA-induced IL-1 production, in contrast to the OVA release accompanying a large initial burst, which reduced IL-1 production. It was concluded that the direct contact of PDLLA granules with M phi and the subsequent sustained release of OVA around M phi effectively activated M phi, resulting in enhanced IL-1 production.

Size effect on the antibody production induced by biodegradable microspheres containing antigen

Poly(L-lactic acid) (PLLA) microspheres containing a model antigen, ovalbumin (OVA), were prepared by the evaporation method using double emulsion, and fractionated into different sizes by counterflow elutriation. Following the intraperitoneal (i.p.) and subcutaneous (s.c.) injection of the microspheres to mice, the titer of anti-OVA antibody in the serum was measured to assess the size effect on the profile of antibody production. OVA was released from the microspheres for 80 days, irrespective of the microsphere size. In both the s.c. and i.p. immunization, the serum level of anti-OVA IgG antibody in the mice induced by the microspheres containing OVA was higher than that of free OVA when compared at the same dose. The serum level of antibody in the mice i.p. injected with the microspheres tended to increase with the decreasing size. On the other hand, in the s.c. immunization, the microsphere size had little influence on the antibody production. It is possible that the injected microspheres tend to aggregate in the s.c. tissue, disappearing the size effect on the antibody production. Since the amount of microspheres injected increases with the decreasing size when their OVA loading is fixed, the increase in the amount will promote the interaction with immune cells, resulting in an enhanced antibody production. The cell interaction with the microspheres in the peritoneal cavity seems to be influenced by their size to a greater extent than in the s.c. tissue, probably because of their more frequent interaction with immune cells.

Liposomes as delivery systems in the prevention and treatment of infectious diseases

Research on the potential application of liposomes in the prevention and treatment of infectious diseases has focussed on improvement of the therapeutic index of antimicrobial drugs and immunomodulators and on stimulation of the immune response to otherwise weak antigens in vaccines composed of purified micro-organism subunits. In this review current approaches in this field are outlined. The improved therapeutic index of antimicrobial drugs after encapsulation in liposomes is a result of enhanced drug delivery to infected tissue or infected cells and/or a reduction of drug toxicity of potentially toxic antibiotics. Liposomal encapsulation of immunomodulators that activate macrophages aims at reducing the toxicity of these agents and targeting them to the cells of the mononuclear phagocyte system in order to increase the nonspecific resistance of the host against infections. Studies on the immunogenicity of liposomal antigens have demonstrated that liposomes can potentiate the humoral and cell mediated immunity to a variety of antigens.

Vaccinating guinea pigs with recombinant glycoprotein D of herpes simplex virus in an efficacious adjuvant formulation elicits protection against vaginal infection

Guinea pigs were immunized with glycoprotein gD-2t in SAF-m or saline, then challenged with herpes simplex virus, type 2 (HSV-2). Animals given gD-2t in SAF-m had higher anti-gD-2t antibodies, fewer and less severe vaginal lesions, and decreased ganglionic latency compared to animals given gD-2t in saline. Leucocytes from animals vaccinated with gD-2t in SAF-m had greater proliferative responses to gD-2t in vitro than cells from control animals. MHC II-restricted, gD-2t-specific cytotoxic T cells were induced in guinea pigs vaccinated with gD-2t in SAF-m. Thus, immunization of guinea pigs with gD-2t in SAF-m markedly reduced the incidence and severity of primary HSV-2 by eliciting both humoral and cell-mediated responses.