Immunostimulating Complexes: Clinical Potential in Vaccine Development

Nanotechnology Platform for Advancing Vaccine Development against the COVID-19 Virus

The COVID-19 pandemic has had a profound impact on societies, public health, healthcare systems, and the world economy. With over 771 million people infected worldwide and a staggering death toll exceeding 6,960,783 as of 4 October 2023 (according to the World Health Organization), the urgency for a solution was paramount. Since the outbreak, the demand for immediate treatment for COVID-19 viral infection, as well as for effective vaccination against this virus, was soaring, which led scientists, pharmaceutical/biotech companies, government health agencies, etc., to think about a treatment strategy that could control and minimize this outbreak as soon as possible. Vaccination emerged as the most effective strategy to combat this infectious disease. For vaccination strategies, any conventional vaccine approach using attenuated live or inactivated/engineered virus, as well as other approaches, typically requires years of research and assessment. However, the urgency of the situation promoted a faster and more effective approach to vaccine development against COVID-19. The role of nanotechnology in designing, manufacturing, boosting, and delivering vaccines to the host to counter this virus was unquestionably valued and assessed. Several nanoformulations are discussed here in terms of their composition, physical properties, credibility, and applications in past vaccine development (as well as the possibility of using those used in previous applications for the generation of the COVID-19 vaccine). Controlling and eliminating the spread of the virus and preventing future recurrence requires a safe, tolerable, and effective vaccine strategy. In this review, we discuss the potential of nanoformulations as the basis for an effective vaccine strategy against COVID-19.

Meta-Analysis on Randomized Controlled Trials of Vaccines with QS-21 or ISCOMATRIX Adjuvant: Safety and Tolerability

BACKGROUND AND OBJECTIVES

QS-21 shows in vitro hemolytic effect and causes side effects in vivo. New saponin adjuvant formulations with better toxicity profiles are needed. This study aims to evaluate the safety and tolerability of QS-21 and the improved saponin adjuvants (ISCOM, ISCOMATRIX and Matrix-M™) from vaccine trials.

METHODS

A systematic literature search was conducted from MEDLINE, EMBASE, Cochrane library and Clinicaltrials.gov. We selected for the meta-analysis randomized controlled trials (RCTs) of vaccines adjuvanted with QS-21, ISCOM, ISCOMATRIX or Matrix-M™, which included a placebo control group and reported safety outcomes. Pooled risk ratios (RRs) and their 95% confidence intervals (CIs) were calculated using a random-effects model. Jadad scale was used to assess the study quality.

RESULTS

Nine RCTs were eligible for the meta-analysis: six trials on QS-21-adjuvanted vaccines and three trials on ISCOMATRIX-adjuvanted, with 907 patients in total. There were no studies on ISCOM or Matrix-M™ adjuvanted vaccines matching the inclusion criteria. Meta-analysis identified an increased risk for diarrhea in patients receiving QS21-adjuvanted vaccines (RR 2.55, 95% CI 1.04-6.24). No increase in the incidence of the reported systemic AEs was observed for ISCOMATRIX-adjuvanted vaccines. QS-21- and ISCOMATRIX-adjuvanted vaccines caused a significantly higher incidence of injection site pain (RR 4.11, 95% CI 1.10-15.35 and RR 2.55, 95% CI 1.41-4.59, respectively). ISCOMATRIX-adjuvanted vaccines also increased the incidence of injection site swelling (RR 3.43, 95% CI 1.08-10.97).

CONCLUSIONS

Our findings suggest that vaccines adjuvanted with either QS-21 or ISCOMATRIX posed no specific safety concern. Furthermore, our results indicate that the use of ISCOMATRIX enables a better systemic tolerability profile when compared to the use of QS-21. However, no better local tolerance was observed for ISCOMATRIX-adjuvanted vaccines in immunized non-healthy subjects. This meta-analysis is limited by the relatively small number of individuals recruited in the included trials, especially in the control groups.

Efficient and Durable Vaccine against Intimin β of Diarrheagenic E. Coli Induced by Clay Nanoparticles

Improved strategies are urgently required to control infections with enterohemorrhagic Escherichia coli and enteropathogenic E. coli, two dominant zoonotic enteric pathogens responsible for a wide spectrum of illnesses as well as deaths of human being, with tremendous financial cost worldwide. The present study investigates the capacity of two clay nanoparticles (NPs) with opposite surface charges, namely synthetic layered double hydroxide (LDH) and hectorite (HEC) NPs as adjuvants to promote strong immune responses against the infections. Here both LDH and HEC NPs are showed to be able to carry an appreciable amount of Intimin β (1.1 and 4.4 mg per mg clay nanomaterials, respectively) and significantly facilitate antigen uptake by antigen-presenting cells. Remarkably, these clay NPs induce strong antibody and cell-mediated immune responses, which are much higher than that by the potent adjuvant, QuilA. Furthermore, these strong immune responses are well maintained for at least four months in the mouse model, during which there are no changes in histopathology of the animal organs. Collectively these data demonstrate the suitability of LDH and HEC NPs as useful adjuvants in new-generation vaccine formulations to control various infectious diseases.

Achieving directed immunostimulating complexes incorporation

In recent years, several studies have been reported with the common aim of generating general expression systems for straightforward production and subsequent coupling of expressed antigens to an adjuvant system. Here, we describe a series of such efforts with a common theme of using gene fusion technology for association of recombinant antigens to immunostimulating complexes (iscoms). In the early stages of vaccine development, uniform antigen preparations are crucial to allow the comparison of immune responses to different antigens, or even subdomains thereof, and we believe that the described systems constitute an important development in this context.

Incorporation of ovalbumin into ISCOMs and related colloidal particles prepared by the lipid film hydration method

The aim of this study was to investigate the incorporation of a model antigen, fluorescently labelled ovalbumin (FITC-OVA), into various colloidal particles including immune stimulating complexes (ISCOMs), liposomes, ring and worm-like micelles, lamellae and lipidic/layered structures that are formed from various combinations of the triterpene saponin Quil A, cholesterol and phosphatidylethanolamine (PE) following hydration of PE/cholesterol lipid films with aqueous solutions of Quil A. Colloidal dispersions of these three components were also prepared by the dialysis method for comparison. FITC-OVA was conjugated with palmitic acid (P) and PE to produce P-FITC-OVA and PE-FITC-OVA, respectively. Both P-FITC-OVA and PE-FITC-OVA could be incorporated in all colloidal structures whereas FITC-OVA was incorporated only into liposomes. The incorporation of PE-FITC-OVA into all colloidal structures was significantly higher than P-FITC-OVA (P < 0.05). The degree of incorporation of protein was in the order: ring and worm-like micelles < liposomes and lipidic/layered structures < ISCOMs and lamellae. The incorporation of protein into the various particles prepared by the lipid film hydration method was similar to those for colloidal particles prepared by the dialysis method (provided both methods lead to the formation of the same colloidal structures). In the case of different colloidal structures arising due to the preparation method, differences in encapsulation efficiency were found (P < 0.05) for formulations with the same polar lipid composition. This study demonstrates that the various colloidal particles formed as a result of hydrating PE/cholesterol lipid films with different amounts of Quil A are capable of incorporating antigen, provided it is amphipathic. Some of these colloidal particles may be used as effective vaccine delivery systems.

CTA1-DD-immune stimulating complexes: a novel, rationally designed combined mucosal vaccine adjuvant effective with nanogram doses of antigen

Mucosally active vaccine adjuvants that will prime a full range of local and systemic immune responses against defined antigenic epitopes are much needed. Cholera toxin and lipophilic immune stimulating complexes (ISCOMS) containing Quil A can both act as adjuvants for orally administered Ags, possibly by targeting different APCs. Recently, we have been successful in separating the adjuvant and toxic effects of cholera toxin by constructing a gene fusion protein, CTA1-DD, that combines the enzymatically active CTA1-subunit with a B cell-targeting moiety, D, derived from Staphylococcus aureus protein A. Here we have extended this work by combining CTA1-DD with ISCOMS, which normally target dendritic cells and/or macrophages. ISCOMS containing a fusion protein comprising the OVA(323-339) peptide epitope linked to CTA1-DD were highly immunogenic when given in nanogram doses by the s.c., oral, or nasal routes, inducing a wide range of T cell-dependent immune responses. In contrast, ISCOMS containing the enzymatically inactive CTA1-R7K-DD mutant protein were much less effective, indicating that at least part of the activity of the combined vector requires the ADP-ribosylating property of CTA1. No toxicity was observed by any route. To our knowledge, this is the first report on the successful combination of two mechanistically different principles of adjuvant action. We conclude that rationally designed vectors consisting of CTA1-DD and ISCOMS may provide a novel strategy for the generation of potent and safe mucosal vaccines.

Immune responses to ISCOM formulations in animal and primate models

ISCOMs are typically 40 nm cage-like structures comprising antigen, saponin, cholesterol and phospholipid. ISCOMs have been shown to induce antibody responses and activate T helper cells and cytolytic T lymphocytes in a number of animal species, including non-human primates. Recent clinical studies have demonstrated that ISCOMs are also able to induce antibody and cellular immune responses in humans. This review describes the current understanding of the ability of ISCOMs to induce immune responses and the mechanisms underlying this property. Recent progress in the characterisation and manufacture of ISCOMs will also be discussed.

Monoclonal antibodies to native HIV type 1 reverse transcriptase and their interaction with enzymes from different subtypes

Recombinant reverse transcriptase (RT) from HIV-1 subtype B was used to produce mouse anti-RT monoclonal antibodies (MAbs). Immunization was done by mixing RT with the ISCOM matrix-forming adjuvant saponin (Quil A). Two different assays, both based on the interaction of native RT and antibodies, were used to monitor the immune response in mice and for screening, selection, and characterization of the MAbs. The first assay measures the capacity of antibodies to inhibit the polymerase activity of the RT and the second assay measures the ability of antibodies to capture enzymatically active RT. Twelve clones with the capacity to inhibit at least 50% of the RT activity and 34 clones with high RT-capturing capacity were found. The MAb panel was utilized to evaluate the immunological properties of 18 different RTs representing 9 different HIV1 subtypes. The RT-inhibitory MAbs could be divided into two groups based on their pattern of cross-reactivity toward the different HIV-1 RTs. The degree of diversity recorded among MAbs with RT-capturing capacity was larger. At least seven groups of MAbs with distinct cross-reactivity patterns were identified. Thus, the degree of isoenzyme specificity varied greatly, from MAbs that were quite specific for subtype B RT to one MAb that was able to capture the RTs from all HIV-1 isolates tested except one of the two group O isolates. In conclusion, our study revealed that there exist surprisingly large immunological differences between RTs from different HIV-1 subtypes as well as from the same subtype.

Induction of antibody and T-cell responses by immunization with ISCOMS containing the 38-kilodalton protein of Mycobacterium tuberculosis

In this study, we investigated the influence of different amounts of N-(palmitoyloxy) succinimide (PA-NHS): attachment of lipid tails to the protein and Quil A on the immunogenicity of the 38-kDa mycobacterial protein incorporated into immunostimulating complexes (ISCOMS; 38-kDa ISCOMS). The addition of higher amounts of Quil A during the ISCOMS preparation increased the amount of protein incorporated into ISCOMS, whereas the use of higher amounts of PA did not influence this parameter. Low antibody responses were observed after primary immunization with all 38-kDa ISCOMS preparations which, however, strongly increased after booster injections. IgG2a is the major subclass IgG induced by these ISCOMS preparations. There were only slight differences between the various ISCOMS formulations in their capacity to induce cytotoxic T-lymphocytes (CTLs). Spleen cells primed with ISCOMS prepared with the highest amount of Quil A produced high levels of IFN-gamma after stimulation with T helper cell type one (Th1) peptide of the 38-kDa protein (aa 70-84), 38-kDa protein or purified protein derivate (PPD). Spleen cells primed with ISCOMS prepared with the lowest amount of Quil A only substantial IFN-gamma levels were detected after stimulation with 38-kDa protein. IL-4 secretion was very low or not detectable with all ISCOM preparations. These results therefore demonstrated that all 38 kDa-ISCOMS preparations were: (1) immunogenic by inducing antibodies, Th1 and CTL responses; (2) that the way in which the ISCOMS were prepared, e.g. the amount of Quil A used, modulates the epitope specificity of the Th1 response.

Improved systems for hydrophobic tagging of recombinant immunogens for efficient iscom incorporation

We have previously reported a strategy for production in Escherichia coli of recombinant immunogens fused to a hydrophobic tag to improve their capacity to associate with an adjuvant formulation [Andersson et al., J. Immunol. Methods 222 (1999) 171]. Here, we describe a further development of the previous strategy and present significant improvements. In the novel system, the target immunogen is produced with an N-terminal affinity tag suitable for affinity purification, and a C-terminal hydrophobic tag, which should enable association through hydrophobic interactions of the immunogen with an adjuvant system, here being immunostimulating complexes (iscoms). Two different hydrophobic tags were evaluated: (i) a tag denoted M, derived from the membrane-spanning region of Staphylococcus aureus protein A (SpA), and (ii) a tag denoted MI consisting of the transmembrane region of hemagglutinin from influenza A virus. Furthermore, two alternative affinity tags were evaluated; the serum albumin-binding protein ABP, derived from streptococcal protein G, and the divalent IgG-binding ZZ-domains derived from SpA. A malaria peptide M5, derived from the central repeat region of the Plasmodium falciparum blood-stage antigen Pf155/RESA, served as model immunogen in this study. Four different fusion proteins, ABP-M5-M, ABP-M5-MI, ZZ-M5-M and ZZ-M5-MI, were thus produced, affinity purified and evaluated in iscom-incorporation experiments. All of the fusion proteins were found in the iscom fractions in analytical ultracentrifugation, indicating iscom incorporation. This was further supported by electron microscopy analysis showing that iscoms were formed. In addition, these iscom preparations were demonstrated to induce M5-specific antibody responses upon immunisation of mice, confirming the successful incorporation into iscoms. The novel system for hydrophobic tagging of immunogens, with optional affinity and hydrophobic tags, gave expression levels that were increased ten to fifty-fold, as compared to the earlier reported system. We believe that the presented strategy would be a convenient way to achieve efficient adjuvant association for recombinant immunogens.

Triterpenoid saponins containing an acetylated branched D-fucosyl residue from Quillaja saponaria Molina

Seven novel saponins were isolated from a bark extract of Quillaja saponaria Molina. the compounds were characterized, using mainly NMR spectroscopy, mass spectrometry and chemical methods, as quillaic acid substituted at C-3 with oligosaccharides consisting of various compositions of D-glucuronic acid D-galactose, D-xylose, and L-rhamnose and at C-28 with complex oligosaccharide structures consisting of various compositions of D-xylose, L-rhamnose, D-apiose and a branched 4-O-acetyl-D-fucose residue.

Separation and structural analysis of some saponins from Quillaja saponaria Molina

A fraction of saponins from Quillaja saponaria Molina, QH-B, was fractionated by consecutive separations on three different reverse-phase HPLC systems. Eight compounds were isolated and the structures of these were elucidated mainly by sugar analysis and NMR spectroscopy. The structures consisted of a quillaic acid substituted with two different trisaccharides at C-3, beta-D-Galp-(1-->2)-[alpha-L-Rhap-(1-->3)]-beta-D-GlcpA and beta-D-Galp-(1-->2)-[beta-D-Xylp-(1-->3)]-beta-D-GlcpA, and a tetra- or pentasaccharide at C-28, beta-D-Xylp-(1-->4)-[beta-D-Glcp-(1-->3)]-alpha-L-Rhap-(1--> 2)-beta-D-Fucp and beta-D-Apif-(1-->3)-beta-D-Xylp-(1-->4)-[beta-D-Glcp-(1-->3) ]-alpha-L- Rhap-(1-->2)-beta-D-Fucp. These compounds were further substituted with an acyl group either at O-3 or O-4 of the fucose residue, which is the sugar linked to C-28 of the quillaic acid.

Quillaja saponin formulations that stimulate proinflammatory cytokines elicit a potent acquired cell-mediated immunity

We examined the ability of various Quillaja saponins in iscom-matrix formulations to induce proinflammatory cytokines, such as interleukin (IL)-1alpha and IL-6, and to stimulate acquired immune responses to influenza virus envelope proteins. The A-fraction of Quillaja saponins (QH-A) was shown to stimulate antigen-presenting cells (APC) to produce proinflammatory cytokines, and elicited a high primary antigen-specific antibody response and potent cell-mediated responses, as measured by T-cell proliferation, production of cytokines and cytotoxic T-lymphocyte (CTL) activity. The C-fraction of Quillaja saponins (QH-C) was shown to have a low capacity to stimulate proinflammatory cytokines and elicited low primary antibody and T-cell responses. However, the QH-C iscom-matrix mediated a potent booster effect, resulting in a high secondary antibody response. The ability of APC to discriminate and to respond to QH-A formulations more efficiently than to QH-C with release of proinflammatory cytokines, which precedes a potent acquired immune response, identifies an important mechanism through which some adjuvants may exert their immunoenhancing activities.

Immunomodulation by iscoms, immune stimulating complexes

The iscom is a uniform stable complex consisting of cholesterol, phospholipid, adjuvant-active saponin, and antigen. The iscom matrix is a particulate complex with identical composition, shape, and morphology, but lacking the incorporated antigen. The assembly of the complex is based on hydrophobic interactions, but antigens that are not hydrophobic can be conjugated with a hydrophobic tail or hidden hydrophobic regions can be exposed, e.g., by acid treatment, to facilitate the incorporation into iscoms. The functional aspects of iscoms are described emphasizing immunomodulation in mouse models. Iscoms prominently enhance the antigen targeting, uptake, and activity of antigen presenting cells including dendritic and B cells and macrophages resulting in the production of proinflammatory cytokines, above all interleukin (IL)-1, IL-6, and IL-12. The expression of costimulatory molecules major histocompatibility complex (MHC) class II, B7.1 and B7.2, is also enhanced. The latter partly explains why the iscom is an efficient adjuvant for elderly mice. Iscoms enhance the Th1 type of response with increased production of IL-2 and interferon gamma. However, with some antigens and particularly in monkeys immunized with HIV iscoms, the production of IL-4 was enhanced. IL-4, IL-2, and interferon gamma (IFNgamma) together with the beta chemokines MIP-1alpha and MIP-1beta correlated with protection against challenge infection with a chimeric virus (simian immunodeficiency virus-human immunodeficiency virus). Iscoms were also shown to induce a potent immune response in the newborn and to be an efficient delivery system for mucosal administration. Technical information is given about formulation of iscoms and about handling of antigens to optimize their incorporation into iscoms.

Iscom and iscom-matrix enhance by intranasal route the IgA responses to OVA and rCTB in local and remote mucosal secretions

Iscoms, with rCTB incorporated via the GM1 receptor, enhanced in mice the mucosal immunogenicity of rCTB as antigen after intranasal (i.n.) administration both by inducing IgA response in the remote intestinal tract mucosa and by a 100-fold increase of the specific IgA locally in the lungs. Iscom-matrix as a separate entity mixed with rCTB enhanced the rCTB-IgA response similarly. While OVA in iscoms induced high mucosal IgA responses, iscom-matrix co-administered with OVA induced low or no mucosal IgA response to OVA. A synergism between iscoms and rCTB could only be seen as an adjuvant targeting effect enhancing the IgA response to OVA in the remote genital tract mucosa. In serum, the immunomodulatory effect of iscoms after i.n. administration was seen as an enhanced serum IgG2a response.

Comparison of immunity generated by nucleic acid-, MF59-, and ISCOM-formulated human immunodeficiency virus type 1 vaccines in Rhesus macaques: evidence for viral clearance

The kinetics of T-helper immune responses generated in 16 mature outbred rhesus monkeys (Macaca mulatta) within a 10-month period by three different human immunodeficiency virus type 1 (HIV-1) vaccine strategies were compared. Immune responses to monomeric recombinant gp120SF2 (rgp120) when the protein was expressed in vivo by DNA immunization or when it was delivered as a subunit protein vaccine formulated either with the MF59 adjuvant or by incorporation into immune-stimulating complexes (ISCOMs) were compared. Virus-neutralizing antibodies (NA) against HIV-1SF2 reached similar titers in the two rgp120SF2 protein-immunized groups, but the responses showed different kinetics, while NA were delayed and their levels were low in the DNA-immunized animals. Antigen-specific gamma interferon (IFN-gamma) T-helper (type 1-like) responses were detected in the DNA-immunized group, but only after the fourth immunization, and the rgp120/MF59 group generated both IFN-gamma and interleukin-4 (IL-4) (type 2-like) responses that appeared after the third immunization. In contrast, rgp120/ISCOM-immunized animals rapidly developed marked IL-2, IFN-gamma (type 1-like), and IL-4 responses that peaked after the second immunization. To determine which type of immune responses correlated with protection from infection, all animals were challenged intravenously with 50 50% infective doses of a rhesus cell-propagated, in vivo-titrated stock of a chimeric simian immunodeficiency virus-HIVSF13 construct. Protection was observed in the two groups receiving the rgp120 subunit vaccines. Half of the animals in the ISCOM group were completely protected from infection. In other subunit vaccinees there was evidence by multiple assays that virus detected at 2 weeks postchallenge was effectively cleared. Early induction of potent type 1- as well as type 2-like T-helper responses induced the most-effective immunity.

The major Epstein-Barr virus (EBV) envelope glycoprotein gp340 when incorporated into Iscoms primes cytotoxic T-cell responses directed against EBV lymphoblastoid cell lines

A recombinant form of the EBV envelope glycoprotein and vaccine candidate gp340, lacking its hydrophobic transmembrane region, was incorporated into Iscoms after coupling to phosphatidyl ethanolamine via carbohydrate residues. Coupling by partial oxidation of gp340 carbohydrate with sodium periodate partly denatured the incorporated gp340 as indicated by its reduced reactivity with monoclonal antibodies that recognise the major neutralising epitope. Immunisation of cottontop tamarins with these Iscoms elicited antibody responses to gp340, but these antibodies only poorly recognised the major neutralising epitope in a competition ELISA and were unable to neutralise EBV in vitro. Despite the lack of neutralising antibody, immunisation with these Iscoms primed significant in vitro proliferative responses to soluble gp340 in lymphocytes from the draining lymph nodes and spleen. T-cell lines were raised from both immunised and control animals by in vitro stimulation of peripheral blood lymphocytes or spleen cells with autologous EBV-transformed lymphoblastoid cell lines. The T-cell lines from control animals had higher numbers of CD4+ T-cells than CD8+ T-cells and were not cytotoxic for autologous lymphoblastoid cell lines (LCL). In contrast the lines from immunised animals contained more CD8+ T-cells than CD4+ T-cells and had marked cytotoxicity for autologous LCL.

General expression vectors for production of hydrophobically tagged immunogens for direct iscom incorporation

A new general strategy for the production of recombinant protein immunogens has been investigated. The rationale involves the production of a recombinant immunogen as fused to a composite tag comprising one domain suitable for affinity purification and a hydrophobic tag designed for direct incorporation through hydrophobic interaction of the affinity-purified immunogen into an adjuvant system, in this case immunostimulating complexes (iscoms). Three different hydrophobic tags were evaluated: (i) a tag denoted IW containing stretches of hydrophobic isoleucine (I) and tryptophan (W) residues; (ii) a tag denoted MI consisting of the transmembrane region of hemagglutinin from influenza A virus; and (iii) a tag denoted PD designed to be pH-dependent in such a way that an amphiphatic alpha-helix would be formed at low pH. As an affinity tag, an IgG-binding domain Z derived from Staphylococcus aureus protein A (SpA) was used, and a malaria peptide M5, derived from the central repeat region of the Plasmodium falciparum blood-stage antigen Pf155/RESA, served as a model immunogen in this study. Three different fusion proteins, IW-Z-M5, MI-Z-M5 and PD-Z-M5, were produced in Escherichia coli, and after affinity purification these were evaluated in iscom-incorporation experiments. Two of the fusion proteins, IW-Z-M5 and MI-Z-M5 were found in the iscom fraction following preparative ultracentrifugation, indicating iscom incorporation. This was further supported by electron microscopy analysis showing that iscoms were formed. Furthermore, these iscom preparations were demonstrated to induce efficient M5-specific antibody responses upon immunization of mice, confirming successful incorporation into iscoms. The implications of these results for the design and production of subunit vaccines are discussed.

Oral vaccination with immune stimulating complexes

There is a need for non-living adjuvant vectors which will induce a full range of local and systemic immune responses to orally administered purified antigens. Here we describe our experience with lipophilic immune stimulating complexes (ISCOMS) containing the saponin adjuvant Quil A. When given orally, ISCOMS containing the model protein antigen ovalbumin (OVA) induce a wide range of systemic immune responses, including Th1 and Th2 CD4 dependent activity, class I MHC restricted cytotoxic T-cell responses and local production of secretory IgA antibodies. More recent results indicate that ISCOMS may act partly by enhancing the uptake of protein from the gut. In addition, intraperitoneal injection of ISCOMS recruits and activates many components of the innate immune system. including neutrophils, macrophages, and dendritic cells. In parallel, there is increased production of nitric oxide (NO), reactive oxygen intermediates (ROI), interleukins (IL) 1, 6, 12, and gamma interferon (gammaIFN). Of these factors, only IL12 is essential for the immunogenicity of ISCOMS in vivo, as mucosal and systemic responses to ISCOMS are reduced in IL12KO mice, but not in IL4KO, IL6KO, inducible NO synthase (iNOS) KO, or gammaIFN receptor KO mice. We propose that ISCOMS act by targetting antigen and adjuvant to macrophages and/or dendritic cells. This pathway may be amenable to exploitation for vaccine development, especially if combined with another vector with a different mucosal adjuvant profile, such as cholera toxin.

Iscom is an efficient mucosal delivery system for Mycoplasma mycoides subsp. mycoides (MmmSC) antigens inducing high mucosal and systemic antibody responses

The purpose of this study was to explore the iscom as a mucosal delivery system for Mycoplasma mycoides subsp. mycoides small colony (MmmSC) antigens. BALB/c female mice were immunised intranasally (i.n.) twice, 8 weeks apart with three different doses (3, 10 and 20 microg) or subcutaneously (s.c.) with 3 microg of M. mycoides antigens incorporated into iscoms. Mycoplasma cells were administered s.c. twice, 8 weeks apart at a dose of 3 microg or i.n. at 10 microg as for iscoms. Both i.n. and s.c. modes of immunisation with iscoms induced prominent primary serum antibody responses in a dose-dependent manner, which were efficiently boosted. Compared to whole mycoplasma cells, iscoms enhanced the total Ig and IgG subclass (IgG1, IgG2a and IgG2b) responses in serum and in lungs greatly, and this enhancement was more prominent after i.n. than after s.c. immunisation. By the i.n. mode of immunisation iscoms containing mycoplasma antigens induced a 60-fold higher IgA response in lungs than the whole cell antigen. Iscoms also induced substantially higher total Ig and IgG subclass responses in the lungs. By Western blot a reduced number of bands (7) were detected in lung secretion after both i.n. and s.c. immunisations with iscoms compared to a high number of bands (more than 30) detected by serum antibodies. Interestingly i.n. immunisation with iscoms induced antibodies in lungs as well as in serum to mycoplasma cell antigens which differed from those induced by s.c. immunisation as revealed by the Western blot patterns.

Vaccination with recombinant Parasite Surface Antigen 2 from Leishmania major induces a Th1 type of immune response but does not protect against infection

Vaccination with the native Parasite Surface Antigen 2 of Leishmania major with Corynebacterium parvum as adjuvant protects mice from leishmaniasis through a Th1 mediated response. Here we show that vaccination with a recombinant form of this protein, purified from Escherichia coli and administered in iscoms or with C. parvum as adjuvant, does not induce protective immunity despite the induction of Th1 responses. The results suggest that protective immunity depends on the ability of the vaccinating antigen to induce Th1-like T cells with ability to be recalled by infection. Therefore, the conformation of antigens may play a more major role for the induction of T cell mediated immunity than originally considered.

The immunostimulating complex (ISCOM) is an efficient mucosal delivery system for respiratory syncytial virus (RSV) envelope antigens inducing high local and systemic antibody responses

ISCOM is an efficient mucosal delivery system for RSV envelope proteins as measured by antibody responses in respiratory tract secretions and in sera of mice following two intranasal (i.n.) administrations. Intranasally administered RSV ISCOMs induced high levels of IgA antibodies both in the upper respiratory tract and in the lungs. In the lungs, a prominent and long-lasting IgA response was recorded, which still persisted 22 weeks after the second i.n. immunization when the experiment ended. Subcutaneous (s.c.) immunization only induced low IgA titres in the upper respiratory tract and no measurable response to RSV was found in the lungs. Differences were also noticed in serum between the i.n. and s.c. modes of immunization. ISCOMs given intranasally induced earlier, higher and longer lasting IgM and IgG1 serum anti-RSV antibody responses than those induced by the s.c. mode of administration. A low serum IgE response was only detectable at 2 weeks after i.n. immunization with ISCOMs and after s.c. immunization with an inactivated virus, but no IgE response was detectable after s.c. injection of ISCOMs. The serum IgA response was more pronounced following s.c. injection of inactivated virus than after i.n. application of ISCOMs, and a clear-cut booster effect was obtained with a second immunization. Virtually no serum IgA response was detected after the s.c. administration of ISCOMs. In conclusion, the high immune responses induced by RSV ISCOMs in the respiratory tract and serum after i.n. administration indicate prominent mucosal delivery and adjuvant properties of the ISCOMs, warranting further studies.

Functional aspects of iscoms

The iscom is a delivery system, designed for both parenteral and mucosal modes of administration, for both antigens and adjuvants, components which are interchangeable. By the parenteral route a prominent systemic Th1 type of response is evoked, but the mucosal immunoglobulin A (IgA) response was insignificant. Intranasal (i.n.) immunization with iscoms evoked potent mucosal IgA response and serum IgG which was much higher than that induced by i.n. administration of the B subunit of cholera toxin (rCTB), both to rCTB itself as well as to co-administered antigen. The immunomodulatory effect on rCTB or co-administered antigens imposed by the iscom was demonstrated by a potent mucosal IgA switch and an enhanced IgG2a serum response. The incorporation of a targeting molecule in the iscom enhanced the remote IgA response in the genital tract mucosa. The capacity to induce CD8-restricted cytotoxic T lymphocytes (CTL) is unique for the iscom as a nonreplicating system, which is facilitated by the delivery of antigens to the cytosol. The immunomodulatory capacity of iscoms also paved the way to override the inhibitory effect of maternally derived antibodies and the relative unresponsiveness of an immature neonatal immune system.

Immune stimulating complexes as mucosal vaccines

There is a need for non-living adjuvant vectors that will allow a full range of local and systemic immune responses to orally administered purified antigens. Here we describe our experience with lipophilic immune-stimulating complexes (ISCOMs) containing the saponin adjuvant Quil A. When given orally, ISCOMs containing the model protein antigen ovalbumin (OVA) induce a wide range of systemic immune responses, including Th1 and Th2 CD4-dependent activity, serum IgG antibodies and class I MHC-restricted cytotoxic T cell responses. In addition, there is local production of secretory IgA antibodies in the intestine itself, as well as priming of CD4 and CD8 T cell responses in the draining lymphoid tissues. Preliminary results indicate that the mucosal adjuvant properties of ISCOMs may reflect their ability to deliver antigen combined with the pro-inflammatory properties of Quil A in a particulate form. Of the many inflammatory mediators induced, interleukin-12, derived from dendritic cells and/or macrophages, appears to be of central importance. These results indicate that ISCOMs may prove to be useful mucosal vaccine vectors with functions which are distinct from existing vectors of this type.

Triterpenoid saponins from Quillaja saponaria

Three new saponins were isolated from a commercial bark extract of Quillaja saponaria Molina. These compounds were also obtained as degradation products from larger saponins in this extract when treated with strong alkali. The compounds were characterized, using mainly NMR spectroscopy, mass spectrometry and chemical methods, as quillaic acid 3-O-¿beta-D-galactopyranosyl-(1-->2)-beta-D-glucopyranosiduronic acid¿, 3-O-¿alpha-L-rhamnopyranosyl-(1-->3)-[beta-D-galactopyranosyl-(1-->2)] -beta-D-glucopyranosiduronic acid¿ and 3-O-¿beta-D-xylopyranosyl-(1-->3)-[beta-D-galactopyranosyl -(1-->2)]-beta-D-glucopyranosiduronic acid¿, respectively.

Induction of a Th1 Immune Response and Simultaneous Lack of Activation of a Th2 Response Are Required for Generation of Immunity to Leishmaniasis

Experimental systems based on immunization with plasmid DNA or immune-stimulating complexes were used to delineate the requirements for generation of protective immunity against murine leishmaniasis. Vaccination with plasmid DNA encoding the host-protective Leishmania major parasite surface Ag-2 primed for an essentially exclusive Th1 response that protected mice against L. major infection. In contrast, parasite surface Ag-2 in immune-stimulating complexes generated an immune response with mixed Th1-like and Th2-like properties that was not protective despite the activation of large numbers of CD4+ T cells secreting IFN-γ. These results indicate that a Th1 response is sufficient to protect against cutaneous leishmaniasis, but the induction of a simultaneous Th2 response abrogates the Th1 effector function. DNA vaccines may therefore have an advantage for diseases in which protection depends on the induction of Th1 responses.

Internalization of iscom-borne antigens and presentation under MHC class I or class II restriction

Exogenous nonreplicating antigens (Ag) incorporated into immunostimulating complexes (iscoms) induce CTL responses under MHC class I restriction. A requirement for inducing CTL responses is that the Ag is delivered to the cytosol of antigen-presenting cells (APC), a route restricted to endogenously produced Ag. To investigate the mechanisms by which iscoms elicit MHC class I-restricted responses, the intracellular distribution of influenza virus envelope proteins incorporated in iscoms (flu-iscoms) or in micelles (flumicelles) was studied in vitro using murine peritoneal cells (PEC). Ultrathin sections of cells pulsed with biotinylated flu-iscoms or flu-micelles were analyzed by electron microscopy after detection of the biotin label by reaction with streptavidin-gold. PEC pulsed with flu-iscoms showed a pattern of scattered gold particles distributed in clear and dense vesicles as well as in the intracellular space but not associated with organelles. In cells pulsed with flu-micelles, Ag was also detected in most cellular compartments but at a considerably lower concentration. The intracellular distribution of particulate Ag in iscom or micelle form was confirmed by lysis and differential centrifugation of Ag-pulsed APC. Furthermore, P815 cells pulsed with flu-iscoms were lysed by specific immune effectors showing that the iscom-Ag was processed and presented by class I-expressing APC. Flu-iscoms were internalized about 50-fold more efficiently than ovalbumin iscoms (ovaiscoms) suggesting that the nature of the protein and/or the presence of cellular receptors are important factors influencing the capacity of APC to take up iscom-borne proteins. PEC accounted for the most active internalization of iscom-borne Ag, although splenic dendritic cells and B cells also took up fluiscoms with remarkable efficiency.

Application of iscom antigen preparations in ELISAs for diagnosis of Neospora and Toxoplasma infections

Immunostimulating complexes (iscoms) are cage-like structures of about 40 nm composed of Quil A, cholesterol, phospholipids and antigen. Their main area of use has been as adjuvants and carriers of immunogens in vaccines. Iscoms can also be used for selection of surface membrane proteins of micro-organisms for use in immunoassays, thus decreasing the number of internal proteins that might cause problems with non-specific binding and cross-reactivity. Enzyme-linked immunoassays (ELISAs) utilising parasite antigens incorporated into iscoms have been developed for demonstration of antibodies directed to the intracellular coccidian parasites Toxoplasma gondii and Neospora caninum. These iscom ELISAs have proved very reliable, with high sensitivity and specificity. The preparation of T. gondii and N. caninum iscoms is described, and ELISAs based on iscom antigen preparations that have so far been used for diagnosis of protozoal infections are reviewed.

ISCOM vaccine against contagious bovine pleuropneumonia (CBPP). 1. Biochemical and immunological characterization

A better vaccine than the existing ones against contagious bovine pleuropneumonia (CBPP) caused by Mycoplasma mycoides subsp. mycoides small colony type (MmmSC) would improve the chances for eradication of CBPP. In such an effort, immunostimulating complexes (ISCOMS) have been prepared from the whole detergent-solubilized cells of MmmSC and characterized biochemically and immunologically. The most efficient detergent for solubilization of the mycoplasma was MEGA-10 which yielded a high recovery of proteins in the ISCOMS. The ISCOMS showed the typical cage-like structure by EM and sedimented as 19S by sucrose gradient centrifugation. The protein pattern of the ISCOMS, analyzed in SDS-PAGE, revealed a great number of bands distributed along the gel as high and low molecular weight polypeptides. The Western blot developed with a serum from a CBPP infected animal detected a reduced number of polypeptides. In samples from whole mycoplasma cells and in ISCOMS, lectin blots revealed more than 20 carbohydrate structures. The ISCOMS induced a strong primary antibody response in mice measured by ELISA and the boost resulted in a 6-fold increase of the serum antibody response. The IgG response was distributed into various IgG subclasses with high IgG1, IgG2a and IgG2b titres while the IgG3 response was low. In cattle the ISCOM vaccine induced strong primary and long lasting secondary antibody responses of similar magnitudes as those of naturally infected animals as recorded by ELISA which persisted more than a year. IgG response was equally distributed in IgG1 and IgG2 subclasses. Also a cell-mediated immune response measured by proliferation assay was induced by low dose of ISCOMS. In the growth inhibition test, sera from vaccinated cattle readily inhibited colony growth already after the first immunization.

Assessment of the immunogenic potential of Rhodococcus equi virulence associated protein (VapA) in mice

The development of immunity to Rhodococcus equi, particularly to a virulence-associated protein (VapA) based antigen preparation, was examined in CD1 and BALB/c mice after intraperitoneal vaccination. Immunization with VapA based antigen without adjuvant markedly enhanced organ clearance in CD1 mice but not in BALB/c mice. Delayed type hypersensitivity response and antibody titres in VapA based antigen immunized BALB/c mice were less than in CD1 mice. By contrast also to CD1 mice, sera from immunized BALB/c mice did not react as strongly with VapA in western blots. Use of adjuvants (aluminium hydroxide, iscoms) interfered markedly with the immunogenic properties of the VapA based antigen, in the case of aluminium hydroxide by apparently driving a Th2 type of response. Unexpectedly, iscom adjuvants also impaired immunity and, despite the highest DTH response, produced a low IgG2a response, suggesting that iscomization of the antigen produced a low interferon gamma and high interleukin 2 response. Passive immunization of BALB/c mice with serum from mice immunized with live virulent strain 103+ resulted in only temporary and slight enhancement of organ clearance, supporting the central importance of cellular immunity to R. equi. Immunization with live virulence plasmid- and VapA-positive R. equi strain 103 resulted in marked liver clearance, in marked DTH response and high antibody titres. By contrast, immunization with live virulence plasmid- and VapA-negative strain 103 resulted in slight but variable enhancement of clearance, but insignificant DTH and antibody. The virulence plasmid, and by implication VapA, was thus shown to be critical in determining a highly effective protection to live organisms.

Iscoms containing purified Quillaja saponins upregulate both Th1-like and Th2-like immune responses

The immune stimulating complex (iscom) is built up by antigen, cholesterol, phospholipids, and adjuvant active Quillaja saponins. Previous studies have shown that iscoms containing Quil A (a semipurified preparation of saponins) efficiently induce antibody and cell-mediated immune responses. In this study, we demonstrate that iscoms containing a mixture of two purified low toxicity Quillaja saponin fractions (ISCOPREP 703) are able to upregulate both Th1-like and Th2-like immune responses. Thus, ovalbumin (OVA) iscoms induced higher levels of antigen-specific IgG1 and IgG2a antibodies and increased the production of both IFN-gamma and IL-4 compared with OVA administered without adjuvant. In contrast, OVA formulated in Al(OH)3 elicited IgG1 and IgE antibodies and primed spleen cells producing IL-4 and IL-10, suggesting the activation of primarily Th2-like cells. These findings underline that adjuvants are able to alter the character of immune responses and may be used to generate responses with desired properties.

Novel adjuvants and vaccine delivery systems

Conventionally the efficiency of an adjuvant is measured by the capacity to induce enhanced antibody serum titres and cell mediated immunity (CMI) to a given antigen. Nowadays the capacity of an adjuvant is also measured by the quality as well as the magnitude of the induced immune response, guided by the protective immune response required. Quality includes isotype and IgG subclass responses, T-helper cell responses characterized by the cytokine profile and cytotoxic T cells (CTL). In the early phase of immunization some adjuvants influence the antigen administration and uptake by a so-called depot effect exemplified by aluminium hydroxide gel and oil adjuvants, which possibly is not as desired as alledged. A modern depot is exerted by slow release formulations continuously releasing the antigen over a period of time or by pulses at intervals aiming at 'single injection' vaccine. Great efforts are made to formulate efficient delivery formulations targeting the antigens from the site of administration, to draining lymph nodes or distant lymphatic tissue or to mucosal surfaces by parenteral or mucosal administrations. Nowadays, non-replicating carriers besides replicating vaccines are formulated to induce mucosal immune responses encompassing secretory IgA and CMI. Efforts to evoke immune responses on mucosal membranes distant from the site of administration have resulted mostly in little success. For a long time it was considered that CTL under the restriction of MHC Class I only could be evoked by replicating viruses or intracellular parasites. However, novel adjuvant delivery systems readily induce CTL by delivering the antigen to the APC resulting in intracellular transport to the cytosol for the MHC Class I presentation system, as well as to the endosomal pathway for the MHC Class II presentation.

Iscoms in parasitological research

During the history of vaccine development, a number of adjuvants and adjuvant formulations have been tested and evaluated for their ability to increase the immunogenicity of different antigens. In this review, Anna Lundén, Karin Lövgren Bengtsson, Anders Sjölander and Arvid Uggla focus on iscoms (immune stimulating complexes), their characteristics and applications to different types of parasitic antigens.

Equine herpesvirus type 2 (EHV-2) as a predisposing factor for Rhodococcus equi pneumonia in foals: prevention of the bifactorial disease with EHV-2 immunostimulating complexes

Equine herpesvirus type 2 (EHV-2), a member of the Gammaherpesvirinae subfamily, was studied in a two-phase respiratory disease complex of young foals as a predisposing factor for the secondary bacterial invasion of lungs with Rhodococcus equi (R. equi). Foals were immunized against EHV-2 on a farm where R. equi pneumonia regularly occurred during the last years. The immunizations were performed by using a subunit vaccine which selectively presents envelope glycoproteins of EHV-2 in a multimeric form of immunostimulating complexes (iscoms). The etiological role of EHV-2 was estimated by observing the occurrence of the respiratory disease complex in groups of foals immunized against the virus, in comparison to non-immunized controls. The immunization trials of young animals revealed that the iscom subunit vaccine formulation is able to overcome the interference of maternal antibodies. Active immunization of foals with a single dose of the iscoms provided a certain degree of protection, while two injections of iscoms yielded complete protection against the disease complex in the majority of the treated animals, by preventing the manifestation of R. equi pneumonia. The present findings strongly support the hypothesis that EHV-2 is a predisposing factor for the R. equi invasion of the respiratory tract. The EHV-2 iscom formulation provides highly specific and effective means to prevent the disease complex.

ISCOMs (immunostimulating complexes): the first decade

A little over a decade ago, novel immunostimulating complexes (ISCOMs) were described. This review examines the position and progress that ISCOM technology has achieved in the fields of vaccine research and medicine over this period. Much of the work on ISCOMs has remained in the area of vaccine research where there is still an urgent need for improved adjuvants to help combat important diseases such as AIDS, malaria and influenza. Currently the only widely licensed adjuvants for human use are the aluminium salts, but with the trend towards highly purified subunit vaccines, which are inherently less immunogenic than some of the older vaccines, potent adjuvants capable of promoting specific immune responses are required. ISCOMs are one such technology that offers many of these requirements and as their use in vaccines enters its second decade clinical trials are commencing that will establish whether these submicron, non-living particles composed of saponin, cholesterol, phospholipid and in many cases protein, are useful components for a range of human vaccines.

Isolation and quantification of Quillaja saponaria Molina saponins and lipids in iscom-matrix and iscoms

In the iscom, multiple copies of antigen are attached by hydrophobic interaction to a matrix which is built up by Quillaja triterpenoid saponins and lipids. Thus, the iscom presents antigen in multimeric form in a small particle with a built-in adjuvant resulting in a highly immunogenic antigen formulation. We have designed a chloroform-methanol-water extraction procedure to isolate the triterpenoid saponins and lipids incorporated into iscom-matrix and iscoms. The triterpenoids in the triterpenoid phase were quantitated using orcinol sulfuric acid detecting their carbohydrate chains and by HPLC. The cholesterol and phosphatidylcholine in the lipid phase were quantitated by HPLC and a commercial colorimetric method for the cholesterol. The quantitative methods showed an almost total separation and recovery of triterpenoids and lipids in their respective phases, while protein was detected in all phases after extraction. The protein content was determined by the method of Lowry and by amino acid analysis. Amino acid analysis was shown to be the reliable method of the two to quantitate proteins in iscoms. In conclusion, simple, reproducible and efficient procedures have been designed to isolate and quantitate the triterpenoids and lipids added for preparation of iscom-matrix and iscoms. The procedures described should also be useful to adequately define constituents in prospective vaccines.