Squalene-Based Oil-in-Water Emulsions | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| MF59 and MF59-like | Squalene-based oil-in-water emulsion containing surfactants (e.g. Tween 80, Span 85) | Creates an antigen depot (slow release of antigen from the injection site) and stimulates influx of monocytes and granulocytes to the injection site Causes muscle cells and resident macrophages to release chemokines (e.g. CCL2) recruiting APCs Also triggers a wave of local cell death that can facilitate antigen uptake and cross-presentation Overall skews toward a robust humoral (Th2-biased) response with some CD4⁺ T cell help Possibly triggers cell death pathways (e.g. RIPK3-dependent necroptosis) that enhance antigen presentation | Excellent safety record: used in seasonal and pandemic flu vaccines in >30 countries Causes mild local reactogenicity (pain, swelling) and occasional short-term systemic effects (fatigue) No serious adverse events definitively attributed; no autoimmune issues observed above background | Significantly enhances antibody responses (quantity and quality) MF59-adjuvanted flu vaccines induce higher titers, longer-lasting and broader immunity than non-adjuvanted controls. Also can promote CD4⁺ Tfh and memory T cell responses, and in mice can support CD8⁺ T cell priming (via necroptosis-driven cross-priming) | |
| AddaVax | MF59-like squalene oil-in-water nanoemulsion containing tocopherol and polysorbate 80 | MF59-like: Enhances antigen presentation and creates an antigen depot; recruits APCs to the site via chemokine induction | Similar to MF59; no significant AEs in animal studies Typical reactions include injection-site tenderness and short-term fatigue. | Induces robust antibody responses and can boost immunity. In studies comparing AddaVax to AS03, each yielded high neutralizing antibody titers and substantial T-helper cell activation (e.g., in a mosaic RBD vaccine, groups with AddaVax or AS03 both showed strong neutralization and Th1-biased responses.) Generally, AS03 may provoke slightly higher cytokine and CD4⁺ T cell responses, while AddaVax is highly effective for antibodies and even CD8⁺ T cells in mice. | |
| SQBA | Squalene-based oil-in-water emulsion (inner squalene oil phase, outer aqueous citrate buffer) | Stimulates innate immunity similar to MF59/AS03, causing a depot effect and APC recruitment Contains α-tocopherol, which amplifies cytokine signals (analogous to AS03) | Phase 2–3 trials (e.g. HIPRA’s PHH-1V/Bimervax) show acceptable safety. Transient reactogenic effects (injection-site pain, fatigue) were mostly mild, with fewer adverse events than a comparator mRNA vaccine booster. | Induces robust antibody and T cell responses As a booster, SQBA-adjuvanted vaccine gave high neutralizing titers and IFN-γ+ T cells. | |
| Montanide ISA 720 | Squalene-based oil-in-water nanoemulsion | Creates an antigen depot Strongly activates local innate cells (non-TLR mechanism) Recruits APCs and is known to drive Th1-biased responses (e.g. strong IFN-γ production) | Associated with injection-site granulomas/nodules (expected from W/O emulsions Mostly local reactogenicity; in trials, all AEs were mild/moderate with no vaccine-related SAEs. Formulation (30% aqueous/70% oil) is designed to reduce reactogenicity relative to older emulsions. | Significantly enhances antibody titers and sustains them Induces high IgG (predominantly IgG1/IgG3) and strong IFN-γ^+ T cells Shown to improve neutralization breadth and induce 100% seroconversion in a COVID-19 RBD-Fc vaccine trial | |
| AS03 | Squalene-based oil-in-water emulsion containing polysorbate 80 and α-tocopherol | Oil-in-water emulsion with α-tocopherol that boosts innate immune activation Triggers local cytokine/chemokine release (via inflammasome/DAMPs), leading to enhanced APC activation Results in strong Th2 and Th1 responses (with CD4⁺ T cell activation) | Generally well tolerated in >55,000 trial subjects Causes transient reactogenic symptoms (injection-site pain, myalgia, fever) more than alum Note: AS03-adjuvanted Pandemrix (H1N1 vaccine) was associated with a rare increased risk of narcolepsy in children, but in general use AS03 has an acceptable safety profile. | Induces high antibody titers and potent, long-lasting CD4⁺ T cell responses In flu vaccines, AS03 dramatically increased neutralizing antibody breadth and magnitude Helps drive a Th1-biased response when combined with antigen (as in malaria and zoster vaccines) | |
| BFA03 (AS03-like) | Oil-in-water emulsion (squalene + surfactant + tocopherol) similar to AS03. Developed as an AS03 benchmark adjuvant | Mimics AS03’s mechanism: squalene emulsion with tocopherol that triggers transient local inflammatory responses α-Tocopherol in BFA03 amplifies the recruitment and activation of APCs, leading to enhanced antigen uptake and a mixed Th1/Th2. Cytokine surges at the injection site and draining nodes activate adaptive immunity. | ReCOV, a SARS-CoV-2 vaccine candidate employing the BFA03 adjuvant, was well-tolerated in both Phase I (New Zealand) and Phase II (Philippines) trials. No vaccine-related SAEs, adverse events of special interest AESIs, or discontinuations were reported. Most AEs were mild to moderate in severity and transient, with median durations ranging from 1–4 days. | Effectively boosts immune responses comparably to AS03 ReCOV elicited robust and durable NAb responses in both Phase I and II trials, and induced strong cross-neutralization against Omicron variants BA.2 and BA.4/5, with titers persisting for at least 6 months. ReCOV also generated Th1-biased cellular immunity without significant CD8+ or Th2 responses. | |
| Sepivac SWE | Oil-in-water emulsion (squalene-based) developed by Seppic (VFI) | Acts similarly to MF59/AS03: creates a depot at injection site and activates innate immunity by inducing local inflammation (without a specific PRR) Recruits APCs and promotes antigen uptake and trafficking to lymph nodes | Tested in flu and COVID-19 vaccine formulations: Demonstrated an acceptable safety profile; mostly mild (grade 1) to moderate (grade 2): injection site pain, muscle aches, headache Reactogenicity is comparable to MF59. No serious adverse events in clinical trials reported; considered safe for pandemic use (stockpiled for influenza) | Proven adjuvant activity: dose-sparing and improved response quality In influenza vaccines, SWE adjuvant enhanced hemagglutination-inhibition titers and provided cross-protection similar to other emulsions. In COVID-19 preclinical and clinical studies, Sepivac SWE-adjuvanted candidates induced higher neutralizing Ab titers and more polyfunctional T cells than non-adjuvanted controls. | |
| EmulsiPan | Proprietary oil-in-water emulsion by Panacea Biotec (limited information available) | Squalene emulsion that creates depots and activates innate cells (similar to MF59) Induces chemokines for APC recruitment | Emulsions generally cause moderate reactogenicity (localized redness, mild fever) and have been used in flu vaccines successfully. Panacea Biotec reports no SAEs in animal studies. | Emulsions allow antigen dose-sparing and yield quicker, higher antibody responses. Significantly increases neutralizing antibody titers and T cell responses | |
Aluminum-based | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| Aluminum hydroxide | Composed primarily of aluminum oxyhydroxide [AlO(OH)]. May be used in adsorbed or precipitated vaccine formats | Creates depot effect (slow antigen release at the infection site to prolong exposure to APCs) Enhances uptake of antigen by DCs and macrophages Activates the NLRP3 inflammasome | Widely used: Generally safe and well-tolerated Rare AEs: Local injection site reactions (e.g., nodules, erythema), granuloma, and macrophagic myofasciitis (MMF) in rare cases Allergic responses possible at doses >4 mg/mL | Induces strong humoral immunity, especially IgG1 (Th2-biased) Can stimulate Th1 and Th2 responses depending on antigen and route Improves antigen-specific antibody titers and prolongs duration of immunity Enhances responses to recombinant protein antigens (e.g., HepB, HPV, EV71) Enables antigen dose-sparing, often allowing 3-4× lower antigen content | |
| Aluminum hydroxide + CpG oligodeoxynucleotides (ODNs) | Alum (Al(OH)_3) gel mixed with CpG ODNs Often co-adsorbed at injection site | CpGs (synthetic TLR9 agonist ODNs) trigger plasmacytoid dendritic cells to produce IFN-α and activate B cells via TLR9, driving strong Th1-biased immunity. Aluminum hydroxide adsorbs antigen and CpG, creating a depot and activating NLRP3 inflammasome. The combination yields synergistic activation of both innate (via TLR9) and depot-based immune stimulation. | Generally well tolerated Adding CpGs to alum can increase reactogenicity (e.g. more injection-site redness, some flu-like symptoms), but no serious AEs in clinical trials. In a flu vaccine trial, alum+CpG7909 was safe and well tolerated with only mild side effects. Advax-CpG55.2 demonstrated a favorable safety profile in trials (SpikoGen® COVID vaccine). | Enhanced Th1 immunity: alum+CpG markedly boosts IFN-γ-producing T cells and IgG2 antibodies. A 2-dose CpG 1018/HBsAg vaccine yielded >95% seroprotection vs ~75% for a 3-dose alum-based vaccine. Human studies showed faster and higher seroconversion rates with alum+CpG (e.g. CpG-adjuvanted HepB vaccine achieved >95% protection in 2 doses vs 3 doses with. In COVID-19 vaccines, Alum+CpG 1018 elicited high neutralizing titers and a dominant Th1 cytokine profile (strong IFN-γ from splenocytes). In humans, two doses of spike+Advax-CpG55.2 achieved ~64% seroconversion by day 35 vs 7% in placebo. | |
| Imject® Alum (AlOH/MgOH) | Aluminum hydroxide + Magnesium hydroxide gel | Functions like standard alum: creates antigen depots and activates the NLRP3 inflammasome in APCs via cell damage signals The magnesium hydroxide may modulate pH or antigen adsorption, but the overall mechanism is the “alum effect;” enhanced uptake by APCs and biased Th2 response with some IL-1β production. | Similar to other alum adjuvants Widely used in animal studies without major issues Magnesium hydroxide component does not introduce additional toxicity; injection-site reactions (swelling) are mild. No systemic toxicity; generally regarded as safe (ThermoFisher reports no notable side effects.) | Effectively raises antibody responses Imject alum has been shown to stimulate strong IgG and is often used as a benchmark adjuvant in preclinical studies. Its performance is comparable to standard alum in provoking humoral immunity, though some studies note minor differences in isotype profile. Questions have been raised about its relevance to human Alum adjuvants. | |
| VAC20 | Aluminum hydroxide adjuvant (veterinary-grade high adsorption capacity) | Alum adjuvant that works via antigen depot and inflammasome activation (promoting IL-1β, Th2-skewed help) | Minimal systemic reaction, decades of preclinical safety data | Boosts immunogenicity over unadjuvanted vaccine Produces strong antibody titers, but more IgG1/IgG4 (Th2-biased) | |
Liposome-based | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| ALFQ | Liposome-based adjuvant (Army Liposome Formulation + QS-21): phospholipid/cholesterol liposomes containing monophosphoryl lipid A and QS-21 saponin | Combines MPLA (TLR4 agonist) and QS-21 (saponin) in liposomes to activate innate immunity MPLA engages TLR4/MD-2, inducing APC maturation and cytokines (but with low toxicity). QS-21 in liposomes triggers inflammasome and enhances antigen presentation, promoting robust cytotoxic T cell responses. | Non-pyrogenic and non-toxic in preclinical studies Designed to avoid QS-21 toxicity by liposomal cholesterol quenching Well-tolerated in animals (no fever, no significant local reactions) and advancing in human trials | Induced higher antibody and T cell responses than alum-based or earlier ALF formulations in rodents and NHPs Promotes high neutralizing antibody titers and robust CD4⁺ and CD8⁺ T cells via cross-presentation | Alving 2020 |
| CAF09b (Poly(I:C) + MMG) | Cationic liposome (DDA-based) containing poly(I:C) electrostatically bound and MMG (monomycolyl glycerol) integrated in the lipid bilayer | Works by combining TLR3 stimulation (Poly(I:C)) with Mincle receptor stimulation (via MMG, a trehalose dibehenate analog) Poly(I:C) activates the TRIF pathway (IFN-I production), while MMG engages the CLEC receptor Mincle on APCs, inducing cytokines like IL-6 and IL-1β. The cationic liposomal delivery system ensures these agonists and the antigen are co-delivered to the same dendritic cells, resulting in robust CD8⁺ T cell priming and Th1-biased responses. | Compared to free Poly(I:C), CAF09b had improved safety in pre-clinical studies; the liposomes prevent systemic dissemination of Poly(I:C), thus reducing side effects. In a Phase 1 study in humans (intranasal CAF09b in a COVID subunit vaccine), no severe adverse events were observed; mild nasal irritation was the main complaint. | Excellent at inducing T cell immunity (especially CTLs) CAF09b reliably generates CD8⁺ T cell responses against subunit antigens. For example, it enhanced an HPV E7 subunit vaccine’s efficacy by inducing strong cytotoxic T cells. It also promotes high antigen-specific IFN-γ and TNF-α from CD4⁺ T cells. Antibody responses with CAF09b are typically moderate but sufficient; the main strength is in cellular immunity. An intranasal CAF09b-adjuvanted COVID vaccine induced protective mucosal immunity and lung-resident memory T cells in mice. | |
TLR Agonists | |||||
| Adjuvant | Preparation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| TLR3 agonists | Synthetic double-stranded RNA analog (e.g. Poly(I:C), stabilized Poly-ICLC, PIKA, or NexaVant) | Activates TLR3, leading to robust Type I interferon and IL-12 production and upregulation of viral RNA sensors MDA5 and RIG-1. This drives strong Th1 polarization and cytotoxic T cell generation. Stimulates DC activation and migration of DCs, neutrophils, and macrophages to draining lymph nodes Often also promotes cross-priming of CD8⁺ T cells (via IFN-α) | Very potent; can cause flu-like symptoms if systemic; to minimize side effects, usually delivered with a carrier or in a localized manner Poly-ICLC (stabilized Poly(I:C)) has been safely used in humans at controlled doses (transient fever, no serious toxicity. Another stabilized Poly(I:C) formulation PIKA, also exhibits an improved safety profile compared to conventional poly(I:C). In clinical trials (e.g. rabies vaccine), PIKA was well tolerated with no serious adverse effects; only mild reactions similar to standard vaccines. Even newer formulations (e.g. NexaVant) have been designed to further improve safety and reduce instability. | Strong inducers of humoral and cellular immunity: high levels of IFN-γ–secreting T cells and enhanced CTL responses Promote Th1-biased antibody isotypes Have shown efficacy in improving viral clearance and long-lived T cell memory | |
| RS09 (TLR4 agonist) | Synthetic TLR4 agonist peptide, derived via phage display | TLR4 agonist that mimics LPS signaling Activates NF-κB pathway, inducing inflammatory cytokine production | In preclinical studies in mice, no local or systemic AEs were observed. | Induces humoral immunity, particularly IgG1/Th2-biased responses In a preclinical SARS-CoV-2 vaccine trial in mice, spike-specific IgG titers persisted for 70-104 weeks. | |
| R848 (Resiquimod; TLR7/8 agonist) | Synthetic imidazoquinoline compound that serves as a TLR7/8 agonist | TLR 7/8 agonist that activates innate immune cells, particularly plasmacytoid DCs and monocytes Engages MyD88-dependent pathway, leading to Type I interferon (IFN-α/β) production, pro-inflammatory cytokines (IL-6, TNF-α), costimulatory molecule upregulation (CD80/CD86 on DCs) | In a preclinical SARS-CoV-2 vaccine study in mice, no significant AEs were reported. | Skews the immune response toward Th1-type, promoting IgG2a/IgG2c antibody production, CD8+ T cell responses, and reduced risk of Th2-associated immunopathology | |
| Infection Mimicking (IM)-Gel (TLR7/8 agonist) | Nanoparticulate TLR7/8 agonist (NP-TLR7/8a) conjugated to albumin | “Infection Mimicking Gel:” Depot formation at the injection site TLR7/8a is transiently masked to prevent early systemic inflammation. Strong germinal center B cell and Tfh responses Potent antigen-specific CD8+ T cell responses Broad and durable serum IgG (IgG1 + IgG2a) | Preclinical only (mice) No significant SAEs No significant weight loss, no cytokine storm or signs of systemic inflammation, normal liver function Transiently elevated IL-6 (proinflammatory), but within normal ranges | In mice: IM-gel elicited higher total IgG, IgG1, and IgG2a titers against sM2HA2 (influenza) and SARS-CoV-2 spike in IM-Gel groups than alum or bolus controls. Titers were durable (>6 months for influenza). Also elicited balanced Th1/Th2 responses (IgG1/IgG2a ratio) Cross-reactivity across multiple influenza subtypes (H1N1, H3N2, H5N2, H7N3, H9N2); Anti-spike antibodies neutralized SARS-CoV-2 VOCs: Alpha, Beta, Gamma, Delta, Kappa, NY510+D614G. Increased CXCR5⁺ PD-1⁺ CD4+ Tfh cells, GL7⁺ CD95⁺ B220⁺ IgD⁻ GC B cells (affinity maturation), and antigen-specific CD8+ T cells). | |
STING Agonists | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| cGAMP (STING agonist) | Cyclic GMP-AMP dinucleotide (natural second messenger). Often delivered encapsulated (e.g. in nanoparticles or a hydrogel) due to cell membrane impermeability | 2’3’-cGAMP is the endogenous ligand for STING. It binds STING in the cytosol of DCs, inducing a potent Type I interferon response and upregulating costimulatory molecules. Enhanced antigen presentation, DC activation, and T cell priming Stimulates CD8⁺ T cell priming and promotes a high-avidity antibody response via IFN-driven class switching | As a natural molecule, rapidly metabolized Preclinical data only Free cGAMP has low toxicity but limited cellular uptake; high doses can cause strong IFN-mediated inflammation. No SAEs reported in animal models Transient innate immune activation No systemic inflammation No changes in body weight or temperature | Potent enhancer of both antibody and T cell responses cGAMP-adjuvanted vaccines induce higher magnitude and longer-lived antibody responses than non-adjuvanted controls. In one study, a cGAMP-containing hydrogel vaccine elicited more potent, durable, and broadly neutralizing antibodies vs the same vaccine given as a bolus. cGAMP also markedly expands antigen-specific CD8⁺ T cells and has shown efficacy in improving protection of mRNA vaccines. | |
| CF501 (STING agonist) | Synthetic cyclic dinucleotide-based STING agonist Designed for improved stability and delivery | CF501 is a cyclic dinucleotide-based STING agonist that activates the STING pathway more potently or persistently than cGAMP, leading to strong induction of type I IFNs and other cytokines. Stimulates B cells to target conserved epitopes and drives T cell responses in Th1-based immunity | Preclinical only; no significant AEs reported in mice, rabbits, or NHPs | Highly immunogenic In a pan-sarbecovirus RBD-Fc vaccine, CF501 adjuvant induced broadly neutralizing antibodies that cross-reacted with diverse sarbecovirus RBDs. hACE2-transgenic mice immunized with CF501/RBD-Fc were almost completely protected from SARS-CoV-2 challenge even 6 months after vaccination. Compared to Alum or cGAMP, CF501 elicited significantly stronger and more broadly neutralizing antibody responses, along with robust T cell immunity. | |
Bacterial Antigen-derived | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity | References |
| LTB | Protein (B subunit of E. coli heat-labile toxin) Pentameric structure that binds cell surfaces | Heat-labile enterotoxin B subunit binds to GM1 ganglioside receptors on mucosal epithelial cells and APCs. This facilitates antigen uptake and can modulate immune responses by targeting antigens to immune inductive sites. While LTB lacks the toxic enzymatic A subunit, it still provides adjuvant signals that can enhance IgA production and B-cell help (likely via activation of APCs in Peyer’s patches). | Non-toxic on its own (the pathogenic effects reside in the A subunit, which is absent) Used in oral and intranasal vaccines with good safety; causes minimal side effects (at most mild diarrhea or nasal irritation at high doses) Overall well tolerated in trials as a mucosal adjuvant | Enhances mucosal and systemic immunity when co-administered LTB has been shown to raise higher IgG titers than alum in some comparisons (especially with mucosal delivery). It promotes mucosal IgA responses if given intranasally/orally and can augment antigen-specific T cell responses (both Th1 and Th2) in mucosal tissues. However, its adjuvant effect is weaker than the full toxin or combination with additional immunostimulators. | |
| HKCC | Killed whole bacterium (Gram-negative Caulobacter) Often delivered as a suspension of inactivated cells | Heat-Killed Caulobacter crescentus provides multiple pathogen-associated molecular patterns (LPS for TLR4, flagellin for TLR5, unmethylated DNA for TLR9, etc.) when introduced. It potently activates dendritic cells through these innate receptors, promoting a broad inflammatory response (Th1- and Th17-supporting cytokines). | Being a non-pathogenic environmental bacterium, C. crescentus is safe when killed; it cannot replicate or cause infection. In studies, HKCC has been well tolerated in mice and hamsters; any reactogenicity is localized (due to innate activation) and not harmful. | Broadly enhances immunity: increases both antibody and T cell responses One study showed that an intranasal SARS-CoV-2 vaccine with HKCC had higher neutralizing antibody titers, including IgA, and greater T cell proliferation than the vaccine without HKCC. It can also augment CD8⁺ T cell responses (in a malaria model, HKCC improved antigen-specific T cell activation). | |
| KFD | E. coli flagellin protein mutant (truncated form for stability) Often formulated as a membrane-bound or particle-bound flagellin for intranasal use | KFD is an engineered flagellin derivative, functioning as a TLR5 agonist. It binds TLR5 on immune and epithelial cells, triggering MyD88-dependent production of IL-8, IL-6, etc. This leads to recruitment and activation of immune cells at mucosal surfaces. KFD-adjuvanted vaccines strongly activate mucosal dendritic cells and promote IgA class switching and tissue-resident memory through TLR5 signaling. | Flagellin-based adjuvants have shown good safety in humans (e.g. flagellin fusion influenza vaccines). KFD in particular has been taken to an open-label trial as a nasal booster with no significant AEs noted. Typical side effects are mild (transient nasal discharge, mild fever in some cases due to IL-6). | Potent inducer of mucosal immunity In mice, a nasal COVID-19 booster with KFD adjuvant (triple RBD + KFD) induced robust nasal IgA and lung IgG, providing broad protection against variants including Omicron in upper and lower airways. KFD also enhances systemic responses: it improves neutralizing antibody titers and T cell responses compared to non-adjuvant controls. Notably, flagellin adjuvants drive mixed Th1/Th2 but are especially effective at improving Th17 and IgA for mucosal defense. | |
| LTA (Lipoteichoic acid) | Glycolipid (lipid-linked teichoic acid from bacterial cell wall) Can be isolated from bacteria or synthesized. | TLR2 agonist It inserts into APC membranes via its lipid moiety and engages TLR2/6, leading to activation of NF-κB and production of proinflammatory cytokines (TNF, IL-1, IL-6). This favors a mixed Th1/Th17 type response and can enhance dendritic cell maturation. | Pure LTA is somewhat pyrogenic, but at adjuvant doses in animals it has been used without severe issues (often in combination with delivery vehicles). It has not been used in licensed vaccines due to reactogenicity concerns. However, analogs of LTA (i.e. genetic detoxification) or small doses have been examined as adjuvants with manageable safety, mostly local inflammation, with risk of fever at high doses. | LTA can boost immune responses by activating monocytes and DCs. Studies have shown that adding LTA or LTA-mimetics to peptide vaccines increases antibody titers and can induce a more balanced IgG subtype profile. It may also promote IL-17 and IFN-γ production, useful for infections where Th17 is needed. Overall, while LTA is immunostimulatory, its adjuvant efficacy in practice has been less documented than other TLR ligands (experiments are ongoing with synthetic LTA glycan conjugates as candidate adjuvants). | |
| Monophosphoryl Lipid A (MPLA) | Lipid A derivative (from Salmonella endotoxin) with one phosphate group and fewer acyl chains Often adsorbed onto alum (AS04) or combined with other adjuvants (e.g. in AS01) | A detoxified TLR4 agonist (derived from LPS) that engages the TLR4–MD-2 receptor on APCs, triggering NF-κB and TRIF pathways Induces upregulation of co-stimulatory molecules and production of cytokines like IL-12, thereby skewing toward a Th1 immune response MPLA retains adjuvant activity but without overproduction of pro-inflammatory signals compared to native LPS. | Well established safety: Used in Cervarix® and Shingrix® vaccines By itself causes minimal systemic symptoms; in humans, MPLA-adjuvanted vaccines have been safe with only mild local reactions (soreness). MPLA’s toxicity is ∼1,000× lower than LPS. In NHPs, no AEs reported | Enhances Th1 responses Significantly increases antigen-specific IFN-γ and IgG2 responses Cervarix (with MPLA) induced higher neutralizing antibodies and CD4⁺ T cell responses than alum-only HPV vaccine. Shingrix (MPL+QS-21) achieved >90% efficacy even in elderly, owing to robust T cell immunity MPLA helps provide. MPLA generally boosts magnitude and longevity of antibody responses. | |
Combination Adjuvants | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| GLA-SE (TLR4 agonist emulsion) | “Glucopyranosyl Lipid A - Stable Emulsion:” a squalene-based oil-in-water emulsion carrying a synthetic lipid A derivative | GLA-SE contains GLA (glucopyranosyl lipid A), a synthetic MPLA analog, embedded in a stable squalene emulsion. GLA is a TLR4 agonist that activates APCs to produce IL-12 and other Th1 cytokines. The squalene emulsion component acts as a depot and further stimulates innate cells. Together, GLA-SE strongly drives Th1 responses. | Exhibited acceptable safety in multiple clinical trials (e.g. ID93+GLA-SE for TB) Dose-escalation studies showed only mild reactogenic events (injection-site tenderness, transient low-grade fever) and no serious toxicity. GLA is detoxified (monophosphorylated), reducing endotoxin-like reactions. | Notably promotes Th1-biased immunity GLA-SE-adjuvanted vaccines induce higher IgG2 (in mice) and robust T cell responses. For example, influenza and TB vaccine studies found GLA-SE boosted IFN-γ^+ T cells and protective efficacy compared to alum. In a Phase 1 study, ID93+GLA-SE elicited multifunctional CD4 T cell responses while remaining immunogenic in 100% of subjects. | |
| CP15 (CpG + Poly(I:C) + IL-15) | Mix of TLR9 agonist (CpG ODN) + TLR3 agonist (Polyinosinic–polycytidylic acid) + recombinant IL-15 cytokine Often formulated with a cationic carrier (e.g. DOTAP liposomes for intranasal delivery) | Triple combination designed to maximize T cell immunity CpG ODN (TLR9 agonist) and Poly(I:C) (TLR3 agonist) together synergize in activating dendritic cells (cross-talk of TLR pathways), resulting in high IL-12 and IFN levels. IL-15 is a cytokine that promotes NK cell and CD8⁺ T cell proliferation and survival. CP-15 thus provides innate signals for Th1 and directly supports CTL and memory T cell development. | Thus far used in animal models Tolerability has been good: no notable weight loss or illness even with repeated intranasal doses (IL-15 is at low dose). The main safety consideration is potential inflammation if IL-15 is too high, but in the tested formulation it did not cause significant adverse effects (no obvious pathology reported). | Extremely potent at inducing cellular immunity, especially mucosally In mice, intranasal SARS-CoV-2 Spike protein with CP15 adjuvant elicited robust lung-resident memory T cells and higher neutralizing IgA titers than even intramuscular immunization. As a booster, CP15-adjuvanted nasal vaccine gave equal or better protection (by viral clearance and T cell readouts) compared to traditional IM boost. This combination drives strong antigen-specific CTLs and IFN-γ^+ CD4 T cells in the respiratory tract. | |
| Aluminum hydroxide + 3M-052 | Small-molecule TLR7/8 agonist (3M-052) adsorbed on alum (Al(OH)_3) | 3M-052 is a synthetic TLR7/8 agonist (imidazoquinoline) engineered with a C18 lipid tail to remain at the injection site. It activates dendritic cells and monocytes via TLR7/8, inducing IL-12, TNF, and IFN, thus promoting Th1 and antibody responses. Alum adsorbs the 3M-052 (preventing systemic spread) and provides inflammasome activation. The combination triggers strong local innate stimulation without systemic cytokine spillover. | 3M-052’s lipidation keeps it localized, avoiding the fever and malaise that free TLR7/8 agonists can cause. In NHP and phase-1 trials, only transient mild reactions were reported, with no SAEs. | Potent immunogenicity In mice, protein + Alum-3M-052 elicited 100-fold higher neutralizing antibody titers than alum alone and provided superior protection. In humans (HIV Env trial), 3M-052+Alum induced robust Env-specific antibody, B-cell, and CD4⁺ T cell responses. | |
| Aluminum phosphate + E6020 | Combination of an inorganic aluminum salt (AlPO₄) with a lipid A analog (E6020). (E6020 is water-soluble and adsorbed onto the alum particles) | E6020 is a synthetic TLR4 agonist (hexa-acylated lipid A analog) that activates the TLR4/MD-2 pathway like MPLA, inducing dendritic cell maturation and cytokine release. Aluminum phosphate provides a depot effect and engages inflammasome (via cell damage signals), skewing toward Th2. Together, they trigger a balanced Th1/Th2 response (TLR4-driven IL-12 plus alum-driven antigen persistence). | Alum has a long record of safety (mild local irritation, extremely low systemic toxicity). E6020 is a relatively non-toxic LPS derivative attenuated compared to natural endotoxin. In animal studies, the combination showed no unexpected toxicity; mostly injection-site inflammation comparable to alum alone. | Markedly improved immune response vs antigen alone or alum alone In mice, alum+E6020 elevated antibody titers and promoted isotype switching to IgG2a/IgG2c (Th1-associated). Comparable adjuvanticity to MPLA was observed, with higher sustained antibody levels and a bias toward a more Th1-balanced response. | Gopalakrishnan 2023 |
Other | |||||
| Adjuvant | Formulation | Mechanism of Action | Safety Profile | Immunogenicity Data | References |
| Chitosan | Polycationic polysaccharide derived from shellfish chitin Often formulated as nanoparticles or gels for intranasal/oral delivery | Chitosan exerts adjuvant effects by mucoadhesion and immune activation. It adheres to mucosal surfaces and opens tight junctions, increasing antigen uptake. Chitosan particles are taken up by antigen-presenting cells and can activate the inflammasome. They stimulate macrophages to produce cytokines and promote IgA responses in local lymphoid tissue. Overall bias is towards enhancing mucosal immunity (Th2/Th17) with some Th1 support (e.g. IL-6, IL-17, IFN-γ induction). | Chitosan is biocompatible and widely used in drug delivery. As an adjuvant it has a good safety profile: non-toxic, non-immunogenic by itself. Intranasal application can cause mild transient nasal irritation. It is not associated with significant systemic effects (since it mostly stays local and is degraded). | Effective at eliciting mucosal antibodies Chitosan nanoparticle adjuvants have been shown to induce strong secretory IgA in the respiratory tract and also elevate systemic IgG levels. They can promote a mix of immune responses (e.g. mice immunized intranasally with chitosan-adjuvanted vaccine had increased IgA, IgG, and cytokines like IFN-γ and IL-6 in NALT). However, chitosan alone may not drive a very high Th1 response unless combined with other immunostimulators. | |