Co-Delivery of Novel Synthetic TLR4 and TLR7/8 Ligands Adsorbed to Aluminum Salts Promotes Th1-Mediated Immunity against Poorly Immunogenic SARS-CoV-2 RBD

Despite the availability of effective vaccines against COVID-19, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to spread worldwide, pressing the need for new vaccines with improved breadth and durability. We developed an adjuvanted subunit vaccine against SARS-CoV-2 using the recombinant receptor-binding domain (RBD) of spikes with synthetic adjuvants targeting TLR7/8 (INI-4001) and TLR4 (INI-2002), co-delivered with aluminum hydroxide (AH) or aluminum phosphate (AP). The formulations were characterized for the quantities of RBD, INI-4001, and INI-2002 adsorbed onto the respective aluminum salts. Results indicated that at pH 6, the uncharged RBD (5.73 ± 4.2 mV) did not efficiently adsorb to the positively charged AH (22.68 ± 7.01 mV), whereas it adsorbed efficiently to the negatively charged AP (-31.87 ± 0.33 mV). Alternatively, pre-adsorption of the TLR ligands to AH converted it to a negatively charged particle, allowing for the efficient adsorption of RBD. RBD could also be directly adsorbed to AH at a pH of 8.1, which changed the charge of the RBD to negative. INI-4001 and INI-2002 efficiently to AH. Following vaccination in C57BL/6 mice, both aluminum salts promoted Th2-mediated immunity when used as the sole adjuvant. Co-delivery with TLR4 and/or TLR7/8 ligands efficiently promoted a switch to Th1-mediated immunity instead. Measurements of viral neutralization by serum antibodies demonstrated that the addition of TLR ligands to alum also greatly improved the neutralizing antibody response. These results indicate that the addition of a TLR7/8 and/or TLR4 agonist to a subunit vaccine containing RBD antigen and alum is a promising strategy for driving a Th1 response and neutralizing antibody titers targeting SARS-CoV-2.

Using Dual Toll-like Receptor Agonism to Drive Th1-Biased Response in a Squalene- and α-Tocopherol-Containing Emulsion for a More Effective SARS-CoV-2 Vaccine

A diversity of vaccines is necessary to reduce the mortality and morbidity of SARS-CoV-2. Vaccines must be efficacious, easy to manufacture, and stable within the existing cold chain to improve their availability around the world. Recombinant protein subunit vaccines adjuvanted with squalene-based emulsions such as AS03™ and MF59™ have a long and robust history of safe, efficacious use with straightforward production and distribution. Here, subunit vaccines were made with squalene-based emulsions containing novel, synthetic toll-like receptor (TLR) agonists, INI-2002 (TLR4 agonist) and INI-4001 (TLR7/8 agonist), using the recombinant receptor-binding domain (RBD) of SARS-CoV-2 S protein as an antigen. The addition of the TLR4 and TLR7/8 agonists, alone or in combination, maintained the formulation characteristics of squalene-based emulsions, including a sterile filterable droplet size (<220 nm), high homogeneity, and colloidal stability after months of storage at 4, 25, and 40 °C. Furthermore, the addition of the TLR agonists skewed the immune response from Th2 towards Th1 in immunized C57BL/6 mice, resulting in an increased production of IgG2c antibodies and a lower antigen-specific production of IL-5 with a higher production of IFNγ by lymphocytes. As such, incorporating TLR4 and TLR7/8 agonists into emulsions leveraged the desirable formulation and stability characteristics of emulsions and can induce Th1-type humoral and cell-mediated immune responses to combat the continued threat of SARS-CoV-2.

Cationic liposomes containing a TLR4 agonist promote the efficient development of cellular immunity against SARS-CoV-2 Spike protein in a subunit vaccine

Subunit vaccines require an adjuvant, as protein antigens alone provoke a very poor immune response. Liposomes can be used to deliver an adjuvant, particularly when it is an amphipathic molecule such as a toll-like receptor (TLR) 4 agonist. TLR4 engagement causes the release of cytokines known to skew the immune response towards Th1-mediated cellular immunity, making it attractive for use in viral vaccines. Additionally, liposomes formulated with a TLR4 agonist can be neutral, positive, or negative in charge, further affecting the immune response. We tested the effect of liposome composition and charge on the immune response against the SARS-CoV-2 Spike protein following vaccination with recombinant Spike trimers and liposomes containing a synthetic TLR4 agonist in mice. Anti-Spike antibody titers were modestly boosted by neutral liposomes, but charged liposomes increased it further. Control groups given liposomes without the TLR4 agonist demonstrated that cationic liposomes alone boosted antibody titers, but gave a strikingly different T cell response. These liposomes evoked a strongly Th2-associated immune profile, with high IgG1 levels and T cells that released IL-5 when stimulated ex vivo. In contrast, the TLR4-stimulating cationic liposomes resulted in high IgG2c levels, along with strong production of IFNγ, some IL-17, and no IL-5 by restimulated T cells. Interestingly, despite similar antibody titers, serum from groups given the TLR4-stimulating liposomes more effectively prevented Spike binding to ACE-2 than that of the control groups in pseudoneutralization assays. These data show a beneficial effect of TLR4-stimulating cationic liposomes on the anti-viral cellular immune response in subunit vaccines.

Supported by a supplement to NIH Adjuvant Discovery Contract 75N93019C00045

A tractable covalent linker strategy for the production of immunogenic antigen-TLR7/8L bioconjugates

Despite the ease of production and improved safety profiles of recombinant vaccines, the inherently low immunogenicity of unadjuvanted proteins remains an impediment to their widespread adoption. The covalent tethering of TLR agonists to antigenic proteins offers a unique approach to co-deliver both constituents to the same cell-enhancing vaccine efficacy while minimizing reactogenicity. However, the paucity of simple and effective linker chemistries continues to hamper progress. Here, we present a modular, PEG-based linker system compatible with even extremely lipophilic and challenging TLR7/8 agonists. To advance the field and address previous obstacles, we offer the most straightforward and antigen-preserving linker system to date. These antigen-adjuvant conjugates enhance antigen-specific immune responses in mice, demonstrating the power of our approach within the context of modern vaccinology.

Co-delivery of antigens with TLR7/8 agonists leads to an improved Th1-mediated immune response to subunit vaccines

When designing an effective and safe vaccine, the induction of a protective immune response to a pathogen without unacceptable levels of immunopathology is critical. The use of purified or recombinant antigens in combination with adjuvants that elicit a defined immune response is a promising strategy for eliciting a productive immune response while minimizing toxicity. Our group has developed libraries of novel toll-like receptor (TLR)7 and 8 ligands with a range of activity. Previous clinical trials have shown that the high potency of TLR7/8 adjuvants can lead to unacceptable toxicity. We hypothesized that direct conjugation of the adjuvant to the antigen will limit systemic distribution of the adjuvant, while boosting the immune response by ensuring co-delivery of antigen and adjuvant to antigen presenting cells. We demonstrate that covalent conjugation of TLR7/8 agonists to antigens results in greatly improved humoral and cell-mediated responses, without apparent systemic or local inflammation. This likely improves safety by preventing off-target immune responses from the engagement of PAMPs in the absence of appropriate antigens. When mice were immunized with an antigen/adjuvant conjugate we observed a switch towards IgG2a and away from IgG1 antigen-specific antibodies, plus greater Th1 cytokine production after restimulation of immune cells ex vivo. We hypothesize that co-delivery of a TLR7/8 agonist with antigen leads to activation of CD4+ T cells in the presence of Th1-skewing cytokines. The TLR7/8 agonist also appears to improve cross-presentation, inducing CD8+ T cells. We find this approach has great potential to enhance vaccine efficacy, while reducing potential immunopathology by TLR7/8 adjuvants.

Combinations of alum and synthetic toll-like receptor agonists as adjuvants for CoVID-19 vaccines

Aluminum-based adjuvants (“alum”) are the most widely-used adjuvants for human vaccines, with an extensive history of safety and efficacy. Alum is known to elicit high antibody titers by driving a Th2-based immune response, which may not be optimal for many viral infections, such as SARS-CoV-2. Therefore, combining the strong safety and efficacy attributes of alum with a Th1-polarizing adjuvant could improve immunity against viral antigens. Here we test the combination of alum with synthetic TLR4- and TLR7/8 -ligands in vaccines against SARS-CoV-2. The combination of alum and TLR agonists resulted in efficient adjuvantation of both humoral and cell-mediated immunity. Alum adsorption studies showed low association between the small, positively-charged receptor binding domain (RBD) antigen and aluminum hydroxide (Alhydrogel) and high adsorption to negatively charged aluminum phosphate (Adju-phos). However, Adju-phos and Alhydrogel both enhanced immunity to the SARS-CoV-2 RBD antigen, suggesting antigen adsorption may not be required for the immune enhancing effects of alum. These data demonstrated that adjuvants combining alum with a TLR agonist result in an improved anti-viral immune response. However, the ability of alum to adsorb to an antigen may not predict its ability to effectively adjuvant.

The comparison of two SARS-CoV-2 spike protein antigens in TLR-adjuvanted subunit vaccines

Adjuvanted subunit vaccines offer improved safety and efficacy over whole inactivated or live attenuated vaccines by incorporating recombinant proteins with highly selective innate immune activators. Selection of an antigen in subunit vaccines can alter immune responses, making it an important consideration. For severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the spike (S)-protein aids in receptor recognition and membrane fusion. Previous studies of SARS-CoV-2 and the closely related SARS-CoV-1 define the receptor binding domain (RBD) portion of the S-protein as the target of most neutralizing antibodies (nAbs). Here the recombinant full spike trimer (FST) and the isolated recombinant RBD are compared as antigens in an adjuvanted subunit vaccine. Induction of Th1-biased CD4+ and CD8+ cellular responses are believed to be important for durable antiviral immunity. Therefore, we used synthetic agonists of the toll-like receptor (TLR)4 or TLR7/8 as vaccine adjuvants for their ability to promote Th1 cellular immunity. We found that a TLR4 agonist was effective at adjuvanting both RBD and FST, while a TLR7/8 ligand provided superior adaptive immune responses with FST. In addition, increased induction of nAbs were demonstrated when vaccinating mice with FST. Thus, we find that early selection and optimization of adjuvant and antigen combinations is critical for the induction of humoral (including nABs) and cell-mediated immunity to SARS-CoV-2 antigens.

Codelivery of antigens with TLR7/8 agonists leads to an improved immune response to influenza vaccination

When designing an effective and safe vaccine, inducing a protective immune response to a pathogen without also inducing unacceptable levels of immunopathology is a major challenge. One promising strategy is the use of purified or recombinant antigens along with adjuvants that specifically engage pathogen-associated molecular pattern (PAMP) receptors. This targeted approach aims to minimize immune-mediated reactogenicity. Our group has developed libraries of novel toll-like receptor (TLR)7 and/or TLR8 ligands which show a range in receptor activity. Previous studies have shown that the high potency of TLR7/8 adjuvants leads to unacceptable toxicity in human clinical trials. One strategy to overcome this toxicity is direct adjuvant-antigen conjugation, which serves to limit systemic distribution of the adjuvant through the codelivery of the antigen and adjuvant to APCs. We have demonstrated that covalent conjugation of a TLR7/8 agonist to an antigen results in improved humoral and cell-mediated responses, with decreased systemic and local inflammation. This approach allows for the use of a reduced adjuvant dose by preventing the engagement of PAMPs in the absence of antigen. We hypothesize that antigen/adjuvant codelivery boosts the engagement of CD4+ T cells and elicits the necessary cytokines to promote a Th1-related switch to antigen-specific IgG2a antibodies. In addition, engagement of TLR7/8 appears to improve cross-presentation, thereby boosting the CD8+ T cell response, which is desirable for improved immunity to influenza and other viruses. We find this approach has great potential to enhance vaccine efficacy, while reducing the immunopathology often associated with highly active TLR7/8 adjuvants.

Influenza vaccines using liposomal formulations of toll-like receptor (TLR) 7/8 and 4 agonists as adjuvants

Influenza virus infection is a global health concern and causes severe illness and thousands of deaths each year. While there are currently licensed vaccines available for both seasonal and pandemic influenza viruses there are several issues to consider; 1) current vaccines do not provide supra-seasonal or universal protection against influenza subtypes, 2) antigenic mismatch between the vaccine and circulating influenza strains impact vaccine efficacy, 3) and the efficacy of influenza seasonal vaccines could be improved in the elderly and immune compromised populations. Therefore, vaccine adjuvants capable of inducing more robust and broadly cross-protective immune responses to various subtypes of influenza virus is a critical unmet need. To address this challenge, we developed a synthetic dual TLR adjuvant combination system (TRAC-478) to improve influenza vaccine immunity. Preliminary data in human peripheral blood mononuclear cells demonstrate that combining our synthetic TLR7/8 and TLR4 agonists results in a synergistic innate immune responses. Influenza vaccine immunogenicity studies in both murine and pig models demonstrate the safety and efficacy of this new dual TLR adjuvant system. Using a co-encapsulated TRAC-478 liposome delivery system with a split-flu vaccine resulted in strong anti-influenza humoral, Th1 cell mediated immunity, and protection from a heterologous influenza challenge in mice. We hypothesize that using the TRAC-478 adjuvant system in combination with current influenza vaccines will induce a more robust and broadly cross-protective immune responses to circulating or pandemic influenza virus.

Salmonella Typhimurium Infection of Human Monocyte-Derived Macrophages

The successful infection of macrophages by non-typhoidal serovars of Salmonella enterica is likely essential to the establishment of the systemic disease they sometimes cause in susceptible human populations. However, the interactions between Salmonella and human macrophages are not widely studied, with mouse macrophages being a much more common model system. Fundamental differences between mouse and human macrophages make this less than ideal. Additionally, the inability of human macrophage-like cell lines to replicate some properties of primary macrophages makes the use of primary cells desirable. Here we present protocols to study the infection of human monocyte-derived macrophages with Salmonella Typhimurium. These include a method for differentiating monocyte-derived macrophages in vitro and protocols for infecting them with Salmonella Typhimurium, as well as assays to measure the extent of infection, replication, and death. These protocols are useful for the investigation of both bacterial and host factors that determine the outcome of infection. © 2018 by John Wiley & Sons, Inc.

Peripheral education of the immune system by colonic commensal microbiota

The instruction of the immune system to be tolerant of self, thereby preventing autoimmunity, is facilitated by the education of T cells in a specialized organ, the thymus, where self-reactive cells are either eliminated or differentiated into tolerogenic Foxp3⁺ regulatory T(Treg) cells¹. However, it is unknown whether T cells are also educated to be tolerant of foreign antigens, such as those from commensal bacteria, in order to prevent immunopathology such as inflammatory bowel disease^2–4. Here, we show that encounter with commensal microbiota results in the peripheral generation of Treg cells, rather than pathogenic effectors. We observed that colonic Treg cells utilized T cell antigen receptors (TCRs)different from those used by Treg cells in other locations, implying an important role for local antigens in shaping the colonic Treg cell population. Many of the local antigens appeared to be derived from commensal bacteria based on the in vitro reactivity of common colon Treg TCRs. Interestingly, these TCRs did not facilitate thymic Treg cell development, implying that manycolonic Treg cells arise instead via antigen-driven peripheral Treg cell development. Further analysis of two of these TCRs by the creation of retroviral bone marrow chimeras and a TCR transgenic linerevealed that microbiota indigenous to our mouse colony was required for the generation of colonic Treg cells from otherwise naive T cells. If T cells expressing these TCRs fail to undergo Treg cell development and instead become effector cells, they have the potential to induce colitis, as evidenced by adoptive transfer studies. These results suggest that the efficient peripheral generation of antigen-specific populations of Treg cells in response to an individual’s microbiota provides important post-thymic education of the immune system to foreign antigens, thereby providing tolerance to commensal microbiota.

Targeting of B and T lymphocyte associated (BTLA) prevents graft-versus-host disease without global immunosuppression

One-time treatment with an antibody against BTLA provides long-term protection against graft-versus-host disease without affecting effector T cell responses to tumors or pathogens.

Graft-versus-host disease (GVHD) causes significant morbidity and mortality in allogeneic hematopoietic stem cell transplantation (aHSCT), preventing its broader application to non–life-threatening diseases. We show that a single administration of a nondepleting monoclonal antibody specific for the coinhibitory immunoglobulin receptor, B and T lymphocyte associated (BTLA), permanently prevented GVHD when administered at the time of aHSCT. Once GVHD was established, anti-BTLA treatment was unable to reverse disease, suggesting that its mechanism occurs early after aHSCT. Anti-BTLA treatment prevented GVHD independently of its ligand, the costimulatory tumor necrosis factor receptor herpesvirus entry mediator (HVEM), and required BTLA expression by donor-derived T cells. Furthermore, anti-BTLA treatment led to the relative inhibition of CD4⁺ forkhead box P3⁻ (Foxp3⁻) effector T cell (T eff cell) expansion compared with precommitted naturally occurring donor-derived CD4⁺ Foxp3⁺ regulatory T cell (T reg cell) and allowed for graft-versus-tumor (GVT) effects as well as robust responses to pathogens. These results suggest that BTLA agonism rebalances T cell expansion in lymphopenic hosts after aHSCT, thereby preventing GVHD without global immunosuppression. Thus, targeting BTLA with a monoclonal antibody at the initiation of aHSCT therapy might reduce limitations imposed by histocompatibility and allow broader application to treatment of non–life-threatening diseases.

Antigen-specific peripheral shaping of the natural regulatory T cell population

Although regulatory T (T reg) cells are thought to develop primarily in the thymus, the peripheral events that shape the protective T reg cell population are unclear. We analyzed the peripheral CD4(+) T cell receptor (TCR) repertoire by cellular phenotype and location in mice with a fixed TCRbeta chain. We found that T reg (Foxp3(+)) cells showed a marked skewing of TCR usage by anatomical location in a manner similar to antigen-experienced (CD44(hi)Foxp3(-)) but not naive (CD44(lo)Foxp3(-)) cells, even though CD44(hi) and T reg cells used mostly dissimilar TCRs. This was likely unrelated to peripheral conversion, which we estimate generates only a small percentage of peripheral T reg cells in adults. Conversion was readily observed, however, during the immune response induced by Foxp3(-) cells in lymphopenic hosts. Interestingly, the converted Foxp3(+) and expanded Foxp3(-) TCR repertoires were different, suggesting that generation of Foxp3(+) cells is not an automatic process upon antigen activation of Foxp3(-) T cells. Retroviral expression of these TCRs in primary monoclonal T cells confirmed that conversion did not require prior cellular conditioning. Thus, these data demonstrate that TCR specificity plays a crucial role in the process of peripheral conversion and in shaping the peripheral T reg cell population to the local antigenic landscape.

A Signal through OX40 (CD134) Allows Anergic, Autoreactive T Cells to Acquire Effector Cell Functions

To study mechanisms of peripheral self-tolerance, we injected small numbers of naive CD4(+) TCR-transgenic T cells into mice expressing the MHC/peptide ligand under the control of an MHC class II promoter. The donor T cells expand rapidly to very large numbers, acquire memory markers, and go out into tissues, but the animals remain healthy, and the accumulated T cells are profoundly anergic to restimulation with Ag in vitro. Provision of a costimulatory signal by coinjection of an agonist Ab to OX40 (CD134), a TNFR family member expressed on activated CD4 T cells, results in death of the mice within 12 days. TCR-transgenic T cells recovered at 5 days from anti-OX40-treated mice have a unique phenotype: they remain unresponsive to Ag in vitro, but they are larger, more granular, and strongly IL-2R positive. Some spontaneously secrete IFN-gamma directly ex vivo, and the majority make IFN-gamma in response to PMA and ionomycin. Although they are anergic by conventional tests requiring Ag recognition, they respond vigorously to cytokines, proliferating in response to IL-2, and secreting IFN-gamma when TCR signaling is bypassed with IL-12 and IL-18. We conclude that the costimulatory signal through OX40 allows otherwise harmless, proliferating, autoreactive T cells to acquire effector cell functions. The ability of these T cells to respond to cytokines by synthesizing additional inflammatory cytokines without a TCR signal may drive the fatal pathogenic process in vivo.