Characterization and Stability of Trypanosoma cruzi 24-C4 (Tc24-C4), a Candidate Antigen for a Therapeutic Vaccine Against Chagas Disease

Chagas disease due to chronic infection with Trypanosoma cruzi is a neglected cause of heart disease, affecting approximately 6-10 million individuals in Latin America and elsewhere. T. cruzi Tc24, a calcium-binding protein in the flagellar pocket of the parasite, is a candidate antigen for an injectable therapeutic vaccine as an alternative or a complement to chemotherapy. Previously, we reported that a genetically engineered construct from which all cysteine residues had been eliminated (Tc24-C4) yields a recombinant protein with reduced aggregation and improved analytical purity in comparison to the wild-type form, without compromising antigenicity and immunogenicity. We now report that the established process for producing Escherichia coli-expressed Tc24-C4 protein is robust and reproducibly yields protein lots with consistent analytical characteristics, freeze-thaw, accelerated, and long-term stability profiles. The data indicate that, like most proteins, Tc24-C4 should be stable at -80°C, but also at 4°C and room temperature for at least 30 days, and up to 7-15 days at 37°C. Thus, the production process for recombinant Tc24-C4 is suitable for Current Good Manufacturing Practice production and clinical testing, based on process robustness, analytical characteristics, and stability profile.

Crystal Structure of Borrelia turicatae protein, BTA121, a differentially regulated gene in the tick-mammalian transmission cycle of relapsing fever spirochetes

Tick-borne relapsing fever (RF) borreliosis is a neglected disease that is often misdiagnosed. RF species circulating in the United States include Borrelia turicatae, which is transmitted by argasid ticks. Environmental adaptation by RF Borrelia is poorly understood, however our previous studies indicated differential regulation of B. turicatae genes localized on the 150 kb linear megaplasmid during the tick-mammalian transmission cycle, including bta121. This gene is up-regulated by B. turicatae in the tick versus the mammal, and the encoded protein (BTA121) is predicted to be surface localized. The structure of BTA121 was solved by single-wavelength anomalous dispersion (SAD) using selenomethionine-derivative protein. The topology of BTA121 is unique with four helical domains organized into two helical bundles. Due to the sequence similarity of several genes on the megaplasmid, BTA121 can serve as a model for their tertiary  structures. BTA121 has large interconnected tunnels and cavities that can accommodate ligands, notably long parallel helices, which have a large hydrophobic central pocket. Preliminary in-vitro studies suggest that BTA121 binds lipids, notably palmitate with a similar order of binding affinity as tablysin-15, a known palmitate-binding protein. The reported data will guide mechanistic studies to determine the role of BTA121 in the tick-mammalian transmission cycle of B. turicatae.

3M-052 as an adjuvant for a PLGA microparticle-based Leishmania donovani recombinant protein vaccine

It is believed that an effective vaccine against leishmaniasis will require a T helper type 1 (T_H 1) immune response. In this study, we investigated the adjuvanticity of the Toll-like receptor (TLR) 7/8 agonist 3M-052 in combination with the Leishmania donovani 36-kDa nucleoside hydrolase recombinant protein antigen (NH36). NH36 and 3M-052 were encapsulated in separate batches of poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs). The loading efficiency for NH36 was 83% and for 3M-052 was above 95%. In vitro stimulation of bone marrow-derived dendritic cells, measured by IL-12 secretion, demonstrated that 3M-052 (free or MP-formulated) had a concentration-dependent immunostimulatory effect with an optimum concentration of 2 µg/mL. In immunogenicity studies in BALB/c mice, MP-formulated NH36 and 3M-052 elicited the highest serum titers of T_H 1-associated IgG2a and IgG2b antibodies and the highest frequency of IFNγ-producing splenocytes. No dose dependency was observed among MP/NH36/3M-052 groups over a dose range of 4-60 µg 3M-052 per injection. The ability of MP-formulated NH36 and 3M-052 to elicit a T_H 1-biased immune response indicates the potential for PLGA MP-formulated 3M-052 to be used as an adjuvant for leishmaniasis vaccines. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1587-1594, 2018.

Crystal Structure of MpPR-1i, a SCP/TAPS protein from Moniliophthora perniciosa, the fungus that causes Witches' Broom Disease of Cacao

The pathogenic fungi Moniliophthora perniciosa causes Witches’ Broom Disease (WBD) of cacao. The structure of MpPR-1i, a protein expressed by M. perniciosa when it infects cacao, are presented. This is the first reported de novo structure determined by single-wavelength anomalous dispersion phasing upon soaking with selenourea. Each monomer has flexible loop regions linking the core alpha-beta-alpha sandwich topology that comprise ~50% of the structure, making it difficult to generate an accurate homology model of the protein. MpPR-1i is monomeric in solution but is packed as a high ~70% solvent content, crystallographic heptamer. The greatest conformational flexibility between monomers is found in loops exposed to the solvent channel that connect the two longest strands. MpPR-1i lacks the conserved CAP tetrad and is incapable of binding divalent cations. MpPR-1i has the ability to bind lipids, which may have roles in its infection of cacao. These lipids likely bind in the palmitate binding cavity as observed in tablysin-15, since MpPR-1i binds palmitate with comparable affinity as tablysin-15. Further studies are required to clarify the possible roles and underlying mechanisms of neutral lipid binding, as well as their effects on the pathogenesis of M. perniciosa so as to develop new interventions for WBD.

Optimization of the Production Process and Characterization of the Yeast-Expressed SARS-CoV Recombinant Receptor-Binding Domain (RBD219-N1), a SARS Vaccine Candidate

From 2002 to 2003, a global pandemic of severe acute respiratory syndrome (SARS) spread to 5 continents and caused 8000 respiratory infections and 800 deaths. To ameliorate the effects of future outbreaks as well as to prepare for biodefense, a process for the production of a recombinant protein vaccine candidate is under development. Previously, we reported the 5 L scale expression and purification of a promising recombinant SARS vaccine candidate, RBD219-N1, the 218-amino acid residue receptor-binding domain (RBD) of SARS coronavirus expressed in yeast-Pichia pastoris X-33. When adjuvanted with aluminum hydroxide, this protein elicited high neutralizing antibody titers and high RBD-specific antibody titers. However, the yield of RBD219-N1 (60 mg RBD219-N1 per liter of fermentation supernatant; 60 mg/L FS) still required improvement to reach our target of >100 mg/L FS. In this study, we optimized the 10 L scale production process and increased the fermentation yield 6- to 7-fold to 400 mg/L FS with purification recovery >50%. A panel of characterization tests indicated that the process is reproducible and that the purified, tag-free RBD219-N1 protein has high purity and a well-defined structure and is therefore a suitable candidate for production under current Good Manufacturing Practice and future phase-1 clinical trials.

Expression and purification of an engineered, yeast-expressed Leishmania donovani nucleoside hydrolase with immunogenic properties

Leishmania donovani is the major cause of visceral leishmaniasis (kala-azar), now recognized as the parasitic disease with the highest level of mortality second only to malaria. No human vaccine is currently available. A 36 kDa L. donovani nucleoside hydrolase (LdNH36) surface protein has been previously identified as a potential vaccine candidate antigen. Here we present data on the expression of LdNH36 in Pichia pastoris and its purification at the 20 L scale to establish suitability for future pilot scale manufacturing. To improve efficiency of process development and ensure reproducibility, 4 N-linked glycosylation sites shown to contribute to heterogeneous high-mannose glycosylation were mutated to glutamine residues. The mutant LdNH36 (LdNH36-dg2) was expressed and purified to homogeneity. Size exclusion chromatography and light scattering demonstrated that LdNH36-dg2 existed as a tetramer in solution, similar to the wild-type recombinant L. major nucleoside hydrolase. The amino acid mutations do not affect the tetrameric interface as confirmed by theoretical modeling, and the mutated amino acids are located outside the major immunogenic domain. Immunogenic properties of the LdNH36-dg2 recombinant protein were evaluated in BALB/c mice using formulations that included a synthetic CpG oligodeoxynucleotide, together with a microparticle delivery platform (poly(lactic-co-glycolic acid)). Mice exhibited high levels of IgG1, IgG2a, and IgG2b antibodies that were reactive to both LdNH36-dg2 and LdNH36 wild-type. While the point mutations did affect the hydrolase activity of the enzyme, the IgG antibodies elicited by LdNH36-dg2 were shown to inhibit the hydrolase activity of the wild-type LdNH36. The results indicate that LdNH36-dg2 as expressed in and purified from P. pastoris is suitable for further scale-up, manufacturing, and testing in support of future first-in-humans phase 1 clinical trials.

Expression, purification, immunogenicity, and protective efficacy of a recombinant Tc24 antigen as a vaccine against Trypanosoma cruzi infection in mice

The Tc24 calcium binding protein from the flagellar pocket of Trypanosoma cruzi is under evaluation as a candidate vaccine antigen against Chagas disease. Previously, a DNA vaccine encoding Tc24 was shown to be an effective vaccine (both as a preventive and therapeutic intervention) in mice and dogs, as evidenced by reductions in T. cruzi parasitemia and cardiac amastigotes, as well as reduced cardiac inflammation and increased host survival. Here we developed a suitable platform for the large scale production of recombinant Tc24 (rTc24) and show that when rTc24 is combined with a monophosphoryl-lipid A (MPLA) adjuvant, the formulated vaccine induces a Th1-biased immune response in mice, comprised of elevated IgG2a antibody levels and interferon-gamma levels from splenocytes, compared to controls. These immune responses also resulted in statistically significant decreased T. cruzi parasitemia and cardiac amastigotes, as well as increased survival following T. cruzi challenge infections, compared to controls. Partial protective efficacy was shown regardless of whether the antigen was expressed in Escherichia coli or in yeast (Pichia pastoris). While mouse vaccinations will require further modifications in order to optimize protective efficacy, such studies provide a basis for further evaluations of vaccines comprised of rTc24, together with alternative adjuvants and additional recombinant antigens.

Yeast-expressed recombinant protein of the receptor-binding domain in SARS-CoV spike protein with deglycosylated forms as a SARS vaccine candidate

Development of vaccines for preventing a future pandemic of severe acute respiratory syndrome (SARS) caused by SARS coronavirus (SARS-CoV) and for biodefense preparedness is urgently needed. Our previous studies have shown that a candidate SARS vaccine antigen consisting of the receptor-binding domain (RBD) of SARS-CoV spike protein can induce potent neutralizing antibody responses and protection against SARS-CoV challenge in vaccinated animals. To optimize expression conditions for scale-up production of the RBD vaccine candidate, we hypothesized that this could be potentially achieved by removing glycosylation sites in the RBD protein. In this study, we constructed two RBD protein variants: 1) RBD193-WT (193-aa, residues 318-510) and its deglycosylated forms (RBD193-N1, RBD193-N2, RBD193-N3); 2) RBD219-WT (219-aa, residues 318-536) and its deglycosylated forms (RBD219-N1, RBD219-N2, and RBD219-N3). All constructs were expressed as recombinant proteins in yeast. The purified recombinant proteins of these constructs were compared for their antigenicity, functionality and immunogenicity in mice using alum as the adjuvant. We found that RBD219-N1 exhibited high expression yield, and maintained its antigenicity and functionality. More importantly, RBD219-N1 induced significantly stronger RBD-specific antibody responses and a higher level of neutralizing antibodies in immunized mice than RBD193-WT, RBD193-N1, RBD193-N3, or RBD219-WT. These results suggest that RBD219-N1 could be selected as an optimal SARS vaccine candidate for further development.