Secretory expression of recombinant diphtheria toxin mutants in B. Subtilis

Hosts and Heterologous Expression Strategies of Recombinant Toxins for Therapeutic Purposes

The production of therapeutic recombinant toxins requires careful host cell selection. Bacteria, yeast, and mammalian cells are common choices, but no universal solution exists. Achieving the delicate balance in toxin production is crucial due to potential self-intoxication. Recombinant toxins from various sources find applications in antimicrobials, biotechnology, cancer drugs, and vaccines. "Toxin-based therapy" targets diseased cells using three strategies. Targeted cancer therapy, like antibody-toxin conjugates, fusion toxins, or "suicide gene therapy", can selectively eliminate cancer cells, leaving healthy cells unharmed. Notable toxins from various biological sources may be used as full-length toxins, as plant (saporin) or animal (melittin) toxins, or as isolated domains that are typical of bacterial toxins, including Pseudomonas Exotoxin A (PE) and diphtheria toxin (DT). This paper outlines toxin expression methods and system advantages and disadvantages, emphasizing host cell selection's critical role.

Heterologous Expression of Recombinant Proteins and Their Derivatives Used as Carriers for Conjugate Vaccines

Carrier proteins that provide an effective and long-term immune response to weak antigens has become a real breakthrough in the disease prevention, making it available to a wider range of patients and making it possible to obtain reliable vaccines against a variety of pathogens. Currently, research is continuing both to identify new peptides, proteins, and their complexes potentially suitable for use as carriers, and to develop new methods for isolation, purification, and conjugation of already known and well-established proteins. The use of recombinant proteins has a number of advantages over isolation from natural sources, such as simpler cultivation of the host organism, the possibility of modifying genetic constructs, use of numerous promoter variants, signal sequences, and other regulatory elements. This review is devoted to the methods of obtaining both traditional and new recombinant proteins and their derivatives already being used or potentially suitable for use as carrier proteins in conjugate vaccines.

High-Level Production of Soluble Cross-Reacting Material 197 in Escherichia coli Cytoplasm Due to Fine Tuning of the Target Gene's mRNA Structure

Cross-reacting material 197 (CRM197) is a non-toxic mutant of the diphtheria toxin and is widely used as a carrier protein in conjugate vaccines. This protein was first obtained from the supernatant of the mutant Corynebacterium diphtheriae strain. This pathogenic bacteria strain is characterized by a slow growth rate and a relatively low target protein yield, resulting in high production costs for CRM197. Many attempts have been made to establish high-yield protocols for the heterologous expression of recombinant CRM197 in different host organisms. In the present work, a novel CRM197-producing Escherichia coli strain was constructed. The target protein was expressed in the cytoplasm of SHuffle T7 E. coli cells without any additional tags and with a single potential mutation-an additional Met [-1]. The fine tuning of the mRNA structure (the disruption of the single hairpin in the start codon area) was sufficient to increase the CRM197 expression level several times, resulting in 150-270 mg/L (1.1-2.0 mg/g wet biomass) yields of pure CRM197 protein. Besides the high yield, the advantages of the obtained expression system include the absence of the necessity of CRM197 refolding or tag removal. Thus, an extensive analysis of the mRNA structure and the removal of the unwanted hairpins in the 5' area may significantly improve the target protein expression rate.

Production and purification of an active CRM197 in Pichia pastoris and its immunological characterization using a Vi-typhoid antigen vaccine

CRM197 is a commonly used glycoconjugate carrier that improves the immunogenicity of vaccines, particularly in infants. Despite the advantages of this diphtheria toxoid mutant, low yields, production in inclusion bodies, and the requirement for specific growth conditions have limited the breadth of successful recombinant protein expression platforms available for its expression. We evaluated Pichia pastoris as a production host, using the methanol inducible AOX1 promoter and a modified α-mating factor signal peptide for secretion into the supernatant. Final purified yields >100 mg L^-1 culture were achieved when produced in a bioreactor, which is equivalent to the productivity obtained from bioprocesses using the native Corynebacterium diphtheriae host. Recombinant CRM197 was purified to ≥95% homogeneity and showed the expected endonuclease activity. Furthermore, mice immunized with a Salmonella enterica serovar Typhi capsular Vi antigen conjugated to our recombinant CRM197 showed greater than 5-fold increase in immune response. Overall, the results demonstrate that Pichia pastoris is a suitable expression host for the production of high quality CRM197 for vaccine applications.

Efficient recovery of recombinant CRM197 expressed as inclusion bodies in E.coli

CRM197, which retains the same inflammatory and immune-stimulant properties as diphtheria toxin but with reduced toxicity, has been used as a safe carrier in conjugated vaccines. Expression of recombinant CRM197 in E. coli is limited due to formation of inclusion bodies. Soluble expression attempts in Bacillus subtilis, P. fluorescens, Pichia pastoris, and E. coli were partially unsuccessful or did not generate yields sufficient for industrial scale production. Multiple approaches have been attempted to produce CRM197 in E. coli, which has attractive features such as high yield, simplicity, fast growth, etc., including expression of oxidative host, concurrent expression of chaperones, or periplasmic export. Recently, alternative methods for recovery of insoluble proteins expressed in E. coli were reported. Compared to traditional denaturation/refolding, these methods used the non-denaturing solubilization agent, N-lauroylsarkosine to obtain higher recovery yields of native proteins. Based on this work, here, we focused on solubilization of CRM197 from E. coli inclusion bodies. First, CRM197 was expressed as inclusion bodies by high-level expression of recombinant CRM197 in E. coli (126.8 mg/g dcw). Then bioactive CRM197 was isolated from these inclusion bodies with high yield (108.1 mg/g dcw) through solubilization with N-lauroylsarkosine including Triton X-100 and CHAPS, and purified by Ni-affinity chromatography and size-exclusion chromatography. In this study, we present a cost-effective alternative for the production of bioactive CRM197 and compare our recovery yield with yields in other production processes.

Molecular Cloning, Structural Modeling and the Production of Soluble Triple-Mutated Diphtheria Toxoid (K51E/G52E/E148K) Co-expressed with Molecular Chaperones in Recombinant Escherichia coli

CRM197 is a diphtheria toxin (DT) mutant (G52E) which has been used as a carrier protein for conjugate vaccines. However, it still possesses cytotoxicity toward mammalian cells. The goal of this project was to produce a non-toxic and soluble CRM197EK through introduction of triple amino acid substitutions (K51E/G52E/E148K) in Escherichia coli. The expression of CRM197EKTrxHis was optimized and co-expressed with different molecular chaperones. The soluble CRM197EKTrxHis was produced at a high concentration (97.33 ± 17.47 μg/ml) under the optimal condition (induction with 0.1 mM IPTG at 20 °C for 24 h). Cells containing pG-Tf2, expressing trigger factor and GroEL-GroES, accumulated the highest amount of soluble CRM197EKTrxHis at 111.24 ± 10.40 μg/ml after induction for 24 h at 20 °C. The soluble CRM197EKTrxHis still possesses nuclease activity and completely digest λDNA at 25 and 37 °C with 8- and 4-h incubation, respectively. Molecular modeling of diphtheria toxin, CRM197 and CRM197EK indicated that substitutions of two amino acids (K51E/E148K) may cause poor NAD binding, consistent with the lack of toxicity. Therefore, CRM197EK might be used as a new potential carrier protein. However, further in vivo study is required to confirm its roles as functional carrier protein in conjugate vaccines.

High level accumulation of soluble diphtheria toxin mutant (CRM197) with co-expression of chaperones in recombinant Escherichia coli

CRM197 is the diphtheria toxin mutant used in many conjugate vaccines. A fusion CRM197 (fCRM197) containing all the tags conferred by the pET32a vector was produced as a soluble protein in Escherichia coli co-expressing several chaperone proteins in conjunction with low temperature cultivation. Trigger factor (Tf) enhanced formation of soluble fCRM197 (150.69 ± 8.95 μg/mL) to a greater degree than other chaperones when fCRM197 expression was induced at 25 °C for 12 h. However, prolonged cultivation resulted in a progressive reduction of fCRM197 accumulation. In contrast, at 15 °C cells, with or without Tf, fCRM197 accumulated to the highest level at 48 h (153.70 ± 13.14 μg/mL and 150.07 ± 8.13 μg/mL, respectively). Transmission electron microscopy (TEM) demonstrated that the formation of inclusion protein as well as cell lysis was reduced in cultures grown at 15 °C. Cell viability was substantially reduced in cells expressing Tf, compared to cultures without Tf, when fCRM197 was induced at 25 °C. The viability of Tf-expressing cells was enhanced when cultured at 15 °C. Both purified fCRM197 and CRM197 efficiently digested lambda DNA (λDNA) at 37 °C (92.78 and 97.45 %, respectively). Digestion efficiency of fCRM197 and CRM197 was reduced at 25 °C (80.80 and 62.73 %, respectively) and at 15 °C (7.34 and 24.79 %, respectively). These results demonstrating nuclease activity, enhanced cell lysis, and reduced cell viability are consistent with the finding of lower fCRM197 yield when cultivation and induction times were prolonged at 25 °C. The present work provides a procedure for the high-level production of soluble fCRM197 using E. coli as a heterologous host.