High-level Expression of Tetanus Toxin Fragment C in Escherichia coli

Fragment C is the C-terminal domain of the heavy chain of tetanus toxin that can promote the immune response against the lethal dose of this toxin. Therefore, this portion can be considered as a candidate vaccine against tetanus infection, which occurs by Clostridium tetani. The present study aimed to compare the expression of tetanus toxin fragment C in Escherichia coli BL21 (DE3) pLysS cells having a high tolerance to toxins between two different expression vectors, namely pET22b and pET28a, using the sodium dodecyl sulfate polyacrylamide gel electrophoresis and western blot analyses. After DNA extraction from Harvard CN49205 strain of C. tetani, the gene of interest was amplified using polymerase chain reaction, and then sequenced and cloned into the expression vectors of pET22b and pET28a, transformed into competent BL21 (DE3) pLysS cells, and finally expressed using an optimized protocol. The cells were induced with isopropyl β-D-1-thiogalactopyranoside (IPTG) at four different incubation temperatures (i.e., 37, 33, 30, and 25 °C) and three different incubation times (i.e., 1, 2, and 3 h). Although the SDS-PAGE and western blot analyses confirmed the expression of the recombinant fragment C (r-fragment C) ligated into both of the expression vectors, pET28a showed a higher r-fragment C expression level than the other vector (38.66 mg/L versus 32.33 mg/L, P<0.05). An optimal expression condition was acquired 3 h after 1 mM IPTG induction at 25 °C. The results demonstrated that E. coli BL21 (DE3) pLysS as an expression host in combination with pET-28a as an expression vector was a more compatible expression system to express the fragment C of tetanus toxin, compared to E. coli BL21 (DE3) pLysS/pET-22b expression system. Overall, these results may represent an opportunity to improve the expression system for the production of tetanus toxin vaccine using recombinant protein strategy.