Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2016
Publication Date: N/A
Citation: N/A

Interpretive Summary: Salmonella Typhi Ty21a is a safe and effective oral typhoid strain, which has served as a valuable vaccine platform for the development of vaccines against foodborne shigellosis. However, Ty21a is administered in a capsule formulation to resist stomach acidity, which moderates efficacy and limits uptake in young children. A robust acid-resistant strain of TY21a was developed by transferring genes that enhance acid resistance. The new strain with improved acid-resistance did not develp virulence and remained a successful and safe vaccine strain. Improving acid survivability of the Ty21a vaccine vector could ultimately lead to an improved delivery method, such as a rapidly dissolvable wafer, that could be more child-friendly and could stimulate a more robust immune response. This information will be of interest to the medical community.

Technical Abstract: The licensed oral, live-attenuated bacterial vaccine for typhoid fever, Salmonella Typhi strain Ty21a, has also been utilized as a vaccine delivery platform for expression of diverse foreign antigens that stimulate protection against shigellosis, anthrax, plague, or human papilloma virus. However, Ty21a is acid-labile and, for effective oral immunization, stomach acidity has to be either neutralized with buffer or by-passed with Ty21a in an enteric-coated capsule (ECC). Several studies have shown that efficacy is reduced when Ty21a is administered in an ECC versus as a buffered liquid formulation, the former limiting exposure to GI tract lymphoid tissues. However, the ECC was selected as a more practical delivery format for both shipping and vaccine administration ease. We have sought to increase Ty21a acid-resistance to allow for removal from the ECC and immune enhancement. To improve Ty21a acid-resistance, glutamate-dependent acid resistance genes (GAD; responsible for Shigella survival at very low pH) were cloned on a low copy plasmid (pGad) under a controllable arabinose-inducible promoter. pGad enhanced acid survival of Ty21a by 5 logs after 3 hours at pH 2.5, when cells were pre-grown in arabinose and under conditions that promote an acid-tolerance response (ATR). For 100% stable expression, we inserted the gad genes into the Ty21a chromosome, using a method that allowed for subsequent removal of a selectable antibiotic resistance marker. Further, bacterial survival assays in cultured human monocytes/macrophages suggest that the acid-resistance conferred by expression of the Gad proteins in Ty21a does not alter the existing attenuation of this vaccine strain.