A Live Plaque Vaccine Candidate with Improved Safety and Immunogenecity

Abstract

The live plague vaccine strain Yersinia pestis EV line NIIEG, a well-known derivative of an attenuated pigmentation-negative strain Y. pestis EV76, has been widely used for human and animal immunization in Russia and the FSU countries. The main advantage of the EV NIIEG is its ability to induce rapidly a high level of specific immunity against both bubonic and pneumonic plague following a single injection. However, the vaccine possesses a “residual virulence”, which causes in vaccinees a number of adverse reactions likely mediated by a highly toxic Y. pestis lipopolysaccharide (LPS) with hexa-acylated lipid A. Recently, we generated a lauroyltransferase gene (lpxM) mutant of strain EV NIIEG, which produced a less toxic, penta-acylated lipid A of the LPS. The mutant was found to be harmless, safe, and low reactogenic, and elicited an enhanced immunity against experimental plague, surpassing in these characteristics the parental EV NIIEG strain. A single injection of the mutant protected outbred (57.1–85.7%) and BALB/c (42.8%) mice from subcutaneous challenge with 2,000 median lethal doses (MLD), as well as 25–50% of guinea pigs from 1,200 MLD of wild-type strain Y. pestis 231. The parental strain was not protective in this range of the challenging doses in both mice and guinea pigs. The lpxM mutation caused a marked alteration in expression, immunoreactivity and epitope specificity of major surface proteins and carbohydrate antigens (F1, LcrV, Ymt, Pla, ECA, LPS), and conferred resistance to the plague diagnostic bacteriophage L-413C. The penta-acylated LPS of the lpxM mutant showed a reduced capacity in stimulating the macrophage-like cell line J774.A1 for production of TNF-α. Therefore, the pleiotropic effects of the lpxM mutation enabled the construction of a new live plague vaccine candidate with improved characteristics.