Effect of deletion of the lpxM gene on virulence and vaccine potential of Yersinia pestis in mice

1 State Research Center for Applied Microbiology and Biotechnology, Obolensk 142279, Moscow Region, Russia 2 Russian Research Anti-Plague Institute ‘Microbe’, Saratov 410071, Russia 3 N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 119991, Russia 4 Research Center Borstel, Leibniz Center for Medicine and Biosciences, D-23845 Borstel, Germany 5 Channing Laboratory, Brigham and Women's Hospital, Harvard Medical School, Boston MA 02115, USA Correspondence Andrey P. Anisimov anisimov{at}obolensk.org Received 8 August 2006 Accepted 12 December 2006 Yersinia pestis undergoes an obligate flea–rodent–flea enzootic life cycle. The rapidly fatal properties of Y. pestis are responsible for the organism's sustained survival in natural plague foci. Lipopolysaccharide (LPS) plays several roles in Y. pestis pathogenesis, prominent among them being resistance to host immune effectors and induction of a septic-shock state during the terminal phases of infection. LPS is acylated with 4–6 fatty acids, the number varying with growth temperature and affecting the molecule's toxic properties. Y. pestis mutants were constructed with a deletion insertion in the lpxM gene in both virulent and attenuated strains, preventing the organisms from synthesizing the most toxic hexa-acylated lipid A molecule when grown at 25 °C. The virulence and/or protective potency of pathogenic and attenuated Y. pestis lpxM mutants were then examined in a mouse model. The lpxM mutation in a virulent strain led to no change in the LD 50 value compared to that of the parental strain, while the lpxM mutation in attenuated strains led to a modest 2.5–16-fold reduction in virulence. LPS preparations containing fully hexa-acylated lipid A were ten times more toxic in actinomycin D-treated mice then preparations lacking this lipid A isoform, although this was not significant ( P >0.05). The lpxM mutation in vaccine strain EV caused a significant increase in its protective potency. These studies suggest there is little impact from lipid A modifications on the virulence of Y. pestis strains but there are potential improvements in the protective properties in attenuated vaccine strains. Abbreviations: CI, 95 % confidence interval; ESI FT-ICR, electrospray ionization Fourier transform ion cyclotron resonance; ImD 50 , 50 % immunizing dose; LA hexa , hexa-acyl lipid A; LA tetra , tetra-acyl lipid A; LPS, lipopolysaccharide.