Deletion of Braun lipoprotein gene (lpp) attenuates Yersinia pestis KIM/D27 strain: Role of Lpp in modulating host immune response, NF-κB activation and cell death

The pathogenic species of yersiniae potently blocks immune responses in host cells by using the type III secretion apparatus and its effector proteins. In this study, we characterized potential mechanisms associated with the Braun lipoprotein (Lpp) that contributed to a further attenuation of a pigmentation locus-minus Yersinia pestis KIM/D27 mutant strain and its ability to generate immune responses in mice. The lpp gene encodes one of the major outer membrane lipoproteins that is involved in inflammatory responses and septic shock. We found that sera and splenocytes from Δlpp mutant-immunized mice, when transferred to naïve animals, provided protection to the latter against challenge with a lethal dose of the Y. pestis parental strain. Further, the Δlpp mutant promoted ex vivo a significantly higher interleukin (IL)-2 and interferon-gamma production from T cells of immunized mice, when compared to those from animals infected with the sub-lethal dose of the parental Y. pestis KIM/D27 strain. Likewise, murine primary macrophages infected with the mutant, when compared to those infected with the parental strain in vitro, produced significantly higher IL-12 levels. Importantly, increased nuclear factor-kappa B activation and decreased apoptosis were noted in splenocytes and primary macrophages of mice challenged with the Δlpp mutant, when compared to those in animals infected with the parental Y. pestis KIM/D27 strain. Finally, significantly higher levels of antibodies specific for the parental Y. pestis antigens were developed in mice first immunized with the Δlpp mutant and then challenged with the parental strain, compared to the antibody levels in animals that were immunized and then infected with the parental KIM/D27 strain. To our knowledge, this is the first report of a mechanistic basis for attenuation and immunological responses associated with deletion of the lpp gene from the Y. pestis KIM/D27 strain.