Immunization of mice with YscF provides protection from Yersinia pestis infections

Abstract

Background: Yersinia pestis, the causative agent of plague, is a pathogen with a tremendous ability to cause harm and panic in populations. Due to the severity of plague and its potential for use as a bioweapon, better preventatives and therapeutics for plague are desirable. Subunit vaccines directed against the F1 capsular antigen and the V antigen (also known as LcrV) of Y. pestis are under development. However, these new vaccine formulations have some possible limitations. The F1 antigen is not required for full virulence of Y. pestis and LcrV has a demonstrated immunosuppressive effect. These limitations could damper the ability of F1/LcrV based vaccines to protect against F1-minus Y. pestis strains and could lead to a high rate of undesired side effects in vaccinated populations. For these reasons, the use of other antigens in a plague vaccine formulation may be advantageous.

Results: Desired features in vaccine candidates would be antigens that are conserved, essential for virulence and accessible to circulating antibody. Several of the proteins required for the construction or function of the type III secretion system (TTSS) complex could be ideal contenders to meet the desired features of a vaccine candidate. Accordingly, the TTSS needle complex protein, YscF, was selected to investigate its potential as a protective antigen. In this study we describe the overexpression, purification and use of YscF as a protective antigen. YscF immunization triggers a robust antibody response to YscF and that antibody response is able to afford significant protection to immunized mice following challenge with Y. pestis. Additionally, evidence is presented that suggests antibody to YscF is likely not protective by blocking the activity of the TTSS.

Conclusion: In this study we investigated YscF, a surface-expressed protein of the Yersinia pestis type III secretion complex, as a protective antigen against experimental plague infection. Immunization of mice with YscF resulted in a high anti-YscF titer and provided protection against i.v. challenge with Y. pestis. This is the first report to our knowledge utilizing a conserved protein from the type III secretion complex of a gram-negative pathogen as a candidate for vaccine development.

Figure 1

Purified HT-YscF was run on SDS-PAGE gels (12.5% for Panels A and B, 15% Panel C) to analyze purity of the recovered protein. Panel A, separated proteins were stained with GelCode Blue stain (Pierce Chemical). Panel B, purified HT-YscF was immunoblotted to PVDF and probed with antibody specific for penta-histidine to identify which bands contained the His-tag. Panel C, purified HT-YscF was immunoblotted to PVDF and probed with post-immune serum from immunized mice.

Figure 2

Derivatives of Yersinia pestis KIM8-3002 (KIM5 pPCP1-minus, Sm^r) were grown in a chemically defined medium at 26°C for 2 h in the presence (lanes 1, 3, and 5) or absence of calcium (lanes 2, 4, and 6). Lanes 1 and 2 contain Y. pestis KIM8-3002. Lanes 3 and 4 contain Y. pestis KIM8-3002 ΔyscF expressing YscF from pBAD18-YscF. Lanes 5 and 6 contain Y. pestis KIM8-3002 ΔyscF gene. After the 2 h growth, the culture was shifted to 37°C to induce expression of the Ysc type III secretion system and the Low Calcium Response. Following 4 h of growth at 37°C cultures, were centrifuged to obtain whole cell fractions and cell-free culture supernatant fractions. Total proteins from each fraction were precipitated with 10% trichloro acetic acid. Dried proteins were resuspended in SDS-PAGE sample buffer and electrophoresed in a 15% SDS-PAGE gel. Proteins were then transferred to a PVDF membrane and immuno-blotted with pooled mouse serum used at a 1:20,000 dilution. Mouse serum was obtained by bleeding mice subsequent to immunization with HT-YscF, serum from several mice was pooled to control for animal specific variation. The position and sizes for the molecular weight markers are indicated and the position of YscF is shown.

Figure 3

Y. pestis strain KIM8-3002 (wt) (lane 1 and lanes 3–6) and KIM8-3002.2 (ΔyopD) (lane 2 and lanes 7–10) both containing plasmid pYopE₁₂₉-Elk were used to infect HeLa cells at an MOI of 10. 4 h following infection the culture supernatant containing non-adherent bacteria were removed the remaining adherent HeLa cells were solubilized in 2X SDS-PAGE buffer. Following solubilization proteins were separated and immunoblotted to triplicate PVDF membranes. The triplicate blots were probed with α-YopE, α-Elk, or α-PO₄-Elk, followed by incubation with an alkaline phosphatase conjugated secondary antibody and developed using NBT/BCIP. To some samples anti-sera specific for HT-YscF (lanes 3–4 and lanes 7–8) or the Pseudomonas protein, PcrG (lanes 5–6 and lanes 9–10), were added at dilutions of 1:10 (lanes 3, 5, 7, and 9) or 1:25 (lanes 4, 6, 8, and 10).