Purification, immunogenicity and protective potency of the F1 antigen from Yersinia pestis.
Yersinia pestis, an organism endemic in much of the world, is the causative agent of
pneumonic and bubonic plague. The disease is severe with a high mortality rate. Most
plague vaccines are formalised whole cells, which give limited and short-lived immunity.
An acellular vaccine could prove to be more effective than using killed whole cells,
without the associated side effects.
The 17.5kDa PI antigen of Y. pestis has been shown to be immunogenic and protective in
mice (Simpson et at., 1990) and therefore it might be a suitable component of an acellular
vaccine. In this study, the conditions for the production and purification of Fl from
culture of Y. pestis were examined with a view to increasing the yield of FI antigen. The
optimal conditions were to grow Y. pestis MRE 1447 in a chemically defined medium at
pH7.4 for 48h at 37°C and these conditions were transferred to a 25-litre fermenter for the
larger-scale production ofFI antigen. Typically, 40mg ofFI antigen of~90% purity was
obtained from lL of culture supernatant. The purified Fl was characterised using
biochemical, structural and immunological methods to confirm the integrity of the
purified antigen. Immunological methods using anti-Fl monoclonal and polyclonal
antibody identified the purified antigen as Fl. Structural studies on the glycosylation of
the protein showed that no post-translational glycosylation of the protein could be
detected by the methods used in this study. The cafl sequence was successfully cloned
into Escherichia coli to produce a recombinant Fl antigen. This recombinant antigen
proved to be both immunogenic and protective but yields of recombinant protein were
Purified culture-derived native Fl and a recombinant V antigen (produced at CBD,
DERA) were used to assess controlled-release vaccine delivery systems in poly(lactideco-
glycolide) PLG micro spheres and liposomes. Recombinant V was used in addition to
FI to assess a combination subunit vaccine with FI given alone as an acellular vaccine.
FI antigen encapsulated in PLG microspheres induced high serum antibody titres when
injected i.p. in mice; mucosal IgA was also detected. Mice immunised with Fl in
Alhydrogel or PLG micro spheres were protected against sub-cutaneous challenge with Y
pestis. FI antigen surface-labeled onto liposome vesicles stimulated high serum titres in
Balb/c mice; a mucosal response was also induced and mice were protected against subcutaneous
challenge with up to 1 x 105 organisms. This strongly indicated that
immunising with Fl formulated in PLG micro spheres and liposomes induced a potent
immune response and that protection was achieved after only one dose. However,
increased protection was observed when a combination vaccine of Fl and rV was
administered in PLG microspheres, liposomes or Alhydrogel given in multiple doses.
The efficacy of different delivery routes against sub-cutaneous Y. pestis challenge was
also investigated. Administration by the i.p. route gave consistent protection against subcutaneous
plague challenge. The i.n. route also proved effective for both PLG
microspheres and liposomes when multiple doses were used. Oral delivery failed to give
adequate protection against sub-cutaneous infection, even after multiple doses.