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Infection and Immunity, April 1999, p. 1694-1701, Vol. 67, No. 4
Division of Infectious Diseases,
Massachusetts General Hospital, Boston, Massachusetts
021141; Department of Microbiology and
Immunology, Tulane University Medical Center, New Orleans, Louisiana
701122; and Department of
Microbiology and Molecular Genetics, Harvard Medical School,
Boston, Massachusetts 021153
Received 20 August 1998/Returned for modification 4 November
1998/Accepted 31 December 1998
Vibrio cholerae secretes cholera toxin (CT) and the
closely related heat-labile enterotoxin (LT) of Escherichia
coli, the latter when expressed in V. cholerae.
Both toxins are also potent immunoadjuvants. Mutant LT molecules that
retain immunoadjuvant properties while possessing markedly diminished
enterotoxic activities when expressed by E. coli have
been developed. One such mutant LT molecule has the substitution of a
glycine residue for arginine-192 [LT(R192G)]. Live
attenuated strains of V. cholerae that have been used
both as V. cholerae vaccines and as vectors for
inducing mucosal and systemic immune responses directed against
expressed heterologous antigens have been developed. In order to
ascertain whether LT(R192G) can act as an immunoadjuvant
when expressed in vivo by V. cholerae, we introduced a
plasmid (pCS95) expressing this molecule into three vaccine strains of
V. cholerae, Peru2, ETR3, and JRB14; the latter two
strains contain genes encoding different heterologous antigens in the
chromosome of the vaccine vectors. We found that LT(R192G)
was expressed from pCS95 in vitro by both E. coli and
V. cholerae strains but that LT(R192G) was detectable in the supernatant fraction of V. cholerae
cultures only. In order to assess potential immunoadjuvanticity, groups of germfree mice were inoculated with the three V. cholerae vaccine strains alone and compared to groups inoculated
with the V. cholerae vaccine strains supplemented with
purified CT as an oral immunoadjuvant or V. cholerae
vaccine strains expressing LT(R192G) from pCS95. We found
that mice continued to pass stool containing V. cholerae strains with pCS95 for at least 4 days after oral
inoculation, the last day evaluated. We found that inoculation with
V. cholerae vaccine strains containing pCS95 resulted
in anti-LT(R192G) immune responses, confirming in vivo
expression. We were unable to detect immune responses directed against
the heterologous antigens expressed at low levels in any group of
animals, including animals that received purified CT as an
immunoadjuvant. We were, however, able to measure increased vibriocidal
immune responses against vaccine strains in animals that received
V. cholerae vaccine strains expressing LT(R192G) from pCS95 compared to the responses in animals
that received V. cholerae vaccine strains alone. These
results demonstrate that mutant LT molecules can be expressed in vivo
by attenuated vaccine strains of V. cholerae and that
such expression can result in an immunoadjuvant effect.
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
In Vivo Expression and Immunoadjuvancy of a Mutant of Heat-Labile
Enterotoxin of Escherichia coli in Vaccine and Vector
Strains of Vibrio cholerae
*
Corresponding author. Mailing address: Division of
Infectious Diseases, Massachusetts General Hospital, Boston, MA 02114. Phone: 617-726-3811. Fax: 617-726-7416. E-mail:
scalderwood{at}partners.org.
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