An experimental vaccine against one type of Group B strep
stimulated strong immune responses in human volunteers. Later, in
laboratory experiments, antibodies isolated from the volunteers
neutralized the same type of Group B strep bacteria and prevented
infection in newborn mice that were exposed to it.
"We are very excited about this important clinical finding," says
Pamela McInnes, D.D.S., project officer for NIAID's Group B
Streptococcal Initiative, a program begun in 1992 to develop a Group
B strep vaccine. "This work provides the most promising evidence to
date that we're getting closer to finding an effective vaccine."
Ten to 30 percent of all women are asymptomatic carriers of
Group B strep, harboring the bacteria in their genital tracts. During
childbirth, the bacteria is transmitted to approximately half of all
infants born to these women. Nearly two of every 1,000 infants in the
United States develop invasive infections, which can cause
pneumonia, meningitis and other serious illnesses, usually within the
first three months of life. Half of all infants who develop Group B
strep meningitis experience long-term neurologic problems, including
seizure disorders and mental retardation. About 10 percent of
infected infants die.
"Studies have shown that babies who get Group B strep
disease are born to women who lack antibodies to the bacteria,"
explains Dennis L. Kasper, M.D., lead author of the current study.
"The good news, however, is that women who have antibodies to
Group B strep pass those antibodies to their infants during
pregnancy. Those antibodies protect the infants from infection after
they are born.
For more than a decade, Dr. Kasper and his colleagues at
Brigham and Women's Hospital in Boston have tried to develop a
vaccine that would protect infants from Group B strep by stimulating
the production of antibodies in pregnant women. Theoretically, the
maternal immunity generated by such a vaccine would cross the
placental membranes and protect the newborn for the first few
months of its life, when most Group B strep disease occurs.
The Group B strep bacterium is enveloped in a complex sugar
molecule called a polysaccharide capsule. Because it is known to
play a key role in stimulating the production of antibodies to Group B
strep, the capsule is a logical vaccine candidate. However, previous
studies supported by NIAID found that immunization with the purified
capsule molecule produced insufficient amounts of antibody in human
volunteers. Those studies led Dr. Kasper and his colleagues to try to
boost the vaccine's performance by chemically linking, or conjugating,
the capsule to tetanus toxoid, a protein that has been used to
increase the immune-stimulating properties of several other vaccines.
In the current study the researchers compared this so-called
conjugate vaccine with its predecessor. Under the direction of Carol
J. Baker, M.D., an NIAID-funded investigator at Baylor College of
Medicine in Houston, 100 women of child-bearing age received either
the conjugate vaccine, the pure polysaccharide vaccine or a placebo
injection. The conjugate vaccine stimulated the production of much
higher levels of antibody than did the pure polysaccharide vaccine. In
addition, linking the capsule to the tetanus toxoid protein did not
affect the function of the resulting antibodies--in test tube experiments,
antibodies produced by either vaccine neutralized Group B strep
Dr. Kasper, Dr. Baker and their colleagues then injected
pregnant mice with antibodies isolated from women immunized with
the conjugate vaccine. Upon exposure to Group B strep bacteria,
nearly three-fourths of the offspring born to these mice were
protected from infection. Offspring born to mice that had been
injected during pregnancy with human serum lacking Group B strep
antibodies died after exposure to the bacteria.
"These findings demonstrate that the antibodies produced by
the conjugate vaccine are able to cross the placental membrane and
could confer protection against Group B strep to the fetus," says Dr.
The vaccine used in the current study was a monovalent
product--designed to protect against just one of the various types of
Group B strep that cause disease in infants. Ultimately, a multivalent
vaccine, providing protection against all types, will be needed.
Acknowledging that much more work remains before a Group B strep
vaccine reaches the marketplace, Dr. Kasper says that the conjugate
vaccine developed by his group provides a blueprint for subsequent
"We're definitely headed in the right direction," he says. "This
is a prototype of what Group B strep vaccines will look like."
Prior to developing a multivalent vaccine, Dr. Kasper and his
colleagues are constructing monovalent conjugate vaccines against
other types of Group B strep and testing them in animals and
humans. Preliminary results, he says, have been encouraging. A
clinical trial using two monovalent vaccines in combination could
begin within the next few months. Such studies, he explains, are
necessary to ensure that the immune response to one type does not
adversely affect the immune response to another.
NIAID, a component of the National Institutes of Health (NIH),
conducts and supports research to prevent, diagnose and treat
illnesses such as AIDS and other sexually transmitted diseases,
tuberculosis, asthma and allergies. NIH is an agency of the Public
Health Service, U.S. Department of Health and Human Services.
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