Salmonella has a bad name, which it richly deserves.
One species of the intestinal bacterium, S. typhimurium, inflicts food poisoning on people who swallow it in contaminated food or drink. Another, S. typhi, brings on typhoid fever.
But for all its crimes and misdemeanors, the Salmonella pathogen, stripped of its virulence, is widely earmarked as a DNA-delivery vector in multi-targeting vaccines of the future.
Such polyvalent vaccines — able to protect against diseases caused by a variety of microorganisms, not just the one being used as a carrier — are on drawing boards in many countries.
"Among the microorganisms that have been used for that purpose," observed microbiologist Jorge Galan, at the State University of New York, in Stony Brook, "Salmonella is probably the one that offers the most hope, and is probably closer to being developed for use in the general public.
"However," he added, "the use of Salmonella as a carrier of other antigens from other microorganisms involves a limitation that stems from the biology of this organism when it encounters the cells of the body. That is, Salmonella has the ability to enter into the cells of its human host and remain sequestered inside a compartment — a phagocytic vacuole."
Research Targets Antibody Plus CTL Response
The problem that poses for vaccinologists, he explained, is the immune system packs a double-barreled response to foreign antigens, one humoral, the other cellular. Its B cells seek out the enemy with antibodies; the T cells with killer CTLs — cytotoxic T lymphocytes.
"When scientists have made Salmonella produce proteins, let's say, from other bacteria, or from viruses," Galan continued, "the type of immune responses one usually gets tend to lead to the production of antibodies, rather than of CTLs. That obviously presents a problem, because for example, in the case of viral infections, in order to achieve protection from infection, you really need to have cytotoxic T cells. Viruses live inside the cells they invade, and an antibody cannot reach those compartments. So you need to generate CTLs that will spot and destroy such virally-infected cells.
"What our work has done," Galan told BioWorld Today, is overcome that limitation. We have made use of a fascinating little organelle that Salmonella has, which we recently discovered. It's called that pathogen's type III secretion apparatus."
Galan is senior author of a paper in today's Science, dated July 24, 1998, titled "Delivery of epitopes by the Salmonella type III secretion system for vaccine development."
He described Salmonella's secret weapon as "basically some sort of biochemical syringe, that is capable of injecting five or six antigenic proteins into the cytoplasm of its target cells, to make the cell do certain things that are good for Salmonella. Its secretion syringe can inject proteins into the host cell when it's outside of the cell.
"Those proteins induce a response in the cell, which will allow the pathogen to go inside," he went on. "But even once it's inside this compartment, Salmonella will continue using its syringe to deliver the proteins to the other side of the vacuole membrane, out into the cytoplasm of the cell.
"What's novel in this work," he added, "is reporting our use of the bacterium's own syringe to deliver antigens into the host's cells."
"In designing our vaccine," Galan continued, "we harnessed those proteins, which usually end up in the cytoplasm of the cell, to fragments of recombinant cDNA protein sequences from viruses. Thus we achieved the delivery of these viral epitopes into the cytoplasm of the infected cells. And therefore, due to the biology of antigen presentation, proteins that end up in that cellular compartment are processed in a way that leads to the production of CTLs directed at those types of epitopes."
First, Galan and his co-authors demonstrated mice vaccinated with a non-virulent S. typhimurium vector carrying a protein from the influenza virus produced CTLs to that viral epitope.
"Then," he recounted, "we determined whether this type of response was able to protect the mouse. We had to go to a different system because influenza virus really doesn't cause disease in mice. We turned to a virus that is very lethal to mice, murine lymphocytic choriomeningitis virus (LCMV)."
They immunized mice via a tube into their stomachs, which completely protected them against a subsequent lethal dose of LCMV.
HIV Vaccines Among Targets
Galan and his team are now embarked on "developing this system, which allows the construction of polyvalent vaccines that hopefully will be able to protect humans against several viral infections."
Their next step in this direction, Galan related, "is trying to put in antigenic protein fragments from other viruses, including HIV. But first, we would like to express proteins from SIV, the simian immunodeficiency virus, and see if we could use a Salmonella system like this in the context of this monkey equivalent of AIDS, to induce protection.
"And of course," Galan concluded, "we would like to test a way we have devised to induce both types of immune responses, CTL and antibody, so it becomes a very versatile system."
His paper ends with this prospect: "In addition, avirulent S. typhimurium expressing tumor-specific antigens may allow use of this system for treatment of neoplastic diseases by induction of cancer-cell-specific. . . CTLs." *