2 December 2002 17:00 GMT

by Clyde Burnham

Transmission electron micrograph of Escherichia coli O157:H7, CDC/Elizabeth H. White, M.S.

A transatlantic research team has transformed the notorious bacterium Escherichia coli into a promising therapeutic agent in the fight against cancer. Darren Higgins, a microbiologist at Harvard Medical School, and colleagues in England, used the novel method to prevent the growth of melanoma cells in mice.

The researchers engineered a recombinant strain of E. coli to express the proteins listeriolysin and ovalbumin. This recombinant bacterium could then induce dendritic cells to efficiently present ovalbumin to cytotoxic T cells in mice. Cytotoxic T cells, the mainstay of so-called cell-mediated immunity, activated in this way then prevent the growth of ovalbumin-labeled melanoma cells in mice.

Cancers succeed by evading cell-mediated immune attack, so Higgins' approach provided a way of preventing that evasion - by efficiently presenting tumor antigen and activating cytotoxic T cells to kill tumor cells. Antigens presented to dendritic cells have to trigger maturation of the dendritic cell, otherwise the result is tolerance of, rather than immune attack on, the antigen. Higgins and colleagues used E. coli which, as with all bacteria, drive dendritic cells to mature. The matured dendritic cells then present the E. coli antigens to cytotoxic T cells. This activates the cytotoxic T cells, which will then attack and lyse cells with those antigens.

Higgins selected a mouse melanoma strain labeled with an ovalbumin marker antigen as their target cell. He and his colleagues engineered E. coli to express ovalbumin and exposed mouse dendritic cells to the recombinant bacteria. The dendritic cells phagocytosed the E. coli and were driven into maturation. The matured dendritic cells then presented the ovalbumin protein to cytotoxic T cells. The cytotoxic T cells thus produced were only slightly more toxic to the ovalbumin-labeled melanoma than were naive cytotoxic T cells.

However, when Higgins engineered E. coli to express ovalbumin and listeriolysin, the outcome changed. Listeriolysin is a pore-forming cytolysin from the bacteria Listeria monocytogenes. When E. coli cells containing ovalbumin and listeriolysin were phagocytosed by dendritic cells the phagosome was perforated by the listeriolysin, and ovalbumin flooded the dendritic cell's cytosol. This presentation of antigen strongly activated cytotoxic T cells against the ovalbumin. Listeriolysin thus appears to moderate the efficient presentation of ovalbumin antigen to cytotoxic T cells.

Mice vaccinated with dendritic cells exposed to ovalbumin and listeriolysin-labeled E. coli mounted a strong immune response when challenged with ovalbumin-labeled melanoma cells. Mice vaccinated with dendritic cells exposed to an E. coli-ovalbumin recombinant without listeriolysin mounted only a minimal immune response to melanoma cells expressing ovalbumin. The researchers then vaccinated the mice with the E. coli-ovalbumin-listeriolysin recombinant without the dendritic cells and saw a stronger immune response. When they vaccinated mice with paraldehyde-fixed E. coli-ovalbumin-listeriolysin they saw an even stronger cell-mediated response against the ovalbumin-labeled melanoma.

William Robinson, professor of medicine and a cutaneous oncologist at the University of Colorado Health Sciences Center, finds the use of listeriolysin to stimulate cell-mediated toxicity an interesting concept. He notes, however, that human melanoma cells are rapidly mutating and have only minimally antigenic surface markers. He thinks it a long step from attacking the very antigenic ovalbumin to the slightly antigenic cells of human cancer, which are so heterogeneous and unstable.

Ralph Reisfeld, a microbiologist at The Scripps Institute, says that using listeriolysin to enhance presentation of antigen to cytotoxic T cells is a welcome advance. He warns that there are many complications between this interesting work in mice and the creation of a vaccine effective against any human cancer. The use of bacteria as a vector is one of his concerns, as the risk of pathogenicity, either acute or late, exists with any bacterium. The bacteria chosen for a vaccine must be maximally innocuous. He also points out that ovalbumin, the marker antigen used by Higgins, is, in mice, a xeno-antigen. Any human tumor antigen chosen for a vaccine in humans will not be a xeno-antigen and additionally must not provoke immune attack on normal cells.

Ronald Levy, professor of medicine at Stanford School of Medicine, agrees that that this paper offers some interesting possibilities for creating human cancer vaccines. He expressed optimism that tumor markers for at least some cancers would prove to be useful vaccine targets.

Higgins, whose primary interest is in bacterial and viral infection not oncology, noted in earlier work that listeriolysin forced macrophages into a much stronger presentation of antigen to cytotoxic T cells. When discussing this finding with his English co-authors they resolved to use the approach against cancer.

While gratified by this proof-of-concept work, Higgins says his group is planning significant follow-up studies using listeriolysin-based vaccines against bacteria, viruses, and cancers. His group has experience with E. coli, and hence used that bacterium for proof of concept. Choosing the bacterial vector for human vaccines is a major issue, which must be determined not by ease of use but rather safety, he says. He notes that choice of the appropriate target antigen for each variety of cancer is another area that will require much work.

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See also:
Cancer vaccination progress
[Research Update]
Graham Pawelec and Robert C. Rees
Trends in Molecular Medicine, 2002, 8:12:545-546

Tuning tumor-specific T-cell activation: a matter of costimulation?
[Opinion]
Hinrich Abken, Andreas Hombach, Claudia Heuser, Kai Kronfeld and Barbara Seliger
Trends in Immunology, 2002, 23:5:240-245

How bacteria could cause cancer: one step at a time
[Review]
Alistair J. Lax and Warren Thomas
Trends in Microbiology, 2002, 10:6:293-299

Immunity against cancer: lessons learned from melanoma
[Review]
Alan N. Houghton, Jason S. Gold and Nathalie E. Blachere
Current Opinion in Immunology, 2001, 13:2:134-140
 

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