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Naked DNA for malaria vaccines
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for malaria vaccines US Department of Defense and the Diego-based biopharmaceutical company al are gearing up to test a new malaria vaccine olunteers, based on naked-DNA vaccine mology. Phase I and II clinical trials are med to start in October 1997. Vlalaria is a complex parasitic infection and mere are no effective vaccines to prevent this disease, which causes up to two million deaths and infects 500 million people annually. Naked-DNA vaccines are already on trial in people with HIV, but this trial is likely to be the first use of this technology against a parasite infection in humans. In naked-DNA vaccines, highly purified double-stranded plasmid DNA, containingthe pathogen's genetic information and a promoter to switch on expression, are designed to instruct the recipient's cells to make antigenic proteins, and protective immune responses result. Stephen L. Hoffman (Head of the Malaria Programme, Naval Medical Research Institute, Bethesda, MD, USA) intends to use plasmids containing four different genes that encode for parasite proteins expressed in the liver stage of the parasite's life cycle. These are: the circumsporozoite protein, sporozoite surface protein 2 (or Trap), Exp-1 and LSAI. Volunteers in the Washington area (DC, USA) will be immunized and then exposed to mosquitoes carrying the malaria parasite. This first-generation vaccine is designed to stimulate a cytotoxic CD8÷ T-cell response to eliminate infected liver cells; in the second phase, Hoffmann plans to target the blood stage and, in a third phase, to combine the two to produce a potentially more comprehensive treatment. 'Eventually, I envision multiple targets at each stage, mixing them together in an injectable cocktail,' says Hoffman, who says that volume, cost and immunogenicity only will ultimately limit the number of plasmids included. A final product that will provide sustainable protection in the field will also need to take into account the many different strains of malaria. 'Once we have a vaccine that gives 50-70% protection, we will move to overseas sites,' says Hoffman. The US Army and Navy have potential testing sites in Indonesia, Thailand, Egypt, Kenya, Brazil and Peru. Hoffman's group have already shown, using this strategy, that 50-80% of mice can be protected against a challenge of malaria parasites, which are virulent in mice; and the mice produce high levels of specific antibodies and cytotoxic T cells [Sedegah, M. et al. (1994)Proc. Natl Acad. Sci. USA 91,9866-9870]. However, only
one inbred strain of mouse was protected with the protein coded by the DNA in the experimental vaccine, demonstrating that other proteins needed to be investigated. 'When we introduced a gene encoding a newly discovered protein and injected it into the mice, we found that it protected a different strain of mouse from the first one. When we mixed the two plasmids together, we were able to protect three out of the five strains,' explains Hoffman. In humans, genetic variations in human leukocyte antigens (HLAs) are expected to cause similar variations in immune responsiveness, as different peptides derived from the malaria-encoding plasmids bind differently to HLAs expressed on the recipient's cells. How many plasmids may eventually be required to confer complete protection in people is not known, although it is thought that if eight HLA types are targeted, 95% of the human population should be covered. Naked-DNA technology may revolutionize vaccine development. In a matter of days, vaccines can be constructed by any competent molecular biology laboratory and in different combinations and formulations; this would be very difficult with recombinant proteins or live recombinant viruses, says Hoffman. This ease of production suggests low cost and, because the vaccine is likely to be stable at room temperature, costly cold chains should be avoidable. Compared with live viral vectors, there is no risk of infection. Studies by other workers on influenza have demonstrated long-lived immunity resulting from DNA vaccination; Hoffman's team are monitoring how long his mice are protected from malaria. Theoretically, DNA from the vaccine's plasmids could integrate with host chromosomal DNA and, via 'insertional mutagenesis', inactivate tumour-suppressor genes or activate oncogenes and initiate cancer formation. Another worry is that the plasmid DNA might leak from the site of injection to integrate into germ cells. Recent studies are helping to allay such fears: in an influenza model, no integration was seen in mice vaccinated with DNA [Nichols, W. (1995) Ann. NYAcad. Sci. 772, 30-39]. In the malaria trials in question, an intramuscular route of administration is proposed: 'From the safety point of view, we want the DNA to remain where we can monitor it, and data suggest it does not stray from the site of intramuscular injection,' says Hoffman.
Janet Fdcker 1
Copyright © 1996 Elsevier Science Ltd. All rights reserved. 1357 - 4310/96/$15.00
91
~ I ~
~O.AY, ~ , ~ .
19~
S
e
w
s
for malaria vaccines US Department of Defense and the Diego-based biopharmaceutical company al are gearing up to test a new malaria vaccine olunteers, based on naked-DNA vaccine mology. Phase I and II clinical trials are med to start in October 1997. Vlalaria is a complex parasitic infection and mere are no effective vaccines to prevent this disease, which causes up to two million deaths and infects 500 million people annually. Naked-DNA vaccines are already on trial in people with HIV, but this trial is likely to be the first use of this technology against a parasite infection in humans. In naked-DNA vaccines, highly purified double-stranded plasmid DNA, containingthe pathogen's genetic information and a promoter to switch on expression, are designed to instruct the recipient's cells to make antigenic proteins, and protective immune responses result. Stephen L. Hoffman (Head of the Malaria Programme, Naval Medical Research Institute, Bethesda, MD, USA) intends to use plasmids containing four different genes that encode for parasite proteins expressed in the liver stage of the parasite's life cycle. These are: the circumsporozoite protein, sporozoite surface protein 2 (or Trap), Exp-1 and LSAI. Volunteers in the Washington area (DC, USA) will be immunized and then exposed to mosquitoes carrying the malaria parasite. This first-generation vaccine is designed to stimulate a cytotoxic CD8÷ T-cell response to eliminate infected liver cells; in the second phase, Hoffmann plans to target the blood stage and, in a third phase, to combine the two to produce a potentially more comprehensive treatment. 'Eventually, I envision multiple targets at each stage, mixing them together in an injectable cocktail,' says Hoffman, who says that volume, cost and immunogenicity only will ultimately limit the number of plasmids included. A final product that will provide sustainable protection in the field will also need to take into account the many different strains of malaria. 'Once we have a vaccine that gives 50-70% protection, we will move to overseas sites,' says Hoffman. The US Army and Navy have potential testing sites in Indonesia, Thailand, Egypt, Kenya, Brazil and Peru. Hoffman's group have already shown, using this strategy, that 50-80% of mice can be protected against a challenge of malaria parasites, which are virulent in mice; and the mice produce high levels of specific antibodies and cytotoxic T cells [Sedegah, M. et al. (1994)Proc. Natl Acad. Sci. USA 91,9866-9870]. However, only
one inbred strain of mouse was protected with the protein coded by the DNA in the experimental vaccine, demonstrating that other proteins needed to be investigated. 'When we introduced a gene encoding a newly discovered protein and injected it into the mice, we found that it protected a different strain of mouse from the first one. When we mixed the two plasmids together, we were able to protect three out of the five strains,' explains Hoffman. In humans, genetic variations in human leukocyte antigens (HLAs) are expected to cause similar variations in immune responsiveness, as different peptides derived from the malaria-encoding plasmids bind differently to HLAs expressed on the recipient's cells. How many plasmids may eventually be required to confer complete protection in people is not known, although it is thought that if eight HLA types are targeted, 95% of the human population should be covered. Naked-DNA technology may revolutionize vaccine development. In a matter of days, vaccines can be constructed by any competent molecular biology laboratory and in different combinations and formulations; this would be very difficult with recombinant proteins or live recombinant viruses, says Hoffman. This ease of production suggests low cost and, because the vaccine is likely to be stable at room temperature, costly cold chains should be avoidable. Compared with live viral vectors, there is no risk of infection. Studies by other workers on influenza have demonstrated long-lived immunity resulting from DNA vaccination; Hoffman's team are monitoring how long his mice are protected from malaria. Theoretically, DNA from the vaccine's plasmids could integrate with host chromosomal DNA and, via 'insertional mutagenesis', inactivate tumour-suppressor genes or activate oncogenes and initiate cancer formation. Another worry is that the plasmid DNA might leak from the site of injection to integrate into germ cells. Recent studies are helping to allay such fears: in an influenza model, no integration was seen in mice vaccinated with DNA [Nichols, W. (1995) Ann. NYAcad. Sci. 772, 30-39]. In the malaria trials in question, an intramuscular route of administration is proposed: 'From the safety point of view, we want the DNA to remain where we can monitor it, and data suggest it does not stray from the site of intramuscular injection,' says Hoffman.
Janet Fdcker 1
Copyright © 1996 Elsevier Science Ltd. All rights reserved. 1357 - 4310/96/$15.00
91