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Image from Margaret Shear
and Ute Frevert
A next-generation malaria vaccine that uses genetically attenuated parasites (GAPs) has demonstrated promise in a phase 1 trial and in experiments with mice, according to researchers.
The team generated a genetically attenuated Plasmodium falciparum parasite by knocking out 3 genes that are required for the parasite to cause malaria in humans.
Healthy volunteers infected with this parasite, Pf GAP3KO, exhibited strong antibody responses, according to researchers, and experienced only mild or moderate adverse events (AEs).
When antibodies from these patients were transferred to humanized mice, they inhibited malaria infection in the liver.
The researchers described this work in Science Translational Medicine.
“This most recent publication builds on our previous work,” said study author Sebastian Mikolajczak, PhD, of the Center for Infectious Disease Research in Seattle, Washington.
“We had already good indicators in preclinical studies that this new ‘triple knock-out’ GAP (GAP3KO), which has 3 genes removed, is completely attenuated. The clinical study now shows that the GAP3KO vaccine is completely attenuated in humans and also shows that, even after only a single administration, it elicits a robust immune response against the malaria parasite. Together, these findings are critical milestones for malaria vaccine development.”
Dr Mikolajczak and his colleagues created GAP3KO by knocking out 3 genes in P falciparum that are required for the parasite to successfully infect and cause disease in humans—Pf p52−/p36−/sap1−.
The researchers then tested Pf GAP3KO sporozoites in 10 healthy human volunteers. The sporozoites were delivered via bites from infected mosquitoes.
All subjects experienced grade 1 AEs, and 70% had grade 2 AEs. There were no grade 3 or higher AEs.
Grade 2 AEs included 1 case of fatigue and several cases of administration-site reactions, including erythema (n=6), pruritus (n=2), and swelling (n=3).
All 10 volunteers completed the 28-day study period without exhibiting any symptoms of malaria. They also remained negative for blood-stage parasitemia throughout the study period.
All subjects experienced antibody responses as well. The researchers collected sera from the volunteers on day 0, 7, 13, and 28 after immunization and analyzed it for anti-circumsporozoite protein (CSP) antibody responses.
The team measured anti-CSP immunoglobulin G (IgG) by enzyme-linked immunosorbent assay (ELISA) using full-length recombinant PfCSP protein.
At day 0, volunteers’ sera showed an average of 1436 ± 257.3 arbitrary units (AU) of anti-CSP titers by ELISA. However, 3 volunteers were slightly above the 2000 AU cutoff for positivity.
At day 7, 8 of the 10 volunteers were positive for anti-CSP IgG, with an average titer of 3821 ± 808.1 AU. At day 13, all volunteers were positive, with an average of 11,547 ± 2084 AU. Positivity was maintained through day 28, with an average of 5774± 840 AU.
To determine whether these antibodies could inhibit Pf sporozoite invasion in vivo, the researchers transferred them to FRG huHep liver chimeric humanized mice, which allow for complete development of liver-stage parasites.
The researchers chose 5 volunteers whose immune serum showed high levels of invasion inhibition in vitro and varying CSP IgG titers. The mice received purified IgG from these volunteers from day 0 and day 13 immune samples (5 mice per volunteer per time point).
The mice were then challenged with bites from mosquitoes infected with sporozoites of a green fluorescent protein–luciferase–expressing P falciparum strain.
The researchers calculated inhibition of infection by comparing the liver-stage burden of mice receiving day 13 IgG samples to the mean value of liver-stage burden in mice receiving day 0 IgG samples.
The day 13 IgG samples inhibited liver infection by an average of 23.18 ± 57%, 32.23 ± 29.16%, 69.38 ± 18.64%, 76.63 ± 19.98%, and 87.99 ± 15.56% (per volunteer).
The 3 volunteers whose serum exhibited the highest inhibition had the lowest Pf CSP titers of the 5 samples.
The researchers said these promising results in mice and humans pave the way to a phase 1b trial of the GAP3KO vaccine candidate using controlled human malaria infection. 
Image from Margaret Shear
and Ute Frevert
A next-generation malaria vaccine that uses genetically attenuated parasites (GAPs) has demonstrated promise in a phase 1 trial and in experiments with mice, according to researchers.
The team generated a genetically attenuated Plasmodium falciparum parasite by knocking out 3 genes that are required for the parasite to cause malaria in humans.
Healthy volunteers infected with this parasite, Pf GAP3KO, exhibited strong antibody responses, according to researchers, and experienced only mild or moderate adverse events (AEs).
When antibodies from these patients were transferred to humanized mice, they inhibited malaria infection in the liver.
The researchers described this work in Science Translational Medicine.
“This most recent publication builds on our previous work,” said study author Sebastian Mikolajczak, PhD, of the Center for Infectious Disease Research in Seattle, Washington.
“We had already good indicators in preclinical studies that this new ‘triple knock-out’ GAP (GAP3KO), which has 3 genes removed, is completely attenuated. The clinical study now shows that the GAP3KO vaccine is completely attenuated in humans and also shows that, even after only a single administration, it elicits a robust immune response against the malaria parasite. Together, these findings are critical milestones for malaria vaccine development.”
Dr Mikolajczak and his colleagues created GAP3KO by knocking out 3 genes in P falciparum that are required for the parasite to successfully infect and cause disease in humans—Pf p52−/p36−/sap1−.
The researchers then tested Pf GAP3KO sporozoites in 10 healthy human volunteers. The sporozoites were delivered via bites from infected mosquitoes.
All subjects experienced grade 1 AEs, and 70% had grade 2 AEs. There were no grade 3 or higher AEs.
Grade 2 AEs included 1 case of fatigue and several cases of administration-site reactions, including erythema (n=6), pruritus (n=2), and swelling (n=3).
All 10 volunteers completed the 28-day study period without exhibiting any symptoms of malaria. They also remained negative for blood-stage parasitemia throughout the study period.
All subjects experienced antibody responses as well. The researchers collected sera from the volunteers on day 0, 7, 13, and 28 after immunization and analyzed it for anti-circumsporozoite protein (CSP) antibody responses.
The team measured anti-CSP immunoglobulin G (IgG) by enzyme-linked immunosorbent assay (ELISA) using full-length recombinant PfCSP protein.
At day 0, volunteers’ sera showed an average of 1436 ± 257.3 arbitrary units (AU) of anti-CSP titers by ELISA. However, 3 volunteers were slightly above the 2000 AU cutoff for positivity.
At day 7, 8 of the 10 volunteers were positive for anti-CSP IgG, with an average titer of 3821 ± 808.1 AU. At day 13, all volunteers were positive, with an average of 11,547 ± 2084 AU. Positivity was maintained through day 28, with an average of 5774± 840 AU.
To determine whether these antibodies could inhibit Pf sporozoite invasion in vivo, the researchers transferred them to FRG huHep liver chimeric humanized mice, which allow for complete development of liver-stage parasites.
The researchers chose 5 volunteers whose immune serum showed high levels of invasion inhibition in vitro and varying CSP IgG titers. The mice received purified IgG from these volunteers from day 0 and day 13 immune samples (5 mice per volunteer per time point).
The mice were then challenged with bites from mosquitoes infected with sporozoites of a green fluorescent protein–luciferase–expressing P falciparum strain.
The researchers calculated inhibition of infection by comparing the liver-stage burden of mice receiving day 13 IgG samples to the mean value of liver-stage burden in mice receiving day 0 IgG samples.
The day 13 IgG samples inhibited liver infection by an average of 23.18 ± 57%, 32.23 ± 29.16%, 69.38 ± 18.64%, 76.63 ± 19.98%, and 87.99 ± 15.56% (per volunteer).
The 3 volunteers whose serum exhibited the highest inhibition had the lowest Pf CSP titers of the 5 samples.
The researchers said these promising results in mice and humans pave the way to a phase 1b trial of the GAP3KO vaccine candidate using controlled human malaria infection.
Image from Margaret Shear
and Ute Frevert
A next-generation malaria vaccine that uses genetically attenuated parasites (GAPs) has demonstrated promise in a phase 1 trial and in experiments with mice, according to researchers.
The team generated a genetically attenuated Plasmodium falciparum parasite by knocking out 3 genes that are required for the parasite to cause malaria in humans.
Healthy volunteers infected with this parasite, Pf GAP3KO, exhibited strong antibody responses, according to researchers, and experienced only mild or moderate adverse events (AEs).
When antibodies from these patients were transferred to humanized mice, they inhibited malaria infection in the liver.
The researchers described this work in Science Translational Medicine.
“This most recent publication builds on our previous work,” said study author Sebastian Mikolajczak, PhD, of the Center for Infectious Disease Research in Seattle, Washington.
“We had already good indicators in preclinical studies that this new ‘triple knock-out’ GAP (GAP3KO), which has 3 genes removed, is completely attenuated. The clinical study now shows that the GAP3KO vaccine is completely attenuated in humans and also shows that, even after only a single administration, it elicits a robust immune response against the malaria parasite. Together, these findings are critical milestones for malaria vaccine development.”
Dr Mikolajczak and his colleagues created GAP3KO by knocking out 3 genes in P falciparum that are required for the parasite to successfully infect and cause disease in humans—Pf p52−/p36−/sap1−.
The researchers then tested Pf GAP3KO sporozoites in 10 healthy human volunteers. The sporozoites were delivered via bites from infected mosquitoes.
All subjects experienced grade 1 AEs, and 70% had grade 2 AEs. There were no grade 3 or higher AEs.
Grade 2 AEs included 1 case of fatigue and several cases of administration-site reactions, including erythema (n=6), pruritus (n=2), and swelling (n=3).
All 10 volunteers completed the 28-day study period without exhibiting any symptoms of malaria. They also remained negative for blood-stage parasitemia throughout the study period.
All subjects experienced antibody responses as well. The researchers collected sera from the volunteers on day 0, 7, 13, and 28 after immunization and analyzed it for anti-circumsporozoite protein (CSP) antibody responses.
The team measured anti-CSP immunoglobulin G (IgG) by enzyme-linked immunosorbent assay (ELISA) using full-length recombinant PfCSP protein.
At day 0, volunteers’ sera showed an average of 1436 ± 257.3 arbitrary units (AU) of anti-CSP titers by ELISA. However, 3 volunteers were slightly above the 2000 AU cutoff for positivity.
At day 7, 8 of the 10 volunteers were positive for anti-CSP IgG, with an average titer of 3821 ± 808.1 AU. At day 13, all volunteers were positive, with an average of 11,547 ± 2084 AU. Positivity was maintained through day 28, with an average of 5774± 840 AU.
To determine whether these antibodies could inhibit Pf sporozoite invasion in vivo, the researchers transferred them to FRG huHep liver chimeric humanized mice, which allow for complete development of liver-stage parasites.
The researchers chose 5 volunteers whose immune serum showed high levels of invasion inhibition in vitro and varying CSP IgG titers. The mice received purified IgG from these volunteers from day 0 and day 13 immune samples (5 mice per volunteer per time point).
The mice were then challenged with bites from mosquitoes infected with sporozoites of a green fluorescent protein–luciferase–expressing P falciparum strain.
The researchers calculated inhibition of infection by comparing the liver-stage burden of mice receiving day 13 IgG samples to the mean value of liver-stage burden in mice receiving day 0 IgG samples.
The day 13 IgG samples inhibited liver infection by an average of 23.18 ± 57%, 32.23 ± 29.16%, 69.38 ± 18.64%, 76.63 ± 19.98%, and 87.99 ± 15.56% (per volunteer).
The 3 volunteers whose serum exhibited the highest inhibition had the lowest Pf CSP titers of the 5 samples.
The researchers said these promising results in mice and humans pave the way to a phase 1b trial of the GAP3KO vaccine candidate using controlled human malaria infection.