• Vaccine Name: P. vivax PVS25 with Montanide ISA-720
• Target Pathogen: Plasmodium spp.
• Target Disease: Malaria
• Vaccine Ontology ID: VO_0000776
• Type: Subunit vaccine
• Antigen: P. vivax protein Pvs25 is the vaccine antigen. It is a protein composed of four cysteine-rich epidermal growth factor–like domains expressed on the surface of zygotes and ookinetes of P. vivax (Arevalo-Herrera et al., 2005).
• Pvs25 gene engineering:
  • Type: Recombinant protein preparation
  • Description: Pvs25 was cloned and purified from yeast (Arevalo-Herrera et al., 2005).
  • Detailed Gene Information: Click Here.
• Adjuvant:
  • Adjuvant name:
  • VO adjuvant ID: VO_0001268
  • Description: Montanide ISA-720 an adjuvant suitable for human vaccination trials (Arevalo-Herrera et al., 2005).
• Preparation: To produce a recombinant protein, Pvs25 was expressed in S. cerevisiae in a secreted form. Briefly, P. vivax genomic DNA from the Salvador I strain was used to amplify the gene fragment encoding the Pvs25 regions (Ala23-Leu195), which was inserted into the yeast episomal plasmid YEpRPEU-3 that encodes a secretory {alpha} factor containing a 6-His tail.12 Supernatants of fermentation were recovered by tangential microfiltration, concentrated by ultrafiltration, and extensively dialyzed. The retentate was incubated overnight at 4°C with Ni-nitrilotriacetic acid agarose. Proteins were purified by chromatography (Arevalo-Herrera et al., 2005).
• Virulence: Not virulent.

Monkey Response

• Host Strain: owl monkey (Aotus lemurinus griseimembra)
• Vaccination Protocol: Male and female adult, malaria-naive Aotus monkeys were randomly allocated into two groups. An experimental group of six animals (group A) were immunized with the recombinant Pvs25 vaccine. A control group of three animals (group B) were immunized with adjuvant alone. Both groups were immunized on days 0, 60, and 120. Group A was inoculated with a total volume of 500 µL of vaccine formulated as 100 µg of the Pvs25 recombinant protein in Montanide ISA-720 in a 7:3 antigen:adjuvant ratio. Group B was injected with distilled water containing no protein and mixed in the same adjuvant following the same procedure. The immunization was performed by the subcutaneous route distributed in five different sites of the thorax and abdomen of each animal (Arevalo-Herrera et al., 2005).
• Immune Response: Antigen-specific antibody responses to the Pvs25 protein as determined by ELISA were evident by day 30 after the first immunization at low levels (61–478 units of anti-Pvs25). By day 60, at the time of the first boosting dose, responses of most animals were similar and by day 90, antibodies were boosted in all but two animals. Only one monkey had an apparent boost with the third antigen injection given on day 120. All animals had maximum antibody levels by day 150. These levels started to decrease by day 180, but were still detectable 10 months after the first immunization (Arevalo-Herrera et al., 2005).
• Side Effects: No adverse side effects were encountered here (Arevalo-Herrera et al., 2005).
• Challenge Protocol: Approximately 10 months after the last immunization (day 440) when specific antibodies to Pvs25 are no longer detected by ELISA, all monkeys were challenged with the P. vivax Salvador I strain by intravenous injection of 105 parasitized red blood cells. Total parasitemia and gametocytemia were followed every other day using thick and thin blood smears stained with Giemsa. Parasite concentrations were expressed as the number of gametocytes per microliter and the percentage of red blood cells parasitized by asexual parasite forms.19 Monkeys were bled post-challenge (days 447–503) to evaluate the presence of antibodies to Pvs25 by ELISA. In addition, the infectivity of circulating gametocytes was tested by feeding of An. albimanus mosquitoes with parasitized monkey red blood cells mixed with normal AB human sera using the MFA on days 460 (Arevalo-Herrera et al., 2005).
• Efficacy: All monkeys developed patent parasitemia by day 453, approximately two weeks after intravenous challenge. The peak of parasitemia for most of the monkeys was observed between days 462 and 464 with parasitemias and ranged from 0.1% to 1.3% as determined by thin blood smear. Gametocytes were first evident between days 458 and 460 and remained at detectable levels in all animals until day 468. Plasma samples obtained on days 447, 462, 482, and 503 after parasite challenge were negative for antibodies directed to the Pvs25 recombinant protein by ELISA. Gametocytes that developed in both groups were infectious to mosquitoes as determined in an MFA conducted with monkey blood drawn on day 460 in which plasma from AB human control sera was replaced by sera from infected monkeys. This result supports the viability and functionality of the circulating gametocytes from both the Pvs25-immunized and the control animals.
  
  Mosquitoes fed with P. vivax gametocyte-carrying human blood in the presence of either normal monkey plasma or normal AB human sera (negative controls) produced positive infections with an arithmetic mean of oocysts per midgut ranging between 0.3 and 3.8 and 0.2 and 1.0 oocysts, respectively. However, plasma from the Pvs25-immunized Aotus tested individually were highly inhibitory and completely blocked the development of oocysts, in all assays (reduction of the oocysts number > 98%) using three different P. vivax human isolates. Plasma from monkeys in the Montanide ISA-720 control group showed similar inhibition to the normal monkey plasma (negative control). Therefore, boosting of antibodies to Pvs25 is not caused by the parasite infection, this Pvs25 vaccine can be used as a malaria transmission-blocking vaccine (Arevalo-Herrera et al., 2005).