Here, we explain how contraceptive vaccines for possums are made and how they work.
Scientists are developing vaccines to make possums less fertile, because possums are a major pest in New Zealand. Vaccines against possum reproductive proteins act as a contraceptive and can decrease possum fertility.
Currently, vaccines against two reproductive proteins are being developed:
- Gonadotrophin-releasing (GnRH) – a fertility hormone (AgResearch)
- Zona pellucida (ZP) proteins, which form the coat protecting the mammalian egg (Landcare Research).
Find out more in the article, Possum vaccines and reproductive proteins.
Vaccine carriers – boosting the immune response
GnRH or ZP proteins are not themselves antigenic. These proteins occur naturally in the possum so will not elicit an immune response.
To encourage a possum’sto attack GnRH and ZP proteins, scientists combine the proteins with an antigenic carrier. The possum’s immune system recognises the and its carrier as foreign and make antibodies to destroy them both. The possum’s immune system is primed to recognise GnRH and ZP proteins as foreign.
After being primed by the vaccine carrier, the possum’s immune system will destroy or neutralise GnRH or ZP proteins when they are produced as part of the reproductive cycle. This blocks the function of the proteins, and the possums are unable to produce offspring.
Types of vaccine carrier
Scientists are testing 3 types of vaccine carrier:
- Bacterial ghosts
- Virus-like particles
Bacterial ghosts are formed fromthat are engineered to expel their contents when heated. An empty , or bacterial ghost, is left. The GnRH or ZP proteins are incorporated into the empty cell membrane, making an antigenic vaccine carrier.
Virus-like particles are made from the structural proteins that make a, so they resemble the virus from which they were derived, but they don’t contain any nucleic acids (DNA, ), so they are not . The GnRH or ZP proteins are attached to the particle or engineered to have the proteins incorporated into the particle.
Lipid matrix (plural: matrices)
Lipids are a type of fat. A cage or matrix of lipid molecules is made with the GnRH or ZP proteins inside so that they are protected as they pass through the gut and can be presented to the immune system to generate an immune response. The lipids are chocolate or aniseed flavoured to appeal to possums. The lipid matrix is prepared in bait that can be fed to possums.