Making contraceptive vaccines for possums

Here, we explain how contraceptive vaccines for possums are made and how they work.

Injecting nematode worms

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Injecting the foreign DNA (the 'transgene') into the ovary of the female nematode worms requires a microscope.

Rights: The University of Waikato Te Whare Wānanga o Waikato

Possums are a major pest in New Zealand, so scientists investigated the development of vaccines to make them less fertile. Vaccines against possum reproductive proteins act as a contraceptive and can decrease possum fertility.

Vaccines against two reproductive proteins were researched:

• Gonadotrophin-releasing hormone (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’s immune system to attack GnRH and ZP proteins, scientists combine the proteins with an antigenic vaccine carrier. The possum’s immune system recognises the protein 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 tested 3 types of vaccine carrier:

• Bacterial ghosts

• Virus-like particles

• Lipid matrix.

Bacterial ghosts

Bacterial ghosts are formed from bacteria that are engineered to expel their contents when heated. An empty cell membrane, 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

Virus-like particles are made from the structural proteins that make a virus, so they resemble the virus from which they were derived, but they don’t contain any nucleic acids (DNA, RNA), so they are not infectious. 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.

An update to the research

In a 2011 report, Dr Jan Wright, Parliamentary Commissioner for the Environment wrote that although significant research effort and resources were put into possum contraceptive vaccines and hormone toxins, funding for the projects finished in September 2010. There was concern about the length of time required to produce results, risks associated with the biocontrol agents and potential controversy over genetic engineering.