New contraceptive vaccine sharply reduces fertility enabling humane wildlife management

A shot that keeps a mare from getting pregnant sounds simple. In practice, it is one of the messier problems in wildlife management, where every extra dose means another chase, another dart, another round of stress for an animal that already lives on the move.

A team at Purdue University thinks it has a cleaner approach: an immunocontraceptive vaccine built around a sperm protein called IZUMO1. In early tests in mice, their formulations cut fertility sharply, in some cases leaving vaccinated females with only a fraction of the pups seen in unvaccinated animals. The work, led by Dr. Harm HogenEsch and Dr. Raluca Ostafe, was published Feb. 6 in the journal Vaccine.

HogenEsch, a distinguished professor of immunopathology in Purdue’s College of Veterinary Medicine, said the goal is a humane tool for animal overpopulation in species such as feral horses, deer and swine. Ostafe directs Purdue’s Molecular Evolution Protein Engineering and Production Facility. Both are members of the Purdue Institute for Inflammation, Immunology and Infectious Disease.

Wildlife contraception already exists, but the options come with tradeoffs. HogenEsch noted that current contraceptive vaccines are built around “self-proteins,” meaning they rely on pushing the immune system to react against the animal’s own molecules. Two major targets are gonadotropin-releasing hormone (GnRH), which sits upstream of reproductive hormones, and zona pellucida (ZP) proteins that surround the egg.

Those approaches can work, but the Purdue team lays out several drawbacks. Some vaccines reduce fertility without lasting long. Injection site reactions can range from swelling to abscesses. A hormone-targeting approach can raise concerns about behavioral changes. One existing vaccine, HogenEsch said, depends on a source that could be vulnerable to supply chain interruptions.

The Purdue strategy takes a different angle. Instead of hormones or egg coatings, the vaccine uses IZUMO1, a protein found on sperm. Female immune systems can treat sperm proteins as foreign, which means a vaccine can aim for a strong response without requiring the breakdown of immune tolerance.

“It is based on a male sperm protein that the female’s immune system recognizes as foreign,” HogenEsch said. “It works exclusively by inhibiting the fusion of a sperm cell and the oocyte.”

That last line matters. Fertilization is not a single switch, it is a sequence: sperm recognize an egg, bind, and then fuse. IZUMO1 plays a key role in binding to a receptor on the egg called JUNO. In mice, deletion of either IZUMO1 or JUNO causes complete sterility. Prior work has also shown antibodies to IZUMO1 or JUNO can inhibit sperm binding in humans and pigs.

The Purdue team produced a recombinant mouse IZUMO1 peptide in Chinese hamster ovary (CHO) cells, then purified it. They confirmed the product’s expected size, about 30 kilodaltons, using SDS-PAGE and a western blot.

They then paired the antigen with different adjuvants, the immune-boosting ingredients that often make or break vaccines. One was AddaS03, a squalene oil-in-water emulsion adjuvant containing alpha-tocopherol and polysorbate 80. The paper notes it is similar in composition to AS03, which has been used in human influenza vaccines.

The other was a nanoparticle-based system called NanoST. It combines modified phytoglycogen nanoparticles derived from corn with ADU-S100, a synthetic cyclic dinucleotide that activates the STING pathway, an intracellular alarm system that can amplify immune responses. Prior studies cited by the team found NanoST could strongly stimulate immune responses in mice and pigs.

For fertility tests, the researchers used outbred CD-1 mice, which have strong reproductive performance. That choice was deliberate. In their control group, females averaged 11.7 pups, a high bar for a contraceptive strategy to clear.

In the first fertility experiment, females were immunized three times, 28 days apart, then bred. The control group, injected with Tris-saline, produced 70 pups across six mice, with all six becoming pregnant.

The group vaccinated with IZUMO1 plus AddaS03 had four pregnancies out of six mice and produced 40 pups. That represented a 43 percent reduction in total pups compared with controls.

The bigger cuts came from NanoST. With IZUMO1 plus NanoST, only three of six mice became pregnant, producing 17 pups total. The combination of NanoST plus AddaS03 yielded two pregnancies and 25 pups.

The team repeated the work with two immunizations instead of three. This time, AddaS03 did not reduce fertility: five of six mice became pregnant and produced 66 pups, compared with five pregnancies and 63 pups in controls.

NanoST again stood out. With IZUMO1 plus NanoST, two of six mice became pregnant and produced 24 pups. With IZUMO1 plus NanoST and AddaS03, three of six mice became pregnant and produced 23 pups. In both NanoST groups, the paper reports a greater than 50 percent reduction in pregnancy rate and pup numbers.

The antibody data did not map perfectly onto fertility outcomes. All immunized groups developed robust anti-IZUMO1 IgG responses. NanoST tended to push a stronger IgG2a response than AddaS03, but the authors caution that it is not obvious that subclass differences explain why NanoST reduced fertility more. They also measured IZUMO1-specific antibodies in the uterus and vagina, because that is where interference with fertilization would have to happen. Both AddaS03 and NanoST groups showed increased IZUMO1-specific IgG1 and IgG2a in reproductive tract fluids compared with controls, yet there was no significant difference between the two adjuvants.

So why did NanoST work better?

The authors point to specificity, not just quantity. Antibody titers measured by ELISA can look impressive while missing the most important parts of a target protein. Adjuvants can shape which epitopes, the specific segments of an antigen, the immune system “chooses” to focus on.

To explore this, the team performed epitope mapping using membranes spotted with selected IZUMO1 peptides. They saw a striking pattern. Serum from immunized mice that still became pregnant tended to react weakly with the peptide set. Serum from immunized mice that were not pregnant reacted strongly with peptides 7, 8 and 9, which correspond to amino acids 148 to 173. That span sits in a beta-hairpin hinge region of IZUMO1.

The hinge region is not a trivial detail. Prior structural and mutation studies cited by the authors suggest the hinge is particularly important for IZUMO1’s interaction with JUNO. In the Purdue mapping, antibodies that locked onto that area correlated with infertility.

The epitope work also came with limitations, which the authors state directly. Their peptide array did not cover the entire IZUMO1 construct, and the method detects linear epitopes, not conformational ones. It is possible that antibodies recognizing three-dimensional shapes on IZUMO1 also contribute to contraceptive effects.

The Purdue group frames this as an early but promising step toward a practical wildlife vaccine. They argue that Freund’s adjuvant, often used in older IZUMO1 vaccination studies, is too inflammatory for real-world use. It can cause necrosis and granulomatous inflammation, and contraceptive vaccines using related adjuvants have been associated with draining abscesses in horses and donkeys. Their study uses adjuvants intended to be safer for broader applications.

HogenEsch said he wants to move the vaccine toward a single-dose, controlled-release formulation, because boosters are hard or impossible in free-ranging wildlife.

“We will develop it into a single-dose, controlled-release formulation,” he said. “A vaccine that induces long-lasting infertility with a single injection will greatly facilitate the utility of the vaccine.”

He also said the team is working to optimize delivery by darts and to explore oral delivery.

Separate from the mouse work, HogenEsch and colleagues have completed the first year of a three-year Bureau of Land Management project testing the vaccine’s effects on horses. If it succeeds, Purdue says it could support wild horse management.

Funding for HogenEsch’s research has come from Humane World for Animals, formerly the Humane Society of the United States, and the Elinor Patterson Baker Trust.

Research findings are available online in the journal Vaccine.

The original story "New contraceptive vaccine sharply reduces fertility enabling humane wildlife management" is published in The Brighter Side of News.

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