Immunocontraception: Where are we with the "spay" vaccine? (Proceedings)

Immunocontraception is the use of the body's own immune response to decrease fertility while permitting normal sexual behavior. Our goal in companion animals is perhaps more properly termed "immunoneutering" or use of the body's own immune response to decrease fertility and minimize or abolish sexual behavior.

The ovaries contain thousands of follicles at birth, each of which contains a single ovum (queens) or one or two ova (bitches). A cohort of follicles is stimulated to begin development with each estrous cycle. Development is promoted by release of hormones from the hypothalamus (gonadotropin releasing hormone [GnRH]) and pituitary (follicle stimulating hormone [FSH] and luteinizing hormone [LH]). As the follicles develop, they secrete estrogen, which causes the physical and behavioral signs of proestrus. In dogs, estrogen concentrations fall about 9 days after the onset of proestrus; at this time, a surge of LH is released, causing ovulation, and the bitch will stand to be bred. In cats, ovulation does not occur until the queen is bred (induced ovulation). The eggs are released from the follicles into the uterine tube, where fertilization occurs. The egg is surrounded by the zona pellucida (ZP) and a layer of cumulus cells. Spermatozoa introduced into the reproductive tract of the bitch or queen undergo capacitation, consisting of the acrosome reaction and acquisition of hypermotility. Capacitated spermatozoa bind to the epithelium of the uterine tube until the ova are present; an undefined signal causes their release and permits binding of the spermatozoa to the ZP. Spermatozoa bind to one of the ZP glycoproteins; this permits cross-linking with other ZP glycoproteins and enacts fusion of proteins in the ZP as one component of the block to polyspermy that occurs as soon as one spermatozoon gets to the inner layer of the ZP.

One can envision many places in this sequence of events where conception could be blocked if stimuli were not present or if tissue processes were physically blocked by antibodies. Research has focused on the use of hormones or tissues unique to reproductive function as antigens. Because these tissues are recognized as "self" they are not generally very immunogenic and must be conjugated with a larger, more antigenic molecule to induce an immune response. Most vaccines created with the conjugates also employ an adjuvant, a non-specific immunostimulant.

The ZP is made of up three glycosylated proteins that are highly conserved between species. Immune response generally is greater to those ZP proteins from animals far removed from a given species phylogenetically. The abundance of pig ovaries available in the United States at abattoirs has made porcine ZP (pZP) a popular candidate. However, pZP does not cause an immune response in all species, presumably because of lack of enough common antigenic sites to stimulate an immune response. Researchers have tried ZP from other species and again have seen variable titres after injection, and variable binding of antibodies formed to reproductive tract tissues. Little cross-reactivity with non-reproductive tissues has been identified, suggesting these vaccines are not detrimental to other organ systems in treated animals and in animals that may unwittingly be exposed, as might occur if oral vaccines were used in baits to control feral dog or cat populations.

Antibody response of species receiving vaccine

The final test of any contraceptive or neutering vaccine is that of pregnancy challenge. Vaccines using pZP in dogs cause erratic estrous cycling and do not consistently prevent pregnancy long-term. Vaccines using recombinant canine ZP proteins conjugated to diphtheria toxin in dogs caused a rise in titers and subsequent inhibition of ovarian follicular development but did not prevent estrous cycling and pregnancy in all cases. Most ZP vaccine studies in dogs report at least short-term infertility in more than 75% of cases but were associated with prolonged proestrus bleeding and estrous behavior and with ovarian cystic disease. In cats, vaccines developed using ZP proteins from dogs, cats, mink, and ferrets all were demonstrated to induce a significant, measurable antibody response but did not protect against pregnancy as the antibodies did not bind to the queen's own ZP in vitro. It may be that variation in sperm binding sites on the ZP vary enough between species to minimize the effect of antibodies raised against ZP proteins.

Another reported problem with immunoneutering vaccines evaluated to date is the adjuvant used. In one study in cats, using Freund's complete adjuvant, 7 of 10 cats developed granulomatous reactions at the injection site and in distant tissues including lymph nodes and brain. One of the 10 cats died of a vaccine-associated sarcoma at the injection site, and 3 of 10 suffered from hypercalcemia and compromised renal function. Granulomatous reactions also have been reported at the injection site in dogs. Current work includes variation in adjuvants used and creation of nucleotide vaccines, which introduce small amounts of antigenic material in plasmids and induce both humoral and cellular immunity without use of adjuvants.

A commercial ZP vaccine with Freund's adjuvant (SpayVac) was available from 2002-2005 through a Canadian company. As of this writing, no ZP vaccine is commercially available for use in companion animal species.

Another "self" antigen used for immunoneutering vaccines is GnRH. This is a highly conserved, very small protein that must be conjugated with a much larger antigen for an immune response to be elicited. In a study in male dogs using GnRH conjugated to tetanus toxoid, rises in antibody titers against the tetanus toxoid but not against the GnRH were demonstrated. A study in bitches using GnRH conjugated to canine distemper virus proteins demonstrated a rise in titers but no inhibition to conception and pregnancy. A recent study in cats using multiple tandem repeats of GnRH conjugated to proteins from Pasteurella showed high titers against GnRH, lack of follicular development, and no estrous cycling or pregnancy for up to 20 months after vaccination. Finally, GnRH conjugated to hemocyanin from the keyhole limpet and adjuvanted with a commercial preparation using Mycobacterium avium (AdjuVac) has been demonstrated to decrease testosterone and sperm count in male dogs and cats; work in bitches and queens in ongoing.

A commercial GnRH vaccine using AdjuVac (GonaCon) is reported to be undergoing registration for use in hoofstock by the USDA. While there are no reports of a commercial vaccine for companion animals as of this writing, a GnRH vaccine of this sort may well be the next immunoneutering vaccine commercially available in the United States.