PZP success. An excerpt from : Frontiers In Bioscience
The efficacy rate that would depend on what type of PZP they use. If you mean the total population for herd production that would depend on how many mares are darted and then applying the efficacy rate. We endorse PZP native, and PZP 22 only. We don’t like Spayvac or Gonacon.
“5. CONTRACEPTIVE VACCINES BASED ON ZONA PELLUCIDA GLYCOPROTEINS
In mammals, egg is surrounded by an extracellular translucent glycoproteinaceous matrix known as zona pellucida (ZP). It acts as a relatively species-specific ‘docking site’ for binding of the spermatozoa to the oocyte, induces acrosomal exocytosis in zona-bound spermatozoa, prevents polyspermy, and plays an important role in the protection of a pre-implanted blastocyst. Due to their critical role in reproduction, ZP glycoproteins have been used as candidate antigens for contraception via immunological intervention. Zona matrix is composed of either 3 or 4 glycoproteins. In mice, it is composed of 3 glycoproteins namely ZP glycoprotein -1 (ZP1), -2 (ZP2), and -3 (ZP3), whereas pig and canine ZP matrix is also composed of 3 glycoproteins but instead of ZP1, ZP glycoprotein-4 (ZP4) is present. In non-human primates and humans, ZP matrix is composed of 4 glycoproteins that are ZP1, ZP2, ZP3, and ZP4. Structure and functions of all the four ZP glycoproteins during fertilization from various species has been investigated by various groups, which has been reviewed elsewhere (60). Characterization and sequencing of ZP glycoproteins from various species revealed that their basic structure is evolutionary conserved but have variable degree of amino acid sequence identity. For example porcine ZP3 has 66% identity at amino acid level with mouse ZP3 and 75% with canine ZP3. The extent of amino acid sequence identity of various porcine ZP glycoproteins with their respective homologues from other species is listed in Table 2. This property of ZP proteins has made heterologous immunization as a feasible proposition. It is further strengthened by the observations that polyclonal antibodies generated against porcine heat solubilised isolated zona pellucida (SIZP) cross-react immunologically with ZP of human, squirrel monkey, rabbit, rat, and mouse (61). Due to easy accessibility of porcine ovaries from abattoirs, contraceptive vaccine based on porcine ZP preparations became the antigen of choice.
5.1. Native porcine zona pellucida (PZP)-based contraceptive vaccine
Contraceptive potential of either native porcine SIZP (PZP) or its purified component (ZP3) has been demonstrated in female rabbits (62), bitches (63), non-human primates (64, 65), domestic and captive wild horses (66), captive white-tailed deer (67) and a variety of zoo animals (68). The field trial of native PZP-based contraceptive vaccine for the management of wild horses population on Assateague Island National Seashore, a barrier island off the coast of Maryland, USA demonstrated its contraceptive efficacy (69). The vaccine was delivered remotely using dart gun. Inhibition of fertility in wild horses could be maintained by giving annual booster of PZP vaccine. It was shown that third consecutive annual booster of porcine ZP led to 79% efficacy in preventing pregnancies in mares (70). Smithsonian Institute’s Conservation and Research Center at Front Royal, VA, USA initiated the first field trials of PZP-based contraceptive vaccine in white-tailed deer inhabitating Fire Island National Seashore, NY, USA (71). Deer were captured, tagged, given primary injection of PZP vaccine, and then released. Subsequent booster injections were given remotely by using dart gun. Between 1993 and 1999, twinning rates among individually known vaccine treated adult female deer decreased by 78.9.% from pre-treatment rates (72). The contraceptive efficacy of PZP vaccine was also demonstrated in free-ranging African elephants (Loxodonta africana) in the Kruger Park, South Africa (73). Thus PZP-based contraceptive vaccine has been successfully used for the management of the population of wild horses (34, 66, 69, 70, 74, 75, 76), urban deer (71, 72, 77, 78), wapti (79), and African elephants (73, 80). Keeping in view the above, currently three commercial vaccines based on PZP are available as listed in Table 3.
5.2. Safety of PZP-based contraceptive vaccine
Ovarian specific expression of zona proteins and lack of cross-reactivity of antibodies generated against zona proteins with other tissues and protein hormones (81, 82) is of an advantage of PZP-based vaccines as compared to GnRH-based contraceptive vaccine. Zona pellucida-based contraceptive vaccine primarily works by either inhibiting fertilization and/or folliculogenesis (Figure 1). Potential changes in ovarian pathology represented by follicular atresia along with sometime depletion of primordial follicles pool in rabbits (62), dogs (63), sheep (83) immunized with porcine zona proteins was one of the main concern for the application of PZP-based contraceptive vaccine for wildlife population management. However, no changes in ovarian histology were found in PZP-based contraceptive vaccine immunized horses and deer (66, 71). Inhibition of fertility was reversible in the captive deer immunized with PZP-vaccine after one to four years of immunization (67). In another study, no changes in ovarian histology of PZP immunized deer were reported. However, breeding season was extended by 1-2 months (71). PZP-vaccinated deer were more active and gained significantly more weight by summer following immunization than untreated deer, presumably because of avoidance of lactation, but by the following fall, all weight differences disappeared (84). Ovarian eosinophilic oophoritis was reported in PZP immunized deer, which; however, was not statistically different as compared to unimmunized group (85). Injection site abscesses were reported only in 0.5.% of PZP immunized deer, but all of them revealed the formation of granuloma at the injection site without any complication (72, 85). In elephants immunized with PZP-based contraceptive vaccine, examination of the ovaries and uteri by ultrasound revealed no adverse changes (73, 80). However, there may be species-specific differences in the ovarian histopathology subsequent to PZP-based contraceptive vaccine immunization, which should be investigated in detail in the respective target species.
Long-term treatment of wild horses with PZP-based contraceptive vaccine did not lead to any permanent or significant changes in ovarian endocrine parameters and oestrous cyclicity as monitored by urinary steroid metabolites (70). It had no adverse outcome when used in pregnant mares. The contraceptive effect was reversible (86). Increased body conditions and longevity were observed in PZP-immunized horses (87). There was no significant change in the social organization or behaviour of PZP-immunized wild horses (88). However, there is some concern over injection site reactions (55). It is likely that the remote delivery of PZP-based contraceptive vaccine by dart may result in surface bacteria and debris being pushed into the injection site resulting in injection site reactions (55).
Few significant changes in blood chemistry such as levels of urea and creatine were observed in PZP immunized deer but none of these were associated with any physiological abnormalities (85, 89). PZP immunized Dall sheep (Ovis dalli) and domestic goats (Capra hircus) also did not reveal any significant changes in blood chemistry (90).
5.3. Recombinant zona proteins as candidates for contraceptive vaccine
Due to limited availability of native porcine zona proteins from pig oocytes and to circumvent the apprehension of their probable contamination by other ovarian-associated proteins, recombinant zona proteins have also been used as candidate immunogens. Immunization of female baboons (Papio anubis) with E. coli-expressed recombinant bonnet monkey (Macaca radiata) ZP4 (previously designated as ZPB) coupled with diphtheria toxoid (DT) led to reversible block of fertility (91). However, immunization of female bonnet monkeys with the above immunogen led to inhibition of fertility which was not reversible. Histology of the ovaries from immunized monkeys revealed follicular atresia (92). Active immunization of female marmosets (Callithrix jacchus) with mammalian-expressed recombinant human ZP3 also led to inhibition of fertility associated with ovarian pathology characterized by depletion of primordial follicle pool (93). Immunization of cynomolgus monkeys (Macaca fascularis) and baboons (Papio cynocephalus) with mammalian expressed recombinant human ZP2, ZP3 and ZP4 respectively showed higher contraceptive efficacy in animals immunized with ZP4 (94). These studies in non-human primates showed the potential of recombinant zona proteins as candidate immunogens for the development of contraceptive vaccine.
In order to control the population of street dogs and thereby reduce the burden of rabies infection, it was demonstrated that non-descript female dogs immunized with E. coli-expressed recombinant dog ZP3 conjugated to DT failed to conceive (95). Ovarian histology of the immunized dogs revealed degenerative changes in the ZP matrix and follicular atresia. Subsequently, to avoid chemical conjugation of recombinant dog ZP3 with DT and to obtain recombinant protein without His6-tag, it has been expressed in E. coli as a fusion protein with promiscuous T cell epitope of tetanus toxoid (TT, amino acid residues 830-844) with dilysine linker (TT-KK-ZP3) (96). Immunization of female mice with recombinant TT-KK-ZP3 led to significant reduction in fertility, which was associated with antibody titres (96). In Australia and New Zealand, the potential of recombinant brushtail possum (Trichosurus vulpecula) ZP3 protein to control the fertility of Koalas (Phascolarctos cinereus) and Eastern Grey Kangaroos (Marcopus giganteus) has also been explored (97, 98).
In light of limited availability of native PZP-based contraceptive vaccine, recombinant porcine ZP3 and ZP4 have also been expressed in E. coli and their contraceptive efficacy was evaluated (99). Immunization of female mice with recombinant porcine ZP3 and ZP4 led to significant inhibition of fertility and decrease in litter size as compared to the adjuvanted control (99). Interestingly, E. coli-expressed recombinant porcine ZP3 and ZP4 boosted the antibody response in female mice primed with porcine native SIZP, suggesting that recombinant proteins can be used as booster doses thereby enabling wider coverage of immunization program in light of limited supply of native PZP-based contraceptive vaccine (99). Contraceptive efficacy of recombinant porcine ZP3 and ZP4 vis-à-vis PZP vaccine has also been evaluated in pony mares. Extended anoestrous was observed in 86% of the PZP immunized mares as compared to 14% in recombinant porcine ZP3/ZP4 immunized group, which correlated with basal serum oestradiol and progesterone levels (100). All mares resumed cyclicity by 10 months post-treatment. None of the PZP-immunized mare conceived whereas all the unimmunized mares became pregnant. Out of 7 pony mares immunized with recombinant porcine ZP3/ZP4, 3 failed to conceive. These experiments suggest the potential of recombinant proteins for controlling fertility in mares, which; however, need further investigations (100).”