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VAP1; A novel ovarian immunocontraceptive target in the common brushtail possum, Trichosurus vulpecula

A Nation, Zoology, The University of Melbourne, Melbourne, Australia
S Cui, Zoology, The University of Melbourne, Melbourne, Australia
L Selwood, Zoology, University of Melbourne, Melbourne, Australia

Correspondence: Angela Nation, Email: a.nation{at}pgrad.unimelb.edu.au

Abstract

Ovarian-based immunological research is currently restricted to proteins of the zona pellucida. This study examined the immunocontraceptive potential of a novel vesicle-associated oocyte protein, VAP1, previously isolated from the vesicle-rich hemisphere of the brushtail possum oocyte. Seven female possums were immunized against recombinant GST-VAP1 fusion protein. Control animals (n=3) received antigen-free vaccinations. Following immunization, regular blood sampling determined the level and duration of immune response. Animals were monitored daily, pre- and post-immunization, to determine oestrous cycling activity and the percentage of reproductive cycles yielding viable young. The reproductive tracts and somatic organs of VAP1 immunized (n=7), control immunized (n=3) and non-immunized (n=5) animals were collected and examined by histology and transmission electron microscopy. VAP1 immunization caused a strong and sustained immune response. Elevated levels of VAP1 antibody binding were detected in sera following initial injections, and immune titres rose as boosters were administered. Immunization had no adverse effect upon animal behaviour or body condition. Immunized females demonstrated no major change in annual oestrous cycling activity, however the percentage of reproductive cycles resulting in pouch young decreased significantly (p<0.05) by 40 %. Histological and ultrastructural analysis revealed an abundance of lipid-like degradation bodies within the ooplasm of developing oocytes and the cytoplasm of failing uterine zygotes. Active macrophage invasion of enlarged endometrial glands was observed in the uteri of two females. Reproductive tract changes are discussed in relation to observed fertility decline. The results of this study indicate that VAP1 has exciting potential as an immunocontraceptive target for possum control in New Zealand.