Immunization of sheep against modified peptides of gonadotropin releasing hormone conjugated to carriers

The efficacy of antigens based on modified GnRH peptides in stimulating the production of antibodies against GnRH in sheep was tested. In the first study cysteine-containing GnRH peptides were conjugated to keyhole limpet haemocyanin (KLH) in 3 different orientations. The 3 conjugates were prepared in an emulsion of Freund's complete adjuvant (FCA) and were injected into 3 groups of 6 castrated male lambs. The 3 vaccines efficiently induced anti-GnRH titers in all the animals treated. The specificity of the GnRH antisera raised varied depending on the orientation of the GnRH molecule in the antigen and on the individual animal.

In a second trial designed to evaluate carrier molecules, a cysteine-containing GnRH peptide was conjugated to either KLH, equine serum albumin, ovalbumin or tetanus toxoid. The conjugates were prepared with FCA and injected into intact male lambs. All 4 vaccines stimulated the production of antibodies against GnRH in all the animals treated. The conjugates prepared with equine serum albumin or ovalbumin were the most effective in raising high anti-GnRH titers. In 18 of 20 lambs treated, anti-GnRH titers resulted in a marked atrophy of the testes.

We conclude that: 1) the different epitopes of the GnRH molecule are equally immunogenic in sheep; 2) the GnRH antibody response is affected by the carrier used; and, 3) anti-GnRH vaccines based on cysteine-substituted GnRH analogues show potential for use in immunocastration of livestock.