Production and biological activity of murine monoclonal antibodies against GnRH

Immunization against GnRH represents a nonsurgical means of castrating domestic species. However, clear target antibody titres for bioactivity have not been established. The aims of this study were to produce characterized anti-GnRH monoclonal antibodies and to determine a threshold titre. Three murine monoclonals were developed which produced IgG2a class immunoglobulins and bound 50% I(125)-GnRH at a 10(6) to 10(7) dilution. The antibodies were specific to GnRH, showed a strong affinity (Ka values from 1.99 to 2.60 x 10(10) litres/mole), and were directed towards the amino terminus. In female mice all 3 antibody clones interrupted ovarian cyclicity, causing an extension in diestrus followed by prolonged estrus/metestrus (12 to 30 d). Throughout this period circulating titres were greater than 15% I(125)-GnRH binding at a 5 x 10(4) dilution. In male mice, immunization with 0.2 ml of ascites significantly reduced testes (P < 0.05), epididymides (P < 0.001) and seminal vesicle (P < 0.01) weights. A 0.1 ml dose (61.4 +/- 18.6% binding at a 10(6) dilution) was ineffective. A serial dilution study indicated that a titre of 50% binding at 2 x 10(6) dilution (antigen binding capacity of 268 +/- 35 ng/ml) was required to completely block GnRH activity. This is a higher tire than threshold levels determined previously. Identification of factors determining the titre required for bioactivity is needed.