The antifertility effect of passive immunization of mice against progesterone is influenced by genetic factors other than H-2 haplotype

The anti-fertility effect of passive immunization against progesterone is influenced by genotype in mice. In order to quantify this finding we determined the effective dose that blocks pregnancy in 50% of treated mice (ED₅₀) for two different anti-progesterone monoclonal antibodies (DB3 and 11/32) in four different strains of mice (BALB/cJ, CBA/Ca, Tuck's no. 1 and F₁C). Efficacy was greatest in the two inbred strains (BALB/cJ and CBA/Ca) with ED₅₀ values of 0.46-1.4 nmol. The F₁C hybrid mice were more resistant to antibody treatment (ED₅₀ 2.2-3.0 nmol), while the outbred Tuck's no. 1 strain required much higher doses (ED₅₀ 7.0-8.2 nmol). There were no intrastrain differences between the two monoclonal antibodies. We have examined the possible role of the H-2 haplotype on antibody efficacy in different strains and crosses. The antibody was highly effective in blocking implantation in three congenic BALB strains of different H-2 haplotype, in another inbred strain CBA/Ca, and in reciprocal BALB/cJ x CBA/Ca crosses. The F₁ hybrid crosses were somewhat resistant, but the C57BL/10Sn and B10.BR congenic strains were most resistant to treatment. The results show that the pregnancy-blocking effect of the anti-progesterone antibody was not influenced by the H-2 haplotype, but rather by background genes.