Immune Response of the Guinea Pig to Bovine Parathyroid Hormone Antigen: Influence of the Booster Dose on Antisera Titer and Sensitivity

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Abstract
Thirty guinea pigs were immunized with 40 μg of bovine parathyroid hormone, bPTH(1–84), and divided into 6 groups with a similar primary response. Each group was boosted twice with a different dose of antigen (0.04 to 40 μg). The titer (30Z binding of tracer) and affinity (% displacement with 320 pg of bPTH(1–84) were studied. The primary response was maximal at 61 days; titers were 1 ± 10−4 or less and affinity was low. The response was maximal 25 days after each booster. After the first, the highest titers, 5.2 ± 2.6 ± 10−4 (mean ± S.D.) were seen in the 40 μg group and a positive correlation was observed between the dose of antigen and the titer (r =.7386, p < 0.001) for doses greater than 0.6 μg. This remained true after the second booster although titers were lower in all groups. Affinity was greater after the first booster (35.85 ± 20.38%, n = 30) than after the second (23.24 ± 20.73, n = 30, p < 0.0025), but was similar in all groups. Selected antisera reacted with antigenic determinants in regions 1–34, 53–84 and 35–53 of the bPTH molecule. Cross-reactivity with human PTH(1–84) was maximal in the 53–84 region. In conclusion, antibodies against all regions of bPTH can be raised in the guinea pig. Once a primary response is elicited, best titers are achieved by boosting with high doses of antigen without any detrimental effect on antisera affinity.