Immune response of the guinea pig to bovine parathyroid hormone antigen: influence of the booster dose on antisera titer and sensitivity.

Thirty guinea pigs were immunized with 40 microgram 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 microgram). The titer (30% 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 x 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 x 10(-4) (mean +/- S.D.) were seen in the 40 microgram group and a positive correlation was observed between the dose of antigen and the titer (r = .7386, p less than 0.001) for doses greater than 0.6 microgram. 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 less than 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.