The interpretation of paradoxical radioimmunoassay standard curves

Computational models in immunological methods: an historical review

The utilization of computational models in immunology dates from the birth of the science. From the description of antibody-antigen binding to the structural models of receptors, models are utilized to bring fundamental understandings of the processes together with laboratory measurements to uncover implications of these data. In this review, an historical view of the role of computational models in the immunology laboratory is presented, and short mathematical descriptions are given of fundamental assays. In addition, the range of current uses of models is explored -- especially as seen through papers which have appeared in the Journal of Immunological Methods from volume 1 (1971/1972) to volume 208 (1997). Each paper which introduced a new mathematical, statistical, or computer simulation model, or introduced an enhancement to an instrument through a model in those volumes is cited and the type of computational model noted.

Interaction of immobilized avidin with an aequorin-biotin conjugate: an aequorin-linked assay for biotin

Biotinylated recombinant aequorin was used in the development of a heterogeneous bioluminescence binding assay for biotin. This assay is based on a competition between a biotinylated aequorin conjugate and biotin for the binding sites of avidin immobilized on solid particles. Dose-response curves were obtained that relate solid-phase aequorin activity to the concentration of biotin. Under certain experimental conditions these curves were biphasic; i.e., as the biotin concentration increased, the solid-phase aequorin activity first increased reaching a maximum and then decreased at higher biotin concentrations. This "hook" effect was observed with four different types of immobilization supports. The effect was more pronounced when low concentrations of aequorin-biotin conjugate were used, and diminished at a high conjugate concentration. This behavior indicates a possible positive cooperativity in the interaction between the immobilized avidin and biotin. Scatchard plot analysis was also consistent with a positive cooperativity mechanism. By using the ascending portion of the dose-response curve, the detection limit of the assay for biotin was 1 x 10(-15) M (100 zmol of biotin in the sample).

Cooperative immunoassays: ultrasensitive assays with mixed monoclonal antibodies

Mixtures of certain monoclonal antibodies appear to bind human chorionic gonadotropin in a "cooperative" fashion because they form circular complexes with the hormone. Experiments illustrate how this property might be exploited to develop very sensitive immunoassays for human chorionic gonadotropin or any other antigen. Since the assays are not based on competitive inhibition between radiolabeled and unlabeled antigen, they are much more sensitive than a traditional radioimmunoassay in which either one of the same antibodies is used alone.

Quantitative explanation for increased affinity shown by mixtures of monoclonal antibodies: importance of a circular complex

Mixtures of some but not all monoclonal antibodies which bind to separate epitopes on human chorionic gonadotropin (hCG) show an increased affinity for the hormone. To find an explanation for the increase in affinity, we developed a mathematical model which predicts the quantities of intermediates formed when pairs of IgG1 mouse monoclonal antibodies having affinities of approximately 10(8) M-1 for hCG are mixed with the hormone. At low antibody concentrations (i.e. less than 1 nM or 0.15 micrograms/ml) analysis of possible antibody-hormone combinations, including linear and circular chains composed of less than 12 molecules of antibody and 12 molecules of hCG, suggests the increase in affinity is due to formation of a circular complex containing two molecules of antibody and two of hCG. Further, the model predicts that the circular complex will be the major species formed at antibody-antigen equivalence. This prediction is supported by experimental observations on the molecular weight of a new complex formed in the presence of hCG and the mixture of the monoclonal antibodies. In addition, based on experimental values of binding constants for individual antibodies to hCG, the model correctly quantifies the loss in complex observed in the presence of excess hCG antigen. At high antibody concentrations (i.e. greater than 10 nM or 1.5 micrograms/ml) the formation of linear chains of antibody hCG pairs becomes appreciable and contributes to the increase in apparent affinity of the mixture for hCG. These results suggest that the observed affinity of complex mixtures of antibody for antigens containing multiple epitopes calculated from Scatchard plots may not be related to the affinity or avidity of any of the antibody species for a given epitope.