The coexistence of antibody with antigen during immunological paralysis to BSA

Nine adult rabbits averaging 2.55 kg were each given 1.275 g bovine serum albumin (BSA Fraction V) 6 days weekly for a period of 66 days and rechallenged with 50 mg BSA or human serum albumin (HSA) 139 days after the last injection of BSA. The sera from all rabbits were studied each week for concentrations of circulating BSA and for capacity to bind ¹³¹I-labelled BSA or HSA (I*BSA or I*HSA). Some sera were also studied after fractionation on Sephadex G-200 columns to remove the BSA which had been present in vivo. Serum levels of BSA were determined and the binding studies performed using variations of the ammonium sulphate method to detect I* antigen—antibody complexes. Some sera were also studied by radioimmunoelectrophoresis.

The observed steady-state serum levels of circulating BSA during the daily infusions were substantially less than the theoretical concentrations. In addition, the measured BSA levels were particularly low throughout two periods of the experiment. The first occurred between days 14 and 35 when the rabbits had significant proteinuria and the second between days 45 and 66 when maximum antibody activity was observed. Following cessation of infusions, BSA disappeared from the circulation with a 5.5 day half-life.

Binding of I*BSA was not detected in whole serum but was measurable between days 14 and 165 after fractionation by gel filtration with Sephadex G-200 to remove the unlabelled BSA. In contrast, Sephadex filtration was not required to demonstrate I*HSA binding in whole serum between days 14 and 198, even in the presence of excess unlabelled BSA.

The immunological specificities of the antibodies present during `immunological paralysis' to BSA were characterized by comparing the effectiveness of graded amounts of unlabelled BSA or HSA to inhibit the I*BSA and I*HSA binding. The binding specificities of sera during `immunological paralysis' were found to be similar in certain respects but distinctly different in others from either normally produced anti-BSA or anti-HSA.

Rechallenge with 50 mg HSA on day 139 produced significant I*HSA binding but only slight I*BSA binding. Specificity studies on these antibodies revealed characteristics more typical of the normally produced anti-HSA than were observed during `immunological paralysis'. However, some characteristics of normally produced anti-BSA remained.

Rechallenge with BSA produced significant binding of either I*BSA or I*HSA, and in some cases both. Specificity studies of these antibodies revealed more of the characteristics of normally produced anti-BSA than were observed prior to rechallenge but some of the characteristics of normally produced anti-HSA remained.

These results indicate that rabbits receiving large daily injections of BSA are `immunologically suppressed' rather than `immunologically paralysed' to BSA. A hypothesis is presented to explain the coexistence of BSA and antibodies having the I* antigen binding characteristics observed in the serum of these `immunologically suppressed' rabbits.