High-dose delay of the immune response. Effect of actinomycin D on continuation of the immune response in vitro.

The continuation of the primary and secondary antibody response to human serum albumin (HSA), induced in vivo, was followed in explanted chicken spleen fragments. The effect of actinomycin D (AMD) on the in vitro response was studied in spleens from chickens injected with various doses of HSA and removed at differing intervals after injection. The antibody response of "early spleen" cultures was AMD-sensitive, while cultures of spleens removed later were AMD-resistant. It was suggested that this shift represented the development of cells with in vivo preformed RNA involved in specific immunoglobulin synthesis. With increasing doses of HSA, the AMD-sensitive phase was prolonged, suggesting the delay of mRNA formation or some other AMD-inhibitable process in vivo. With large doses of HSA, the immune response in vitro was decreased, starting after a 1-2 day delay and not occurring in the presence of AMD. Massive doses of HSA completely inhibited the continuation of the response in vitro by spleen fragments removed between the 2nd and 5th day after injection. The results point to the controlling role of antigen dose in determining the onset of macromolecular synthesis during immunocyte maturation.