Number and activity of free and membrane-bound spleen ribosomes during the course of the immune response

Abstract

Mice were immunized by one intraperitoneal injection of Salmonella typhi, and the number and activity of free and membrane-bound ribosomes of the spleen were studied during the immune response. Free and bound spleen ribosomes were separated from the post-nuclear supernatant, and analyzed directly on linear sucrose gradients. An average of 25 % of the total spleen ribosomes were membrane-bound in both immunized and control mice; this is in contrast to a value of 75–80 % for mouse liver. No polyribosomes were observed in the membrane-bound fractions, unless nuclease-containing organelles were removed from the post-nuclear supernatant before disruption of the membranes with detergent. The number of ribosomes, which had been actively engaged in protein synthesis at the time of cell fractionation, was determined by their resistance to dissociation at high ionic strength; this assay was not affected by the presence of ribonuclease, and did not require that the ribosomes catalyze polypeptide synthesis in vitro. We found that 36–55 % of free ribosomes and 56–77 % of total membrane-bound ribosomes were active by this criterion (90–95 % of liver membrane-bound ribosomes were active). Polyribosome profiles provided similar estimates for the number of active free ribosomes. After immunization, the numbers of spleen cells and of bound ribosomes per spleen were maximal at 3–4 days and at 8 days; each peak was preceded by a peak in the number of free ribosomes per cell, suggesting the extensive formation of ribosomes prior to cell division. Apparently the increased protein synthetic capacity of the spleen is primarily the result of an increased synthesis of new free ribosomes, and, to a lesser degree, of an activation of free ribosomes which were inactive in protein synthesis before immunization. The bulk of the splenic response appears to be concerned with a generalized tissue activation not strictly related to immunoglobulin synthesis, since the number and activity of membrane-bound ribosomes, the primary site of immunoglobulin synthesis, did not increase per 10 8 cells.