Kinetics of macrophages and reticuloendothelial cells in mouse lymph nodes during secondary antibody responses to tetanus toxoid

Abstract

The proliferative activity and rate of cell renewal of, and incorporation of tritiated nucleosides into, macrophages, reticuloendothelial cells and fibroblasts were studied in lymph nodes of mice before and after a regional second antigenic stimulus with tetanus toxoid. Histological and autoradiographic examinations revealed that in non-stimulated nodes of animals primed 6 months previously, none of the above cell-types was found to synthesize DNA. Throughout a period of 10 days following secondary antigenic stimulation via the hind leg foot pads, only four of approximately five thousand of these cells counted in the popliteal lymph nodes of thirty-four mice were labelled initially by [³H]thymidine. Incorporation of [³H]cytidine could, therefore, be interpreted as reflecting almost exclusively RNA synthesis. Mean grain counts of non-proliferating cells 1 hour after injection of this radioactive precursor started to rise first in small lymphocytes, reached a peak around 6 hours following the booster injection of antigen and returned to slightly elevated values at 12 hours. At this time, the mean labelling intensity of macrophages, reticuloendothelial cells and fibroblasts had barely begun to increase. Peak values for the latter cell-types were attained only on the third day after secondary antigenic stimulation. These findings are interpreted as indicating consecutive waves of enhanced RNA synthesis first in lymphocytes and then in macrophages or the other elements mentioned above. This lends further support to the hypothesis that, following a second injection of tetanus toxoid, elevated rates of RNA synthesis in macrophages are not a prerequisite for triggering lymphocytes to enter proliferation and differentiation. Macrophages having ingested various materials showed significantly higher mean grain counts than cells not containing cytoplasmic inclusions. One early response of the macrophage system to the booster injection of antigen consisted of a more rapid and increased turnover, i.e. replacement of unlabelled by labelled cells with increasing time after injection of [³H]thymidine. The results are discussed in relation to macrophage functions in immune responses and possible cellular transformations in the macrophage series.