Depopulation of lymphocyte migration sites in the lymph node by irradiation and colloidal carbon

The purpose of the present investigation was to examine, in the light of recent histological findings, whether irradiation and colloidal carbon can have a lymphocyte depopulating effect on preferentially particular structures of the rat lymph nodes. Normal eight-week-old Sprague-Dawley rats received a 500 R whole body irradiation or a subcutaneous injection of 0.02 ml of India ink. The animals were then sacrificed at various time-intervals. The histological analysis of the irradiated and draining nodes revealed that both treatments almost completely eliminated small lymphocytes from the affected nodal structures, except in the center of the deep cortex units. The affected structures had been predominantly populated by recirculating lymphocytes.Thus, the treatment had a rather preferential depleting effect on a node population of recirculating lymphocytes. This finding provides another possible explanation for the carbon-induced augmentation of a GVH reaction in nodes. This augmentation had previously been attributed to a stimulation by the carbon of host macrophages, which would mediate the proliferation of antigen-reactive donor cells. From our present findings, it appears that carbon, like irradiation, could act by depleting a node of recirculating lymphocytes, thereby weakening its immunological potential against the inoculated lymphocytes.