The Carriage and Delivery of Substances to Lymphatic Tissues by Recirculating Lymphocytes. II. Long-term Selective Irradiation of the Spleen and Lymph Nodes by Deposition of Indium-114m

Recirculating lymphocytes were used to carry an isotope of indium, In-114m. The isotope has a half-life of 50 days, and emits both gamma and beta particles, properties that may be useful clinically. In-114m-labelled rat thoracic duct lymphocytes (TDL), injected intravenously, migrated to traffic areas in spleen and lymph nodes. The radioactivity was transferred selectively to a resident, radioresistant phagocytic population located within the marginal zone and red pulp of spleen, where it remained for at least 6 weeks. That the isotope was relatively concentrated in traffic areas, but not elsewhere, suggested that In-114m-TDL could be used as a means of producing total lymphoid irradiation without damaging bone marrow. Recipients of In-114m-TDL developed a profound lymphocytopenia within 2-3 weeks of injection but gained weight at a comparable rate to control rats. The effect of In-114m-TDL injection was also examined in rats injected with a rapidly growing, malignant rat leukaemia. In-114m-TDL delayed the onset of the proliferative phase of the leukaemia from 10 days to 20 days in a dose-dependent fashion. Repeated weekly injections of In-114m-TDL prevented the onset of the leukaemia until the injections ceased after 11 weeks. The potential use of In-114m-labelled lymphocytes in diagnosis (by external imaging) or in treatment (by localized lymphoid irradiation) is considered.