In order to study the role of blood-borne small lymphocytes in the sensitization of rats to renal homografts 2 techniques for the perfusion of isolated rat kidneys were employed: (a) the in vitro perfusion of kidneys with thoracic duct cells suspended in either an artificial medium or in blood; the perfusates were then injected into rats syngeneic with the lymphocyte donors; (b) the in vivo perfusion of kidneys with blood issuing from the femoral artery and returning to the femoral vein of living rats. The degree of sensitization conferred on the recipients by the perfusates was assessed by applying a skin homograft from the kidney donor and scoring the epithelial necrosis at 6 days. The in vitro experiments indicated that parental strain thoracic duct cells, which had passed through an F1 hybrid kidney could confer upon a parental rat sensitivity to an F1 skin graft. Several perfusions with radioactively labelled lymphocytes showed that the injected cells migrated to the lymph nodes and spleen of the recipients Labelled large pyroninophilic cells were occasionally seen in the spleen and lymph nodes of recipients, and it was suggested that these had arisen from the injected cells. Although the in vitro perfusions with blood indicated that renal homografts might sensitize their hosts within 1 hour, the in vivo perfusions suggested that about 5 to 12 hours were required. The more rapid sensitization in vitro was possibly due to the more frequent opportunity for contact between lymphocytes and kidney vascular endothelium which was afforded by the conditions in vitro.

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