Radioautographs of infiltrative cells in the kidneys of (Lewis x BN)F1 rats labeled with tritiated thymidine (TdR3H) before the subcapsular injection of parental (Lewis) thoracic duct lymphocytes (TDL) showed a predominantly host-proliferative response by 4 days after grafting. The immediate renal incorporation of TdR3H was used to measure the local graft-vs.-host (GVH) reactions. Substantial reactions could still be induced in the face of the considerable degree of leukopenia after 400 R whole body γ-irradiation. These results suggest that radioresistant cells are capable of carrying on the appropriate host activities and that the weakness of GVH reactions induced after higher doses of irradiation may be due to impairment of the mitotic mechanism of host cells. The importance of circulating leukocytes as a source of immunogenic stimulation was nevertheless substantiated by inducing local GVH reactions with Lew TDL in chimeric parental-type rats that had been repopulated with F1 bone marrow. This result also emphasizes the nonspecific nature of tissue destruction in the renal GVH reaction in confirmation of Elkins. In this and other situations in which B cells were the predominant F1 type elements available for interaction with parental-type TDL the reactions were nearly equivalent or equivalent to those in the appropriate controls.

Typical local GVH reactions could be induced in heavily irradiated hosts by an inoculum of combined parental and F1-type TDL in the apparent absence of mononuclear phagocytes.

The possible relationship between the activation of host lymphocytes, the involvement of B cells, and the nonspecific nature of tissue damage in the renal GVH is discussed.

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