The lymphocyte proliferation in repeatedly studied mixed leukocyte cultures of peripheral white blood cells from a skin graft donor and 2 recipients was significantly increased at the time of graft rejection. This was determined from the increased proportions of mononuclear cells labeling with tritiated thymidine, increased mitotic indices, and the appearance of increased numbers of transformed lymphocytes after rejection of 1st and 2nd skin grafts. The temporarily enhanced response occurred sooner and was of shorter duration after the second than after the first graft, but was quantitatively similar each time. The cell proliferation in the mixed leukocyte cultures of the two recipients was similarly affected by the homograft rejections. The cultures containing three cell populations usually manifested a greater lymphocyte response than corresponding cultures of leukocytes from only two unrelated subjects.

An increase in the ratio of female recipient to male graft donor metaphases in the cultures at the time of enhanced lymphocyte transformation indicated that proliferation of the graft recipient lymphocytes was responsible for the above findings. Unmixed, unstimulated control cultures grown in autologous, the other subjects plasma, or heterologous calf serum failed to support significant lymphocyte transformation. The role of humoral factors and relationship of the in vitro cellular responses to the in vivo homograft reaction are discussed.

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