EFFECT OF REPEATED EXPOStrRE TO ALLOANTIGENS ON THE CYTOTOXIC ACTIVITY OF LONG-TERM MIXED LEUKOCYTE CULTURES* BY H. ROBSON MACDONALD,~t HOWARD D. ENGERS,$ JEAN-CHARLES CEROTTINI, AND K. THEODOR BRUNNER

In recent years, many studies have dealt with the generation of cytotoxic thymusderived lymphocytes (CTL) 1 in response to an allogeneic stimulus either in vivo or in vitro (see reference 1 for review). However, very few attempts have been made to determine the ultimate fate of these effector cells. Previous studies of the physical characteristics of splenic CTL in mice undergoing tumor aUograft rejection suggested that a transition from large, low density CTL to small dense CTL may occur with time (2, 3). In other experiments, an anamnestic CTL response was suggested by the demonstration of earlier appearance and higher peak levels of CTL activity in the spleens of mice injected twice with allogeneic tumor ceils (4). Furthermore, in the preceding paper (5), we showed that the cell-mediated cytotoxic response in mixed leukocyte cultures (MLC) was increased when alloimmune spleen cells were used as responding cells. Each of these findings is consistent with the possibility that CTL may differentiate into putative "memory" cells capable of mediating a specific secondary response to aUoantigens. However, since the initial interaction with alloantigen in each of these model systems takes place in vivo, where cellular migration may account for the presence or absence of CTL in any tissue at any given time, it is extremely difficult to conclude that any such lymphocyte differentiation pathway exists. To critically evaluate this hypothesis then it is necessary to generate putative "memory" CT under defined in vitro conditions.