In Vitro Generation of Memory Lymphocytes

After rejection of tissue allografts, animals acquire immunological memory to the transplantation antigens of the graft (1). The memory thus acquired manifests itself in several distinct reactions: (a) Accelerated rejection of a second graft antigenically similar to the first transplant (second set effect) (1). (b) Enhanced rejection of tissue grafts by normal animals receiving lymphocytes from immunized donors (1,:2). (c) Greater efficiency in breakdown of tolerance in animals receiving lymphocytes from immunized donors (3). (d) Accelerated transformation of lymphocytes re-exposed to the sensitizing cells in vitro (4). The immunological memory has been attributed to the clonal progeny of lymphocytes which responded to the antigens of the primary graft. Thus, the increase in number of specific antigen-reactive cells seemed to account for the enhanced response in sensitized animals. On the basis of this, one would expect lymphocytes from animals presensitized to histocompatibility antigens to react more vigorously than normal lymphocytes in both graft vs. host and the mixed lymphocyte culture reactions. However, this is not the case (4, 5), suggesting that immunological memory may be based on changes in the lymphocytes themselves, rather than on an increased number of specific antigen-reactive lymphocytes (6). To analyze the behavior and properties of memory cells, one needs pure populations of such cells, preferably in an in vitro system. In previous studies (7-9) we showed that blast cells, obtained by stimulation of rat lymphocytes with phytomitogens, redifferentiate, after removal of the mitogen, to small lymphocytes termed "secondary lymphocytes." Studies on the pattern of responsiveness of these lymphocytes led us to the conclusion that the secondary lymphocytes obtained by stimulation with pokeweed mitogen (PWM) 1 represented a population of memory cells specific to PWM (9, 10). Blast cells have also been obtained by sensitizing rat lymphocytes to xenogeneic or allogeneic fibroblast monolayers (11, 12). In this paper we show that these blasts, when removed from the foreign sensitizing monolayers and replated on syngeneic fibroblasts, transformed to secondary lymphocytes.