Properties of memory T lymphocytes isolated from the mixed leukocyte reaction.

During the primary mixed leukocyte reaction, T lymphocytes of the lyt-2- helper subclass proliferate in response to transplantation antigens on allogeneic dendritic cells. We have isolated populations of antigen-specific proliferating lymphoblasts and recultured them in fresh medium. Within 2 days, the blasts become smaller in size, lose responsiveness to T-cell growth factor or interleukin 2, but retain vigorous reactivity to the original alloantigen. Two new biologic properties of these "memory" lymphocytes have been noted. First, they primarily respond to alloantigen on dendritic cells, whereas freshly sensitized lymphoblasts react to allogeneic dendritic cells, macrophages, and B lymphocytes. Second, the memory lymphocytes quickly aggregate with dendritic cells that are either syngeneic or allogeneic, but not with B cells. The aggregates that form with syngeneic dendritic cells disassemble within hours and do not release interleukin 2 or proliferate. The aggregates that form with allogeneic dendritic cells remain intact, release large amounts of interleukin 2 on the first day of culture, and synthesize DNA on the second day. Therefore, dendritic cells actively cluster memory lymphocytes by an antigen-independent mechanism, and this may underlie the heightened functional activity of each cell type.