Long-term maintenance in vitro of human T cells by repeated exposure to the same stimulator cells. Differences when using repeated stimulation in allogeneic mixed leukocyte culture and when using stimulation with autologous lymphoblastoid cells.

Cells from one-way human mixed leukocyte cultures (MLC) which had reverted to small lymphocytes after 2 weeks' incubation responded with accelerated kinetics and higher thymidine incorporation on restimulation with lymphocytes or lymphoblastoid cell line (LCL) cells having relevant antigens. In contrast to fresh lymphocytes, they did not respond to autologous LCL cells. Cultures could be restimulated every second week with relevant allogeneic lymphocytes and could thus be maintained for periods of up to 4 months. Almost all these cultured cells had T-cell characteristics, during stimulation as well as in their reverted phase. The response to phytohemagglutinin (PHA) successively disappeared with repeated allogeneic restimulation, whereas the response to the relevant lymphocytes and cells of related donors was maintained. When lymphocytes had been stimulated with autologous LCL cells, the restimulation response was accelerated, although lower than after the primary stimulation. Restimulated cultures could not be maintained by further restimulation. Allogeneic and autologous LCL were equally efficient restumulators. A low level of stimulation was also achieved with allogeneic lymphocytes. The PHA response was usually reduced.