Resistance to suppression and acquisition of memory by human T cells.

Human memory cells acquire resistance to several types of suppressor cells, including MLR generated suppressor cells. These data suggest one possible mechanism of that resistance, namely retention of IL-2 receptors in the resting state. Cells from a 7-day MLR were separated on a single step percoll gradient. All proliferating cells were found in the interface. Pellet cells were nondividing. Interface and pellet cells had equivalent memory function in a secondary MLR. Thus, there appear to be at least two subpopulations of memory cells, including one that is primed without undergoing division. These subpopulations are functionally distinct. Interface memory cells were 30-50% more resistant than pellet memory cells to MLR generated suppressor cells. On culture day 10, neither pellet nor interface cells displayed significant spontaneous proliferation but exogenous Interleukin 2 (IL-2) produced up to five times as much proliferation in interface cells as in pellet cells. Further, FACS analysis with an anti-TAC equivalent antibody also showed that significantly more interface cells have surface receptors for IL-2. Thus, cells that had previously divided continue to have more and/or higher affinity receptors for IL-2 even after return to the resting state. If a mechanism of suppression in the mixed lymphocyte reaction is to reduce the synthesis/release of Il-2, memory cells may acquire their relative resistance to this suppression by virtue of the increased IL-2 sensitivity of this discrete subpopulation.