Role of macrophages as modulators but not as stimulators in primary mixed leukocyte reaction

The role of macrophages (M phi) and that of splenic dendritic cells (DC) in the allogeneic mixed leukocyte reaction (MLR) in the mouse have been investigated. In contrast with the high stimulatory capacity of DC, we obtained no evidence in favor of the competence of M phi, whether Ia + or Ia-, as an autonomous stimulator of MLR. However, M phi were found to modulate the level of MLR. Thus, M phi amplified the low level MLR to low dose DC and apparently suppressed the high level MLR to high dose DC. Ia + M phi seemed superior to Ia- M phi in the MLR-enhancing effect. M phi syngeneic to the responder and those to the stimulator suggest that M phi are modulators of immune responses triggered through the mediation of DC.