Macrophage subpopulations and the murine F1 x parent mixed leukocyte reaction

The ability of subpopulations of murine spleen cells enriched for macrophages to function as stimulator cells in the F1 x parent-hybrid mixed leukocyte reaction (F1 x P MLR) was assessed. Suspensions of F1 splenic adherent cells--depleted of T cells, B cells, and dendritic cells--were separated into four density-dependent subpopulations on discontinuous gradients of Percoll, and the cells were examined for macrophage characteristics, the presence of la antigen, and their ability to stimulate in the F1 x P MLR. The least dense subpopulation was the most highly enriched with nonspecific esterase-positive (NSE+), phagocytic, and Fc and complement receptor-bearing cells, by comparison with the denser subpopulations. All subpopulations contained similar proportions of Ia+ cells. When the MLR stimulatory activity of the subpopulations was tested with suspensions of parental responder cells, the level of stimulation was directly proportional to the content of NSE+la+ cells in the subpopulations. The densest subpopulation contained no NSE+ cells but did contain la+ cells, which did not induce a MLR. Thus, although necessary for an MLR, la-bearing splenocytes were not by themselves adequate to stimulate the reaction. Rather, cells had to be both NSE+ and la+, indicating that a NSE+ cell-derived factor is involved in this in vitro correlate of cell-mediated immunity. For MLR stimulatory activity, the subpopulations of NSE+la+ cells appear to be functionally homogeneous.