Evidence that agarose must be internalized to stimulate mouse macrophages in vitro

Agarose stimulation of macrophages in vitro was studied. Under conditions where agarose was ingested, stimulation was detected during 24-48 h of incubation at a time when the agarose increasingly was concentrated in the perinuclear region. Removal of extracellular agarose after 24 h when endocytosis had reached a plateau did not reduce the stimulatory effect. Preincubation for 4 days with dextran sulphate in concentrations reported to inhibit phagosome-lysosome fusion potentiated strongly the stimulatory effect. In all situations in which agarose was not internalized--in teflon tubes where the cells remain in suspension, on glass cover slips with inhibitors (2-deoxy-D-glucose, cytochalasin B), or with large, noningestible Sepharose beads--no stimulation was recorded. The possibility is discussed that stimulation of macrophages by agarose may be related to complement activation in phagosomes.