The use of frozen erythrocytes in macrophage studies. 2. Attachment and phagocytosis mediated by the two immunological receptors of the macrophages.

Sheep erythrocytes opsonized with IgG or C3b were frozen in various cryoprotective agents, thawed, and compared to corresponding unfrozen erythrocytes exposed to the cryoprotectants and to unfrozen erythrocytes not exposed to the cryoprotectants (controls) as test particles in macrophage attachment and phagocytosis assays. Fc-receptor-mediated attachment and phagocytosis were not influenced by the use of any cryoprotective agent tested or by freezing the erythrocytes. This was also the case with C3b-receptor-mediated attachment. Phagocytosis via this receptor was negligible in normal macrophages, but tended to be slightly more effective when the test particles had been treated with cryoprotective agents. In vitro stimulation of the macrophages with Escherichia coli endotoxin, however, triggered the capacity to internalize treated and untreated erythrocytes equally.