C3-mediated release of prostaglandin from human monocytes. Behaviour in short-term culture

C3b and C3bi, cleavage products of the third component of human complement (C3), stimulate purified human monocytes to release immunoreactive prostaglandin (PG) and thromboxane B2 (TxB2) in vitro. The stimulant must be present in culture for more than 4 hr to achieve maximal PG release during a 24 hr culture period. Preculturing monocytes for 24 hr or more greatly diminishes the capacity of such cells to be stimulated to release PG or TxB2. This diminished capacity is not simply due to loss of complement receptors by a large percentage of cultured cells, since the percentage of C3b receptor-bearing cells is similar at the inception (85%) and after 120 hr of culture (84%). The addition of arachidonic acid (AA) at a concentration of 2.5 micrograms/ml at the beginning of the culture period enhances the PG release induced by C3b or C3bi. However, addition of the same concentration of AA to precultured cells fails to restore the ability of these cells to respond to C3b, C3bi, or AA. The diminished release of PG and TxB2 into the culture medium is not accompanied by an increase in release of PGF2 alpha or other eicosanoids, as determined by thin layer chromatography. Cells that were cultured in the presence of C3b for 24 hr will, however, respond to the addition of AA during the second 24 hr of culture by releasing PG and TxB2. Thus, engagement of C3 receptors by soluble ligands influences the expression of the PG-secretory phenotype by cultured human monocytes.