We have studied the effect of IgA- and IgG-containing immune complexes
on the production of complement proteins C3, factor B and C2 by human
monocyte-derived macrophages, using biosynthetic labelling, immunopre
cipitation, sodium dodecyl sulphate-polyacrylamide gel(SDS-PAGE) and a
utoradiography. There was a consistent increase in C3 production and s
ecretion with both IgA and IgG immune complexes. This increase appeare
d after a 24-hr incubation period of the macrophages in the presence o
f immune complexes. No change in the biosynthesis of factor B and C2 p
roteins was observed in these experiments. Concomitant with the enhanc
ed C3 biosynthesis, the immune complexes caused an increase in macroph
age tumour necrosis factor (TNF) production; 310 + 24 U/ml/5 x 10(5) c
ells and 430 + 51 U/ml/5 x 10(5) cells for IgA and IgG immune complexe
s, respectively, versus 12 + 8 U/ml/5 x 10(5) cells in the control cel
ls. The presence of prednisolone (2 x 10(-5) M) or dexamethasone (1 x
10(-7) M) inhibited the immune complex-induced TNF production, but had
no effect on C3-increased synthesis, suggesting that the effect of im
mune complexes was not mediated by endogenous TNF production. These fi
ndings may be relevant to the local inflammatory response in IgA immun
e complex-mediated diseases, including IgA nephropathy.