Monocyte lysosomal enzyme release in response to naturally occurring circulating immune complexes

To investigate the influence of naturally occurring immune complexes (ICs) on monocyte lysosomal enzyme release, blood monocytes have been isolated from normal human volunteers, purified, and maintained in culture for 5 days. These cells have been exposed in vitro to sera containing ICs and to the same sera depleted of ICs with 2% polyethylene glycol (PEG). IC-containing sera have been shown to result in increased lysosomal enzyme release (N-acetyl-beta-glucosaminidase (NAG) and beta-glucuronidase) sustained for up to 5 days even after brief (24-h) exposure (NAG activity at 120 h, nmol substrate hydrolysed ml-1 culture supernatant: control, 98 +/- 27; 2% PEG control, 70 +/- 10; IC serum exposure for 120 h, 305 +/- 73; IC serum exposure for 24 h, 330 +/- 95; exposure to IC-depleted sera 120 h, 158 +/- 93). Further analysis showed no relationship between the total IC concentration and enzyme release, nor was there any relationship between the complement component (C3) composition of the complexes and degree of enzyme release. These results confirm the potential importance of circulating IC on monocyte activation in man and indicate that this could result in changes in macrophage activity at the sites of inflammation.