Diminished uptake and degradation of soluble aggregates of IgG by monocytes of patients with rheumatoid arthritis and vasculitis.

We investigated the capacity of monocytes to degrade soluble aggregates of IgG in vitro in the absence (Fc receptor [FcR] mediated) and presence of complement (FcR and C3 receptor mediated). Adherent monocytes from 33 patients with active rheumatoid arthritis (RA) and rheumatoid vasculitis, 32 patients with inactive RA alone, and 20 healthy controls were incubated with 125I-aggregated IgG (125I-AIgG) of restricted size with or without fresh serum. Normal monocytes degraded 9.8% of 125I-AIgG via FcR alone and the presence of complement enhanced degradation to 2.7%. Degradation of 125I-AIgG via FcR from patients without active RA suggested a depressed function of FcR. The maximal amount of 125I-AIgG which was bound by monocytes from patients with inactive and active RA, however, was increased compared to normals, suggesting a defect in intracellular processing in patients with RA. The degradation of 125I-AIgG in the presence of complement was also significantly depressed for both groups of patients. The monocytes from the patients also had decreased numbers of C3b receptors (CR1). Since CR1 are involved in the enhanced uptake of immune complexes bearing complement, the depressed capacity of monocytes from patients with RA to degrade 125I-AIgG in vitro may be caused both by a diminished uptake as well as a diminished capacity to degrade soluble AIgG.