Effect of polymeric IgG on human accessory cell function

Circulating immune complexes are considered to have a profound effect on host defense mechanisms against invading pathogens and to modulate cellular interactions required for an appropriate course of the immune response. In this study we have investigated the influence of polymeric IgG (used as a model system for immune complexes) on accessory functions of human monocytes. We show that a short (1 hr) incubation of human monocytes in the presence of polymeric IgG (16 hr prior to antigen pulsing) led to a significant decrease of these cells' antigen-presenting capacity while accessory functions in alloantigen- or mitogen-driven proliferation systems remained unimpaired. The polymeric IgG-induced impairment of antigen presentation, which was assessed by diminished proliferation of antigen-reactive T cells following stimulation by antigen-pulsed polymeric IgG-treated or Dulbecco's phosphate-buffered saline (PBS-D)-treated control monocytes, could not be attributed to the generation of suppressor mechanisms (no release of soluble suppressor factors, no induction of suppressive monocytes). The release of interleukin-1 by polymeric IgG-treated monocytes and PBS-D-treated monocytes was comparable and polymeric IgG did not down modulate major histocompatibility complex (MHC) class II molecules already expressed in the monocyte plasma membrane. Profound changes in the monocyte plasma membrane occurring subsequent to polymeric IgG treatment possibly accompanied by altered kinetics of MHC class II reexpression are likely to contribute to the observed decrease of antigen presentation.