The role of complement in the stimulation of lysosomal enzyme release by polymorphonuclear leucocytes induced by immune complexes of IgG and of IgM.

The effect of complement components incorporated into precipitated immune complexes (IC) of IgG or of IgM on their capacity for stimulating lysosomal enzyme release from rabbit polymorphonuclear leucocytes was studied in vitro. We have found that: (i) complement causes an amplification in the stimulatory capacity of both classes of IC, the effect being dependent on the concentration of the IC, and higher for the IgM class; (ii) dose-effect experiments of competition by fluid-phase immunoglobulins have shown that IgG (in physiological or smaller concentrations) can inhibit greatly the stimulation by this class of immune complex; this inhibition can be prevented, however, by the presence of complement in the IC (a situation expected to occur in vivo); for IgM immune complexes there was no such competitive inhibition, so complement would not be necessary; (iii) the relevant complement factors must be located in the range C1-C3. These results help us to understand the importance of complement (besides the well-known generation of chemotactic factors) in the mechanisms of tissue injury produced by neutrophils in immune complex diseases.