Precipitated immune complexes of IgM as well as of IgG can bind to rabbit polymorphonuclear leucocytes but only the immune complexes of IgG are readily phagocytosed.

We have shown by in vitro experiments, using immunofluorescence techniques, that precipitated immune complexes of IgM antibodies and ovalbumin (ICIgM) are able to bind to rabbit blood polymorphonuclear leucocytes (PMN), as well as immune complexes of IgG antibodies (ICIgG). This binding capacity for both classes of immune complexes is exhibited by more than 80% of the PMN cell population and is independent of Ca2+ in the medium. For ICIgG the binding to PMN can be completely inhibited by preincubation of the cells with soluble IgG used at physiological concentrations (competition for the Fc gamma receptors) while for ICIgM there is no such inhibition by fluid-phase IgM. After binding to the leucocytes there was a striking difference in the fate of ICIgM and ICIgG: whereas the ICIgG was readily phagocytosed (endocytosed), the ICIgM remained mostly on the cell surface, being only poorly endocytosed after 1 hr incubation at 37 degrees. This was demonstrated by a quantitative fluorimetric method developed to assay phagocytosis of immune complexes, and was confirmed by a qualitative fluorescence quenching technique. These results may have implications for understanding the fate of these classes of immune complexes formed in circulation or deposited in tissues, and the participation of PMN in inflammatory reactions and tissue injury in immune complex diseases.