Complement and Fc receptor‐mediated phagocytosis of normal and stimulated mouse peritoneal macrophages

Normal and stimulated macrophage populations induced by six different stimuli were tested for their IgG and complement (C)‐mediated phagocytic capacity. A similar phagocytic capacity for sheep red cells (E) coated with IgG was exhibited by normal and stimulated macrophage populations with the exception of thioglycollate‐stimulated macrophages which exhibited a lower phagocytic potential. The extent of attachment and ingestion of E coated with IgM and C 5‐deficient mouse serum (EIgMC) was shown to depend on the concentration of IgM and serum in the opsonizing solutions. A further interaction of macrophage‐EIgMC rosettes with C5‐deficient mouse serum led to ingestion of a high proportion of the attached ElgMC. Heat‐killed yeast cells coated with C5‐deficient mouse complement were ingested to a similar extent by normal and stimulated macrophage populations except for thioglycollate‐stimulated macrophage populations which exhibited about 30% of the phagocytic response. The data support the notion that complement receptors on macrophage surface are not limited in their function to the attachment phase but that they also are capable of mediating phagocytosis. It is suggested that the prevalent protocols of sequential opsonization of E with IgM and C5‐deficient mouse serum do not provide enough bridging moieties between the coated E and macrophages to allow for effective ingestion; the EIgMC complex formed is only capable of attachment to macrophage surfaces.