C1q-independent activation of neutrophils by immunoglobulin M-coated surfaces

Neutrophil granulocytes are known to rapidly adhere and undergo frustrated phagocytosis upon contact with immunoglobulin and/or complement protein opsonized artificial surfaces. In this study, we examined the relation between serum protein deposition and human neutrophil activation on hydrophobic glass and silicon model surfaces that were coated with immunoglobulin G or M (IgG/IgM), both initiators of the classical complement pathway. Protein adsorption from normal human serum (NHS) was quantified with null-ellipsometry combined with antibody techniques. The neutrophil oxygen radical production was registered by luminol-amplified chemiluminescence (CL) and the morphology, as well as changes in the content of filamentous actin (F-actin), were documented by fluorescence microscopy. Complement factor 3 (C3) bound to both IgG- and IgM-coated surfaces, but surprisingly C1q was found only on IgG-coated surfaces. Both immunoglobulins triggered complement dependent neutrophil activation. However, CL and F-actin accumulation were found sensitive to the presence of C1q in the serum only at the IgG-coated surface. We suggest that spontaneously adsorbed IgM activates the complement system and interacts with neutrophils by C1q-independent mechanisms.