C-reactive protein does not opsonize early apoptotic human neutrophils, but binds only membrane-permeable late apoptotic cells and has no effect on their phagocytosis by macrophages.

BACKGROUND: It has been reported that C-reactive protein (CRP) binds both leukocyte FcgammaRIIA (CD32) and the plasma membrane of apoptotic cells. Since FcgammaRIIA becomes functionally enabled during neutrophil apoptosis, we sought to determine whether CRP bound to apoptotic neutrophils via FcgammaRIIA. METHODS: We prepared directly labelled CRP and demonstrated that it was essentially free of IgG. We looked for evidence of CRP binding to intact, membrane impermeable apoptotic human neutrophils and to FcgammaRIIA-transfected Jurkat cells. We examined the functional consequences of incubation with CRP upon phagocytosis of apoptotic cells by human monocyte-derived macrophages. RESULTS: We could not detect binding of purified soluble CRP to classical early apoptotic human neutrophils or to FcgammaRIIA-transfected Jurkat cells. In contrast, membrane-permeable late apoptotic neutrophils exhibited strong CRP binding, which comprised both Ca2+-dependent and heparin-inhibitable Ca2+-independent components. However, there was no effect of CRP binding upon phagocytosis of late apoptotic neutrophils by macrophages. CONCLUSION: Potential apoptotic cell opsonins such as CRP may bind only to intracellular structures in cells with leaky membranes that have progressed to a late stage of apoptosis.