Opsonophagocytosis of Chlamydia pneumoniae by Human Monocytes and Neutrophils

The human respiratory tract pathogen , which causes mild to severe infections, has been associated with the development of chronic inflammatory diseases. To understand the biology of infections, several studies have investigated the interaction between and professional phagocytes. However, these studies have been conducted under nonopsonizing conditions, making the role of opsonization in infections elusive. Thus, we analyzed complement and antibody opsonization of and evaluated how opsonization affects chlamydial infectivity and phagocytosis in human monocytes and neutrophils. We demonstrated that IgG antibodies and activation products of complement C3 and C4 are deposited on the surface of elementary bodies when incubated in human serum. Complement activation limits infectivity and has the potential to induce bacterial lysis by the formation of the membrane attack complex. Coculture of and freshly isolated human leukocytes showed that complement opsonization is superior to IgG opsonization for efficient opsonophagocytosis of in monocytes and neutrophils. Neutrophil-mediated phagocytosis of was crucially dependent on opsonization, while monocytes retained minor phagocytic potential under nonopsonizing conditions. Complement opsonization significantly enhanced the intracellular neutralization of in peripheral blood mononuclear cells and neutrophils and almost abrogated the infectious potential of In conclusion, we demonstrated that complements limit infection by interfering with entry into permissive cells by direct complement-induced lysis and by tagging bacteria for efficient phagocytosis in both monocytes and neutrophils.