Surfactant protein A binds to IgG and enhances phagocytosis of IgG-opsonized erythrocytes.
Surfactant protein (SP)-A and SP-D, immunoglobulins, and complement all modulate inflammation within the lung by regulating pathogen clearance. For example, SP-A binds to and opsonizes a variety of bacteria and viruses, thereby enhancing their phagocytosis by innate immune cells such as alveolar macrophages. Immunoglobulins, which bind to antigen and facilitate Fc receptor-mediated phagocytosis, can also activate complement, a family of soluble proteins with multiple host defense functions. Previous studies showed that SP-A and complement protein C1q interact. Since complement protein C1q binds to IgG and IgM immune complexes, the hypothesis tested in this study was that SP-A, which is structurally homologous to C1q, also binds to IgG and affects its functions. SP-A binds to the Fc, rather than the Fab, region of IgG. Binding is calcium dependent but not inhibited by saccharides known to bind to SP-A's carbohydrate recognition domain. The binding of SP-A does not inhibit the formation of immune complexes or the binding of IgG to C1q. In contrast, SP-A enhances the uptake of IgG-coated erythrocytes, suggesting that SP-A might be influencing Fc receptor-mediated uptake. In summary, this study shows a novel interaction between SP-A and IgG and a functional consequence of the binding.
Lin
Peggy M
PM
Department of Cell Biology, Duke University Medical Center, Durham, NC 27710, USA.
Wright
Jo Rae
JR
D000818:Animals; D019921:Biotinylation; D002118:Calcium / Q000502:physiology; D004912:Erythrocytes / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007074:Immunoglobulin G / Q000502:physiology*; D007700:Kinetics; D009895:Opsonin Proteins / Q000502:physiology; D010587:Phagocytosis; D037662:Pulmonary Surfactant-Associated Protein A / Q000276:immunology / Q000502:physiology*; D011817:Rabbits
D007074:Immunoglobulin G; D009895:Opsonin Proteins; D037662:Pulmonary Surfactant-Associated Protein A; D002118:Calcium