Natural IgM mediates complement-dependent uptake of Francisella tularensis by human neutrophils via complement receptors 1 and 3 in nonimmune serum.
A fundamental step in the life cycle of Francisella tularensis is bacterial entry into host cells. F. tularensis activates complement, and recent data suggest that the classical pathway is required for complement factor C3 deposition on the bacterial surface. Nevertheless, C3 deposition is inefficient and neither the specific serum components necessary for classical pathway activation by F. tularensis in nonimmune human serum nor the receptors that mediate infection of neutrophils have been defined. In this study, human neutrophil uptake of GFP-expressing F. tularensis strains live vaccine strain and Schu S4 was quantified with high efficiency by flow cytometry. Using depleted sera and purified complement components, we demonstrated first that C1q and C3 were essential for F. tularensis phagocytosis, whereas C5 was not. Second, we used purification and immunodepletion approaches to identify a critical role for natural IgM in this process, and then used a wbtA2 mutant to identify LPS O-Ag and capsule as prominent targets of these Abs on the bacterial surface. Finally, we demonstrate using receptor-blocking Abs that CR1 (CD35) and CR3 (CD11b/CD18) acted in concert for phagocytosis of opsonized F. tularensis by human neutrophils, whereas CR3 and CR4 (CD11c/CD18) mediated infection of human monocyte-derived macrophages. Altogether, our data provide fundamental insight into mechanisms of F. tularensis phagocytosis and support a model whereby natural IgM binds to surface capsular and O-Ag polysaccharides of F. tularensis and initiates the classical complement cascade via C1q to promote C3 opsonization of the bacterium and phagocytosis via CR3 and either CR1 or CR4 in a phagocyte-specific manner.
Inflammation Program, University of Iowa and Veterans Administration Medical Center, Iowa City, IA 52242, USA.
Justin T
JT
Schwartz
Jason H
JH
Barker
Matthew E
ME
Long
Justin
J
Kaufman
Jenna
J
McCracken
Lee-Ann H
LA
Allen
D000328:Adult; D000818:Animals; D005604:Francisella tularensis / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D007106:Immune Sera / Q000502:physiology*; D007075:Immunoglobulin M / Q000097:blood / Q000502:physiology*; D016177:Macrophage-1 Antigen / Q000502:physiology*; D009504:Neutrophils / Q000276:immunology* / Q000378:metabolism / Q000382:microbiology*; D009895:Opsonin Proteins / Q000378:metabolism; D010587:Phagocytosis / Q000276:immunology; D011951:Receptors, Complement / Q000502:physiology*; D017463:Receptors, Complement 3b / Q000502:physiology*; D012756:Sheep
C509991:CR1 protein, human; D007106:Immune Sera; D007075:Immunoglobulin M; D016177:Macrophage-1 Antigen; D009895:Opsonin Proteins; D011951:Receptors, Complement; D017463:Receptors, Complement 3b