Tracing uptake of C3dg-conjugated antigen into B cells via complement receptor type 2 (CR2, CD21)

Abstract

Electron microscopy was used to study the internalization and delivery of ligands for complement receptor type 2 (CR2, CD21) to endocytic compartments of B-lymphoblastoid Raji cells. Opsonized antigen was mimicked with purified C3dg conjugated to colloidal gold. C3dg-gold bound specifically to the cell surface in a time-dependent manner, and preincubation of the cells with a monoclonal antibody blocking the CR2 ligand-binding site completely inhibited any C3dg-gold binding. Notably, the binding of C3d-gold was confined to cell surface protrusions, eg, microvilli. C3dg-gold was apparently internalized through coated pits located at the bases of microvilli and could be traced to different compartments of the endocytic pathway. The morphologic characteristics and intracellular distribution of these multivesicular or multilaminar structures were compatible with those of compartments known to harbor major histocompatibility complex (MHC) class II molecules. Immunolabeling showed that the internalized C3dg-gold colocalized with MHC class II in these structures. These data provide the first ultrastructural evidence that complement-coated antigens are endocytosed by antigen-nonspecific B cells by CR2 and are delivered to the compartments in which peptide loading for antigen presentation occurs. They support the notion that CR2 may play a role in antigen presentation by B cells regardless of B-cell receptor specificity.

Pivotal Advance: Characterization of mouse liver phagocytic B cells in innate immunity

“…In addition, the phagocytosis-promoting activity of fresh mouse serum was significantly inhibited by anticomplement C3 antibody, which reacts with mouse C3, as well as breakdown C3 products, C3b, iC3b, and C3dg. C3dg reportedly binds to the CD21 of B cells [16]. Therefore, B cells might phagocytose E. coli more efficiently in the liver via opsonization of E. coli by serum factors, including complement components.…”

Impaired Antibody Responses but Normal Proliferation of Specific CD4+ T Cells in Mice Lacking Complement Receptors 1 and 2

“…In vitro, B cells can take up C-Ag complexes via CR1 ⁄ 2 and present the Ag to T cells [26][27][28][29][30][31][32] and we cannot exclude that this takes place also in vivo in some situations. However, the important conclusion from the present study is that there are indeed situations where this mechanism cannot explain the impaired Ab responses seen in the absence of CR1 ⁄ 2.…”

“…In some reports, an important role for follicular dendritic cells has been demonstrated [21,22], suggesting that increased Ag retention in germinal centres explains the contribution of CR1 ⁄ 2 to Ab responses. In other studies, B cells were the major effector cells [11,12,18], an observation that could be explained by at least three different mechanisms: (i) transportation of C-coated Ag from the marginal zone to B cell follicles by CR1 ⁄ 2 + marginal zone B cells [23,24], (ii) increased signalling to the B cell caused by co-crosslinking of BCR and CR2 [19,25] or (iii) increased uptake of C-coated Ag via CR1 ⁄ 2 on B cells, followed by more efficient Ag presentation to T cells [26][27][28][29][30][31][32].…”

“…In C3 ) ⁄ ) mice, tested in the ex vivo approach, T-cell priming was either impaired [35] or normal [34]. On the other hand, in vitro observations unanimously show that B cells can indeed endocytose C-containing immune complexes via CR1 ⁄ 2 and present antigenic peptides to T cells [26][27][28][29][30][31][32].…”

Rod-shaped and fluorine-substituted hydroxyapatite free of molecular immunopotentiators stimulates anti-cancer immunity in vivo

“…The uptake of antigens by the immune cell is the first step that eventually leads to a specific immune response. 25,26 Maximizing the efficiency of antigen presentation is considered to be the key point for the successful development of a therapeutic immune response. 27 In vitro immunogenic activity and cytotoxicity of rod-shaped HA and FHA nanoparticles were evaluated by culturing the nanoparticles with BMDCs (Fig.…”