Mapping the topographic epitopes of a model antigen

The thesis describes the development of a method for determining the antigenicity of different regions of the surface of a protein. The method involved the construction of a set of mutant antigens in which cysteine residues had been introduced at different points in the surface of the antigen. Each mutant protein was tethered through the surface cysteine residue and a bifunctional chemical cross-linker to solid phase, allowing the creation of an array of mutant antigens, each oriented so as to expose or to mask different topological regions of the antigen surface. Indeed, as expected, binding of a set of monoclonal antibodies proved sensitive to the orientation of the antigen. By use of this array of oriented antigen it became possible to compare the different regions of the antigen surface for binding to rabbit antiserum, and thereby to create a topological map of the antibody response. When the immunisations were undertaken with use of Freund's adjuvant, the antibody response to β-lactamase was mainly directed against a region centered on a flexible loop. Furthermore it comprised antibodies cross-reactive to both native and denatured protein. By contrast, in the absence of Freund's adjuvant, the antibody response was more evenly distributed, and with few cross reactive antibodies. This suggested that Freund's adjuvant denatures β-lactamase (as was confirmed experimentally), and that the appearance of dominant epitopes may follow the presentation of denatured protein to the immune system. These observations have implications for understanding the basis for dominance of B-cell epitopes, and also for design of vaccines

Guidance of B Cells by the Orphan G Protein-Coupled Receptor EBI2 Shapes Humoral Immune Responses

SummaryHumoral immunity depends on both rapid and long-term antibody production against invading pathogens. This is achieved by the generation of spatially distinct extrafollicular plasmablast and follicular germinal center (GC) B cell populations, but the signals that guide responding B cells to these alternative compartments have not been fully elucidated. Here, we show that expression of the orphan G protein-coupled receptor Epstein-Barr virus-induced gene 2 (EBI2, also known as GPR183) by activated B cells was essential for their movement to extrafollicular sites and induction of early plasmablast responses. Conversely, downregulation of EBI2 enabled B cells to access the center of follicles and promoted efficient GC formation. EBI2 therefore provides a previously uncharacterized dimension to B cell migration that is crucial for coordinating rapid versus long-term antibody responses

High affinity germinal center B cells are actively selected into the plasma cell compartment

A hallmark of T cell–dependent immune responses is the progressive increase in the ability of serum antibodies to bind antigen and provide immune protection. Affinity maturation of the antibody response is thought to be connected with the preferential survival of germinal centre (GC) B cells that have acquired increased affinity for antigen via somatic hypermutation of their immunoglobulin genes. However, the mechanisms that drive affinity maturation remain obscure because of the difficulty in tracking the affinity-based selection of GC B cells and their differentiation into plasma cells. We describe a powerful new model that allows these processes to be followed as they occur in vivo. In contrast to evidence from in vitro systems, responding GC B cells do not undergo plasma cell differentiation stochastically. Rather, only GC B cells that have acquired high affinity for the immunizing antigen form plasma cells. Affinity maturation is therefore driven by a tightly controlled mechanism that ensures only antibodies with the greatest possibility of neutralizing foreign antigen are produced. Because the body can sustain only limited numbers of plasma cells, this “quality control” over plasma cell differentiation is likely critical for establishing effective humoral immunity