CELL INTERACTIONS IN THE IMMUNE RESPONSE IN VITRO : III. SPECIFIC COLLABORATION ACROSS A CELL IMPERMEABLE MEMBRANE

Tissue cultures with two compartments, separated by a cell impermeable nuclepore membrane (1 µ pore size), were used to investigate the mechanism of T-B lymphocyte cooperation. It was found that collaboration was as effective when the T and B lymphocyte populations were separated by the membrane as when they were mixed together. Critical tests were performed to verify that the membranes used were in fact cell impermeable. The specificity of the augmentation of the B cell response by various T cell populations was investigated. Only the response of B cells reactive to determinants on the same molecule as recognized by the T cells was augmented markedly. Specific activation of thymocytes by antigen was necessary for efficient collaboration across the membrane. The response of both unprimed and hapten-primed spleen cells was augmented by the T cell "factor" although, as expected, hapten-primed cells yielded greater responses. The T cell factor acted as efficiently if T cells were present or absent in the lower chamber. Thus the site of action of the T cell factor was not on other T cells, but was either on macrophages or the B cells themselves. The T cell-specific immunizing factor did not pass through dialysis membranes. The experiments reported here help rule out some of the possible theories of T-B cell collaboration. Clearly T-B cell contact was not necessary for successful cooperation to occur in this system. Possible theoretical interpretations of the results and their bearing on the detailed mechanism of T-B lymphocyte cooperation are discussed

THE RELATIONSHIP BETWEEN ANTIGENIC STRUCTURE AND THE REQUIREMENT FOR THYMUS-DERIVED CELLS IN THE IMMUNE RESPONSE

Certain antigens such as polymerized flagellin are capable of producing relatively normal antibody levels in thymectomized mice, whereas others, including heterologous erythrocytes require the presence of T cells in a helper capacity. The mechanism of thymus-independent antibody production was investigated by comparing the primary IgM responses of spleen cells from ATXBM, XBM, and normal mice to various physical forms of the flagellar antigens of Salmonella adelaide in vitro. No reduction in antibody-forming cell levels to polymerized flagellin over a wide dose range was observed in ATXBM cultures, although the same spleen cells did not respond to an optimal dose of sheep red cells. In contrast, when flagellar determinants were presented in a monomeric form or as flagellin-coated donkey red cells, a highly significant difference was observed between the antibody responses of spleen cells from ATXBM mice and XBM or normal controls. The results suggested that the requirement for T cells in antibody production is not a property of specific antigenic determinants, but depends on the mode of antigenic presentation. The validity of this conclusion was confirmed by using another antigenic determinant (DNP) coupled either to the thymus-independent carrier, POL, or to the thymus-dependent carrier, DRC. Spleen cells from XBM mice produced comparable AFC levels to both forms of DNP, but the results from ATXBM cultures showed a marked difference. The anti-DNP response to DNP-DRC was greatly reduced compared to controls, whereas that to DNP-POL was normal even after prolonged thoracic duct drainage of the ATXBM donors and pretreatment of their spleen cells with anti-θ-serum and complement. The data presented here imply that the role of T cells in humoral immunity is the presentation of antigen to B cells in such a manner as to initiate optimal antibody synthesis

CELL INTERACTIONS IN THE IMMUNE RESPONSE IN VITRO : IV. COMPARISON OF THE EFFECTS OF ANTIGEN-SPECIFIC AND ALLOGENEIC THYMHS-DERIVED CELL FACTORS

The role of soluble factors in cell collaboration was investigated by means of a tissue culture system in which populations of T and B cells were either incubated together or separated from each other by cell impermeable membranes. Histoincompatible T cells were found to augment antibody responses to both thymus-dependent and thymus-independent antigens, whether they were in contact with B cells or not. The properties of the factor released by the T cells in the allogeneic mixture were compared with those of the previously reported antigen-specific mediator found in syngeneic collaborative antibody responses. Unlike the latter, the factor made in allogeneic responses failed to display any degree of antigen specificity either in its induction or in its action, enhancing responses to all the antigens present in the cultures to a similar degree. It was of lower molecular weight than the antigen-specific factor, because it could pass through dialysis membranes as well as nuclepore membranes, whereas the antigen-specific factor could only penetrate nuclepore membranes. Furthermore, the factor made in allogeneic reactions had a different site of action. It acted directly on B lymphocytes, whereas the antigen-specific component acts through macrophages. Although antigen in the presence of the allogeneic factor did not initiate antibody production, it augmented responses once they had been induced by a matrix of antigenic determinants, either mediated by the antigen-specific factor or directly by a thymus-independent antigen. It was therefore considered to act at a later stage of the antibody response, probably as a nonspecific stimulator of immune B cell proliferation. Observations that the effect on the allogeneic factor are more pronounced 2 days after the beginning of the response are in keeping with this interpretation

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

B Cell Receptor–independent Stimuli Trigger Immunoglobulin (Ig) Class Switch Recombination and Production of IgG Autoantibodies by Anergic Self-Reactive B Cells

In both humans and animals, immunoglobulin (Ig)G autoantibodies are less frequent but more pathogenic than IgM autoantibodies, suggesting that controls over Ig isotype switching are required to reinforce B cell self-tolerance. We have used gene targeting to produce mice in which hen egg lysozyme (HEL)-specific B cells can switch to all Ig isotypes (SWHEL mice). When crossed with soluble HEL transgenic (Tg) mice, self-reactive SWHEL B cells became anergic. However, in contrast to anergic B cells from the original nonswitching anti-HEL × soluble HEL double Tg model, self-reactive SWHEL B cells also displayed an immature phenotype, reduced lifespan, and exclusion from the splenic follicle. These differences were not related to their ability to Ig class switch, but instead to competition with non-HEL–binding B cells generated by VH gene replacement in SWHEL mice. When activated in vitro with B cell receptor (BCR)-independent stimuli such as anti-CD40 monoclonal antibody plus interleukin 4 or lipopolysaccharide (LPS), anergic SWHEL double Tg B cells proliferated and produced IgG anti-HEL antibodies as efficiently as naive HEL-binding B cells from SWHEL Ig Tg mice. These results demonstrate that no intrinsic constraints to isotype switching exist in anergic self-reactive B cells. Instead, production of IgG autoantibodies is prevented by separate controls that reduce the likelihood of anergic B cells encountering BCR-independent stimuli. That bacteria-derived LPS could circumvent these controls may explain the well-known association between autoantibody-mediated diseases and episodes of systemic infection