Autoreactive B cells are present in the lymphoid tissues of healthy individuals, but typically remain quiescent. When this homeostasis is perturbed, the formation of self-reactive antibodies can have serious pathological consequences. B cells expressing an antigen receptor specific for self-immunoglobulin-gamma (IgG) make a class of autoantibodies known as rheumatoid factor (RF). Here we show that effective activation of RF+ B cells is mediated by IgG2a-chromatin immune complexes and requires the synergistic engagement of the antigen receptor and a member of the MyD88-dependent Toll-like receptor (TLR) family. Inhibitor studies implicate TLR9. These data establish a critical link between the innate and adaptive immune systems in the development of systemic autoimmune disease and explain the preponderance of autoantibodies reactive with nucleic acid-protein particles. The unique features of this dual-engagement pathway should facilitate the development of therapies that specifically target autoreactive B cells.

Chromatin–IgG complexes activate B cells by dual engagement of IgM and Toll-like receptors

Cyclophilin, a specific cytosolic binding protein responsible for the concentration of the immunosuppressant cyclosporin A by lymphoid cells, was purified to homogeneity from bovine thymocytes. Cation-exchange high-performance liquid chromatography resolved a major and minor cyclophilin species that bind cyclosporin A with a dissociation constant of about 2 X 10(-7) moles per liter and specific activities of 77 and 67 micrograms per milligram of protein, respectively. Both cyclophilin species have an apparent molecular weight of 15,000, an isoelectric point of 9.6, and nearly identical amino acid compositions. A portion of the NH2-terminal amino acid sequence of the major species was determined. The cyclosporin A-binding activity of cyclophilin is sulfhydryl dependent, unstable at 56 degrees C and at pH 4 or 9.5, and sensitive to trypsin but not to chymotrypsin digestion. Cyclophilin specifically binds a series of cyclosporin analogs in proportion to their activity in a mixed lymphocyte reaction. Isolation of cyclophilin from the cytosol of thymocytes suggests that the immunosuppressive activity of cyclosporin A is mediated by an intracellular mechanism, not by a membrane-associated mechanism.

https://science.sciencemag.org/content/sci/226/4674/544.full.pdf

Cyclophilin: A Specific Cytosolic Binding Protein for Cyclosporin A

Publisher Summary Lectins play an important role in the development of immunology. Lectins also find application in serological laboratories for typing blood and determining secretor status, separating leucocytes from erythrocytes, and agglutinating cells from blood in the preparation of plasma. They serve as reagents for the detection, isolation, and characterization of carbohydrate-containing macromolecules, including blood-group antigens. In their interaction with saccharides, lectins serve as models for carbohydrate-specific antibodies, with the important advantage to purify lectins in gram quantities. Lectins are classified according to their carbohydrate-binding specificity that includes D-mannose(D-glucose)-binding lectins and 2-acetamido-2-deoxy-D-glucose-binding lectins. The chapter considers only those lectins that have been purified to homogeneity, and studied with regard to their biophysical, biochemical, and carbohydrate-binding specificity. The chapter also describes the cell-binding and biological properties of lectins. The chapter concludes with the description of several glycopeptide structures showing the carbohydrate-binding loci with which various lectins interact.

The lectins : carbohydrate-binding proteins of plants and animals

B lymphocytes: how they develop and function

Publisher Summary The term “cell-mediated cytotoxicity” applies to lytic reactions that require the participation of lymphoid or non-lymphoid cells but not of added complement. It has been clearly established that several pathways, including different cell types, are involved in cytotoxic reactions in vitro. With membrane-associated antigens, such as transplantation, and tumor-associated antigens, the cytotoxic effect of effector cells on adequate target cells is most often highly specific and requires intimate contact rather than release of diffusible toxic factors. Specific cell-mediated cytotoxicity in vitro can be divided into three categories according to the nature of the effector cells.

Cell-Mediated Cytotoxicity, Allograft Rejection, and Tumor Immunity

Lipopolysaccharides (LPS) from E. coli bacteria were found to be mitogenic for bone marrowderived (B) lymphocytes, but had no effect on thymusderived (T) lymphocytes. When added to normal spleen cells in culture, LPS selectively stimulated the secretion of 19 S proteins, whereas there was no demonstrable increase of 7 S protein synthesis. Spleen cell cultures treated with various doses of LPS exhibited atypical dose response curve with regard to induction of DNA synthesis, 10 μg/ml being optimal, higher and lower concentrations giving lower responses. When direct antibody producing cells to horse and sheep red cells were studied in normal spleen cell cultures exposed to different concentrations of LPS in vitro, it was found that their number increased in parallel with stimulation of DNA synthesis. In contrast, the number of antibody producing cells to LPS itself did not parallel activation of DNA synthesis. Spleen cells from LPS tolerant animals responded with increased numbers of antibody producing cells to heterologous red cells after treatment with LPS in vitro to the same extent as normal spleen cells.



Thus, a B cell mitogen, such as LPS, could activate division in B cells, resulting in an increased number of cells producing antibodies to non-cross-reacting antigens, mimicing the effect of specific antigen.

Induction of immunoglobulin and antibody synthesis in vitro by lipopolysaccharides.

Selective induction of DNA synthesis in T and B lymphocytes

Publisher Summary Paralysis is seriously considered only in the “two specific signal” hypothesis by postulating that the first signal, generated by the combination of antigen with the specific receptor, results in tolerance rather than induction, except if the cell is simultaneously exposed to the second specific signal, generated by the recognition of the antigen by associative antibody. This model of associative recognition is designed in order to explain self-nonself discrimination and denies the existence of truly thymus-independent (TI) antigens as well as the possibility of triggering B cells by any mechanism not involving the first specific signal. The main general conclusion concerning induction of paralysis by TI antigens is that specific B cells are turned off at higher concentrations of the TI antigen than those causing activation. TI antigens do induce specific B cells directly and one cannot exclude that at least some of them also activate other cell types, including T cells, nor that some or possibly all of them interact with macrophages. It is suggested that the effects on other cell types represent trivial or optimizing events on top of the basic, fundamentally effective mechanism of direct B-cell activation. All TI antigens are competent to activate B cells polyclonally and this nonspecific property seems to be responsible for induction of specific cells. The inactivation of competent specific cells is also a result of the interaction between nonspecific surface receptors and the competent ligand.

Thymus-independent B-cell induction and paralysis.

CERTAIN antigens are known to be immunogenic in the absence of thymus-derived (T) lymphocytes1–5. One extreme explanation of this is that all antigens are fundamentally thymus-independent, whereas the contrary view is to deny the experimental evidence for the existence of this type of antigen6. Another explanation for thymus-independency is based on the structure of these molecules, which are characterized by repeating antigenic determinants. This special structure would make it possible for the molecules to establish multiple interactions with the immunoglobulin receptors of the B cells and thus cause their activation7.

B cell mitogenic properties of thymus-independent antigens.

The present experiments were performed in order to analyze the mechanism by which thymus-independent antigens (nonspecific B-cell mitogens) can induce specific immune responses to antigenic determinants present on the same molecule. The hapten NNP was coupled to the B-cell mitogen, lipopolysaccharide (LPS). The conjugate retained full mitogenic activity and bound specifically to NNP-reactive cells. NNP-LPS activated polyclonal as well as specific anti-NNP antibody synthesis, but the optimal concentrations for induction of specific anti-NNP cells were several orders of magnitude lower than the concentrations required for polyclonal activation. These low concentrations failed to activate nonspecific cells, but they induced specific thymus-independent responses of high-avidity NNP-specific cells with the typical kinetics of antigenic responses in vitro. Furthermore, hapten-specific cells were paralyzed by NNP-LPS concentrations that were optimal for induction of polyclonal activation. Specific activation and paralysis could be abolished by free hapten indicating that selective binding of NNP-LPS to hapten-specific cells was responsible for the specificity of the response. However, the triggering signal lacked specificity, since high-avidity specific anti-NNP cells could still be activated by stimulating concentrations of NNP-LPS in the presence of free hapten, even though the Ig receptor combining sites were presumably occupied by NNP. The findings show that B cells with specific Ig receptors for the antigenic determinants on mitogen molecules preferentially bind these molecules and become activated at concentrations still unsufficient to trigger other B cells that lack specific receptors. It is suggested that activation for primary IgM responses in B cells is the result of "one nonspecific signal." This nonspecific signal is provided by the mitogenic properties of some antigens (highly thymus independent or, alternatively, by nonspecific T-cell factors (for highly T cell-dependent antigens), or both, and the surface structures responsible for triggering are not the Ig receptors. The specific Ig receptors only act as passive focusing devices for nonspecific stimuli, entitling the cell to be selectively activated, even though both the signal and the receptors for the triggering are nonspecific.

https://rupress.org/jem/article-pdf/139/1/74/1392889/74.pdf

Göran Möller

Papers

Induction of immunoglobulin and antibody synthesis in vitro by lipopolysaccharides.

Selective induction of DNA synthesis in T and B lymphocytes

Thymus-independent B-cell induction and paralysis.

B cell mitogenic properties of thymus-independent antigens.

Mechanism of thymus-independent immunocyte triggering : mitogenic activation of b cells results in specific immune responses