The influence of T lymphocytes on antibody-induced B-cell suppression

Negative feedback regulation of IgE synthesis by murine CD23

Immunoglobulin E is found in nanogram amounts in normal human and mouse serum. It is increased during parasitic infestations and mediates allergy. CD23, the low-affinity receptor for IgE (Fc epsilon RII), has been proposed as an important regulator of IgE synthesis. The type-II transmembrane lectin CD23 is expressed in the mouse on B cells and follicular dendritic cells. In humans there are two forms of CD23 which differ in their intracellular amino-terminal 6/7 amino acids; expression of the A-form corresponds to that of murine CD23, whereas the B-form is also found on T and other haematopoietic cells. CD23 has been implicated in cellular adhesion, antigen presentation, as a growth and differentiation factor for human B, T and plasma cells, and as a signal transduction molecule (reviewed in refs 3, 8). Here we disrupt the gene coding for murine CD23 (ref. 9) to clarify the role of CD23 in vivo and find that B- and T-cell development is normal in these CD23-deficient mice. Immune responses to the helminth Nippostrongylus brasiliensis are unaffected. In contrast, immunization with thymus-dependent antigens leads to increased and sustained specific IgE antibody titres compared with controls. Formation of germinal centres is normal. These results suggest that murine CD23 acts as a negative feedback component of IgE regulation.

Circulating immune complexes may play a regulatory and pathogenic role in experimental autoimmune uveoretinitis

We compared the time course of changes in serum levels of circulating immune complexes (CICs) and of IgG antibody after sensitization of albino Lewis and pigmented Lister strain rats with uveitogenic (retinal S-antigen) and non-uveitogenic (ovalbumin) protein antigens of comparable molecular weight. Normal levels of CICs were far lower in Lewis rats in which experimental autoimmune uveoretinitis (EAU) takes the form of a severe panuveitis, than in Lister rats, in which the disease is mild, focal, confined to the posterior segment, and of lower incidence. After sensitization with either S-antigen or ovalbumin, polyethylene-glycol-precipitable CIC (PEG-CIC) peaked and fell as IgG antibody levels rose in both rat strains. However, peak levels of PEG-CIC were lower and subsequent IgG antibody levels were higher in the Lewis strain than in the less susceptible Lister strain. In both strains of rat these linked PEG-CIC/IgG antibody responses occurred earlier after sensitization with uveitogenic (S-) antigen than with ovalbumin, whether or not individual S-antigen-sensitized Lister rats developed EAU. In contrast, complement-binding CIC rose substantially only in those rats of both strains displaying EAU in response to S-antigen and not in response to ovalbumin. We suggest that immune complex (idiotypic) regulation of IgG antibody responses may be more readily perturbed by a pathogenic autoantigen (S-antigen) than by a bland antigen (ovalbumin). We also suggest that differences between the balance of regulatory and pathogenic CIC responses to uveitogenic retinal antigen may underlie or reflect strain differences in susceptibility to and severity of EAU.

How many signals are enough?

The many signals that control the progress of various immune responses to both foreign and self antigens can be divided into no less than three major groups. The first group is the initial positive stimulus, associated with activation events through antigen receptors and their associated proteins. These signals launch lymphocytes in their response to antigen, either foreign or self. The second group of signals is negative and involves various end products and interactions between cells, all recognizing antigen. These signals are endogenous to the reacting cell, or nearly so (two interacting cells from the same clone, daughter cells, which are in the same locale and bind to the same ligand). The third group (the prevention of end product feedback, involving various forms of antigen presentation, T cell contributions, rheumatoid factor activity, and other mechanisms) is more likely to occur with nonself antigens, which are temporally and spatially more restricted than self antigens. Experimental evidence for this immunological schema is summarized and clarified in its relationship to the Bretscher-Cohn theory of self-nonself recognition and to suppressor cell and idiotype-antiidiotypic theories.

Clonalism--the myth?

Two regulatory mechanisms, based on the contrasting concepts of imprinting (clonal theories and idiotypic networks) and of ongoing regulation of immune responses (by antigen and end products with specificity for antigen), give rise to different predictions and approaches to the question of autoimmunity and autoimmune disease. Both concepts have legitimacy, however, if a ranking in terms of explicative power must be given, ongoing regulation is more plausible since it accounts more fully for basic events in immune responses and in autoimmune phenomena. Many instructive findings have emerged from experiments based on this latter concept, furthermore, the approach has only received limited notice and, thus, has not yet been exhausted.

Rheumatoid factor blocks regulatory Fc signals

Immunosuppressed cultures of murine spleen cells, partly deprived of T cells and antigen-stimulated, can be reconstituted to near full activity in their antibody-forming cell response with murine rheumatoid factors (RF). The dose of RF required for recovery of 50% of the reconstitutable immune response was 10-100 ng and reconstitution was blocked by intact murine IgG added to the cultures. IgG subclass specificity of RF was demonstrated; RF specific for IgG2a was more potent than RF specific for IgG1 in reconstituting the response. Synergy was observed between RF added at culture initiation and late-acting B-cell differentiation factors. The greater the degree of T-cell deprivation, the more stringent the conditions needed for reconstitution. Suitable conditions for reconstitution with profound T-cell depletion included the limited reconstitution by specific RF, the synergistic action of RF with late-acting T-cell-replacing supernatants, and multiple additions of a number of RFs to the cultures on Days 0, 1, and 2. RF was also shown to block Fc-dependent immunosuppression by added antigen-antibody complexes. These results are interpreted as favoring the hypothesis put forward previously that the normal production of RF acts to reduce T-cell dependency by preventing negative Fc signal transmission by immune complexes on the B-cell surface. Abnormal production of RF may be a primary destabilizer of the immune responses leading to autoimmunity.

Murine Fc gamma receptor proteins: identification of a previously unrecognized molecule with a monoclonal antibody (12-15)

Previously we studied differential expression of cell surface molecules between the metastatic murine lymphoma ESb and an adhesion variant ESb-MP. Here we describe the specificity of a monoclonal antibody (12-15) that showed strong binding to the adhesion variant and weak reactivity against ESb cells. The antibody also reacted to lymphoid but not to macrophage-derived cell lines and immunoprecipitated a molecule of approx. 60-69 kDa from ESb-MP cells. N-terminal sequencing of the antigen revealed identity to the beta protein of mouse Fc gamma receptors. Using monoclonal antibodies against Fc gamma receptors (2.4G2 and K9.361) in immunofluorescence assays and cDNA probes specific for alpha, beta 1 and beta 2 Fc receptor transcripts in Northern blot experiments the differential expression of Fc receptors in ESb and ESb-MP cells was confirmed. Biochemical analysis of endoglycosidase F-treated precipitates revealed that antibody 12-15 reacted to products of all three transcripts with molecular masses for the protein core of 38.5 kDa (beta 1), 34 kDa (beta 2) and 31 kDa alpha). In addition, an unknown protein of 37 kDa (termed beta 3) was identified by antibody 12-15 which could also be detected in ESb cells and EL4 cells. Antibodies 2.4G2 and K9.361 did not react to the beta 3 chain but reacted to varying extents to the other Fc proteins in macrophage and lymphoid cells. Comparison by peptide mapping of the novel beta 3 chain to beta 1, beta 2 and alpha proteins revealed similar, but also distinct peptides. The tissue-specific reactivity of monoclonal antibody 12-15 is likely to be due to a carbohydrate epitope associated with all Fc gamma receptors in lymphoid but not macrophage cell lines.

Interference with Fc signals increases an antibody response by T-cell-deprived cultures to a T-dependent antigen

Affinity column purified goat anti-mouse immunoglobulin antibodies specific for the Fc portion of IgG increased an in vitro antibody response to a T-dependent antigen when T cells were limiting. Picogram amounts of specific anti-Fc antibody at culture initiation and nanogram quantities up to 3 days were required to demonstrate this effect. The demonstration of reconstitution by anti-Fc antibodies requires that the cultures be T-cell depleted and stimulated by antigen. These results support the concept that anti-Fc antibody and T cells block endogenously generated negative Fc signals.

Retention of immune complexes by murine lymph node or spleen follicular dendritic cells. Role of antibody isotype

Using monoclonal anti-trinitrophenyl (TNP) antibodies complexed to TNP-myoglobin-coated gold particles, we analysed at the ultrastructural level the retention by follicular dendritic cells (FDC) of immune complexes containing various antibody isotypes. Gold-labelled immune complexes were injected subcutaneously or intravenously into naive mice and, after 24 h, germinal centres of draining lymph nodes or spleen were examined by electron microscopy. FDC generally retained complexes containing IgG2a and IgG2b better than those formed with IgG1 or IgG3. IgM was rarely retained. FDC isolated from lymph nodes or spleens were incubated in vitro with gold-labelled complexes in a serum-free medium. IgG2a and IgG2b complexes were also retained in vitro in large quantities by FDC; IgG1 and IgG3 complexes were retained in smaller quantities or in highly variable quantities compared with IgG2; IgM complexes were rarely seen on FDC. There was no difference between FDC isolated from lymph nodes or from spleen with respect to the Ig isotypes required for Fc-mediated retention of immune complexes.

Lymphokines active in B cell proliferation and differentiation

Activation of B cells from a resting to an Ig-secreting cell is a multistep event regulated by T cells and their products. These steps include (1) induction of the clonally selected B cell, (2) growth, and (3) maturation of the proliferating cell to Ig secretion. In model systems, induction can be achieved either by direct signalling to the B cells by T cells recognizing surface structures without Ig involvement or, alternatively, via Ig only. Under physiologic conditions a cooperativity of the two events is assumed. Following induction, B cells are supposed to express receptors for growth and maturation factors produced by the neighboring T cell. Growth and differentiation are mediated by distinct molecules.