Immunoregulation by Fc signals

Beginnings of coinhibition

Costimulatory and coinhibitory mechanisms appear to be involved throughout immune responses to control their specificity and level. Many mechanisms operate; therefore, various theoretical models should be considered complementary rather than competing. One such coinhibitory model, pictured in 1971, involves the crosslinking of antigen receptors with inhibitory Fc receptors by antigen/antibody complexes. This model was prompted by observations that the Fc portion of antibody was required for potent suppression of immune responses by antibody. The signal via the antigen receptor wakes up T or B cells, providing specificity, while costimulators and coinhibitors stimulate or inhibit these awoken cells. The recent observations that administration of monoclonal anti-SARS-CoV-2 spike antibodies in early COVID-19 patients inhibits the induction of clinically damaging autoimmune antibodies suggest they may provide negative Fc signals that are blocked in COVID-19 patients. Furthermore, the reduced ability of SARS-CoV-2 antigen to localize to germinal centres in COVID-19 patients also suggests a block in binding of the Fc of antibody bound to antigen on FcγRIIb of follicular dendritic cells. The distinction between self and foreign is made not only at the beginning of immune responses but also throughout, and involves multiple mechanisms and models. There are past beginnings (history of models) and current and future beginnings for solving serious clinical problems (such as COVID-19) and different types of models used for understanding the complexities of fundamental immunology.

Rheumatoid Factor: A Novel Determiner in Cancer History

Simple Summary

Rheumatoid factors are autoantibodies that characterize different autoimmune diseases, in particular rheumatoid arthritis, but that can also be found in the sera of the general healthy population. They have been mainly studied in the context of autoimmune diseases, but some evidence have suggested an association between their presence and the predisposition to develop cancer as well as a facilitation of cancer growth and progression in oncologic patients. In this review, for the first time we thus analyze and discuss the possible roles that these autoantibodies can assume in tumor history, from determiners of a heightened susceptibility of developing cancer to drivers of a reduced response to immunotherapies.

Abstract

The possible interplay between autoimmunity and cancer is a topic that still needs to be deeply explored. Rheumatoid factors are autoantibodies that are able to bind the constant regions (Fc) of immunoglobulins class G (IgGs). In physiological conditions, their production is a transient event aimed at contributing to the elimination of pathogens as well as limiting a redundant immune response by facilitating the clearance of antibodies and immune complexes. Their production can become persistent in case of different chronic infections or diseases, being for instance a fundamental marker for the diagnosis and prognosis of rheumatoid arthritis. Their presence is also associated with aging. Some studies highlighted how elevated levels of rheumatoid factors (RFs) in the blood of patients are correlated with an increased cancer risk, tumor recurrence, and load and with a reduced response to anti-tumor immunotherapies. In line with their physiological roles, RFs showed in different works the ability to impair in vitro anti-cancer immune responses and effector functions, suggesting their potential immunosuppressive activity in the context of tumor immunity. Thus, the aim of this review is to investigate the emerging role of RFs as determiners of cancer faith.

A longitudinal study of the relationship between galactosylation degree of IgG and rheumatoid factor titer and avidity during long-term immunization of rabbits with BSA

Although IgG with reduced content of galactose has been implicated as important in the autoimmune rheumatoid factor (RF) response in rheumatoid arthritis (RA), relatively little is known about the temporal relationship between RF and the degree of galactosylation of IgG in vivo. We established an experimental model for studying the dynamic association between changes in the relative extent of galactosylation of IgG antigen(s) and the main parameters of RF activity, such as the titer, specificity and functional affinity/avidity. Rabbits hyperimmunized with BSA were used for examining the influence of long-term antigenic stimulation on the galactosylation status of IgG and rheumatoid factor production. The results showed that the galactosylation profile of IgG varied during the humoral anti-BSA response in rabbits and that the accompanying RF response fluctuated in titer and binding avidity for differently galactosylated IgG. The immune complexes (IC) were found to be composed of differently galactosylated IgG differing in capacity to inhibit the agglutination activity of RF. Moreover, the ability of circulating RF to react avidly with rather small IC was associated with a lower content of galactose in complexed IgG. The results suggest that a certain dynamic relationship exists between the oligosaccharide moiety of IgG and the titer and avidity of RF during the normal anti-BSA response of rabbits.

Circulating cat allergen and immune complexes in cat-allergic children

BACKGROUND

The first encounters with allergens seem to influence the development of allergy. Food antigens have been detected in sera as free antigens and in complexes with IgG but less is known about the presence of inhalant allergens.

OBJECTIVE

To investigate the presence of the major cat allergen Fel d 1, either as free allergen and/or in complexes with IgG and IgE antibodies in sera from atopic children.

METHODS

Serum samples from 33 cat allergic asthmatic children, 7-17 years old, and 15 non-allergic controls were investigated for the presence of Fel d 1 by ELISA (detection limit 0. 13 microg/L). To detect immune complexes (IC), the IgG fraction from Fel d 1 positive sera was purified by affinity chromatography. Purified and non-absorbed material was then analysed for allergen content and specific IgG antibody levels. Immune complexes with Fel d 1 IgE were detected by coupling anti-Fel d 1 MoAb to paramagnetic particles.

RESULTS

Fel d 1 was detected (0.15-1.8 microg/L) in 23 of the 33 patients (70%) but not from any of the controls. Eighteen samples contained IgE-Fel d 1 IC and two of four tested samples contained Fel d 1 in the IgG fraction. Electrophoresis and Western blotting of IgG purified material using anti-Fel d 1 MoAb corroborated the presence of IgG-Fel d 1 IC.

CONCLUSION

Free-circulating inhalant allergen and IC with allergens may contribute to maintaining immune responsiveness and sensitivity.

Binding of immune complexes to Atlantic salmon peripheral blood leucocytes

The ability of peripheral blood leucocytes (PBL) of Atlantic salmon to bind immune complexes in an antibody-dependent fashion was investigated. Immune complexes were labelled with fluorescein isothiocyanate and binding of these complexes to isolated PBL was determined by flow cytometry. The data show that a high proportion (up to 65%) of PBL were capable of binding immune complexes, and this binding did not occur when immune serum was replaced with normal serum. The presence of fresh normal serum inhibited or abrogated immune complex-binding of PBL. This is the first report of high levels of immune complex receptors on leucocytes in fish, and the dependence of complex binding on the presence of antibody suggests that these receptors may be similar to Fc receptors which are widely distributed on immunocytes of mammals.

Mediator release is altered in immunotherapy-treated patients: a 4-year study

In recent years, it has been possible to demonstrate mediator release into the nasal secretion after nasal allergen challenge in patients with allergic rhinitis. Using the nasal provocation model, we determined whether the mediator release was altered in immunotherapy-treated patients. Seventeen grass-pollen-allergic patients were examined under controlled, reproducible conditions. Serial challenges with increasing doses of grass pollen produced increasing numbers of clinical symptoms and release of mediators such as kinins, TAME-esterase activity, and histamine. Ten patients received a semidepot perennial grass-pollen extract for 4 years. Seven patients served as controls and did not receive immunotherapy during the observation period. Data from the group of patients receiving immunotherapy over the first year already showed a partially significant decline in the maximal mediator release after nasal allergen challenges compared to the results of pretreated challenges, whereas controls did not show any significant changes. Nasal allergen challenges after termination of 4 years' immunotherapy significantly modified the mediator release compared to pretreatment values (TAME-esterase activity P < 0.05, kinins P < 0.01, and histamine P < 0.01). Decrease of mediator release paralleled the symptom-medication scores and quantitative skin prick test. Finally, we could demonstrate a significant correlation between specific IgG increase and mediator decrease in the treated group.

Negative signaling via FcgammaRIIB1 in B cells blocks phospholipase Cgamma2 tyrosine phosphorylation but not Syk or Lyn activation

Crosslinking of the B cell antigen receptor surface immunoglobulin induces tyrosine phosphorylation and activation of the Src family and Syk tyrosine protein kinases, tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) and increases in intracellular second messengers inositol phosphates and Ca2+. These activation events, in conjunction with other pathways, culminate in the induction of B cell proliferation and differentiation. In contrast, co-crosslinking surface Ig with the B cell IgG Fc receptor prevents many of these activation events, including B cell proliferation and differentiation. The precise nature of the negative signal(s) derived from Fc receptors that prevent B cell activation is not known. Here, early activation events were examined in B cells stimulated via the antigen receptor alone or under co-crosslinking conditions. The data indicated a selective block in the tyrosine phosphorylation and activation of PLCgamma2 but not in activation of the upstream kinases, Syk and Lyn, under co-crosslinking conditions. We conclude that the negative signal acts directly on PLCgamma2 and is consistent with recent studies describing an activation-induced association of a phosphotyrosine phosphatase with tyrosine-phosphorylated B cell Fc receptor.

Clinical implications of IgA rheumatoid factor subclasses

OBJECTIVES

To evaluate the diagnostic and pathogenetic significance of IgA rheumatoid factor (RF) subclasses in rheumatoid arthritis (RA).

METHODS

Rheumatoid factors of the IgA class and IgA1 and IgA2 subclasses were measured by enzyme linked immunosorbent assay in 58 patients with RA, 31 patients with other rheumatic diseases, 30 non-rheumatic individuals with increased concentrations of IgA RF, and in 100 randomly selected healthy controls.

RESULTS

Using a 95% cut off for the controls, 55% of the RA patients had increased total IgA RF, 64% IgA1 RF, and 60% IgA2 RF. RA patients with extraarticular manifestations more often had increased concentrations of IgA RF and both subclasses than patients without such manifestations (p < or = 0.01). Nearly all (31/32) RA patients with increased IgA RF had increases in both IgA RF subclasses, compared with 67% (20/30 of nonrheumatic symptom free individuals with increased IgA RF (p = 0.002).

CONCLUSION

Increased concentrations of the IgA2 RF subclass appears to be more specific for RA than increased IgA1 RF. Measurement of IgA RF subclasses may be clinically useful.

Feedback suppression of B cell colony formation by supernatants of B colony cells: role of immunoglobulin

We have reported previously that CD5+ B cells from mature B cell colonies provide a negative feedback signal to the growth of autologous B cell colonies. Now we have observed that supernatants from mature B cell colonies also provide a negative feedback signal to the growth of autologous B cell colonies. We investigated the mechanism of this effect by growing B cell colonies physically separated by a 0.45 micron filter from T cells in millicell-CM chambers. Addition of colony supernatants to the T cell compartment reduced the number of B cell colonies by 28 +/- 6%. Colony numbers were reduced by 11 +/- 2 and 17 +/- 5% when the supernatants were added to the B cell or to both compartments, respectively. Pulsing T cells with the B cell colony supernatants before adding them to the colonies also decreased colony numbers by 33 +/- 13%. The addition of exogenous Ig classes and IgG subclasses to B cells decreased B cell colony numbers, although the effect was variable. In the presence of T cells, IgG had the greatest suppressive activity and the subclass IgG4 was most suppressive. In the absence of T cells, high concentrations of IgG almost abolished B cell colony formation. We conclude that these supernatants provide a negative feedback signal either directly to B cells, or via T cells which may be mediated at least in part by Ig.

The role of immunoglobulin binding factors in the pathogenesis and therapy of AIDS

The human immunodeficiency virus (HIV) gp120 and gp41 envelope proteins and Staphylococcus aureus protein A (SPA) all have Fc receptor (FcR)-like immunoglobulin binding factor (IBF) activity for the Fc constant fragments of human immunoglobulin G (IgG). Viral IBF may contribute to the pathology of HIV by jamming the network of FcR signals that control FcR-dependent immunity. Conversely, the bacterial IBF SPA has anti-retroviral activity that may involve antagonism of the immunopathological action of viral IBF, strongly suggesting IBF may act as a double-edged sword that might be turned against viral invaders.

Protection and suppression of the humoral immune response in mice mediated by a monoclonal antibody against the M epitope of Brucella

The capacity of the BmE10-5 monoclonal antibody (mAb), with specificity for the Brucella spp. M epitope, to confer protection against infection with B. abortus 2308 (A-dominant strain) has been evaluated. Injected before infection, the BmE10-5 mAb diminished the bacterial counts in spleen from week 1 to week 8 postinfection and in liver from week 4 to week 7. Thus, protection mediated by the BmE10-5 mAb, as measured by a reduction in the bacterial counts in both spleen and liver, was demonstrated from week 2 to week 8 postinfection. The humoral immune response of IgG, IgM, IgG1 and IgG3 antibodies, specific against the B. abortus 2308 smooth lipopolysaccharide, was clearly suppressed in all the mice protected with the BmE10-5 mAb, thus demonstrating the importance, in protecting against infection, of the existence in serum of M-epitope-specific antibodies at the same time the infection is acquired. The development of subcellular vaccines including the Brucella M epitope could constitute an interesting alternative to attenuated living vaccines.

Suppression of antibody responses to ricin A chain (RTA) by monoclonal anti-RTA antibodies

Balb/c mice treated with an immunotoxin constructed by conjugation of murine monoclonal antibody 791T/36 via a disulfide linker to ricin A chain generate a pronounced antibody response to peptide epitopes on ricin A chain. Monoclonal anti-RTA antibodies which recognize peptide epitopes have been developed and these have been used to down-regulate anti-RTA antibody responses in 791T/36-RTA immunotoxin-treated Balb/c mice. Of the five MAB tests, two (608/7 and 596/134) proved most effective, inhibiting anti-RTA antibody formation by up to 73%. MAB treatment was effective when initiated up to 3 days after immunotoxin treatment. Pharmacokinetic studies with 791T/36-RTA have shown that the immunotoxin is rapidly eliminated from the circulation, with no more than 4% remaining in blood after 24 hr. It is proposed that the down-regulation of anti-RTA antibodies is effected by MAB interfering with antigen processing.

The disulfide bridges of the immunoglobulin-like domains of Fc gamma RIIIB are essential for efficient expression and biological activity

The immunoglobulin G receptor Fc gamma RIIIB belongs to the immunoglobulin superfamily as two extracellular domains show homology to the immunoglobulin domains. Since some residues in these domains, such as the two cysteines, are supposed to form an intrachain disulfide bridge are so commonly conserved, they may be of importance for correct folding. Site-directed mutagenesis and expression in BHK21 confirmed this supposition for the Fc gamma RIIIB. Replacing both cysteines in the first and/or second domain by serines reduced the surface expression level by 50%, whereas the ligand binding capability was 20-30% of that seen in cells expressing the wild-type receptor. Replacing one of the four cysteines resulted in the loss of surface expression. Exchanging the conserved tryptophan in the first domain by phenylalanine only slightly affected the ligand binding (25%), whereas the surface expression remained unchanged.

Differential responsiveness of immature- and mature-stage murine B cells to anti-IgM reflects both FcR-dependent and -independent mechanisms

Mature and immature B cells differ in their responses to antigen receptor crosslinking. Whereas mature B cells enter cell cycle in response to such stimulation, immature B cells exhibit proliferative unresponsiveness and undergo induced tolerance following surface immunoglobulin (sIg) engagement. Previous studies evaluating antigen receptor-mediated negative signaling have utilized intact goat anti-immunoglobulin (anti-Ig) antibodies as polyclonal ligands based upon observations that the Fc portion of these reagents does not interact with and mediate negative signaling through the FcR on mature B cells. Thus, the negative effects of goat anti-Ig on immature B cells have been attributed solely to signals mediated via their antigen receptors. In the studies reported here we show that the activation unresponsiveness inherent to immature B cells is FcR independent. However, we also show that immature B cells are sensitive to FcR-mediated inhibition and that these effects can be mediated by intact goat antibodies at concentrations that promote positive activation signals in mature B cells. Our results demonstrate that inhibition of immature B cell LPS responses by anti-Ig antibodies, used in previous studies as an in vitro model for B cell tolerance induction, is an FcR-mediated phenomenon. We show that developmentally associated anti-Ig-mediated inhibition of LPS requires the use of intact antibodies, and that this inhibition can be blocked by the anti-FcR monoclonal antibody 2.4G2. Flow cytometric analysis of FcR-positive B cells indicates that both mature and immature B cells express equivalent levels of FcR gamma. Therefore, the sensitivity of immature, but not mature, cells to intact goat anti-mu antibodies suggests that either FcRs or their associated inhibitory pathways change during B cell development.

Modulation of IgM to IgG class switch by protein A

The Fc binding property of soluble protein A (SpA) from Staphylococcus aureus has been utilized to form IgG-SpA complexes which enabled an increase or decrease of IgG from the host depending on the dose of SpA administered. When 5 micrograms SpA was administered the IgG-SpA complexes were rapidly catabolized and, hence, low plasma IgG levels were observed. In contrast 25 micrograms SpA resulted in a significant increase in the IgG level in the host plasma. Based on these observations, the present investigation attempted to study the effect of IgG depletion/increase on the primary and secondary B-cell response to sheep erythrocyte (SRBC) antigen in Balb/c mice. Introduction of 5, 10 and 25 micrograms SpA at the time of the primary antigenic challenge inhibited both the primary IgM and the secondary IgM and IgG responses in a dose-dependent manner. Administration of 5 and 10 micrograms SpA at the time of the secondary antigenic challenge enabled the host to maintain the otherwise depressed secondary IgM response equivalent to the normal primary response. In contrast, 25 micrograms SpA at the time of the secondary antigenic challenge inhibited both the IgM and IgG PFC responses. These results extend our understanding of the mechanism of switch in immunoglobulin class expression during antigen driven maturation of the B-cell response.

Slow bacterial infections or autoimmunity?

In this article, Graham Rook and John Stanford propose that a group of idiopathic diseases that are often associated with a degree of autoimmunity and arthritis, including rheumatoid arthritis, inflammatory bowel disease, sarcoidosis and psoriasis, are caused by extremely slow-growing bacteria. They suggest that these diseases are one end of a continuous spectrum caused by related slow-growing genera, which ranges from rheumatoid arthritis, through Takayasu's arteritis and Whipple's disease, to reach the conventional mycobacterioses such as tuberculosis and leprosy.

Anti-IgG autoantibodies in HIV-infected hemophilia patients

Sera of 76 HIV-negative hemophilia patients, 103 HIV-positive (HIV+) hemophilia patients free of AIDS or AIDS related complex (ARC), and 32 HIV+ hemophilia patients with AIDS/ARC were tested for four different anti-IgG activities. IgG-anti-F(ab')2 gamma, IgM-anti-F(ab')2 gamma, and IgG-anti-Fc gamma serum activities were significantly associated with the clinical stage of HIV infection, whereas IgM-anti-Fc gamma was not. IgG-anti-F(ab')2 gamma activity was found to be caused by cross-reaction of anti-HIV antibody with an epitope within the constant CH1 domain of human IgG. HIV+ hemophilia patients with severe thrombocytopenia (less than 50,000/microliters platelet counts) had significantly higher IgM-anti-IgG activity than patients with greater than 50,000/microliters platelets. Because anti-IgG antibodies possess immunoregulatory properties, our results may serve as a possible explanation for the frequent B cell disorders encountered in HIV-infected patients.

Rheumatoid factor isotypes and cancer prognosis

The relationship between rheumatoid factor (RF) and cancer was studied during a long-term health survey done in the Reykjavik area of Iceland since 1967. A total of 16,299 blood samples from 13,858 persons attending this health survey between 1974 and 1983 were screened for the presence of RF. In 1987, RF-positive participants in this study (n = 270) and matched RF-negative control subjects (n = 223) were evaluated for incidence and prognosis of cancer with information obtained from the comprehensive Icelandic Cancer Registry. The average observation time for this cohort was 9.3 years. Participants with raised immunoglobulin A RF in their original blood sample showed an increased risk of having cancer compared with both other members of the cohort and the national cancer incidence. Their total mortality was also higher during the study period. By contrast, patients with cancer and elevated immunoglobulin M RF before the diagnosis of cancer were more likely to survive than those who were immunoglobulin M RF negative. Cancer incidence in the RF-negative control group was not different from the expected national incidence of cancer in Iceland for that age group. It is suggested that elevation of immunoglobulin A RF is an adverse phenomenon in relation to cancer; elevation of immunoglobulin M RF is associated with a favorable prognosis.

Cell kinetics of the germinal center reaction--a stathmokinetic study

The changes in splenic germinal center (GC) cell proliferation were measured during primary and secondary responses to a T-dependent antigen in vivo to examine the regulation of the GC reaction. Adult C3H/HeN mice were immunized with sheep red blood cells and boosted 7 or 21 days later. GC cell proliferation was assessed by measurement of GC cell birth rates using a stathmokinetic technique. Actual GC growth and regression were assessed in terms of total splenic volume and number. Pre-existing GC had a mean cell birth rate of 33 cells/1000 cells/h. The GC reactions following each immunization showed a biphasic pattern of changes in cell birth rate, comprising an initial fall immediately succeeded by a transient, but significant, increase. These fluctuations occurred earlier in secondary compared to primary responses. Significant increases in total GC volumes succeeded the peaks of cell birth rate following both primary and early secondary immunization. However, there was a substantially smaller increase following later secondary immunization. We propose that the initial cell birth rate reduction is due to inhibition of pre-existing GC clones and represents one component of the phenomenon of GC dissociation. The succeeding peak birth rate represents early, massive proliferation of newly activated antigen-specific clones. The different patterns of GC expansion, despite similar proliferative responses, may reflect different pathways of differentiation dependent on the timing of antigenic stimulation.

Structural requirements of the cytoplasmic domains of the human macrophage Fc gamma receptor IIa and B cell Fc gamma receptor IIb2 for the endocytosis of immune complexes

Two isotypes of the monocyte/macrophage as well as B cell Fc gamma receptor type II (FcRIIa and FcRIIb2, respectively) mainly differ in the length (76 vs. 44 amino acids) and amino acid sequence of their cytoplasmic domains. Only the eight amino acids just behind the putative transmembrane region are identical. Despite this marked difference, both FcRII mediate endocytosis of immune complexes. To determine the functional significance of the cytoplasmic domains, we expressed truncated FcRIIa and FcRIIb2 in FcR- BHK-21 cells. Mutants of both receptors containing only one amino acid (tail-minus) of the cytoplasmic domain failed to mediate immune complex uptake. The significance of the cytoplasmic domain of the receptors could be further demonstrated using a chimeric FcRIII-FcRIIa construct. Therefore we expressed an FcRIII lacking the hydrophobic carboxyl terminus (containing the putative phosphatidyl - inositol - glycan anchor site) fused inframe to the transmembrane and cytoplasmic domain of the FcRIIa in BHK-21 cells. In contrast to the wild type FcRIII, this chimeric receptor mediated immune complex uptake indistinguishable from that mediated by the FcRIIa. Receptor mutants with relatively short cytoplasmic domains (FcRIIb2: 13, and FcRIIa: 16 amino acids) revealed, that these short amino acid stretches are sufficient to allow reduced receptor-mediated endocytosis of bound ligand. Furthermore, using FcRIIa deletion mutants with a cytoplasmic domain consisting of 62, 46, and 28 amino acids, respectively, we found that the capability of these mutants to mediate immune complex uptake decreased gradually with the truncation of the cytoplasmic tails. Thus, only short amino acid sequences of the cytoplasmic domain are sufficient to enable an, albeit reduced, receptor-mediated endocytosis.

Modulation of type II Fc gamma receptor expression on activated human B lymphocytes

We have monitored Fc gamma RII expression during the activation of human B lymphocytes by simultaneous analysis of monoclonal antibody (mAb) binding and EA rosetting. The expression of Fc gamma RII showed a biphasic time course. Initially, a transient increase of Fc gamma RII with no ligand-binding capacity was observed with mAb staining as early as 10 min after stimulation by the F(ab')2 fragment of anti-human IgM or phorbol 12-myristate 13-acetate and then after 3 to 24 h a decrease in the number of Fc gamma RII+ cells was seen. Trypsin-like serine protease activity also appeared in the lysate of activated B cells at this time. On the 2nd day of activation a significant enhancement of Fc gamma RII expression was observed, mainly on enlarged blast cells as monitored by both mAb and by ligand binding (EA rosette). At the same time, soluble fragments of Fc gamma RII with the ability to bind human Fc were detected in the supernatant of activated B cells, probably as a result of proteolytic cleavage. These findings suggest that activated B cells are identical with the population of mononuclear cells which shed Fc gamma R when incubated at 37 degrees C. The ability of activated but not resting B cells to release Fc gamma RII correlates with the expression of early activation markers and with the appearance of a trypsin-like serine protease activity of the same cells; thus, the release of Fc gamma RII occurs in the early G1 phase of cell cycle as a result of proteolysis. Later the release of Fc gamma RII is accompanied by the enhancement of Fc gamma RII expression before the cells reach the S phase. The fragments of cleaved Fc gamma RII had an apparent molecular mass of 33 and 14-18 kDa under nonreducing conditions, and upon reduction fragments of smaller size were observed.

CD4/immunoglobulin interaction: implications for immune physiology and autoimmunity

CD4 has an important role in T cell activation events that depend on its binding to non-polymorphic MHC class II determinants on antigen-presenting cells. Here, we provide evidence that CD4 also interacts with immunoglobulins (Ig). The Ig-binding region lies within residues 21-49 of V1 domain of CD4. Immunochemical studies suggest that this property of CD4 does not depend on the three-dimensional folding of the CD4 molecule. Synthetic peptides (p) encompassing amino acid residues 16-49 and 21-49 of CD4 bind immunoglobulins in comparable way to the intact molecule. In vitro p 16-49 enhances significantly idiotype/anti-idiotype and some weak antigen-antibody interactions. Antigen antibody complexes formed in antigen excess bind CD4 peptides with much higher avidity then non-complexed antibodies. The possible role of the CD4/Ig interaction in T-B cell cooperation is discussed.

Role of anti-IgG autoantibodies in kidney transplantation

To demonstrate whether anti-IgG autoantibodies of different isotypes and specificities play a role in kidney transplantation, pretransplant sera of recipients (a) with well-functioning grafts (n = 40); (b) with reversible rejection episodes that were treated successfully (n = 63); and (c) with irreversible graft rejection (n = 40) were tested for four different anti-IgG autoantibody activities. A protective effect of IgG-anti-F(ab)2 gamma antibodies on graft survival was observed (p less than 0.01). High pretransplant IgG-anti-Fc gamma activity was found to be associated with a low kidney graft survival rate (p less than 0.02). Pretransplant IgM-anti-F(ab)2 gamma and IgM-anti-Fc gamma activities showed no effect on kidney graft outcome.

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.

The immune complex: possible ways of regulating the antibody response

B cells express antigen, Fc and complement receptors on their surfaces and can thus bind all three components of an immune complex. In addition to the direct effects that they exert on cells, immune complexes may affect localization, presentation and digestion of antigen. In this article, Birgitta Heyman discusses recent developments in antibody-mediated regulation of the humoral immune response, with emphasis on in vivo systems where antigens are injected together with highly purified IgM or IgG antibodies in the absence of adjuvants.

Fc receptor phosphorylation during receptor-mediated control of B-cell activation

It is well known that Fc receptors for IgG (FcRII) on macrophages mediate the endocytosis of antibody-antigen complexes and signal the release of inflammatory and cytotoxic agents. FcRII are also expressed at high levels on B cells where they are less involved in endocytosis than in modulating B-cell activation by membrane immunoglobulins. Although crosslinking of membrane immunoglobulins can result in B-cell differentiation and proliferation through stimulation of phospholipase C, mobilization of intracellular Ca2+, and activation of protein kinase C, crosslinking FcR with membrane immunoglobulins confers a dominant inhibitory signal that prevents or aborts activation. This form of regulation may have a role in the induction of tolerance by IgG and in controlling the B-cell repertoire by anti-idiotypes. The different functions of FcR on B cells and macrophages may reflect the fact that these cell types express closely related but distinct FcR isoforms. We have recently found that the main lymphocyte FcR isoform, FcRII-B1, is unable to mediate endocytosis by way of coated pits and coated vesicles owing to an in-frame insertion of 47 amino acids in its cytoplasmic tail. Here we show that this insert, absent from the FcRII-B2 macrophage isoform, also contains serine phosphorylation sites that may have a role in the ability of FcR to regulate B-cell activation through membrane immunoglobulins.

Distribution, inducibility and biological function of the cloned and expressed human beta Fc receptor II

A cDNA encoding the human beta Fc gamma receptor II (FcRII) was isolated from a placental cDNA library. Analysis of the predicted amino acid sequence indicates that this receptor is synthesized with a 42-amino acid leader sequence. The mature protein consists of 249 amino acids. The leader sequence and the cytoplasmic domain are strikingly different from the CDw32 antigen but show great homology to the mouse beta 2FcR. RNA blot analysis of human cells using CDw32 and beta FcRII-specific DNA fragments demonstrated one beta FcRII transcript (1.7 kb) in B cells and in HL-60 cells which were induced to differentiate along a monocyte-macrophage pathway by phorbol 12-myristate 13-acetate treatment. Under these conditions the CDw32 transcripts (2.5 and 1.7 kb) are induced to a minor extent in HL-60 cells. In contrast, the 2.5-kb CDw32 transcript is strongly induced in HL-60 cells which have been induced to differentiate into granulocytes by exposure to dimethylsulfoxide. To determine the biological properties of the beta FcRII, we expressed the antigen in FcR- hamster cells. Only immune complexes but not monomeric human IgG were bound significantly. Bound ligand was efficiently internalized within 15 min and it was then found in vesicular structures. Thus the low-affinity beta FcRII is able to internalize ligands without cooperation with any other FcR.

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.

Rheumatoid factor and Fc signaling: a tale of two Cinderellas

Signaling to lymphocytes depends not only upon the interaction of receptors with specific antigen, but also upon antigen nonspecific receptors which receive input from two classes of molecules, immunologically specific end products and lymphokines. These represent the central elements in the physiologic stimulation of immune responses to both self and foreign antigens. This review is dedicated primarily to describing the function of physiologically active receptors for end product which are central to end product feedback in normal immune responses. The emergence of autoimmunity as a pathologic process resultant from a defective end product signaling mechanism is described.

Cellular changes during the infusion of high dose intravenous immunoglobulin

With the ever widening group of autoimmune conditions that are beneficially affected by infusions of high dose immunoglobulin the possible mechanisms of action of such therapy appear increasingly complex. Fc mediated blockade of the mononuclear phagocyte system is an acknowledged early effect. This is, however, accompanied by a decrease of neutrophil counts which suggests that IgG binding to the neutrophil may be a mechanism of action. The decrease of neutrophil counts is transient but in immune thrombocytopenia is inversely proportional to the platelet response observed. In parallel to the effect on the neutrophil there are changes in the lymphocyte subsets with reversal of the T helper/suppressor ratio and alterations in the individual cellular constituents of each subset that correlate with the clinical response. The observed changes in B cell numbers and function suggest that T dependent and independent antibody production is effected by intravenous immunoglobulin. It is increasingly clear that in ITP at least the clinical response to IV IgG is a summation of several cellular events and their balance reflects the ultimate outcome. It may eventually be possible to use these observations to predict the likely outcome in the individual patient of this mode of therapy.

IgA rheumatoid factor and IgG dietary protein antibodies are associated in rheumatoid arthritis

This study sought to determine whether patients with rheumatoid arthritis (RA) were immunologically sensitised to dietary protein (DP). Using an enzyme linked immunosorbent assay (ELISA), antibodies to milk and wheat proteins were measured in 93 unselected out-patients with classical or definite RA. Of these 93, 53 had raised levels of IgG antibodies to one or both dietary proteins (DP). In the DP antibody positive group, 48 patients (90%) also had raised levels of IgA rheumatoid factor (measured by ELISA) while only 7 (17%) of the 40 DP antibody negative patients had detectable IgA RF; P less than 0.02. There was no association between IgM rheumatoid factor and dietary protein antibodies. These results demonstrate that in RA, raised levels of IgA RF are associated with an increased IgG response to antigens which enter the body through the gastrointestinal tract. A breakdown in gastrointestinal tolerance to dietary antigens may play a role in the immunopathogenesis of RA in these patients who might therefore benefit from dietary manipulation.

Co-cross-linking of surface immunoglobulin Fc gamma receptors on B lymphocytes uncouples the antigen receptors from their associated G protein

Cross-linking of surface Ig receptors (sIg) by mitogenic forms of anti-Ig antibodies (e.g. F(ab')2 fragments of rabbit anti-Ig) causes the rapid, and prolonged breakdown of phosphatidylinositol 4,5-bisphosphate. This response involves an unidentified guanine nucleotide regulatory protein (termed Gp), which couples sIg to the polyphosphoinositide-specific phosphodiesterase. Intact (IgG) rabbit anti-Ig antibodies, which co-cross-link sIg and Fc gamma receptors on B cells, only induce short-lived inositol phospholipid breakdown and abortive B cell activation. We show here that in permeabilized B cells intact anti-Ig inhibits the reconstituted breakdown of inositol phospholipids given by a combination of F(ab')2 anti-Ig and the non-hydrolyzable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S), but not the basal stimulation of Gp induced by GTP gamma S alone. These results therefore indicate that co-cross-linkage of sIg and Fc gamma receptors on B cells uncouples the antigen receptors from the associated G protein, but does not affect coupling between Gp and the phosphodiesterase. These observations therefore provide further insight into the mechanisms whereby engaging Fc receptors on B cells, by antigen-antibody complexes for example, could modulate antigen-induced B cell activation.

Detection and isolation of antigen-specific B cells by the fluorescence activated cell sorter (FACS)

A method is described for the isolation of antigen-specific B cells from immunized and subsequently boosted mice. Antigen-specific B cells were stained by incubation with fluorescein isothiocyanate (FITC)-labelled antigen and then detected and isolated in a fluorescence activated cell sorter (FACS). Ovalbumin (OVA) and Helix pomatia haemocyanin (HPH) were used as antigens in this procedure, yielding relative amounts of antigen-FITC-binding lymphocytes of 0.9 +/- 0.4% and 3.5 +/- 3.1%. The FITC-positive cells were visible as distinct cell populations in the FACS-generated histograms. All antigen-FITC-binding cells were B cells, as shown by double staining with phycoerythrin-conjugated anti-mouse Ig In addition, as tested in a spot-ELISA, the sorted, antigen-FITC-binding cell population contained almost the entire population of antigen-specific antibody-producing B cells. However, sorting had a negative influence on the antibody production capability of the sorted cells. Through washing of isolated spleen cells in the procedure before labelling with antigen-FITC proved to be essential for the specific detection of antigen-specific B cells, since staining without prior washing resulted in antigen-FITC binding to all B cells. This 'nonspecific' staining phenomenon was caused by the presence of antibodies, specific for the immunizing/boosting antigen, which were also present in the spleen cell suspension. These antibodies formed immune complexes with antigen-FITC and bound to Fc receptors present on all B cells, interfering in this way with any specific binding of antigen-FITC to sIg on the B cells.

Inhibition of IgG-mediated immunosuppression by a monoclonal anti-Fc receptor antibody

IgG-antibodies can efficiently suppress the antibody response against their specific antigen. The suppressive capacity is dependent on intact Fc regions. However, it is not clear which of the Fc-mediated effector functions are necessary for the induction of immunosuppression. The monoclonal antibody 24G2, which binds to murine Fc receptors on macrophages and B cells, was used in the present study to address the question of whether IgG-mediated suppression is in fact dependent on the binding of IgG antibodies to Fc receptors on splenocytes. In a murine in vitro immunization system, 24G2 is shown to reverse efficiently the suppression of the sheep erythrocyte-specific antibody response. The role of B cells or macrophages as effector cells is discussed.

Idiopathic thrombocytopenic purpura (ITP): immunomodulation by intravenous immunoglobulin (IVIg)

ITP is a destructive thrombocytopenia. Platelets are coated with antibodies and these opsonized platelets are rapidly removed by phagocytes. On the other hand, therapeutic application of antibody concentrates (IVIg) rapidly raise platelet counts, and in some patients, sustained platelet recovery has been observed. The mechanism of action of IVIg is far from being clear. Several possible mechanism of action of IVIg treatment have been described. The immediate effect of IVIg seems to be a decrease in (unspecific) Fc mediated mononuclear phagocytosis, the long term effect might be a change in the complex network of the regulatory function of the immune response. Both types of interactions seem to play a keyrole in the immunomodulation. The various possible modes of actions evoke investigation of IVIg in a wide range of diseases with similar ineffective immune response. Controlled clinical studies have to be done to prove or disapprove the use of IVIg in other indications.

Immunomodulatory effect of synthetic peptides corresponding to sequences within the CH2 and CH3 domain of human IgG1

The Fc region of IgG is known to be the source of small peptides possessing immunomodulatory function. Results are summarized showing the effect of synthetic peptides composed of surface exposed residues of C gamma 2 or C gamma 3 domains on different steps of human B lymphocyte activation cycle. Both the CH2 (289Thr-301Arg) and CH3 (407Tyr-416Arg) peptides as well as the whole Fc fragment enhanced the IgM synthesis of PWM or PMA + CaI activated lymphocytes. This effect was exerted at the early phase of B cell activation. The incubation of separated resting B cells with Fc fragments or CH2 peptides resulted in increase of cell volume and in expression of HLA-DR antigen. On the other hand, LIF production was induced both by CH2 and CH3 peptides. It was also shown that Fc peptides induce IL-1 release from monocytes. The results suggest that the CH2 and CH3 domain peptides exert their effect partly directly, by activating resting B cells, rendering the cells more susceptible to other stimuli; and moreover, by enhancing the humoral response by triggering the release of IL-1.