A role for membrane IgD in the tolerance of pathological human rheumatoid factor B cells

Unraveling the role of Breg cells in digestive tract cancer and infectious immunity

Over the past two decades, regulatory B cells (Breg cells or Bregs) have emerged as an immunosuppressive subset of B lymphocytes playing a key role in inflammation, infection, allergy, transplantation, and cancer. However, the involvement of Bregs in various pathological conditions of the gastrointestinal tract is not fully understood and is the subject of much recent research. In this review, we aimed to summarize the current state of knowledge about the origin, phenotype, and suppressive mechanisms of Bregs. The relationship between the host gut microbiota and the function of Bregs in the context of the disturbance of mucosal immune homeostasis is also discussed. Moreover, we focused our attention on the role of Bregs in certain diseases and pathological conditions related to the digestive tract, especially Helicobacter pylori infection, parasitic diseases (leishmaniasis and schistosomiasis), and gastrointestinal neoplasms. Increasing evidence points to a relationship between the presence and number of Bregs and the severity and progression of these pathologies. As the number of cases is increasing year by year, also among young people, it is extremely important to understand the role of these cells in the digestive tract.

The role of basophils as innate immune regulatory cells in allergy and immunotherapy

Basophils are circulating cells that are associated quite exclusively with allergy response and hypersensitivity reactions but their role in the immune network might be much more intriguing and complex than previously expected. The feasibility of testing their biology in vitro for allergy research and diagnosis, due fundamentally to their quite easy availability in the peripheral blood, made them the major source for assessing allergy in the laboratory assay, when yet many further cells such as mast cells and eosinophils are much more involved as effector cells in allergy than circulating basophils. Interestingly, basophil numbers change rarely in peripheral blood during an atopic response, while we might yet observe an increase in eosinophils and modification in the biology of mast cells in the tissue during an hypersensitivity response. Furthermore, the fact that basophils are very scanty in numbers suggests that they should mainly serve as regulatory cells in immunity, rather than effector leukocytes, as still believed by the majority of physicians. In this review we will try to describe and elucidate the possible role of these cells, known as "innate IL4-producing cells" in the immune regulation of allergy and their function in allergen immunotherapy.

What do we know about the role of regulatory B cells (Breg) during the course of infection of two major parasitic diseases, malaria and leishmaniasis?

Parasitic diseases, such as malaria and leishmaniasis, are relevant public health problems worldwide. For both diseases, the alarming number of clinical cases and deaths reported annually has justified the incentives directed to better understanding of host's factors associated with susceptibility to infection or protection. In this context, over recent years, some studies have given special attention to B lymphocytes with a regulator phenotype, known as Breg cells. Essentially important in the maintenance of immunological tolerance, especially in autoimmune disease models such as rheumatoid arthritis and experimentally induced autoimmune encephalomyelitis, the function of these lymphocytes has so far been poorly explored during the course of diseases caused by parasites. As the activation of Breg cells has been proposed as a possible therapeutic or vaccine strategy against several diseases, here we reviewed studies focused on understanding the relation of parasite and Breg cells in malaria and leishmaniasis, and the possible implications of these strategies in the course of both infections.

IgD attenuates the IgM-induced anergy response in transitional and mature B cells

Self-tolerance by clonal anergy of B cells is marked by an increase in IgD and decrease in IgM antigen receptor surface expression, yet the function of IgD on anergic cells is obscure. Here we define the RNA landscape of the in vivo anergy response, comprising 220 induced sequences including a core set of 97. Failure to co-express IgD with IgM decreases overall expression of receptors for self-antigen, but paradoxically increases the core anergy response, exemplified by increased Sdc1 encoding the cell surface marker syndecan-1. IgD expressed on its own is nevertheless competent to induce calcium signalling and the core anergy mRNA response. Syndecan-1 induction correlates with reduction of surface IgM and is exaggerated without surface IgD in many transitional and mature B cells. These results show that IgD attenuates the response to self-antigen in anergic cells and promotes their accumulation. In this way, IgD minimizes tolerance-induced holes in the pre-immune antibody repertoire.

Self-reactive B cells that are anergic express mainly IgD, yet the function of IgD is not clear. Here the authors analyse primary B cells from mice to show that IgD signalling attenuates self-antigen induced gene expression and promotes survival of anergic B cells that might go on to reactivate to foreign antigens and mutate away from self-reactivity.

Control of B cells expressing naturally occurring autoantibodies

Naturally occurring autoantibodies (NAbs) are typically polyreactive, bind with low affinity to a discrete set of autoantigens and are encoded by variable region genes in germline configuration. They differ from disease-associated autoantibodies (autoAb), which are mostly monoreactive, somatically mutated and of high affinities. Structure-function studies have shown that polyreactivity of NAbs relies on the somatically generated complementarity determining region, CDR3, of the heavy chain. This finding suggested that NAb-producing B cells were positively selected from the pre-immune B-cell repertoire. The biological significance of this selection remains, however, unclear. Data originating mainly from transgenic mice have shown that mature NAb-producing B cells are frequently ignorant toward their antigen, possibly due to their low affinity, though active tolerance mechanisms are not excluded. An important issue is whether NAb-producing B cells constitute the pool from which pathologic auto Ab emerge after autoantigen-driven maturation. We summarize results obtained in mouse models, showing that some infectious agents are able to induce an autoantigen-driven activation of certain NAb-producing B cells. However direct proof that selection by autoantigen may lead to somatic hypermutation are still lacking. Other data tend to suggest that pathologic auto Abs may derive from non-autoimmune B cells that have diversified by somatic hypermutation of their variable region genes.

The absence of immunoglobulin D B cell receptor-mediated signals promotes the production of autoantibodies and exacerbates glomerulonephritis in murine lupus

Immunoglobulin (Ig)D is the major antigen receptor isotype co-expressed with IgM on the surface of most peripheral B cells in mice and humans. However, the biological role of IgD as B cell receptor (BCR) has remained unclear. Previous studies have indicated that IgD may play a role in B cell tolerance. To understand the role of IgD in B cell tolerance and autoimmunity, we have examined the development of autoimmune syndrome in lpr mice deficient for IgD. The present study showed that IgD deficiency did not alter lymphoproliferation and lymphocyte activation in lpr mice. The survival and proliferation of B cells were not affected by the absence of IgD, indicating that IgD BCR-mediated signals do not have an important role in negative selection of autoreactive B cell clones. Interestingly, compared to IgD-competent littermates, lpr mice with IgD deficiency had elevated autoantibody production, increased deposition of immune complex in the kidney and more severe nephritis. Accumulation of abnormal CD4(-) CD8(-) αβ(+) T cells was accelerated in IgD(-/-) lpr mice compared to lpr mice. These results suggest that IgD BCR-mediated signals may be involved in the differentiation of autoreactive B cells into plasma cells and abnormal T cell expansion.

IgM, IgG, and IgA rheumatoid factors in pigeon hypersensitivity pneumonitis

The association of rheumatoid factor (RF) and lung disease in several immunologically mediated conditions has suggested that it may be physiopathologically relevant. Since previous reports in hypersensitivity pneumonitis (HP) have dealt mainly with the immunoglobulin M (IgM) RF measurement, we studied such antibody activity in other immunoglobulins, to determine the IgG and IgA RF levels in pigeon-HP, and in asymptomatic breeders (AB) and rheumatoid arthritis (RA) as controls. RFs were measured in 35 HP patients, 41 AB, 31 RA controls, and 55 healthy donors by enzyme-linked immunosorbent assay (ELISA) using human or rabbit immunoglobulin G (IgG), anti-IgM, F(ab')2 of IgG, and IgA F(ab')2 conjugates. An affinity chromatography, fragment crystallizable (Fc) preparations of IgG, pepsin digestion, and Western blots were used to confirm RF specificity. We also evaluated anti-avian antibodies (AA) and cross-reacting antibodies. The HP group revealed positive IgM (51.4%), IgG (31.4%), and immunoglobulin A (IgA) (34.2%) RF tests, and these antibody values exceeded the AB reference levels (P<0.02). HP and RA showed a similar frequency and distribution of RFs. Possible immunoassay interferences were excluded. As in other immunologically mediated diseases, IgG and IgA RFs may play a pathogenic role in HP, amplifying the inflammatory reaction, immune-complex formation, and complement activation. IgM-RF producing cells that have been implicated in the presentation of self and foreign antigens, and T-cell activation might induce the isotype switching of RFs.

Influenza virus-induced type I interferon leads to polyclonal B-cell activation but does not break down B-cell tolerance

The link between infection and autoimmunity is not yet well understood. This study was designed to evaluate if an acute viral infection known to induce type I interferon production, like influenza, can by itself be responsible for the breakdown of immune tolerance and for autoimmunity. We first tested the effects of influenza virus on B cells in vitro. We then infected different transgenic mice expressing human rheumatoid factors (RF) in the absence or in the constitutive presence of the autoantigen (human immunoglobulin G [IgG]) and young lupus-prone mice [(NZB x NZW)F(1)] with influenza virus and looked for B-cell activation. In vitro, the virus induces B-cell activation through type I interferon production by non-B cells but does not directly stimulate purified B cells. In vivo, both RF and non-RF B cells were activated in an autoantigen-independent manner. This activation was abortive since IgM and IgM-RF production levels were not increased in infected mice compared to uninfected controls, whether or not anti-influenza virus human IgG was detected and even after viral rechallenge. As in RF transgenic mice, acute viral infection of (NZB x NZW)F(1) mice induced only an abortive activation of B cells and no increase in autoantibody production compared to uninfected animals. Taken together, these experiments show that virus-induced acute type I interferon production is not able by itself to break down B-cell tolerance in both normal and autoimmune genetic backgrounds.

Auto-reactive B cells in transgenic mice

In order to understand how the natural occurrence of autoreactive B cells is controlled in normal individuals, and how self reactive B cells can escape this control during diverse clinical situations, many different transgenic mice have been generated expressing self reactive antibodies. In this review, we focus our attention on disease-associated self reactive transgenic models which show the variety of the tolerization mechanisms. The same transgenic lines are also used to analyse the effects of the autoimmune genetic background on the self reactive B cell fate, as well as to study the influence of infectious agents on the behaviour of the auto-reactive transgenic B cells.

The riddle of the dual expression of IgM and IgD

Signalling through the B cell antigen receptor (BCR) is required for peripheral B lymphocyte maturation, maintenance, activation and silencing. In mature B cells, the antigen receptor normally consists of two isotypes, membrane IgM and IgD (mIgM, mIgD). Although the signals initiated from both isotypes differ in kinetics and intensity, in vivo, the BCR of either isotype seems to be able to compensate for the loss of the other, reflected by the mild phenotypes of mice deficient for mIgM or mIgD. Thus, it is still unclear why mature B cells need expression of mIgD in addition to mIgM. In the current review we suggest that the view that IgD has a simply definable function centred around the basic signalling function should be replaced by the assumption that IgD fine tunes humoral responses, modulates B cell selection and homeostasis and thus shapes the B cell repertoire, defining IgD to be a key modulator of the humoral immune response.

Autoantigen, innate immunity, and T cells cooperate to break B cell tolerance during bacterial infection

Autoantibody production during infections is considered to result from nonspecific activation of low-affinity autoreactive B cells. Whether this can lead to autoimmune disease remains uncertain. We show that chronic infection by Borrelia burgdorferi of Tg animals expressing human rheumatoid factor (RF) B cells (of low or intermediate affinities) in the absence or in the constitutive presence of the autoantigen (represented here by chimeric IgG with human constant region) breaks their state of immunological ignorance, leading to the production of RFs. Surprisingly, this production was more pronounced in intermediate-affinity RF Tg mice co-expressing the autoantigen. This overproduction was mediated by immune complexes and involved synergistic signaling between the B cell receptor and Toll-like receptors and T cell help. These findings indicate that chronic infection can activate autoreactive B cells with significant affinity and creates conditions that can drive them to differentiate into memory cells. Such cells may have some physiological yet undetermined role, but in autoimmune-prone individuals, this scenario may initiate autoimmunity.

Differential effects of granulocyte colony-stimulating factor on marrow- and blood-derived hematopoietic and immune cell populations in healthy human donors

A recent phase III trial comparing granulocyte colony-stimulating factor (G-CSF)-stimulated bone marrow (G-BM) and G-CSF-mobilized peripheral blood (G-PB) in matched sibling allograft recipients showed that G-BM produced a similar hematologic recovery but a reduced incidence of extensive chronic graft-versus-host disease, indicating differences in the cell populations infused. As a first step toward identifying these differences, we treated a group of healthy adult humans with 4 daily doses of G-CSF 10 microg/kg and monitored the effects on various hematopoietic and immune cell types in the PB and BM over 12 days. G-CSF treatment caused rapid and large but transient increases in the number of circulating CD34+ cells, colony-forming cells, and long-term culture-initiating cells and in the short-term repopulating activity detectable in nonobese diabetic/severe combined immunodeficiency/beta2-microglobulin-null mice. Similar but generally less marked changes occurred in the same cell populations in the BM. G-CSF also caused transient perturbations in some immune cell types in both PB and BM: these included a greater increase in the frequency of naive B cells and CD123+ dendritic cells in the BM. The rapidity of the effects of G-CSF on the early progenitor activity of the BM provides a rationale for the apparent equivalence in rates of hematologic recovery obtained with G-BM and G-PB allotransplants. Accompanying effects on immune cell populations are consistent with a greater ability of G-BM to promote tolerance in allogeneic recipients, and this could contribute to a lower rate of chronic graft-versus-host disease.

Progressive Surface B Cell Antigen Receptor Down-Regulation Accompanies Efficient Development of Antinuclear Antigen B Cells to Mature, Follicular Phenotype

Previous studies have suggested that B cell Ag receptor (BCR) down-regulation by potentially pathological autoreactive B cells is associated with pathways leading to developmental arrest and receptor editing, or anergy. In this study we compare the primary development of B cells in two strains of mice expressing transgenic BCRs that differ by a single amino acid substitution that substantially increases reactivity for nuclear autoantigens such as DNA. Surprisingly, we find that both BCRs promote efficient development to mature follicular phenotype, but the strongly autoreactive BCR fails to promote marginal zone B cell development. The follicular B cells expressing the strongly autoreactive BCR do not appear to be anergic, as they robustly respond to polyclonal stimuli in vitro, are not short-lived, and can participate in germinal center reactions. Strikingly however, substantial and progressive down-modulation of surface IgM and IgD takes place throughout their primary development in the BM and periphery. We propose that BCR-autoantigen interactions regulate this pathway, resulting in reduced cellular avidity for autoantigens. This process of "learned ignorance" could allow autoreactive B cells access to the foreign Ag-driven memory B cell response, during which their self-reactivity would be attenuated by somatic hypermutation and selection in the germinal center.