Abatacept (cytotoxic T lymphocyte antigen 4-immunoglobulin) improves B cell function and regulatory T cell inhibitory capacity in rheumatoid arthritis patients non-responding to anti-tumour necrosis factor-α agents.

The use of biological agents combined with methotrexate (MTX) in rheumatoid arthritis (RA) patients has strongly improved disease outcome. In this study, the effects of abatacept on the size and function of circulating B and T cells in RA patients not responding to anti-tumour necrosis factor (TNF)-α have been analysed, with the aim of identifying immunological parameters helpful to choosing suitable tailored therapies. We analysed the frequency of peripheral B and T cell subsets, B cell function and T regulatory cell (Treg ) inhibitory function in 20 moderate/severe RA patients, according to the European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) criteria, primary non-responders to one TNF-α blocking agent, who received abatacept + MTX. Patients were studied before and 6 months after therapy. We found that abatacept therapy significantly reduced disease activity score on 44 joints (DAS)/erythrocyte sedimentation rate (ESR) values without causing severe side effects. The size of the circulating B and T cell compartments in RA patients was not significantly different from healthy donors, but B cell proliferation and plasma cell differentiation was impaired before therapy and restored by abatacept. While Treg cell frequency was normal, its inhibitory function was absent before therapy and was partially recovered 6 months after abatacept. B and Treg cell function is impaired in RA patients not responding to the first anti-TNF-α agent. Abatacept therapy was able to rescue immune function and led to an effective and safe clinical outcome, suggesting that RA patients, in whom anti-TNF-α failed, are immunologically prone to benefit from an agent targeting a different pathway.

Effects of PARP-1 deficiency on Th1 and Th2 cell differentiation.

T cell differentiation to effector Th cells such as Th1 and Th2 requires the integration of multiple synergic and antagonist signals. Poly(ADP-ribosy)lation is a posttranslational modification of proteins catalyzed by Poly(ADP-ribose) polymerases (PARPs). Recently, many reports showed that PARP-1, the prototypical member of the PARP family, plays a role in immune/inflammatory responses. Consistently, its enzymatic inhibition confers protection in several models of immune-mediated diseases, mainly through an inhibitory effect on NF-κB (and NFAT) activation. PARP-1 regulates cell functions in many types of immune cells, including dendritic cells, macrophages, and T and B lymphocytes. Our results show that PARP-1KO cells displayed a reduced ability to differentiate in Th2 cells. Under both nonskewing and Th2-polarizing conditions, naïve CD4 cells from PARP-1KO mice generated a reduced frequency of IL-4(+) cells, produced less IL-5, and expressed GATA-3 at lower levels compared with cells from wild type mice. Conversely, PARP-1 deficiency did not substantially affect differentiation to Th1 cells. Indeed, the frequency of IFN-γ (+) cells as well as IFN-γ production, in nonskewing and Th1-polarizing conditions, was not affected by PARP-1 gene ablation. These findings demonstrate that PARP-1 plays a relevant role in Th2 cell differentiation and it might be a target to be exploited for the modulation of Th2-dependent immune-mediated diseases.

Hyper-IgM, neutropenia, mild infections and low response to polyclonal stimulation: hyper-IgM syndrome or common variable immunodeficiency?

A young woman presenting respiratory infections, polyarthritis, severe neutropenia, and increased serum IgM was treated with intravenous immunoglobulin (IVIG) with good clinical and laboratory outcome followed by a loss of efficacy. The increased serum IgM associated to recurrent infections and autoimmune manifestations suggested the diagnosis of a hyper-IgM syndrome (HIGMs). The frequency of peripheral T cells, the expression of CD40 on the patients' B cells and CD40L on T cells and the activation-induced cytidine deaminase (AID) and uracil-DNA glycosylase (UNG) at mRNA level was comparable to controls. In contrast, the frequency of B cells was one half of the healthy control and all cells showed an atypical phenotype. Although AID and UNG were normal, class-switch recombination was not very efficient because circulating switched memory were reduced and, once stimulated with CpG, generated less antibody-secreting cells than controls. An increase in serum B Lymphocytes stimulator (BLyS) was also found. The patient presented a peculiar clinical and immunological phenotype fitting for many aspects of both HIGM4 and Common Variable Immunodeficiency (CVID). These findings underline the need to better explore the complex link between these two diseases.

From the fetal liver to spleen and gut: the highway to natural antibody.

The film of sIgA lining the intestinal epithelium plays a role in the regulation of the commensal microflora and prevention of pathogen invasion. We show that, in the absence of intentional immunization, all sIgA in the gut is produced by B-1a B cells. We also show that B-1a B cells and sIgA derive from lineage-negative precursors found in the fetal liver and located in the spleen after birth. The splenic precursors do not generate B cells of the adaptive immune system in bone marrow, spleen, and lymph nodes, but efficiently replenish the cells producing the natural antibodies. Therefore, B-1a B cells with their splenic progenitors and their progeny of plasma cells fill the same function of the primordial immune system of lower vertebrates. The natural antibodies in the serum and on the intestinal epithelium may be an evolutionary ancient tool for the immediate protection against commensal and pathogenic bacteria.

Immunoglobulin M memory B cell decrease in inflammatory bowel disease.

BACKGROUND & OBJECTIVES: Memory B cells represent 30-60% of the B cell pool and can be subdivided in IgM memory and switched memory. IgM memory B cells differ from switched because they express IgM and their frequency may vary from 20-50% of the total memory pool. Switched memory express IgG, IgA or IgE and lack surface expression of IgM and IgD. Switched memory B cells derive from the germinal centres, whereas IgM memory B cells, which require the spleen for their survival and/or generation, are involved in the immune response to encapsulated bacteria. Since infections are one of the most frequent comorbid conditions in inflammatory bowel disease, we aimed to verify whether IgM memory B cell pool was decreased in Crohn's disease and ulcerative colitis patients. PATIENTS & METHODS: Peripheral blood samples were obtained from 22 Crohn's disease patients, 20 ulcerative colitis patients, 22 healthy controls and 18 splenectomized patients. To analyse peripheral blood lymphocytes, flow cytometry was performed using anti-CD19, anti-CD22, anti-CD27, anti-IgM, anti-IgD and anti-CD38 monoclonal antibodies. RESULTS: Circulating IgM memory B cells were significantly lower in Crohn's disease (median 7.1%, range 1.8-20.7) and ulcerative colitis patients (median 8.1%, range 2.1-18.8) in comparison to control subjects (median 14.0%, range 6.8-31.1). As expected, there was a highly significant difference in the proportion of IgM memory B cells between splenectomized patients (median 2.4%, range 0.9-6.9) and healthy controls. Crohn's disease patients with abscesses showed the lowest frequency of IgM memory B cells. DISCUSSION: Our findings show that peripheral IgM memory B cells are reduced in inflammatory bowel disease patients. Further studies are necessary to answer the question of whether high risk of infection (abscess development) is promoted by the reduction/depletion of IgM memory B-cell pool in inflammatory bowel disease.

Peripheral B cell survival.

Recent findings suggest that lymphocyte survival is a continuous active process and support the role of B cell receptor engagement in B cell survival. In this context the conflict of survival interests between the diverse B cells gives rise to a pattern of interactions which mimics the behavior of complex ecological systems. In response to competition lymphocytes modify their survival requirements and diverge to occupy different immunological niches through differentiation. Thus naive and memory-activated B cell populations show independent homeostatic regulation. We discuss how niche differentiation allows the coexistence of different cell types and guarantees both repertoire diversity and efficient immune responses.

B cell positive selection by self antigens and counter-selection of dual B cell receptor cells in the peripheral B cell pools.

The presence of B cells expressing two B cell receptors (BCR), described in BCR-transgenic, gene-targeted and normal mice, may represent an autoimmune hazard. We generated RAG-2-deficient mice bearing two complete rearranged immunoglobulin transgenes. In these mice most mature resting B cells express chains from the two transgenes. We studied selection of these dual receptor B cells in the presence of self antigens. In spite of the reduced surface density of the anti-self receptor, self-reactive B cells are deleted in the presence of membrane-bound self antigens. In contrast, the presence of soluble self antigen positively selects single receptor B cells expressing the self-reactive receptor. At the periphery these positively selected B cells down-regulate surface IgM expression and become unresponsive. A few dual receptor cells, however, escape tolerance induction. We examined the peripheral fate of the dual receptor B cells and showed that they are poorly selected into the activated B cell compartment and show a poor competitive capacity when in presence of populations of single receptor B cells. These results indicate that peripheral selection contributes to the very low frequencies of dual receptor B cells in normal mice and that multiple safeguard mechanisms operate to minimize the autoimmune hazard that allelically included B cells could represent.

The role of the B cell receptor V region in peripheral B cell survival.

We investigate the role of the antigen-specific B cell receptor (BCR) in the establishment and maintenance of the peripheral B cell pools. We studied the fate of a population of transgenic B cells expressing a BCR without V region (Tg(deltaVmu)). We found that the Tg(deltaVmu) B cells can populate the peripheral B cell pools in the absence of other B cells, but when in the presence of a second population of non-transgenic B cells, they are virtually absent from the mature B cell compartments. By studying the rate of accumulation of 5-bromo-2'-deoxyuridine we show that the peripheral Tg(deltaVmu) B cells have a shorter life-span compared to non-transgenic B cells. By directly comparing the fate of two populations of transgenic B cells, either lacking or expressing a V region, we were able to assign the poorest competitive ability and the short peripheral survival of the Tg(delatVmu) B cells to the lack of an antigen-binding site. The results obtained support the involvement of the V region in the persistence of peripheral B cell populations.

Independent homeostatic regulation of B cell compartments.

In the present study we used mice with a developmental arrest of B cell production to study the ability of a limited number of normal B cell precursors to populate peripheral B cell pools. In chimeras reconstituted with mixtures of bone marrow (BM) cells from normal and B cell-deficient donors, we show that the rate of BM B cell production is a constant function of the number of BM pre-B cells and is not modified by the peripheral B cell pool size, i.e. there is no feedback regulation of the central pre-B cell compartment by the number of mature B cells. We also show that the physiological number of peripheral B cells requires a minimum continuous input of newly formed cells, but is not determined by the number of B cell precursors. Chimeras with a threefold reduced rate of BM B cell production have normal numbers of peripheral B cells. Parabiosis between normal and B cell-deficient mice showed that the BM B cell production of one mouse suffices to replenish the B cell pool of three mice. Finally, we show that the compartment of activated IgM-secreting B cells is homeostatically autonomous since the number of cells it comprises is regulated independently of the size of the mature B cell pool. The results presented here support a model of the immune system in which the size of the different B cell compartments, i.e. pre-B, resting B and IgM-secreting, is autonomously regulated.

Resource competition as a mechanism for B cell homeostasis.

Cellular competition for survival signals offers a cogent and appealing mechanism for the maintenance of cellular homeostasis [Raff, M. C. (1992) Nature (London) 356, 397-400]. We present a theoretical and experimental investigation of the role of competition for resources in the regulation of peripheral B cell numbers. We use formal ecological competition theory, mathematical models of interspecific competition, and competitive repopulation experiments to show that B cells must compete to persist in the periphery and that antigen forms a part of the resources over which B cells compete.

Lymphocyte homeostasis.

B- and T-lymphocyte populations have an independent homeostatic regulation of resting (B and T) and activated (B) or memory (T) cell compartments. This organization may provide an efficient mechanism to ensure simultaneously a first natural barrier of protection against common pathogens, the maintenance of immunological T-cell memory and a reservoir of repertoire diversity capable of dealing with new antigenic challenges.

The role of cellular competition in B cell survival and selection of B cell repertoires.

We studied the competitive repopulation by different B cells of irradiated mice reconstituted with bone marrow from either congenic or Ig-transgenic (TG) mice mixed at different ratios. We found that after reconstitution, the number of B cells recovered in the different chimeras is similar and independent of the ratio of injected cells. In chimeras hosting TG and non-TG cells, the relative representation of the donor cell lineages diverges from the ratios present in the inoculum, i.e. at the periphery, non-TG cells are preferentially selected. Selection of non-TG cells only occurs when population growth plateaus, i.e. when resources become limiting and competition starts to operate. Selection of non-TG cells depends on surface Ig expression, and they are selected because they have a longer survival. Finally, the life-expectancy of the same B cell population differs depending upon the second population present. The present results show that the life-span and the population size of each B cell clone can be altered (interfered with) by the presence of a second cell population, demonstrating the existence of cellular competition among B cells. Our findings establish the role of cellular competition in the selection of B cell repertoires and the existence of a hierarchy of B cell selection in the absence of antigenic stimulation. The implications of cellular competition on our understanding of the immune system are discussed.