The Epstein-Barr virus-induced production of IgE by human B cells

Human Monoclonal IgE Antibodies-a Major Milestone in Allergy

PURPOSE OF REVIEW

Bound to its high affinity receptor on mast cells and basophils, the IgE antibody molecule plays an integral role in the allergic reaction. Through interactions with the allergen, it provides the sensitivity and specificity parameters for cell activation and mediator release that produce allergic symptoms. Advancements in human hybridoma technologies allow for the generation and molecular definition of naturally occurring allergen-specific human IgE monoclonal antibodies.

RECENT FINDINGS

A high-resolution structure of dust mite allergen Der p 2 in complex with Fab of the human IgE mAb 2F10 was recently determined using X-ray crystallography. The structure reveals the fine molecular details of IgE 2F10 binding its 750 Å2 conformational epitope on Der p 2. This review provides an overview of this major milestone in allergy, the first atomic resolution structure of an authentic human IgE epitope. The molecular insights that IgE epitopes provide will allow for structure-based design approaches to the development of novel diagnostics, antibody therapeutics, and immunotherapies.

Immune Activation: Contribution to AIDS-Associated Non-Hodgkin Lymphoma

HIV infection is associated with a greatly elevated risk for the development of non-Hodgkin lymphoma (NHL), which while diminished, remains elevated in the highly active antiretroviral therapy (HAART) era. Chronic B cell activation, driven by contact with HIV virions, B cell-stimulatory cytokines, viruses (EBV, HPV, HCV), and by high levels of antigenic stimulation occurs in HIV infected persons, and it is seen at even higher levels in those who go on to develop AIDS-NHL. Evidence from multiple studies indicates that elevated serum levels of several B cell-stimulatory cytokines and biomarkers are seen preceding AIDS-NHL, as well as in immunocompetent persons that develop NHL. Phenotypic changes in circulating B cells also are seen preceding AIDS-NHL, including the expression of AICDA, and of cell-surface molecules and miRNA that are associated with activated B cells. HAART only partially normalizes the immune system of treated HIV⁺ persons as they still show clear evidence for ongoing inflammation and immune activation in, even those who show complete suppression of HIV viremia. Together, this provides ample evidence to support the notion that chronic activation of B cells contributes to the genesis of B cell lymphomas.

Characterization of baseline intestinal mucosal indices of injury and inflammation in men for use in rectal microbicide trials (HIV Prevention Trials Network-056)

OBJECTIVES

The purpose of this study was to evaluate the biologic stability of mucosal parameters that might be used as endpoints in phase 1 rectal safety studies.

METHODS

Sixteen male participants were enrolled into 4 groups defined by HIV status, viral load, and sexual activity. Each participant underwent 3 flexible sigmoidoscopies at 2-week intervals with collection of blood, intestinal biopsies, and rectal secretions. Intestinal histology, phenotypic characterization of mucosal mononuclear cells, cytokine messenger RNA (mRNA) profiles (RANTES, interferon-gamma [IFNgamma], and interleukin-10), and immunoglobulin secretion were assessed. Intraclass correlation (ICC) was calculated to assess endpoint stability.

RESULTS

Qualitative histology demonstrated minimal inflammation in >95% of biopsies and remained stable throughout the study period. ICC for the tissue cytokine mRNA measurements and several T-cell phenotypic markers was >0.7, indicating stability over time. Mucosal CD4 lymphopenia was seen in the HIV-positive participants and was more pronounced in those with higher viral loads. Modest differences were observed for cytokine expression (IFNgamma) and T-cell phenotype (CD3, CD4, CD8, CD19, CD4/CCR5, and CD4/CD38) between the tissue samples collected at 10 and 30 cm.

CONCLUSIONS

These data help to provide a rationale for the selection of endpoints for future phase 1 rectal safety studies.

An increased frequency of IgE-producing B cell precursors contributes to the elevated levels of plasma IgE in atopic subjects

BACKGROUND

The production of specific IgE, which underlies the allergic response, may be a normal correlate of the immune response to a certain class of antigen (allergens), or could represent a unique response driven by regulatory signals that are absent in non-allergic individuals. If atopic subjects do possess a regulatory environment favoring IgE production, they may display not only allergen-specific IgE, but also higher levels of total IgE and higher frequencies of IgE-producing B lymphocytes.

OBJECTIVE

To address the contribution of antibody-producing cell number to the circulating IgE titre in atopic vs non-atopic subjects.

METHODS

Frequency determination by limiting dilution of EBV transformants and Poisson distribution analysis. Titres of total and allergen-specific IgM, IgG, and IgE by specific ELISA.

RESULTS

In contrast to findings reported by others, the atopic subjects had a significantly higher frequency of IgE-producing B cells than non-atopics (0.79% of total Ig-producing cells, as compared with 0.17% for the control group; P < 0.01), suggesting that one factor contributing to the high plasma IgE titres in atopic subjects is the high frequency of B lymphocytes with the potential to produce IgE. Although only the atopic subjects produced allergen-specific IgE, the frequency of specific IgE-producing B cells was undetectable in both groups.

CONCLUSION

Atopic subjects have higher frequencies of IgE-producing B cell precursors than non-atopics. A correlation exists between IgE-producing B cell frequency and levels of circulating IgE.

Quantitative analysis of peripheral allergen-specific B lymphocytes in asthma

We have recently shown that the number of peripheral allergen-specific T cells can, in part, predict, together with methacholine hyperresponsiveness, the bronchial response to inhaled allergen in asthmatic patients. This study was designed to explore the role of blood B cells committed to produce allergen-specific IgE in asthma. Twenty-three asthmatic patients sensitized to Dermatophagoides pteronyssinus and 11 control subjects were studied. Peripheral blood B cells, committed to produce allergen-specific IgE, were enumerated by limiting dilution microcultures of Epstein-Barr virus (EBV)-transformed B cells. An allergen inhalation challenge was performed in all asthmatic subjects. No difference was found in the frequency of B cells committed to produce allergen-specific IgE either between asthmatic patients and controls or between asthmatic patients with or without late-phase bronchial response to allergen. No correlation was found between the frequency of B cells committed to produce allergen-specific IgE and the bronchial response to the allergen inhalation challenge. We conclude that, in quantitative terms, peripheral allergen-specific B cells are not as relevant as T cells to the development of the asthmatic response in the model of provoked asthma.

Defective IL-6 secretion in HIV-infected haemophilia patients

To study the role of IL-6 in HIV-induced B cell defects, in vitro B cell responses and IL-6 secretion were determined simultaneously in 67 haemophilia patients. Twenty-three patients were HIV- (Group 1), 27 HIV+ stage CDC II, III (Group 2), and 17 were HIV+ stage CDC IV (Group 3). Pokeweed mitogen (PWM) was used for T cell-dependent and Staphylococcus aureus Cowan I (SAC I) for T cell-independent B cell stimulation. B cell differentiation was assessed in a reverse haemolytic plaque assay and by ELISA determination of IgG and IgM in culture supernatants. An ELISA was used to measure IL-6 in plasma and culture supernatants. HIV- patients showed impaired immunoglobulin-secreting cell (ISC) responses after T cell-independent and T cell-dependent stimulation (P < 0.0001 and P < 0.01, respectively), whereas IL-6 secretion, IgM and IgG responses were comparable to those in healthy controls. HIV+ patients at stage CDC II, III or IV demonstrated significantly reduced mitogen-stimulated IL-6 secretion (P < 0.05, PWM; P < or = 0.001, SAC I) as well as impaired ISC and IgG responses (P < 0.01, PWM; P < or = 0.0001, SAC I). CDC IV patients showed reduced IgM responses in addition (P < 0.02, PWM; P < 0.0005, SAC I). Plasma IL-6 levels were elevated both in HIV+ patients (CDC II, III patients: 165 +/- 73 pg/ml, P < 0.005; CDC IV patients: 58 +/- 18 pg/ml, P < 0.0001) and in HIV- patients (283 +/- 65 pg/ml, P < 0.0001) which appeared to be a T cell effect induced by treatment with haemophilia factor concentrates. Our data provide evidence for different types of B cell deficiencies in HIV- patients (impaired ISC response only) and HIV+ patients (impaired ISC as well as IL-6 and IgM/IgG responses). The defective IL-6 secretion in HIV+ patients is likely to affect terminal B cell differentiation and this may explain the reduced immunoglobulin secretion in these patients in response to antigenic challenge.