B cell-stimulating activity of lymphoid cell membrane fractions

Cytokines Involved in the Pathophysiology of IgE Response

Recent data in both mouse and human systems have clearly indicated that IL-4 plays an essential role in the induction of IgE response, whereas IFN-γ exerts an opposite effect on this phenomenon. However, IL-4 alone is unable to induce IgE synthesis. A prior membrane contact signal delivered by activated T cells is required by B cells to synthesize IgE in response to IL-4. A cognate interaction between B and T cells producing IL-4 (but not IFN-γ) is optimal for induction of IgE synthesis. However, when such IL-4 producing T cells are activated, they may provide B cells with a membrane signal, through a non-cognate interaction, which also results in induction of IgE synthesis. Other cytokines, such as IL-2 and IL-6, play an auxiliary role in IL-4 dependent IgE production.

Analyses at the clonal level of the profile of cytokine production by helper T cells have shown that imbalances between IL-4 and IFN-γ producing T cells can be detected in patients with hyperproduction of IgE. In children with hyper-IgE syndrome a strong reduction of circulating T cells able to produce IFN-γ (and TNF-α) was found, whereas both, reduction of IFN-γ-producing and increase of IL-4 producing T cells were found in patients with parasitic infestation. In allergic inflammatory infiltrates, such as those found in vernal conjuctivitis, helper T cells able to produce IL-4 and to induce IgE synthesis accounted for the large majority of infiltrating T cells. It is reasonable to suggest that systemic or microenvironmental alterations of the balance between IL-4- and IFN-γ-producing T cells may be involved, at least in part, in the pathophysiology of the increased igE response occurring in a number of human diseases.

B-T cell interactions modulate inhibitory effects of interleukin-4 on human B cell proliferation

In this study, we investigated the role of B-T cell contacts in interleukin Ia-mediated inhibition of human B lymphocyte proliferation induced by mitogenic doses of soluble anti-mu monoclonal antibody (mAb). We show that additional cross-linking of B cell antigens, using Sepharose beads coated with anti-mu, anti-(IL)-4 mAb (but not soluble mAb) or anti-CD40 antigen counteracted the inhibitory activity of IL-4. More importantly, cell contacts between B cells and activated T cells (but not unstimulated T cells) were sufficient to counteract IL-4-mediated inhibition of DNA synthesis. In addition, the inhibitory activity of IL-4 on chronic lymphocytic leukemia B cells stimulated with anti-mu and IL-2 was itself reduced by the presence of fixed activated T cells. Our data suggest that a major role for IL-4 would be to prepare B cells to receive additional mitogenic signals through cell contact interactions with activated T lymphocytes. When such interactions do not occur IL-4 may block DNA synthesis, preventing uncontrolled B cell proliferation.

Identification of a source of biologically active CD40 ligand

We have identified the murine thymoma line EL4 as a source of biologically active CD40 ligand. Using a biotin-labeled soluble CD40.Fc fusion protein, consisting of the extracellular domain of human CD40 and the Fc region of human IgG1, EL4 cells were subjected to repeated flow cytometric cell sorting to select for cells with enhanced biotinylated CD40.Fc binding. After nine rounds of sorting, the number of CD40.Fc binding sites/cell had risen from 450 on the unsorted parental EL4 cells to 15,000 on EL40.9 cells (EL4 cells sorted with biotinylated CD40.Fc for nine rounds). Scatchard analysis of radiolabeled CD40.Fc binding revealed that the surface-expressed CD40 ligand on parental EL4 and EL40.9 cells bound its receptor with a single class of high-affinity sites (Kd = 0.5 nM). Supernatant (SN) from the sorted EL40.9 cells was found to contain human and murine B cell stimulatory activity which could be removed by preclearing with immobilized CD40.Fc, confirming the presence of soluble CD40 ligand in the preparations. EL40.9 supernatant enhanced soluble CD23 (sCD23) release and induced IgE secretion from interleukin 4-stimulated human B cells. In addition, EL40.9 SN contained proliferative activity for anti-IgM-activated murine B cells which could be removed by treatment with immobilized CD40.Fc. However, the same SN had no demonstrable activity on the proliferation of human B cells. The results presented here describe, for the first time, a source of membrane-bound and soluble CD40 ligand. The soluble form of this murine ligand has activity on murine and human B cells and induces some of the functional responses predicted for the ligand based on the action of stimulatory antibodies directed against the human CD40 surface molecule.

Identification of a novel surface protein on activated CD4+ T cells that induces contact-dependent B cell differentiation (help)

CD4+ T lymphocytes provide contact-dependent stimuli to B cells that are critical for the generation of specific antibody responses in a process termed T helper function. The surface structures on activated CD4+ T cells that mediate this function are not fully known. We previously reported the isolation of a functionally unique subclone of the Jurkat leukemic T cell line (D1.1) that constitutively expressed contact-dependent helper effector function. To identify T cell surface molecules that mediate contact-dependent T helper function, a monoclonal antibody (mAb), designated 5c8, was generated that inhibits D1.1-mediated B cell activation and immunoprecipitates a novel 30-kD protein structure from surface-iodinated D1.1 cells. Normal CD4+ T cells express 5c8 antigen (Ag) transiently 5-6 h after activation by phorbol myristate acetate and phytohemagglutinin with maximal expression 5-6 h after activation and absence of expression by 24 h. In contrast, neither resting nor activated CD8+ T cells express 5c8 Ag. In functional studies, mAb 5c8 inhibits the ability of fixed, activated CD4+ T cells to induce B cell surface CD23 expression. In addition, mAb 5c8 inhibits the ability of CD4+ T cells to direct terminal B cell differentiation driven by pokeweed mitogen. Taken together, these data suggest that 5c8 Ag is a novel, activation-induced surface T cell protein that is involved in mediating a contact-dependent element of the helper effector function of CD4+ T lymphocytes.

Autoreactive T cells from MRL-lpr/lpr mice secrete multiple lymphokines and induce the production of IgG anti-DNA antibodies

The study of T cells in individuals with systemic lupus erythematosus has been limited because a specific marker for the disease has not been identified. To approach this issue, we isolated autoreactive T cell clones from lupus-prone MRL mice, a strain that develops an accelerated form of lupus. These CD4+ T cell clones grew spontaneously from unimmunized mice, and were maintained in culture by intermittent stimulation with syngeneic antigen presenting cells in the absence of exogenous antigen. One autoreactive T cell clone, termed ARTC-1, previously reported to have atypical MHC requirements for activation (both I-Ak and I-Ek were required) and to stimulate B cell proliferation and Ig production in vitro, was found to have an unrestricted pattern of lymphokine secretion. Following stimulation, it produced IL-4, IFN-gamma and IL-2. ARTC-1 induced B cell proliferation both by cell contact and through secretion of soluble lymphokines. B cell proliferation by cell-cell contact was MHC restricted in a manner analogous to ARTC-1 activation by APCs; the B cell response was inhibited by both anti-I-Ak and anti-I-Ek antibodies. The ARTC-1 B cell interaction was also found to result in the production of IgG autoantibodies. These observations suggest that cells such as ARTC-1, if unregulated, could lead to B cell stimulation and autoantibody production in vivo, in the absence of exogenous stimulation. Furthermore, IFN-gamma production by ARTC-1 could also result in enhanced class II expression, leading both to additional T-B cell interactions and to T cell interactions with endogenous cells capable of expressing class II antigens in other organs.

Regulation and deregulation of human IgE synthesis

The IgE antibody system is among the most sophisticated of immune defense mechanisms. IgE molecules bind specifically and avidly to receptors localized on the surface of tissue mast cells and circulating basophils. These cells can produce and release several potent mediators after antigen interaction with a small number of surface-bound IgE molecules. The enormous amplification power of the IgE antibody system not only provides an important defense mechanism against parasites, but is also responsible for a number of clinical disorders. The human pathological condition most commonly associated with hyperproduction of IgE is atopy, the familial allergic disorder of immediate-type hypersensitivity to environmental allergens. In this paper Sergio Romagnani summarizes current understanding of the mechanisms involved in IgE regulation. Such studies are of primary importance in defining alterations responsible for the pathological conditions characterized by hyperproduction of IgE.

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.

Interleukin 4 induces synthesis of IgE and IgG4 in human B cells

Interleukin (IL)4 has been shown to regulate the IgG subclasses and induce IgE production in splenic mouse B cells. Here we show that IL4 and phorbol 12-myristate 13-acetate (PMA) induce, on a per cell basis, very high IgE secretion in purified human B cells by using a mouse thymoma (EL4) co-culture method. In addition, a marked increase in the number of IgG4-producing cells was also observed. Furthermore, IL2 could synergize with IL4 and PMA in the production of IgE. By using limiting dilution analysis, a considerable increase in the precursor frequency for IgE was found when IL4 and PMA were added to cultures as compared to cultures with PMA only. This indicates that IL4 induces an isotype switch in human B cells.