The molecular interactions with helper T cells which limit antigen-specific B cell differentiation

The 1st step initiation essential for allergen-specific IgE antibody production upon the 2nd step: Induction of non-specific IgE+ small B cells containing secondly-sensitized allergen-specific ones in mice firstly-sensitized with an allergen

There was a significant amount of non-specific, but not of allergen (e.g., papain, mite feces and four kinds of pollen)-specific, IgE antibodies (Abs) in the sera of normal mice. An i.n. injection of each allergen without adjuvant into mice caused an increase in total IgE Ab titers with a similar time course in the serum. However, the stage of initiation of allergy varied from allergen to allergen. Submandibular lymph node cells from normal mice contained papain-, but not mite feces- or pollen-specific IgE⁺ cells and an i.n. injection of papain induced papain-specific IgE Abs in the serum. In contrast, one (i.n.) or two (i.n. and s.c) injections of mite feces induced neither mite feces-specific IgE⁺ cells in the lymph nodes nor mite feces-specific IgE Abs in the serum. I.n. sensitization with cedar pollen induced cedar pollen-specific IgE⁺ small B cells in the lymph nodes on Day 10, when non-specific IgE Ab titers reached a peak in the serum, implying induction of related allergen-specific IgE⁺ small cells as well. In fact, a second (s.c.) injection of ragweed (or cedar) pollen into mice sensitized i.n. once with cedar (or ragweed) pollen, but not with mite feces, induced a large amount of ragweed (or cedar) pollen-specific IgE Abs in the serum. These results indicate that when firstly-sensitized non-specific IgE⁺ small B cells in mouse lymph nodes include some secondly-sensitized allergen-specific ones, mice produce IgE Abs specific for the secondly-injected allergen.

Humoral immune responses in CD40 ligand-deficient mice

Individuals with X-linked hyper-IgM syndrome fail to express functional CD40 ligand (CD40L) and, as a consequence, are incapable of mounting protective antibody responses to opportunistic bacterial infections. To address the role of CD40L in humoral immunity, we created, through homologous recombination, mice deficient in CD40L expression. These mice exhibited no gross developmental deficiencies or health abnormalities and contained normal percentages of B and T cell subpopulations. CD40L-deficient mice did display selective deficiencies in humoral immunity; basal serum isotype levels were significantly lower than observed in normal mice, and IgE was undetectable. Furthermore, the CD40L-deficient mice failed to mount secondary antigen-specific responses to immunization with a thymus-dependent antigen, trinitrophenol-conjugated keyhole limpet hemocyanin (TNP-KLH). By contrast, the CD40L-deficient mice produced antigen-specific antibody of all isotypes except IgE in response to the thymus-independent antigen, DNP-Ficoll. These results underscore the requirement of CD40L for T cell-dependent antibody responses. Moreover, Ig class switching to isotypes other than IgE can occur in vivo in the absence of CD40L, supporting the notion that alternative B cell signaling pathways regulate responses to thymus-independent antigens.

Small splenic B cells that bind to antigen-specific T helper (Th) cells and face the site of cytokine production in the Th cells selectively proliferate: immunofluorescence microscopic studies of Th-B antigen-presenting cell interactions

Antigen (Ag)-specific T helper (Th) cells regulate the proliferation and differentiation of Ag-specific B cells by secreting cytokines and by expressing activating receptors like gp39. In vitro, the cytokines and the activating receptors function in an Ag-nonspecific manner. It is unclear, therefore, how Ag specificity is imposed on B cell responses in physiological Th-B cell interactions. Here we studied, at the single cell level, the interactions between cloned Th cells and small splenic B cells, which served as Ag-specific antigen-presenting cells (APCs) to the Th cells. Digital confocal immunofluorescence microscopy of Th-B cell conjugates revealed significant variability in the molecular and cellular properties of these interactions, in spite of the fact that all the interactions in this system were expected to be Ag specific. After 30 h of incubation B cells began to divide, and this process was entirely dependent on the presence of both Th cells and Ag. Immunofluorescence microscopic studies showed that essentially all the mitotic B cells were bound to Th cells and faced the microtubule organizing center (MTOC) in the Th cells where interleukin 4 was highly concentrated. Other B cells that were bound to the same Th cells but were not close to the Th-MTOC remained in interphase. These results provide the first direct structural and functional evidence that the site of interaction of B cells with Th cells affects their immune response. We propose that, during Ag-induced Th-B cell interactions, B cells that are bound facing the Th-MTOC proliferate preferentially because they are the recipients of locally secreted cytokines. In addition, these B cells may interact with newly expressed receptors, which may also be locally inserted into the Th membrane. The polarized delivery of activating molecules towards the Th-bound APCs may impose functional specificity on effector molecules that otherwise are not Ag specific.

Biochemistry of B lymphocyte activation

The activation of B lymphocytes from resting cells proceeds from the events of early activation to clonal proliferation to final differentiation into either an antibody-secreting plasma cell or a memory B cell. This is a complex activation process marked by several alternative pathways, depending on the nature of the initial antigenic stimulus. Over the past 5-10 years, there has been an explosion of studies examining the biochemical nature of various steps in these pathways. Some of that progress is reviewed here. In particular, we have described in detail what is known about the structure and function of the AgR, as this molecule plays a pivotal role in B cell responses of various types. We have also reviewed recent progress in understanding the mechanism of action of contact-dependent T cell help and of the cytokine receptors, particularly the receptors for IL-2, IL-4, and IL-6. Clearly, all of these areas represent active areas of investigation and great progress can be anticipated in the next few years.

The induction of resting B cell differentiation does not require T cell contact

A T cell clone as well as immediately ex vivo CD4+ lymph node T cells are shown to support the differentiation of co-cultured resting B cells in the absence of T cell-B cell contact. Antibodies specific for class II products of the major histocompatibility complex inhibit the transactivation of resting but not activated B cells. This differential inhibition pattern indicates that the responses obtained from resting B cell populations are not due to their contamination with B cell blasts. Further, supernatants prepared from an activated T cell clone induce resting B cell differentiation. Two lines of evidence suggest that the activity contained in these supernatants can be attributed to interleukin (IL)-5. Activity is neutralized by monoclonal anti-IL-5; and both recombinant and affinity-purified IL-5 induce the differentiation of immediately ex vivo resting and activated B cells with comparable efficiency. Taken together, these results demonstrate that contact with T cells does not provide prerequisite signals for the induction of resting B cell differentiation.

Membranes of activated CD4+ T cells expressing T cell receptor (TcR) alpha beta or TcR gamma delta induce IgE synthesis by human B cells in the presence of interleukin-4

In the present study it is demonstrated that human B cells can be induced to switch to IgE production following a contact-mediated signal provided by activated T cell receptor (TcR) gamma delta+, CD4+ and TcR alpha beta+, CD4+ T cell clones and interleukin (IL)-4. The signal provided by these T cell clones was antigen nonspecific, indicating that the TcR alpha beta/CD3 or TcR gamma delta/CD3 complexes were not involved in these T-B cell interactions. Activated TcR alpha beta+, CD8+, and TcR gamma delta+, CD4-CD8-, or resting CD4+ T cell clones were ineffective. Intact TcR alpha beta+ or TcR gamma delta+, CD4+ T cell clones could be replaced by plasma membrane-enriched fractions isolated from these activated CD4+ T cell clones. In contrast, membranes isolated from resting TcR alpha beta+, CD4+, TcR gamma delta+, CD4+ T cell clones or an Epstein-Barr virus (EBV)-transformed B cell line (EBV-LCL) failed to provide the costimulatory signal that, in addition to IL-4, is required for induction of IgE synthesis. As described for intact CD4+ T cells, CD4+ T cell membranes induced purified surface IgM+ B cells to switch to IgG4- and IgE- but not to IgA-producing cells, excluding the possibility of a preferential outgrowth of IgG4- and IgE-committed B cells. The membrane activity was inhibited by protease or heat treatment. Induction of IgE synthesis by B cells co-cultured with both TcR alpha beta+, CD4+ and TcR gamma delta+, CD4+ T cell clones and membrane preparations of these cells was blocked by anti-class II major histocompatibility complex (MHC) monoclonal antibodies (mAb), whereas various anti-CD4 mAb had differential blocking effects. Murine L cells, or EBV-LCL transfected with CD4 could not replace CD4+ T cell clones. These results indicate that, although CD4 and class II MHC antigens are required for productive CD4+ T cell clone-B cell interactions, an additional signal, provided by a membrane associated (glyco)protein that is induced by activation of both TcR alpha beta and TcR gamma delta, CD4+ T cells, is needed for induction of IgE production in the presence of IL-4.

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.

Requirement for noncognate interaction with T cells for the activation of B cell immunoglobulin secretion by IL-2

The mechanism whereby noncognate contact with activated IL-2-producing Type 1 helper T cells (TH1) induces B cell activation was examined. Small resting B cells from C57B1/6 mice were cultured, in the absence of any ligand for surface Ig, with irradiated cells of the hapten-specific, CBA-derived, F23.1+ TH1 clone E9.D4 in F23.1 (anti-T cell receptor V-beta 8)-coated microwells. This induced polyclonal B cell activation to enter cell cycle (thymidine incorporation) at 2 days and to secrete immunoglobulin at 5 days. An anti-IL-2 mAb (S4B6) inhibited antibody production completely. Anti-IL-2 did not inhibit either LPS-induced B cell responses, or T cell activation (measured as IL-3 secretion). Anti-IL-2 receptor (anti-Tac) mAbs also inhibited T-dependent B cell responses, without affecting LPS responses. An anti-IFN-gamma mAb partially inhibited Ig secretion, without affecting entry into cycle. LPS responses or T cell activation. Other antibodies (anti-IL-3, IL-4, IL-5, Thy-1.2, CD5) were not inhibitory. After 2 days of culture with F23.1-activated T cells, B cells appeared to have become responsive to IL-2, in that they could be driven to immunoglobulin production by the addition of IL-2. Flow cytometry showed no expression by these B cells of 55-kDa (Tac) IL-2 receptors. Also, rigorous removal of T cells from 2-day cocultures prevented the response to IL-2, and readdition of T cells restored it. Because the reconstituted responses were inhibited both by anti-IL-2 and by anti-Tac, IL-2 must have acted indirectly, via the T cells that were present in these cultures. Continued contact with T cells was therefore necessary for the progression of B cells to antibody secretion.

Cognate interactions between helper T cells and B cells

The mechanism by which mammals produce an antibody response after exposure to antigen has intrigued biologists for over a hundred years. Here, Randolph Noelle and Charles Snow review some of the experimental findings since the early 1970s that have advanced understanding of the mechanisms operating during B-cell activation by thymus-dependent (TD) antigens. They also propose a model for B-cell activation that emphasizes the critical role played by direct cellular interactions between B cells and helper T(TH) cells and seek to place into perspective the role played by the membrane immunoglobulin (mlg) receptor in cognate responses.

The staphylococcal toxic shock syndrome toxin 1 triggers B cell proliferation and differentiation via major histocompatibility complex-unrestricted cognate T/B cell interaction

The Staphylococcus aureus exotoxin toxic shock syndrome toxin 1 (TSST-1) is a potent activator of T cells and monocytes. We have recently demonstrated that TSST-1 is a superantigen that binds monomorphic determinants on MHC class II molecules. In the present study, we have examined the effect of TSST-1 on the activation and differentiation of high density human tonsillar B cells. TSST-1 bound to tonsilar B cells with high affinity and saturation kinetics. This binding was effectively inhibited by a combination of anti-HLA-DR and anti-HLA-DQ mAbs. Treatment of purified B cells with TSST-1 failed to induce B cell proliferation or Ig production. However, in the presence of irradiated T cells, TSST-1 induced resting B cells to proliferate and differentiate into Ig secretory cells. TSST-1 mimicked nominal antigen in that its induction of B cell responses was strictly dependent on physical contact between T and B cells, and was profoundly inhibited by anti-MHC class II mAbs, anti-CD3 mAbs, and, to a lesser extent, by anti-CD18 mAbs. However, unlike nominal antigen, TSST-1-mediated T/B cell interactions were MHC unrestricted. These results suggest that TSST-1 induces T cell-dependent B cell proliferation and differentiation by virtue of its ability to mediate MHC-unrestricted cognate T/B cell interaction via the TCR/CD3 complex and MHC class II antigens.

Immune status of a mu, kappa transgenic mouse line. Deficient response to bacterially related antigens

We have examined the immune repertoire and immune response of a mouse that carries transgenes for a mu heavy chain and kappa light chain. The expression of these genes is under the regulation of their own controlling elements. The transgenes are expressed early in ontogeny and are easily detectable from day 13 of gestation onwards. The pre-B cells seem to function normally as they generate IgM-secreting colonies at normal frequencies. Colonies show predominantly the transgenic specificity. Expression of the transgenes is not limited to B cells since around 10%-20% of peripheral T cells and 50% of thymocytes express the mu transgene as an intracellular protein. Ectopic expression of kappa was not seen. The spleen size of the transgenic mouse is decreased by around 20%; this reduction is largely caused by a reduction of the B cell pool. Almost all B cells express the transgenes, only 30% co-express endogenous heavy chain genes and all co-express endogenous light chain genes. Serum Ig levels for IgM and IgA were normal, 20% of the IgM consist of the transgenic product. Serum IgG levels were decreased. T cell functions (helper and cytotoxic) were normal. Immune responses to conventional antigens were impaired, especially in the early phases of the immune response, but after boosting they were virtually normal, except for IgG3 which remained low. Primary antibody responses to T cell-independent antigens of the class II type (bacterially related antigens) were absent, although precursor frequencies for these antigens were within the expected range. The significance of this finding, as it relates to allelic exclusion of Ig genes, is discussed.

Molecular dynamics in the membranes of helper T cells

We provide evidence that redistributions and interactions of integral proteins in the fluid membranes of helper T (Th) cells may play important roles in Th-cell activation. A particular monoclonal antibody, 3D3, directed to a clonotypic determinant on the T-cell receptor (TCR) of the cloned Th-cell line D10, had previously been shown to be distinctively capable of directly activating D10 cells at low concentrations. We demonstrate here by immunofluorescence experiments that it is also distinctively able itself to produce a clustering (capping) of the TCRs on the D10 cell surface. Simultaneously, by means of double-immunofluorescence experiments, we find that the 3D3-induced clustering of the TCRs distinctively produces a co-clustering of the accessory molecule CD4 with the TCR clusters, although the CD4 and TCR molecules are normally independent of one another in the D10 cell membrane. These results, and related ones previously obtained from studies of the interactions of D10 Th cells with antigen-presenting cells, are analyzed to suggest that the membrane clustering of TCRs and the induced TCR-CD4 interactions are critical to the signaling events in Th-cell activation.