Non-antigen signals for B-cell growth and differentiation to antibody secretion

Indirect IL-4 pathway in type 1 immunity

Recall Ag-specific IL-4 was detected in the spleen and in the blood, but not in lymph nodes of mice in which polarized type 1 immunity was induced. This IL-4 was not produced by T cells, but soluble factors secreted by the recall Ag-activated T cells, including IL-3, triggered cells of the innate immune system, primarily mast cells, to secrete IL-4. This notion has profound implications for immunodiagnostics: the detection of apparently recall Ag-specific IL-4 does not necessarily reflect the presence of Th2 or Th0 memory T cells with long-term cytokine commitment as is of interest for assessing adoptive immunity. We found that in vivo the indirect IL-4 pathway did not suffice to trigger IgE isotype switching, but promoted IgG1 production and inhibited type 1 T cell differentiation. Therefore, the indirect IL-4 pathway can explain partial type 2 immune response phenotypes in vivo in face of unipolar Th1 T cell immunity. The representation of mast cells in different tissues may explain why immune responses in certain organs are more type 2 biased. Therefore, the indirect pathway of IL-4 production represents a novel type of interaction between the innate and the adoptive immune system that can contribute to the outcome of host defense and immune pathology.

Activated murine B lymphocytes and dendritic cells produce a novel CC chemokine which acts selectively on activated T cells

Genes were isolated using the suppression subtractive hybridization method by stimulation of pro/pre B cells with anti-CD40 and interleukin (IL)-4 to mature S mu-Sepsilon-switched cells. One of the strongly upregulated genes encodes a novel murine CC chemokine we have named ABCD-1. The ABCD-1 gene has three exons separated by 1. 2- and 2.7-kb introns. It gives rise to a 2.2-kb transcript containing an open reading frame of 276 nucleotides. Two polyadenylation sites are used, giving rise to cDNAs with either 1550 or 1850 bp of 3' untranslated regions. The open reading frame encodes a 24 amino acid-long leader peptide and a 68 amino acid-long mature protein with a predicted molecular mass of 7.8 kD. ABCD-1 mRNA is found in highest quantities in activated splenic B lymphocytes and dendritic cells. Little chemokine mRNA is present in lung, in unstimulated splenic cells, in thymocytes, and in lymph node cells. No ABCD-1 mRNA is detected in bone marrow, liver, kidney, or brain, in peritoneal exudate cells as well as in the majority of all unstimulated B lineage cells tested. It is also undetectable in Concanavalin A-activated/IL-2-restimulated splenic T cells, and in bone marrow-derived IL-2-induced natural killer cells and IL-3-activated macrophages. Recombinant ABCD-1 revealed a concentration-dependent and specific migration of activated splenic T lymphoblasts in chemotaxis assays. FACS(R) analyses of migrated cells showed no preferential difference in migration of CD4(+) versus CD8(+) T cell blasts. Murine as well as human T cells responded to ABCD-1. Freshly isolated cells from bone marrow, thymus, spleen, and lymph node, IL-2-activated NK cells, and LPS-stimulated splenic cells, all did not show any chemotactic response. Thus, ABCD-1 is the first chemokine produced in large amounts by activated B cells and acting selectively on activated T lymphocytes. Therefore, ABCD-1 is expected to play an important role in the collaboration of dendritic cells and B lymphocytes with T cells in immune responses.

Complement component 3 interactions with coxsackievirus B3 capsid proteins: innate immunity and the rapid formation of splenic antiviral germinal centers

Innate immunity is central to the clearance of pathogens from hosts as well as to the definition of acquired immune responses (D. T. Fearon, and R. M. Locksley, Science 272:50-53, 1996). Coxsackievirus B3 (CVB3), a human cardiopathic virus, was evaluated for the ability to activate the alternative and classical pathway of complement. CVB3 proteins interact with complement component 3 (C3, a soluble protein effector of innate immunity) after either in vitro exposure to mouse serum or in vivo murine infection and activate the alternative pathway of complement. In addition, we demonstrate that viral antigen retention and localization in germinal centers is dependent on C3, while virus antigen retention in extrafollicular regions in the spleen is not. In vivo depletion of native C3 abolished the rapid formation of virus-specific germinal centers (by day 3 post-CVB3 infection) in the absence of serum anti-CVB3 antibodies. These studies demonstrate that innate immune mechanisms, such as C3 interaction with CVB3, are essential for splenic antiviral germinal center formation in naive (antigen nonsensitized) mice resistant (C57BL/6J strain) and susceptible (A/J strain) to CVB3-induced myocarditis.

Modulation of B-cell activation by the B subunit of Escherichia coli enterotoxin: receptor interaction up-regulates MHC class II, B7, CD40, CD25 and ICAM-1

The B subunits of cholera toxin (CtxB) and Escherichia coli heat-labile enterotoxin (EtxB) are non-toxic lectins that bind and cross-link a ubiquitous cell glycolipid receptor, ganglioside GM1, and are recognized as potent mucosal and systemic immunogens. Here we examine the role of EtxB receptor occupancy in modulating the activation of B cells, in vitro, in primary lymphocyte cultures containing B and T cells. When 48-hr spleen cell cultures containing EtxB were compared with those in the presence of a non-receptor binding mutant, EtxB(G33D), a marked shift in the ratio of CD4+ T cells: B cells was noted. Evidence suggested that this was the result of either enhanced survival or proliferation of B cells associated with receptor occupancy by EtxB. Investigation revealed that EtxB induced only a minimal increase in proliferation above that of EtxB(G33D), in mixed cell cultures, and failed to induce any cell division of purified B cells or T cells. In contrast, receptor-binding by EtxB markedly up-regulated the expression of major histocompatability complex (MHC) class II, B7, intracellular adhesion molecule-1 (ICAM-1), CD40 and CD25 on the B-cell surface. These results indicate that the polyclonal effects of EtxB on B cells are not associated with wide-scale proliferation, but more likely with maintenance of B-cell survival by activation of molecules essential for B-cell differentiation. The findings also highlight the essential role of GM1-interaction with EtxB in the regulation of lymphocyte responses.

Jak3 is associated with CD40 and is critical for CD40 induction of gene expression in B cells

CD40 is a receptor that is critical for the survival, growth, differentiation, and isotype switching of B lymphocytes. Although CD40 lacks intrinsic tyrosine kinase activity, its ligation induces protein tyrosine phosphorylation, which is necessary for several CD40-mediated events. We show that engagement of CD40 induces tyrosine phosphorylation and activation of Jak3 as well as of STAT3. Jak3 is constitutively associated with CD40, and this interaction requires a proline-rich sequence in the membrane-proximal region of CD40. Deletion of this sequence abolishes the capacity of CD40 to induce expression of CD23, ICAM-1, and lymphotoxin-alpha genes in B cells. These results indicate that signaling through Jak3 is activated by CD40 and plays an important role in CD40-mediated functions.

Polyreactive antigen-binding B cells in the peripheral circulation are IgD+ and B7-

Polyreactive antibodies are naturally occurring antibodies, primarily of the IgM isotype, that are capable of reacting with a wide variety of different self and non-self antigens. Previously, we reported that a B cell capable of making polyreactive antibody has Ig receptors on its surface that can bind different antigens. The present investigation was initiated to characterize these polyreactive antigen-binding B cells further. A panel of fluorescein isothiocyanate-labeled antigens (insulin, IgG Fc fragment or beta-galactosidase) served as probes to select polyreactive antigen-binding B cells by cell sorting. Our experiment revealed that these polyreactive antigen-binding B cells were mainly of the IgD isotype. They expressed high levels of CD40 and major histocompatibility complex class II molecules, but little or no B7-1, B7-2, or Fas. In contrast to the binding of antigens to monoreactive receptors (usually high affinity), the binding of antigens to polyreactive receptors (usually moderate or low affinity) did not up-regulate the expression of B7-1 or B7-2. Antigens that bound to polyreactive receptors, however, were internalized and degraded, although not as efficiently as antigens that bound to monoreactive receptors. Despite the ability of these B7- cells to process antigens, they were not able to activate T cells in a mixed leukocyte reaction. It is concluded that polyreactive antigen-binding B cells have properties that are consistent with the ability to induce immunological tolerance.

The immune response: the efferent arm

Once naive T cells encounter antigen, they become primed effector cells. The scope of effector functions mediated by these cells defines the efferent arm of the immune response. The change from naive to primed effector cell is known as adaptive immunity and takes 2 forms: cell mediated, in which T cells mediate effector function, and humoral, in which antibodies are the effector molecules. There are 3 types of effector T cells: inflammatory CD4 T cells, which activate macrophages; helper CD4 T cells, which help B lymphocytes produce antibody; and cytotoxic CD8 T cells, which kill their target cells. The interaction of primed effector cells with their targets results in phenotypic changes in the cells and the secretion of cytokines. These cytokines may be secreted by the primed effector T cell, the target cell, or both. Cytokines function in either autocrine (secreted and used by the same cell) or paracrine (secreted by 1 cell and used by a different cell) circuits and have marked regulatory effects on cells in both the immune and skeletal systems. Many of these cytokines, which were once thought to be products exclusively of immune cells, are now known to be produced by cells of the skeletal system. Both the specific and nonspecific components of the immune response have profound effects on remodeling of the musculoskeletal system during normal and pathologic states.

The identification of germinal centres and follicular dendritic cell networks in rheumatoid synovial tissue

We document here the identification of germinal centres with dark and light zones, a follicular dendritic cell network and clonal expansion in the synovium of rheumatoid arthritis patients. Synovial tissue from 24 patients suffering from rheumatoid arthritis or the polyarticular form of juvenile rheumatoid arthritis were screened for the presence of lymphoid follicles. The synovial tissues of 14 patients contained follicles and four of these had germinal centres and a follicular dendritic cell network. There was a statistically significant association between follicles in the synovium and the presence of rheumatoid factor autoantibodies in the patients' serum indicating a link between local germinal centre formation and the presence of pathological rheumatoid factors. Nucleotide sequencing of monoclonal rheumatoid factors from one of the patients' synovial tissue which contained germinal centres clearly supports the possibility that these rheumatoid factors have gone through a germinal centre reaction. While rheumatoid factors from healthy immunized donors are regulated through a tolerization mechanism which selects against replacement mutations and does not allow affinity maturation, synovial rheumatoid factors seem to lack this tolerization mechanism. The formation of germinal centres where B cells affinity mature and expand at the central site of disease in rheumatoid arthritis may explain why rheumatoid factors in rheumatoid arthritis develop into auto-aggressive antibodies.

Human keratinocytes regulate their expression of B7/BB-1 antigen by a unique, calcium-dependent mechanism

Previous studies of normal human keratinocytes have indicated that these cells express BB-1 antigen, an important adherence molecule usually associated with "professional" antigen-presenting cells. We studied freshly isolated epidermal cells and noted that the frequency of BB-1-positive cells in normal human skin varied from 2.6 to 7.4% of total epidermal cells. Two-color flow cytometry confirmed that keratinocytes were the major cell in the epidermis that expressed BB-1, because less than 10% of total epidermal Langerhans cells were positive for BB-1. Northern blot analysis of RNA extracted from normal human epidermis revealed low levels of 1.7-kb B7-1 transcripts, which were independent of the presence of epidermal Langerhans cells, again indicating that such transcripts were derived from keratinocytes. Keratinocytes cultured in medium containing low concentrations of extracellular calcium (0.07 mM) expressed low levels of cell-surface BB-1. However, keratinocytes cultured in medium with higher levels of extracellular calcium (1.5 mM) lost cell-surface expression of BB-1. Similarly, low-calcium keratinocytes expressed the 1.7- and 2.9-kb B7-1 transcripts, whereas high-calcium keratinocytes expressed only the 1.7-kb transcript. Studies of the cell-surface expression of BB-1 by plastic adherent monocytes indicated that such cells do not respond to similar changes in extracellular calcium concentrations. Calcium-induced differentiation of keratinocytes regulates the expression of BB-1 antigen as well as transcripts, which is a novel mechanism for the regulation of this molecule.

CD40 ligand has potent antiviral activity

For B cells to make antibodies against most antigens, they require help from T cells. T cell help is delivered as two signals to the B cell, one of which is via CD40 and the other can be through receptors for any of a variety of soluble cytokines. We have constructed recombinant vaccinia viruses that express the ligand for CD40 and have shown that the growth of these viruses is dramatically controlled in vivo, even in mice that lack T or B cells. In this paper, we also describe our attempts to analyse the CD40 ligand-mediated antiviral activity by studying the clearance of these viruses in mice that are deficient in important antiviral mechanisms. Thus, the antiviral activity of CD40L may represent a surprising and potent effector mechanism of T cells activated during a virus infection.

Organ culture of human lymphoid tissue. I. Characteristics of the system

The major aim of three-dimensional tissue culture is to preserve the natural architecture of the tissue and thereby allow the cells to retain their original functions during in vitro cultivation. Here we describe a method for the rapid preparation of three-dimensional tissue explants from human lymphoid organs. The precision-cut tissue slices are of uniform size and thickness and can be cryopreserved and stored in liquid nitrogen without substantial loss of viability or functionality of the cells. Upon in vitro culture, cells within the explants survived as well as their counterparts cultured in single cell suspension. However, spontaneous immunoglobulin (Ig) production in explants started more promptly and often reached considerably higher levels than that in suspension cultures run in parallel. Lymphocytes within the slices could be activated by polyclonal stimuli such as PHA, as shown by the upregulation of the activation markers CD23 and CD25 on B and T cells, respectively. However, approximately five-fold higher concentrations of mitogen than those used for suspension cultures were needed. Taken together, the system presented here constitutes a potent tool for the investigation of the complex interactions leading to activation and differentiation of B and T cells in lymphoid organs.

A novel member of the TRAF family of putative signal transducing proteins binds to the cytosolic domain of CD40

CD40 is a member of the tumor necrosis factor receptor (TNF-R) family that regulates B-lymphocyte proliferation, immunoglobulin class-switching, and apoptosis through poorly defined signal transduction mechanisms. Using a yeast two-hybrid method, cDNAs were obtained that encode a novel protein, CD40-associated protein-1 (CAP-1), which binds specifically to the cytosolic domain of CD40 but not TNF-R1, TNF-R2, or Fas. The CAP-1 protein contains a C-terminal domain that shares strong amino acid sequence homology with a unique domain found recently in two putative signal transducing proteins that bind to the TNF-R2 cytosolic tail, TRAF1 and TRAF2. This C-terminal region of CAP-1 was sufficient to mediate binding to CD40 and homodimerization of CAP-1 proteins. The N-terminal portion of CAP-1 contains a RING finger motif and three zinc finger-like domains similar to those found in several regulatory proteins that interact with DNA or RNA. CAP-1 thus represents a new member of a family of potential signal transducing proteins that contain a conserved domain (the TRAF domain), bind to the cytosolic regions of particular members of TNF-R family proteins, and that can form homo- and heterotypic dimers.

CD27: marker and mediator of T-cell activation?

CD27 is a lymphocyte-specific member of the tumour necrosis factor receptor (TNF-R) family, expression of which is tightly regulated during T-cell ontogeny. Recently, the ligand for CD27 was identified and was shown to be identical to CD70, a novel member of the TNF family. Functional experiments show that the interaction between CD27 and its ligand generates a co-stimulatory signal for T-cell activation. Here, Rogier Hintzen and colleagues integrate the phenotypic and functional data available on CD27 and its ligand, and propose a role for CD27 in the amplification of T-cell responses.

Cell-cell interactions that regulate the development of B-lineage cells

Significant progress has been made recently in our understanding of the functions of lymphocyte-associated surface proteins. The latest developments involve the identification of ligands or co-receptors for many of these surface proteins. The signal transduction mechanisms utilized by these molecules are also beginning to be elucidated.