Control of the cell cycle of murine B lymphocytes: the nature of alpha- and beta-B-cell growth factors and of B-cell maturation factors

Germinal centre cell kinetics

The germinal centre is a fundamental component of the humoral immune response, representing a unique microenvironment where antigen-activated B lymphocytes undergo clonal expansion, mutate their immunoglobulin, and are subject to a stringent selection process based on their antigen affinity. This review highlights recent advances in the understanding of the cell kinetic process of activation, proliferation, differentiation, and death of germinal centre cells, which are beginning to provide important insights into the regulation of this highly complex reaction. Their definition may have considerable pathological import given the involvement of the germinal centre in non-Hodgkin's lymphomas and recent evidence suggesting that abnormal germinal centre reactions may be involved in the pathogenesis of Hodgkin's disease and some autoimmune diseases.

Regulation of murine germinal centre cell proliferation in vivo: a stathmokinetic study examining the effect of differently timed doses of cyclosporin A

Although studies have identified factors which affect germinal centre cell proliferation in vitro, their relative contributions in vivo remain largely undetermined. In this study, the proliferative rate of germinal centre cells was measured in sheep red blood cell-immunized C3H/HeN mice exposed to variously timed doses of cyclosporin A. Germinal centre (GC) cell proliferation was measured by a stathmokinetic technique to determine GC cell birth rates at specific time points after immunization. Changes in total GC volume were determined by morphometry in order to assess actual growth and regression of the GC cell population. An estimate of the absolute rate of GC cell proliferation was derived from these two values. Following exposure to antigen, there was an initial inhibition of proliferation within pre-existing germinal centres, followed by a rapid rise, then a sustained phase of increased GC cell proliferation. By comparing the effects of the different cyclosporin A treatment regimes, it was possible to deduce that the initial inhibition of proliferation was mediated by a T-cell-derived cytokine, as was the final sustained phase of the proliferative response. The intervening rise in GC cell proliferation, however, was attributable to a contact-dependent signalling mechanism.

The structure of an alternate form of complement C3 that displays costimulatory growth factor activity for B lymphocytes

In this study, the structure of a novel 1.9-kb transcript coding for complement component 3 (C3) is described. This alternate C3 is identical to the 3' end of the C3 message beginning at position 3300 of the C3 cDNA. Its transcription appears to be driven by an alternate promoter located within intron 8 of the C3 gene. This alternate C3 message contains an open reading frame that may encode a 536-amino acid-long protein identical to the 3' part of the C3 alpha chain. The resulting protein contains the complement receptor CR2 binding site. The suggested 5' end of coding region of the alternate C3 includes information for a potential hydrophobic leader peptide that would allow secretion of the protein. In vitro assays with macrophage-depleted mouse splenic B cells indicate that an activity is secreted from cell lines transfected with the alternate C3 cDNA. Together with Sepharose-bound immunoglobulin M-specific monoclonal antibodies and interleukin 2, it costimulates the proliferation of B cells. Implications for possible in vivo functions are discussed.

Kinetics of the pleiotropic effect of interleukin 4 on the surface properties of human B-lymphoma cells

Striking antigenic changes were elicited by interleukin 4 (IL-4) in the Farage human B-cell lymphoma line. After 2 days of incubation with IL-4 the expression of CD23, CD54 (ICAM-1), CD58 (LFA-3) was increased while the levels of CD21, CD22, CD38 were diminished. Prolonged incubation of Farage cells with IL-4 for 6-8 days led to increased expression of CD11a (LFA-1) CD39, CD40, and to disappearance of CD21 and CD38. The modulation of antigenic properties of Farage cells was associated with enhancement of their homotypic adhesiveness and the formation of giant clumps of cells. The recovery of Farage cells which had been exposed to IL-4 for six days was not complete and eleven days after withdrawal of the cytokine, these cells still displayed a lower level of CD21 and of CD38 than control cells. Cycling and non-cycling cells did not appear to differ in their antigenic properties, indicating that modification of the antigenic profile did not result from cell selection or cell arrest. These results showed that the pleiotropic effect of IL-4 on various cell surface structures on malignant human B cells proceeds at different rates suggesting that distinct metabolic pathways may regulate their expression.

The effect of IL-4 on the phenotype of a human B-cell lymphoma line (Farage) lacking immunoglobulin expression

Farage cells do not express surface immunoglobulins (sIg) but display a high level of CD19, CD21, CD22, CD23, CD39, CD40 B-cell antigens, and various adhesion proteins, such as CD11a (LFA-1), CD29 (VLA-4), CD44, CD54 (ICAM-1), and CD58 (LFA-3). The phenotype of Farage resembled that of EBV-LCL but differed from the phenotype of Burkitt's lymphoma lines, which were CD39- CD44-, expressed a high level of CD38, and either lacked CD21 or were weakly positive. Exposure to IL-4 augmented the concentrations of CD23 and of adhesion proteins on the surface of Farage cells but diminished the expression of CD21, CD22, and CD38. IL-4 did not induce the expression of sIg on Farage cells and failed to affect the level of HLA-DR. IL-2 and TPA did not alter the level of CD21 and adhesion proteins on Farage cells. Although IL-4 induced unique changes of the antigenic pattern in Farage, no significant effect on the phenotype of Burkitt's lymphoma lines was detected after IL-4 treatment. The present study indicates that the responsiveness of B cells to IL-4 is not determined by the expression of sIg but rather is associated with the antigenic profile characteristic for non-germinal center B lymphocytes.

Cell kinetics of the germinal center reaction--a stathmokinetic study

The changes in splenic germinal center (GC) cell proliferation were measured during primary and secondary responses to a T-dependent antigen in vivo to examine the regulation of the GC reaction. Adult C3H/HeN mice were immunized with sheep red blood cells and boosted 7 or 21 days later. GC cell proliferation was assessed by measurement of GC cell birth rates using a stathmokinetic technique. Actual GC growth and regression were assessed in terms of total splenic volume and number. Pre-existing GC had a mean cell birth rate of 33 cells/1000 cells/h. The GC reactions following each immunization showed a biphasic pattern of changes in cell birth rate, comprising an initial fall immediately succeeded by a transient, but significant, increase. These fluctuations occurred earlier in secondary compared to primary responses. Significant increases in total GC volumes succeeded the peaks of cell birth rate following both primary and early secondary immunization. However, there was a substantially smaller increase following later secondary immunization. We propose that the initial cell birth rate reduction is due to inhibition of pre-existing GC clones and represents one component of the phenomenon of GC dissociation. The succeeding peak birth rate represents early, massive proliferation of newly activated antigen-specific clones. The different patterns of GC expansion, despite similar proliferative responses, may reflect different pathways of differentiation dependent on the timing of antigenic stimulation.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

Induction of interleukin-5 responsiveness in resting B cells by engagement of the antigen receptor and perception of a second polyclonal activation signal

Experiments were performed to examine the nature of agents which could induce IL-5 responsiveness in small, resting splenic B lymphocytes. First, IL-5 increased plaque forming cell responses to the TI-1 antigen TNP-LPS. A second set of experiments using anti-IgM + LPS which allowed limiting dilution analysis showed induction of IL-5 responsiveness in about 20% of the resting B cell population. In the same system, IL-4 increased the percentage of proliferating cells by about 40%. A third system using the TI-2 analog conjugate anti-IgD-dextran (anti-delta-dextran) also rendered small, resting B cells responsive to IL-5. An additional system employing anti-IgM plus dextran sulfate, which also allowed limiting dilution analysis, induced IL-5 responsiveness in at least 10% of resting B cells. The features common to all four systems inducing B cell IL-5 responsiveness are at least twofold. Each system directly accesses the B cell antigen receptor and causes crosslinking. Second, each system also provides an additional polyclonal activating moiety, some of which may be similar to those in thymus independent antigens. These results suggest that some resting B cells may become IL-5 receptive after perception of at least two kinds of signals one of which perturbs sIg and the second being nonspecific and polyclonally activating.

Differential activity of recombinant lymphokines on mouse B cell proliferation and cell cycle progression are revealed by 5-bromo-2'-deoxyuridine/Hoechst 33258 dye flow cytometry

Activation of resting mouse B cells with anti-mu chain antibodies (anti-mu) leads to cell proliferation. We have investigated the effect of recombinant T cell interleukins (IL 2 to IL 6) on such anti-mu-induced proliferation. No proliferative response was detected when IL 2, IL 3 and IL 6, either alone or in combination with anti-mu, were studied. Furthermore, neither IL 4 nor IL 5 could induce proliferation when added alone to B cell cultures. However, when combined with anti-mu, IL 4 as well as IL 5 stimulated cell growth. Analysis by 5-bromo-2'-deoxyuridine/Hoechst 33258 flow cytometry revealed distinct effects of IL 4 and IL 5 on B cell growth. In the presence of anti-mu, both IL 4 and IL 5 co-stimulated unfractionated splenic B cells. However, when B cells were separated into subpopulations by density, IL 4 proved to be a cell cycle progression factor, stimulating the majority of resting B cells to enter the cell cycle. In contrast, IL 5 had little effect on the resting fraction of B cells. Rather, IL 5 acted as a co-competence factor, stimulating predominantly low-density B cells. Following exposure of anti-mu alone, most B cells accumulated in the G1 of the second cycle. Upon addition of IL 4, the cells acquired the ability to progress into the next S phase compartment. Contrary to what is seen when B cells are stimulated by other mitogens, very few cells are in the G2 compartments after anti-mu plus IL 4 stimulation. This phenomenon was not due to a differential cell cycle progression rate. Our findings provide an analytical basis for fractionating cell-cycle-compartment-specific B cells for their molecular study.

Immunoregulators in the nervous system

The nervous system, through the production of neuroregulators (neurotransmitters, neuromodulators and neuropeptides) can regulate specific immune system functions, while the immune system, through the production of immunoregulators (immunomodulators and immunopeptides) can regulate specific nervous system functions. This indicates a reciprocal communication between the nervous and immune systems. The presence of immunoregulators in the brain and cerebrospinal fluid is the result of local synthesis--by intrinsic and blood-derived macrophages, activated T-lymphocytes that cross the blood-brain barrier, endothelial cells of the cerebrovasculature, microglia, astrocytes, and neuronal components--and/or uptake from the peripheral blood through the blood-brain barrier (in specific cases) and circumventricular organs. Acute and chronic pathological processes (infection, inflammation, immunological reactions, malignancy, necrosis) stimulate the synthesis and release of immunoregulators in various cell systems. These immunoregulators have pivotal roles in the coordination of the host defense mechanisms and repair, and induce a series of immunological, endocrinological, metabolical and neurological responses. This review summarizes studies concerning immunoregulators--such as interleukins, tumor necrosis factor, interferons, transforming growth factors, thymic peptides, tuftsin, platelet activating factor, neuro-immunoregulators--in the nervous system. It also describes the monitoring of immunoregulators by the central nervous system (CNS) as part of the regulatory factors that induce neurological manifestations (e.g., fever, somnolence, appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic pathological processes.

Analysis of cell proliferation and mu-RNA processing during activation of mouse B-cells by anti-mu and T lymphokines

Anti-immunoglobulin (anti-Ig, anti-mu is commonly used) activates resting mouse B-cells to proliferate but not to differentiate and secrete Ig. Differentiation requires additional help from T-cells including soluble factors such as lymphokines. The capability of lymphokines, alone and in combination, to promote the differentiation of anti-mu activated B-cells has been investigated. Some lymphokines, like interleukin (IL) 2 and 3, as well as human-interferon beta-2 (IL-6), have no significant effect on differentiation. IL-4 and 5 maintain cell growth but do not lead to differentiation, which requires multiple factors present in ConA supernatant or partially purified TRF. Anti-mu and interferon-gamma (IFN-gamma) exert both positive and negative effects on B-cell maturation. Anti-mu induces cell proliferation. IFN-gamma enhances Ig transcription, but it has no apparent proliferation or differentiation activity. Anti-mu and IFN-gamma inhibit Ig secretion by causing the accumulation of nuclear mu-RNA precursors. Although phorbol ester plus ionomycin induce cell proliferation, the negative effect of anti-mu in RNA processing could not be mimicked by these reagents. I show that anti-mu and IFN-gamma interfere with the splicing of nuclear hnRNA. This phenomenon is independent of known 2'-5'(A)n synthetase activity. The data suggest that post-transcriptional regulation of mu-RNA processing might be a critical event in controlling the generation of the plasma cells (which secrete IgM), memory precursor cells or abortive cells (both of which do not secrete IgM).

Ion channels and B cell mitogenesis

Given the presence of ionic channels at the membrane of lymphocytes, we have analyzed the effect of various channels blockers on B lymphocytes activation. TEA and 4-AP, two K+ channels blockers, quinine, a blocker of Ca2(+)-activated K+ channels, nickel and verapamil, two Ca2+ channels blockers, all inhibited LPS-induced B cell proliferation. However, these drugs neither inhibited the induction of Ia and Fc gamma RII expression nor cell enlargement and early RNA synthesis, indicating that the entry of B lymphocytes into G1 phase was not affected. In contrast, both late RNA synthesis and the induction of the TfR, which occur while the cell progress through G1, were inhibited by these blockers. These data show that TEA, quinine and verapamil block B lymphocyte activation during the G1 phase, probably between G1A and G1B. To question whether these effects were due to the block of voltage-activated K+ channels, we compared the ability of TEA, quinine, verapamil, 4-AP and nickel to block proliferation and K+ channels. A striking correlation was found for all the drugs but less for 4-AP. Moreover, TMA, a TEA analog unable to block K+ currents, did not affect B cell proliferation. Taken together, our data suggests that functional voltage-gated K+ channels are required at a precise stage of the G1 phase of the B cell cycle.

Immunomodulators and feeding regulation: a humoral link between the immune and nervous systems

Cells of the nervous and immune systems have specific receptors for humoral substances that originate in both systems. These elements establish a bidirectional information exchange network between the nervous and immune systems. In particular, neuroregulators (neurotransmitters and neuromodulators) can modulate specific immune system function(s) and immunoregulators (immunomodulators) can modulate specific nervous system function(s). Modulation of immune functions by neuroregulators has been receiving considerable attention; however, modulation of nervous system functions by immunomodulators has been little studied. The presence of immunomodulators in the brain and cerebrospinal fluid may represent local synthesis by astrocytes, microglia, endothelial cells, intrinsic macrophages and blood-derived lymphocytes which cross the blood-brain barrier, or the concentration of substances derived from the peripheral blood. Acute and chronic inflammatory processes, malignancy, and immunological reactions stimulate the synthesis and release of immunomodulators in various cell systems. These immunomodulators have pivotal roles in the coordination of the host defense mechanisms and repair and induce a series of endocrine, metabolic, and neurologic responses. This paper focuses on the effects of immunomodulators (interleukins, tumor necrosis factor, tuftsin, platelet activating factor, and others) on the central nervous system (CNS), in particular, on feeding regulation. It is proposed that an immunomodulatory system regulates food intake by a direct action in the CNS through a specific neuro-immuno interaction. This regulatory system may be operative during acute and chronic disease.

The effects of bacterial lipopolysaccharide, anti-receptor antibodies and recombinant interferon on mouse B cell cycle progression using 5-bromo-2'-deoxyuridine/Hoechst 33258 dye flow cytometry

Polyclonal stimulation of resting B cells with anti-antigen receptor antibodies [anti-IgM mu chain antibody (anti-mu)] or with bacterial lipopolysaccharide (LPS) stimulates a proportion of B cells to proliferate. Exposure of resting B cells to both LPS and anti-mu activates a larger population of resting B cells than either alone, suggesting a synergistic effect of these two stimuli. Although recombinant interferon (rIFN) either alone or in combination with anti-mu has no apparent proliferative activity (as measured by [3H]thymidine incorporation), application of 5-bromo-2'-deoxyuridine/Hoechst 33258 dye flow cytometry reveals a distinct effect of rIFN on B cell growth. In the presence of anti-mu plus LPS, rIFN causes the majority of B cells to enter the cell cycle (CC), but a subset of B cells remains in the resting stage. Another subset of B cells has extremely rapid CC transit times, with a CC duration of less than 10 h. These studies show that both anti-mu and LPS are competence factors (which move cells from the G0 phase to the G1 phase). LPS acts also as a CC progression factor, while rIFN is a CC potentiating factor.

Developmental biology of T cell receptors

T cell receptors are the antigen-recognizing elements found on the effector cells of the immune system. Two isotypes have been discovered, TCR-gamma delta and TCR-alpha beta, which appear in that order during ontogeny. The maturation of prothymocytes that colonize the thymic rudiment at defined gestational stages occurs principally within the thymus, although some evidence for extrathymic maturation also exists. The maturation process includes the rearrangement and expression of the T cell receptor genes. Determination of these mechanisms, the lineages of the cells, and the subsequent thymic selection that results in self-tolerance is the central problem in developmental immunology and is important for the understanding of autoimmune diseases.

T cell-replacing factor (TRF)/interleukin 5 (IL-5): molecular and functional properties

TRF has originally been defined as a T-cell-derived lymphokine that triggers activated B cells for a terminal differentiation into Ig-secreting cells. HPLC-purified TRF from Sup of a murine TRF-producing B151 cell is an acidic glycoprotein, exerts BCGF II activity and induces expression of IL-2 receptors. It does not show IL-1, IL-2, IL-3, BSF-1/IL-4, or IFN gamma activity. We prepared monoclonal TB13 and NC17 antibodies against HPLC-purified B151-TRF which are specific for and can inhibit TRF as well as BCFG II activity of B151-TRF. Moreover, TB13 as well as NC17 antibody can immunoprecipitate the 46 Kd molecule from B151 Sup which exerts TRF as well as BCGF II activity. Complementary DNA (pSP6K-mTRF23) encoding for murine TRF/IL-5 was cloned and its entire nucleotide sequences were determined. The murine TRF/IL-5 cDNA encodes 133 amino acids including N-terminal strongly hydrophobic regions. Secreted recombinant TRF/IL-5 (apparent m.w. of 46 Kd) has 113 amino acid residues and also comprises homodimers of a molecule with an apparent m.w. of 25 to 30 Kd. TRF/IL-5 mRNA is constitutively expressed in constitutively TRF-producing B151 and is inducible in some T cell lines upon stimulation with PMA or Con A. TRF/IL-5 mRNA is also expressed in Tbc-primed T cells upon the stimulation with PPD, whereas its expression is not effectively induced in non-primed spleen cells by stimulation with Con A or PMA plus calcium ionophore. The translation product of murine TRF/IL-5 cDNA triggers resting as well as activated (DNP-primed or LPS-stimulated) murine B cells for terminal differentiation into Ig-secreting cells (IgM, IgG1, or IgA) accompanied by increased mRNA expression for secreted forms of relevant Ig heavy chain (mu, gamma, or alpha). Among these, increases in the level of mu, and alpha-specific mRNA for the secreted form of IgM and IgA, respectively, are prominent. Moreover, TRF/IL-5 induces maturation of resting B cells into IgM-secreting cells. TRF/IL-5 promotes growth of activated B cells as well as BCL1 cells. TRF/IL-5 is, therefore, a growth as well as a differentiation inducing factor for B cells. Moreover, it induces functional IL-2 receptors on resting as well as activated B cells, besides TRF and BCGF II activities.(ABSTRACT TRUNCATED AT 400 WORDS)

Establishment of mouse cell lines which constitutively secrete large quantities of interleukin 2, 3, 4 or 5, using modified cDNA expression vectors

Mouse cell lines of different lineages have been established which constitutively secrete large quantities of recombinant mouse interleukins (mIL2, mIL3, mIL4 or mIL5). An existing bovine papilloma virus-based expression vector, pBV-1MTHA, was modified to allow transformed X63Ag8-653 myeloma cells, NIH 3T3 fibroblasts and C127 mammary tumor cells to stably carry multiple copies of the vector, to express the inserted cDNA encoding a single interleukin constitutively, and to secrete the interleukin in high quantities. Cell lines transformed with mIL2 cDNA stably carried 30-100 copies of the plasmid per cell and constitutively secreted biologically active mIL2 in quantities similar to those produced by murine EL4 thymoma cells or rat spleen cells stimulated with mitogens. Deletion of the 3' untranslated region containing AT-rich sequences from the mIL2 cDNA resulted in a 100-fold increase in the constitutive production and secretion of mIL2 by the transformants. Addition of a heavy metal further increased the production 2 to 6-fold. Cells transformed with 3'-deleted mIL3 cDNA constitutively secreted 300-1000 times higher activities of mIL3 than the myelomonocytic leukemia line WEHI3. mIL4 produced by the similar transformants induced [3H]thymidine uptake of a T cell line, a mast cell line and B leukemia cells, and enhanced the production of IgG1 by B cells. IL4 titers were 150 times higher than those produced by the concanavalin A-stimulated T cell line 2.19. mIL5 was secreted by similar transformants at 10-fold higher titers than those produced by concanavalin A-stimulated 2.19 T cells, as judged by the proliferation and maturation of B cell leukemia BCL1. The expression vectors should be useful in establishing eukaryotic cell lines producing proteins from full length cDNA clones at higher rates. The established cell lines secreting IL2, 3, 4 or 5 at high rate should be useful sources for these interleukins in the investigation of their function in the immune system.