T-independent activation of single B cells: an orderly analysis of overlapping stages in the activation pathway

Bronchus-Associated Lymphoid Tissue (BALT) Histology and Its Role in Various Pathologies

The lower respiratory tract is in direct communication with the external environment for gas exchange to occur. Therefore, it is constantly exposed to allergens, antigens, bacteria, viruses, and a wide variety of airborne foreign bodies. Bronchus-associated lymphoid tissue (BALT), which develops in response to these exposures and is one of the most prominent representatives of mucosa-associated lymphoid tissue (MALT), is important for generating rapid and specific bronchopulmonary adaptive immune responses. Therefore, this chapter focuses on the lymphoid architecture of BALT, which was first discovered in the bronchial wall of rabbits, its inducible form called inducible BALT (iBALT), its immunological response mechanisms, and its roles in certain pathologies including infectious and autoimmune diseases as well as in allergic and malignant conditions. In conclusion, it is hypothesized that BALT plays an important role in maintaining health and in the development of lower respiratory tract diseases; thanks to the pulmonary immune system in which it functions as a functional lymphoid tissue.

Analysis by Flow Cytometry of B-Cell Activation and Antibody Responses Induced by Toll-Like Receptors

Toll-like receptors (TLRs) are expressed in B lymphocytes and contribute to B-cell activation, antibody responses, and their maturation. TLR stimulation of mouse B cells induces class switch DNA recombination (CSR) to isotypes specified by cytokines, and also induces formation of IgM(+) as well as class-switched plasma cells. B-cell receptor (BCR) signaling, while on its own inducing limited B-cell proliferation and no CSR, can enhance CSR driven by TLRs. Particular synergistic or antagonistic interactions among TLR pathways, BCR, and cytokine signaling can have important consequences for B-cell activation, CSR, and plasma cell formation. This chapter outlines protocols for the induction and analysis of B-cell activation and antibody production by TLRs with or without other stimuli.

Identification of epitopes recognised by mucosal CD4(+) T-cell populations from cattle experimentally colonised with Escherichia coli O157:H7

Vaccines targeting enterohaemorrhagic Escherichia coli (EHEC) O157:H7 shedding in cattle are only partially protective. The correlates of protection of these vaccines are unknown, but it is probable that they reduce bacterial adherence at the mucosal surface via the induction of blocking antibodies. Recent studies have indicated a role for cellular immunity in cattle during colonisation, providing an impetus to understand the bacterial epitopes recognised during this response. This study mapped the epitopes of 16 EHEC O157:H7 proteins recognised by rectal lymph node CD4(+) T-cells from calves colonised with Shiga toxin producing EHEC O157:H7 strains. 20 CD4(+) T-cell epitopes specific to E. coli from 7 of the proteins were identified. The highly conserved N-terminal region of Intimin, including the signal peptide, was consistently recognised by mucosal CD4(+) T-cell populations from multiple animals of different major histocompatibility complex class II haplotypes. These T-cell epitopes are missing from many Intimin constructs used in published vaccine trials, but are relatively conserved across a range of EHEC serotypes, offering the potential to develop cross protective vaccines. Antibodies recognising H7 flagellin have been consistently identified in colonised calves; however CD4(+) T-cell epitopes from H7 flagellin were not identified in this study, suggesting that H7 flagellin may act as a T-cell independent antigen. This is the first time that the epitopes recognised by CD4(+) T-cells following colonisation with an attaching and effacing pathogen have been characterised in any species. The findings have implications for the design of antigens used in the next generation of EHEC O157:H7 vaccines.

Regulation of Aicda expression and AID activity

Activation-induced cytidine deaminase (AID) is expressed in a B cell differentiation stage-specific fashion and is essential for immunoglobulin (Ig) gene class switch DNA recombination (CSR) and somatic hypermutation (SHM). CSR and SHM play a central role in the maturation of antibody and autoantibody responses. AID displays a mutagenic activity by catalyzing targeted deamination of deoxycytidine (dC) residues in DNA resulting in dU:dG mismatches, which are processed into point-mutations in SHM or double-strand breaks (DSBs) in CSR. Although AID specifically targets the Ig gene loci (IgH, Igκ and Igλ), it can also home into a wide array of non-Ig genes in B-and non-B-cell backgrounds. Aberrant expression of AID is associated with multiple diseases such as allergy, inflammation, autoimmunity and cancer. In autoimmune systemic lupus erythematosus, dysregulated AID expression underpins increased CSR, SHM and autoantibody production. As a potent mutator, AID is under stringent transcriptional, post-transcriptional and post-translational regulation. AID is also regulated in its targeting and enzymatic function. In resting naïve or memory B cells, AID transcripts and protein are undetectable. These, however, are readily and significantly up-regulated in B cells induced to undergo CSR and/or SHM. Transcription factors, such as HoxC4 and NF-κB, which are up-regulated in a B cell lineage-and/or differentiation stage-specific manner, regulate the induction of AID. HoxC4 induces AID expression by directly binding to the AID gene promoter through an evolutionarily conserved 5'-ATTT-3' motif. HoxC4 is induced by the same stimuli that induce AID and CSR. It is further up-regulated by estrogen through three estrogen responsive elements in its promoter region. The targeting of AID to switch (S) regions is mediated by 14-3-3 adaptor proteins, which specifically bind to 5'-AGCT-3' repeats that are exist at high frequency in S region cores. Like HoxC4, 14-3-3 adaptors are induced by the same stimuli that induce AID. These include "primary" inducing stimuli, that is, those that play a major role in inducing AID, i.e., engagement of CD40 by CD154, engagement of Toll-like receptors (TLRs) by microbial-associated molecular patterns (MAMPs) and cross-linking of the BCR, as synergized by "secondary" inducing stimuli, that is, those that synergize for AID induction and specify CSR to different isotypes, i.e., switch-directing cytokines IL-4, TGF-β or IFN-γ. In this review, we focus on the multi-levels regulation of AID expression and activity. We also discuss the dysregulation or misexpression of AID in autoimmunity and tumorigenesis.

B cell TLRs and induction of immunoglobulin class-switch DNA recombination

Toll-like receptors (TLRs) are a family of conserved pattern recognition receptors (PRRs). Engagement of B cell TLRs by microbe-associated molecular patterns (MAMPs) induces T-independent (TI) antibody responses and plays an important role in the early stages of T-dependent (TD) antibody responses before specific T cell help becomes available. The role of B cell TLRs in the antibody response is magnified by the synergy of B cell receptor (BCR) crosslinking and TLR engagement in inducing immunoglobulin (Ig) class switch DNA recombination (CSR), which crucially diversifies the antibody biological effector functions. Dual BCR/TLR engagement induces CSR to all Ig isotypes, as directed by cytokines, while TLR engagement alone induces marginal CSR. Integration of BCR and TLR signaling results in activation of the canonical and non-canonical NF-κB pathways, induction of activation-induced cytidine deaminase (AID) and germline transcription of IgH switch (S) regions. A critical role of B cell TLRs in CSR and the antibody response is emphasized by the emergence of several TLR ligands as integral components of vaccines that greatly boost humoral immunity in a B cell-intrinsic fashion.

Bronchus-associated lymphoid tissue (BALT) structure and function

Bronchus-associated lymphoid tissue (BALT) is a constitutive mucosal lymphoid tissue adjacent to major airways in some mammalian species, including rats and rabbits, but not humans or mice. A related tissue, inducible BALT (iBALT), is an ectopic lymphoid tissue that is formed upon inflammation or infection in both mice and humans and can be found throughout the lung. Both BALT and iBALT acquire antigens from the airways and initiate local immune responses and maintain memory cells in the lungs. Here, we discuss the development and function of BALT and iBALT in the context of pulmonary immunity to infectious agents, tumors, and allergens as well as autoimmunity and inflammatory diseases of the lung.

Toll-like receptors and B-cell receptors synergize to induce immunoglobulin class-switch DNA recombination: relevance to microbial antibody responses

Differentiation of naïve B cells, including immunoglobulin class-switch DNA recombination, is critical for the immune response and depends on the extensive integration of signals from the B-cell receptor (BCR), tumor necrosis factor (TNF) family members, Toll-like receptors (TLRs), and cytokine receptors. TLRs and BCR synergize to induce class-switch DNA recombination in T cell-dependent and T cell-independent antibody responses to microbial pathogens. BCR triggering together with simultaneous endosomal TLR engagement leads to enhanced B-cell differentiation and antibody responses. Te requirement of both BCR and TLR engagement would ensure appropriate antigen-specific activation in an infection. Co-stimulation of TLRs and BCR likely plays a significant role in anti-microbial antibody responses to contain pathogen loads until the T cell-dependent antibody responses peak. Furthermore, the temporal sequence of different signals is also critical for optimal B cell responses, as exemplified by the activation of B cells by initial TLR engagement, leading to the up-regulation of co-stimulatory CD80 and MCH-II receptors, which result in more efficient interactions with T cells, thereby enhancing the germinal center reaction and antibody affinity maturation. Overall, BCR and TLR stimulation and the integration with signals from the pathogen or immune cells and their products determine the ensuing B-cell antibody response.

Responses to the soluble flagellar protein FliC are Th2, while those to FliC on Salmonella are Th1

Features of the Th1 or Th2 phenotype start to develop during CD4 T cell priming. This study of the response to the bacterial flagellar protein FliC shows that either Th1 or Th2 responses can be induced in mice depending upon how FliC is presented. This is shown by assessing the cytokine mRNA and class of FliC-specific plasma cells induced in situ. Soluble recombinant (r)FliC and polymerized FliC are strongly Th2 polarizing, inducing IL-4, NIP45 and c-Maf mRNA as well as epsilon and gamma1 switch transcripts and switching to IgG1. CD28-requirement for this switching shows its T cell dependence. rFliC was unable to induce markers of Th1 activity including IL-12, T-bet and IFN-gamma. Conversely, when FliC is presented in its native context surface-bound on live, flagellated Salmonella, switching is predominantly to IgG2a (IgG2c in C57BL/6 mice), reflecting Th1 activity. The development of divergent FliC-specific polarization to either Th1 or Th2 indicates that the context in which this antigen is encountered rather than its intrinsic immunostimulatory properties determines the direction of Th polarization.

Rel-dependent induction of A1 transcription is required to protect B cells from antigen receptor ligation-induced apoptosis

In response to different extracellular signals, Rel/NF-kappaB transcription factors are critical regulators of apoptosis in a variety of cell types. Here we show that in normal B and T cells, expression of the Bcl-2 prosurvival homolog, A1, is rapidly induced in a Rel-dependent manner by mitogens. In B-cell lines derived from c-rel-/- mice, which like primary cells lacking Rel undergo apoptosis in response to antigen receptor ligation, constitutive expression of an A1 transgene inhibits this pathway to cell death. These findings are the first to show that Rel/NF-kappaB regulates physiologically the expression of a Bcl-2-like protein that is critical for the control of cell survival during lymphocyte activation.

The use of reporter antigens in the popliteal lymph node assay to assess immunomodulation by chemicals

Various drugs and other chemicals can induce T-cell-dependent B-cell activation which may lead to allergic or autoimmune-like diseases. Because the nature of the relevant (neo-) antigens is generally not known and probably depends on the chemical, we have explored the potential use of reporter antigens to determine T-cell-dependent B-cell activation by chemicals. TNP-Ficoll and TNP-OVA were used for this purpose because they are recognized by the same TNP-specific B cells, but these cells require distinct costimulation for specific antibody production. It was found that HgCl2, phenytoin, nitrofurantoin, and D-penicillamine stimulated IgG1 production to both antigens, incomplete Freund's adjuvant, silica, and dimethylsulfoxide to TNP-OVA only, and LPS and hydroxyl-amino procainamide to TNP-Ficoll alone. The diabetogene streptozotocin did not enhance IgG1 production, but may enhance a cellular response instead. Tolerogens and a T-cell antigen without intrinsic adjuvant activity did not influence the responses. The IgG1 production to TNP-Ficoll was local and transient, and did not always require T cells. In contrast, responses to TNP-OVA could be measured in serum, led to specific memory, and were strictly T-cell dependent. These results demonstrate that specific antibody production to reporter antigens indicates immunostimulatory effects of chemicals more sensitive than PLN cell count and provides important mechanistic information. Moreover, with TNP-OVA as reporter antigen the kinetics and regulation of chemically enhanced immune responses can be studied without the need to know the relevant neo-antigens for each individual compound.

T cell-independent antibody-mediated clearance of polyoma virus in T cell-deficient mice

Polyomavirus (PyV) infection of SCID mice, which lack functional T and B cells, leads to a lethal acute myeloproliferative disease (AMD) and to high levels of virus replication in several organs by two wk after infection. This is in contrast to infection of T cell-deficient athymic nude mice, which are resistant to acute PyV-induced disease and poorly replicate the virus in their organs. This major difference in the virus load and in the outcome of PyV infection between SCID and nude mice suggested that an efficient, T cell-independent antiviral mechanism operates in T cell-deficient, PyV infected mice. To investigate this possibility, mice with different genetically engineered T and/or B cell deficiencies and SCID mice adoptively reconstituted with B and/or T cells were infected with PyV. The results indicated that the presence of B cells in the absence of T cells protected mice from the AMD, and this was accompanied by a major reduction of PyV in all organs tested. Sera from PyV-infected T cell receptor (TCR) alpha beta knockout or TCR alpha beta gamma delta knockout mice contained IgG2a antibodies to PyV. Sera or purified immunoglobulin fractions from PyV-infected TCR alpha beta knockout mice protected SCID mice from the PyV-induced AMD. To our knowledge, this is the first report of an effective T cell-independent antibody response clearing a virus and changing the outcome of infection from 100% mortality to 100% survival.

Germinal-center cell proliferation in response to T-independent antigens: a stathmokinetic, morphometric and immunohistochemical study in vivo

Although the nature of the germinal center reaction during responses to T-dependent antigens has been well documented, much less is known regarding the relationship between germinal centers and T-independent antigens. In this study, germinal-center cell proliferation was determined at specific time points in spleens of C3H/HeN mice following immunization with either the type-1, T-independent antigen dinitrophenol-lipopolysaccharide (DNP-LPS), or the type-2, T-independent antigen DNP-Ficoll. A stathmokinetic technique was employed to assess proliferation in terms of germinal center cell birth rate and morphometry was used to measure actual growth and regression of the germinal center cell population. An estimate of the absolute rate of germinal-center (GC) cell proliferation was derived from these two values. In addition, immunohistochemistry was performed to correlate changes in GC cell proliferation with the presence or absence of antigen within GC. Following immunization with both antigens, there was an initial reduction of proliferation within pre-existing germinal centers which manifested as either GC dissociation (DNP-LPS) or a suppression of birth rates (DNP-Ficoll). This was followed by a period of increased GC cell proliferation in animals immunized with DNP-LPS, but not in those exposed to DNP-Ficoll. GC cell proliferation was then measured in mice treated with cyclosporin A from 1 day before to 2 days after immunization with DNP-LPS. In these animals, the expected increase in GC cell birth rates did not take place. Immunohistochemistry showed that DNP-Ficoll and DNP-LPS were present in GC from 1 day after immunization until the end of the experiment on day 7. Treatment with cyclosporin A did not affect the deposition of DNP-LPS in GC. These results show that only some T-independent antigens are able to stimulate GC cell proliferation, and we propose that this is related to their ability to recruit precursors of GC B cells into the GC reaction. In addition, the results indicate that GC proliferation seen in response to a so-called T-independent antigen is at least partly driven by T cell-derived cytokines.

B-cell stimulation

Recent studies have identified CD40 ligand (CD40L) as the critical membrane-expressed molecule responsible for T cell dependent B-cell activation. CD40L co-operates with various cytokines to induce B-cell activation, proliferation, and immunoglobulin isotype switching. Some antigens, however, can also stimulate B-cell activation and isotype switching in the absence of CD40L or T cells. Recent studies have suggested that cytokines derived from non-T cells, such as natural killer cells, macrophages and mast cells, are responsible for isotype switching in T cell independent responses.

Interferon-gamma downregulates the proliferative response of hapten-specific B cells stimulated by antigen and cytokines

IFN-gamma plays a role in many aspects of cellular interactions, both positive and negative. Among its functions during the immune response, the antagonistic effects of IFN-gamma and IL-4 are well documented. Observations in our laboratory suggested that IFN-gamma could also interfere with the activation of single, antigen-specific B cells by antigen and other cytokines. Closer examination revealed that IFN-gamma reduced the number of proliferating cell clones in response to antigen and a variety of cytokines, alone or in combination. Cell viability remained at the initial level and the cells were still able to produce Ig, albeit to a lesser extent than in the absence of IFN-gamma. On the other hand, the frequency of IgM secreting clones was not affected, whereas the total amount of secreted IgM was lower in the presence of IFN-gamma, probably due to the reduced cell number and a decrease in Ig production. In addition, proliferation was prevented when B cells were pre-incubated with IFN-gamma and then stimulated by other cytokines. Kinetic studies revealed that INF-gamma had to be present from the onset of culture because delayed addition did not inhibit the proliferation of the B cells. After its initial action, IFN-gamma could be removed without abolishing the negative signal for proliferation. From these results it can be concluded that IFN-gamma transmits a signal that causes B cells to stop proliferating and prevents them from forming large clones.

T cell-dependent activity of ganglioside GM1-specific B cells in Guillain-Barré syndrome and multifocal motor neuropathy in vitro

Synthesis of anti-ganglioside GM1 antibodies of the IgM class by peripheral blood mononuclear cells (PBMNC) from patients with immune-mediated neuropathies and motor neuron diseases and from normal controls was stimulated by Pokeweed mitogen (PWM) in vitro. In patients with acute Guillain-Barré syndrome or multifocal motor neuropathy and high serum titers of IgM anti-ganglioside GM1 antibodies this culture response was greatly enhanced as compared to controls and already detectable in unstimulated cultures. Limiting dilution analysis demonstrated high frequencies of GM1-specific B cells in these patients. Anti-ganglioside GM1 antibodies of the IgG and IgA class were only produced by PBMNC from patients with Guillain-Barré syndrome corresponding to serum titers. In cultures taken at intervals over 6 months in vitro B cell activity for IgM, IgG and IgA anti-ganglioside GM1 antibodies in two Guillain-Barré syndrome patients declined accompanied by clinical improvement and falling serum titers. We conclude that GM1-specific PWM-responsive B cells pre-exist in peripheral blood and respond to T cell-dependent stimulation in Guillain-Barré syndrome and multifocal motor neuropathy.

Association of IgG anti-GD1a antibody with severe Guillain-Barré syndrome

We earlier reported cases of 2 patients with severe acute Guillain-Barré syndrome (GBS) associated with high-IgG anti-GD1a antibody titer. We now have investigated the autoantibody against GD1a or GM1 in 37 GBS patients using the enzyme-linked immunosorbent assay and have found a statistically significant association between IgG anti-GD1a antibody and the severity of the disease (need of a respirator for more than 1 month and a poor functional prognosis 3 months after neurologic onset). An autopsy which showed severe GBS associated with IgG anti-GD1a antibody produced the following findings: (1) severe axonal degeneration and segmental demyelination of peripheral nerves; (2) lymphocytic infiltration; and (3) marked central chromatolysis of the lower motoneurons.

Severe acute axonal form of Guillain-Barré syndrome associated with IgG anti-GD1a antibodies

We report cases of 2 patients with pure motor Guillain-Barré syndrome of explosive onset who required mechanical ventilation for more than 2 months. Their electrophysiologic findings and poor clinical recoveries suggested severe axonal degeneration involving the motor nerves. Enzyme-linked immunosorbent assay and thin-layer chromatogram-immunostaining showed the sera of both patients had high IgG antibody titer against GD1a ganglioside. Their titers decreased with the clinical course of the illness. GD1a as well as GM1, appears to be the target pathogenic antigen in motor axon disorders. Elevated IgG anti-GD1a antibody titer may prove useful for predicting severe GBS.

Cellular and molecular mechanisms of B lymphocyte tolerance

A paradox of immunology is that the immune system is distributed so widely in the body, as a large number of cells that discharge most of their effector functions as single cells; but, at the same time, the elements of the system are so very interdependent, not only via specialized cell clusters and microenvironments, but also by mobile feedback loops, cellular and molecular. The end result is that one cannot really understand one element of the system without understanding every other, at least to a degree. Certainly, tolerance cannot be isolated from immune activation, nor B cell from T cell tolerance, rendering the task of the reviewer somewhat thankless. This being said, the last few years have seen wonderful progress in our grasp of B cell tolerance, to which the transgenic revolution has contributed a great deal. The fact that B cell tolerance exists as an important component of self-tolerance has been firmly established, as have the limits of the process in terms of both the survival of low-affinity antiself clonotypes and the question of location and concentration of antigen required for tolerance induction. Two processes have been identified as key alternatives: clonal abortion/maturation arrest/deletion and induction of clonal anergy. The latter requires a less strong Ig receptor crosslinking signal, may be partial, and is reversible. Recognition of these facts has prompted both experimentation and speculation on possible functions of the anergic cell. One unsatisfactory area, which we have not addressed because nothing like a consensus has been reached, is T cell-mediated suppression and its possible effects on tolerant states, including anergy induction in B cells. The phenomenology of suppression is too striking to sweep under the carpet, and suppressor T cell memory in particular (Adelstein et al., 1990) requires much more investigation; however, suppression has not been shown to play a major role in any of the best-studied transgenic models. These can readily be explained on the basis of direct interactions between the B cell target for abortion or anergy and the self antigen in question. The biochemical basis of discrimination between immunity and tolerance has also progressed, but not as fast. This is understandable, as so many signaling pathways have to come together for full immune induction, and as immaturity of the signal transduction pathway plays a profound role that must be studied in normal cells, with all the attendant difficulties of cell separation.(ABSTRACT TRUNCATED AT 400 WORDS)

The interleukin network and lymphoid development

Lymphoid development differs sharply between the primary and secondary lymphoid organs. In the former, lymphocytes arise from precursors by antigen-independent processes under thymic or bone marrow microenvironmental influences and undergo extensive selective processes before being allowed to leave. In the latter, lymphocytes with receptors relevant to particular antigens undergo a second wave of proliferation and differentiation leading to the emergence of immunocytes with effector functions. Each of the two sets of events are profoundly dependent on cellular interactions. In the primary lymphoid organs, the "action" centres on stromal cell-lymphoid precursor interactions, and artificial systems permitting B cell formation are much more advanced than those for T cell development. For B cells, IL-7 and c-kit ligand (KL) are clearly important but so are as yet undefined stromal cell-derived activities. For thymic development, only fragments of the complex 3-week process of T cell formation can be mimicked in vitro and no IL has unequivocally been shown to be critical. Within the secondary lymphoid organs, where lymphocytes react to the antigenic universe, the key to regulation lies in interactions between accessory cells (dendritic cells, macrophages and their various relatives) T cells and B cells. Efforts to squeeze the relevant cytokines into sharp compartments such as activation factors, growth factors and differentiation factors have been largely unsuccessful.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Spontaneous and cytokine-inducible 'natural' immunoglobulin secreting cells in organized lymphoid tissues of mice

The number and frequency of spontaneous and cytokine-inducible 'natural' immunoglobulin-secreting cells (ISC) were determined in bone marrow (BM), spleen and Peyer's patch (PP), in vitro. Cells were cultured at limiting dilution in the presence or absence of exogenous recombinant cytokines and supernatants then assayed for total immunoglobulin (Ig) and Ig isotype using an ELISA. Most spontaneous ISC were found in the spleen and BM, with fewer in PP. The addition of recombinant interleukin 5 (rIL-5) promoted a marked increase in both the ISC frequency and the amount of Ig secreted/ISC whereas recombinant IL-6 (rIL-6) promoted only a marginal increase. Recombinant IL-4 (rIL-4) promoted a marginal increase in ISC frequency only. The isotype profile of ISC was in the order IgM greater than IgG2 greater than IgA greater than IgG3 greater than IgG1. The exposure of cells to 1200 rad of gamma-radiation resulted in decreased numbers of spontaneous ISC in all tissues, but the addition of rIL-5 or rIL-6 to the irradiated cells increased both the ISC frequency and Ig secreted. The Ig isotype profile was similar to that of non-irradiated ISC with a few minor exceptions. This large population of potential cytokine-inducible ISC could contribute to 'natural' Ig secretion in vivo.

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.

Clonal repertoire analysis of murine B cells specific for repeat sequence antigens of Plasmodium falciparum

Clonal analysis of the murine B-cell repertoire has been used to investigate the possible role of tandem repeat sequence epitopes of Plasmodium falciparum in immune evasion. A limiting dilution culture system was used whereby murine spleen cells were stimulated with the B-cell mitogen lipopolysaccharide (LPS) in the presence of 3T3 fibroblast filler cells. One in three B cells were shown to produce clones secreting immunoglobulin measurable by an ELISA. The frequency of antibody forming cell precursors (AFCp) specific for the 3' repeat epitopes of the ring injected erythrocyte surface antigen (RESA) was estimated in non-primed mice and found to be low. However, an accurate frequency determination was not possible using this method since the detection of the few positive cultures was found to depend on the presence of more than one AFCp or its products. Limiting dilution analysis was used to assess the frequency and repertoire of splenic AFCp at various times after immunization with a synthetic peptide of the RESA 3' repeat epitope (8 x 4-mer), presented in various ways. There was no marked increase in LPS-responsive AFCp specific for this antigen at the level of either IgM or IgG secretion. This was in marked contrast to the antibody response in vivo, where moderate IgG antibody titres, normally indicative of a secondary response, were seen in the serum of the same mice used for AFCp assay. This discrepancy between serum titre and AFCp frequency following immunization was not apparent with a non-malarial antigen, keyhole limpet haemocyanin (KLH). It was concluded that the LPS-stimulated limiting dilution culture system was not registering RESA-specific memory AFCp. These results raise the possibility that the malarial antigens are deficient in memory B-cell generation, or that secondary responses to these determinants may arise from a distinct B-cell progenitor which is non-responsive to LPS in vitro.

The peritoneal Ly-1 (CD5) B cell repertoire is unique among murine B cell repertoires

Ly-1 (CD5) B cells and conventional B cells represent two distinct lineages of murine B cells which are distinguishable by expression of surface molecules, organ location, ontogeny and development and antibody production in vivo. In order to assess whether the different developmental pathways of Ly-1 B cells and conventional B cells result in different antibody repertoires, we have used limiting dilution analyses to determine frequencies of B cells making antibodies capable of binding to a range of antigens including haptens, proteins, bacterial polysaccharides and bromelain-treated mouse red blood cells. Starting populations of B cells were purified from spleen, peritoneum and bone marrow of adult BALB/c mice or from spleens of newborn mice by use of the fluorescence-activated cell sorter. The peritoneal Ly-1 B cell repertoire was found to be different from that of conventional B cells, with between 5- and 100-fold higher frequencies of clones producing IgM antibodies capable of binding to the antigens tested. However, when tested, the majority of Ly-1 B cell anti-haptenic antibodies did not show the high affinity binding or fine specificity characteristics of specific antibodies elicited in immune responses in vivo. The high frequencies of antigen-reactive antibodies within the Ly-1 B repertoire are most likely explained by the presence of clones secreting low-affinity or multireactive antibodies. The Ly-1 B cell repertoire is not mirrored in repertoires from either newborn B cells or virgin B cells in adult bone marrow. Therefore, either Ly-1 B cells develop from distinct precursors with intrinsically different mechanisms of V gene usage and recombination, or newly formed Ly-1 B are heavily selected on specificity for entry into this peritoneal lineage. If the second alternative is true, bacterial antigens in the gut are not required for selection of this unique repertoire, as Ly-1 B cells in germ-free mice also show the multireactive repertoire characteristic of this B cell lineage in normal mice.

Combinations of interleukins 2, 4 and 5 regulate the secretion of murine immunoglobulin isotypes

Immunoglobulin (Ig) production by lipopolysaccharide (LPS)-stimulated cells in the presence of various combinations of interleukin (IL)2, IL4 and IL5 was examined. IgG1, IgM and IgE secretion was studied using a 3T3-fibroblast filler cell-supported B cell culture system, either at low cell density to support maximal Ig secretion, or at limiting dilution to determine isotype-specific precursor frequencies. In the presence of optimal concentrations of IL5 (2%) and IL2 (3 U/ml), the addition of 1 U/ml of IL4 resulted in the production of 4 ng of IgG1 per input B cell. In contrast, 1000 U/ml of IL4 alone was required to produce equivalent levels of IgG1. IL5 and IL2 increased both the precursor frequency and the amount of IgG1 secreted per clone in the presence of low levels of IL4. On the other hand, IgM secretion was decreased 10-fold by the addition of 10 U/ml IL4 or greater. This was not seen when IL5 was present. The IgM-secreting precursor frequency was unaffected by any of the lymphokines, either singly or in combination. The inhibition of IgM production and subsequent relief of this with IL5 was shown to affect the amount of IgM secreted per clone. IgE secretion was shown to be highly IL4 dependent with only minor reduction in the required concentration following addition of IL5 and IL2. At the clonal level, the majority of IgE-secreting clones (93%) at high IL4 concentrations (200 U/ml) arose from precursors which were able to produce IgM and IgG1. Furthermore, only 3% of the clones secreted IgG1 alone, with a further 3% secreting IgE alone. These results suggest that B cells in vivo are predominantly uncommitted in terms of isotype to be produced, the choice of isotype secreted being dependent on the nature of the stimulus. Overall, this work shows that the isotype secreted by B cells can be regulated using combinations of IL2, IL4 and IL5, and that major effects can be achieved by very small quantities of lymphokines acting in synergy.

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.

Immunologic tolerance: collaboration between antigen and lymphokines

Immunologic tolerance is the process whereby limits are placed on the degree to which lymphocytes respond to an animal's inherent antigens. It is a quantitative rather than an absolute term, as some autoantibody formation is common. Contrary to early hopes, it is not due to some single, simple causative mechanism confined to early developmental stages of the fetal immune system. Rather, self-tolerance results from a variety of complementary mechanisms and feedback loops in the immune system and is thus best seen as part of the general process of immunoregulation.

Interleukins 4 and 5 control expression of IL-2 receptor on murine B cells through independent induction of its two chains

Resting B cells express few, if any, receptors for interleukin-2 (IL-2), whereas activated B cells can express receptors for and respond to IL-2. IL-2 receptors can exist on the cell surface in three different forms; the complete high-affinity receptor, a heterodimer consisting of a chain of relative molecular mass (Mr) 70-75,000 (70-75K) and a chain of Mr 55K; the 70-75K chain alone, with intermediate affinity for IL-2; or the 55K chain alone, with low affinity for IL-2. We have previously reported that IL-5-stimulated B cells are induced to express the 55K chain. We report here evidence for the differential regulation of the expression of the two chains, namely that IL-4 and IL-5 can independently induce expression of the 70-75K and 55K chains respectively on murine B cells. As expected, cells stimulated to express the 55K chain alone are unresponsive to IL-2, whereas cells stimulated to express either the 70-75K chain or the 70-75/55K heterodimer respond to IL-2, at a high and low ligand concentration respectively, with a marked increase in proliferation. This orchestration of receptor expression and factor responsiveness may represent a novel activation pathway for B cells, where the two chains of a compound receptor are shown to be independently regulated.

Sudden appearance of anti-protein IgG1-forming cell precursors early during primary immunization

The anti-keyhole limpet hemocyanin (KLH) B-cell repertoire of unimmunized adult mice was examined by culture of splenocytes (generally 100-3000) at limiting dilution. Cells were polyclonally stimulated with Escherichia coli lipopolysaccharide (LPS) and an interleukin-4-containing lymphokine mixture in the presence of 3T3 fibroblast filler cells. After 7 days of culture, supernatants were examined for their content of anti-KLH IgM and IgG1 antibody by an enzyme-linked immunosorbent assay (ELISA). Parallel cultures of smaller numbers (generally 1-15) of splenocytes were examined to determine the cloning efficiency of B cells in terms of total IgM and IgG1 production. Whereas one spleen cell in 370 produced clones secreting anti-KLH IgM, only 1% of these produced IgG1 that could bind to KLH, despite the fact that about half of the clones switched to IgG1 production with these stimuli. In mice immunized with KLH, this situation did not change until day 5, when there was a sudden, explosive emergence of B cells that could form clones secreting anti-KLH IgG1. The absolute number of such cells in the spleen was found to rise by a factor of 350 between days 3 and 7 of immunization. Moreover, the median amount of IgG1 antibody formed per clone and binding to KLH also rose markedly. In contrast, neither the numbers nor the median KLH-binding antibody content of anti-KLH IgM clones changed significantly after immunization. The results show that the repertoire of anti-protein B cells detected through IgM formation in ELISA consists chiefly of cells producing antibody of low avidity and of doubtful in vivo significance. Assuming that the small proportion of these cells making antibody that is of sufficient avidity to bind as the IgG1 isotype are the ancestors of the many such cells found on day 7 of the primary immune response, one would have to postulate a very high recruitment and/or division rate to account for the increase in numbers and avidity that occurs. It is possible that the anti-KLH IgG1 precursors that suddenly emerge are the results of early variable region gene (V) mutations in B cells. Moreover, it is not excluded that they represent products of a subset of B cells different from those that give rise to the primary in vitro anti-KLH IgM response. The findings have implications for theories of B-cell tolerance.

Inhibition of antibody production at high cell density following mitogen stimulation and isotype switching in vitro

We have investigated antibody production in 3T3 filler cell-containing murine B lymphocyte cultures, stimulated with LPS and an IL-4-containing lymphokine mixture. At low cell density cultures produced 1.8 +/- 0.6 ng of IgM and 4.2 +/- 1.7 ng of IgG1 per input B cell. It was found that 21.7 +/- 3.5% of spleen cells, or approximately 43% of B cells, produce IgM under these conditions, and 11.9 +/- 5.5% spleen cells, approximately 24% of B cells, produced IgG1. Therefore, the mean IgM production per IgM-positive clone was 4.2 ng, and the mean IgG1 production per IgG1-positive clone was 17.6 ng. A cell density of about 10,000 B cells/ml was found to produce maximal antibody per input cell. A 32-fold increase above the maximum cell density resulted in a 600-fold decrease in IgG1 production per B cell. IgM production was also found to be inhibited above this concentration of cells, but to a six-fold lesser extent. Cell proliferation in dense cultures was also found to be diminished in a cell concentration-dependent manner, partially accounting for the observed inhibition phenomenon. The replenishment of media, LPS and growth factors was able to lessen the inhibition of dense cultures, but not to maximal levels. Overall, this work identified the upper limit of cell density for in vitro cloning of B lymphocytes for isotype switch and repertoire analysis. The most important conclusion is that antibody production is grossly suboptimal at the cell densities frequently used in the literature.

Interleukin 4 causes isotype switching to IgE in T cell-stimulated clonal B cell cultures

Although it has been established that IL-4 enhances both IgG1 and IgE secretion in LPS-stimulated B cell cultures, these studies failed to determine whether IL-4 preferentially induces isotype switching or preferentially allows for the maturation of precommitted precursor cells. To distinguish between these possibilities, it is necessary to ascertain the effect of IL-4 on the isotypes secreted by individual precursor cells during clonal expansion. Therefore, clonal cultures of B cells stimulated with a Th2 helper cell line specific for rabbit Ig and rabbit anti-mouse IgM were established. The majority of B cells are capable of undergoing clonal expansion under these conditions. To vary the level of IL-4 present, either IL-4 or anti-IL-4 was added to cultures. In the presence of IL-4 there was an increase in the proportion of clones that secreted IgE and a decrease in the proportion of clones that secreted IgM. The addition of IL-4 to cultures also increased the amount of IgE secreted by individual clones. Thus, these experiments definitively prove that IL-4 causes specific heavy chain class switching to IgE in Th2-stimulated B cell cultures. In contrast, IL-4 does not affect the proportion of clones secreting IgG1, suggesting that other consequences of Th cell-B cell interactions play a role in the generation of an IgG1 response.

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)