A model system for peptide hormone action in differentiation: interleukin 2 induces a B lymphoma to transcribe the J chain gene

A promoter element that exerts positive and negative control of the interleukin 2-responsive J-chain gene

In a primary immune response a signal from interleukin 2 (IL-2) induces B lymphocytes to express the gene for the IgM joining component, the J chain. The signaling mechanism was pursued in this study by examining the J-chain gene 5' flanking region for regulatory sequences and interacting nuclear factors. The analyses identified a major control region located between -75 and -45 that encodes two adjacent elements: a T-rich sequence (JA) containing a single positive regulatory motif and an A+G-rich sequence (JB) containing overlapping positive and negative regulatory motifs. Dissection of the two elements indicated that the bifunctional JB sequence is the likely target of the IL-2 signal. The evidence was based on findings that (i) JB activity correlated with J-chain gene transcription--i.e., JB acts as a repressor in J-chain-silent B cells and as an activator in J-chain-expressing cells, and (ii) JB activator function is mediated by a B-cell-specific nuclear protein, NF-JB, that exhibits an IL-2-responsive binding pattern.

Immunophysiological studies of interleukin-2 and canine lymphocytes

Interleukin-2-dependent pathways of lymphocyte activation were investigated in canine peripheral blood lymphocytes (PBL) following stimulation with T-cell mitogens including phytohemagglutinin, phorbol ester (TPA), calcium ionophore (ionomycin), and human recombinant interleukin-2 (hrIL-2). The ability of the stimulated cells to produce interleukin-2 (IL-2) was determined using murine indicator cell lines. IL-2 receptor expression by mitogen-stimulated canine PBL was confirmed by the binding of hrIL-2 with high affinity, and with characteristics comparable to those of the human and murine IL-2 receptor. Examination of serum and PBL from two dogs that were treated with hrIL-2 and human recombinant tumor necrosis factor for systemic mast cell tumors showed that in one dog, IL-2 could be measured in the serum. Concurrently, the in vitro mitogenic response of this dog's PBL to hrIL-2 occurred earlier, possibly reflecting an increase in the relative number of IL-2-responsive cells within the PBL population.

Role and regulation of interleukin (IL)-2 receptor alpha and beta chains in IL-2-driven B-cell growth

Substantial proportions of resting B cells constitutively express low levels of IL-2 receptor (IL-2R) alpha and/or beta chains. The expression of these chains is differentially regulated by anti-IgM and IL-2/IL-4. The anti-IgM induces IL-2R alpha chain expression, whereas each of the two cytokines induces IL-2R beta chain expression in a dose-dependent manner. Moreover, IL-2 induces the growth of B cells, when the cells were pretreated with IL-2 or IL-4 for 24 h. The magnitude of this IL-2-driven B-cell growth depends upon the level of IL-2R beta chain expression. Costimulation of the B cells with IL-2 and anti-IgM shifts the dose-response curve, and the cells proliferate at an IL-2 concentration as low as 40 pM. These results indicate that the levels of anti-IgM-induced IL-2R alpha chain and IL-2-induced IL-2R beta chain determine the sensitivity of the cells to IL-2.

Interleukin 2 regulates the activity of the lyn protein-tyrosine kinase in a B-cell line

Recently, interleukin 2 (IL-2) has been shown to induce increased activity of the p56lck protein-tyrosine kinase (PTK) in T-cell and natural killer cell lines, and evidence for a direct interaction between the p75 subunit of the IL-2 receptor (IL-2R) and this src-family kinase has been reported. Though these findings suggest a central role for lck in IL-2 signal transduction, one problem with this idea is that not all IL-2-responsive cells express the lck gene. For this reason, we examined the effects of IL-2 on the activity of src-like kinases in a pro-B cell line, F7, that lacks p56lck but that displays high-affinity IL-2Rs and vigorously proliferates in response to this lymphokine. Of the eight known src-family PTKs, F7 cells were shown to contain only p53/56lyn, p59fyn, and a small amount of p62yes. Stimulation of resting F7 cells with IL-2 induced a rapid (detectable within 1 min and maximal at 15 min) and concentration-dependent increase in the specific activity of p53/56lyn kinase, as assessed by in vitro kinase assays. This effect of IL-2 on p53/56lyn kinase was specific, since no IL-2-inducible changes were detected in the activities of the p59fyn and p62yes kinases. Furthermore, by using a monoclonal antibody specific for the approximately 75-kDa beta subunit of the IL-2R (referred to as p75/IL-2R beta), evidence for physical association between the lyn kinase and the IL-2R complex was obtained, in that a small proportion of the p53/56lyn kinase in F7 cells, but no detectable p59fyn kinase, was coimmunoprecipitated with p75/IL-2R beta. When combined with the recent evidence that IL-2 regulates p56lck in T cells, these results indicate that some flexibility exists in the ability of various src-like PTKs to participate in IL-2 signal transduction mechanisms and raise the possibility that lineage-specific (T-versus B-cell) responses to IL-2 may be determined at least in part by the repertoire of src-like PTKs expressed in the cell.

Human CD8+ herpes simplex virus-specific cytotoxic T-lymphocyte clones recognize diverse virion protein antigens

The role of the HLA class I-restricted, CD8+, herpes simplex virus (HSV)-specific cytotoxic T lymphocytes (CTL) in the control of human HSV infections is controversial because previous reports suggest that a substantial portion of the antigen-specific lytic response is mediated by CD4+ cells. To address this question directly, we isolated HSV-specific CD8+ CTL clones from a patient with recurrent genital herpes. These CTL were cloned by coculturing responder peripheral blood mononuclear cells (PBMC) with phytohemagglutinin-stimulated PBMC that had been infected with live HSV-2 and then irradiated prior to the addition of responder cells. After 1 week, CTL were cloned by limiting dilution using phytohemagglutinin stimulation and allogeneic feeder PBMC. Seven clones were isolated; all seven clones were CD8+ CD4- CD3+ DRbright, six lysed only HSV-2-infected targets, and one lysed both HSV-1- and HSV-2-infected targets. Antigen presentation was restricted by two to three different HLA class I loci. To determine the antigens recognized by these HSV-specific CTL, target cells were infected with HSV in the presence of acyclovir, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, or cycloheximide in a series of drug block/release protocols to limit the repertoire of viral gene expression to select transcriptional classes. Five of the clones exhibited a different pattern of cytotoxicity, suggesting that each recognized a distinct HSV antigen. One of the clones appears to be directed against an immediate-early antigen; six of the clones recognize virion proteins. Five of these clones recognized internal virion proteins that could be introduced into target cells by HSV infection in the absence of virus gene expression. Antigen specificity was further tested by using vaccinia virus vectors that express glycoproteins gD2 and gB2 or the tegument protein VP16. One clone lysed vaccinia virus/gD2-infected target cells; the remaining clones did not recognize any of these gene products. The diversity of the CD8+ response from a single individual indicated that several different antigens are recognized when presented in the context of a variety of class I HLA alleles, a pattern that markedly differs from that described for another human herpesvirus, cytomegalovirus.

J chain synthesis and secretion of hexameric IgM is differentially regulated by lipopolysaccharide and interleukin 5

Two functional polymeric forms of IgM can be produced by antibody-secreting B cells. Hexameric IgM lacks detectable J (joining) chain and activates complement 17-fold better than pentameric IgM, which usually contains one J chain per pentamer. Using the inducible B-cell lymphoma CH12, we determined if the synthesis of a particular polymeric form of IgM is a fixed property of B cells or can be altered. Lipopolysaccharide (LPS)-stimulated CH12 cells produced mixtures of IgM hexamers and pentamers, resulting in antibody with high complement-fixing activity. In contrast, interleukin-5-stimulated CH12 cells secreted predominantly pentameric IgM, with a correspondingly lower lytic activity. Differences in lytic activity were due only to the amount of hexameric IgM in the secreted antibody. Interleukin 5 stimulated higher production of J chain RNA and protein than LPS, while LPS induced the highest levels of the secretory form of mu protein. The amount of hexameric IgM secreted was therefore inversely proportional to the level of intracellular J chain protein in the responding B cells. We conclude that the biologic function of IgM produced by B cells differs depending on how they are stimulated and that this difference may be regulated by the relative availabilities of J chain and secretory mu proteins during IgM polymerization.

Clustering of IgA-producing immunocytes related to HLA-DR-positive ducts in normal and inflamed salivary glands

Ig-producing immunocytes and epithelial expression of secretory component (SC) and HLA-DR were evaluated by two-colour immunofluorescence staining in 10 normal and 20 inflamed salivary glands; the latter included specimens from 10 patients with obstructive sialadenitis and 10 with Sjögren's syndrome (SS). Epithelium adjacent to T-cell infiltrates showed extensive co-expression of SC and HLA-DR, suggesting that leucocyte-derived cytokines were responsible for this concurrent up-regulation. Clusters (greater than 2 positive cells) of IgA-producing cells were spatially related to DR-positive ducts. The possibility is discussed that DR-expressing epithelium contributes to local terminal differentiation of IgA-producing plasma cells. A cytokine-mediated up-regulation of SC that simultaneously increases the transport capacity for polymeric IgA would constitute an efficient enhancement of secretory immunity in diseased glandular tissue.

Interleukin 2- and interleukin 5-induced changes in the binding of regulatory factors to the J-chain gene promoter

In a primary immune response, B cells require signals from the T-cell lymphokines interleukins 2 and 5 (IL-2 and IL-5) to develop into IgM-secreting cells. One role of IL-2 and IL-5 is to activate transcription of the gene encoding the IgM joining component, the J chain. In this study the activation mechanism was investigated by using an inducible beta-lymphoma cell line to examine J-chain RNA expression and factor binding to the J-chain promoter. The analyses revealed that both IL-2 and IL-5 trigger a decrease in the binding of two promoter-specific nuclear proteins that precedes the appearance of J-chain RNA. In combination the two lymphokines effected nearly additive changes in factor binding and J-chain RNA abundance. Both effects were reversed upon withdrawal of the lymphokine stimulus and both were inhibited in the presence of the T-cell lymphokine IL-4. These findings indicate that the IL-2 and IL-5 signal pathways converge to deliver a common signal that regulates the repressor activities of two lymphokine-responsive promoter elements.

A novel method for synchronizing a B cell lymphoma

Certain sub-lines of the murine B cell lymphoma BCL1 can be maintained in vitro and respond to cytokines including IL-2 and IL-5. BCL1 cells, as well as other B lymphomas, are difficult to synchronize using conventional techniques such as thymidine block or DNA synthesis inhibition. We have found that BCL1 cells maintained in Dulbecco's minimum essential medium (DMEM) with non-essential amino acids (NEAA) can be readily synchronized by culture in DMEM lacking NEAA. Within 10-18 h of medium replacement, 98% of BCL1 cells are 2 N in DNA content, suggesting that these cells are arrested in G0/G1. This population of BCL1 cells is viable and can be stimulated to enter S phase by culture in media containing NEAA; however, arrested cells did not appear to return synchronously into the cell cycle on addition of NEAA. A transient increase in levels of c-fos and c-myc mRNA was not detected after arrested BCL1 cells were stimulated to enter S phase, suggesting that arrested cells are in the G1 phase of the cell cycle, rather than G0. This technique for obtaining G1 arrested B lymphoma cells may prove useful in the analysis of molecular events that occur in B cells as a function of cell cycle position.

IL-2 and IL-5 both induce mu S and J chain mRNA in a clonal B cell line, but differ in their cell-cycle dependency for optimal signaling

We have found that a neoplastic Lyl+ B cell clone (BCL1-3B3) can be stimulated to secrete IgM by a Th1-derived cytokine, IL-2, and/or by a Th2-derived cytokine, IL-5. At suboptimal concentrations these interleukins acted synergistically to enhance IgM secretion. Both IL-2 and IL-5 induced increases in microseconds and J chain mRNA levels. In the presence of both ILs, increases in microseconds and J chain mRNA were additive and paralleled increases in IgM secretion. Using cells synchronized at the G1/S border with excess thymidine or in early G1 using isoleucine-deficient media, IL-2 and IL-5 differed in their cell-cycle dependency for signal transmission. IL-5 appeared to act preferentially in late G1 of the cell cycle. In contrast, IL-2 stimulated S and G2 phase cells slightly more efficiently than cells in G1 of the cell cycle. Furthermore, a twofold increase in high-affinity IL-2R was observed as the cells entered S phase. The results suggest that although IL-2 and IL-5 can independently and additively induce differentiation of the Lyl+ BCL1-3B3 cells, they differ in their point of action during the cell cycle.

Interaction of the IL-2 receptor with the src-family kinase p56lck: identification of novel intermolecular association

In the interleukin-2 (IL-2) system, intracellular signal transduction is triggered by the beta chain of the IL-2 receptor (IL-2R beta); however, the responsible signaling mechanism remains unidentified. Evidence for the formation of a stable complex of IL-2R beta and the lymphocyte-specific protein tyrosine kinase p56lck is presented. Specific association sites were identified in the tyrosine kinase catalytic domain of p56lck and in the cytoplasmic domain of IL-2R beta. As a result of interaction, IL-2R beta became phosphorylated in vitro by p56lck. Treatment of T lymphocytes with IL-2 promotes p56lck kinase activity. These data suggest the participation of p56lck as a critical signaling molecule downstream of IL-2R via a novel interaction.

Glycosylphosphatidylinositol: a candidate system for interleukin-2 signal transduction

The mechanism of interleukin-2 (IL-2) signal transduction was analyzed by use of an inducible B lymphoma. Like normal antigen-activated B lymphocytes, the lymphoma cells respond to IL-2 by proliferating and differentiating into antibody-secreting cells; both responses are blocked by a second interleukin, IL-4. Analyses of the signaling pathway showed that IL-2 stimulated the rapid hydrolysis of an inositol-containing glycolipid to yield two possible second messengers, a myristylated diacylglycerol and an inositol phosphate-glycan. The myristylated diacylglycerol response exhibited the same IL-2 dose dependence as the growth and differentiative responses, and the generation of both hydrolysis products was inhibited by IL-4. These correlations implicate the glycosyl-phosphatidylinositol system in the intracellular relay of the IL-2 signal.

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).

The biological effects of IgM hexamer formation

The inducible B cell lymphoma, CH12, and its in vitro adapted subclone, CH12-LBK, produce immunoglobulins of identical sequence, specificity and isotype, with equivalent affinities for the hapten trimethyl ammonium. However, the hemolytic efficiencies of the antibody secreted by the two cell lines are quite different. Antibody preparations from lipopolysaccharide-stimulated CH12 cells lyse erythrocytes six- to ten times more effectively than antibody preparations of the same concentration from CH12-LBK cells. Both cell lines secrete polymeric IgM, but while CH12-LBK cells secrete predominantly the canonical pentameric IgM, CH12 cells secrete a mixture of pentamers and hexamers. High-efficiency complement-dependent cytolysis is associated with hexameric IgM, which has a specific activity that is approximately twenty times higher than that of the pentameric form. J chain protein is found in the secreted IgM of both cell lines, but is associated only with the pentameric IgM and not with the hexameric form, nor with any intermediate polymers smaller than a pentamer. A deficit in, or the inaccessibility of, J chain protein appears to facilitate hexamer formation. These experiments confirm previously published data showing that J chain is not necessary either for assembly or secretion of polymeric IgM, and suggest instead that J chain may be important in regulating the lytic efficiency of polymeric IgM by controlling the IgM pentamer/hexamer ratio. The experiments further suggest a mechanism, in addition to isotype switching and somatic mutation, by which the biological efficiency of antibodies from a single clone of B cells can be regulated.

Improvement in the antibody productivity of human-human hybridomas by transfection with Tac gene

The presence of a highly purified recombinant interleukin-2 (rIL-2) increased the production of immunoglobulin (IgM or IgG) by human-human hybridomas to 1.5-2.0 times the production by untreated cells. However, these cells did not react with anti-Tac (IL-2 receptor alpha) antibody. To enhance the response of the hybridoma cells to rIL-2, Tac gene was introduced by co-transfection with Tac gene expression plasmid pTB459 and G418 resistant gene expression plasmid pRSVneo. Tac cDNA transfected hybridoma (HBW-4.16.459-6-126) was induced to produce 6 times as much IgG by rIL-2 as was the control. This antibody production promoting phenomenon mediated by rIL-2 was depressed by anti-Tac antibody.

Human interleukin 2 receptor beta-chain gene: chromosomal localization and identification of 5' regulatory sequences

Interleukin 2 (IL-2) binds to and stimulates activated T cells through high-affinity IL-2 receptors (IL-2Rs). Such receptors represent a complex consisting of at least two proteins, the 55-kDa IL-2R alpha chain and the 70-kDa IL-2R beta chain. The low-affinity, IL-2R alpha chain cannot by itself transduce a mitogenic signal, whereas IL-2 stimulates resting lymphocytes through the intermediate-affinity, IL-2R beta receptor. We report here identification of the genomic locus for IL-2R beta. The exons are contained on four EcoRI fragments of 1.1, 9.2, 7.2, and 13.7 kilobases. The 1.1-kilobase EcoRI fragment lies at the 5'-most end of the genomic locus and contains promoter sequences. The promoter contains no TATA box-like elements but does contain the d(GT)n class of middle repetitive elements, which may play an interesting regulatory role. The IL-2R beta gene is localized to chromosome 22q11.2-q12, a region that is the locus for several lymphoid neoplasias.

Regulation of microM vs microS mRNA expression in an inducible B cell line

During the course of B lymphocyte differentiation into immunoglobulin secreting cells the abundance of mRNA for the heavy chain of secreted IgM (microS) increases dramatically. In order to understand the regulatory events responsible for the selective increase in micS mRNA we have looked for transcriptional alterations of VDJC mu gene segments as well as changes in the relative stability of microM and microS mRNA in BCL1 lymphoma cells which can be stimulated to increase the expression of microS mRNA. These experiments showed that although the transcriptional level of the mu gene locus is not preferentially augmented after stimulation, an alteration in the sites of polymerase termination is a significant factor contributing to the higher microS to microM ratio. This switch is dependent on new RNA synthesis. In addition, although the half-life of microS mRNA is not selectively increased, stimulation of the cells does result in a specific enhancement of the half-lives of both species of mu mRNA, which accounts for the higher steady state levels of total mu message.

Interleukin-2 receptor beta chain gene: generation of three receptor forms by cloned human alpha and beta chain cDNA's

Interleukin-2 (IL-2) binds to two distinct receptor molecules, the IL-2 receptor alpha (IL-2R alpha, p55) chain and the newly identified IL-2 receptor beta (IL-2R beta, p70-75) chain. The cDNA encoding the human IL-2R beta chain has now been isolated. The overall primary structure of the IL-2R beta chain shows no apparent homology to other known receptors. Unlike the IL-2R alpha chain, the IL-2R beta chain has a large cytoplasmic region in which a functional domain (or domains) mediating an intracellular signal transduction pathway (or pathways) may be embodied. The cDNA-encoded beta chain binds and internalizes IL-2 when expressed on T lymphoid cells but not fibroblast cells. Furthermore, the cDNA gives rise to the generation of high-affinity IL-2 receptor when co-expressed with the IL-2R alpha chain cDNA.

Interleukin 2 regulates antigen-specific immunoglobulin response of naive murine B cells

Activation of mouse B cells with lipopolysaccharide in conjunction with anti-immunoglobulin (Ig) antibodies results in interleukin 2 (IL2) receptor (IL2-R) expression and IL2 responsiveness. In most studies on the effect of IL2 on antibody production by B cells, polyclonally activated normal B cells or B cell lines established in vitro have been used as indicator cells, thus allowing no direct correlation between the experimental findings and the actual physiological mechanism of IL2 action in antigen-specific B cell response. We employed the splenic fragment culture technique, which measures antibody response on the clonal level, to analyze the effect of purified human recombinant IL2 (rIL2) on the primary antigen-specific Ig response of mouse B cells. Here we report that rIL2 increased the frequency of dinitrophenyl (DNP)-responsive splenic B cells and the amount of Ig secreted per clone. The anti-DNP antibody response was dependent upon interaction of naive B cells with carrier-primed T cells, which apparently provided the signal for IL2-R expression. Recombinant IL2 also facilitated Ig isotype switching by individual clones, suggesting a role for IL2 in activation, maturation, and differentiation of antigen-specific naive B cells in their response to T-dependent antigens.

IgM--molecular requirements for its assembly and function

The conventional model of IgM structure depicts a unique, array of mu, L and J chains, held together by well-defined disulfide bonds and other interactions. Some, but not all, recent data support this model. Here Ann Davis and Marc Shulman review recent, as well as older, studies of IgM and consider their implications for our understanding of IgM structure and function.

Characterization of a presecretory phase in B-cell differentiation

We have identified and characterized an inducible in vitro subclone of the CH12 B-cell lymphoma, CH12-LBK, which appears to represent a transitional phase in the B-cell differentiation pathway. This phase, which we call the "presecretory" phase, falls between replicating B cells that are not secreting antibodies and B cells that secrete antibody at a high rate. Presecretory cells are characterized by abundant steady-state levels of immunoglobulin and joining (J) chain transcripts and of protein but low levels of mouse mammary tumor virus envelope transcripts and low rates of immunoglobulin secretion. Additional stimulation is required for presecretory cells to differentiate into cells that secrete antibodies at a high rate. The existence of cells with this phenotype suggests that high-level expression of immunoglobulin and J-chain protein does not necessarily commit a B cell to polymerize and secrete multimeric immunoglobulin. Rather, other gene products, expressed after immunoglobulin and J-chain transcripts have been upregulated late in B-cell differentiation, appear responsible for inducing high rates of antibody secretion.

Mechanism of interleukin-2 signaling: mediation of different outcomes by a single receptor and transduction pathway

The T cell lymphokine, interleukin-2 (IL-2), plays a pivotal role in an immune response by stimulating antigen-activated B lymphocytes to progress through the cell cycle and to differentiate into antibody-secreting cells. An IL-2 inducible B lymphoma line, in which the growth and differentiation responses are uncoupled, provides a model system for dissecting the signaling mechanisms operating in each response. This system was used to show that both signals are initiated by IL-2 binding to a single, unifunctional receptor complex. Moreover, both signals are transduced by a pathway that does not involve any known second messenger system and that can be blocked by a second T cell lymphokine, interleukin 4. These findings suggest that the pleiotrophic effects of IL-2 are determined by different translations of the signal in the nucleus.

Induction of differentiation in human leukemic B cells by interleukin 2 alone: differential effect on the expression of mu and J chain genes

The effects of interleukin 2 (IL2) on the proliferation and differentiation of B cells were analyzed separately using cells from two patients suffering from B-type chronic lymphocytic leukemia. The monoclonal B cells from these patients exhibited an opposite pattern of responsiveness upon in vitro culture with IL2 in the absence of other stimuli. In the first patient, IL2 alone was able to induce DNA synthesis and no Ig production. In the second patient, although no DNA synthesis was detected, B lymphocytes synthesized IgM upon stimulation with IL2 alone. Analysis of mRNA levels was performed on the cells of this latter patient after culture without or with IL2. In the presence of IL2 we observed a strong enhancement of C mu gene expression associated with an increase of the ratio between the secreted form and the membrane-bound form of mu mRNA. In contrast IL2 induced only a marginal enhancement of J chain mRNA. Thus, terminal B cell differentiation of selected monoclonal B cells can be obtained in the absence of DNA synthesis and IL2 alone can mediate this process. Moreover, IL2 can act at selective steps of the molecular events associated with IgM production. These results document the multiple effects of a given IL on the events leading to antibody production and strongly suggest that they can be conditioned by the maturation stage of a given responding cell.

Lymphoepithelial interactions in the mucosal immune system

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Interleukin-2: inception, impact, and implications

Interleukin-2 (IL-2), the first of a series of lymphocytotrophic hormones to be recognized and completely characterized, is pivotal for the generation and regulation of the immune response. A T lymphocyte product, IL-2 also stimulates T cells to undergo cell cycle progression via a finite number of interactions with its specific membrane receptors. Because T cell clonal proliferation after antigen challenge is obligatory for immune responsiveness and immune memory, the IL-2-T cell system has opened the way to a molecular understanding of phenomena that are fundamental to biology, immunology, and medicine.

T-cell proliferation involving the CD28 pathway is associated with cyclosporine-resistant interleukin 2 gene expression

CD28 is a homodimeric glycoprotein expressed on the surface of a major subset of human T cells that has recently been identified as a member of the immunoglobulin supergene family. The binding of monoclonal antibodies to the CD28 antigen on purified T cells does not result in proliferation; however, previous studies have shown that the combination of CD28 stimulation and protein kinase C activation by phorbol myristate acetate (PMA) results in T-cell proliferation that is independent of both accessory cells and activation of the T-cell receptor-CD3 complex. In the present study, effects of stimulation by anti-CD28 on cell cycle progression and on the interleukin 2 (IL-2) and IL-2 receptor system have been investigated on primary cultures of purified peripheral-blood CD28+ T cells. There was no measurable effect on cell size or on DNA synthesis after stimulation of resting (G0) cells by CD28 alone. After 3 h of activation of T cells by PMA alone, a slight (8%) increase in cell volume occurred that did not progress to DNA synthesis. In contrast, T-cell stimulation by CD28 in combination with PMA resulted in a progressive increase in cell volume in approximately 100% of cells at 12 to 14 h after stimulation. Northern blot (RNA blot) analysis revealed that CD28 stimulation alone failed to cause expression of the alpha chain of the IL-2 receptor or of IL-2 mRNA, and in accord with previous studies, stimulation by PMA alone resulted in the accumulation of IL-2 receptor transcripts but no detectable IL-2 mRNA. In contrast, T-cell stimulation by the combination of CD28 and PMA resulted in the appearance of IL-2 transcripts and enhanced expression of IL-2 receptor mRNA. Functional studies revealed that the proliferation induced by CD28 and PMA stimulation was entirely resistant to cyclosporine, in contrast to T-cell activation induced by the CD3-T-cell receptor complex. Cyclosporine was found not to affect the accumulation of IL-2 mRNA after CD28 plus PMA stimulation, although there was no detectable IL-2 mRNA after stimulation by CD3 in the presence of the drug. Furthermore, stimulation by CD28 in combination with immobilized CD3 antibodies caused a striking enhancement of IL-2 mRNA expression that was, in part, resistant to the effects of cyclosporine. These studies indicate that the CD28 molecule synergizes with protein kinase C activation to induce IL-2 gene expression and demonstrate that stimulation by the CD28 pathway can cause vigorous T-cell proliferation even in the presence of cyclosporine and that cyclosporine does not prevent transcription of 16-2 mRNA, as has been suggested previously. Moreover, these findings suggest that a potential role for the CD28 molecule in vivo may be to augment IL-2 production after stimulation of the CD3-T-cell receptor molecular complex and thereby to amplify an antigen-specific immune response. Finally, these results provide further evidence that the CD28 molecule triggers T-cell proliferation in a manner that differs biochemically from CD3-T-cell receptor-induced proliferation.

T-cell proliferation involving the CD28 pathway is associated with cyclosporine-resistant interleukin 2 gene expression

CD28 is a homodimeric glycoprotein expressed on the surface of a major subset of human T cells that has recently been identified as a member of the immunoglobulin supergene family. The binding of monoclonal antibodies to the CD28 antigen on purified T cells does not result in proliferation; however, previous studies have shown that the combination of CD28 stimulation and protein kinase C activation by phorbol myristate acetate (PMA) results in T-cell proliferation that is independent of both accessory cells and activation of the T-cell receptor-CD3 complex. In the present study, effects of stimulation by anti-CD28 on cell cycle progression and on the interleukin 2 (IL-2) and IL-2 receptor system have been investigated on primary cultures of purified peripheral-blood CD28+ T cells. There was no measurable effect on cell size or on DNA synthesis after stimulation of resting (G0) cells by CD28 alone. After 3 h of activation of T cells by PMA alone, a slight (8%) increase in cell volume occurred that did not progress to DNA synthesis. In contrast, T-cell stimulation by CD28 in combination with PMA resulted in a progressive increase in cell volume in approximately 100% of cells at 12 to 14 h after stimulation. Northern blot (RNA blot) analysis revealed that CD28 stimulation alone failed to cause expression of the alpha chain of the IL-2 receptor or of IL-2 mRNA, and in accord with previous studies, stimulation by PMA alone resulted in the accumulation of IL-2 receptor transcripts but no detectable IL-2 mRNA. In contrast, T-cell stimulation by the combination of CD28 and PMA resulted in the appearance of IL-2 transcripts and enhanced expression of IL-2 receptor mRNA. Functional studies revealed that the proliferation induced by CD28 and PMA stimulation was entirely resistant to cyclosporine, in contrast to T-cell activation induced by the CD3-T-cell receptor complex. Cyclosporine was found not to affect the accumulation of IL-2 mRNA after CD28 plus PMA stimulation, although there was no detectable IL-2 mRNA after stimulation by CD3 in the presence of the drug. Furthermore, stimulation by CD28 in combination with immobilized CD3 antibodies caused a striking enhancement of IL-2 mRNA expression that was, in part, resistant to the effects of cyclosporine. These studies indicate that the CD28 molecule synergizes with protein kinase C activation to induce IL-2 gene expression and demonstrate that stimulation by the CD28 pathway can cause vigorous T-cell proliferation even in the presence of cyclosporine and that cyclosporine does not prevent transcription of 16-2 mRNA, as has been suggested previously. Moreover, these findings suggest that a potential role for the CD28 molecule in vivo may be to augment IL-2 production after stimulation of the CD3-T-cell receptor molecular complex and thereby to amplify an antigen-specific immune response. Finally, these results provide further evidence that the CD28 molecule triggers T-cell proliferation in a manner that differs biochemically from CD3-T-cell receptor-induced proliferation.