Studies of surface immunoglobulin-dependent B cell activation

A simple method for the purification of human peripheral blood antigen presenting cells (dendritic cells, monocytes/macrophages, and B lymphocytes)

An adherence method was developed that enriches for antigen presenting cells (dendritic cells, monocytes/macrophages, and B lymphocytes) from peripheral blood mononuclear cell (PBMC) preparations. This method utilizes the cells' natural adherence to polystyrene tissue culture dishes and their subsequent removal with K3EDTA after incubation at 4 degrees, with gentle pipeting. Flow cytometric analysis revealed that on average, the enrichment of CD83+ dendritic cells, CD14+ monocytes/ macrophages, and CD19+ B cells increased by 12.5 to 20, 2, and 4 fold, respectively, compared to their initial numbers present in PBMC preparations. Cell viability, determined by trypan blue exclusion, was between 90 and 98%. After the enrichment procedure, the cells could still be activated by tetanus toxoid and this was shown by flow cytometric analysis, as enhancement of class II major histocompatibility complex (MHC) (31% increase) (after antigen treatment). This is a fast and economical alternative to other established methods for the preparation of pure, functionally competent antigen presenting cells derived from peripheral blood.

Increased p27Kip1 Cyclin-Dependent Kinase Inhibitor Gene Expression Following Anti-IgM Treatment Promotes Apoptosis of WEHI 231 B Cells

Engagement of the B cell receptor of WEHI 231 immature B cells leads sequentially to a drop in c-Myc, to induction of the cyclin-dependent kinase inhibitor p27Kip1, and finally to apoptosis. Recently we demonstrated that the drop in c-Myc expression promotes cell death, whereas the induction of p27 has been shown to lead to growth arrest. In this paper, we demonstrate that increased p27 expression also promotes apoptosis of WEHI 231 B cells. The rescue of WEHI 231 cells by CD40 ligand engagement of its receptor prevented the increase in p27 induction. Inhibition of p27-ablated apoptosis induced upon expression of antisense c-myc RNA. Furthermore, specific induction of p27 gene expression resulted in apoptosis of WEHI 231 cells. Lastly, inhibition of expression of c-Myc, upon induction of an antisense c-myc RNA vector, was sufficient to induce increased p27 levels and apoptosis. Thus, these findings define a signaling pathway during B cell receptor engagement in which the drop in c-Myc levels leads to an increase in p27 levels that promotes apoptosis.

Roles of the tumor suppressor p53 and the cyclin-dependent kinase inhibitor p21WAF1/CIP1 in receptor-mediated apoptosis of WEHI 231 B lymphoma cells

Treatment of WEHI 231 immature B lymphoma cells with an antibody against their surface immunoglobulin M (anti-IgM) induces apoptosis and has been studied extensively as a model of self-induced B cell tolerance. Since the tumor suppressor protein p53 has been implicated in apoptosis in a large number of cell types and has been found to be mutated in a variety of B cell tumors, here we sought to determine whether p53 and the p53 target gene cyclin-dependent kinase inhibitor p21(WAF1/CIP1) were involved in anti-IgM-induced cell death. Anti-IgM treatment of WEHI 231 cells increased expression of p53 and p21 protein levels. Ectopic expression of wild-type p53 in WEHI 231 cells induced both p21 expression and apoptosis. Ectopic expression of p21 similarly induced apoptosis. Rescue of WEHI 231 cells from apoptosis by costimulation with CD40 ligand ablated the increase in p21 expression. Lastly, a significant decrease in anti-IgM-mediated apoptosis was seen upon downregulation of endogenous p53 activity by expression of a dominant-negative p53 protein or upon microinjection of an antisense p21 expression vector or antibody. Taken together, the above data demonstrate important roles for p53 and p21 proteins in receptor-mediated apoptosis of WEHI 231 B cells.

Use of isolated immature-stage B cells to understand negative selection and tolerance induction at the molecular level

Encounter with antigen by newly developing antigen receptor-positive B cells leads to negative selection. This process positions the B cell antigen receptor (BCR) in a central role for initiating the process of negative selection and suggests developmental regulation of its signaling. The observation that immature B cells are more susceptible to negative selection than are mature B cells has been demonstrated in a number of in vitro and in vivo model systems and support the idea of developmental regulation of BCR-initiated responses. Since identical antigen receptors are expressed on immature and mature B cells, the critical fate-determining distinction between these developmental stages must lie downstream of the receptor-ligand interaction itself, in the form of different BCR-linked signaling processes or with different secondary events occurring subsequent to BCR cross-linking. To address the first possibility, our laboratory and others have sought to define the differences in BCR-mediated signal transduction in immature and mature B lymphocytes. In this review article we will discuss current in vitro systems to study this question in primary, nontransformed murine B lymphocytes. In addition, we will discuss our previously published work in order to illustrate how these model systems have been useful in beginning to unravel the molecular basis for immune B cell negative selection and tolerance.

Inhibition of c-myc expression induces apoptosis of WEHI 231 murine B cells

Treatment of WEHI 231 immature B-lymphoma cells with an antibody against their surface immunoglobulin (anti-Ig) induces apoptosis and has been studied extensively as a model of B-cell tolerance. Anti-Ig treatment of exponentially growing WEHI 231 cells results in an early transient increase in c-myc expression that is followed by a decline to below basal levels; this decrease in c-myc expression immediately precedes the induction of cell death. Here we have modulated NF-kappaB/Rel factor activity, which regulates the rate of c-myc gene transcription, to determine whether the increase or decrease in c-Myc-levels mediates apoptosis in WEHI 231 cells. Addition of the serine/threonine protease inhibitor N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), which blocks the normally rapid turnover of the specific inhibitor of NF-kappaB/Rel IkappaBalpha in these cells, caused a drop in Rel-related factor binding. TPCK treatment resulted in decreased c-myc expression, preventing the usual increase seen following anti-Ig treatment. Whereas inhibition of the induction of c-myc expression mediated by anti-Ig failed to block apoptosis, reduction of c-myc expression in exponentially growing WEHI 231 cells induced apoptosis even in the absence of anti-Ig treatment. In WEHI 231 clones ectopically expressing c-Myc, apoptosis induced by treatment with TPCK or anti-Ig was significantly diminished and cells continued to proliferate. Furthermore, apoptosis of WEHI 231 cells ensued following enhanced expression of Mad1, which has been found to reduce functional c-Myc levels. These results indicate that the decline in c-myc expression resulting from the drop in NF-kappaB/Rel binding leads to activation of apoptosis of WEHI 231 B cells.

TGF beta 1 inhibits NF-kappa B/Rel activity inducing apoptosis of B cells: transcriptional activation of I kappa B alpha

TGF beta 1 treatment of B cell lymphomas decreases c-myc gene expression and induces apoptosis. Since we have demonstrated NF-kappa/Rel factors play a key role in transcriptional control of c-myc, we explored the effects of TGF beta1 on WEHI 231 immature B cells. A reduction in NF-kappa B/Rel activity followed TGF beta 1 treatment. In WEHI 231 and CH33 cells, we observed an increase in I kappa B alpha, a specific NF-kappa B/Rel inhibitor, due to transcriptional induction. Engagement of surface CD40 or ectopic c-Rel led to maintenance of NF-kappa B/Rel and c-Myc expression and protection of WEHI 231 cells from TGF beta 1-mediated apoptosis. Ectopic c-Myc expression overrode apoptosis induced by TGF beta 1. Thus, downmodulation of NF-kappa B/Rel reduces c-Myc expression, which leads to apoptosis in these immature B cell models of clonal deletion. The inhibition of NF-kappa B/Rel activity represents a novel TGF beta signaling mechanism.

Role of Rel-related factors in control of c-myc gene transcription in receptor-mediated apoptosis of the murine B cell WEHI 231 line

Treatment of immature murine B lymphocytes with an antiserum against their surface immunoglobulin (sIg)M results in cell death via apoptosis. The WEHI 231 B cell line (IgM, kappa) has been used extensively as a model for this anti-Ig receptor-mediated apoptosis. Anti-sIg treatment of WEHI 231 cells causes an early, transient increase in the levels of c-myc messenger RNA and gene transcription, followed by a rapid decline below control values. Given the evidence for a role of the c-myc gene in promoting apoptosis, we have characterized the nature and kinetics of changes in the binding of Rel-related factors, which modulate c-myc promoter activity. In exponentially growing WEHI 231 cells, multiple Rel-related binding activities were detectable. The major binding species was identified as p50/c-Rel heterodimers; only minor amounts of nuclear factor kappa B (NF-kappa B) (p50/p65) were detectable. Cotransfection of an inhibitor of NF-kappa B (I kappa B)-alpha expression vector reduced c-myc-promoter/upstream/exon1-CAT reporter construct activity, indicating the role of Rel factor binding in c-myc basal expression in these cells. Treatment with anti-sIg resulted in a rapid transient increase in the rate of c-myc gene transcription and in the binding of Rel factors. At later times, formation of p50 homodimer complexes occurred. In cotransfection analysis, p65 and c-Rel expression potently and modestly transactivated the c-myc promoter, respectively, whereas, overexpression of the p50 subunit caused a significant drop in its activity. The role of activation of Rel-family binding was demonstrated directly upon addition of the antioxidant pyrrolidinedithiocarbamate, which inhibited the anti-sIg-mediated activation of the endogenous c-myc gene. Similarly, induction after anti-sIg treatment of a transfected c-myc promoter was abrogated upon cotransfection of an I kappa B-alpha expression vector. These results implicate the Rel-family in Ig receptor-mediated signals controlling the activation of c-myc gene transcription in WEHI 231 cells, and suggest a role for this family in apoptosis of this line, which is mediated through a c-myc signaling pathway.

Differential responsiveness of immature- and mature-stage murine B cells to anti-IgM reflects both FcR-dependent and -independent mechanisms

Mature and immature B cells differ in their responses to antigen receptor crosslinking. Whereas mature B cells enter cell cycle in response to such stimulation, immature B cells exhibit proliferative unresponsiveness and undergo induced tolerance following surface immunoglobulin (sIg) engagement. Previous studies evaluating antigen receptor-mediated negative signaling have utilized intact goat anti-immunoglobulin (anti-Ig) antibodies as polyclonal ligands based upon observations that the Fc portion of these reagents does not interact with and mediate negative signaling through the FcR on mature B cells. Thus, the negative effects of goat anti-Ig on immature B cells have been attributed solely to signals mediated via their antigen receptors. In the studies reported here we show that the activation unresponsiveness inherent to immature B cells is FcR independent. However, we also show that immature B cells are sensitive to FcR-mediated inhibition and that these effects can be mediated by intact goat antibodies at concentrations that promote positive activation signals in mature B cells. Our results demonstrate that inhibition of immature B cell LPS responses by anti-Ig antibodies, used in previous studies as an in vitro model for B cell tolerance induction, is an FcR-mediated phenomenon. We show that developmentally associated anti-Ig-mediated inhibition of LPS requires the use of intact antibodies, and that this inhibition can be blocked by the anti-FcR monoclonal antibody 2.4G2. Flow cytometric analysis of FcR-positive B cells indicates that both mature and immature B cells express equivalent levels of FcR gamma. Therefore, the sensitivity of immature, but not mature, cells to intact goat anti-mu antibodies suggests that either FcRs or their associated inhibitory pathways change during B cell development.

Developmentally regulated association of a 56-kD member of the surface immunoglobulin M receptor complex

Immature and mature B cells differ in the signals generated and transduced through their antigen receptor, surface immunoglobulin M (sIgM). Whereas signals generated through sIgM on mature B cells initiate a program leading to the positive activation of these cells, signaling through this receptor at the immature stage of development leads to a state of induced unresponsiveness or tolerance. Our previous studies have described developmental differences in sIgM transmembrane signaling that are independent of ligand-receptor affinity. In an attempt to understand the molecular basis for signaling differences between immature and mature B cells, we have analyzed the sIgM receptor complex in neonatal and adult mouse splenic B cells. While previously described components of this complex do not exhibit marked developmentally regulated differences in their association with sIgM, we have identified a 56-kD protein that associates with sIgM in mature (antigen-responsive), but not immature (tolerance-sensitive) B cells. This protein (p56) associates with sIgM as a homodimer, is constitutively phosphorylated on tyrosine, and is coimmunoprecipitated with IgM but not IgD. The observed inability to iodinate p56 suggests it is an intracellular component of the receptor complex. Based upon its migration in one- and two-dimensional gel electrophoresis we show, however, that p56 is distinct from the blk, lyn, or fyn src family kinases that have been shown to be associated with sIgM in mature B cells. The developmentally regulated participation of p56 in the B cell antigen receptor complex suggests a role in the differential signaling mediated via sIgM on immature and mature B cells.

Muramyl dipeptide (MDP) synergizes with interleukin 2 and interleukin 4 to stimulate, respectively, the differentiation and proliferation of B cells

The synthetic immunomodulator muramyldipeptide (MDP) can stimulate B cells. MDP, when used alone, was apparently unable to induce the differentiation or proliferation of resting B cells. In contrast, MDP appeared to synergize with a single recombinant interleukin (IL) to stimulate either their differentiation or proliferation. We used single interleukins to avoid synergistic and antagonistic effects inherent in the use of several factors. IL-2 was found to be sufficient to restore the specific immune response of resting B cells to sheep erythrocytes; MDP greatly increased the number of plaque-forming cells of such IL-2-stimulated B cells. In contrast, IL-4 and interferon-gamma (IFN-gamma), either alone or in the presence of MDP, had no effect in this differentiation assay. MDP was also able to stimulate polyclonally activated B cells. IL-4 increased the proliferation of anti-IgM-stimulated B cells, leading to enlargement and driving more cells into the cell cycle; these effects were further enhanced by MDP, more cells being induced to proliferate, to enlarge, and to progress into the cycle with a higher frequency of cells in the G1B, S, and G2/M compartments. Intracellular free calcium levels were not increased by IL-4 and/or MDP, and the two compounds did not modify the anti-IgM-induced calcium mobilization. Therefore, MDP appears to amplify cytokine effects in B cell activation, by a mechanism which does not appear to involve free calcium mobilization.

Involvement of a specific guanine nucleotide binding protein in receptor immunoglobulin stimulated inositol phospholipid hydrolysis

The role of a specific guanine nucleotide binding (G protein) protein in coupling murine B lymphocyte receptor immunoglobulin to inositol phospholipid hydrolysis was investigated. Using an in vitro system with isolated membranes, we have observed specific enhancement of GTP binding subsequent to ligand-induced receptor crosslinking. Induced increases were inhibited by pretreatment with pertussis toxin which catalyzed ADP-ribosylation of a 43 kDa substrate. Involvement of this G protein with receptor immunoglobulin-induced inositol phospholipid hydrolysis was evidenced by the ability of pertussis toxin to block this response. This report, then, indicates that the B lymphocyte antigen receptor belongs to a family of receptors which are linked to inositol phospholipid hydrolysis through a G protein.

Proliferation of resting B cells is modulated by CR2 and CR1

Absence of the third component of complement, C3, is associated with impaired ability to synthesize antibody, particularly in the presence of limiting antigen [1-9]. The mature B lymphocyte bearing the surface immunoglobulin receptor transduces signals for proliferation and differentiation upon binding of specific antigen. This mature B cell also bears two related membrane proteins, CR2 (the C3d/Epstein-Barr virus receptor) (CD35) [15], which can mediate the binding of ligands to which appropriate cleavage fragments of C3 have become attached [16]. It has been suggested that these receptors play a direct role(s) in B cell activation [17-25]. In light of previous in vivo observations we decided to assess the function of CR2 and CR1 in relation to B cell activation through the membrane IgM receptor. Highly purified splenic B cells were prepared. No contaminating T cells or macrophages were detected by flow cytometric analysis and no proliferative activity was present upon PHA or ConA stimulation of the purified cells. The B cells were separated into low (activated), medium (preactivated) and high density (resting) fractions by Percoll gradient density centrifugation [26]. The responses of the B cell subpopulations to various concentrations of anti mu (DA4.4 monoclonal antibody) [27] were examined for proliferation at 72 h and for IgM/IgG production at 7 days. Low density B cells were maximally stimulated and no concentration of anti-mu was effective in enhancing their responses. High density B cells proliferated to anti-mu in a concentration dependent manner. When substimulatory concentrations of anti-mu were employed, concomitant crosslinking of CR2 (with either of 2 distinct monoclonal antibodies HB-5 [28] or OKB7 [17]) resulted in a 45% enhancement of B cell proliferation above that observed by crosslinking of SIgM alone. In these studies, total IgM and IgG did not increase in the absence of T cells or T cell factors, indicating that terminal differentiation did not occur. In contrast, when a monoclonal antibody to CR1(44D) [29] was employed in an identical experiment, B cell proliferation was completely inhibited. Antibodies to CR2 or CR1 either alone or in crosslinked form did not enhance B cell proliferation. Immune complexes may crosslink the B cell surface in a manner analogous to our model when the immunoglobulin receptor and CR2 are simultaneously engaged. This activation signal may be particularly important in eliciting antibody responses when the quantity of specific antigen or the affinity for antigen is low. The marked inhibition of proliferation induced by CR1 suggests an alternate role for this receptor in modulation of B cell responses.