Interleukin 4 induces cellular adhesion among B lymphocytes

Homotypic aggregates contribute to heterogeneity in B cell fates due to an intrinsic gradient of stimulant exposure

Monocultures of several cell types result in the formation of robust clusters called homotypic aggregates (HAs). How this physical aggregation affects cell fates in immune cell cultures, is poorly understood. We studied anti-CD40-stimulated primary B cell cultures, where cells assembled into large three-dimensional LFA1-driven HAs by 72 h. The dense packing in these aggregates restricts the infiltration of stimulants, such as antibodies, to cells inside the clusters. This creates a concentration gradient of stimulant availability across the cross-section of HAs. We describe a method to retain this positional information even after the disruption of HAs, for analysis by flow cytometry. Comparison of stage-specific cell-surface markers showed that the extent of stimulant-binding affected multiple fates non-uniformly. While germinal center and lineage markers were moderately upregulated, immunoglobulins and markers associated with memory were more than doubled in the peripheral cells binding more anti-CD40. These cells also experienced a strong repression of the plasma cell regulator Prdm1 and an upregulation of the oncogene Myc. Thus, cells at different locations in HAs are subjected to unequal doses of stimulants, leading to a hitherto unreported source of heterogeneity in cell fates. These findings can be extrapolated to understand the dose-dependent effects of stimulants in other three-dimensional cell clusters.

B cell receptor-induced phosphorylation of Pyk2 and focal adhesion kinase involves integrins and the Rap GTPases and is required for B cell spreading

Signaling by the B cell receptor (BCR) promotes integrin-mediated adhesion and cytoskeletal reorganization. This results in B cell spreading, which enhances the ability of B cells to bind antigens and become activated. Proline-rich tyrosine kinase (Pyk2) and focal adhesion kinase (FAK) are related cytoplasmic tyrosine kinases that regulate cell adhesion, cell morphology, and cell migration. In this report we show that BCR signaling and integrin signaling collaborate to induce the phosphorylation of Pyk2 and FAK on key tyrosine residues, a modification that increases the kinase activity of Pyk2 and FAK. Activation of the Rap GTPases is critical for BCR-induced integrin activation as well as for BCR- and integrin-induced reorganization of the actin cytoskeleton. We now show that Rap activation is essential for BCR-induced phosphorylation of Pyk2 and for integrin-induced phosphorylation of Pyk2 and FAK. Moreover Rap-dependent phosphorylation of Pyk2 and FAK required an intact actin cytoskeleton as well as actin dynamics, suggesting that Rap regulates Pyk2 and FAK via its effects on the actin cytoskeleton. Importantly B cell spreading induced by BCR/integrin co-stimulation or by integrin engagement was inhibited by short hairpin RNA-mediated knockdown of either Pyk2 or FAK expression and by treatment with PF-431396, a chemical inhibitor that blocks the kinase activities of both Pyk2 and FAK. Thus Pyk2 and FAK are downstream targets of the Rap GTPases that play a key role in regulating B cell morphology.

Regulation of adhesion and motility in B lymphocytes

During the differentiation process of B lymphocytes, they go through changes in adhesion and motility. In order to investigate the molecular mechanism of such changes, in vitro culture systems are necessary. When B cells are activated by various stimuli, they form different types of homotypic aggregates. In addition, they might also spread and express microvilli and/or become polarized, the latter being a sign of motility. In this review, we summarize our own research in this area. We give evidence for involvement of different adhesion and signalling molecules, and by the end, we speculate on the in vivo significance of our findings.

Efficient antigen presentation of soluble, but not particulate, antigen in the absence of Wiskott-Aldrich syndrome protein

B cells and dendritic cells, lacking functional Wiskott-Aldrich syndrome protein (WASP), have aberrant formation of membrane protrusions. We hypothesized that protrusions may play a role in antigen presentation, and consequently, that impaired antigen presentation may be an underlying factor of the immune deficiency in patients with Wiskott-Aldrich syndrome. In this paper, we investigated the antigen presentation capacity of B cells and dendritic cells from WASP knockout mice, using soluble and particulate antigen, to CD4+ T cells from T-cell receptor transgenic DO11.10 mice. As antigen we used soluble ovalbumin (OVA), a peptide thereof (amino acids 323-339) or bacteria expressing OVA. We found that WASP-deficient B cells and dendritic cells efficiently processed and presented soluble OVA protein as well as its peptide in vitro, inducing proliferation and cytokine production from CD4+ T cells. Antigen presentation of soluble protein was efficient also in vivo, because immunization of WASP-deficient mice with OVA elicited proliferation of transferred, fluorescent-labelled, CD4+ T cells. Although we could detect uptake of bacteria in dendritic cells, processing and presentation of bacterial-expressed OVA was impaired in WASP-deficient dendritic cells. In conclusion, our data suggest that WASP is not needed for processing and presentation of soluble antigen, but that efficient presentation of particulate antigen require WASP.

Cdc42, Rac1, and the Wiskott-Aldrich syndrome protein are involved in the cytoskeletal regulation of B lymphocytes

Patients with the immunodeficiency disorder Wiskott-Aldrich syndrome (WAS) have lymphocytes with aberrant microvilli, and their T cells, macrophages, and dendritic cells are impaired in cytoskeletal-dependent processes. WAS is caused by a defective or a missing WAS protein (WASP). Signal mediators interleukin-4 (IL-4) and CD40 are important for actin-dependent morphology changes in B cells. A possible function of WASP and its interacting partners, Cdc42 and Rac1, was investigated for these changes. It was found that active Cdc42 and Rac1 induced filopodia and lamellipodia, respectively, in activated B cells. Evidence is given that IL-4 has a specific role in the regulated cycling of Cdc42 because IL-4 partially and transiently depleted active Cdc42 from detergent extract of activated B cells. WASP-deficient B lymphocytes were impaired in IL-4-- and CD40-dependent induction of polarized and spread cells. Microvilli were expressed on WASP-deficient B cells, but they appeared shorter and less dense in cell contacts than in wild-type cells. In conclusion, evidence is provided for the involvement of Cdc42, Rac1, and WASP in the cytoskeletal regulation of B lymphocytes. Aberrations in WASP-deficient B lymphocytes, described here, provide further evidence that WAS is a cytoskeletal disease of hematopoietic cells. (Blood. 2001;98:1086-1094)

T(h)1 versus T(h)2 cytokine profile determines the modulation of in vitro T cell-independent type 2 responses by IL-4

We have previously demonstrated that stimulation of B cells by multivalent membrane Ig cross-linking, using dextran-conjugated anti-IgD mAb (alpha delta-dex), in the presence of cytokines, is an in vitro model for T cell-independent type 2 (TI-2) Ig secretory responses. Earlier studies have shown that IL-4 enhances IgM secretion upon stimulation with alpha delta-dex plus IL-5 and induces IgG1 isotype-switching, without altering the proliferative response to alpha delta-dex. Here we show that IL-4 can have both stimulatory and inhibitory effects on alpha delta-dex-induced Ig secretion. Both the kinetics and time of exposure to IL-4, and the nature of the cytokine additions, T(h)1 versus T(h)2, determine whether stimulation or inhibition is observed. Preincubation of sort-purified B cells with IL-4 caused a 6- to 8-fold increase in Ig secretory responses to subsequent stimulation with alpha delta-dex plus IL-1, IL-2 or a combination of both. However, the continued presence of IL-4 during B cell stimulation suppressed responses to all cytokine combinations tested, except for those which included IL-5. Of 11 cytokines tested, only IL-4 showed this dual effect of enhancement and suppression. The stimulatory effect of IL-4 required a minimum of 4 h of preincubation and could be inhibited by the addition of IFN-gamma. Thus stimulation of non-MHC class II-dependent T or non-T cells by multivalent antigens to secrete IL-4 may regulate the response to these antigens, such that early and brief exposure of B cells to IL-4 will enhance a subsequent TI-2 response in the presence of T(h)1-dependent cytokines, while continuous exposure will result in inhibition of the response.

STAT6 is required for the regulation of IL-4-induced cytoskeletal events in B cells

During lymphocyte activation, changes in cell morphology are commonly observed. This reflects cell functions important for the regulation of immune responses such as cell adhesion or cell migration. Notably, IL-4 has been shown to induce adhesion and locomotion in B cells, and we have recently described that IL-4 causes dramatic changes in B cell morphology. Thus, such B cells spread with dendritic cell protrusions and produce microvilli-like structures. The molecular mechanisms by which IL-4 induces these complex changes are currently unknown. Two signal transduction pathways are well described for IL-4, i.e. one involving insulin receptor substrate (IRS)-2 and a Janus kinase (JAK)/ signal transducer and activator of transcription (STAT) pathway mediated by STAT6. In this study we therefore used B cells from STAT6-deficient mice to address the question of a possible STAT6 dependence in IL-4-induced morphology changes. By light and electron microscopy, cell spreading and polarization were found to be severely impaired and microvilli formation was reduced. In contrast, only mild impairment was observed in cell adhesion in B cells from STAT6-deficient mice. Our results show that adhesion can be induced in the absence of STAT6. However, expression of STAT6 is necessary for optimal responses in both cell adhesion and microvilli induction. STAT6 is also essential to allow an IL-4-dependent spreading or polarization response. A possible interpretation of our results is that STAT6-dependent expression of a specific gene or genes is required for IL-4 to affect changes in B cell morphology.

Regulation of Cell Morphology in B Lymphocytes by IL-4: Evidence for Induced Cytoskeletal Changes

Lymphocyte activation is often accompanied by changes in cell morphology, for example, in cell adhesion or motility. IL-4 is a cytokine exerting many effects on B lymphocytes. In this study, we show that stimulation with LPS in combination with IL-4, but not LPS or IL-4 alone, results in a pronounced dendritic morphology of B cells. Using a culture system in which Abs directed to B cell surface markers are immobilized on the tissue culture plastic, we find that cell spreading can be mediated by a variety of Abs, including anti-CD44, -CD23, -LFA-1, -VLA-4, -ICAM-1, and -Ig. B cells stimulated with anti-Ig Abs plus IL-4, or anti-CD40 Abs in the presence or absence of IL-4, are also induced to spread, while IL-2, IL-5, or IL-10 in combination with LPS or alone fail to induce this. Spreading correlates with induction of tight cell aggregation. It is sensitive to cytochalasin B, indicating a requirement for intact actin cytoskeleton. CD44 is selectively detected in the detergent-insoluble fraction of cell lysates prepared from LPS plus IL-4-stimulated B cell cultures after Ab cross-linking of CD44, suggesting a membrane protein-cytoskeleton interaction. Interestingly, electron microscopy studies reveal induction of microvilli-like structures on LPS plus IL-4-stimulated blasts, suggesting that IL-4 can influence cell morphology on an ultra-structural level. In summary, our data show that stimulation with LPS plus IL-4 or ligation of CD40 is capable of inducing dramatic morphologic changes in murine B cells, which correlates with in vitro induction of strong cell adhesion.

Homotypic aggregation of murine B lymphocytes is independent of CD23

CD23 is a low-affinity receptor for IgE (Fc epsilon RII). Functions attributed to CD23 not involving IgE suggest that it interacts with ligands other than IgE. CD21 has recently been described as a counter ligand for CD23. A number of lines of evidence have implicated CD23 as an adhesion molecule in human B cells. We have investigated the role of CD23 in homotypic B cell aggregation in the mouse, using lipopolysaccharide plus interleukin-4-induced aggregation as a model system. In this system high levels of aggregation are accompanied by a massive up-regulation of CD23 expression. However, in contrast to what has been observed in human B cells, we find no evidence of a role for CD23 in homotypic adhesion of murine B cells.

Properties of mouse CD40: the role of homotypic adhesion in the activation of B cells via CD40

Stimulation of human B cells via CD40 is known to induce their homotypic aggregation. We show here that anti-mouse CD40 monoclonal antibodies (mAb) also induce B cells to form large, spherical, extremely stable clusters. This clustering is markedly enhanced by co-stimulation with either interleukin-4 (IL-4) or anti-immunoglobulin (Ig). The aggregation is slow in onset, and is largely (but not completely) abrogated by anti-LFA-1 mAb, but not by mAb directed against other potentially important adhesion molecules on B cells. Anti-LFA-1 mAb also partially suppressed DNA synthesis induced by anti-CD40, but not by other B cell mitogens, suggesting that clustering is an important component of B cell activation via CD40. This concept is supported by analyses of the phenotype of clustered B cells: the cells within clusters express higher levels of various activation markers, and also more of them are in cell cycle than non-clustered cells. These results therefore suggest that CD40 stimulation may either induce B cells to secrete soluble factors which act in an autocrine way to promote B cell activation, or that clustering generates cell contact-mediated signals which are important in the activation cascade.

CD23 and CD21 function as adhesion molecules in homotypic aggregation of human B lymphocytes

We have previously found that interleukin-4 and CD40 monoclonal antibodies (mAb) are strong potentiators of homotypic B cell aggregation which is dependent on LFA-1. We show here that CD23 mAb were also able to inhibit aggregation to a similar extent as LFA-1 antibodies. This inhibition was restricted to the MHM6 epitope of CD23 and antibodies to other epitopes [Epstein-Barr virus (EBV) CS-1, EBV CS-2, EBV CS-5 and mAb 25] or occupation of the Fc-binding site by IgE had no or a slightly enhancing effect on aggregation. When testing two antibodies to CD21, the recently defined ligand for CD23, one of these (BU32) was found to be inhibitory whereas the other (THB5) had no effect. By combining antibodies to LFA-1 and CD23, aggregation was often completely inhibited. These data suggest that LFA-1/ICAM-1 and CD23/CD21 are the major molecules involved in homotypic aggregation of human B cells.

T and B cell collaboration: induction of motility in small, resting B cells by interleukin 4

In this report we investigate if IL 4 can work as a chemoattractant factor by inducing locomotion in B cells. We found that murine recombinant IL 4 (rIL 4) induced motile morphology and migration through polycarbonate micropore filters of murine, splenic B cells at an optimal concentration of 3 ng/ml. Kinetic studies revealed optimal migration at 8-16 h, although a significant response could be detected already after 1 h. Flow cytometric studies confirmed that the migrated cells were indeed B cells. We also compared the activity of small, dense B cells and large, low-density B cells, based on Percoll gradient separation. We found no difference in IL 4-induced motility among the two groups. Furthermore, we looked at B cells activated in vitro by preculture in lipopolysaccharide (LPS) or IL 4. Our data indicate that both LPS and IL 4 can increase the general capacity for motility in B cells after preculture for 24 h. T and B cell collaboration requires close cell-cell contacts in order for T cell help to be administered to the B cell. One way of enhancing such cell contacts could be through directional cell migration induced by helper factors (chemotaxis). We suggest that IL 4 can play a role as a chemoattractant factor that enhances cell contacts between T helper cells and B cells.