Death-receptor contribution to the germinal-center reaction

Both helper T cells and follicular dendritic cells play crucial roles in the germinal-center (GC) reaction. One of their key functions is to provide GC B cells with anti-apoptotic signals during their growth, diversification of antibody repertoire and positive selection. Dysregulation of the mechanisms that control B-cell apoptosis in the GC could cause hyperplasia, endanger self-tolerance or impair dramatically the efficiency of the humoral response. This article discusses how the death receptor Fas and components of its signaling machinery contribute to the GC reaction.

Expression of macrophage inflammatory protein-3alpha, stromal cell-derived factor-1, and B-cell-attracting chemokine-1 identifies the tonsil crypt as an attractive site for B cells

The expression of 3 lymphoid chemokines-macrophage inflammatory protein-3alpha (MIP-3alpha), stromal cell-derived factor-1 (SDF-1), and B-cell-attracting chemokine-1 (BCA-1)-in the tonsil and the possible correlation between their sites of expression and B-cell localization within this tissue were studied. The results show that all 3 chemokines are produced in the crypts but differ by the nature of the cells that produce them and their location within the crypt. SDF-1 and MIP-3alpha are produced by epithelial cells, but their secretion is mutually exclusive. Both MIP-3alpha- and SDF-1-expressing cells are in close contact with memory B cells. By contrast, BCA-1-producing cells in the crypt are not epithelial and form clusters colocalized with plasma cells. Altogether, these data suggest that the chemokines produced in the tonsillar crypt may (1) attract memory B cells to antigen and (2) recruit and retain plasma cells and memory B cells within the supportive epithelial microenvironment of the crypt. (Blood. 2001;97:3992-3994)

Correlation between apoptosis microarray gene expression profiling and histopathological lymph node lesions

AIMS

Microarray technology has recently led to the identification of molecular prognostic subgroups in non-Hodgkin's lymphomas. To determine the usefulness of ready made macroarrays as routine diagnostic tools in haematopathology, lymph node biopsies were analysed using a cDNA macroarray containing genes involved in apoptosis, including caspases.

METHODS

Nine biopsy specimens were analysed using total frozen tissues: four samples of B cell follicular lymphoma, two of B cell diffuse large cell lymphoma, and three of non-neoplastic lymph nodes from benign lymphadenitis. Nine cell populations were sorted from fresh tissues: malignant B cells from two patients with follicular lymphoma and two with diffuse large cell lymphoma, reactive B cells from two benign lymph nodes, reactive T cells from one benign lymph node, and virgin (mantle zone) B cells and germinal centre B cells from benign tonsils. Immunohistochemistry (IHC) on paraffin wax sections was performed for the localisation of caspases 2, 3, 4, 7, 8, and 9.

RESULTS

In the clustered array data, sorted cells from samples sharing common histological lesions were grouped together, whereas the array/histology correlation was less satisfactory for tissues. The expression profiles of both the array and IHC methods correlated for most caspases and samples.

CONCLUSIONS

Variations in array profiles of sorted cell populations can be associated with specific histological features, suggesting a possible diagnostic application of ready made apoptosis macroarrays in haematopathology.

Frequent nuclear localization of ICAD and cytoplasmic co-expression of caspase-8 and caspase-3 in human lymphomas

Lymphoma cells often display in vitro resistance to FAS-induced apoptosis, in which caspases act as crucial cell death effectors. Following FAS stimulation, caspase-8 activates caspase-3, which in turn activates the caspase-activated DNAse (CAD) by proteolysis of its inhibitor (ICAD). To investigate the mechanism of FAS resistance, the expression of caspase-8 was analysed by immunohistochemistry, together with that of the substrates caspase-3 and ICAD, in 52 representative samples from non Hodgkin's lymphoma (NHL), 12 from Hodgkin's disease (HD), and eight benign lymphoid tissues. In benign tissues, caspase-8 was co-expressed with caspase-3 in the cytoplasm in germinal centre (GC) cells and was co-expressed with ICAD in the nuclei of the mantle and marginal zone cells. ICAD expression was weak or absent in GC cells. Cytoplasmic staining for both caspase-8 and caspase-3 was present in 11/12 cases of diffuse large cell B-NHL. Caspase-8 positivity was nuclear and cytoplasmic in 9/9 follicular NHLs, in 5/5 mantle cell NHLs and in 6/6 marginal zone NHLs. Five out of six peripheral T-cell NHLs expressed cytoplasmic caspase-8. Ten out of the 12 HD cases lacked significant cytoplasmic staining for caspase-3 and caspase-8 in the majority of Reed-Sternberg cells. All lymphoma cases exhibited predominant nuclear ICAD positivity. Subcellular fractionation analysis of three lymphoma samples and normal mantle zone cells confirmed that ICAD and caspase-8 were at least partly localized in the nucleus. These results show that the profile of caspase-8 expression is correlated with histological lymphoma subtypes; that caspase-8 is co-expressed with caspase-3 in GC cells and their neoplastic counterparts; that ICAD has an immunohistochemical nuclear localization in vivo; and that caspase-8 and ICAD can be co-expressed in the nuclei of mantle zone and marginal zone cells; their unexpected nuclear localization allows a reappraisal of the biochemical cascade of caspase activation.

Regulation of the Fas death pathway by FLICE-inhibitory protein in primary human B cells

The Fas/Fas ligand (L) system plays an important role in the maintenance of peripheral B cell tolerance and the prevention of misguided T cell help. CD40-derived signals are required to induce Fas expression on virgin B cells and to promote their susceptibility to Fas-mediated apoptosis. In the current study, we have analyzed the early biochemical events occurring upon Fas ligation in CD40L-activated primary human tonsillar B cells with respect to Fas-associated death domain protein (FADD), caspase-8/FADD-like IL-1beta-converting enzyme (FLICE), and c-FLICE inhibitory protein (FLIP). We report here that Fas-induced apoptosis in B cells does not require integrity of the mitochondrial Apaf-1 pathway and that caspase-8 is activated by association with the death-inducing signaling complex (DISC), i.e., upstream of the mitochondria. We show that both FADD and the zymogen form of caspase-8 are constitutively expressed at high levels in virgin B cells, whereas c-FLIP expression is marginal. In contrast, c-FLIP, but neither FADD nor procaspase-8, is strongly up-regulated upon ligation of CD40 or the B cell receptor on virgin B cells. Finally, we have found that c-FLIP is also recruited and cleaved at the level of the DISC in CD40L-activated virgin B cells. We propose that c-FLIP expression delays the onset of apoptosis in Fas-sensitive B cells. The transient protection afforded by c-FLIP could offer an ultimate safeguard mechanism against inappropriate cell death or allow recruitment of phagocytes to ensure efficient removal of apoptotic cells.

Mitochondria connects the antigen receptor to effector caspases during B cell receptor-induced apoptosis in normal human B cells

We have previously reported that CD40 stimulation sensitizes human memory B cells to undergo apoptosis upon subsequent B cell receptor (BCR) ligation. We have proposed that activation stimuli connect the BCR to an apoptotic pathway in mature B cells and that BCR-induced apoptosis of activated B cells could serve a similar function as activation-induced cell death in the mature T cell compartment. Although it has been reported that caspases are activated during this process, the early molecular events that link the Ag receptor to these apoptosis effectors are largely unknown. In this study, we report that acquisition of susceptibility to BCR-induced apoptosis requires entry of memory B cells into the S phase of the cell cycle. We also show that transduction of the death signal via the BCR sequentially proceeds through a caspase-independent and a caspase-dependent phase, which take place upstream and downstream of the mitochondria, respectively. Furthermore, our data indicate that the BCR-induced alterations of the mitochondrial functions are involved in activation of the caspase cascade. We have found both caspases-3 and -9, but not caspase-8, to be involved in the BCR apoptotic pathway, thus supporting the notion that initiation of the caspase cascade could be under the control of the caspase-9/Apaf-1/cytochrome c multimolecular complex. Altogether, our findings establish the mitochondria as the connection point through which the Ag receptor can trigger the executioners of apoptotic cell death in mature B lymphocytes.

Activation sensitizes human memory B cells to B-cell receptor-induced apoptosis

The outcome of antigen receptor (B-cell receptor; BCR) ligation on B-cell survival can be influenced by multiple parameters. They are linked to the physical properties of the antigen itself, the maturational stage of the cells and the costimuli provided by different components of the innate and acquired immunity. Here we report that apoptosis prevails over stimulation when a BCR agonist is applied to human memory B cells which have been preactivated by CD40 ligand or anti-immunoglobulin antibodies. The susceptibility of activated memory B cells to BCR-induced killing is correlated with their enhanced expression of the transcripts encoding the pro-apoptotic molecules Bax, c-Myc and p53. The BCR-mediated apoptosis of activated memory B cells does not require extensive cross-linking of the antigen receptors and relies neither on engagement of the FcgammaRII nor on the Fas/Fas ligand (Fas-L) system. Our findings suggest that activation stimuli open the BCR-induced apoptotic pathway in memory B cells. Therefore we propose that the concept of activation-induced cell death (AICD), originally described for T cells, also applies to mature B lymphocytes. The functions fulfilled by the AICD of mature B cells in the regulation of B-cell responses are discussed.

Modulation and functional involvement of CB2 peripheral cannabinoid receptors during B-cell differentiation

Two subtypes of G-protein-coupled cannabinoid receptors have been identified to date: the CB1 central receptor subtype, which is mainly expressed in the brain, and the CB2 peripheral receptor subtype, which appears particularly abundant in the immune system. We investigated the expression of CB2 receptors in leukocytes using anti-CB2 receptor immunopurified polyclonal antibodies. We showed that peripheral blood and tonsillar B cells were the leukocyte subsets expressing the highest amount of CB2 receptor proteins. Dual-color confocal microscopy performed on tonsillar tissues showed a marked expression of CB2 receptors in mantle zones of secondary follicles, whereas germinal centers (GC) were weakly stained, suggesting a modulation of this receptor during the differentiation stages from virgin B lymphocytes to memory B cells. Indeed, we showed a clear downregulation of CB2 receptor expression during B-cell differentiation both at transcript and protein levels. The lowest expression was observed in GC proliferating centroblasts. Furthermore, we investigated the effect of the cannabinoid agonist CP55,940 on the CD40-mediated proliferation of both virgin and GC B-cell subsets. We found that CP55,940 enhanced the proliferation of both subsets and that this enhancement was blocked by the CB2 receptor antagonist SR 144528 but not by the CB1 receptor antagonist SR 141716. Finally, we observed that CB2 receptors were dramatically upregulated in both B-cell subsets during the first 24 hours of CD40-mediated activation. These data strongly support an involvement of CB2 receptors during B-cell differentiation.

Triggering of CD40 antigen inhibits fludarabine-induced apoptosis in B chronic lymphocytic leukemia cells

We analyzed the effect of CD40 triggering on the fludarabine-induced apoptosis of B chronic lymphocytic leukemia (B-CLL) cells. Peripheral blood samples obtained from 15 patients were incubated with fludarabine in the absence or the presence of the anti-CD40 monoclonal antibody (MoAb) G28-5. In 12 patients a significant proportion of apoptotic cells, ranging from 22% to 38% (mean +/- SE: 28.5 +/- 1.6), were detected after 3 days of culture. In 9 of these samples, the addition of G28-5 reduced apoptosis by at least 30.1% and by 57.1% +/- 7.8% on average (P = .0077). Because the CD40 antigen activates NF-kappaB/Rel transcription factors in B cells, and NF-kappaB/Rel complexes can inhibit cell apoptosis, we investigated whether the antiapoptotic effect of G28-5, in our system, could be related to modulation of NF-kappaB/Rel activity. As expected, B-CLL cells displayed significant levels of nuclear NF-kappaB/Rel activity; p50, RelA, and c-Rel components of the NF-kappaB/Rel protein family were identified in these complexes. After exposure to fludarabine, NF-kappaB/Rel complexes were decreased in the nuclei. The addition of G28-5 upregulated the NF-kappaB/Rel levels. To determine the involvement of NF-kappaB/Rel activity in the G28-5-mediated inhibition of apoptosis, we blocked the transcription factor with a decoy oligonucleotide, corresponding to the NF-kappaB/Rel consensus sequence. Cells incubated with the anti-CD40 MoAb in the presence of the decoy oligonucleotide but not a control oligonucleotide displayed a complete impairment of the G28-5 antiapoptotic effect, indicating that NF-kappaB/Rel activity was required for the inhibition of apoptosis. These results suggest that CD40 triggering in vivo could counteract the apoptotic effect of fludarabine on B-CLL cells and that its neutralization, or the use of NF-kappaB/Rel inhibitors, could improve the therapeutic effect of fludarabine.

A role for the VLA-4 integrin in the activation of human memory B cells

It is generally recognized that activation through membrane effector molecules such as CD40 or the B cell receptor (BCR) is mandatory to allow B cells to proliferate and differentiate into antibody (Ab)-secreting cells in response to cytokines. We show here that purified tonsillar B cells can be stimulated directly by a cytokine combination to proliferate and secrete immunoglobulins when cultures are performed at high cell density. The contact-mediated activation of B cells in this experimental system is strongly inhibited both by anti-very late antigen (VLA)-4 monoclonal Ab and by a peptide containing the LDV sequence specifically recognized by the alpha 4 integrin binding site. These reagents also significantly suppressed the B cell responses elicited by engagement of the BCR or CD40. Our data reveal that memory B cells but not virgin or germinal center B cells are sensitive to the direct stimulatory effect of cytokines in high-density cultures. Finally, we found that the dual expression of the alpha and beta chains of VLA-4 is a distinctive feature of the memory B cell population. Collectively, our findings support the notion that VLA-4-dependent homotypic B cell interactions can mediate a co-stimulatory signal to human memory B cells and might participate in the B cell activation triggered through the BCR and CD40.

Measles virus nucleocapsid protein binds to FcgammaRII and inhibits human B cell antibody production

Despite the development of an efficient specific immune response during measles virus (MV) infection, an immunosuppression occurs contributing to secondary infections. To study the role of nucleocapsid protein (NP) in MV-induced immunosuppression, we produced recombinant MV NP. Purified recombinant NP exhibited biochemical, antigenic, and tridimensional structure similar to viral NP. By flow cytometry, we showed that viral or recombinant NP bound to human and murine B lymphocytes, but not to T lymphocytes. This binding was specific, independent of MHC class II expression, and dependent of the B lymphocyte activation state. The murine IIA1. 6 B cell line, deficient in the Fc receptor for IgG (FcgammaRII) expression, did not bind NP efficiently. Transfected IIA1.6 cells expressing either murine FcgammaRIIb1 or b2, or human FcgammaRIIa, b1*, or b2 isoforms efficiently bound NP. Furthermore, this binding was inhibited up to 90% by monoclonal antibodies 2.4G2 or KB61 specific for murine and human FcgammaRII, respectively. Finally, the in vitro Ig synthesis of CD40- or Ig-activated human B lymphocytes in the presence of interleukin (IL)-2 and IL-10 was reduced by 50% in the presence of recombinant NP. These data demonstrate that MV NP binds to human and murine FcgammaRII and inhibits in vitro antibody production, and therefore suggests a role for NP in MV-induced immunosuppression.

Antigen receptor-induced apoptosis of human germinal center B cells is targeted to a centrocytic subset

The outcome of the signals transduced through the B cell antigen receptor (BCR) depends both on their maturational stage and on the extent of receptor cross-linking. It is established that the BCR-mediated apoptosis of immature B cells represents an important mechanism for tolerance induction in the pre-immune B cell compartment. We show here that mature germinal center (GC) B cells can re-acquire sensitivity to BCR-induced cell death following CD40 ligation. In contrast, neither virgin nor memory B cells become susceptible to antigen receptor-triggered apoptosis upon CD40 stimulation, suggesting that this phenomenon may play a role in the shaping of the mature B cell repertoire in GC. Our data reveal that the death signal evoked through the BCR does not involve the Fcgamma receptors, does not operate through the Fas/Fas ligand system, and can be blocked by interleukin-4. Finally, we found that the acquisition of sensitivity to the death-promoting effect of anti-Ig antibodies in CD40-stimulated GC B cell cultures correlates with the induction of a centrocytic phenotype. We propose that negative regulatory signals via the BCR may delete somatically mutated centrocytes with self-reactivity.

Identification of a tonsil IgD+ B cell subset with phenotypical and functional characteristics of germinal center B cells

We have identified and isolated a subpopulation of IgD+ B cells (IgD+CD38+ B cells) from human tonsils which expresses the germinal center (GC)-associated surface markers CD10, CD38, CD75, CD77 and Cd95/Fas. The heterogeneity of expression of several markers on IgD+ CD38+ B cells suggests that this population can be further subdivided into two discrete subtypes. On a functional basis, IgD+ CD38+ B cells behave as GC B cells as they rapidly and spontaneously undergo apoptosis in vitro and cannot be stimulated to synthesize DNA upon cross-linking of the antigen receptor. However, in contrast with most GC B cells, IgD+ CD38+ B cells have not completed Ig class switching since they predominantly secrete IgM following stimulation in vitro and lack surface expression of secondary isotypes. Immunoenzymatic staining performed on tonsil tissue sections revealed that IgD+ CD38+ B cells are located in two distinct histological structures: within the GC of a few classical secondary follicles, in which they appear as scattered cells, and within rare atypical GC, homogeneously constituted of IgD+ B cells. Taken together, our findings indicate that IgD+ CD38+ B cells constitute a novel subset of GC B cells. The possibility that these cells could represent an early stage of the follicular reaction or be generated in response to certain bacterial carbohydrate antigens is discussed.

Concurrent engagement of CD40 and the antigen receptor protects naive and memory human B cells from APO-1/Fas-mediated apoptosis

Naive and memory B cells were isolated from human tonsils and examined for expression of APO-1/Fas and for their sensitivity to the APO-1-dependent apoptosis. APO-1 was found to be constitutively expressed on memory but not on naive B cells. The susceptibility of both cell types to the APO-1 apoptotic pathway was acquired upon CD40 triggering and was correlated with increased expression of the APO-1 receptor. Both naive and memory B cells were protected from the APO-1-mediated death signal after dual ligation of the Ag receptor adn CD40. Our findings suggest that the APO-1 pathway controls the specificity of B cell responses to T-dependent Ags and that occupancy of the Ag receptor dictates the outcome of APO-1-ligation on B cell survival.

The differentiation of human memory B cells into specific antibody-secreting cells is CD40 independent

It is generally accepted that memory B cells can be defined by their ability to produce, upon antigenic challenge, somatically mutated antibody molecules characterized by an increased affinity and by the expression of a downstream heavy chain isotype. However, the inability to isolate this particular B cell compartment has precluded the study of memory B lymphocyte physiology in man. We previously reported on the identification of an IgD- B cell subset in human tonsils that we defined as CD38- B cells, whose phenotype is highly reminiscent of that of memory B lymphocytes from the splenic marginal zone of rodents. In the present study, we developed a model of the measles virus (MV)-specific secondary antibody response in vitro to assess the presence of memory B lymphocytes in different B cell subsets isolated from human tonsils and explore the activation requirements of human memory B cells. Our findings show that the memory B cell pool resides in the CD38- B cell subpopulation and that the differentiation of MV-activated memory B cells into antibody-secreting cells can be achieved upon co-stimulation with interleukin (IL)-2 and IL-10, but does not require engagement of CD40. Interestingly, the CD40-mediated signal was found to synergize with Ig-cross-linking agents for the proliferation of memory B cells, but strongly suppressed their capacity to differentiate along the plasmacytoid pathway. Collectively, our results suggest that the CD40 signaling pathway is instrumental for the clonal expansion of the memory B cell pool, but does not operate in the later phase of the response, which allows their maturation into antibody-secreting cells.

Regulation of germinal center B cell differentiation. Role of the human APO-1/Fas (CD95) molecule

We previously described the existence of a tonsillar IgD- B cell subset with memory B cell features. To test the possibility that these cells could derive from germinal center (GC) B cell precursors, we examined the proliferation, differentiation, and phenotype of GC B cells after culturing with either anti-CD40 Abs or activated T cells, presumably mimicking the signals received by centrocytes in the light zone of GC. We show in this work that GC B cells proliferate and secrete Igs in both activation systems, thus indicating that CD40 ligation is also required for differentiation of GC B cells along the plasmacytoid pathway. T cell-dependent activation of GC B cells induced down-regulation of most GC-related markers (CD10, CD38, and CD77) and up-regulation of CD44 and CD62-L which are both expressed on the putative memory B cells subset. Moreover, T cell-mediated stimulation of GC B cells resulted in the strong induction of CD5 and up-regulation of APO-1/Fas (CD95). In contrast, stimulation performed with immobilized anti-CD40 Abs did not affect expression of CD10 and CD38 and failed to induce CD62-L and CD5, suggesting that the CD40 signaling pathway is necessary but not sufficient for the development of memory B cells. CD95 ligation on GC B cells was found to antagonize the stimulatory effect of immobilized anti-CD40 Abs on their proliferation, survival, and Bcl-2 expression. The possible role of CD95 in the expansion and selection of the Ag-activated B cell clones in GC is discussed.

Regulation of germinal center B cell differentiation. Role of the human APO-1/Fas (CD95) molecule

We previously described the existence of a tonsillar IgD- B cell subset with memory B cell features. To test the possibility that these cells could derive from germinal center (GC) B cell precursors, we examined the proliferation, differentiation, and phenotype of GC B cells after culturing with either anti-CD40 Abs or activated T cells, presumably mimicking the signals received by centrocytes in the light zone of GC. We show in this work that GC B cells proliferate and secrete Igs in both activation systems, thus indicating that CD40 ligation is also required for differentiation of GC B cells along the plasmacytoid pathway. T cell-dependent activation of GC B cells induced down-regulation of most GC-related markers (CD10, CD38, and CD77) and up-regulation of CD44 and CD62-L which are both expressed on the putative memory B cells subset. Moreover, T cell-mediated stimulation of GC B cells resulted in the strong induction of CD5 and up-regulation of APO-1/Fas (CD95). In contrast, stimulation performed with immobilized anti-CD40 Abs did not affect expression of CD10 and CD38 and failed to induce CD62-L and CD5, suggesting that the CD40 signaling pathway is necessary but not sufficient for the development of memory B cells. CD95 ligation on GC B cells was found to antagonize the stimulatory effect of immobilized anti-CD40 Abs on their proliferation, survival, and Bcl-2 expression. The possible role of CD95 in the expansion and selection of the Ag-activated B cell clones in GC is discussed.

The vitronectin receptor (alpha v beta 3) is implicated, in cooperation with P-selectin and platelet-activating factor, in the adhesion of monocytes to activated endothelial cells

In this study we have investigated the presence on endothelial cells of potential glycoprotein receptors, other than P-selectin, which are involved in the adhesion of monocytes at the early stages of activation. We report that the majority of cells binding to thrombin-activated endothelial cells from a peripheral blood mononuclear cell (PBMC) preparation are monocytes. The adhesion of PBMC to thrombin-activated, but not resting, endothelial cells was inhibited (66%) by a monoclonal antibody (mAb) directed against alpha v beta 3. Elutriated monocytes or a monocytic cell line (U937) were also inhibited by this antibody, its F(ab)'2 fragments and three other anti-(alpha v beta 3) mAbs. alpha v beta 3 isolated from endothelial-cell lysates significantly inhibited the adhesion of monocytes and U937 cells to endothelial cells. A peptide motif (RGDF) known to interact with alpha v beta 3 inhibited U937 cell adhesion to activated endothelial cells by 53%. Finally, an anti-(P-selectin) mAb (LYP20) or a platelet-activating factor (PAF)-receptor antagonist (WEB 2086) inhibited monocyte adhesion to activated endothelial cells. This study shows for the first time that alpha v beta 3 is implicated, in addition to P-selectin and PAF, in the adhesion of monocytes to activated endothelial cells.

Moabs MAS516 and 5B5, two fibroblast markers, recognize human follicular dendritic cells

Follicular dendritic cells (FDC) are only located within follicles of secondary lymphoid tissues. The origin of this peculiar cell type is not clearly defined. To contribute to this study, we applied two monoclonal antibodies (MAS516 and 5B5) considered as specific for fibroblasts to tonsil cryosections and to isolated follicular dendritic cells. On the basis of an enzyme cocktail digestion of human tonsils and a fractionation procedure on albumin gradients, FDC can be prepared in the form of cell aggregates with associated lymphoid cells. MAS516 reacts with surface membrane molecules expressed by human fibroblasts, tissue macrophages and peripheral blood monocytes. With immunoperoxidase assays on tonsil cryosections connective tissue cells and macrophages are stained. Inside germinal centres, heavy labelling of the light zone was found. The MAS516 staining pattern is very similar to that of specific FDC markers DRC-1 or BU10. All isolated FDC reacted with MAS516 antibody. 5B5, considered as a typical fibroblast marker, reacts with human prolyl-4-hydroxylase which is an intracellular enzyme related to collagen biosynthesis. In cryosections, interfollicular and capsular areas showed 5B5 positive connective tissue fibroblasts. In germinal centres, some cells presenting features of FDC were 5B5 positive. After cell separation, 25%-50% of the isolated FDC were labelled with this antibody. This positivity of some FDC for 5B5 antibody may support the idea of their fibroblastic origin. The combination of observations realized in situ and after cell purification ensured an unequivocal recognition and identification of FDC.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin 13 is a B cell stimulating factor

The recently cloned human interleukin 13 (IL-13) is a novel cytokine expressed in activated T cells that has been shown to inhibit inflammatory cytokine production by lipopolysaccharide-activated monocytes. The protein encoded by the IL-13 cDNA is the human homologue of a mouse Th2-product called P600. Here, we show that IL-13 acts at different stages of the B cell maturation pathway: (a) it enhances the expression of CD23/Fc epsilon RII and class II MHC antigens on resting B cells; (b) it stimulates B cell proliferation in combination with anti-Ig and anti-CD40 antibodies; and (c) it induces IgE synthesis. Thus, the spectrum of the biological activities of IL-13 on B cells largely overlaps that previously ascribed to IL-4. The present observations suggest that IL-13 may be an important factor, in addition to IL-4, in the development of allergic diseases.

Phenotypic and functional heterogeneity of the IgD- B cell compartment: identification of two major tonsillar B cell subsets

Two major B cell subpopulations were identified in the IgD- compartment of tonsils and subsequently isolated. They displayed the following phenotypes: CD10+CD38+CD44- (CD38+ B cells) and CD10-CD38-CD44+ (CD38- B cells). Of the CD38- B cells, 70% also expressed CD24 and CD39, whereas CD77 was specifically distributed on 40% of CD38+ B cells, suggesting an additional level of heterogeneity in the cellular composition of these two B cell types. Whereas the majority of CD38+ B cells were in cycle, most CD38- B cells were quiescent. Conversely, Bcl-2 was expressed in CD38- B cells but was not detected in CD38+ B cells. Of the CD38- B cells, 30% bore the homing receptor Leu-8/Mel-14, whereas CD38+ B cells lacked this marker. Thus, CD38- B cells have both survival capacity and migratory competence. Both subsets expressed surface (s) Igs which were mainly of the IgG class, implying that most of these cells have already undergone isotype switching. CD38- B cells proliferated vigorously and produced large amounts of IgG in response to cytokines, following ligation of slgs or CD40. In contrast, CD38+ B cells were only stimulated for DNA synthesis by a combination of IL-4 and anti-CD40 antibodies, and failed to differentiate into Ig-secreting cells regardless of the stimulus applied. We propose that CD38- B cells represent an extra-follicular mature B cell population which has been positively selected and rescued from apoptosis, whereas the CD38+ B cell subset is composed of germinal centre B cells.

Responsiveness of chronic lymphocytic leukemia B cells activated via surface Igs or CD40 to B-cell tropic factors

Recent studies performed in the laboratory have established that interleukin-4 (IL-4) used in combination with anti-CD40 monoclonal antibody (MoAb) 89 presented on Ltk- mouse fibroblasts stably expressing human Fc gamma RII/CDw32 (referred to as the CD40 system) sustains long-term proliferation of normal human B cells. In the present study, B-cell chronic lymphocytic leukemias (B-CLLs) activated through slgs or CD40 were examined for their capacity to proliferate and differentiate in response to various cytokines. Our results indicate that the outcome of IL-4 stimulation on the in vitro growth of B-CLL depends on the signalling pathway used for their activation. Whereas IL-4 did not display any growth-stimulatory effect on B-CLL activated by Ig cross-linking agents, it could stimulate DNA synthesis and enhance the viable cell recovery when leukemic B cells were cultured in the CD40 system. Most B-CLL samples were induced for IgM synthesis upon Staphylococcus aureus strain Cowan I stimulation. This Ig response was potentiated by IL-2 and antagonized by IL-4. Anti-CD40 MoAb used alone or in combination with cytokines (IL-1 alpha to IL-6, interferon gamma, tumor necrosis factor gamma, and transforming growth factor beta) failed to induce Ig secretion from B-CLL cells. No evidence for Ig isotype switching was obtained with the cytokines listed above, regardless of the mode of activation. Taken together, our results suggest that B-CLL cells can be partially released from their apparent maturation block by IL-2 and Ig cross-linking agents. In contrast, combinations of IL-4 and cross-linked anti-CD40 antibodies induced entry of B-CLL cell into cycle, but poorly stimulated their differentiation into Ig secreting cells.

Proliferation and differentiation of human CD5+ and CD5- B cell subsets activated through their antigen receptors or CD40 antigens

The pan-T cell antigen CD5 has been shown to delineate two different mouse B cell subsets, originating from distinct progenitors. In man, on average, 30% of the tonsillar B cell pool expresses this antigen. In the present report, a detailed comparison of the CD5+ and CD5- B cell response to cytokines, following activation via surface immunoglobulins (sIg) or CD40 antigen, was undertaken. CD5+ B cells were positively selected by panning or by sorting from tonsils. Two-color immunofluorescence analysis performed on tonsillar B cell populations showed that CD5+ B cells displayed most of the phenotypic features of mantle zone B cells. CD5+ B cells could be stimulated for DNA synthesis by mitogenic concentrations of Staphylococcus aureus, Cowan I strain (SAC), insolubilized anti-IgM antibodies, immobilized anti-CD40 antibodies and phorbol 12-myristate 13-acetate (PMA). The growth-response of small dense CD5- B cells to these T cell-independent mitogens was comparable to that of CD5+ B cells, whereas the low-density, in vivo-activated, CD5- B cells were only marginally stimulated by Ig-cross-linking agents and PMA. Following ligation of sIg, both B cell subsets proliferated essentially in response to interleukin (IL)-2 and IL-4. When used in co-stimulation with immobilized anti-CD40 antibodies, IL-4 promoted growth of CD5+ and CD5- B cells, whereas IL-2 displayed only moderate stimulatory effects. CD5+ and CD5- B cells differentiated into Ig-secreting cells when they were co-cultured with SAC or cross-linked anti-CD40 antibodies and IL-2. However, IgM constituted the major component of the Ig response of CD5+ B cells, whereas high levels of IgG were secreted by CD5- B cells.

Interleukin 10 is a potent growth and differentiation factor for activated human B lymphocytes

Interleukin 10 (IL-10), originally identified as a TH2 helper T-cell product able to inhibit cytokine production by TH1 cells, is highly homologous to BCRF1 (viral IL-10), an open reading frame in the Epstein-Barr virus genome. Here, we show that human and viral IL-10 stimulate DNA replication of B lymphocytes activated either via their antigen receptor or via their CD40 antigen. IL-4 and IL-10 display additive effects and induce a strong increase in the number of viable cells. Moreover, IL-10 induces activated B cells to secrete large amounts of IgG, IgA, and IgM, and the combination of IL-10 and IL-4 results in the secretion of the four immunoglobulin isotypes. Thus, IL-10 may play an important role in the amplification of humoral responses.

Interleukin 10 and transforming growth factor beta cooperate to induce anti-CD40-activated naive human B cells to secrete immunoglobulin A

In the present report, we have investigated the in vitro differentiation of surface(s) sIgD+ and sIgD- human B cells into Ig- secreting cells in response to various stimuli. sIgD+ B cells homogeneously expressed some of the antigens identifying mantle zone B cells, but lacked expression of germinal center markers, thus confirming that the B cell populations positively selected on the basis of sIgD expression were highly enriched for naive B lymphocytes. Conversely, sIgD- B cells expressed some of the antigens specifically associated with germinal center B cells. T cell-independent differentiation of sIgD+ and sIgD- B cells could be achieved by simultaneous crosslinking of sIgs and CD40 in the presence of a mouse Ltk- cell line stably expressing human CDw32/Fc gamma RII (CDw32 L cells). In this experimental system, sIgD+ B cells were exclusively proned for IgM synthesis, whereas sIgD- B cells produced IgG, IgM, and IgA. Both the human and viral forms of interleukin 10 (IL-10) strongly increased the Ig secretion by sIgD+ and sIgD- B cells simultaneously activated through sIgs and CD40. IgM and IgG constituted the predominant Ig isotype produced by sIgD+ and sIgD- B cells, respectively, in response to IL-10. sIgD+ B cells could be induced for IgA synthesis upon co-culturing with transforming growth factor beta (TGF-beta) and IL-10, in the presence of an anti-CD40 monoclonal antibody presented by the CDw32 L cells. In contrast, TGF-beta suppressed the IL-10-mediated IgG, IgM, and IgA secretions by sIgD- B cells. sIgD+ B cells could not be induced for IgA synthesis by TGF-beta and IL-10 after crosslinking of their sIgs, suggesting that ligation of CD40 was one of the obligatory signals required for commitment of naive B cells to IgA secretion. Limiting dilution experiments indicated that the IgA-potentiating effect of TGF-beta was due to its capacity to increase the frequency of IgA-producing cells, most likely as a consequence of class switching. Taken together, our data strongly suggest that TGF-beta is involved in the regulation of IgA isotype selection in humans.

Antiproliferative effects of interleukin-4 on freshly isolated non-Hodgkin malignant B-lymphoma cells

The pattern of in vitro growth response of freshly isolated non-Hodgkin malignant lymphoma B cells (NHML) to cytokines was investigated. Ten tumor specimens of low- or intermediate-grade malignancy were selected for study. To assess their proliferative capacity in vitro, B-lymphoma cells were activated through ligation of their surface Ig receptor with insolubilized anti-IgM antibodies or Staphylococcus aureus strain Cowan I (SAC). In the great majority of cases, interleukin-2 (IL-2) was the sole factor that significantly and reproducibly stimulated DNA synthesis in NHML activated through their surface Igs. Other B-cell tropic factors, including IL-4, IL-5, IL-6, and tumor necrosis factor-alpha (TNF-alpha), failed to elicit a growth response in most of the IL-2-responsive neoplastic samples. However, one specimen among 10 exhibited the opposite pattern of response and proliferated following culture with IL-4 and anti-Ig reagents, but not after IL-2 stimulation. Three specimens could also be induced for DNA synthesis on cross-linking of their surface Igs in the absence of exogenous growth factors. Although IL-4 could not support the in vitro growth of the majority of NHML cases, it strongly suppressed the proliferative signals delivered to these cells by anti-Ig reagents used alone or in combination with IL-2. Our data suggest that, in most cases, IL-4 essentially provides growth-inhibitory signals to NHML when they are activated through their surface Ig receptors and as such may be considered to be a valid candidate for future therapy of this type of mature B-cell malignancy.

Growth factor requirements for the stimulation of germinal center B cells: evidence for an IL-2-dependent pathway of development

Germinal center (GC) B cells readily undergo apoptosis, a tendency which can be suppressed in vitro by immobilized anti-Ig; mAb to CD40 and soluble CD23 (in synergy with IL-1 alpha) also effect rescue of GC cells from programmed cell death. In the present study, the signals which stimulate rescued GC populations to DNA synthesis have been examined and compared to those established for the activation of follicular mantle (FM) B cells. On co-culture with anti-Ig, optimal responses in FM B cells can be achieved with a combination of IL-4 and CD40 antibody; these activities also provided a modest stimulus to GC cells but, for this population, anti-Ig was ineffective at augmenting the response further. Stimulations of GC B cells were enhanced, however, when performed on a support of primary fetal lung fibroblasts; a major influence of stroma was to promote, by direct cell-cell contact, the CD40-dependent survival of GC B cells. FM B cells were relatively independent of such stromal support. In marked contrast to FM cells, GC B cells were found to respond by enhanced DNA synthesis to IL-2 even when quite low concentrations of the factor were present (IC50 = 2 U/ml). Stimulation of GC cells via this pathway was augmented almost 2-fold on the inclusion of anti-Ig whereas neither fibroblasts, IL-4, nor CD40 antibody made any additional contribution to the IL-2-dependent response. The requirements found for stimulating GC cells in vitro are discussed with reference to the signals that this population may encounter in appropriate microenvironments in vivo: the variety of options apparently available could reflect changing priorities at different stages of a developing GC response.

Activation of human B lymphocytes through CD40 and interleukin 4

We have produced and characterized a new CD40 monoclonal antibody, mAb 89, which in the presence of anti-IgM antibodies co-stimulates to induce B cell proliferation. mAb 89 activates resting B cells as shown by an increase in cell volume and an enhanced subsequent proliferation of B cells in response to anti-IgM antibody. However, mAb 89 does not prepare B cells to respond to the growth-promoting activity of interleukin (IL) 2 or IL 4. Unlike IL 2 and IL 4, mAb 89 only weakly stimulates the proliferation of anti-IgM pre-activated B cells. Thus, the activating properties of anti-CD40 are likely to explain its co-stimulatory effect on B cells. Interestingly, the anti-CD40 mAb 89 was found to act in synergy with IL 4, but not with IL 2, in co-stimulation and restimulation assays. In this respect, anti-CD40 does not induce a significant increase of B cell surface IL 4 receptors while IL 4, but not IL 2, induces a twofold increase of the CD40 antigen expression. Thus the synergistic interaction between IL 4 and anti-CD40 may be related to the IL 4-dependent increase of CD40 antigen expression.

Analysis of the B-cell growth-promoting activity of human IL-4, the co-stimulatory assay with anti-immunoglobulin antibodies. Comparison with the B-cell growth-promoting activity of other lymphokines

Human recombinant interleukin-4 (rIL-4) was assessed for its ability to promote the proliferative response of purified human B cells co-stimulated with submitogenic concentrations of soluble F(ab')2 fragments of anti-immunoglobulin (Ig) antibodies. The growth-promoting activity of rIL-4 was usually as potent as, or even more potent than, that of recombinant interleukin-2 (rIL-2), and more potent than that of recombinant interferon-gamma (rIFN-gamma). Preincubation with rIL-4 did not cause enhancement of the proliferative response of B cells to the subsequent addition of rIL-4 and anti-IgM antibody. In contrast, the proliferative response of B cells preincubated with anti-IgM antibody and rIL-4 was potentiated by the subsequent addition of rIL-4. The simultaneous addition of rIFN-gamma and rIL-2 or rIFN-gamma and rIL-4 had an additive effect in comparison with the response induced by rIL-2 or rIL-4 alone, respectively, whereas simultaneous addition of rIL-2 and rIL-4 induced a response equal or lower than that stimulated by rIL-2 or rIL-4 alone. The addition of rIFN-gamma at the beginning of culture or preincubation of B cells with rIFN-gamma and anti-IgM antibody potentiated the proliferative response of B cells to the subsequent addition of either rIL-2 or rIL-4. Taken together, these data suggest that rIL-4 acts as a growth factor for activated human B cells and displays on such cells a growth-promoting activity similar to that of rIL-2.

Interleukin (IL) 4 counteracts the helper effect of IL2 on antigen-activated human B cells

We tested the effect of interleukin (IL) 4 on the specific IgM antibody response induced by trinitrophenylated-polyacrylamide beads (TNP-PAA) in cultures of human B cells. T cell help was provided by exogeneous IL2. IL4 profoundly suppressed the response to optimal concentrations (50 U/ml) of IL2, with a 50% inhibitory concentration of 6 U/ml. This was due neither to a shift in the kinetics nor to a switch to an IgG response. The production of anti-TNP antibody (as measured by an enzyme-linked immunosorbent assay in the culture supernatant) was inhibited to the same extent as the generation of plaque-forming cells. The effect of IL4 was completely abolished by a neutralizing antibody toward IL4. Kinetic studies showed that IL4 had to be present during the first 48 h of culture to fully inhibit the response. The sequential stimulation of B cells by antigen and by IL2 showed that IL4 does not negatively interfere with signaling through membrane Ig but counteracts the effect of IL2 on antigen-activated B cells.

Human interleukin 4 down-regulates the surface expression of CD5 on normal and leukemic B cells

CD5 is a pan-T cell antigen which is expressed on a minor subset of normal B lymphocytes and on most chronic lymphocytic leukemia B cells (B-CLL). In this study, it is demonstrated that interleukin (IL) 4 down-regulates the surface expression of CD5 on tonsil B cells and B-CLL cells, but not on T cells. IL 4 inhibits both the spontaneous and the phorbol myristate acetate (PMA)-induced hyperexpression of CD5 on tonsil B cells. In contrast, IL 4 only suppresses the PMA-induced hyperexpression of CD5 on B-CLL, whereas the spontaneous CD5 expression is essentially unaffected. The IL 4 concentrations required to down-regulate CD5 are the same as those required to up-regulate CD23. The IL 4-mediated down-regulation of CD5 is an intrinsic property of this IL, since an anti-IL 4-neutralizing antiserum blocks this effect. Interferon (IFN)-gamma, which inhibits the IL 4 induced CD23 expression, does not block the IL 4-induced down-regulation of CD5. Recombinant IL 1 alpha, IL 2, IL 3, IL 5, IL 6, tumor necrosis factor (TNF)-alpha, IFN-gamma and granulocyte macrophage colony-stimulating factor neither inhibit nor stimulate CD5 expression on B cells, therefore suggesting that the capacity to down-regulate CD5 expression on B cells is specific to IL 4.

Interleukin 4 inhibits the proliferation but not the differentiation of activated human B cells in response to interleukin 2

The combined effect of IL-4 and IL-2 on proliferation of anti-IgM antibody or Staphylococcus aureus strain Cowan I (SAC)-preactivated B cells was investigated. It was observed that in most cases, rIL-2 used at optimal concentration induced higher levels of tritiated thymidine ([3H]TdR) uptake than rIL-4 used at optimal concentration. When rIL-4 and rIL-2 were added together, it was repeatedly found that B cell proliferation induced by rIL-2 was significantly reduced and was, in most cases, comparable with the proliferation induced by rIL-4 alone. Cell cycle studies demonstrated that rIL-4 significantly reduced the number of cells entering S and G2/M phases of the cell cycle upon rIL-2 stimulation. B cell blasts preincubated for 24 or 48 h with rIL-4 displayed a reduced proliferation in response to rIL-2. In contrast, preculture of resting B cells with rIL-4 did not impair their subsequent proliferation in response to rIL-2 plus insolubilized anti-IgM antibody. This suggests that rIL-4 can only exert its inhibitory effect once B cells have received an activation signal. The differentiative activity of rIL-2 measured on B cell blasts preactivated for 2 d with SAC was not altered by rIL-4, which suggests that rIL-4 did not exert its inhibitory activity on rIL-2-induced B cell proliferation by enhancing rIL-2-mediated differentiation. Delayed addition of a neutralizing anti-IL-4 antiserum demonstrated that a period of contact of at least 24 h between IL-4 and B cell blasts was necessary for the development of the antagonistic effect of IL-4 on IL-2-mediated growth of activated B cells. These data demonstrate that IL-4 antagonizes the B cell growth-promoting effect of IL-2 without affecting the differentiation of preactivated B cells in response to IL-2.

Human recombinant IL-4 induces activated B lymphocytes to produce IgG and IgM

In this report, we describe a novel biologic activity of IL-4 namely, its ability to induce activated human B cells to produce IgM. Staphylococcus aureus Cowan I-activated blasts prepared from high density tonsil B cells were found to secrete IgG and IgM, but no IgE, when cultured in the presence of rIL-4. The differentiating activity of rIL-4 was totally blocked by a neutralizing anti-IL-4 antiserum, therefore demonstrating that the IgG/IgM-inducing activity of rIL-4 was an intrinsic property of IL-4. rIL-4 was only minimally inducing Ig production of blasts prepared from low density B cells, whereas it induced B cell blasts prepared from high density B cells to secrete a high amount of Ig. Delayed additions of the neutralizing anti-IL-4 antiserum demonstrated that a 48-h contact between IL-4 and B cell blasts was required for optimal Ig production. The IL-4-mediated IgG and IgM production was neither suppressed by IFN-gamma nor by anti-CD23 mAb 25, whereas these agents have been shown earlier to inhibit IgE production of enriched B cells cultured in the presence of IL-4. These data indicate that the IgG/IgM-inducing activity of IL-4 is not regulated like the IL-4-induced IgE production by enriched B cells.

Human recombinant IL-4 induces activated B lymphocytes to produce IgG and IgM

In this report, we describe a novel biologic activity of IL-4 namely, its ability to induce activated human B cells to produce IgM. Staphylococcus aureus Cowan I-activated blasts prepared from high density tonsil B cells were found to secrete IgG and IgM, but no IgE, when cultured in the presence of rIL-4. The differentiating activity of rIL-4 was totally blocked by a neutralizing anti-IL-4 antiserum, therefore demonstrating that the IgG/IgM-inducing activity of rIL-4 was an intrinsic property of IL-4. rIL-4 was only minimally inducing Ig production of blasts prepared from low density B cells, whereas it induced B cell blasts prepared from high density B cells to secrete a high amount of Ig. Delayed additions of the neutralizing anti-IL-4 antiserum demonstrated that a 48-h contact between IL-4 and B cell blasts was required for optimal Ig production. The IL-4-mediated IgG and IgM production was neither suppressed by IFN-gamma nor by anti-CD23 mAb 25, whereas these agents have been shown earlier to inhibit IgE production of enriched B cells cultured in the presence of IL-4. These data indicate that the IgG/IgM-inducing activity of IL-4 is not regulated like the IL-4-induced IgE production by enriched B cells.

Regulation of Fc receptor for IgE (CD23) and class II MHC antigen expression on Burkitt's lymphoma cell lines by human IL-4 and IFN-gamma

The effect of rIL-4 on the expression of low affinity receptor for the Fc part of IgE (Fc epsilon R2/CD23) and class II MHC antigens on Burkitt's lymphoma (BL) cell lines was investigated. Some of the BL lines contained low percentages of CD23 and HLA-DQ-positive cells, but virtually all cells expressed HLA-DR. IL-4 induced CD23 and class II MHC Ag expression on 7 of 9 BL. Optimal CD23 and class II MHC expression was observed after 48-72 h of incubation. Induction of CD23 and class II MHC Ag in the BL cell line BL2 by IL-4 was confirmed at the specific mRNA level. Significant activation of HLA-DQ mRNA was obtained after 6 h of incubation with IL-4 and gradually increased during prolonged incubation. Maximal induction of mRNA transcription occurred after 48 to 72 h. Optimal induction of HLA-DR and CD23 transcription in BL2 was also observed after 48 to 72 h. The induction of CD23 and class II MHC Ag seems to be specific for IL-4, because rIL-1, rIL-2, rIFN-gamma, recombinant granulocyte-macrophage-CSF, and a commercial source of low m.w. B cell growth factor were ineffective. In addition, the expression of class I MHC Ag, the transferrin receptor, CD38, CD25, CD10, CD20, and CD21 were not affected by IL-4. Interestingly, IFN-gamma and PGE2 suppressed the IL-4-induced membrane expression of CD23 and class II MHC Ag in a dose-dependent way. IFN-gamma also blocked IL-4-induced CD23 mRNA transcription in BL2 completely, whereas PGE2 (10(-7) M) was partially inhibitory. The induction of CD23 and class II MHC Ag by IL-4 required intact protein synthesis as shown by its inhibition by cycloheximide. These results indicate that the induction of CD23 and class II MHC Ag by IL-4 is regulated in a coordinated way.

Regulation of Fc receptor for IgE (CD23) and class II MHC antigen expression on Burkitt's lymphoma cell lines by human IL-4 and IFN-gamma

The effect of rIL-4 on the expression of low affinity receptor for the Fc part of IgE (Fc epsilon R2/CD23) and class II MHC antigens on Burkitt's lymphoma (BL) cell lines was investigated. Some of the BL lines contained low percentages of CD23 and HLA-DQ-positive cells, but virtually all cells expressed HLA-DR. IL-4 induced CD23 and class II MHC Ag expression on 7 of 9 BL. Optimal CD23 and class II MHC expression was observed after 48-72 h of incubation. Induction of CD23 and class II MHC Ag in the BL cell line BL2 by IL-4 was confirmed at the specific mRNA level. Significant activation of HLA-DQ mRNA was obtained after 6 h of incubation with IL-4 and gradually increased during prolonged incubation. Maximal induction of mRNA transcription occurred after 48 to 72 h. Optimal induction of HLA-DR and CD23 transcription in BL2 was also observed after 48 to 72 h. The induction of CD23 and class II MHC Ag seems to be specific for IL-4, because rIL-1, rIL-2, rIFN-gamma, recombinant granulocyte-macrophage-CSF, and a commercial source of low m.w. B cell growth factor were ineffective. In addition, the expression of class I MHC Ag, the transferrin receptor, CD38, CD25, CD10, CD20, and CD21 were not affected by IL-4. Interestingly, IFN-gamma and PGE2 suppressed the IL-4-induced membrane expression of CD23 and class II MHC Ag in a dose-dependent way. IFN-gamma also blocked IL-4-induced CD23 mRNA transcription in BL2 completely, whereas PGE2 (10(-7) M) was partially inhibitory. The induction of CD23 and class II MHC Ag by IL-4 required intact protein synthesis as shown by its inhibition by cycloheximide. These results indicate that the induction of CD23 and class II MHC Ag by IL-4 is regulated in a coordinated way.

Response of LFA-1-deficient B cells to interleukin 4 (BSF-1) and low molecular weight B cell growth factor (BCGFlow)

T cell-depleted B cells from a patient with LFA-1 deficiency were tested in costimulation assays for responsiveness to recombinant human IL4 (BSF-1) and purified low molecular weight B cell growth factor (BCGFlow). In both cases the response of LFA-1-deficient B cells was comparable with normal controls. Monoclonal antibodies to LFA-1 alpha (CD11a) and beta (CD18) chains were unable to mimic the action of IL4 on normal B cells in costimulation assays with anti-IgM, and did not inhibit normal B cell proliferation in response to IL4 and anti-IgM. Epstein-Barr virus-transformed lymphoblastoid B cell lines (LCL) from normal and LFA-1-deficient donors both responded in proliferation assays to BCGFlow but not IL4. Similarly, both normal and LFA-1-deficient LCL increased IgM secretion in response to BCDF, BCGFlow and, interestingly, IL4. The normal LCL also increased IgG secretion in response to these factors, but no IgG was detected in supernatants from the LFA-1-deficient LCL. These results show that LFA-1 expression is not essential for B cell responses to B cell growth and differentiation factors.

Human recombinant interleukin 4 induces normal B cells to produce soluble CD23/IgE-binding factor analogous to that spontaneously released by lymphoblastoid B cell lines

Surface-labeled Epstein-Barr virus (EBV)-transformed lymphoblastoid RPMI 8866 cells release in their supernatant a radiolabeled 25-kDa polypeptide which reacts with the Fc epsilon RL/CD23-specific monoclonal antibody (mAb) 25 and which binds to IgE but not IgG (IgE BF/sCD23). IgE BF/sCD23 had an isoelectric point of 4.5-5.0. The reactivity of mAb 25 with IgE BF/sCD23 allowed us to set up a radioimmunoassay for detection of IgE BF/sCD23 in cell culture supernatants. Supernatants from Fc epsilon RL/CD23+ cell lines were found to contain IgE BF/sCD23. Addition of human recombinant interleukin 4 (IL 4) to normal human B cells cultures induced the production of IgE BF/sCD23. Activation of B cells with anti-IgM antibody coupled to beads enhanced the IL 4-induced production of IgE BF/sCD23 when compared to nonactivated B cells. This correlates with the finding that anti-IgM antibody-activated B cells cultured with IL 4 express more Fc epsilon RL/CD23 than B cells cultured with IL 4 alone. The biochemical characteristics of radiolabeled IgE BF/sCD23 immunoprecipitated by mAb 25 from the supernatants of normal B cells cultured with IL 4 were identical to those of the IgE BF/sCD23 isolated from EBV-transformed cell line supernatants. Addition of interferon-gamma to B cells cultured with IL 4 strongly decreased the level of IgE BF/sCD23 in culture supernatants correlating with the observed decrease of Fc epsilon RL/CD23 on B cell surface. These data demonstrate that normal human B cells cultured in the presence of IL 4 produce an IgE-binding factor (sCD23) biochemically and antigenically equivalent to that spontaneously produced by EBV-transformed lymphoblastoid cell lines.

B cell growth-promoting activity of recombinant human interleukin 4

Human interleukin 4 (IL-4), also known as B cell stimulatory factor 1, is a T cell-derived glycoprotein consisting of 129 amino acids for which a cDNA has been recently isolated. IL-4 displays little or no B cell growth factor (BCGF) activity in the standard anti-IgM costimulatory assay using suboptimal concentrations of soluble anti-IgM antibody whereas the low m.w. BCGF is very active. When insolubilized anti-IgM was used as the costimulating agent, both IL-4 and the low m.w. BCGF were found to promote B cell proliferation. Human IL-4 is able to induce the proliferation of B lymphocytes preactivated for either 1 day with insolubilized anti-IgM antibody or for 3 days with Staphylococcus aureus strain Cowan I. However, IL-4 is poorly mitogenic for B cells preactivated for 1 day with the Staphylococcus strain whereas the low m.w. BCGF strongly enhances the proliferation of these B cells. These two findings demonstrate that the preactivation signal necessary to induce human B cells to proliferate in response to IL-4 is critical. The increased tritiated thymidine ([3H]dThd) uptake in preactivated B cell cultures with IL-4 reflects cel proliferation because cell cycle analysis demonstrates that IL-4 induces activated B cells to enter the S and G2/M phases of the cell cycle and the addition of IL-4 to preactivated B cell cultures permits the recovery of three- to fourfold more B cells after 4 days of culture. IL-4 and the low m.w. BCGF act in concert to induce the proliferation of anti-IgM-preactivated B cells as demonstrated by [3H]dThd uptake and cell cycle analysis. In striking contrast to the demonstrated antagonistic effect of interferon-gamma on the IL-4-induced expression of the low affinity receptor for IgE (Fc epsilon RL/CD23), on B cells, it was found that interferon-gamma enhanced the IL-4-induced proliferation of anti-IgM-preactivated B cells. Finally, it was found that IL-4 had to be present continuously during the culture period to exert an optimal growth-promoting effect on B cell blasts. As a conclusion, IL-4 is able to induce the proliferation of an appropriately activated subpopulation of human B cells.

B cell growth-promoting activity of recombinant human interleukin 4

Human interleukin 4 (IL-4), also known as B cell stimulatory factor 1, is a T cell-derived glycoprotein consisting of 129 amino acids for which a cDNA has been recently isolated. IL-4 displays little or no B cell growth factor (BCGF) activity in the standard anti-IgM costimulatory assay using suboptimal concentrations of soluble anti-IgM antibody whereas the low m.w. BCGF is very active. When insolubilized anti-IgM was used as the costimulating agent, both IL-4 and the low m.w. BCGF were found to promote B cell proliferation. Human IL-4 is able to induce the proliferation of B lymphocytes preactivated for either 1 day with insolubilized anti-IgM antibody or for 3 days with Staphylococcus aureus strain Cowan I. However, IL-4 is poorly mitogenic for B cells preactivated for 1 day with the Staphylococcus strain whereas the low m.w. BCGF strongly enhances the proliferation of these B cells. These two findings demonstrate that the preactivation signal necessary to induce human B cells to proliferate in response to IL-4 is critical. The increased tritiated thymidine ([3H]dThd) uptake in preactivated B cell cultures with IL-4 reflects cel proliferation because cell cycle analysis demonstrates that IL-4 induces activated B cells to enter the S and G2/M phases of the cell cycle and the addition of IL-4 to preactivated B cell cultures permits the recovery of three- to fourfold more B cells after 4 days of culture. IL-4 and the low m.w. BCGF act in concert to induce the proliferation of anti-IgM-preactivated B cells as demonstrated by [3H]dThd uptake and cell cycle analysis. In striking contrast to the demonstrated antagonistic effect of interferon-gamma on the IL-4-induced expression of the low affinity receptor for IgE (Fc epsilon RL/CD23), on B cells, it was found that interferon-gamma enhanced the IL-4-induced proliferation of anti-IgM-preactivated B cells. Finally, it was found that IL-4 had to be present continuously during the culture period to exert an optimal growth-promoting effect on B cell blasts. As a conclusion, IL-4 is able to induce the proliferation of an appropriately activated subpopulation of human B cells.

Human recombinant interleukin 4 induces Fc epsilon receptors (CD23) on normal human B lymphocytes

Human rIL-4 is able to induce the expression of low-affinity receptors for IgE (Fc epsilon RL/CD23) on resting B lymphocytes, as determined by the binding of either the anti Fc epsilon RL/CD23-specific mAb 25 or IgE. Stimulation of B cells with insolubilized anti-IgM antibody increases the number of cells expressing Fc epsilon RL/CD23 upon culturing with IL-4 and enhances the level of Fc epsilon RL/CD23 expression on these cells. Fc epsilon RL/CD23 induction is specific for IL-4 since IL-1 alpha, IL-2, IFN-gamma, B cell-derived B cell growth factor (BCGF), and a low-molecular-weight BCGF were ineffective. IFN-gamma strongly inhibited the induction of Fc epsilon RL/CD23 by IL-4.