Enhancement of human B cell proliferation and differentiation by tumor necrosis factor-alpha and interleukin 1

The role of tumor necrosis factor-alpha (TNF-alpha) in human B cell responses was examined and compared with that of interleukin (IL) 1 by assessing the ability of each cytokine to support proliferation and differentiation. Recombinant TNF-alpha (rTNF-alpha) and recombinant IL-1 (rIL-1) each enhanced the generation of immunoglobulin-secreting cells (ISC) in cultures of pokeweed mitogen-stimulated B cells incubated with T cells. To examine the direct effect of rTNF-alpha and rIL-1 on the responding B cell, highly purified peripheral blood B cells were stimulated with Cowan I Staphylococcus aureus (SA). In the absence of T cell factors, proliferation was minimal and there was no generation of ISC. Recombinant IL-2 (rIL-2) supported both responses. Although rTNF-alpha alone did not support SA-stimulated generation of ISC, it did increase SA-stimulated B cell DNA synthesis by two- to eightfold. In addition, rTNF-alpha augmented B cell proliferation in rIL-2 supported SA-stimulated cultures. Moreover, rTNF-alpha enhanced the generation of ISC stimulated by rIL-2 alone or rIL-2 and SA. rIL-1 also augmented DNA synthesis and generation of ISC by B cells stimulated with SA and rIL-2. However, rTNF-alpha enhanced proliferation and ISC generation in SA + rIL-2-stimulated cultures even when they were supplemented with saturating concentrations of rIL-1. Utilizing a two-stage culture system, it was found that the major effect of rTNF-alpha was to enhance responsiveness of SA-activated B cells to rIL-2, whereas it exerted only a minimal effect during initial stimulation. These results indicate that TNF-alpha as well as IL-1 augment B cell responsiveness. Moreover, the ability of rTNF-alpha to enhance B cell responsiveness was not an indirect effect resulting from the induction of Il-1 production by contaminating monocytes, but rather resulted from the delivery of a signal by rTNF-alpha directly to the responding B cell that promoted both proliferation and differentiation after initial activation. The data therefore indicate that human B cell responsiveness can be independently regulated by the action of two separate monocyte-derived cytokines.     

IL-2 dependence of the promotion of human B cell differentiation by IL-6 (BSF-2)

The effect of rIL-6 on the growth and differentiation of highly purified human peripheral blood B cells was examined. IL-6 alone induced minimal incorporation of [3H]thymidine by unstimulated or Staphylococcus aureus (SA)-stimulated B cells and did not augment proliferation induced by SA and IL-2. Similarly, IL-6 alone did not support the generation of Ig-secreting cells (ISC) or induce the secretion of Ig by unstimulated or SA-stimulated B cells. However, IL-6 did augment the generation of ISC and the secretion of all isotypes of Ig induced by SA and IL-2. Maximal enhancement of B cell responsiveness by IL-6 required its presence from the initiation of culture. Delaying the addition of IL-6 to B cells stimulated with SA and IL-2 beyond 24 h diminished its effect on ISC generation. However, increased Ig production but not ISC generation was observed when IL-6 was added to B cells that had been preactivated for 48 h with SA and IL-2. This effect was most marked when the activated B cells were also stimulated with IL-2. IL-6 in combination with other cytokines such as IL-1 and IL-4 did not induce the secretion of Ig or generation of ISC in the absence of IL-2. Moreover, antibody to IL-6 did not inhibit the effect of IL-2 on the growth and differentiation of B cells stimulated with SA, but did inhibit the IL-6-induced augmentation of Ig secretion by B cells stimulated with SA and IL-2. IL-6 alone enhanced T cell dependent induction of B cell differentiation stimulated by PWM. Part of this enhancement was related to its capacity to increase the production of IL-2 in these cultures. These results indicate that IL-6 has several direct enhancing effects on the differentiation of B cells, all of which are at least in part dependent on the presence of IL-2. In addition, IL-6 can indirectly increase B cell differentiation by increasing IL-2 production by T cells.     

Immunomodulatory role of IL-4 on the secretion of Ig by human B cells

The effect of IL-4 on the production of Ig by human B cells was examined. Highly purified B cells were stimulated with Staphylococcus aureus (SA) and IL-4 alone or in combination with various other cytokines and the supernatants assayed for Ig by isotype-specific ELISA. IL-4 (10 to 100 U/ml) did not support Ig secretion by SA-stimulated blood, spleen, or lymph node B cells, whereas IL-2 supported the production of all isotypes including IgE. Moreover, IL-4 suppressed the production of all isotypes of Ig by B cells stimulated with SA and IL-2 including IgG1, IgG2, and IgE. IL-4-mediated suppression was partially reversed by IFN-gamma or -alpha and low m.w. B cell growth factor. TNF-alpha and IL-6 did not reverse the IL-4-induced suppression of Ig production. The inhibitory action of IL-4 on Ig production appeared to depend on the polyclonal activator used to stimulate the B cells. Thus, Ig secretion by B cells activated by LPS and supported by IL-2 was not inhibited by IL-4. Whereas IL-4 alone supported minimal Ig production by LPS-activated B cells, it augmented production of all Ig isotypes in cultures stimulated with LPS and supported by IL-2. IFN-gamma further enhanced production of Ig in these cultures. When the effect of IL-4 on the responsiveness of B cells preactivated with SA and IL-2 was examined, it was found not to inhibit but rather to promote Ig production modestly. A direct effect of IL-4 on the terminal differentiation of B cells was demonstrated using B lymphoblastoid cell lines. IL-4 was able to enhance the Ig secreted by an IgA-secreting hybridoma, 219 and by SKW6-CL-4, an IL-6-responsive IgM-secreting EBV transformed B cell line. These results indicate that IL-4 exerts a number of immunoregulatory actions on human B cell differentiation. It interferes with the activation of B cells by SA and IL-2, but promotes the differentiation of preactivated B cells, B cell lines, and B cells activated by LPS without apparent isotype specificity.     

Effects of interferon-alpha on human B cell responsiveness: biphasic effects in cultures stimulated with Staphylococcus aureus.

Although interferon-alpha (IFN-alpha) has been found to be involved in the immune regulation in vivo, the effects of IFN-alpha on human B cells have not yet been clarified because of conflicting results in the literature. The present study therefore examined the effects of several subtypes of IFN-alpha (natural, alpha 1, alpha 2a, alpha 2b) on B cell responsiveness in detail by comparing different experimental conditions. Highly purified B cells from normal human individuals were cultured with Staphylococcus aureus (SA) + IL-2 or with immobilized anti-CD3-activated T4 cells in the presence or absence of IFN-alpha. IFN-alpha enhanced the immunoglobulin (Ig) production induced by immobilized anti-CD3-activated T4 cells. By contrast, IFN-alpha (5-50,000 IU/ml) suppressed the Ig production induced by SA + IL-2. The suppression by IFN-alpha was dependent on the concentration of SA. The inhibitory effects of IFN-alpha in SA-stimulated cultures were exerted in the first 72 hr of cultures and required the presence of IL-2, whereas IFN-alpha enhanced the maturation of B cells when it was added after 72 hr of cultures. The suppressive effects of IFN-alpha were overcome by addition of immobilized anti-CD3-preactivated T cells that had been treated with mitomycin C, but not by the addition of fresh T cells or soluble factors produced by activated T cells. Of interest, IFN-alpha did not inhibit the expression of IL-2R, but inhibited that of intercellular adhesion molecule-1 (ICAM-1) on B cells after stimulation with SA + IL-2, suggesting that the suppressive effects of IFN-alpha might be related to the regulation of B cell-B cell contacts through ICAM-1. There was no significant difference in effects on B cells among various subtypes of IFN-alpha. These results suggest that the effects of IFN-alpha on human B cell responsiveness may be different depending on the nature of stimulation. Moreover, the data indicate that IFN-alpha enhances the differentiation of activated B cells irrespective of the activation signals.     

Pre-exposure of human B cells to recombinant IL-1 enhances subsequent proliferation

The reported effects of the monocyte-derived cytokine IL-1 on human B lymphocytes are both varied and controversial. IL-1 has been reported to augment both proliferation and Ig secretion of previously activated human B cells. In the present study highly purified splenic B cells were cultured with rIL-1 before, simultaneously with, and after the addition of the polyclonal B cell mitogen, anti-Ig. rIL-1 had no significant effect on B cell proliferation when added simultaneously with or after B cell activation with anti-Ig. However, incubation of splenic B cells with rIL-1 for 24 h before stimulation with anti-Ig appeared to enhance mitogenesis. With the observation that rIL-1 exerted effects on resting B cells, the effect of rIL-1 on several events which accompany B cell activation was examined. rIL-1 failed to stimulate RNA synthesis, effect increases in cell size or intracellular Ca2+ levels, or lead to the hyperexpression of MHC class II or B cell activation Ag. These studies suggest that rIL-1 does not activate B cells but primes them to respond to subsequent activation.     

Murine lymphocytes exhibit heterogeneity in their proliferative responsiveness to calcium ionophore.

The calcium ionophore, A23187, when used alone was found to induce proliferation of murine T cells, at concentrations of 0.5-1 mM. This response required the presence of syngeneic splenic adherant cells (SAC) as a source of accessory cells. Interestingly, only CD4+ T cells but not CD8+ T cells or B cells responded to the calcium ionophore by proliferation. The inability of CD8+ T cells or B cells to respond was not related to decreased elevation in the intracellular ionized calcium [Ca2+]i concentration induced by the ionophore, because activated CD4+ T, CD8+ T and B cells all exhibited similar elevation in [Ca2+]i. The inability of CD8+ T cells to respond to calcium ionophore was probably due to insufficient production of autocrine growth factors, such as IL-2, inasmuch as the addition of exogenous IL-2 could completely restore the CD8+ T cell responsiveness. Also, exogenous rIL-1 could partially restore purified T cell response to calcium ionophore, whereas, rIL-6 failed to do so. IL-2, but not IL-4, acted as an autocrine growth factor for T cells responding to the calcium ionophore in the presence of SAC, since, antibodies against IL-2 or IL-2 receptor (IL-2R) but not against IL-4, could inhibit the T cell proliferation. Furthermore, exogenous rIL-2 but not rIL-4 supported the proliferation of T cells to calcium ionophore in the absence of accessory cells. Our results suggest that murine lymphocytes exhibit heterogeneity in their proliferative responsiveness to calcium ionophore and that this may not depend on the early activation signal such as the elevation in [Ca2+]i) induced by the ionophore but may depend on subsequent signals which regulate endogenous growth factor production.     

Monoclonal antibody against the human peripheral lymph node homing receptor homologue (Leu 8) inhibits B cell differentiation but not B cell proliferation.

Previous studies have shown that a subpopulation of circulating human B cells expresses the Leu 8 peripheral lymph node homing receptor homologue and that these B cells are capable of producing Ig in response to staphylococcus A Cowan I (SAC). In the present study the effect of a signal delivered via the Leu 8 molecule (using anti-Leu 8 mAb) on B cells was examined. Initially, it was shown that immobilized anti-Leu 8 suppressed IgM and IgG secretion of B cells activated by SAC + IL-2 but not that by PWM-prestimulated B cells or B cells stimulated with PWM in the presence of CD4+, Leu 8- T cells (a source of helper cells). It was also shown that anti-Leu 8 did not suppress SAC + IL-2-stimulated B cell proliferation or expression of IL-2R alpha-chain or c-myc mRNA in B cells. The addition of T cells, monocytes, purified IL-2, rIL-1, rIL-6, or human B cell growth factor did not overcome the inhibitory effect of anti-Leu 8 on SAC-stimulated B cell Ig production, and the inhibitory effect of anti-Leu 8 was not blocked by anti-TGF-beta. Finally, inhibition of B cell differentiation occurred even when anti-Leu 8 was added up to 72 hrs after initiation of cell culture. Thus, anti-Leu 8 is unique among inhibitors of B cell function in that it can down-regulate immunoglobulin synthesis without affecting B cell proliferation. These findings suggest that a natural ligand for Leu 8 could affect not only homing of B cells, but also B cell differentiation.     

Interleukin-2 promotes antigenic reactivity of rested T cells but prolongs the postactivational refractory phase of activated T cells

IL-2 is a principal autocrine growth factor that promotes T-cell activation and proliferation. However, IL-2 has also been implicated as a key intermediate in the induction and maintenance of self-tolerance in vivo. The purpose of this study was to assess whether the differential regulatory activity of IL-2 was related to the activation status of responder T cells. In cultures of rested myelin basic protein (MBP)-specific T cells, IL-2 not only induced IL-2R alpha but also augmented surface expression of several other activation-associated glycoproteins including OX40, LFA-1, B7.1, B7.2, TCR, and CD4. Pretreatment of T cells with IL-2 also up-regulated subsequent antigen reactivity in assays of MBP-stimulated proliferation and IL-2 production and also promoted proliferative responsiveness to IL-2. In cultures of activated T cells, however, IL-2 inhibited subsequent reactivity to antigen or IL-2 and thereby prolonged a phase of postactivational refractoriness. Exposure of preactivated T cells to IL-2 also inhibited subsequent responses to the mitogenic combination of PMA, ionomycin, and IL-2 without enhancing cell death. These data support the concept that the inhibitory activity of IL-2 is dependent upon the activation status of T cells and is manifest as impaired cell cycle progression in response to a variety of IL-2-dependent stimuli.     

Recombinant murine IL-5 induces high rate IgA synthesis in cycling IgA-positive Peyer's patch B cells

Recent studies have shown that purified IL-5 from T cell lines and clones enhances IgA synthesis in LPS-triggered splenic B cell cultures, and that this effect is augmented by IL-4. In this study we have examined the ability of rIL-5 and rIL-4 to support spontaneous Ig synthesis in normal Peyer's patch (PP) B cell cultures. The rIL-4 supported proliferation of the HT-2 and in vivo adapted BCL-1 cell lines, increased Ia expression on normal spleen B cells, co-stimulated splenic B cell proliferation in the presence of anti-mu and enhanced IgG1 synthesis in LPS triggered splenic B cell cultures. The rIL-5 supported BCL-1 proliferation, co-stimulated splenic B cell proliferation in the presence of dextran sulfate, and increased IgA synthesis in LPS-stimulated splenic B cell cultures. Markedly enhanced IgA responses occurred in PP B cell, but not splenic B cell cultures supplemented with rIL-5 in the absence of an added B cell trigger. However, rIL-4 alone did not enhance IgA synthesis or increase the IgA synthesis of PP B cell cultures stimulated with rIL-5. The rIL-5 receptive PP B cells were present in the blast cell subpopulation, inasmuch as a low density fraction isolated on Percoll gradients accounted for the enhanced IgA synthesis. Further, cell cycle analysis of whole PP B cells using propidium iodide in conjunction with staining for surface B220, demonstrated that approximately 12 to 16% of the B cells were in the S and G2/M stages of cell cycle, the remainder being in Go + G1. The surface IgM+ B cells were predominantly in Go + G1, whereas the sIgA+ B cell subpopulation was enriched for cells in the S and G2/M compartments. The PP B cell subset responsible for enhanced IgA synthesis in the presence of rIL-5 was sIgA-positive because FACS-depletion of the sIgA+ B cells resulted in the total loss of rIL-5 enhanced IgA synthesis. Further, when PP B cells were enriched for sIgA+ B cells by cell sorting, these cells responded to rIL-5 with increased IgA synthesis in a dose-dependent manner. When the actual numbers of IgA secreting cells were assessed in PP B cell cultures with supplemental rIL-5, no significant increase in total IgA-producing cells was noted when compared with B cells cultured without rIL-5.(ABSTRACT TRUNCATED AT 400 WORDS)     

1,25-Dihydroxyvitamin D3 modulates the effects of interleukin 2 independent of IL-2 receptor binding

Previous studies have shown that 1,25-dihydroxyvitamin D3 (calcitriol) is a macrophage-derived cytokine and a potent inhibitor of IL-2 and interferon-gamma (IFN-gamma) production and T lymphocyte proliferation. The growth inhibitory effect of calcitriol is only partially reversed by IL-2 addition, suggesting IL-2 independent effects. In this report we characterize the IL-2-independent effects of calcitriol on lymphocyte activation. Calcitriol inhibited cellular transition from early to late G1 (G1A-G1B transition) in both the absence and presence of IL-2. Exogenous IL-2 did not increase either IFN-gamma production or transferrin receptor (TfR) expression in the presence of calcitriol despite increases in cell entry into late G1 and proliferation. Calcitriol treatment reduced TfR expression by activated T lymphocytes independent of their location in the cell cycle, further suggesting its independence from IL-2-mediated events. Combinations of rIL-2 and rIL-4 did not reverse calcitriol-dependent inhibition of proliferation and TfR expression to any greater degree than rIL-2 alone. Northern blot analysis demonstrated the decrease in IFN-gamma and TfR mRNA accumulation with calcitriol treatment was unaffected by exogenous IL-2. In contrast, IL-2R mRNA and protein were increased by IL-2, with superinduction in the presence of calcitriol, demonstrating that the lack of effect on IFN-gamma and TfR was not due to IL-2 insensitivity. Moreover, equivalent numbers of high-affinity IL-2R were expressed by both control and calcitriol-treated T lymphoblasts. Thus, lectin-activated T lymphocyte responsiveness to IL-2, as measured by IL-2R expression and proliferation, can be partly to completely dissociated from IFN-gamma production and TfR expression in the presence of calcitriol. Finally, IL-2-induced proliferation of unstimulated mononuclear cells and purified T lymphocytes was inhibited by calcitriol. These data indicate that local production of calcitriol by activated macrophages is capable of regulating T lymphocyte activation not only through suppression of IL-2 production, but also through additional mechanism(s), that are mediated at a post-IL-2R level.     

The effects of IL-4 on human natural killer cells. A potent regulator of IL-2 activation and proliferation

NK cells are directly activated by rIL-2 and subsequently undergo rIL-2-dependent proliferation in vitro. Herein, we report that rIL-4 is a potent regulator of human NK cells. Although rIL-4 had no effect on the cytotoxic activity of resting NK cells, it was capable of inhibiting in a concentration-dependent manner the rIL-2-induced cytolytic activation of NK cells against NK cell-resistant tumor cell targets. rIL-4 acted directly on NK cells and did not require accessory cells. rIL-4-induced inhibition of NK cell activation was specific for rIL-2 in that activation of NK cell cytolysis by IFN-alpha was not affected. These results represent the first direct evidence that rIL-2 and IFN-alpha activate NK cells by different pathways. rIL-4 also effectively blocked the rIL-2-dependent proliferation of NK cells. The results presented in this study clearly demonstrate that rIL-4 is a potent regulator of IL-2-dependent mechanisms of NK cell activation and proliferation and thus may play an important physiologic role in vivo.     

T cell-dependent activation of B cell proliferation and differentiation by immobilized monoclonal antibodies to CD3

The capacity of mAb directed at the CD3 molecular complex (64.1) to induce T cell-dependent B cell proliferation and differentiation was examined. Coculture of B cells with mitomycin C-treated T4 cells (T4 mito) stimulated by immobilized 64.1 resulted in marked B cell proliferation and Ig-secreting cells (ISC) generation in the absence of any additional stimulation. The magnitude of the B cell responses induced by immobilized 64.1-stimulated T4 mito was far greater than that induced by other stimuli, such as Staphylococcus aureus plus factors produced by mitogen-activated T cells, PWM, or soluble 64.1. The induction of maximal B cell responsiveness required direct contact between activated T cells and responding B cells. Of note, immobilized 64.1 also induced B cell proliferation and ISC generation in the presence of mitomycin C-treated T8 cells. By contrast, immobilized 64.1 stimulated T4 or T8 cells that had not been treated with mitomycin C induced very modest ISC generation and suppressed B cell responses supported by T4 mito even in the presence of exogenous IL-2 or factors produced by mitogen-activated T cells. The interactions between T and B cells in these cultures not only induced B cell responses, but also enhanced the production of IL-2 by activated T cells. Increased IL-2 production was facilitated when culture conditions afforded the opportunity for contact between B cells and activated T cells. These results indicate that the establishment of interactions between B cells and anti-CD3-stimulated T4 or T8 cells provides all of the signals necessary for proliferation and differentiation of B cells without other stimuli and also augments the production of lymphokines by the activated T cells. The data emphasize the role of Ag-nonspecific interactions between B cells and T cells in promoting polyclonal responses of both cell types.     

Modulation of IL-2-induced human B cell proliferation in the presence of human 50-kDa B cell growth factor and IL-4

Several human T cell derived factors capable of stimulating human B cells to synthesize DNA have been previously described. One such factor exhibits an apparent m.w. of 50,000 Da and has been termed 50-kDa-B cell growth factor (BCGF). In this report, we show that a human B cell proliferation pathway based on the sequential action of anti-mu antibody, 50-kDa-BCGF and IL-2 is inhibited in the presence of human rIL-4. Although IL-4 itself is capable of triggering B cell DNA synthesis as measured at 72 h, this lymphokine inhibits, in a dose-related manner, the 50-kDa-BCGF driven response of B cells to IL-2 when such proliferation is determined after 144 h. This inhibition takes place at an early step of the B cell activation and does not require the presence of IL-4 during the whole culture period. Such inhibitory activity of IL-4 is specific to the IL-2-induced B cell proliferation because DNA synthesis measured in the presence of semi-purified human 12-kDa-BCGF is not affected by the presence of IL-4. Our results suggest that a particular pathway of human B cell activation leading to the proliferation of these cells in the presence of IL-2 could be either up- or down-modulated by 50-kDa-BCGF and IL-4, respectively.     

Phenotypic and functional analysis of the cellular response in regional lymphoid tissue during an acute virus infection

A phenotypic and functional analysis has been made of the cellular response in regional lymphoid tissue of C57BL/6J mice infected with lymphocytic choriomeningitis virus. Massive recruitment of nondividing cells occurred from 3 days after infection, with total numbers of CD8+ T lymphocytes, B220+ B cells, and Thy-1- B220- null cells being high from day 4 to day 6. In contrast, the peak counts for CD4+ T cells were recorded on day 4 and declined dramatically thereafter. Enhanced expression of IL-2R and Ly-24, both of which can be regarded as T cell activation markers, was found for both the CD4+ and the CD8+ subsets, being most prominent for the CD8+ T cells on day 6. Evidence of T cell proliferation was not recognized until days 5 and 6, coincident with enhanced responsiveness of the lymphocytes to rIL-2 and the development of virus-specific cytotoxic activity. Elimination of the CD4+ T cells by treatment of mice with mAb did not modify either the pathogenesis of lymphocytic choriomeningitis, or the expression of activation markers on the CD8+ T cells which are known to be the key effectors in this disease. Thus, the pattern of responsiveness for the CD8+ population is of recruitment to the lymph node, progressive increase in the expression of activation markers and enhanced sensitivity to rIL-2, with late proliferation and generation of cytotoxic activity. This model provides a system for the rigorous in vivo analysis of parameters influencing lymphocyte differentiation and activation in a virus infection.     

Regulation of TGF-beta response during T cell activation is modulated by IL-10.

TGF-beta1 is an important pleiotropic cytokine that has been described to have both stimulatory and inhibitory effects on cell growth and differentiation. For several cell types, the effect of TGF-beta1 was found to correlate with the differentiation stage of the cells and the presence of other cytokines. In this report, we address the influence of TGF-beta1 on CD4(+) T cell activation by evaluating the effect of TGF-beta1 on the proliferative and cytokine responses of purified resting and activated human or mouse CD4(+) T cells. TGF-beta1 inhibits proliferation and cytokine secretion on resting CD4(+) T cells but has no inhibitory effect on activated T cells. Moreover, TGF-beta1 unresponsiveness of activated T cells was correlated with a down-regulation in the expression of the TGF-beta receptor type II. Interestingly, IL-10 addition enhances TGF-beta receptor type II expression and restores TGF-beta responsiveness on activated T cells. These results indicated that TGF-beta responsiveness is sequentially regulated on T cells by the modulation of the of TGF-beta receptor type II chain expression. Moreover, we have identified a novel regulatory role of IL-10 on TGF-beta-dependent T cell growth that can explain the control of T cell activation on chronic vs acute inflammatory sites.     

Interleukin 2 induces proliferation of normal "resting" human T cells in the absence of other known external stimulation

Interleukin 2(IL-2) is known to stimulate the progression of activated T cells from G1 through the rest of the cell cycle. We have demonstrated that addition of purified recombinant human IL-2 (rIL-2) to fresh normal human peripheral blood mononuclear cells (PBM), which were IL-2 receptor (Tac) negative by FACS analysis, stimulated marked proliferation of the PBM. IL-2-induced proliferation was also observed with umbilical cord blood mononuclear cells. Monocyte depletion of PBM resulted in a marked reduction of rIL-2-induced proliferative response which could be restored by adding back autologous irradiated monocytes but not by interleukin 1. The T cells preincubated with rIL-2 showed a five to six times enhanced autologous mixed-lymphocyte reaction (AMLR) compared to controls. The rIL-2-induced proliferative response of PBM was inhibited in a concentration-dependent fashion by preincubation of PBM with an anti-HLA-DR framework monoclonal antibody. The proliferating cells were shown by two-color flow cytometric analysis to be primarily Leu-1+ and Leu-4+ T cells (both leu-3+ and Leu-2+ subsets); however, 6 to 19% of responding cells had surface markers for B cells or NK cells. The data demonstrate that rIL-2 can induce proliferation of "resting" human T cells. The phenomenon may be related to a monocyte-dependent AMLR which induces IL-2 receptors and IL-2 responsiveness in a subset of T cells.     

IL-6 is a potent cofactor of IL-1 in IgM synthesis and of IL-5 in IgA synthesis

In these studies we determined the capacity of IL-6 to act as a differentiation cofactor for murine Peyer's patch B cells producing different Ig classes and subclasses. In preliminary studies we determined that sufficient endogenous IL-6 was produced in LPS-induced cell systems to obscure responses to exogenous IL-6. We therefore studied IL-6 effects on Peyer's patch B cells (T cell-depleted cell populations) in the absence of LPS, relying on responses of in vivo-activated cells. rIL-1 alpha or purified IL-6 only slightly enhanced synthesis of IgM over minimal baseline levels in Peyer's patch T cell-depleted cell cultures; however, when IL-6 was added to cultures also containing rIL-1, IgM synthesis was very substantially increased. In addition, rIL-5 alone gave rise to a modest increase in IgM synthesis and its effect was not enhanced by either rIL-1 or IL-6. IgG production (mainly IgG3) followed a similar pattern. In contrast, IgA production was only modestly increased above baseline by rIL-1, rIL-5, or IL-6 alone or by rIL-1 and IL-6 in combination, but was greatly increased by rIL-5 and IL-6 in combination. The effect of IL-6 on Ig synthesis in the above studies was not due to an effect on cell proliferation. In summary, these data indicate that B cells differ in respect to the cytokines supporting maximal terminal differentiation and thus the class of Ig produced may depend on the presence of a particular combination of cytokines and lymphokines.     

Role of IL-2 in the generation of CD4+ suppressors of human B cell responsiveness

The capacity of human T4 cells stimulated by immobilized monoclonal antibodies to the CD3 molecular complex (64.1 and OKT3) to induce and regulate B cell responsiveness was examined. T4 cells stimulated by low concentrations of immobilized 64.1 (3.0 ng/well) and all concentrations of immobilized OKT3 supported B cell proliferation and differentiation. High concentrations of immobilized 64.1 (200 ng/well) failed to stimulate help but rather induced suppression by T4 cells. Suppression was prevented by treating the T4 cells with mitomycin C. Suppression could not be accounted for by deprivation of IL-2. In contrast, induction of suppressor T4 cell activity was closely related to the amount of IL-2 produced by anti-CD3 stimulated T4 cells. Moreover, IL 2 appeared to facilitate the generation of suppressor T4 cell activity. Suppressor cell activity could be generated from unseparated T4 cells as well as from highly purified T4 cell subsets, including Leu 8-, CD45R+, and CD45R- T4 cells, after stimulation with immobilized 64.1. A primary action of suppressor T4 cells appeared to be the direct inhibition of B cell function, as evidenced by the finding that immobilized anti-CD3 activated T4 cells directly suppressed B cell responses stimulated by Staphylococcus aureus and IL-2. Anti-CD3 activated T4 cells did not inhibit initial B cell activation, but suppressed the capacity of the activated B cells to differentiate into ISC. The suppressive influence of anti-CD3 activated T4 cells was reversible as evidenced by the finding that removal of the activated T4 cells from the culture permitted B cell differentiation to proceed. Moreover, anti-CD3-activated T4 cells were able to stimulate initial B cell activation that became apparent when the T cells and B cells were separated. Inhibition of B cell responsiveness by 64.1-activated T4 cells was the result of a block at the G1-S interphase of the cell cycle. The data indicate that anti-CD3-stimulated T4 cells directly and reversibly suppress human B cell function. Moreover, IL 2 appears to play an important role in the differentiation of functionally effective suppressor cells from activated T4 cells.     

Promotion of human T lymphocyte proliferation by IL-4

The capacity of human rIL-4 to support the proliferation of mitogen-stimulated T cells directly as well as by increasing IL-2 production or enhancing IL-2 responsiveness was investigated. IL-4 augmented proliferation of T cells stimulated with PHA, Con A, immobilized mAb to the CD3 molecular complex (OKT3), or PMA. IL-4 increased the number of mitogen-stimulated cells entering the cell cycle as well as enhancing ongoing proliferation of mitogen-activated lymphoblasts. Facilitation of initial activation by IL-4 was not inhibited by mAb to the p55 component of the IL-2R, anti-Tac, and, therefore, was not dependent on endogenous IL-2 activity. However, IL-4-mediated enhancement of ongoing T cell proliferation stimulated by PHA or OKT3 was partially but not completely blocked by anti-Tac. Analysis of the supernatants from PHA-stimulated T cell cultures indicated that IL-4 increased the production of IL-2 by mitogen-activated cells. Moreover, IL-4 increased the amount of IL-2 mRNA that accumulated in mitogen-stimulated T cells. In addition, IL-4 markedly augmented IL-2R expression by PHA-stimulated T cells. Although IL-4 promoted ongoing DNA synthesis of mitogen-stimulated T cells in an IL-2-dependent manner, it was also able to sustain their proliferation directly. Thus, IL-4 supported proliferation of PMA-activated T cells in a manner that was not inhibited by anti-Tac. Furthermore, IL-4 could augment proliferation and IL-2R expression of T cells stimulated with PHA in the presence of cyclosporin A, which blocks endogenous cytokine production or anti-Tac. Finally, IL-4 was noted to enhance proliferation of both CD4+ and CD8+ T cell subsets. The results indicate that IL-4 enhances proliferation of mitogen-activated human T cells by a number of mechanisms, including the direct promotion of cell cycle entry and subsequent DNA synthesis, enhanced production of IL-2, and increased responsiveness to IL-2 in part by up-regulation of IL-2R expression.     

The role of IL-5 in IgA B cell differentiation

IL-5 enhances secretion of IgA by B cells. The stage of B cell differentiation at which IL-5 enhances IgA secretion and the mechanism by which it exerts this effect are unknown. We examined these issues by separating Peyer's patch (PP) B cells into membrane IgA (mIgA)-positive and mIgA-negative cells with panning or cell sorting. When LPS was used to activate these cells, mIgA-positive PP B cells were induced by IL-5 (either as crude T cell supernatant or rIL-5 to secrete large amounts of IgA. In contrast mIgA-negative PP B cells showed no significant amount of IgA secretion with IL-5. In addition, rIL-5 did not cause expression of mIgA by mIgM-bearing B cells. The mechanism involved in enhancement of IgA secretion was evaluated by utilizing an ELISPOT assay to quantitate IgA secreting cells. Both unsorted PP B cells and mIgA-positive PP B cells, when incubated with IL-5, showed an increase in the number of IgA-secreting cells that was proportional to the increase in total secreted IgA. However, LPS-activated PP mIgA-positive B cells, when incubated with rIL-5, showed no increase in proliferation, as measured by [3H]thymidine incorporation indicating that the increase in IgA-secreting cells after incubation with IL-5 occurred not as a result of proliferation but rather through promotion of terminal differentiation. Thus, IL-5 acts as a differentiation factor on B cells which have already undergone isotype switch to IgA B cells, promoting differentiation into IgA-secreting cells with resultant increased IgA secretion.