B-cell Growth Factor Activity Research Articles

Soluble interleukin-2 receptor, interleukin-2 and interleukin-4 in sera and supernatants from patients with progressive systemic sclerosis

We studied the sera of patients with progressive systemic sclerosis (PSS) for elevated levels of soluble interleukin-2 receptor (sIL-2R), interleukin-2 (IL-2) and interleukin-4 (IL-4). We also measured IL-2, IL-4 and B cell growth factor (BCGF) activity in supernatants of peripheral blood mononuclear cells from the same patients. The finding of elevated serum sIL-2R and IL-2, and the increased levels of IL-2, IL-4 and BCGF activity in culture supernatants indicates that T lymphocyte hyperactivity likely play a major role in PSS. The failure to detect under our experimental conditions a direct proliferative effect of recombinant IL-2 on enriched normal B cells might suggest that IL-4 is the cytokine mainly responsible of the BCGF activity recovered in PSS supernatants.

Abnormal production of B cell growth factor in patients with systemic lupus erythematosus.

In order to clarify the role of B cell growth factor (BCGF) in the pathogenesis of systemic lupus erythematosus (SLE), BCGF production by peripheral blood mononuclear cells (PBMC) and T cells was studied using a new bioassay for BCGF activity. For this purpose, we established an Epstein-Barr (EB) virus-transformed B cell line KS-3.F10 that proliferates only in response to two B cell-specific BCGF, low-mol. wt BCGF (LMW-BCGF) and high-mol. wt BCGF (HMW-BCGF). PBMC from active SLE patients produced less BCGF when stimulated with phytohaemagglutinin (PHA) compared with controls. The decreased BCGF production by PHA-stimulated PBMC from active SLE reverted to control values when SLE became inactive. However, PHA-stimulated T cells from active SLE patients produced more BCGF compared with controls, whereas those from inactive SLE showed normal BCGF production. Spontaneous BCGF production by T cells was not observed in active SLE patients. These findings suggest that decreased BCGF production by SLE PBMC is due to excessive BCGF consumption by B cells in vitro and that SLE T cells produce large amounts of BCGF with appropriate immune stimuli in vivo to promote polyclonal B cell activation.

The Apoptosis and Proliferation of SAC-Activated B Cells by IL-10 Are Associated with Changes in Bcl-2, Bcl-xL, and Mcl-1 Expression

Interleukin-10 (IL-10), a cytokine from mouse Th2 cells and macrophage that inhibits IL-2 and IFN-γ production by Th1 cells, has been reported to stimulate growth and differentiation of B cells activated by CD40 or antigen receptor crosslinking. Our early observation revealed that IL-10 had B cell growth factor (BCGF) activity in human B cells preactivated with SAC or anti-Ig. The responsiveness of the preactivated B cells to IL-10 greatly increased when B cells were activated in the presence of IL-2, whereas IL-10 has no BCGF activity when added at the initiation of activation by SAC. To investigate the dual effects (proliferation and apoptosis) of IL-10 on B cells, the expression of a panel ofbcl-2protoncogene family members,bcl-2, bcl-x, mcl-1,andbax,was analyzed when B cells were activated by SAC. Bcl-xLprotein was not expressed in the small resting B cells but was induced by SAC stimulation, reaching its peak at 48 hr. The addition of IL-2 further augmented the Bcl-xLexpression with the same kinetics, whereas Bcl-2 and Mcl-1 were expressed by resting B cells and enhanced by SAC stimulation. However, the addition of IL-10 at the initiation of activation down-regulated Bcl-xL, Bcl-2, and Mcl-1 expression. At the same time, B cell proliferation was inhibited and apoptotic cell number increased, suggesting the growth arrest and/or apoptosis of B cells. The apoptosis of SAC-activated B cells by IL-10 was further confirmed by propidium iodide-staining and Annexin V–FITC-staining methods. In contrast, IL-10 failed to down-regulate the Bcl-xLand Bcl-2 expression but rather augmented the expression of Mcl-1 of B cells after preactivation for 48 hr with SAC and IL-2. Under this culture condition, B cells responded to IL-10 to proliferate and differentiate, while IL-2 and IL-10 had an additive or synergistic effect. Taken together, our data suggest that IL-10 acts on the induction stage of Bcl-xLexpression and regulates the apoptosis and proliferation of SAC-activated B cells through theirbcl-2family gene expression.

Interleukin-12 Stimulates B Cell Growth by Inducing IFN-γ

Human interleukin-12 (IL-12) is a 70-kDa polypeptide that activates human natural killer cells. It has been purified from the culture supernatant of a human Epstein–Barr virus-transformed B cell line and cloned. We show that native as well as recombinant IL-12 promoted growth ofStaphylococcus aureusCowan I strain (SAC) or anti-μ antibody-activated B cells in a dose-dependent manner. IL-12 also acted in synergy with IL-2 in growth and differentiation of SAC-activated B cells. Since anti-interferon (IFN)-γ antibody completely abrogates B cell growth factor (BCGF) activity of IL-12, the BCGF activity is mediated by IFN-γ. This conclusion is clearly supported by the results that IL-12 indeed induced IFN-γ production by activated B cells. These results suggest that the B cell proliferative effect of IL-12 may be mediated by autocrine IFN-γ.

Production of factors with B-cell growth and differentiation activities by peripheral blood mononuclear cells from patients with hypogammaglobulinemia

Background: The maturation of normal B lymphocytes proceeds through a growth phase and a differentiation phase. These two phases appear to be under the influence of mediators released by immune cells, B-cell growth factor(s), which induce proliferation of B cells; and B-cell differentiation factor(s), which induce B-cell differentiation. Methods: We analyzed the ability of peripheral blood mononuclear cells from patients with hypogammaglobulinemia to produce B-cell growth factor and B-cell differentiation factor activity in comparison with normal peripheral blood mononuclear cells. Results: Of 27 patients tested, 26 had normal production of B-cell growth factor activity. A quantitative but not absolute defect in B-cell growth factor production was demonstrable in one boy with hypogammaglobulinemia. Interleukin-2 and interleukin-4 levels, as determined antigenically in these supematants, had a similar distribution pattern from patients' or from control peripheral blood mononuclear cells; that is, undetectable levels of interleukin-2 were produced by cells from 4 of 16 patients tested and from 4 of 13 control subjects, and undetectable levels of interleukin-4 produced by cells from 6 of 16 patients and 4 of 13 control subjects. B-cell differentiation factor activity was absent in only one child tested but present in all other patients. Two patients had quantitatively low secretion of B-cell differentiation factor, but all others were within normal range. The two patients with quantitatively depressed B-cell differentiation factor activity had normal levels of B-cell growth factor activity, interleukin-2, and interleukin-4 produced from their cells. Conclusion: Peripheral blood mononuclear cells from the majority of patients with hypogammaglobulinemia appear to have the capacity to produce B-cell growth factors and B-cell differentiation factor activity in vitro.

T‐cell chronic lymphocytic leukemia: T‐cell function and lymphokine secretion

The leukemic T-cells of the six patients with T-cell chronic lymphocytic leukemia (T-CLL), four with CD4 and CD45R-positive (CD4+ CD45R*) T-CLL and two with CD8 and CD45R-positive (CD8+ CD45R+) T-CLL phenotype were studied for detailed immunologic phenotypic and functional characteristics. The levels of soluble interleukin-2 receptors were elevated significantly in the serum of all four patients with CD4+ CD45R+ T-CLL. Moreover, the CD4+ CD45R+ T-CLL patients' T-cells, after in vitro stimulation with phytohemagglutinin and concanavalin A, expressed elevated percentages of interleukin-2 receptors on cells and secreted high interleukin-2 activity. The B-cell growth factor (BCGF) activity from three patients with CD4+ CD45R+ T-CLL was enhanced, but B-cell differentiation factor (BCDF) activity of the all T-CLL patients was decreased. Reduced BCGF and BCDF activity of the leukemic T-cells was one possible mechanism of hypogammaglobulinemia detected in two patients with T-CLL. All T-CLL patients' leukemic T-cells had diminished immunoregulatory functional activity in allogeneic mixed lymphocyte reactions. These observations suggest that leukemic T-cells from T-CLL patients have many immunologic functional defects that may be important in their proliferative potential.

Humoral immunodeficiency in t-cell chronic lymphocytic leukemia. An immunologic assessment

The humoral antibody immunodeficiency in two patients with T-cell chronic lymphocytic leukemia (T-CLL) appeared to be the result of immunoregulatory abnormalities in the leukemic T-cell populations. Both patients had CD4+ CD45R+ "virgin" or suppressor-inducer T-CLL, but Patient 1 had hypogammaglobulinemia and Patient 2, immunoglobulin (Ig) M hypergammaglobulinemia. Although, CD25+ interleukin-2 (IL-2) receptors were present on leukemic T-cells of both patient, OKT9+ (CD71) transferrin receptors and OKT10 (CD38) activation antigens were found only on Patient 2's cells. Highly elevated amounts of IL-2 was secreted from phytohemagglutinin-stimulated and concanavalin A-stimulated T-cells in both patients. In Patient 1 with hypogammaglobulinemia, immune defects involve T-cells, first an intense suppressor activity on B-cell-induced IgM and IgG synthesis and, second, deficient production of B-cell growth factor (BCGF) and B-cell differentiation factor (BCDF). In Patient 2, highly elevated BCGF and IgM-specific BCDF was secreted by T-cells, a mechanism leading to IgM hypergammaglobulinemia in this patient. These studies stress the importance of BCGF and BCDF activity of leukemic T-cells in humoral antibody immunodeficiency disorders in T-CLL cases.

Spontaneously increased B cell growth factor and B cell differentiation factor activities in the synovial fluid of patients with rheumatoid arthritis.

The presence of factors implicated in B cell proliferation and differentiation was studied in synovial fluid (SF) from patients with rheumatoid arthritis (RA) and from patients with ankylosing spondylitis (AS) and traumatic joint injury. Culture with Staphylococcus aureus Cowan I B cell blasts showed strong B cell growth factor (BCGF) activity in the SF from patients with RA. This BCGF activity was significantly greater than that found in SF from patients with traumatic joint injury and similar to that of patients with AS. The presence of B cell differentiation factor (BCDF) for IgM(mu) in the SF from patients with RA was also demonstrated and was significantly greater than that found in SF from patients with AS and traumatic joint injury. Moreover, a significantly increased BCDF for IgG(gamma) was also found in the SF from patients with RA compared with that observed in those patients with traumatic joint injury, which, however, was similar to that of patients with AS.

Isolation and Characterization of Growth Factor(s) From a Human B-Cell Lymphoma

We demonstrate that human neoplastic B cells (Br cells) contain a cytoplasmic protein of molecular mass 60 Kd that exhibits B-cell growth factor (BCGF) activity on growth factor-dependent long-term human B cells as well as on autochthonous tumor cells. This 60-Kd protein is recognized by antibodies against a similar intracellular 60-Kd protein derived from normal human lymphocytes. These results demonstrate that the two proteins share epitope homology. Microculture bioassays indicate that neoplastic and normal 60-Kd proteins are capable of driving neoplastic B cells through S-phase. Western immunoblot analysis indicates that neoplastic B cells secrete 60- as well as 14-Kd protein. Immunoaffinity-purified proteins secreted by Br cells exhibit BCGF activity in anti-IgM or dextran sulfate-preactivated human B cells. In addition, a double-antibody immunofluorescence staining technique was used to demonstrate that Br cells express cell surface receptors for BCGF molecule(s). These studies provide support for the autocrine growth model for neoplastic human B cells and suggest that the autocrine growth factor derived from such tumor cells is similar if not identical to normal BCGF molecules.

Isolation and characterization of growth factor(s) from a human B-cell lymphoma

We demonstrate that human neoplastic B cells (Br cells) contain a cytoplasmic protein of molecular mass 60 Kd that exhibits B-cell growth factor (BCGF) activity on growth factor-dependent long-term human B cells as well as on autochthonous tumor cells. This 60-Kd protein is recognized by antibodies against a similar intracellular 60-Kd protein derived from normal human lymphocytes. These results demonstrate that the two proteins share epitope homology. Microculture bioassays indicate that neoplastic and normal 60-Kd proteins are capable of driving neoplastic B cells through S-phase. Western immunoblot analysis indicates that neoplastic B cells secrete 60- as well as 14-Kd protein. Immunoaffinity-purified proteins secreted by Br cells exhibit BCGF activity in anti-IgM or dextran sulfate-preactivated human B cells. In addition, a double-antibody immunofluorescence staining technique was used to demonstrate that Br cells express cell surface receptors for BCGF molecule(s). These studies provide support for the autocrine growth model for neoplastic human B cells and suggest that the autocrine growth factor derived from such tumor cells is similar if not identical to normal BCGF molecules.

Human B cell proliferation in response to IL-4 is associated with enhanced production of B cell-derived growth factors.

To investigate the capacity of human IL-4 to function as a B cell growth factor (BCGF), we studied its ability to promote proliferation of a selected B cell line. We show that the cell line, designated A4, proliferated in response to IL-4 in a dose-dependent manner. The A4 cells also proliferated in response to their own B cell derived growth factor (B. BCGF), suggesting autocrine-mediated growth. The ability of IL-4 to induce proliferation of the A4 cell line was dependent on the level of autocrine growth. At low cell density, IL-4 induced marked dose-dependent proliferation. However, as A4 cell density increased, the ability of IL-4 to induce proliferation was diminished. The possibility that IL-4 may be mediating the autocrine growth of A4 cells was ruled out, because A4 cell-derived BCGF failed to induce CD23/low affinity receptors for the Fc region of IgE on activated tonsillar B cells and anti-IL-4 antibody did not block B. BCGF activity. We found that IL-4 stimulation of A4 cells and activated tonsillar B cells is associated with enhanced production of B. BCGF. These data indicate that human IL-4 has the capacity to promote proliferation of the B cell line A4, and that the ability of IL-4 to function as BCGF is associated with enhanced autocrine growth of activated B cells.

Human B cell growth factor enhances proliferation and glial fibrillary acidic protein gene expression in rat astrocytes

The proliferation and differentiation of astrocytes are fundamental events in the normal development and function of the central nervous system (CNS), and may also contribute to the pathogenesis of a number of neurological diseases. Products of T lymphocytes can stimulate proliferation of astrocytes, but the nature of the T lymphocyte-derived molecule(s) responsible for this response is unknown. The present study was undertaken to examine several well-characterized T lymphocyte-derived factors for their ability to stimulate cultured primary rat astrocytes. While recombinant human interleukin-2 (IL-2), interleukin-3 (IL-3), interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-6 (IL-6), and rat or human recombinant interferon-gamma (IFN-gamma) have no proliferative effect on astrocytes, a human T cell-derived B cell growth factor (BCGF) does. This BCGF, termed 2B11, had previously been characterized by its ability to enhance the proliferation of anti-mu-stimulated human B cells, while not influencing B cell immunoglobulin synthesis. High performance liquid chromatography (HPLC)-purified 2B11-BCGF (MW approximately 20,000 daltons) stimulates the proliferation of astrocytes in a dose-dependent fashion. Purified 2B11-BCGF also induced morphological differentiation and increased mRNA transcripts for glial fibrillary acidic protein (GFAP) in rat astrocytes. In addition to demonstrating the absence of effect of other known lymphokines, the effect on astrocytes attributed to 2B11-BCGF was confirmed by blocking its activity with a monoclonal antibody specific for 2B11-BCGF. Absorption experiments demonstrated that when BCGF activity was absorbed out by large, activated human B cells, astrocyte-stimulatory activity was also depleted. Rat astrocytes were able to partially absorb out both BCGF and astrocyte-stimulatory activity. These results suggest that 2B11-BCGF is responsible for stimulating astrocyte proliferation, and that human B cells and rat astrocytes may share a similar receptor for BCGF. These findings indicate that the growth and differentiation of astrocytes can be influenced by a T cell-derived lymphokine, 2B11-BCGF, whose activity thus far appears to be distinct from other reported cytokines.

B cell growth factor (BCGF) secreting human T cell clones reactive to a soluble glycoprotein antigen, streptococcal antigen (SA)

A panel of human T cell clones bearing exclusively the helper (T4) phenotype and showing reactivities to a soluble glycoprotein antigen (185,000 dalton Mol. Wt. Streptococcal antigen, SA) is described. Two of these clones namely, SA 1.53 and SA 1.82, are found to co-produce B cell growth factor (BCGF) and interferon-gamma (IFN-γ) in the absence of interleukin 2 (IL 2) upon stimulation with phytohaemagglutinin (PHA) or the specific antigen in the presence of irradiated autologous antigen-presenting cells (APC). Secretion of the lymphokines is genetically restricted in part by DR molecules that are expressed on the cloned cells and APC. Produced BCGF is differentiated from the BCGF-promoting property of IFN-γ in that only IFN-γ activity, but not BCGF activity is removed and inhibited by anti-IFN-γ antibodies. Exogenous IL 2 induces secretion of BCGF and IFN-γ of the cloned cells, an observation which involves interaction of IL 2 with IL 2 receptors. An analysis of the proliferative responses to antigen of the T cell clones shows that BCGF-producing clones, unlike those that secrete IL 2, fail to proliferate significantly to specific antigen restimulation.

Generation of large granular lymphocytes and lymphocyte subset changes linked with cyclophosphamide-induced eosinophilia in rats--and the effects of ciclosporin.

Administration of cyclophosphamide (Cy: 150 mg/kg i.p.) to rats 48 h before immunization with a T-dependent antigen (ovalbumin) resulted in a striking absolute eosinophilia in blood, bone marrow, and secondary lymphoid organs after 10 to 14 days. This eosinophilia was preceded by a significant increase in the W3/25+/OX-8+ (T helper/inducer to T cytotoxic/suppressor) ratio in lymph nodes and spleen and accompanied by a pronounced rise in splenic OX-12+ (B cell) numbers. There was also a concomitant increase in cells with the morphology and immunophenotype (OX-8+, OX-19-) of large granular lymphocytes (LGL). It is suggested that the eosinophilia linked with the B lymphocytosis may be due to cell-derived soluble factors, including a possible equivalent of eosinophil differentiation factor (EDF = interleukin 5), which also has B-cell growth factor activity (BCGF II) in mice. Ciclosporin (CsA; 25 mg/kg/day per os) from the time of immunization, did not affect the incidence of W3/25+ cells in spleen or lymph nodes, but abrogated Cy-induced eosinophilia and reduced the extent of B-cell proliferation. In addition, CsA caused a further, marked increase in the incidence of OX-8+, OX-19-LGL within the spleen. The functional role(s) of these latter cells remains to be defined.

Identification of a major 50-kDa molecular weight human B-cell growth factor with Tac antigen-inducing activity on B cells

A bioassay was developed using human small B cells adherent to anti-human IgM (anti-μ)-coated wells. These B cells were stimulated to proliferate by culture supernatants of concanavalin A (Con A)-activated human peripheral blood lymphocytes (Con A Sup) even in the presence of high concentrations of anti-μ coated on assay wells. Human B-cell growth factor (BCGF) activities were partially purified from Con A Sup. Preparative chromatography (Sephacryl S-200 and isoelectrofocusing) yielded a major peak of BCGF activity for B cells adherent to anti-μ-coated wells with a molecular weight of 50,000 (50 kDa) and a p I 7.6. The 50-kDa BCGF was further purified by sequential chromatography using DEAE-Sephacel, CM-Sepharose, Sephacryl S-200, CM-high performance liquid chromatography (HPLC), and hydroxyapatite (HA)-HPLC. The HA-HPLC-purified 50-kDa BCGF was free of interleukin-1 (IL-1), interleukin-2 (IL-2), and interferon activities, but could support growth of BCL 1 cells, similar to BCGF-II. Neither IL-1 nor interferon-γ had any growth-stimulating effect in our B-cell proliferation assay with or without BCGF in Iscove's synthetic assay medium. BCGF-induced proliferation of B cells adherent to anti-μ-coated wells could be markedly augmented by the simultaneous or sequential addition of recombinant human IL-2 (rIL-2). When cultured for 3 days with 50-kDa BCGF, about 40% of B cells adherent to anti-μ-coated wells expressed Tac antigen, and monoclonal anti-Tac antibody inhibited rIL-2 enhancement of proliferation of 50-kDa BCGF-preactivated B cells. In addition, 50-kDa BCGF could induce Tac antigen on an Epstein-Barr virus-transformed B-cell line (ORSON) in the presence of a suboptimal dose of phorbol myristate acetate (PMA) and also on a natural killer-like cell line (YT cells). We have therefore identified a major 50-kDa BCGF activity with Tac antigen-inducing activity that also has a synergistic effect with IL-2 on normal B-cell proliferation.

Anti‐proliferative effects of interferons on Daudiburkitt Lymphoma cells: Induction of cell differentiation and loss of response to autocrine growth factors

Treatment of Daudi B-lymphoblastoid cells with low concentrations of either natural or recombinant human alpha-interferons inhibits cell proliferation and modulates the expression of a number of cell-surface antigens. Using a panel of monoclonal antibodies (MAbs) identifying determinants expressed at the surface of normal plasma cells, and polyclonal antibodies against surface and cytoplasmic immunoglobulin, we have found that growth inhibition is accompanied by plasmacytoid differentiation. Assays of growth stimulation of heterologous cells indicate that the culture medium from interferon-treated Daudi cells contains substantially more B-cell growth factor activity than that from control cells. However, the interferon-treated cells exhibit an impaired ability to respond to both these autocrine factors and exogenous factors produced by another Burkitt lymphoma line. These findings show that, in the case of Daudi cells, growth inhibition by interferons is closely associated with both terminal differentiation and a refractoriness to growth factors. In this system IFN-alpha may therefore be considered to be a B-cell differentiation factor, suggesting a possible basis for the anti-proliferative effects observed with certain human B-cell malignancies.

Distinct culture requirements for activation and proliferation of human peripheral blood B lymphocytes by Staphylococcus aureus cowan strain and growth factors

The concentration of fetal calf serum (FCS) and the culture period were the crucial culture conditions in measuring B cell growth factor (BCGF) and IL-2 activity in vitro. Higher concentrations of FCS (10–15%) significantly inhibited BCGF activity; whereas, lower concentrations of FCS (<2.5%) were not sufficient for the response to IL-2. Kinetic experiments showed that the culture period for BCGF should be shorter than that for IL-2, while BCGF in combination with IL-2 induced a synergistic proliferation of B cells in a longer culture period. Adding BCGF after 48 h of SAC stimulation reduced the reaction. Hence, the conventional method using preactivated B cells does not measure BCGF but mostly IL-2. Furthermore, minute amounts of BCGF activity can be more sensitively determined by co-culturing with fixed amounts of IL-2.

Recombinant human interleukin 5 is an eosinophil differentiation factor but has no activity in standard human B cell growth factor assays

Following the observation that mouse interleukin 5 (IL5) is active as a B cell growth factor (BCGF) as well as an eosinophil differentiation factor, this work was carried out to test recombinant human IL5 for BCGF activity. A highly active, partially purified batch of recombinant human IL5 was prepared and tested for BCGF activity in four laboratories. This batch gave a 50% endpoint of 1:77,450 in the human eosinophil differentiation assay, 1:983 in the mouse eosinophil differentiation assay and 1:42 in the mouse BCL1 assay, thus demonstrating that, like mouse IL5, human IL5 has cross-species activity. By comparison with the assays in the mouse this batch would be expected to have 50% maximal human BCGF activity of about 1:4000. In each assay a known positive factor was used as a positive control, and there was no inhibitory activity in the preparation. However, despite the activity towards the mouse B cell lymphoma, the results showed no detectable activity in a panel of assays used to identify human BCGF and B cell differentiation factors. These assays included (a) proliferation assays with tonsillar or splenic B cells in the presence of the co-stimulators anti-mu or phorbol myristate acetate; (b) a restimulation assay in which tonsillar B cells are first activated with either Staphylococcus aureus Cowan 1 or a mixture of phorbol dibutyrate and ionomycin, or splenic B cells are first activated with anti-mu; (c) production of immunoglobulin by B cells in a restimulation assay with Staphylococcus aureus Cowan 1; (d) production of immunoglobulin by the Epstein-Barr virus-transformed B lymphoblastoid CESS cell line; (e) the ability to stimulate proliferation of chronic lymphocytic leukemia (B-CLL) cells freshly explanted from three different patients; (f) the ability to stimulate the B lymphoma (L4) cell line and the mature B cell (HBF1) line, and (g) the ability to replace T cells in specific antibody responses. It therefore seems unlikely that recombinant human IL5 is either a growth or a differentiation factor for human B cells, and raises the interesting question of the biological significance of the BCGF activity of this factor in the mouse.

Human interferon-gamma acts as a B cell growth factor in the anti-IgM antibody co-stimulatory assay but has no direct B cell differentiation activity.

In this study it is illustrated that recombinant human interferon-gamma (IFN-gamma) acts as a B cell growth factor (BCGF) in the anti-IgM antibody co-stimulatory assay. A monoclonal antibody that specifically inhibits the biological activities of IFN-gamma blocks its BCGF activity supporting the specificity of the IFN-gamma effect. Various IFN-gamma obtained from different sources displayed the same BCGF activity. Nonactivated B lymphocytes do not proliferate in response to IFN-gamma. IFN-gamma acts directly on B cells because highly purified B cells obtained after standard purification procedures coupled to cell sorting could still proliferate in response to IFN-gamma. Blood B lymphocytes were found to be more sensitive to the BCGF activity of IFN-gamma than B cells obtained from spleens or tonsils. The IFN-gamma-induced proliferation of B cells was short lasting when compared with that of recombinant IL 2 or BCGF containing T cell clone supernatants. B cells preactivated with either Staphylococcus aureus strain Cowan I (SAC) or optimal concentrations of anti-IgM antibodies coupled to beads did not proliferate in response to IFN-gamma, whereas they proliferated in response to IL 2 or T cell clone supernatants. IFN-gamma did not stimulate nor inhibit the proliferative response of human B lymphocytes stimulated with optimal concentrations of anti-IgM antibodies or SAC. Additionally none of the different IFN-gamma tested had B cell differentiation factor activity in the standard SAC assay. These results indicate that IFN-gamma sensitizes B cells to suboptimal mitogenic concentrations of anti-IgM antibody.

Production of B cell growth factor by normal human B cells.

Although it has been demonstrated that malignant human B cell lines are capable of producing B cell growth factor (BCGF), production of BCGF by normal B cells has not been shown. In this study, we demonstrate BCGF production by normal B cells, achieved by using human peripheral blood B cells prepared by a positive selection technique and stimulated with Staphylococcus aureus Cowan I (SAC) for 12 hr. SAC was removed from the supernatants by anti-SAC-coupled Sepharose. Supernatants absorbed with this antibody were functionally free of SAC, as demonstrated by their inability to activate resting B cells. B cells stimulated with SAC for 12 hr produced BCGF activity that was generally unmeasurable in supernatants by 36 hr. Characterization of BCGF produced by SAC-stimulated B cells revealed a m.w. of 32,000 by high-performance liquid chromatography sieving and sodium dodecyl sulfate-polyacrylamide gel electrophoresis; this BCGF was found to have an isoelectric point of 6.7. Furthermore, this BCGF lacked interleukin 1, interleukin 2, interferon, and B cell differentiation factor activity. This observation that BCGF can be produced by normal human B cells is significant because it demonstrates for the first time that normal B cells have the ability to provide their own growth factors or the growth factors for other B cells.