T cell-replacing factor in specific antibody responses to influenza virus by human blood B cells

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.

Human T cell-replacing factor(s): a comparison of recombinant and purified human B cell growth and differentiation factors

Conditioned medium from phytohemagglutinin-activated T cells contains T cell-replacing factor(s) (TRF) able to restore specific antibody responses by human blood or tonsillar B cells which have been thoroughly depleted of T cells. Of twelve recombinant cytokines tested as possible candidates for TRF in conditioned media, namely human recombinant interleukin (hrIL) 1 alpha and beta, hrIL2, hrIL3, hrIL4, hrIL5, hrIL6, hrIFN-alpha and -gamma, hr granulocyte macrophage colony-stimulating factor (hrGM-CSF) and tumor necrosis factor (hr TNF)-alpha and -beta only IL2 was found to have TRF activity. In addition, a semi-purified low molecular weight B cell growth factor (BCGFlow) also had TRF activity. As the commercially available BCGFlow is known to contain low concentrations of IL2, IFN-gamma, TNF and GM-CSF as impurities, it was important to exclude these as being responsible for the TRF activity. At the concentrations present in BCGFlow (less than 0.2 U/ml), IL2 was not active in the TRF assay. In contrast, a combination of IL2 (0.2 U/ml), IFN-gamma (50 U/ml), TNF-alpha (50 U/ml) and TNF-beta (100 U/ml) did have TRF activity suggesting that B cells could be made to respond to low doses of IL2 by the presence of other cytokines. Although this finding raises important questions about the nature of TRF in conditioned medium, the TRF activity of BCGFlow was unlikely to be due to such a synergistic combination of cytokines for the following reasons. First, in several experiments, responses were obtained with BCGFlow, but not with IL2 or combinations of IL2 with IFN and TNF. Second, antibody to IL2 was found to inhibit the TRF activity of IL2 but not of BCGFlow. Taken together these findings show that two distinct cytokines (IL2 and BCGFlow) are TRF for human B cells. However, some combinations of cytokines can also have TRF activity underlining the complexities which can arise from working with semi-purified rather than recombinant factors.

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.

Interleukin 2 and low molecular weight B cell growth factor are T cell-replacing factors for different subpopulations of human B cells

Both recombinant human interleukin 2 (rhIL 2) and low molecular weight B cell growth factor (BCGFlow) were shown to be T cell-replacing factors (TRF) in specific antibody responses to influenza virus by human blood and tonsillar B cells. When B cells were separated into high and low-density populations on Percoll gradients at 1.074 kg/l, IL 2 was found to act as a TRF only on the low-density B cells, whereas BCGFlow was a TRF for high-density B cells with a lesser effect on low-density B cells. Both populations of B cells responded well in the presence of T cells. The high-density B cells could not be activated to respond to IL 2 by either IL 1, rhIL 4 or by a CD22 monoclonal antibody known to enhance B cell activation. In contrast, a 24-h preincubation with T cells and antigen appeared to prime high-density B cells to respond to IL 2. These results show that high-density B cells can in fact respond to TRF, and that IL 2 and BCGFlow act on different populations of B cells which may be defined by prior exposure to T cells.

Anti-CD3 antibodies induce T helper function for human B cell differentiation in vitro by an interleukin 2-independent pathway

Previously we have shown that interleukin 2 (IL 2) is an essential mediator in T cell-dependent B cell differentiation induced by pokeweed mitogen. Here we show that activation with monoclonal anti-CD3 antibodies of peripheral blood T cells led to the induction of helper activity for IgM secretion by human B cells from a prolymphatic leukemia. With the use of monoclonal antibodies against the IL 2 receptor and CD3+CD4+CD8- chronic lymphatic leukemia T cells with a strongly reduced capacity to produce IL 2, it was demonstrated that the anti-CD3-driven Ig secretion was obtained by an IL 2-independent pathway. The T cell help in this system is mediated by soluble factors.

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.

Analysis of T cell-replacing factor-like activity: potent induction of T helper activity for human B cells by residual concanavalin A and interleukin 2

At least two factors with the capacity to induce IgM synthesis in human B cells were found to be present in the 15-20-kDa fraction of the supernatant of mononuclear cells activated with concanavalin A (Con A) and phorbol ester. Previously, it has been shown (Sauerwein, R. W. et al., Eur. J. Immunol. 1985. 15: 611) that interleukin 2 (IL2) in this material is able to induce T cell-dependent IgM secretion in normal B cells. Evidence was obtained for the presence of another factor distinct from IL2 that could replace T cells in the induction of B cell differentiation. We have analyzed this factor with the use of a neoplastic B cell population of prolymphocytic origin that was functionally nonresponsive to IL2. T cell-replacing factor (TRF)-like activity and IL2 could be separated by ion-exchange chromatography, although a small amount of IL2 was recovered in the TRF fractions. This small amount of IL2 was found to be crucial for the observed TRF activity. Moreover, a substantial amount of monomeric Con A was detected in the TRF preparation. Our studies show that Con A in the presence of IL2 can act as a potent inducer of helper function in lower numbers of T cells for normal and neoplastic B cells. Functional assays for T cell contamination in B cell suspensions are therefore of limited value because they are determined by the efficiency of the stimulating signal. Particularly in those B cell factor preparations, obtained from mitogen-activated T cells with an obligatory or unidentified role of IL2, the possible effect of a contaminating mitogen must be considered.

Functional heterogeneity of nonresting B cells in human blood

Among human peripheral blood B cells we localized the precursors of two interleukin-dependent B cell activation processes: the specific response to a particulate antigen, trinitrophenylated polyacrylamide beads (TNP-PAA) and the polyclonally induced response to pokeweed mitogen. In both cases the precursors belong to the OKB7+, sIgD-, mouse red blood cell- subpopulation. However, they differ when cell density, reflecting the stage of activation reached by B cells in peripheral blood, is considered. Only B cells of intermediate density respond to TNP-PAA, whereas the optimal response to pokeweed mitogen is obtained with the cells displaying the lower density. The lack of response of the more dense (resting) B cells to TNP-PAA suggests that the T dependency of this antigen is not based on linked recognition, and fits with our demonstration that this particulate antigen can trigger B cells in the presence of T cell factor. More importantly, our results show that nonresting B cells are functionally heterogeneous according to their degree of preactivation: the responsiveness to specific signals provided by a nonmitogenic hapten-carrier conjugate would be acquired before that to polyclonal activators.

T cell help in human antigen-specific antibody responses can be replaced by interleukin 2

Recombinant IL 2, and immunosorbent/high performance liquid chromatography-purified interleukin 2 (IL 2) obtained from the human T cell leukemic line Jurkat, but not interferon-alpha or -gamma, were able to substitute for T cells in specific antibody responses to influenza virus by T cell-depleted (E-) human peripheral blood mononuclear cells, and resulted in antibody formation equivalent to that obtained in the presence of T cells. The antibody response was shown to be antigen specific by using two non-cross-reacting strains of influenza virus (A/X31 and B/HK). IL 2 in this assay therefore functions as a T cell-replacing factor. Less than 1% of T (UCHT1+) cells were present in the E- preparations, and this number did not increase during the 7-day culture with antigen and IL 2. Because the frequency of T helper cells for X31 is known to be less than 5 X 10(-5), this low number of contaminating cells excluded indirect action of IL 2 through antigen-specific T helper cells. Three to four times less IL 2 was required for antibody production by E- cells than was needed for optimal proliferation by an IL 2-dependent T cell line. Moreover, the concentration of anti-Tac required for 50% inhibition of the IL 2-induced antibody response was 50 times less than required for 50% inhibition of IL 2-dependent proliferation by the T cell line. But when T cells were added back to the E- cells, the anti-Tac inhibition curve shifted back to that obtained with the T cell line. In cell labeling experiments, Leu 11+ cells but not HNK1+ cells were increased in E- cells cultured with antigen and IL 2. This increase in Leu 11+ cells was abolished by prior passage of the E- cells through Sephadex G-10 columns without affecting the IL 2-induced antibody response. From these experiments we conclude that IL 2 can replace T cells in specific antibody responses, and that the IL 2 effect is not mediated indirectly through T cells or large granular lymphocytes.

Differentiation factors for human specific B cell response

Nonspecific T cell factors produced by lectin-activated human peripheral blood lymphocytes (PBL) were used to restore the T-dependent B cell response to trinitrophenyl-polyacrylamide (TNP-PAA). Preincubation experiments with the particulate antigen TNP-PAA and/or a soluble TNP-protein conjugate show that a first specific signal provided by the antigen and nonspecific lymphokines sequentially acts on B cells. By gel filtration the T cell-replacing factor (TRF) activity is present in the 30-15-kDa fraction of T cell supernatants and is associated to interleukin 2 (IL2). However, absorption of IL2 does not abolish the TRF activity. Moreover, chromatofocusing of this 30-15-kDa material allows the obtaining of an IL2-free fraction containing a differentiation factor (with an isoelectric point of 5.7 +/- 0.2). The ability of this fraction to restore the anti-TNP response is manifest in the presence of a 50-kDa B cell growth factor. This latter, prepared by a combination of absorption on concanavalin A-Sepharose and gel filtration, was IL2 free and unable to support the anti-TNP response. We thus directly demonstrate that in the absence of IL2 three separate signals (the antigen, T cell-derived growth and differentiation factors) are involved in human-specific B cell response.

Antigen-specific suppression of human antibody responses by allogeneic T cells. II. Cell interactions involved in the generation of suppression

Specific antibody responses to influenza virus were obtained in vitro from human blood mononuclear cells (PBMC). Antibody production in these cultures was profoundly suppressed by the addition of allogeneic T cells with the surface phenotype Leu2a+ (CD8+), Leu8-. Suppression by allogeneic T suppressor (Ts) cells required interactions only between T-depleted B (E-) cells and allogeneic Leu2a+. No evidence was obtained for T-T cell interactions, or for Ts inducer cells similar to those described for nonspecific antibody responses to pokeweed mitogen. Moreover, allogeneic E+, or allogeneic Leu2a+ cells were able to suppress specific antibody responses by E- cells when help was provided by T cell-replacing factor showing that the target of suppression was the responding E- cells, and not T helper cells. In contrast to allogeneic T cells, allogeneic E- cells did not suppress antibody production when added to cultures of unfractionated PBMC (E- + E+). That is, Ts cells activated to allogeneic E- were unable to suppress antibody production by the syngeneic E- cells present in the same culture tube. This result shows that alloactivated Ts cells were specific for the allogeneic E- target cells, and that suppression was not mediated by nonspecific allogeneic effects. Allogeneic Ts cells therefore differ from Ts cells in pokeweed mitogen responses by their specificity, and by their activation in the absence of Ts inducer cells.