Intrahepatic, nucleocapsid antigen-specific T cells in chronic active hepatitis B

Hepatitis B core antigen (HBcAg)-specific T cell lines were established from hepatic lymphomononuclear cells derived from five patients with chronic active hepatitis B. No hepatitis B virus envelope antigen-specific cell lines were established. Proliferation in response to recombinant and native HBcAg, but not to native hepatitis B surface antigen containing the pre-S(2) region, confirmed the specificity of the five T cell lines. All cell lines represented mixed populations of CD4+ and CD8+ T cells. The CD4+ subset provided antigen-specific help to autologous B cells with respect to anti-HBc production and to CD8+ cells with regard to HBcAg-induced proliferation and suppressor activity. The CD8+ subset contained suppressor cells that selectively inhibited the proliferative response of autologous HBcAg-specific CD4+ cells without inhibiting CD4+ cells of unrelated specificity (tetanus toxoid). Moreover, the CD8+ cells were also capable of suppressing HBcAg-stimulated antibody to HBcAg production without showing inhibition of total immunoglobulin production stimulated by pokeweed mitogen. The cytotoxic potential of the T cell lines was established in a lectin-dependent cytotoxicity system; natural killer cytotoxicity was completely absent. Our data suggest that the lesional T cells present at the site of hepatocellular injury in chronic active hepatitis B are primarily HBcAg-specific lymphocytes of the helper and suppressor/cytotoxic phenotypes and that both are functionally competent.     

Influence of human T lymphocytes identified by antibodies to dipeptidyl peptidase IV on differentiation of human B lymphocytes stimulated with Staphylococcus aureus Cowan I and pokeweed mitogen

The influence of human T lymphocytes expressing the enzyme dipeptidyl peptidase IV (DPP IV) was investigated with respect to human peripheral B-lymphocyte differentiation. B cells stimulated with pokeweed mitogen in the presence of DPP IV-positive T cells produced high amounts of immunoglobulin. Moderate amounts of immunoglobulin could be measured when B cells were cultured in the presence of DPP IV-negative T cells. DPP IV defines a T-cell subset partially overlapping the subsets characterized by the differentiation antigens Leu 3a (helper/inducer) and Leu 2a (suppressor/cytotoxic). DPP IV-positive T cells exert, in contrast to DPP IV-negative T cells, high interleukin-2 activity after stimulation with phytohemagglutinin and pokeweed mitogen. To further functionally characterize DPP IV-positive and DPP IV-negative T cells, the helper effects of Leu 3a-positive T-cell subsets, differing in DPP IV expression, were investigated in pokeweed mitogen- and Staphylococcus aureus-driven B-cell differentiation systems. After pokeweed mitogen stimulation, immunoglobulin production was markedly reduced when B cells were cultured in the presence of Leu 3a-positive T cells expressing DPP IV (DPP IV+/Leu 3a+). In contrast, high amounts of immunoglobulin were produced in cultures with Leu 3a-positive but DPP IV-negative T cells (DPP IV-/Leu 3a+). This difference in immunoglobulin production of B cells cultured with DPP IV+/Leu 3a+ and DPP IV-/Leu 3a+ T cells could not be observed in Staphylococcus aureus-stimulated cultures. Here, both T-cell subsets supported terminal differentiation of B cells. We conclude that in the pokeweed mitogen-driven culture systems, DPP IV+/Leu 3a+ and DPP IV-/Leu 3a+ T cells may differ in the production of growth and/or differentiation factors distinct from interleukin-2.     

Analysis of the antigen- and mitogen-induced differentiation of B lymphocytes from asymptomatic human immunodeficiency virus-seropositive male homosexuals. Discrepancy between T cell-dependent and T cell-independent activation

Five asymptomatic human immunodeficiency virus (HIV)-seropositive male homosexuals were immunized with the recall antigens tetanus toxoid (TT) and the three types of poliovirus present in diphtheria, tetanus, and polio vaccine. Four weeks after immunization, the in vivo response to booster immunization, the in vitro pokeweed mitogen (PWM)-induced IgG secretion, and the in vitro T cell-dependent and T cell-independent antigen-induced antibody response were assayed. Increase in serum antibody titer to TT and poliovirus was low and normal, respectively. In all five subjects studied, a high rate of spontaneous IgG production, including antibodies directed toward HIV was observed. Addition of PWM to the cultures induced suppression of the spontaneous IgG secretion. Only one donor showed a slightly increased IgG production after stimulation with PWM. Peripheral blood mononuclear cells of four of the five HIV-seropositive individuals did not produce TT, or poliovirus-specific antibodies when stimulated with the respective T cell-dependent antigens. However, stimulation of these peripheral blood mononuclear cells with TT coupled to agarose beads, which was shown to be T cell-independent, resulted in the generation of IgG anti-TT antibody-forming cells.     

Regulatory interactions governing the proliferation of T cell subsets stimulated with pokeweed mitogen

Serial phenotyping of human peripheral blood mononuclear cells (PBMC) cultured with pokeweed mitogen (PWM) demonstrated an excess of T8+ cells after stimulation. Preferential expansion of the T8+ cell compartment was a result of T8+ cell blast transformation while T4+ cells generated fewer blasts and tended to remain as small resting cells. When the proliferative behavior of T cell subsets in PWM-stimulated PBMC with physiologic proportions of T4+ and T8+ cells was compared with that of cultures depleted of T4+ or T8+ cells, two levels of regulation of proliferation were found: without T4+ cell help, T8+ cells were unable to divide; however, in the presence of T4+ cells, PWM-stimulated T8+ cells became potent feedback inhibitors of T4+ cell proliferation. The mechanism of suppression by PWM-activated T8+ cells of T4+ cell proliferation, not only to PWM, but also to tetanus toxoid, was pursued by measuring decreased interleukin 2 (IL2) recovery from cultures containing suppressors. Although passive absorption of IL2 by PWM-activated cells could contribute to the suppression of fresh proliferative responses, as shown directly with isolated T4+ cells induced by PWM to express IL2 receptors, a much more profound suppression was mediated by PWM-activated T8+ cells. The regulation of proliferative responses of helper and suppressor T cell subsets may determine the magnitude of their subsequent interactions and thus control the ultimate outcome of in vivo physiologic and pathologic immune responses.     

Antigen-specific human B-cell responses: modulation by immunoregulatory T-cell subsets

In vitro T-cell requirements for and modulation of human B-cell responses were studied in individuals immunized in vivo to the protein antigen keyhole limpet hemocyanin or tetanus toxoid. T cells were required for antibody synthesis in both antigen-driven and pokeweed mitogen (PWM)-driven cultures. T cells were separated into T4+ and T8+ subpopulations using monoclonal antibodies, and their modulation of antibody synthesis was studied. T4+ cells functioned as helper cells in both antigen-driven and PWM-driven cultures in a dose-dependent manner. Whereas T8+ cells suppress both total and specific immunoglobulin secretion in PWM-stimulated cultures, in antigen-stimulated cultures T8+ cells do not suppress unless activated by another cell population present in peripheral blood mononuclear cells (PBMNC). This cellular requirement was further investigated by prestimulation of cells prior to addition to optimally stimulated antigen-driven cultures of PBMNC or B cells, monocytes, and helper T cells. No suppression of these optimally stimulated cultures was seen when T8+ cells were precultured with antigen or PWM. However, after 3-5 days preculture of total T cells with PWM or antigen and then selection of T4+ cells, these cells were able to induce fresh autologous T8+ cells to suppress optimally stimulated antigen-driven cultures. Addition of a precultured mixture of T8+ cells with 20% T4+ cells also resulted in antigen-induced suppression. These data indicate that T8+ cells can suppress antigen-driven cultures but require the presence of preactivated T4+ cells for induction of this suppression of antigen-specific T-cell-dependent human B-cell responses.     

In vitro human antibody production to the Haemophilus influenzae type b capsular polysaccharide

In vitro production of human antibody to the Haemophilus influenzae type b capsular polysaccharide (PRP) and to tetanus toxoid (TT) and diphtheria toxoid was measured in culture supernatants of peripheral blood mononuclear cells and by enumeration of antibody secreting cells (AbSC) in an enzyme-linked immunosorbent-plaquing assay. Normal adult peripheral blood mononuclear cells stimulated with Epstein-Barr virus secreted anti-PRP antibody with a frequency of 1/552 to 1/1190 relative to total Ig secreting cells; the frequency of AbSC to tetanus toxoid (TT) was 7.5 times higher (p less than 0.05). These frequencies did not change significantly after in vivo immunization, although the isotype distribution shifted toward increased IgG for TT and increased IgG and IgA for PRP. At 8 days postimmunization, spontaneous AbSC to PRP and TT were detected; frequencies for total anti-TT AbSC again being higher than anti-PRP, but there were significantly more IgA plaques among anti-PRP AbSC. Spontaneous AbSC were suppressed in culture by pokeweed mitogen and enhanced by cyclosporine. Three wk after in vivo immunization with PRP and TT, in vitro stimulation with pokeweed mitogen, Staphylococcus aureus Cowan 1 bacteria, or antigen induced anti-TT but not anti-PRP in vitro antibody secretion, although Epstein-Barr virus induced both. These data suggest that PRP, a polysaccharide, and TT, a protein, differ in their requirements for in vitro activation with antigen and mitogens.     

Sodium periodate-induced suppressor T cells of the human in vitro antibody response

Combinations of T and B lymphocytes from normal individuals booster immunized 14–30 days previously with a combination of diphtheria and tetanus toxoids, synthesized IgG antitetanus toxoid, and IgG antidiphtheria toxoid antibodies when stimulated by pokeweed mitogen in vitro. The addition of 5 μg of soluble tetanus toxoid to the cultures during the first 2 days incubation resulted in greater than 90% suppression of the subsequent production of IgG antitetanus toxoid antibodies. The synthesis of IgM antitetanus toxoid antibodies, total IgG, total IgM, and IgG antidiphtheria toxoid antibodies were unaffected. Similarly, the addition of 5 μg of soluble diphtheria toxoid suppressed the synthesis of IgG antidiphtheria toxoid antibodies with no effect on the synthesis of IgG antitetanus antibodies. Allogeneic combinations of B and T lymphocytes were capable of mediating the suppression, and irradiation of the T cells caused only a partial and variable reversal of the suppression. The antigen-induced specific suppression of antibody synthesis could not be demonstrated in cultures stimulated with soluble T-cell-derived helper factors.     

Inhibition of human antigen-induced lymphoblastoid B-cell function by an in vivo-induced suppressor T cell

Lymphoblastoid (LB) B cells which spontaneously produce antitetanus toxoid IgG antibodies (Tet-IgG) in short-term cultures (3 days) appear in the circulation 5-7 days after immunization with tetanus toxoid. Addition of pokeweed mitogen (PWM), normally a stimulator of antibody production, caused instead a reduction in the in vitro synthesis of Tet-IgG by the LB cells. In order for this inhibition of antibody production to occur, T cells had to be present, and the inhibition was proportional to the number of T cells added to the culture, demonstrating the existence of PWM-inducible suppressor cells. The cells mediating the suppression had the OKT8 phenotype and also exhibited the following characteristics: (1) a PWM pretreatment period as little as 14 hr was enough to complete activation; (2) conventional inhibitors of suppressor T cells as hydrocortisone and cyclosporin A only partially reversed its effect; and (3) DNA synthesis was not required. The T-suppressor activity was detectable in the circulation before immunization, increased two- to fourfold by 5-12 days after boosting, and waned after 3 weeks. The mechanism of action of this suppression does not appear to involve conventional cytotoxic T cells as (1) the suppression was mediated across allogeneic barriers and (2) the suppression could not be reversed by inclusion of anti-Leu-2a antibodies in the culture. These results suggest that this suppressor T-cell subset may be important in the normal regulation of activated stages of human B lymphocytes.     

Suppression of immune cell function in vitro by recombinant human transforming growth factor-beta

The influence of recombinant human transforming growth factor-beta (rHuTGF-beta) on B-cell function and antigen-specific T-cell responses in vitro was investigated. The addition of 0.1 ng/ml of rHuTGF-beta to cultures of peripheral blood mononuclear cells (PBMC) stimulated with tetanus toxoid resulted in a 50% inhibition of the PBMC proliferative response as determined by [3H]thymidine incorporation. Further, rHuTGF-beta at 0.37 ng/ml caused a greater than 50% reduction in the number of immunoglobulin G-secreting cells among PBMC induced by pokeweed mitogen. rHuTGF-beta also inhibited the generation of allospecific cytotoxic T lymphocytes (CTL) in the mixed-lymphocyte reaction but had no effect on the cytolytic function of CTL generated in the absence of exogenous HuTGF-beta. The results indicate additional immunoregulatory activities for HuTGF-beta and suggest that this factor may play an important role in the regulation of the antigen-dependent immune response.     

IL-2 secretion by soluble antigen-reactive human T-cell clones

Human tetanus toxoid specific T-cell lines and clones capable of producing IL-2 were established. IL-2 production occurred only when the antigen-specific T cells were cultured with both tetanus toxoid antigen and an autologous, irradiated adherent cell population. The T-cell lines and clones remained strictly dependent on exogenous IL-2 for proliferation at all other times. Phenotypic characterization with monoclonal antibodies recognizing T-cell subsets revealed that the antigen-specific lines and clones bore predominantly OKT3 and OKT4 markers with essentially no OKT8 positive cells present. T-cell clones which were demonstrated to secrete IL-2 activity could also partially deplete media of IL-2 if cultured in the absence of soluble antigen and irradiated adherent cells.     

Immunoregulatory human T lymphocytes triggered as a consequence of viral infection: clonal analysis of helper, suppressor inducer and suppressor effector cell populations

The regulatory functions of a series of human T cell clones specific for an autologous Epstein-Barr virus transformed B lymphoblastoid cell line were examined. Two T4+ T cell clones, termed AT4II and AT4IV, and one T8+ clone, AT8III, were maintained in culture for greater than or equal to 9 months and were characterized in detail. Both T4+ clones provided helper function for autologous B cell immunoglobulin production when added to unstimulated peripheral blood mononuclear cells. In addition, these same clones produced soluble inducer factors after specific antigenic stimulation. However, when AT4II, AT4IV and their subclones were tested on pokeweed mitogen stimulated peripheral blood mononuclear cells, it was found that AT4IV provided help for immunoglobulin production whereas AT4II cells were strongly suppressive. This suppression by AT4II was indirect and required the presence of fresh, autologous, unirradiated T8+ cells. In contrast, the T8+ AT8III clone markedly inhibited Ig production by autologous B cells in the absence of any additional T8+ cells from peripheral blood and produced a soluble suppressor factor upon specific antigenic triggering. Thus, after stimulation with autologous Epstein-Barr virus transformed cells, at least three discrete regulatory human T cell populations can be defined at the clonal level: helper, inducer of suppression and suppressor effector clones.     

The necessity for T cell help for human tonsil B cell responses to pokeweed mitogens: induction of DNA synthesis, immunoglobulin, and specific antibody production with a T cell helper factor produced with pokeweed mitogen.

Human B lymphocytes obtained from tonsils do not proliferate when stimulated with pokeweed mitogen. A soluble factor produced from T cells cultured with pokeweed mitogen stimulates B cells to synthesize DNA and differentiate into immunoglobulin producing cells. This PWM produced supernatant induced a PFC response to SRBC. The T cell supernatant activity is produced within 12 hr of stimulation in the presence of serum and without a requirement for T cell division. Optimal stimulation of B cells occurred at 7 to 9 days of culture. This helper factor activity eluted postalbumin from a column of Sephadex G-200. Insolubilized pokeweed mitogen was not mitogenic for B cells. The continuous presence of the lectin in culture was not required for B cell proliferation or for immunoglobulin synthesis.     

Induction of CD4 suppressor T cells with anti-Leu-8 antibody

To characterize the conditions under which CD4 T cells suppress polyclonal immunoglobulin synthesis, we investigated the capacity of CD4 T cells that coexpress the surface antigen recognized by the monoclonal antibody anti-Leu-8 to mediate suppression. In an in vitro system devoid of CD8 T cells, CD4, Leu-8+ T cells suppressed pokeweed mitogen-induced immunoglobulin synthesis. Similarly, suppressor function was induced in unfractionated CD4 T cell populations after incubation with anti-Leu-8 antibody under cross-linking conditions. This induction of suppressor function by anti-Leu-8 antibody was not due to expansion of the CD4, Leu-8+ T cell population because CD4 T cells did not proliferate in response to anti-Leu-8 antibody. However, CD4, Leu-8+ T cell-mediated suppression was radiosensitive. Finally, CD4, Leu-8+ T cells do not inhibit immunoglobulin synthesis when T cell lymphokines were used in place of helper CD4 T cells (CD4, Leu-8- T cells), suggesting that CD4 T cell-mediated suppression occurs at the T cell level. We conclude that CD4 T cells can be induced to suppress immunoglobulin synthesis by modulation of the membrane antigen recognized by anti-Leu-8 antibody.     

人淋巴细胞的体外抗体诱发及其在杂交瘤中的应用

Sources of immunized lymphocytes constitute one of the main obstacles in the production of human monoclonal antibodies. We tried to get them through in vitro immunization. Cells from excised tonsils or trauma spleens were used for the induction of antibody responses in vitro. Antibodies to different antigens including sheep red blood cells, ovalbumin, tetanus toxoid, and hepatitis B surface antigens were induced in 7-14 days' cultures. Taking tetanus toxoid as antigen, we analysed the various factors required for antibody induction with statistics analysis, which included cell separation method, T cell conditioned medium, antigen dosage, serum content, and concentration of mitogen PWM and LPS. The results showed: (1) The cell separation method influenced the antibody production significantly in comparison with other factors. It signified that immune cells' combination was the most influential factor. (2) Serum also constituted quite important influencing factor especially in the later period of culture. However, it did not make much differences if it attended 10% or so. The antigens and mitogens tended to be used at low concentration. (3) Due to the significant variation among individuals and among different antigens, it is suggested to set up the culture system with some flexibility so as to adapt to the variation in cells and antigens from different sources. The present culture system we use includes nylon wall column separation of cells, suitable range of antigens (three doses instead of one), and either 10% T cell conditioned medium or a mixture of 1 microgram PWM/ ml + 0.1 microgram LPS/ml. The human B lymphocytes stimulated in vitro with tetanus toxoid were used for the construction of human hybridomas.     

NK and T cell subsets regulate antibody production by human in vivo antigen-induced lymphoblastoid B cells

This study demonstrates the existence of two different suppressive systems for the regulation of antitetanus toxoid antibody production by human lymphoblastoid (LB) B cells. These B cells appear in the circulation 5 to 7 days after in vivo immunization and spontaneously secrete antibody during a 3-day in vitro culture. One suppressive system was mediated by large granular lymphocytes that exhibited high natural killer activity. This suppressive cell subset spontaneously inhibited the antibody production by autologous LB cells, and this effect could be enhanced by the addition of interferon. This inhibition of antibody synthesis could be readily reversed by the addition of as few as 10(2) K-562 cells to the culture. Additionally, the activity of this suppressive cell population could be reduced by complement (C)-mediated lysis with Leu-7 antibody. These results strongly suggest that this autologous suppression was mediated by NK cells. The other suppressor system was contained in the fraction of high density T cells depleted of Fc receptor-bearing cells, which was low in NK activity. This subset inhibited LB function in the presence of pokeweed mitogen but not interferon, and even the addition of up to 10(6) K-562 NK target cells only minimally reversed this inhibition. These results indicate that two distinct subsets of cells share regulatory functions on the in vivo induced B lymphoblastoid cells. The observation that NK cells can inhibit these highly differentiated B cells expands our view of the spectrum of natural targets recognized by NK cells.     

Distinctive expression of the T4 antigen in normal and stimulated lymphocytes

Correlated light scatter and fluorescence flow cytometric analysis of human peripheral blood lymphocytes showed that the expression of the T4 antigen was higher in the larger lymphocytes than in the smaller lymphocytes. A similar expression pattern was observed for HLA Class I antigens but not for T3 and T8, whose expression was independent of cell size. Results with lymphocytes from spleen, lymph node, and tonsil were comparable to those of peripheral blood. Thymocytes, however, were smaller and expressed less T4 and T8 than peripheral lymphocytes. In studies of lymphocytes stimulated in vitro with allogeneic cells or pokeweed mitogen, two populations of T4-positive cells were observed: one of large cells expressing high amounts of T4 and one of small cells expressing low amounts of T4. Similar patterns were seen with T8, although less consistently. In contrast, the expression of T3 was the same in both large and small cells. The large cells expressing high amounts of T4 were not restricted to cells engaged in DNA synthesis or mitosis. This was established by selectively analyzing cells in the G0G1 phases of the cell cycle and by studying stimulated lymphocytes no longer undergoing proliferation. Taken together, these results suggest that immature T lymphocytes are small and express low amounts of T4 and T8. We postulate that as they differentiate, cell size and T4 expression increase proportionally, both parameters increasing even further after antigenic or mitogenic stimulation. The quantitative expression of T4, and probably of T8 but not of T3, is therefore intimately related to maturation and activation of lymphocytes, a fact that may conceivably be related to a functional role of these surface molecules.     

Abnormalities of the in vitro cellular and humoral responses to tetanus and influenza antigens with concomitant numerical alterations in lymphocyte subsets in Down syndrome (trisomy 21)

Peripheral blood leukocytes (PBL) from noninstitutionalized individuals with trisomy 21, paired with closely age-matched and/or family members as controls, were analyzed for different aspects of their cellular and humoral immune responses, and were phenotypically characterized by means of various monoclonal antibodies. Both the in vitro PBL proliferative and antibody responses to a bacterial antigen (tetanus toxoid) and to viral antigens (influenza A/Bangkok and B/Singapore) were significantly decreased in trisomy 21. In addition, bacterial and viral antigen-induced in vitro interleukin 2 (IL 2) production was markedly reduced, although mitogen (PHA)-stimulated IL 2 production was not impaired. The functional abnormalities observed in trisomy 21 PBL occur concomitantly with numerical alterations in circulating lymphocyte subsets in these same individuals. Although no difference was observed between the trisomic and control groups in the percentage of total T and B lymphocytes, a decreased level of Leu-3a + 3b-positive cells (T helper/inducer cells) and an increased level of Leu-2a-positive cells (T suppressor/cytotoxic cells) that co-expressed Leu-15 (suppressor alone) were noted.     

Differential synthesis of IgM and IgA anti-tetanus toxoid antibody in vitro following in vivo booster immunization of humans.

The in vitro syntheses of IgM and IgG anti-tetanus toxoid antibody by human peripheral blood leukocytes were compared prior to and at various intervals following in vivo booster immunization with soluble tetanus toxoid. Prior to booster immunization, the in vitro synthesis of IgG anti-tetanus toxoid antibody by combinations of B cells and irradiated T lymphocytes was negligible following pokeweed mitogen stimulation. Within 2 weeks after booster immunization, the quantity of IgG anti-tetanus toxoid antibody synthesized in vitro increased 5- to 20-fold. There was no comparable increase in total IgG synthesis. In contrast to the synthesis of IgG antibody, in vitro synthesis of IgM anti-tetanus toxoid antibody occurred prior to booster immunization and did not increase significantly following booster immunization. This dichotomy in anti-tetanus antibody production was further demonstrated in an individual with common variable hypogammaglobulinemia whose lymphocytes synthesized normal quantities of total IgG, IgM, and IgM anti-tetanus toxoid antibody in vitro, but failed to synthesize IgG anti-tetanus antibody following in vivo booster immunization.     

Subpopulations of human T lymphocytes. VII. Cellular basis of concanavalin A-induced T cell-mediated suppression of immunoglobulin production by B lymphocytes from normal humans.

Purified peripheral blood T lymphocytes from normal subjects were pretreated with varying concentrations of concanavalin A (Con A) for a period of 18 or 48 hr. Following treatment, these T lymphocytes were examined for the proportions of Tμ and Tγ cells and their regulatory effect on immunoglobulin production by normal allogeneic B cells in presence of pokeweed mitogen. A significant decrease in the proportions of Tμ cells and increase in Tγ cells were observed when concentration of Con A, 40 μg/ml, was used to treat purified T cells for either 18 or 48 hr. Significant suppression of in vitro immunoglobulin synthesis was observed at a similar concentration in mixing experiments. The mechanism of Con A-induced T cell-mediated suppression of immunoglobulin secretion by allogeneic B cells is discussed.     

Modulation of lymphocyte proliferation and immunoglobulin synthesis by interferon-gamma and "type I" interferons

Interferon (IFN)-alpha and IFN-beta ("type I" IFNs), but not IFN-gamma reduced phytohemagglutinin- or pokeweed mitogen (PWM)-induced proliferation in cultures of human mononuclear leukocytes. Proliferation induced by specific antigens (tuberculin PPD or tetanus toxoid) or by exogenous interleukin 2 (IL-2) was strongly inhibited by type I IFNs and, to a lesser extent, by IFN-gamma as well. Inhibition of proliferation in mitogen-stimulated cultures was not due to a reduced production of IL-2 or to an inhibition of IL-2 receptor expression. Type I IFNs inhibited immunoglobulin (Ig) production in PWM-stimulated unseparated mononuclear cells, whereas IFN-gamma enhanced Ig production in such cultures. In cultures of purified B cells type I IFNs caused a stimulation of Ig production and this B-cell differentiation factor (BCDF)-like activity of IFNs was synergistically enhanced in the presence of IL-2. IFN-gamma produced less BCDF-like activity than type I IFNs. These results show that in some instances type I IFNs can be more potent in affecting functions of cells of the immune system than IFN-gamma.