Reduction of regulatory T cells in skin lesions but not in peripheral blood of patients with systemic scleroderma.

OBJECTIVE: To determine the frequency and suppressive capacity of regulatory T cells (T(reg)) and their association with clinical parameters in patients with systemic scleroderma (SSc). METHODS: Peripheral blood from 25 patients with SSc, 15 patients with localised scleroderma (LS) and 29 healthy controls (HC) was studied. Analysis of CD4(+) forkhead box P3 (Foxp3)(+) and CD4(+)CD25(++)Foxp3(+) T(reg) subpopulations was carried out by flow cytometry and cell proliferation was quantified by (3)H-thymidine incorporation. Quantitative analysis of T(reg) was further performed in skin biopsies from 17 patients with SSc and 21 patients with LS using anti-CD4 and anti-Foxp3 monoclonal antibodies for immunohistochemistry. RESULTS: The frequency of CD4(+)Foxp3(+) and CD4(+)CD25(++)Foxp3(+) T(reg) in peripheral blood from patients with SSc was not significantly different from that of patients with LS or HC. The suppressive capacity of CD4(+)CD25(++) T(reg) in SSc was also found to be similar to that of HC. Phenotypic and functional data revealed no significant difference between the limited or diffuse form of SSc. Moreover, therapy with bosentan showed no significant effect on the frequency of T(reg) during the course of the disease. However, the frequency of T(reg) in skin lesions from patients with SSc or LS, determined as the percentage of CD4(+) cells expressing Foxp3 in the inflammatory infiltrate, was significantly reduced compared with other inflammatory skin diseases. CONCLUSION: These results indicate that although the authors found no defect in the frequency or function of peripheral T(reg) subpopulations, the reduction of CD4(+)Foxp3(+) T(reg) in the skin of patients with SSc may be important in the pathogenesis of the disease.

Involvement of the CD95 (APO-1/FAS) receptor/ligand system in drug-induced apoptosis in leukemia cells.

Cytotoxic drugs used in chemotherapy of leukemias and solid tumors cause apoptosis in target cells. In lymphoid cells the CD95 (APO-1/Fas)/CD95 ligand (CD95-L) system is a key regulator of apoptosis. Here we describe that doxorbicin induces apoptosis via the CD95/CD95-L system in human leukemia T-cell lines. Doxorubicin-induced apoptosis was completely blocked by inhibition of gene expression and protein synthesis. Also, doxorbicin strongly stimulates CD95-L messenger RNA expression in vitro at concentrations relevant for therapy in vivo. CEM and jurkat cells resistant to CD95-mediated apoptosis were also resistant to doxorbicin-induced apoptosis . Furthermore, doxorbicin-induced apoptosis was inhibited by blocking F(ab')2 anti-APO-1 (anti-CD95) antibody fragments. Expression of CD95-L mRNA and protein in vitro was also stimulated by other cytotoxic drugs such as methotrexate. The finding that apoptosis caused by anticancer drugs may be mediated via the CD95 system provides a new molecular insight into resistance and sensitivity toward chemotherapy in malignancies.

Lymphocyte apoptosis induced by CD95 (APO-1/Fas) ligand-expressing tumor cells--a mechanism of immune evasion?

The CD95 (APO-1/Fas) system is an important mediator of T-cell cytotoxicity. We investigated this system in 22 hepatocellular carcinomas (HCCs) from patients. All HCCs had partially or completely lost the expression of the CD95 receptor constitutively expressed by normal liver cells and might thus evade CD95-mediated killing. We also considered a new mechanism of immune evasion, namely, the active destruction of T-lymphocytes by tumor cells expressing CD95 ligand (CD95L). CD95L messenger RNA and protein could be detected in the HCCs. In coculture experiments, HepG2 hepatoblastoma cells, expressing CD95L mRNA after treatment with cytostatic drugs, killed CD95+ Jurkat lymphocytes. Our data suggest that tumor cells can evade immune attack by down-regulation of the CD95 receptor and killing of lymphocytes through expression of CD95L.

Hepatic failure and liver cell damage in acute Wilson's disease involve CD95 (APO-1/Fas) mediated apoptosis.

Wilson's disease can result in fulminant liver failure due to hepatic copper overload. The CD95 system mediates apoptosis and has been demonstrated to be involved in liver disease. In this study CD95 mediated apoptosis was investigated in patients with fulminant hepatic failure in the course of Wilson's disease and in an in vitro model of copper treated human hepatoma cells. In patients, hepatic expression of CD95 and CD95L mRNA and apoptosis were detected. Copper overload in vitro resulted in hepatocytic apoptosis which could be reduced with a neutralizing anti-CD95L antibody. Copper treatment of hepatocytes results in activation of the CD95 system and induction of apoptosis which is operative during the course of hepatic failure in acute Wilson's disease.

Alloantigen receptors on activated T cells in mice. I. Binding of alloantigens and anti-idiotypic antibodies to the same receptors.

B6 alloantigens in supernates from one-way mixed lymphocyte reaction (MLR) cultures of AKR T cells against B6 lymph node cells rebound specifically to (B6)AKR-T-cell blasts after overnight incubation and recovery of these blasts from trypsin treatment. A similar specificity was observed with the binding of SJL alloantigens to (SJL)AKR-T-cell blasts. In both cases, the corresponding alloantigens were rebound several-fold more efficiently than control alloantigens. In a different assay system, T-cell receptors were studied with anti-idiotypic sera. These antisera were raised by repeated injection of purified (B6)AKR-T-cell blasts into (AKR X B6)F1 hybrid mice. These F1a(AKRaB6) sera reacted with the majority of (B6)AKR-T and (B6)(AKR X SJL)F1-T-cell blasts. They also reacted with a lower, though sizable, number of (SJL)AKR-T- and (SJL)(AKR X B6)F1-T-cell blasts. No reaction was observed with (B6)SJL-T, concanavalin A-activated AKR T-cell blasts, normal B6, (AKR X B6)F1, and (AKR X SJL)F1 T cells. Normal AKR T cells were positive only minimally above background. F1a(AKRaB6) sera could be made specific for (B6)AKR-T and (B6)(AKR X SJL)F1-T-cell blasts by absorption of contaminating antibodies with (SJL)AKR-T-cell blasts. Finally, it was shown by competition experiments that the receptors on MLR-activated T-cell blasts that bind alloantigens were the same as those binding anti-idiotypic antibodies. In addition, it was found that at least a fraction of alloantibodies share common idiotypic determinants with receptors on MLR-activated T-cell blasts.

Most influenza A virus-specific memory cytotoxic T lymphocytes react with antigenic epitopes associated with internal virus determinants.

This paper shows that most murine (C57BL/6) influenza A virus-specific memory cytotoxic T lymphocyte (CTL) clones tested in limiting dilution did not react with the influenza A virus surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). This lysis of syngeneic target cells infected with the influenza A virus strains, Aichi (H3N2), PR8 (H1N1), or recombinant strain X31 (H3N2) indicates that most antigenic epitopes recognized are associated with internal virus determinants. X31 and PR8 share the internal, and X31 and Aichi the external, viral determinants. Extensive CTL cross-reactivity was observed in experiments with target cells infected with virus carrying internal determinants homologous with the priming virus. In contrast, when the internal viral determinants differed between the priming virus and the virus used to infect the target cells, and although HA and NA were homologous, we found almost complete CTL-specificity for the priming virus. Thus, the predominant reactivity of influenza A virus-specific CTL differs from that of anti-influenza A antibodies, which are primarily directed towards epitopes on the virus surface glycoproteins. This finding may be relevant for the role of influenza A virus-specific CTL in recurrent infections with different influenza A viruses.

Antigen receptors on major histocompatibility complex-restricted T lymphocytes. I. Preparation and characterization of syngeneic antisera against trinitrophenyl-activated T cell blasts and demonstration of their specificity for idiotypes on cytotoxic T lymphocytes.

This paper describes the specificity of AKR anti-(a) [AKR anti-trinitrophenylated AKR (AKR-TNP)] [AKRa (AKRaAKR TNP)] antisera raised in syngeneic AKR mice against AKRaAKR-TNP cell populations enriched for H-2-restricted aTNP cytotoxic lymphocytes (CTL) by blast-cell isolation. The activity of the antisera resided in the Ig fraction. All antisera were shown to reproducibly react with AKRaAKR-TNP-CTL-containing cell populations in indirect immunofluorescence and all removed the major fraction of CTL in complement-dependent lysis causing a considerable depression of cell-mediated lympholysis. The antisera were nonreactive with alloreactive AKRaC57BL/6 CTL and other H-2-restricted AKR CTL against fluorescein-isothiocyanate-conjugated AKR-target cells. It could be excluded that the antisera contained contaminating antibodies against TNP, TNP-neoantigenic determinants (NAD), or processed CTL-receptor-bound TNP-NAD, thus demonstrating specificity for determinants on T cell receptors of AKRaAKR-TNP CTL. These receptors were produced by the CTL themselves. These observations are interpreted to suggest that AKRa (AKRaAKR-TNP) antisera contain anti-idiotypic antibodies directed against specificity-associated determinants (idiotypes) on T cell receptors of H-2-restricted AKRaAKR-TNP CTL. The antisera provide a new tool to study the genetic control of idiotype expression on H-2-restricted CTL, the biochemistry of T cell receptors, and the regulation of the generation of H-2 restricted CTL on the idiotype level.

Fc-receptors, Ia-antigens, and immunoglobulin on normal and activated mouse T lymphocytes.

Using antibody coated bovine erythrocytes we were unable to demonstrate Fc-receptors on either thymus cells or T cells prepared from lymph node cell suspensions by anti-Ig column filtration. However, if parental thymus or lymph node T cells were transferred to X-irradiated F1 hybrids, activated donor T cells recovered from the recipient's spleen (ATC-spleen) were shown to express Fc-receptors. Fc-receptors were also demonstrable on ATC-spleen prepared between strain combinations differing at the M-locus. In marked contrast, Fc-receptors were not detected on ATC recovered from thoracic duct lymph (T.TDL). This applied to (a) H-2-activated T.TDL derived from normal thymus cells, (b) H-2-activated T.TDL derived from thymus cells depleted of B cells, and (c) M-locus-activated T.TDL. Of these three populations, surface Ig (of B cell origin) was detected on a large proportion of the first but not on the second and third populations. Thus, the failure to detect Fc-receptors on any of these populations could not be attributed to blocking by adsorbed surface Ig. In addition, various T-cell populations were examined by a microcytotoxicity assay for the presence of cell surface Ia-antigens. 5--10% of the thymus cells, 20--30% of cortisone-resistant thymus cells, 60--70% of lymph node cells, and 60--80% of ATC-spleen and T.TDL showed Ia.

FLICE-inhibitory protein is a key regulator of germinal center B cell apoptosis.

Affinity maturation of the B cell response to antigen (Ag) takes place in the germinal centers (GCs) of secondary follicles. Two sequential molecular mechanisms underpin this process. First, the B cell repertoire is diversified through hypermutation of the immunoglobulin (Ig) variable region genes. Second, mutant B cell clones with improved affinity for Ag are positively selected by Ag and CD40 ligand (L). This selection step is contingent upon "priming" of GC B cells for apoptosis. The molecular means by which B cell apoptosis is initiated and controlled in the GC remains unclear. Here, we show that GC B cell apoptosis is preceded by the rapid activation of caspase-8 at the level of CD95 death-inducing signaling complex (DISC). We found that GC B cells ex vivo display a preformed inactive DISC containing Fas-associated death domain-containing protein (FADD), procaspase-8, and the long isoform of cellular FADD-like IL-1beta-converting enzyme-inhibitory protein (c-FLIP(L)) but not the CD95L. In culture, c-FLIP(L) is rapidly lost from the CD95 DISC unless GC B cells are exposed to the survival signal provided by CD40L. Our results suggest that (a) the death receptor signaling pathway is involved in the affinity maturation of antibodies, and (b) c-FLIP(L) plays an active role in positive selection of B cells in the GC.

Interferon gamma and lymphotoxin or tumor necrosis factor act synergistically to induce macrophage killing of tumor cells and schistosomula of Schistosoma mansoni.

Macrophages play a crucial role in the defense against tumors and parasites. Activation of tumoricidal and microbicidal effector mechanisms requires stimulation of macrophages with macrophage-activating factors (MAF). One such MAF is interferon gamma (IFN-gamma). In some assays, substantial activity of IFN-gamma on murine macrophages, however, is only observed in synergy with lipopolysaccharide (LPS) or other cytokines (1). In addition, certain cytokines have been shown to induce monocyte or macrophage activation in the absence of IFN-gamma (2-5). We previously described lymphokines in the supernatant of a murine T cell clone that synergized with IFN-gamma in the induction of tumoricidal and schistosomulicidal murine macrophages (1). We called this lymphokine(s) macrophage cytotoxicityinducing factor 2 (MCIF2)(1). A candidate for MCIF2 was lymphotoxin (LT), because the T cell clone supernatant contained high amounts of LT. LT is functionally homologous and structurally related to the macrophage product tumor necrosis factor (TNF). Therefore, we tested whether recombinant (r) LT or rTNF can function as MAF. We report here that rLT or rTNF synergize with rIFN-gamma in the induction of tumoricidal and schistosomulicidal murine macrophages.

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

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

T cell-derived B cell differentiation factor(s). Effect on the isotype switch of murine B cells.

Culturing BALB/c B cells for 6 d at low cell density in the presence of lipopolysaccharide (LPS) results in the appearance of a small number of IgG plaque-forming cells (PFC). The addition of supernatants from concanavalin A (Con A)-induced alloreactive (AKR anti-B6) long-term T cell lines (PK 7.1.1a and 7.1.2) or a T cell hybridoma (FS7-6.18) to LPS-treated B cells resulted in a marked increase in IgG PFC (3--10-fold higher than in cultures treated with LPS alone. The number of induced IgG PFC was not affected by removing IgG-bearing cells on the fluorescence-activated cell sorter, indicating that T cell-derived B cell differentiation factor enhances isotype switching of sIgG- cells, rather than selecting and expanding pre-existing subpopulations of sIgG+ cells. We also investigated the subclass of IgG produced in the absence or presence of T cell factors and found that PK 7.1.1a, PK 7.1.2, and FS7-6.18 supernatants selectively increased IgG1 production. Several other T cell supernatants containing a variety of lymphokines had no effect, suggesting that PK 7.1.1a, PK 7.1.2, and FS7-6.18 lines produce factor(s) that can specifically enhance the recovery of IgG secreting cells in culture in the presence of LPS. These factors, which we have termed B cell differentiation factors, are different from interleukin 1, interleukin 2, T cell-replacing factor, colony-stimulating factor, macrophage-activating factor, and immune interferon. Our results suggest that soluble factors produced by T cell lines and hybridomas can markedly influence both the class and subclass of Ig produced by B cells.

Specific binding of alloantigens to T cells activated in the mixed lymphocyte reaction.

Immunoglobulin (Ig) is present on a large fraction of T cells from unfractionated lymphocytes activated by in vitro stimulation with H-2-incompatible cells (mixed lymphocyte reaction [MLR]). Removal of bursa equivalent-derived (B) cells from the responder cell population before mixed culture, by filtration through nylon wool columns, reduces the percentage of Ig-bearing responder T blasts to background levels. Thus, Ig on the T blast is probably of B cell origin. A large fraction of T blasts activated against the stimulator cells. This staining occurs with "early" and hyperimmune alloantisera, including the 7S fraction of the latter. B-depleted responder cells were activated against a mixture of two different stimulator cells and the resulting T blasts stained with different concentrations of sera directed either against one or both stimulator cells. We obtained results which strongly suggest that most or all responder T blasts stain with only one antistimulator serum. When antisera directed against different segments of the H-2 complex of the stimulator cells were used, it seemed that most responder T cells only bound antibody directed against a single segment. We propose that T cells activated in MLR carry stimulator alloantigens on their surface, and that this is due to specific antigen binding, not requiring the presence of B-cell-derived antibody. These histocompatibility antigen-binding T blasts can be detected by appropriate antistimulator alloantibodies.

B cell-stimulatory factor 1 (BSF-1) promotes growth of helper T cell lines.

T cell-derived supernatants (SN) that contain B cell-stimulatory factor 1 (BSF-1) and lack IL-2 promote the growth of the IL-2-dependent T cell line, HT-2, as well as three other clones or lines of T cells that can provide help to B cells. The BSF-1 purified from these SNs promotes growth of HT-2 cells approximately 50% as effectively as purified IL-2. A potential involvement for contaminating IL-2 in the BSF-1 preparations was excluded by the demonstration that anti-BSF-1 mAbs blocked the BSF-1-induced growth of HT-2 cells; in contrast, these antibodies did not block the IL-2-induced proliferation of the HT-2 cells. In addition, anti-IL-2 mAbs or anti-IL-2-R antibodies blocked the HT-2 growth-promoting activity of purified IL-2, but not BSF-1. Finally, BSF-1 promoted only a very modest growth of Con A-induced T cell blasts, and failed to induce significant growth in seven other cytotoxic, alloreactive, and long-term T cell lines. Taken together, these results indicate that in addition to its known effects on resting and LPS-stimulated B cells, BSF-1 can promote growth of certain subsets of activated T cells, in particular, those that provide help to B cells.

T cell-mediated lethal shock triggered in mice by the superantigen staphylococcal enterotoxin B: critical role of tumor necrosis factor.

Because mice are more resistant than humans to the pathogenic effects of bacterial toxins, we used D-Galactosamine- (D-Gal) sensitized mice as a model system to evaluate potential toxic shock symptoms triggered by the superantigen staphylococcal enterotoxin B (SEB). We show that similar to endotoxin (lipopolysaccharide) [LPS], the exotoxin SEB causes lethal shock within 8 h in D-Gal-sensitized mice, inducing 100% and about 50% lethality with 20 and 2 micrograms SEB, respectively. The lethal shock triggered by the superantigen SEB is mediated by T cells, a conclusion based on the observation that T cell repopulation of SCID mice conferred sensitivity to SEB. Since CSA also conferred protection, the role of T cell-derived lymphokines in mediating lethal shock was evaluated. Within 30-60 min after SEB injection, serum tumor necrosis factor (TNF) levels peaked, followed immediately by interleukin-2 (IL-2). Serum-borne lymphokines were detected well in advance of signs of T cell activation, as assessed by IL-2 receptor expression of SEB-reactive V beta 8+ T cells. Passive immunization with anti-TNF-alpha/beta-neutralizing monoclonal antibody also conferred protection, indicating that it is TNF which is critical for initiating toxic shock symptoms. Taken together, this study defines basic differences between endotoxin (LPS)- and exotoxin (SEB)-mediated lethal shock, in that the former is mediated by macrophages and the latter by T cells. Yet the pathogenesis distal to the lymphokine/cytokine-producing cells appears surprisingly similar in that TNF represents a key mediator in inducing shock.

Clonal analysis of B cells induced to secrete IgG by T cell-derived lymphokine(s).

To gain insight into how T cell-derived lymphokines induce the secretion of IgG in activated B cells, we performed a limiting dilution analysis, using murine splenic B cells incubated with lipopolysaccharide (LPS) and a T cell-derived B cell differentiating factor for IgG (BCDF gamma)-containing supernatant (SN). The results of this analysis indicate that such a SN induces a marked increase in the precursor frequency of IgG1-secreting cells and a modest increase in clone size. The precursors lack surface IgG and are committed to the differentiation pathway for IgG1 secretion after LPS activation, but before the addition of BCDF gamma-containing SN. The majority of IgG1-secreting clones arise independently from precursors of cells that secrete IgG3. Taken together, these results indicate that BCDF gamma directs differentiation of activated B cells to IgG1 secretion.

Herpes simplex virus type 1 infection of activated cytotoxic T cells: Induction of fratricide as a mechanism of viral immune evasion.

Herpes simplex virus type 1 (HSV1), a large DNA-containing virus, is endemic in all human populations investigated. After infection of mucocutaneous surfaces, HSV1 establishes a latent infection in nerve cells. Recently, it was demonstrated that HSV1 can also infect activated T lymphocytes. However, the consequences of T cell infection for viral pathogenesis and immunity are unknown. We have observed that in contrast to the situation in human fibroblasts, in human T cell lines antigen presentation by major histocompatibility complex class I molecules is not blocked after HSV1 infection. Moreover, HSV1 infection of T cells results in rapid elimination of antiviral T cells by fratricide. To dissect the underlying molecular events, we used a transgenic mouse model of HSV1 infection to demonstrate that CD95 (Apo-1, Fas)-triggered apoptosis is essential for HSV1-induced fratricide, whereas tumor necrosis factor (TNF) also contributes to this phenomenon but to a lesser extent. By contrast, neither TRAIL (TNF-related apoptosis-inducing ligand) nor perforin were involved. Finally, we defined two mechanisms associated with HSV1-associated fratricide of antiviral T cells: (a) T cell receptor-mediated upregulation of CD95 ligand and (b) a viral "competence-to-die" signal that renders activated T lymphocytes susceptible to CD95 signaling. We propose that induction of fratricide is an important immune evasion mechanism of HSV1, helping the virus to persist in the host organism throughout its lifetime.

Serological, biochemical, and functional identity of B cell-stimulatory factor 1 and B cell differentiation factor for IgG1.

By three criteria, we have demonstrated that B cell stimulatory factor (BSF-1) and B cell differentiation factor (BCDF-gamma) are the same lymphokine. Highly purified preparations of high performance liquid chromatography-purified or affinity-purified BSF-1 had BCDF-gamma activity but not BCDF-mu activity. A monoclonal anti-BSF-1 antibody coupled to Sepharose depleted both BSF-1 and BCDF-gamma activity but not BCDF-mu activity from two different T cell supernatants. Soluble monoclonal anti-BSF-1 blocked the BSF-1 and BCDF-gamma but not the BCDF-mu responses. These results suggest that BSF-1 acts on both resting and activated B cells to induce different effects.

Involvement of the CD95 (APO-1/Fas) receptor and ligand in liver damage.

Apoptosis occurs in the normal liver and in various forms of liver disease. The CD95 (APO-1/Fas) (CD95) receptor mediates apoptosis, and liver cells in animal models are acutely sensitive to apoptosis initiated by this receptor. We have used primary human hepatocytes as a model system to investigate CD95-mediated apoptotic liver damage. Treatment of fresh human hepatocytes with low concentrations of agonistic antibodies against CD95 resulted in apoptosis of > 95% of the cultured liver cells within 4 and 7.5 h. Immunohistology of a panel of explanted liver tissues revealed that hepatocytes in normal livers (n = 5) and in alcoholic cirrhosis (n = 13) expressed low constitutive levels of CD95. CD95 receptor expression was highly elevated in hepatocytes in hepatitis B virus-related cirrhosis (n = 9) and in acute liver failure (n = 8). By in situ hybridization CD95 ligand messenger RNA expression was absent in normal liver but detected at high levels in livers with ongoing liver damage. In cases of hepatitis B virus-related cirrhosis and acute hepatic failure, ligand expression was found primarily in areas with lymphocytic infiltration. In contrast, in patients with alcoholic liver damage, high CD95 ligand messenger RNA expression was found in hepatocytes. These findings suggest that liver destruction in hepatitis B may primarily involve killing of hepatocytes by T lymphocytes using the CD95 receptor-ligand system. In alcoholic liver damage, death of hepatocytes might occur by fratricide and paracrine or autocrine mechanisms mediated by the hepatocytes themselves.

CD40 ligation induces Apo-1/Fas expression on human B lymphocytes and facilitates apoptosis through the Apo-1/Fas pathway.

The Apo-1/Fas antigen (CD95) mediates programmed cell death of lymphocytes when bound by Fas ligand or anti-Apo-1/Fas antibody. In contrast, the CD40 antigen provides a potent activation and survival signal to B lymphocytes when it is engaged by its T cell ligand (CD40L, gp39) or cross-linked by anti-CD40 antibody. In this study, we use human tonsillar B cells and the Ramos Burkitt's lymphoma B cell line, which serves as a model for human germinal center B lymphocytes, to study the effectors of Apo-1/Fas expression and apoptosis of human B cells. We found that Apo-1/Fas expression was upregulated on both malignant and normal human B lymphocytes after CD40 ligation induced by (a) cognate T helper-B cell interaction mediated by microbial superantigen (SAg); (b) contact-dependent interaction with CD40L+, but not CD40L- Jurkat mutant T cell clones; and (c) monoclonal anti-CD40, but not any of a panel of control antibodies. Enhanced B cell Fas/Apo-1 expression is functionally significant. Coculture of Ramos Burkitt's lymphoma line cells with irradiated SAg-reactive CD4+ T cells with SAg or CD40L+ Jurkat T cells results in B cell apoptosis, evidenced by reduced cell viability and DNA laddering. This process is augmented by the addition of anti-Apo-1/Fas monoclonal antibody, consistent with an acquired susceptibility to Apo-1/Fas-mediated apoptosis. These data support an immunoregulatory pathway in which seemingly contradictory signals involving the B cell proliferation/survival antigen CD40, as well as the Apo-1/Fas molecule, which mediates programmed cell death of lymphocytes, are linked in the process of human B cell activation.