How does B cell depletion therapy work, and how can it be improved?

The past few years have seen a surge of interest in B cell depletion therapy for patients with rheumatoid arthritis. This paper outlines the possible mechanism(s) by which B cell depletion therapy works. It is likely there is more than one mechanism and the relative importance of each mechanism depends on the target cell. These include CD20-induced apoptosis, complement dependent cytotoxicity, antibody dependent cell-mediated cytotoxicity, and selective targeting and depletion of B cell subsets. The implications of these mechanisms in the further improvement of B cell depletion therapy in rheumatoid arthritis and other autoimmune diseases are discussed.

Tumor necrosis factor-alpha regulates the expression of inducible costimulator receptor ligand on CD34(+) progenitor cells during differentiation into antigen presenting cells.

The inducible costimulator receptor (ICOS) is a third member of the CD28 receptor family that regulates T cell activation and function. ICOS binds to a newly identified ligand on antigen presenting cells different from the CD152 ligands CD80 and CD86. We used soluble ICOSIg and a newly developed murine anti-human ICOS ligand (ICOSL) monoclonal antibody to further characterize the ICOSL during ontogeny of antigen presenting cells. In a previous study, we found that ICOSL is expressed on monocytes, dendritic cells, and B cells. To define when ICOSL is first expressed on myeloid antigen presenting cells, we examined ICOSL expression on CD34(+) cells in bone marrow. We found that CD34(bright) cells regardless of their myeloid commitment were ICOSL(-), whereas ICOSL was first expressed when CD34 expression diminished and the myeloid marker CD33 appeared. However, acute myeloid leukemia cells were ICOSL-negative, whereas among B-cell malignancies only some cases of the most mature tumors such as prolymphocytic leukemia and hairy cell leukemia were positive. Next, we investigated purified CD34(+) hematopoietic progenitor cells that did not constitutively express ICOSL but were induced to express ICOSL within 12 h after granulocyte/macrophage colony-stimulating factor/tumor necrosis factor alpha (TNF-alpha) stimulation. Interestingly, ICOSL was induced prior to CD80/CD86 induction on CD34(+) cells so that ICOSL was expressed in the absence of CD80/CD86. This suggests that ICOSL is an early differentiation marker along the monocytic/dendritic maturation pathway. Induction of ICOSL was dependent on TNF-alpha and was regulated via NF-kappa B as revealed by use of inhibitors specific for I kappa B alpha phosphorylation such as BAY 11-7082 and BAY 11-7085. The antigen presenting capacity of TNF-alpha stimulated CD34(+) cells was strongly inhibited by ICOSIg fusion proteins or by NF-kappa B inhibition. Thus, TNF-alpha-induced ICOSL expression seemed to be functionally important for the costimulatory capacity of CD34(+) hematopoietic progenitor cells.

ICOS ligand costimulation is required for T-cell encephalitogenicity.

The interaction of ICOS with its ligand on APC provides a costimulatory signal to previously activated T-cells. In these studies, we blocked the ICOS:ICOS ligand interaction with ICOS-Ig during the in vitro activation of MBP-reactive transgenic CD4(+) T-cells. The presence of ICOS-Ig in these cultures inhibited the ability of the transgenic T-cells to transfer EAE, although they entered the brains of the recipient mice. ICOS-Ig increased apoptosis in the transgenic T-cells, especially in the memory population. This enhanced apoptosis was accompanied by an increase in the BAX/BCL-2 mRNA ratio. ICOS-Ig did not prevent IL2 production, demonstrating that IL-2 production is ICOS ligand independent. IFN-gamma and IL-10 production by the transgenic T-cells, however, was suppressed. Finally, ICOS-Ig injection into mice after the first signs of EAE ameliorated clinical disease. Therefore, ICOSL provides a signal distinct from CD28 costimulation that is required for the activation and viability of encephalitogenic T-cells.

Overview of the Third International Workshop on Swine Leukocyte Differentiation Antigens.

The aim of the Third International Workshop on Swine Leukocyte Differentiation Antigens (CD workshop), supported by the Veterinary Immunology Committee (VIC) of the International Union of Immunological Societies (IUIS), was to standardize the assignment of monoclonal antibodies (mAb) reactive with porcine leukocyte differentiation antigens and to define new antibody clusters, using nomenclature in accordance with human and ruminant CD nomenclature, as agreed at the summary meeting of the Second International Swine CD Workshop in Davis, 1995: only mAb with proven reactivity for the orthologous porcine gene product or cross-reactivity for the human gene products, were given the full CD nomenclature, all other allocations were prefixed with "w". As in previous workshops, the overall organization was entrusted to the chair and first author, with support by the chair of the previous workshop and second author. In addition to the existing 26 pig leukocyte CD/SWC determinants established in previous workshops, this workshop established/confirmed another 11 CDs for pig leukocytes, identified by a total of 21 mAb: CD11R1 (2 mAb), CD11R2 (1 mAb), CD11R3 (4 mAb), wCD40 (1 mAb), wCD46 (4 mAb), wCD47 (3 mAb), wCD49d (1 mAb), CD61 (1 mAb), wCD92 (1 mAb), wCD93 (1 mAb) and CD163 (2 mAb).

Summary of the first round analyses of the Third International Workshop on Swine Leukocyte Differentiation Antigens.

The reactivity of 155 monoclonal antibodies submitted to the Third International Workshop on Swine Leukocyte Differentiation Antigens, together with 41 internal standards, was analysed by flow cytometry on 29 different pig cell targets as well as two human cell targets as a means of establishing suitable panels of monoclonal antibodies for more detailed clustering analyses by the various subsections of the workshop. Results were collected either without further gating, with gating based on FS/SS characteristics or with gating based on the co-expression of a reference antibody in two-colour flow cytometry. The CD or SWC reactivity of the internal standards had been established in previous workshops. Data sets were subsequently analysed by statistical clustering using the Leucocyte Typing Database IV software. The resulting 18 cluster groups were allocated to the appropriate second round sections of the workshop, after reviewing the overall cellular reactivity of each cluster as well as the specificity of known standards which clustered in a group.

Monoclonal antibodies raised to human cells--specificity for pig leukocytes.

A total of 27 monoclonal antibodies raised to human targets were included in the present Pig CD workshop. 14 of these had been tested in previous workshops and had been reported as cross-reactive, a further 13 had been reported as cross-reactive during the Human Leukocyte Differentiation Antigens Workshop VI (HLDA VI) and/or by the donor (a commercial company submitting these mAb for validation by the workshop community). Of the 27 antibodies, three antibodies with previously reported reactivity for pig cells were eliminated from the workshop following preliminary tests due to lack of reactivity. Nine antibodies, although initially positive, gave inconsistent results during the course of the workshop. We found consistent reactivity for 15 antibodies. However, the cellular distribution of the target molecules on pig and human cells was shown to be different for three of these antibodies. These findings have important implications for the usefulness of these antibodies as research tools in the pig.

CD3-mediated activation of tumor-reactive lymphocytes from patients with advanced cancer.

Lymphocytes from blood or tumors of patients with advanced cancer did not proliferate and produced very low levels of tumor necrosis factor and IFN-gamma when cultured with autologous tumor cells. Proliferation and lymphokine production dramatically increased in the presence of beads conjugated with mAbs to CD3 plus mAbs to CD28 and/or CD40, and the lymphocytes destroyed the tumor cells. Expression density of CD3 concomitantly increased from low to normal levels. Furthermore, beads providing a CD3 signal (in combination with CD28 or CD28 plus CD40) gave partial protection against the inhibitory effect of transforming growth factor type beta1 on lymphocyte proliferation and production of tumor necrosis factor and IFN-gamma. MHC class I-restricted cytolytic T cells lysing autologous tumor cells in a 4-h Cr(51) release assay were generated when peripheral blood leukocytes were activated in the presence of autologous tumor cells and anti-CD3/CD28 or anti-CD3/CD28/CD40 beads. Experiments performed in a model system using anti-V-beta1 or anti-V-beta2 mAbs to activate subsets of T cells expressing restricted T cell receptor showed that lymphocytes previously activated by anti-V-beta can respond to CD3 stimulation with vigorous proliferation and lymphokine production while retaining their specificity, also in the presence of transforming growth factor type beta1. Our results suggest that T lymphocytes from cancer patients can proliferate and form Th1 type lymphokines in the presence of autologous tumor cell when properly activated, and that antigen released from killed tumor cells and presented by antigen-presenting cells in the cultures facilitates the selective expansion of tumor-directed, CD8(+) cytolytic T cells.

CD83 is an I-type lectin adhesion receptor that binds monocytes and a subset of activated CD8+ T cells [corrected].

To help determine CD83 function, a cDNA encoding a soluble protein containing the CD83 extracellular domain was fused with a mutated human IgG1 constant region (CD83Ig) and expressed by stable transfection of Chinese hamster ovary cells. Purified CD83Ig bound to peripheral blood monocytes and a subset of activated CD3(+)CD8(+) lymphocytes but did not bind to FcR. Monocytes that had adhered to plastic lost their ability to bind to CD83Ig after 90 min of in vitro incubation. CD83Ig bound to two of five T cell lines tested, HPB-ALL and Jurkat. The binding to HPB-ALL cells significantly increased when they were grown at a low pH (pH 6.5), whereas binding to Jurkat cells increased after apoptosis was induced with anti-Fas mAb. B cell and monocytic lines did not bind CD83Ig and neither did CD56(+) NK cells or granulocytes. Full-length CD83 expressed by a transfected carcinoma line mediated CD83-dependent adhesion to HPB-ALL cells. CD83Ig immunoprecipitated and immunoblotted a 72-kDa protein from HPB-ALL cells. Binding of CD83Ig to HPB-ALL cells was eliminated by neuraminidase treatment of the cells. We conclude that CD83 is an adhesion receptor with a counterreceptor expressed on monocytes and a subset of activated or stressed T lymphocytes, and that interaction between CD83 and its counterreceptor is dependent upon the state of glycosylation of a 72-kDa counterreceptor by sialic acid residues. In view of the selectivity of the expression of CD83 and its ligand, we postulate that the interaction between the two plays an important role in the induction and regulation of immune responses.

A D-amino acid peptide inhibitor of NF-kappa B nuclear localization is efficacious in models of inflammatory disease.

The transcription factor NF-kappa B regulates many genes involved in proinflammatory and immune responses. The transport of NF-kappa B into the nucleus is essential for its biologic activity. We describe a novel, potent, and selective NF-kappa B inhibitor composed of a cell-permeable peptide carrying two nuclear localization sequences (NLS). This peptide blocks NF-kappa B nuclear localization, resulting in inhibition of cell surface protein expression, cytokine production, and T cell proliferation. The peptide is efficacious in vivo in a mouse septic shock model as well as a mouse model of inflammatory bowel disease, demonstrating that NF-kappa B nuclear import plays a role in these acute inflammatory disease models.

Characterization of human inducible costimulator ligand expression and function.

The inducible costimulator (ICOS) is the newest member of the CD28/CD152 receptor family involved in regulating T cell activation. We constructed a soluble-Ig fusion protein of the extracellular domain of human ICOS and used it as a probe to characterize expression patterns of the ICOS ligand (ICOSL). ICOSIg did not bind to CD80- or CD86-transfected Chinese hamster ovary cell lines, demonstrating that ICOSL is distinct from those ligands identified for CD28/CD152. ICOSIg showed selective binding to monocytic and B cell lines, whereas binding was undetectable on unstimulated monocytes and peripheral blood T and B cells. Expression of ICOSL was induced on monocytes after integrin-dependent plastic adhesion. Pretreatment of monocytes with mAb to the beta2-integrin subunit CD18 decreased adhesion and abolished ICOSL up-regulation but had no effect on CD80/86 (CD152 ligand (CD152L)) expression. Both ICOSL and CD152L were up-regulated on monocytes by IFN-gamma but by distinct signaling pathways. Unlike CD152L expression, ICOSL expression did not change when monocytes were differentiated into dendritic cells (DCs) or after DCs were induced to mature by LPS, TNF-alpha, or CD40 ligation. Addition of ICOSIg to allogeneic MLRs between DCs and T cells reduced T cell proliferative responses but did so less efficiently than CTLA4Ig (CD152Ig) did. Similarly, ICOSIg also blocked Ag-specific T cell proliferation to tetanus toxoid. Thus, ICOSL, like CD80/86, is expressed on activated monocytes and dendritic cells but is regulated differently and delivers distinct signals to T cells that can be specifically inhibited by ICOSIg.

IL-2 and IL-15 regulate CD154 expression on activated CD4 T cells.

The cellular and humoral immune system is critically dependent upon CD40-CD154 (CD40 ligand) interactions between CD40 expressed on B cells, macrophages, and dendritic cells, and CD154 expressed primarily on CD4 T cells. Previous studies have shown that CD154 is transiently expressed on CD4 T cells after T cell receptor engagement in vitro. However, we found that stimulation of PBLs with maximal CD28 costimulation, using beads coupled to Abs against CD3 and CD28, led to a very prolonged expression of CD154 on CD4 cells (>4 days) that was dependent upon autocrine IL-2 production. Previously activated CD4 T cells could respond to IL-2, or the related cytokine IL-15, by de novo CD154 production and expression without requiring an additional signal from CD3 and CD28. These results provide evidence that CD28 costimulation of CD4 T cells, through autocrine IL-2 production, maintains high levels of CD154 expression. This has significant impact on our understanding of the acquired immune response and may provide insight concerning the mechanisms underlying several immunological diseases.

Signaling events involved in anti-CD20-induced apoptosis of malignant human B cells.

Anti-CD20 monoclonal antibodies have been successfully employed in the clinical treatment of non-Hodgkin's lymphomas in both unmodified and radio-labeled forms. Previous publications have demonstrated that the antitumor effects of unmodified anti-CD20 mAb are mediated by several mechanisms including antibody-dependent cellular cytotoxicity, complement-mediated cell lysis, and induction of apoptosis by CD20 cross-linking. In this report, we demonstrate induction of apoptosis by three anti-CD20 monoclonal antibodies [1F5, anti-B1, and C2B8 (Rituximab)]. The magnitude of apoptosis induction was greater with the chimeric Rituximab antibody than with the murine 1F5 and anti-B1 antibodies. Apoptosis could be enhanced with any of the antibodies by cross-linking with secondary antibodies (or Fc-receptor-bearing accessory cells). The signaling events involved in anti-CD20-induced apoptosis were investigated, including activation of protein tyrosine kinases, increases in intracellular Ca2+ concentrations, caspase activation, and cleavage of caspase substrates. Our results indicate that anti-CD20-induced apoptosis can be attenuated by PP1, an inhibitor of protein tyrosine kinases Lck and Fyn, chelators of extracellular or intracellular Ca2+, and inhibitors of caspases, suggesting that anti-CD20-induced apoptosis may involve modulation of these signaling molecules. We also demonstrated that varying the expression of Bc1-2 did not affect the magnitude of anti-B1-induced apoptosis, possibly because of the sequestering effects of other Bc1-2 family members, such as Bad. These studies identify several of the signal-transduction events involved in the apoptosis of malignant B cells that transpire following ligation of CD20 by anti-CD20 antibodies in the presence of Fc-receptor-expressing cells or secondary goat anti-(mouse Ig) antibodies and which may contribute to the tumor regressions observed in mouse models and clinical trials.

Characterization of scFv-Ig constructs generated from the anti-CD20 mAb 1F5 using linker peptides of varying lengths.

The heavy (VH) and light (VL) chain variable regions of the murine anti-human CD20 mAb 1F5 were cloned, and four single-chain Ab (scFv) molecules were constructed using linker peptides of variable lengths to join the VH and VL domains. Three constructs were engineered using linker peptides of 15, 10, and 5 aa residues consisting of (GGGGS)3, (GGGGS)2, and (GGGGS)1 sequences, respectively, whereas the fourth was prepared by joining the VH and VL domains directly. Each construct was fused to a derivative of human IgG1 (hinge plus CH2 plus CH3) to facilitate purification using staphylococcal protein A. The aggregation and CD20 binding properties of these four 1F5 scFv-Ig derivatives produced were investigated. Both size-exclusion HPLC column analysis and Western blots of proteins subjected to nonreducing SDS-PAGE suggested that all four 1F5 scFv-Ig were monomeric with m.w. of approximately 55 kDa. The CD20 binding properties of the four 1F5 scFv-Ig were studied by ELISA and flow cytometry. The 1F5 scFv-Ig with the 5-aa linker (GS1) demonstrated significantly superior binding to CD20-expressing target cells, compared with the other scFv-Ig constructs. Scatchard analysis of the radiolabeled monovalent GS1 scFv-Ig revealed a binding avidity of 1.35 x 108 M-1 compared with an avidity of 7.56 x 108 M-1 for the native bivalent 1F5 Ab. These findings suggest that the GS1 scFv-Ig with a short linker peptide of approximately 5 aa is the best of the engineered constructs for future studies.

Construction, expression, and characterization of anticarcinoma sFv fused to IL-2 or GM-CSF.

Local production of cytokines by genetically engineered tumor cells decreases their tumorigenicity and elicits protective immune responses against the parental tumor cells. An alternative approach to elicit a therapeutic immune response is to use fusion proteins that can target tumor cells and simultaneously activate effector cells. Fusion proteins between human IL-2, murine or human GM-CSF, and sFv of antihuman carcinoma antibody L6 have been constructed, expressed in both COS and Chinese hamster ovary (CHO) cells, and purified by affinity chromatography. The biologic activity of L6 sFV-hIL-2, L6 sFv-mGM-CSF, and L6 sFv-hGM-CSF was tested on human T cell blasts, factor-dependent FDCP-1, and TF-1 cells, respectively. The ability of soluble L6 sFv-hIL-2, L6 sFv-mGM-CSF, and L6 sFv-hGM-CSF to stimulate the proliferation of the indicator cells was found to be comparable to that of recombinant hIL-2, mGM-CSF, or hGM-CSF. Tumor cells coated with L6 sFV-mGM-CSF or L6 sFv-hGM-CSF were also tested in this way and were found to be potent stimulators, indicating that the cytokines were functionally active when bound to the tumor cell surface. This work demonstrates the feasibility of targeting sFv-cytokine fusion proteins for the activation of effector cells as an alternative to cytokine gene therapy.

A role for tumour necrosis factor-alpha, Fas and Fas-Ligand in marrow failure associated with myelodysplastic syndrome.

Apoptosis of haemopoietic cells in the marrow of patients with myelodysplastic syndrome (MDS) has been suggested as a mechanism for peripheral cytopenias. We determined the expression of Fas (CD95), Fas-Ligand (Fas-L) and TNF-alpha factors known to be involved in apoptosis, in the marrow of 44 patients with MDS and characterized their functional relevance in in vitro assays of haemopoiesis. Multidimensional flow cytometry revealed phenotypically aberrant blasts as defined by orthogonal light scatter and CD45 expression in the marrow of 24/44 patients. Among those blasts Fas expression was increased on CD34-positive cells and on cells co-expressing HLA-DR. In addition, Fas-L was expressed on some CD34+ cells of MDS patients but was never detected on CD34+ cells in normal marrow. Fas and Fas-L mRNAs as well as mRNA for TNF-alpha, known to increase Fas expression in normal marrow, were up-regulated in patients with MDS. TNF-alpha protein and sTNF-R1 levels in marrow plasma were higher in MDS patients than in controls (P<0.002 and <0.003, respectively). However, results were dependent upon disease category: TNF-alpha levels were significantly higher in patients with refractory anaemia (RA) than in patients with RA with excess blasts (RAEB) or RAEB in transformation (RAEB-T) (P=0.043). Conversely, the proportion of Fas-L-positive cells was lowest in patients with RA (P=0.037). In marrow cultures, Fas-Ig, rhuTNFR:Fc or anti-TNF-alpha antibody, by blocking Fas or TNF mediated signals, respectively, significantly increased the numbers of haemopoietic colonies compared to untreated cells (P<0.001, P<0.003, P<0.001, respectively). These results show significant dysregulation in the expression of TNF-alpha, Fas and Fas-L in the marrow from MDS patients. Altered expression of these molecules appears to be of functional relevance in the dysregulation of haemopoiesis in MDS and may be amenable to therapeutic interventions.

A role for CD5 in cognate interactions between T cells and B cells, and identification of a novel ligand for CD5.

CD5 is a glycoprotein expressed at a high level on the surface of mature T lymphocytes. Studies with CD5 mAb and CD5-deficient mice have shown that the CD5 molecules have a significant role in T cell growth response. However, the precise role of CD5 in immune cell interactions is still unclear. The present study provides evidence that CD5 plays a direct role in providing growth signals during the contact-dependent activation and proliferation of splenic B cells. An anti-CD5 mAb inhibited Th1- and Th2-type cell-induced B cell proliferation. CD5-Ig, a chimeric fusion protein, induced proliferation of resting B cells. Flow cytometric analyses using CD5-Ig and mAb to CD72 demonstrated that CD5 bound to a ligand (CD5L), and this binding was not blocked by a variety of anti-CD72 mAb. Also, CD5-Ig did not bind to CD72+-transfected cells. Immunoprecipitation of surface labeled B cell molecules with CD5-Ig showed that CD5L was composed of 77-80 and 38-40 kDa polypeptide chains, distinct from CD72. CD5L was expressed on activated splenic B cells, but not T cells, whereas its expression was constitutive on peritoneal B cells and on B lymphoma cell lines.

Apoptosis of malignant human B cells by ligation of CD20 with monoclonal antibodies.

CD20 is a nonglycosylated 33 to 37 kD phosphoprotein involved in B-cell signaling that subserves important functions in the regulation of B-cell proliferation and differentiation. In addition, this B-cell surface antigen has been shown recently to be an effective target for immunotherapy of B-cell malignancies using chimeric (mouse/human) or radiolabeled murine monoclonal anti-CD20 antibodies. In this report we show that extensive crosslinking of CD20 with murine anti-CD20 monoclonal antibodies (MoAbs) in the presence of either goat anti-mouse IgG or Fc receptor (FcR)-expressing cells directly inhibits B-cell proliferation, induces nuclear DNA fragmentation, and leads to cell death by apoptosis. The apoptotic effects of these MoAbs can be inhibited by chelation of extracellular or intracellular Ca2+ by EGTA or Bapta AM, indicating that anti-CD20-mediated apoptosis may be related to changes in Ca2+ concentration. These findings suggest that ligation of CD20 in vivo by anti-CD20 antibodies in the presence of FcR-expressing cells may initiate signal transduction events that induce elevation of [Ca2+]i and lead to apoptosis of malignant B cells, thereby contributing to the impressive tumor regressions observed in mouse models and clinical trials using anti-CD20 MoAbs.

Mitogenic properties of a bispecific single-chain Fv-Ig fusion generated from CD2-specific mAb to distinct epitopes.

The combination of anti-CD2 mAb 9.6 and 9-1, specific for distinct epitopes, induces proliferation of resting human T cells. The mitogenic activity of this mAb mixture depends upon accessory cells and the 9-1 mAb Fc domain. To further study the functional properties of these mAb, their variable regions were cloned and expressed as monospecific single-chain Fv (scFv) proteins fused to the human IgG1 Fc domain (scFvIg). A novel bispecific scFvIg was constructed by cloning the two monospecific scFv binding sites in tandem, with the 9.6 scFv placed N-terminal to the 9-1 scFvIg. Monospecific scFvIg binding to CD2 was comparable to that of the corresponding parental mAb, while the bispecific scFvIg exhibited binding activity similar to that of the 9-1 scFvIg. The combination of 9.6 scFvIg and 9-1 mAb was mitogenic, whereas mixtures including the 9-1 scFvIg were non-stimulatory, confirming the unique properties of the 9-1 IgG3 Fc. Without the IgG3 tail, the bispecific 9.6/9-1 scFvIg was directly mitogenic and was a more potent mitogen than the mAb mixture, but was accessory cell dependent. Unlike the combination of mAb, the bispecific reagent did not directly mobilize calcium in T cells. In comparison to the mAb mixture, bispecific 9.6/9-1 scFvIg-mediated stimulation of a mixed lymphocyte reaction was significantly more resistant to inhibition of the CD28 co-stimulatory pathway by the inhibitor CTLA-4-Ig. These results show that expression of the 9.6 and 9-1 binding sites together on a bispecific scFvIg increased the mitogenic properties of the mAb and altered the degree of accessory cell signals required for T cell activation.

HLA-DR-triggered inhibition of hemopoiesis involves Fas/Fas ligand interactions and is prevented by c-kit ligand.

The function of MHC class II (HLA-DR) Ags in hemopoiesis is not well defined. Here we investigated the effect of anti-HLA-DR mAb H81.9 on human marrow cells. mAb H81.9 inhibited colony formation from purified CD34+ marrow cells in long term culture-initiating cell assays. Inhibition was prevented, however, if c-kit ligand (stem cell factor (SCF)) was added to cultures concurrently with H81.9. DNA histograms from cultured untreated marrow mononuclear cells showed 2+/-1.2% apoptotic nuclei, whereas 14.1+/-5.4% were apoptotic after 12-h exposure to mAb H81.9. The apoptotic peak was reduced to 1.2+/-0.8% when SCF was added to cultures concurrently with mAb H81.9. The addition of Fas-Ig, a fusion protein that neutralizes Fas ligand (Fas-L), also prevented mAb H81.9-induced apoptosis. As determined by terminal deoxynucleotidyl transferase assays, agonistic anti-Fas mAb also induced apoptosis (in 13+/-4% of cells), and combined treatment with anti-Fas mAb and H81.9 was additive (27% apoptotic nuclei). The extent of apoptosis induced by anti-Fas mAb was significantly reduced by SCF. After H81.9 exposure, Fas was up-regulated on CD34+ cells, and Fas-L expression was 2.5-fold higher than in controls or CD34- cells, particularly within a small cell window with low orthogonal scatter (lymphocyte gate). These findings show that HLA-DR-mediated signals inhibit hemopoiesis in human marrow by a mechanism involving Fas/Fas-L-dependent signals that are blocked by c-kit ligand. These data suggest a possible role for MHC class II molecules in the regulation of hemopoiesis.

Human monocytic cells contain high levels of intracellular Fas ligand: rapid release following cellular activation.

Human monocytes express both Fas and Fas ligand (FasL) on the cell surface, and the interaction of these molecules induces spontaneous apoptosis. In this report we present a study of monocytic cells by FACS and confocal microscopy using anti-FasL Abs that reveals high levels of preformed FasL within the intracellular compartment. Further analysis by immunoblotting of cell cytoplasmic proteins confirmed the presence of a 37-kDa protein recognized by anti-FasL Abs. Stimulation of the monocytic cells with immune complexes, PHA, or superantigen gave rise to the rapid release of soluble FasL from within the cells. The presence of high levels of FasL within human monocytes suggests that, upon stimulation, the cells can rapidly translocate intracellular FasL to the cell surface and release it into the extracellular milieu. These findings indicate a novel mechanism for monocytes to respond rapidly to environmental changes, resulting in the release of active, soluble FasL.